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PROFESSORIAL INAUGURAL LECTURE SERIES

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					                                                  Kamarul ‘Asri Ibrahim / 1


     PROFESSORIAL INAUGURAL LECTURE SERIES

              DISTILLATION: THE WORKHORSE OF
                  VAPOR-LIQUID SEPARATION

                                 by
                 Professor Dr. Kamarul ‘Asri Ibrahim




                            INTRODUCTION


In the chemical process industry, numerous operations are carried out
before products are produced from feed. Separation is one of the key
operations in any modern chemical plant. The separation process
involves a mixture being sent into a separation device together with a
separation agent to produce pure components. Examples of separation
agents are heat, solvent, pressure and mechanical force. The type of
separation agent used in a separation process depends heavily on the
mixture that needs to be separated. For heterogeneous mixtures
(mixtures with two distinctive phases: solid-liquid, solid-solid),
mechanical or gravity force is adequate for a satisfactory separation
while for homogeneous mixtures (mixtures with no distinctive phases:
liquid-gas, liquid-liquid, gas-gas), heat, solvent and pressure are
normally used as separation agents.
     Distillation is a common separation process for homogenous
mixtures in the chemical industry [1]. The homogenous mixtures
commonly separated using distillation is consist of liquid-vapor, liquid-
liquid or vapor-vapor. The main separation agents in distillation are
pressure and heat. Distillation are used in processes such as refinery
plants, gas processing plants, fractionation plants, air separation plants
and almost any type of chemical plants [2]. In Malaysia, distillation is
used widely in gas processing plants, refineries and air separation
plants. For example, gas-processing plants in Kerteh processes ethane
and its derivatives, propane (C3) and butane (C4) are processed in
plants in Gebeng and in Pasir Gudang, Titan Petrochemicals produces
plastic products from polyethylene.
2 / Professorial Inaugural Lecture Series 31



Besides distillation, other separation techniques in the industry are
absorption, adsorption, crystallization, extraction and membrane-based
technologies. However, distillation is clearly the most preferred choice,
accounting for more applications than all the other methods combined
[3]. On a worldwide scale, 95% of all separations are made with
distillation and more than 95% of the energy consumed by the
separation processes in the chemical process industries is from
distillation [3]. Distillation is definitely the main separation technique
in the chemical process industries nowadays.
     As a summary, separation using distillation involves separating the
components in the feed mixture in a system involving two phases, liquid
and vapor, through manipulation of heat. The concept of equilibrium
and vapor/liquid equilibrium (VLE) are important for general
understanding of the distillation process. These concepts are covered
in the following section.


2.0 EQUILIBRIUM AND VAPOR/LIQUID EQUILIBRIUM (VLE)

In distillation, two phases of a component are brought into contact.
When the phases are not in equilibrium, mass transfer occurs between
the phases. The rate of mass transfer of each species in the distillation
process depends on the departure of the system from equilibrium.
Equilibrium is a static condition in which no changes occur in the
macroscopic properties of a system with time [4]. Example, in the
reboiler for a distillation column, vapor and liquid phases is in
equilibrium. The equilibrium concept is stated in the Duhem’s theorem
as [4]: For any closed system formed initially from given mass of
prescribed chemical species, the equilibrium state is completely
determined when any two independent variables are fixed.
      This theorem is closely related to the concept of degrees of freedom
in equilibrium studies. The degrees of freedom are defined as the
number of phase-rule variables which must be arbitrarily specified in
order to fix the intensive state of a system at equilibrium. This parameter,
F, is determined as shown in Equation (1).
                       F =2 – ! +N                                    …(1)

where ! is the number of phases in equilibrium and N is the number of
chemical species. Vapor/liquid equilibrium (VLE), refers to systems in
which a single liquid phase is in equilibrium with its vapor [4, 5]. For
example, when N = 2 (there are two chemical species in the system),
the phase rule becomes F = 4 – !. There must be at least one phase
                                                   Kamarul ‘Asri Ibrahim / 3



in any system (! = 1), the maximum number of phase-rule variables
which must be specified to fix the intensive state of the system is three:
namely, pressure (P), x and y (xi refers to mole fraction of species-ij n
liquid phase and yi refers to mole fraction of species-i in vapor phase).
Figure 1 shows a threoretical P-xy diagram for a binary system [4].
     These diagrams (P-xy and T-xy diagrams) are important in distillation
operation. They provide important VLE data that are used in the
separation process in distillation. An important VLE parameter in
distillation operation is the relative volatility, "ij, of a species. This
parameter is calculated as in Equation (2).

                                                                      …(2)
                            yi / xi
                   " ij =
                            yj / x j
where yi and xi are the mole fraction/composition of species-i in the
vapor and liquid phase, respectively. This parameter gives the indication
of how volatile a component relatively to another component. If "ij =
1, then species, i and j, form an azeotrope (concept of azeotrope will
be discussed later on), and conventional distillation can not separate
the two species at their azeotropic composition. In distillation
operations, the relative volatility of a component over another
component is preferred to be more than one as this will indicate good
separation of the two components. If the relative volatility of the first
component over the second component is nearly one, than these two
components are difficult to separate as they have similar vapor-liquid
equilibrium characteristics. These examples indicate how vital the
relative volatility of a species is in distillation processes. The following
section will discuss about the general separation mechanism of
distillation.
     Normal distillation process involves two main sections: enriching
and stripping section [6]. Figure 3 shows the process flow of a
conventional distillation process [7].
     From Figure 3, the stages (referred as sieve or plate trays) in a
distillation tower (or column) is arranged vertically. In each stage, a
vapor stream and a liquid stream enter, are mixed and equilibrated,
and a vapor and a liquid stream leave in equilibrium. The feed enters
the column in Figure 3 somewhere in the middle of the column. If the
feed is liquid, it flows down to a sieve tray (stage). Vapor enters the
tray and bubbles through the liquid on this tray as the entering liquid
flows across. The vapor and liquid leaving the tray are essentially in
equilibrium. The vapor continues to the next tray, where it is again
contacted with a down flowing liquid. In this case, the concentration of
the more volatile component (component with a lower boiling point) is
being increased in the vapor from each stage going upward (enriched)
4 / Professorial Inaugural Lecture Series 31



                   11
                           Constant Temperature

                   10


                   9


                   8
                                           LIQUID
                   7
           P/atm




                   6


                   5


                   4       Bubble Line                                                 VAPOR


                   3
                                                 Dew Line
                   2
                     0.0     0.1    0.2     0.3     0.4       0.5     0.6      0.7     0.8         0.9         1.0
                                                             x,y
                                                             x, y



           Figure 1 P-xy diagram for constant temperature for a
           binary system


               390
                                                                            Constant Pressure

               380
                                                                                  VAPOR
               370


               360


               350
         T/K




                                                                                       Dew Line
               340
                                         LIQUID


               330


               320                                                                   Bubble Line



               310
                     0.0     0.1   0.2     0.3     0.4      0.5     0.6     0.7      0.8     0.9         1.0
                                                             x,y
                                                            x, y



                   Figure 2 T-xy diagram for constant pressure
                                                     Kamarul ‘Asri Ibrahim / 5




                              Condenser



                                                   Reflux Drum
               Vapor                              (Accumulator)




                                                         Distillate
                              Reflux                    (Overhead)



        Feed



                    Stage

                            Boilup

                                       Reboiler


                                                  Bottoms
               Liquid




       Figure 3 Proses flow of a conventional distillation process

and decreased in the liquid from each stage going downward (stripped).
Thus, the upper part of the tower above the feed entrance is known as
the enriching section and lower part of the tower below the feed is
known as the stripping section [7]. The final vapor product coming
overhead is condensed in a condenser and a portion of the liquid
product (distillate) is removed and the remaining liquid from the
condenser is returned (reflux) as a liquid stream to the top tray.
     The liquid leaving the bottom tray enters a reboiler, where it is
partially vaporized, and the remaining liquid, is withdrawn as liquid
product (bottom product). The vapor from the reboiler is sent back to
the bottom stage of the column. The reboiler can be considered a
stage since the vapor and liquid leaving the reboiler are in equilibrium
6 / Professorial Inaugural Lecture Series 31



[6]. From Figure 3, a distillation process consists of three main parts:
condenser, column tower and reboiler. Each of this main part will be
discussed in further details in the following section.


4.0 COLUMN TOWER, CONDENSER AND REBOILER

In the first part of this section, the column tower (column internal) will be
discussed followed by condenser and reboiler in the next two parts.


4.1 Column Tower

The column tower can be made from sections (from plate,valve sieve
trays) or packed sections. According to Sinnot [8], some considerations
to be made when choosing plate or packed columns:

1.    Plate columns can be designed to handle a wider range of liquid
      and gas flow rates than packed columns.
2.    For corrosive liquids, a packed column will usually be cheaper
      than the equivalent plate column.
3.    Packed columns are not suitable for very low liquid rates.
4.    The liquid hold-up is appreciably lower in packed column than a
      plate column. This is important when the liquids in the column
      are toxic or flammable; they need to be kept as little as possible
      for safety reasons.
5.    Packing should always be considered for small diameter columns,
      where plates would be difficult to install and expensive.

For plate columns, cross-flow plates are the most common type of plate
contactor used in distillation. Figure 4 shows a typical cross-flow plate [7].
     From Figure 4, the liquid flows across the plate and the vapor up
through the plate. The flowing liquid is transferred from plate to plate
through vertical channels called “downcomers”. A pool of liquid is
retained on the plate by an outlet weir. Three principal types of cross-
flow tray are used, classified according to the method used to contact
the vapor and liquid [7]. Figure 5 shows the plan and cross-section
view of each plate type [8].
     Sieve plate is the simplest type of cross-flow plate. The vapor
passes up through perforations in the plate, and the liquid is retained
by vapor flow. There is no positive liquid seal, and at low flow rates,
liquid “weep” through the holes, reducing the plate efficiency. From
Figure 5, vapor passes up through short pipes, called risers, covered
                                                      Kamarul ‘Asri Ibrahim / 7




                       Figure 4 Typical cross-flow plate

by a cap with a serrated edge for bubble-cap plates. The most
significant feature of the bubble-cap plate is that the use of risers ensures
that a level of liquid is maintained on the tray at all vapor flow rate.
Valve plates (floating cap plates) are essentially sieve plates with large-
diameter holes covered by movable flaps, which lifts as the vapor flow
increases. Valve plates can operate efficiently than sieve plates at lower
flow rates as the area of vapor flow varies with the flow rate. The type
of trays used affects greatly the fluid dynamics of the column; careful
selection of tray is important [9]. Selection of plate type is based on
the following factors [8]:

(i)     Cost: Bubble-cap plates are appreciably more expensive than
        sieve or valve plates.
(ii)    Capacity: There is little difference in the capacity rating of the
        three types; the ranking is sieve, valve, and bubble-cap.
(iii)   Operating range: This is the most significant factor. Operating
        range refers to the range of vapor and liquid rates over which the
        plate will operate satisfactorily (stable operation). The ratio of
        the highest to the lowest flow rates is often referred to as the
        “turn-down” ratio. Bubble-cap plates have a positive liquid seal
        and therefore operate efficiently at very low vapor rates. Valve
        plates are intended to give greater flexibility than sieve plates at a
        lower cost than bubble-cap plates. Sieve plates rely on the flow
        of vapor through the holes to hold liquid on the plate: can not
        operate at very low vapor rates.
8 / Professorial Inaugural Lecture Series 31




                                      Sieve plate




                     Bubble-cap                       Valve plates


       Figure 5 Plan and cross-section view of sieve, bubble-cap and simple
       valve plates


(iv)     Pressure drop: The pressure drop over the plates can be an
         important design consideration, particularly for vacuum columns.
         In general, sieve plates give the lowest pressure drop, followed
         by valve plates, and then bubble-cap plates.
                                                    Kamarul ‘Asri Ibrahim / 9


According to Kister [6], principal requirements of a packing should

(i)     Provide a large surface area: high interfacial area between the
        gas and liquid.
(ii)    Have an open structure: low resistance to gas flow.
(iii)   Promote uniform liquid distribution on the packing surface.
(iv)    Promote uniform gas flow across the column cross-section.

Packing in packed columns can be arranged regularly or randomly. In
stacked packing, such as grids, have an open structure, and are used
for high gas rates, where a low pressure drop is essential [8]. Random
packing is more commonly used in the process industries. The principal
types of random packing are shown in Figure 6.




Figure 6 Types of random packing (source: www.wikipedia.org/distillation/
packings)
     Raschig rings are one of the oldest manufactured types of random
packing, and are still in general use. Pall rings are essentially Raschig
rings in which openings have been made by folding strips of the surface
into the ring. This feature improves the liquid distribution characteristics.
Berl saddles were developed to give improved liquid distribution
compared to Raschig rings; Intalox saddles are considered an improved
version of Berl saddles. Intalox saddle has a shape that is easier to be
10 / Professorial Inaugural Lecture Series 31



made than Berl saddle [8]. Rings and saddles packing are available in
a variety of materials: ceramics, metals, plastics and carbon. Metal
and plastic (polypropylene) rings are more efficient than ceramic rings,
as it is possible to make the walls thinner.
     The choice of material will depend on the nature of the fluids and
the operating temperature. Ceramic packing will be the first choice for
corrosive liquids; but ceramics are unsuitable for use with strong
alkaline. Plastic packing can be used up to moderate temperatures:
unsuitable for distillation columns. Metal rings should be utilized when
the column operation is likely to be unstable. In general, the largest
size of packing that is suitable for the size of column should be used,
up to 50 mm [8]. Small sizes are appreciably more expensive than the
large sizes. The lower cost per cubic meter does not normally
compensate for the lower mass transfer efficiency for size packing of
more than 50 mm. The usage of very large packing in size in a small
column can cause poor liquid distribution.


4.2 Condenser

The main objective of condenser in distillation process is to condense
the overhead vapor to produce products (distillate). Condensers are
most commonly constructed from shell and tube heat exchangers. In
fact, the shell and tube heat exchanger is the most important type of
exchanger in use in process industries [7]. In Malaysia, a lot of
condensers are consist of fan type condenser. This is due to the fact
that fan type condensers are cheaper than other conventional
condensers and work accordingly to the needed requirements. Figure
7 shows the fluid flows in a conventional shell and tube condenser.
     In a shell and tube condenser, the flows are continuous. Many
tubes in parallel are used where one fluid flow inside these tubes. These
tubes, arranged in a bundle, are endorsed in a single shell and the
other fluid flows outside the tubes in the shell side. Condensation can
take place in the shell, tubes or both.
     Horizontal shell-side and vertical tube-side are the most commonly
used types of condensers [8]. A horizontal exchanger with
condensation in the tubes is rarely used as a process condenser. For
distillation involving a multi-component feed, the design of a condenser
is a difficult task. Some of the features that one must consider in the
design for distillation condensers are:

(i)   The condensation will not be isothermal. As the heavy component
                                                   Kamarul ‘Asri Ibrahim / 11




                                                           Refrigerant
                                                           In
            Vapor
            Out




                                                           Vapor
                                                           In
        Refrigerant
        Out




                                                         Condensate
                                                         Out


                Figure 7 A Conventional shell and tube condenser

        condenses out, the composition of the vapor, and therefore, its
        dew point, change.
(ii)    Since the condensation is not isothermal, there will be a transfer
        of sensible heat from the vapor to cool the gas to the dewpoint.
        There will be also a transfer of sensible heat from the condensate,
        as it must be cooled from the temperature of which it condensed
        to the outlet temperature. The transfer of sensible heat from the
        vapor can be particularly significant.
(iii)   The composition of the vapor and liquid change throughout the
        condenser, their physical properties vary.
(iv)    The heavy component must diffuse through the lighter
        components to reach the condensing surface. The rate of
        diffusion and the rate of heat transfer will govern the rate of
        condensation.

   All these aspects have to be carefully studied before designing a
condenser for a multi-component distillation process.
12 / Professorial Inaugural Lecture Series 31


4.3 Reboiler

Reboilers are used in distillation processes to vaporize a fraction of the
bottom product. Figure 8 and 9 shows the structure of external reboilers
and internal reboiler, respectively.
    Referring to Figure 8, in a forced-circulation reboiler, the fluid is




                                                      Steam




                      Bottoms



                                 Thermosyphon
                                 Reboiler




                                 Vapour
                                                     Steam




                                 Bottoms

                                 Kettle Reboiler

                      Liquid and vapor                   Steam
                      mixture to tower




                        Condensate
                                                     Bottoms
                     Tower                           Product
                    Bottoms
                              Pump
                                Forced-circulation
                                reboiler



  Figure 8 Types of external reboiler (Source: www.wikipedia.org/reboilers)
                                                    Kamarul ‘Asri Ibrahim / 13



pumped through the heat exchanger and the vapor formed is separated
in the base of the column. For thermosyphon reboiler (natural circulation
reboiler), vertical exchangers with vaporization in the tubes, or horizontal
exchangers with vaporization in the shell are used. The liquid circulation
through the heat exchanger is maintained by difference in density
between the two-phase mixture of vapor and liquid in the heat exchanger
and the single-phase liquid in the base of the column. In kettle type
reboiler, boiling takes place on tubes immersed in a pool of liquid;
there is no circulation of liquid throughout the heat exchanger. This
type of reboiler is also known as submerged bundle reboiler. In Figure
9, an internal reboiler is essentially a kettle reboiler without the shell. It
is bundled in the base of the column. The choice of reboiler to be used
in a distillation process depends on the following factors [8]:

(i)     The nature of the process fluid: particularly its propensity to fouling
        and viscosity.
(ii)    The operating pressure: pressure or vacuum.
(iii)   The equipment layout: particularly the headroom available.




                                                   STEAM




                                  BOTTOMS



Figure 9 A conventional internal reboiler (source: www.wikipedia.org/reboilers)
14 / Professorial Inaugural Lecture Series 31


      Forced-circulation reboilers are suitable for handling viscous and
heavily fouling process fluids. The circulation rate is predictable and
high velocities can be used. They are also suitable for low vacuum
operations. The major disadvantages of this type of operation is; a
pump is required and the pumping cost will be high and a possibility of
leakage of hot fluid will occur at the pump seal. Thermosyphon reboilers
are the most economical type for most applications, but are unsuitable
for high viscosity fluids or high vacuum operation. They would not
normally be specified for pressures below 0.3 bar [8]. A distinct
disadvantage of this type is that the column base must be elevated to
provide hydrostatic head required for the thermosyphon effect. This
will increase the cost of the column supporting-structure.
      Horizontal reboilers require less headroom than vertical types, but
have more complex pipe work. Horizontal exchangers are more easily
maintained than vertical, as tube bundle can be more easily withdrawn.
Kettle reboilers have lower heat-transfer coefficients than the other types,
as there is no liquid circulation. They are not suitable for fouling
materials and have a high residence time. They are generally more
expensive than an equivalent thermosyphon type as a larger shell is
needed. However, if the heat duty is such that the bundle can be
installed in the column base, the cost will be competitive with the other
types. As a separate vapor-liquid disengagement vessel is not needed,
kettle reboilers are often used as vaporizers. This section has covered
the main sections of a distillation column. The next section will focused
on different types of distillation operations.


5.0 DIFFERENT TYPES OF DISTILLATION OPERATIONS

There are various types of distillation operations. Among the common
ones are: stripping, Petlyuk distillation, azeotropic distillation, flash
distillation, reactive distillation and fractionators. The general operations
and mechanisms of each type of these columns will be discussed briefly.


5.1 Stripping Column

In this process, the feed to be distilled is added to the top of the stripping
column as shown in Figure 10.
     The feed is usually a saturated liquid at the boiling point and the
overhead product, the vapor rising from the top tray, goes to a
condenser with no feflux returned back to the tower. A stripping column
is a common unit operation found in many chemical processes [10-
13]. The stripper in Figure 10 can maintain its operation without any
                                                 Kamarul ‘Asri Ibrahim / 15




                                            Vapor




                 Feed




                                                    Liquid




                Figure 10 A conventional stripping column

feflux (or usage of condenser) because the feed is in the form of
saturated liquid. If the feed is a mixture of vapor and liquid, the
application of a condenser and reflux is a must.

5.2 Azeotropic Distillation

An azeotrope is a mixture of two or more volatile components having
identical vapor and liquid compositions at equilibrium [14]. The relative
volatility of the components is nearly one. Its composition, therefore,
cannot changed by distrillation. It is a constant-boiling-point mixture.
This property precludes its being separated by simple distillation, an
example of an azeotropic mixture is isopropyl alcohol (IPA) and water
(H2O). An azeotropic distillation was used to separate this mixture, with
cyclohexane (C6H12) as an entrainer [15]. Figure 11 shows the process
flow of the column.
    The mixture (IPA + H2O azeotrope) will be fed into the column
and another feed (C6H12) will also be fed into the column. The entrainer
(C6H12) has the objective of forming a lower-boiling-point ternary mixture
with the azeotropic mixture. The principle effect of this entrainer is to
remove one of the binary components overhead (in this case, water)
allowing the other to be concentrated in the bottom of the column. The
ternary azeotrope must be heterogeneous if the valuable product and
the entrainer in the overhead vapor are to be recovered. The overhead
vapor will be cooled and sent to a decanter. The decanter will separate
16 / Professorial Inaugural Lecture Series 31




                                            CyII Makeup

                                          On/Off
     Feed                                                            LC

                                                          Decanter        FC
  (IPA + H2O
  Azeotrope)                                LC
                                                                                Distillate
                                     FC
                                           Organic                             (H2O rich)
                                            Reflux



               LC                                    FC


                                                           Steam
       FC
                                          Reboiler


                    Bottom Product
                         (IPA)


            Figure 11 Process flow of a azeotropic distillation process

the mixture (H2O + C6H12) into C6H12 rich stream (which will be refluxed
into the column) and H2O rich stream (distillate). IPA will be recovered
int he bottom product. The example discussed involved a
heterogeneous azeotropic mixture. For homogeneous azeotrope
mixtures, extractive distillation is used to separate them. Heterogeneous
azeotropes separate into two liquid phases when condensed from vapor
while homogeneous azeotropes do not [16]. The mechanisms and
operations of extractive distillation are discussed in detail in [14, 16-18).


5.3 Flash Distillation

Flash distillation (equilibrium distillation) occurs in a single stage, the
liquid mixture is partially vaporized. The vapor is allowed to come to
equilibrium with the liquid, and the vapor and liquid phases are then
separated. Figure 12 shows a flash distillation process [7].
      In Figure 12, the pressure in the vessel, P1,is less than the pressure
at the valve, P2. The reduction in pressure from the valve to the vessel
will caused the vapor and liquid the vessel to separate. A flash distillation
process can be done continuously or batch wise. Conventional
                                                    Kamarul ‘Asri Ibrahim / 17




                                                       Vapor




                                  P2           P1
               Feed




                                                      Liquid



                  Figure 12 Flash (equilibrium) distillation

distillation column is in reality a series of flash distillation arranged in
a vertical fashion.


5.4 Reactive Distillation

In this type of distillation, the column serves both as a reactor and a
separator device. Feed (reactants) are fed into the column, reaction
took place in the column, and products are then separated and later
purified in subsequent unit operations. A catalytic distillation (catalytic
distillation is a reactive distillation with catalyst in the case) process
was used to produce methyl tert-butyl ether (MTBE) from methanol
and isobutylene [19]. Figure 13 shows the schematic diagram of the
MTBE process.
     From Figure 13, the isobutylene mixture (mixed C4) passes through
a mixer, a pre-reactor (R-1) (90% of isobutylene is converted) and heat
exchanger (E-1), before being fed into the catalytic distillation column
(CD-1). The methanol from another stream will enter CD-1 (reactive
distillation) and react with isobutylene to form MTBE, which will be
recovered in the bottom product of CD-1. The unreacted C4+ methanol
will pass through a separation process (D-1 & D-2) to recover C4 and
methanol in rich in streams. Other examples of reactive distillation:
chlorination of toluene in a semi-batch mode [20], treatment of
wastewater polluted with acetic acid [21].
18 / Professorial Inaugural Lecture Series 31




                                                                Unreacted C4
    Water

                                Unreacted C4                         Methanol
                                  Methanol
               V-1

  Methanol



                         R-1
                                                         D-1          D-2
    Mixed C4
                                                CD-1


                                                  Unreacted
                                                  Methanol           Recycling
                                                                       water
               V-2
                                                          E-2


                                E-1
                                                                       MTBE


             Figure 13   Schematic diagram of the MTBE process


5.5 Fractionator

Fractionation process is a process involving a series of distillation
columns used to separate a multi-component feed. An example of a
fractionator is presented in Wong [22]. In this work, a fractionator in an
oleo chemical plant is used as a case study. Figure 14 shows the
fractionation column.
     From Figure 14, the column is a packed column operating under
vacuum. One distinctive difference between this column and other
discussed types of columns is the concensing portion of the column.
A side draw stream is drawed from the column as a cooling liquid to
cool down the overhead vapor, prior from release from the top stage.
The “condenser” in this case is the top ‘stage’ in the column. The side
draw liquid stream will be sprayed into the column at the top to condense
the rising vapor in the top ‘stage’. An example of application of fractionator
in the petrochemical industry is the reforming of naphtha [23].
     Numerous examples of types of different distillation operations were
discussed in this section. One can take note the complexity of the
                                                                                                                     Kamarul ‘Asri Ibrahim / 19




                                                   To vent


                                                                                                               1.4
                                                                                                                                              Cooling
                                                   1.3                                                                                        Utility In
                                                                                      D
                                                                                                                         TIC
                                                                                1.6
                                                                                          FIC          Pumparound              HX-2     1.5
                                                                          PIC                             Point
                                                                                                                                      Cooling Utility
                                                                          P                                                                Out
                                                                               LIC                                       Reflux
                                                                                                                         Point
                                                                                                        FIC
                                                                          Sd

                                                                         Re
                                                                                                1.7

                          F


                                                                                                      Pump-2



      1.2

                                                                                            Controller
    Hot                    Hot                                                LIC
   Utility         HX-1   Utility                                                           TIC : Temperature Indicator Controller
     In                    Out                                                              FIC : Flow Indicator Controller
                                    TIC
                                                                                            LIC : Level Indicator Controller
                                                                                            PIC : Pressure Indicator Controller
                                                         PreCut Column
                                                                                            Equipment
                                                                                            HX : Heat Exchanger
     B
             1.1
                                          Pump-1                                            Stream
                                                                                            B : Bottom Flow Rate
                                                                                            Re : Reflux Flow Rate
                                                                                            Sd : Sidedraw Flow Rate
                                                                                            P : Pumparound Flow Rate
                                                                                            F : Feed Flow Rate
                                                                                            D : Distillate Flow Rate

                                                                                            Symbol
                                                                                            CL : Control Loop




                                                   Figure 14 Fractionation column


operation of distillation columns. The next section will discuss on the
subject of common column operation difficulties and troubleshooting.


5.6 Petlyuk Distillation

High energy consumption is a major drawback in distillation operation.
In order to achieve efficient energy operation in distillation, good design
of column operation is vital. Petlyuk column (fully thermally coupled
distillation column) is a complex column arrangement developed to
save energy usage in distillation. Figure 15 shows a typical Petlyuk
column [24].
     From Figure 15, the Petlyuk column consists of a prefractionator
and a main column. In the prefractionator, the components, A and C,
20 / Professorial Inaugural Lecture Series 31




                               A
                               B                               A




                                                    B
           A
           B
           C




                               B                         C
                               C




                      Figure 15 Typical Petlyuk column

are first separated; they are the extremes in relative volatility and are
easily separated. The other component, B, is distributed both up and
down. This technique is called a sloppy split, in contrast to a sharp
separation, where two components of adjacent relative volatility are
separated. The components, A and B, are then separated in the upper
part of the main column. Similarly, the components, B and C, are
separated in the lower part of the main column. The main column has
three product streams and supplies the reflux and vaporstreams
required by the prefractionator. The Petlyuk column can be modified
into a divided-wall column (DWC) as shown in Figure 16 [25].
     As shown in Figure 16, the prefractionator is constructed inside
the main column by introducing a vertical partition that divides the
column shell into a prefractionator and a side-draw section so it performs
a job of two columns in a single column shell saving valuable capital
investment. The major advantage of the Petlyuk column and DWC over
                                                 Kamarul ‘Asri Ibrahim / 21




                                                        A
                         AB




              A                            B
              B
              C




                         BC

                                                C




                      Figure 16 Divided-wall column

conventional distillation scheme (for separation of a mixture of three
components, two distillation columns (each with a condenser and a
reboiler) are used in sequence) is that there is no remixing of the middle
component B, in the prefractionator. In conventional distillation scheme,
the most volatile component A, is separated from components, B and
C, in the first column. The concentration of component B raises to a
maximum some way up the first column. However, when it exits at the
bottom, it has been diluted with a higher concentration of the least
volatile component C. This remixing means that energy has been wasted
on separating B partway up the column [25].
22 / Professorial Inaugural Lecture Series 31



6.0 COLUMN OPERATION DIFFICULTIES AND TROUGLESHOOTING

Distillation columns are complex unit operations with a lot of difficulties
in their operations. Capacity and efficiency are the major column
performance criteria. Field tests are by far the means for evaluating
these criteria. Field test data are used as basis for column performance
evaluation, optimixzation, debottlenecking and trougleshooting. Poor
test data have caused many troubleshooting and optimization effects
fall short of their expectations. When testing column performance in
the field, it is essential to recognize and avoid potential pitfalls.


6.1 Flooding

Column capacity is usually restricted by the onset of flooding [6, 8].
Flooding is excessive accumulation of liquid inside the column. Flooding
can be caused by various mechanisms such as spray entrainment
flooding and froth entrainment flooding. Figure 17 shows two other
types of mechanisms of flooding [8].

                                                       Liquid backed up
                                                       due to DC entrance
                                                       restriction
             P2


                              Backup due to tray
                              pressure drop, P1–P2
             P1               Backup due to friction
                              under DC, hd
        hf        hd          Backup due to
                              tray froth height hf
        Downcomer                                       Downcomer
        backup                                          choke
        flooding                                        flooding


              Figure 17 Downcomer backup and choke flooding

     From Figure 17, downcomer backup flooding is caused by aerated
liquid is backed up into the downcomer because of tray pressure drop,
liquid height on the tray, and frictional losses in the downcomer apron.
All of these will increase when liquid flow rate is raised, while tray
pressure drop also increases when vapor flow rate is raised. When the
backup of aerated liquid in the downcomer exceeds the tray spacing,
                                                   Kamarul ‘Asri Ibrahim / 23



liquid accumulates on the tray above, causing downcomer backup
flooding.
     In downcomer choke flooding, as liquid flow rate increases, so
does the velocity of aerated liguid in the downcomer. When this velocity
exceeds a certain limit, friction losses in the downcomer below. This
causes liquid accumulation on the tray above. Overcoming a flooding
limitation usually involves changes to the hardware of the column [6].
At other times, steps such as on-line cleaning, anti-foam injection and
solvent injection to dissolve frozen particles are methods of overcoming
specific problems which cause premature flooding. Techniques useful
for increasing column capacity during operation and when flooding
occurs at normal conditions are listed in brief below [6]:

(i)     Unloading - Most common technique, column unloading can be
        achieved either by reducing plant rated, changing feed
        composition, or by diverting one of the feed streams or portion
        of it away from the column.
(ii)    Prehating/precooling changes - Feed temperature can be varied
        (using heat exchangers) in order to unload the section above or
        below the feed. when the flooding limitation occurs below the
        feed, a hotter feed can reduce the reboiler heat load and the
        vapor and liquid traffic in the section below the feed at the expense
        of higher vapor and liquid traffic above the feed. Conversely,
        reducing feed temperature unloads the section above the feed
        at the expense of higher loads in the section below the feed [26].
(iii)   Pressure changes - Column capacity gains can be achieved either
        from raising or loweing the pressure. Raising the pressure reduced
        gas density. Thus allowing a greater vapor flow rate through the
        column, but it also reduces relative volatility, causing a higher
        reflux and reboil requirement for the same separation. Either
        factor may predominated, or the dominant factor dictates the
        direction in which pressure should be changed.
(iv)    Improved stability - When columns operate close to their operating
        capacity limits, even small disturbances can carry the column
        beyond the flood point. In order to operate the column at a point
        close to maximum capacity, stable operation and reduction of
        the magnitude and frequency of outside disturbances are
        essential. It is known that improved stability of a column can
        usually enchanced its capacity by 2 to 5 percent, and sometimes
        by up to 10 percent [6].
24 / Professorial Inaugural Lecture Series 31



6.2 Foaming

Foaming in fractionation and absorption columns can chronically lower
capacity and lead to premature flooding, liquid carryover and solvent
losses. In packed columns, poor distributor and redistributors action
are caused by foaming. Foam forms when bubbles rise to the surface
of a liquid and persist without coalescence with one another, or without
rupture into the vapor space. When a foam is stabilized, it can persist
for 2 to 3 minutes [6]. Four common mechanics that cause foam
stabilization are Ross-type foaming, Marangoni effect,mass-transfer-
induced Marangoni and formation of a gelatinous surface layer. Some
guidelines in identifying foam formation are presented below:

(i)     Foaming is a common problem in absorbers using aqueous
        solutions of high-molecular-weight organic solvents. Typical
        examples of such solvents are ethanolamine, glycol and
        potassium carbonate.
(ii)    Corrosion inhibitors are surface-active agents and generally
        severe foamers. For example, corrosion inhibitors injected into
        natural gas-gathering systems are known to have caused
        severefoaming problems in gas plant amine absorbers [6].
(iii)   Foaming is often experienced in some extractive distillations.
        Example: extractive distillation using acetonitrile solvent used in
        butadiene plants and sulpholane extractive stripping. This foaming
        is attributed to mass-transfer-induced Marangoni effect, is more
        likely to occur under stripping conditions.
(iv)    Foaming is a common problem in the stripping system of refinery
        crude atmospheric columns [6]. This scenario is often related to
        the long residence time of the residue at high temperature and
        the presence of trace impurities.
(v)     Foaming is sometime experienced in moderate pressure strippers
        that strip light from heavy hydrocarbons. The presence of small
        quantities of water may prompt this foaming.

   Foaming usually occurs when there is premature flooding and
massive entrainment [6]. Thus, foaming should be cured as soon as
possible once the earliest symptoms of foaming are detected. Some
general guidelines for curing foaming problems:

(i)     Choosing the correct foam inhibitor is important.
(ii)    It is vital to correctly inject the inhibitor. Injecting the inhibitor
        upstream of a point of high turbulence such as pump suction or
                                                 Kamarul ‘Asri Ibrahim / 25



       ahead of a pump letdown valve has been recommended. An
       injection past a long distance upstream of the column should be
       avoided whenever possible. Dispersing the inhibitor correctly is
       also important. Avoid massive injection without effective dispersal.
(iii) It is essential to inject the amount of inhibitor recommended by
       the supplier or found optimum in experimental tests. Excessive
       inhibitor injection can be harmful.
(iv) Increasing downcomer size and reducing the potential for
       downcomer flooding often eliminates foaming problems [27].
(v) Heavy hydrocarbons may condense out of warm saturated gases
       upon contact with a cold aqueous solution, promoting foaming.
       Upstream removal of the heavy constituents can reduce foaming.
(vi) Whenever possible, an effort should be made to identify the cause
       of the foaming and to minimize its effects upstream of the column.
       This can drastically reduce the cost of inhibitor and the adverse
       effects of the inhibitor on the product or downstream units.
(vii) In some applications, it may be feasible to replace an absorption
       solvent by one that is less likely to foam. For example, replacing
       light cycle oil by heavy naphtha was found the most effective
       means of preventing foaming in refinery absorbers [6].
(viii) In column piping, it is best to avoid oils or greases that have a
       soap or detergent basis.
(ix) After a new catalyst is installed in an upstream unit, catalyst dust
       maybe carried over into the column and induces foaming.
(x) In extractive distillations, foaming caused by stripping off of light
       components that lower the surface tension can often be
       suppressed by adding a heavy homologue. In one case, adding
       a minor quantity of kerosene to the feed of a sulpholane extractive
       stripper effectively suppressed foaming [6].

6.3   Reboiler Operation diffculties and Troubleshooting

Reboiler operating problems can be divided into process side problems
and heating side difficulties. Heating side problems are common
operating problems experienced by all types of reboilers while process
side problems are more specific towards the reboiler type.


6.3.1 Thermosyphon Reboiler (Process Side)

Excessive circulation occurs when a vertical reboiler sump level is too
high and cannot be lowered. This will restrict heat transfer rate and
26 / Professorial Inaugural Lecture Series 31



happen commonly in vacuum and atmospheric reboilers. An excessive
circulation problem can be cured by adding a restriction to the reboiler
inlet line. This can be implemented by installing a throttling valve in the
inlet line to the reboiler. The throttling valve should be located as close to
the reboiler as possible to prevent flashing in the reboiler inlet line [27].
         Insufficient circulation forms a mist flow one in the upper portions
of the reboiler tubes. This give rise to poor heat transfer, accelerated
fouling rates, and possible tube overheating. This problem is usually
caused by plugging, a leaking reboiler preferential baffle or draw pan,
or by insufficent liquid head. Leakage across the preferential baffle is
implied when the bottom sump level influences reboiler heat transfer
rate despite the presence of a baffle. Remedies for a deficient liquid head
are raising liquid level or cutting down pressure drop in the reboiler [6].
      Surging is an instability caused by depletion of lights and
consequent drop in heat transfer and boilup rate. Columns that are
prone to surging are those with bottom liquid consisting mainly of high
boilers, together with a small fraction of lights. Surging may also occur
when the column bottom contains water-insoluble componenets along
with a small quantity of water. The water acts as a light component
because of steam distillation. Surging in reboilers can be prevented by
the following general guidelines [6]:

(i)     Avoid subjecting heavy componenets to repeated thermal
        contact: polymerization causes surging problem.
(ii)    Periodically flash the reboiler with light component.
(iii)   Consider a forced-circulation reboiler.
(iv)    Consider a hotter heating medium.
(v)     For surging caused by presence of small quantity of water,
        elevating the reboiler liquid offtake and converting the section
        below the offtake into a reservoir will eliminate surging problem.

     Other types of thermosyphon reboiler operating problems (process
side) such as oscillations, temperature pinch, fouling, film boiling and
liquid distribution are covered extensively in Kister [27].


6.3.2 Forced-Circulation Reboilers (Process Side)

This type of reboilers is similar to vertical thermosyphon reboilers, but
do not depend on the natural thermosyphon action and commonly
operate at high circulation rates (using a pump). A major consideration
with these reboilers is pump-system compatibility. Net positive suction
                                                     Kamarul ‘Asri Ibrahim / 27



head (NPSH) is critical since the liquid is near its boiling point and
liquid head is costly. Oversized pumps could be detrimental to NPSH
and should be avoided. To obtain good heat transfer coefficients, it
has been advised to maintain moderate liquid velocities in the reboiler
[6]. Low velocities promote fouling and overheating, while high
velocities erode tubes and add little to improve the heat transfer rate.


6.3.3 Kettle Reboilers (Process Side)

In kettle reboilers, the reboiler liquid level plus the head for overcoming
reboiler circuit friction sets the liquid level at the column base. Some
common operating problems with kettle reboilers:

(i)     Fouling - Low velocities, high retention time in the heated zone
        and high fractional vaporization rates are conducive to fouling.
        Buildup of degradation products in the reboiler increase the
        boiling point and aggravate the problem. Adequate purging is
        frequently needed to purge out degradation products and avoid
        residue accumulation [28].
(ii)    Film boiling - In kettle reboilers, the effect of fluid velocity is much
        smaller and film boiling may be a severe problem [6]. It is
        important to ensure that vapor can escape faster than it is
        generated.
(iii)   Disengagement - Sufficient disengagement space needs to be
        provided above the bundle to disentrain liquid droplets. Demisters
        are used to improve disengagement.
(iv)    Bottom product surge - The liquid draw compartment of kettle
        reboilers is much smaller than most column bottom sumps, and
        usually provides less liquid residence time and product surge.
        Since it is often impractical to incorporate the desired residence
        time in this draw compartment, one can add a surge drum
        downstream of the reboiler.

6.3.4 Internal Reboilers (Process Side)

This type of reboilers is similar in principle to kettle reboilers. Some of the
main operating difficulties experienced with internal reboilers are:

(i)     Liquid level - Measurement and control of liquid level is a major
        problem in columns equipped with internal reboilers. As vapor
        bubbles through the liquid, froth rather than pure liquid exists at
        and above the bundle. The froth aeration tends to increase with
28 / Professorial Inaugural Lecture Series 31



        reboil rate and is difficult to predict. Thus, it is difficult to relate
        the apparent to actual liquid level. In order to minimize this
        problem, sufficient height should be incorporated above the top
        of the bundle for avoiding froth carry over into the column or
        bottom seal pan. Performing fields tests to determine the apparent
        liquid levels at points where flooding initiates and where reboiler
        tubes become unflooded at various reboil rates will define the
        satisfactory operating range as a function of the boilup rate.
(ii)    Distribution - Liquid distribution to reboiler is uneven in tray towers.
        Light compounds can be depleted near the inlet or the top of the
        bundle, causing temperature pinches. However, distribution is
        not a major problem because the internal reboiler has a large
        temperature difference and the bundle is small.
(iii)   Fouling - A less severe problem in internal reboilers than kettle
        reboilers because of the down flow movement of liquid. It is severe
        if tubes are unflooded. The isolating chamber must be blown
        down (perforating the chamber floor) to avoid it from becoming a
        dirt trap.


6.3.5 All Reboilers (Heating Side)

Some common operating problems experienced with all types of
reboilers (on the heating side) are:

(i)     Inerts - Accumulation of inerts can drastically reduce heat transfer,
        especially in steam reboilers. Accumulation of oxidizing or acidic
        inerts such as carbon dioxide is known to have caused severe
        corrosion [6]. Venting inerts must be carried out through adequate
        inert venting facilities.
(ii)    Condensate removal - Adequate removal of condensate is
        important to prevent flooding of the tube surface. In order to
        avoid condensate accumulation problems, steps such as sloped
        lines leading to the trap, avoiding undersized steam traps, reboiler
        provided with its own trap (when more than one reboiler is used)
        and adequate sizing of condensate outlet line can be taken.
(iii)   Blown condensate seal - When this happen, uncondensed vapor
        blows and channels right through the reboiler and out the
        condensate drain line. Heat transfer slumps and water hammer
        may follow. Throttling the reboiler outlet and installation of a
        condensate seal drum can overcome this problem [27].
                                                   Kamarul ‘Asri Ibrahim / 29



6.4 Condenser Operation difficulties and Troubleshooting

Inert accumulation, condenser fouling and condensate removal are by
far the most common problems that severely affect condenser operation
[6]. These problems and some troubleshooting guidelines are
discussed briefly in the following sections.


6.4.1 Inert Accumulation

Condensation heat transfer can be impaired even by accumulation of
a small fraction of non-condensable. Inert problems are most common
in shell side condensation, where gases can segregate in pockets,
and are difficult to remove unless sufficient pressure drop is used to
force them to the vent outlet [6]. Cross-flow condensers are prone to
inerts accumulation. Adequate venting facilities are needed at all
locations where non-condensable are likely to accumulate.


6.4.2 Condensate Removal

When condensate is removed at an insufficient rate or the condenser
traps condensate, heat transfer area will be flooded. This will lower
condenser heat transfer rates. Some guidelines to address this problem
in condensers are:

(i)     Adequate sizing of outlet condensate lines is important.
(ii)     If no liquid level is maintained in the condenser, the liquid outlet
        line should enter the vapor space of the reflux drum and should
        not be submerged.
(iii)   When a condensate pot is shared by a number of condensers,
        separate lines should lead each condenser to the pot.
(iv)    Unless it is intended to maintain a liquid level in the condenser,
        all liquid outlet piping should leave the condenser at the bottom
        and slope toward the reflux drum with no high points. This practice
        is essential for gravity draining.

6.4.3 Condenser Fouling

Fouling in condenser usually occurs on the coolant side [28]. Fouling
on the condensing side is seldom troublesome. They have been caused
by sticky or viscous materials which condense near the inlet. Flushing
tubes near the inlet with lighter material or proper external solvent can
minimize this type of fouling [6].
30 / Professorial Inaugural Lecture Series 31



6.4.4 Other Common Condenser Operation Problems

Aside from the discussed operation problems in the previous sections,
there are other types of operation problems involving condensers:

(i)     Flooding - This and liquid carryover are often experienced in
        vertical up flow, tube or shell side. Carryover occurred whenever
        the vent control valve opened excessively. Installing a valve limiter
        is sufficient to prevent carryover problems.
(ii)    Slug flow - When partial condensers are located below the reflux
        drum and the velocity in the risers is too slow, vapor and liquid
        segregate in the riser. A head of liquid builds up and exerts back
        pressure against the column. Periodically, a slug of liquid breaks
        through and releases the back pressure. The riser then gradually
        fills up with liquid and the cycle repeats itself. The column
        pressure and accumulator level will experience fluctuations. To
        prevent slug flow, dual risers are often installed. The small-
        diameter riser is operated at low throughputs, while the larger
        one is operated at higher throughputs [6].

     Various distillation operation problems, difficulties and
troubleshooting guidelines were discussed. One important fact that
can be stated: distillation operations are highly complex and should
be properly controlled to ensure optimum and stable operations. The
next section will discuss about the common control techniques applied
in distillation operations.


7.0 CONTROL ASPECTS OF DISTILLATION OPERATION

Distillation control is too wide a topic to be comprehensively covered
here. The coverage here emphasizes on operational aspects, various
control schemes for stable operations (not necessarily optimum
operation), recognizing and avoiding troublesome control schemes
and troubleshooting procedures in distillation control. Generally, a
column control system has three main objectives:

(i)     To set stable conditions for column operation.
(ii)    To regulate conditions in the column so that the product always
        meet the required specifications.
(iii)   To achieve the above objectives most efficiently.

     Table 1 shows the conventional pairing of control and manipulated
variables in distillation control
                                                     Kamarul ‘Asri Ibrahim / 31



Table 1 Conventional pairing of control and manipulated variables in
distillation control

    Control Variable                     Manipulated variable
    Bottom level                         Bottom flow rate
    Accumulator Level                    Distillate
    Column Pressure                      Condensation rate
    Composition (bottom)*                Boilup rate
    Composition (top)*                   Reflux flow rate


*Only composition of one product is controlled at a time (either bottom or
top), and not both products as controlling the two product compositions
simultaneously will cause serious coupling between the two composition
controllers [18].

      Figure 18 shows the controlled variables and manipulated streams
in a typical column control scheme.
      From Figure 18, variables typically controlled in a column include
flows, pressure, bottom level, accumulator level, top and bottom
product compositions. A stream is manipulated by varying the opening
of its control valve. The stream flow rate is varied to control a desired
variable. Five manipulated streams in Figure 18: top and bottom
product flow rates, reflux flow rate, condensation rate and boilup rate.
Some ground rules can be applied for initial screening out of
undesirable control schemes [6, 29]:


              PC




                                         LC




                                              D
  Feed
                                    QC




                                                  Figure 18 Controlled
         LC




                                                  variables and manipulated
                                                  streams in a typical column
                               QC
                                              B

                                                  control scheme
32 / Professorial Inaugural Lecture Series 31



(i)    Ideally, both product compositions should be controlled to
       maintain each within its desired specifications. However, in
       practice, simultaneous composition control of both products
       suffers from serious coupling between the two composition
       controllers. This interaction must be decoupled or instability will
       occur. In basic control systems, it is avoided by controlling only
       one of the two product compositions.
(ii)   Pressure is often considered the prime distillation control variable
       as it affects vaporization, condensation, temperatures, volatilities,
       compositions and almost any process in the column. Pressure
       is therefore paired with a manipulated stream that is most effective
       for providing tight control. When the top product is vapor, this stream
       is almost always the top product rate while when the top product is
       liquid, this stream is almost always the condensation rate.

       Distillation control scheme can be based on Material Balance (MB)
control or Energy Balance Control (EB). Each of these concepts is
discussed in the following sections.
       In a MB control scheme, product compositions are controlled by
manipulating the flow of material into and out of the column. This concept
is illustrated by a common MB control scheme shown in Figure 19.
       Suppose the composition of lights rises in the column feed. This
will caused a temperature drop and the temperature controller will
increase boilup. This will raise column pressure, and the pressure
controller will step up condensation. Accumulator level will rise, and
the level controller will increase distillate rate. The increased boilup
will reduce the amount of liquid reaching the bottom sump, and the
level controller will lower bottom product rate. The net result of the
control action (stepwise actions) is a shift in the material balance, so
that more of the feed leaves with the distillate and less with the bottom.
The shift transports the light component up the column and keeps
bottom (and top) compositions constant. The vast majority of distillation
columns use MB control schemes [6, 18].
       One implication of the MB control is that a product stream can not
be flow-controlled (or be the free stream). If flow rates of both the feed
and one product are fixed, then the flow rate of the other product must
be the difference between them (or accumulation will occur). This fixes
the material balance and precludes shifting it for product composition
control. Therefore, running a product stream on flow control leads to
poor product purity control.
       MB control schemes can be divided into direct and indirect MB
control. In indirect MB control, composition (temperature) controller
                                                        Kamarul ‘Asri Ibrahim / 33




                                              FC




                                                                       LC
                                                   FC




                                                                            D

       Feed


                                              TC




              LC




                                                                            B



                   Figure 19 Common MB control scheme

does not directly regulate a material balance stream. Instead, it
regulates boilup rate, condensation rate or reflux flow rate. The product
streams are controlled by level or pressure. Adjustments to the material
balance are thus performed indirectly (by working through the pressure
or levels). The control scheme shown in Figure 18 is an example of
indirect MB control scheme. In direct MB control, composition
(temperature) controller directly regulates a material balance stream.
The other product is regulated by pressure or a level. Figure 20 shows
a common direct MB control setup.
      The action (stepwise) of the control setup in Figure 20 is as follows:
if the concentration of lights rises in the column feed; a drop in column
34 / Professorial Inaugural Lecture Series 31



temperature will caused the temperature controller to increase distillate
flow. Accumulator level will fall, and the level controller will reduce
reflux. This action will lower the bottom level, and the level controller
will reduce bottom flow. Guidelines for choosing the most suitable MB
control scheme (direct or indirect) are discussed in detailed in literature
[6, 8, 18, 30, 31].


                                              PC




                                         LC




                                                           D
             Feed

                                              TC




                                                      FC
                    LC




                                                           B



             Figure 20 Common indirect MB control setup

7.2 Energy Balance (EB) Control

In an EB control scheme, energy balance variations control product
composition and the free variable is one of the product flow rate. A
common EB control scheme is shown in Figure 21.
    The main disadvantage of an EB control scheme is that material
balance variations interact with the controls. For this reason, EB control
schemes are only used if a satisfactory MB control scheme cannot be
implemented. The stepwise action of the EB control scheme shown in
Figure 21 is as follows: consider a rise in composition of the light
component in feed. The bottom section temperature will drop, and the
                                                    Kamarul ‘Asri Ibrahim / 35




                                               FC




                                          LC




                                                                    D
                                                            FC




         Feed


                                               TC




                LC




                                                                    B




                     Figure 21 Common EB control scheme

temperature controller will raise boilup. Column pressure will rise, and
the pressure controller will increase the condensation rate. The
accumulator level will rise, and the level controller will introduce more
reflux into the column. This in turn will reduce control tray temperature,
and the temperature controller will raise boilup again. This will continue
until reflux and boilup sufficiently rise to keep the bottom temperature
up. In this scheme, the bottom column temperature is assumed to be
an exact indicator of the corresponding product compositions (tray
temperature is usually selected to infer the tray compositions [32]).
36 / Professorial Inaugural Lecture Series 31



However, the temperature variation is very small at the column end
and may be difficult to distinguish from measurement noise [33]. For
multi-component columns, tray temperatures do not correspond exactly
to product compositions. This EB control scheme is not recommended
due to the stated limitations. The next sections will focused on research
work involving distillation processes carried out in Universiti Teknologi
Malaysia (UTM), Skudai and in the industries around the world.

8.0 EXAMPLES OF APPLICATION OF DISTILLATION

This section will highlight some of distillation applications in the
academic field and the industry. For the academic field, examples of
research work involving distillation carried out in the Department of
Chemical and Natural Resources Engineering (FKKKSA), UTM, Skudai,
will be presented. Applications of distillation in the chemical industries
will be discussed in Section 8.2.


8.1 Research Work Involving distillation

Distillation is considered a highly complex unit operation with multiple
variables changing rapidly throughout its operation. In the study of
process fault detection and diagnosis (FDD), distillation columns are
usually chosen as the case study due to their multivariate aspects. A
fractionation column in an oleochemical plant was chosen as the case
study in a FDD algorithm development project [34]. The fractionation
column is as shown in Figure 16.
     In this research, the fractionator was modeled and simulated in a
program. This program will generate data of the fractionation process
to be analyzed in the development of the FDD algorithm. Multivariate
Statistical Process Control (MSPC) together with multivariate projection
techniques such as Principal Component Analysis (PCA) and Partial
Correlation Analysis (PcorrA) was used to derive the relationship
between the variables of the process. Correlation coefficients were
derived using PCA and PcorrA to construct limits of monitoring charts
(Shewhart Control Chart and Range Control Chart) for FDD purposes.
Correlation coefficients represent the nature and strength of relationship
between the selected key process variables and the quality variables
of interest [35]. The developed algorithm was implemented on “fault
data” generated from the column model. Results show that the
developed FDD algorithm was able to detect and diagnose the pre-
designed faults in the process.
                                                  Kamarul ‘Asri Ibrahim / 37



     Distillation is also chosen as case study in modeling and simulation
work. A dynamic simulation algorithm was developed for simulation
of distillation column using Matlab [36]. The chosen case study was
a column used to remove butane from the feed (this column is also
known as debutanizer; its objective is to minimize butane (C4) in the
column bottom [37]). Some of the major assumptions made in the
algorithm are:

(i)     Liquid on the tray is perfectly mixed and incompressible.
(ii)    Vapor and liquid are in thermal equilibrium.
(iii)   Tray vapor hold-ups are negligible because the column operating
        pressure is less than 10 bar.
(iv)    Coolant and steam dynamics are negligible.

     The developed algorithm was able to simulate the column and
producing satisfactorily results (column profile from the algorithm was
compared to column profile from commercial simulator; satisfactorily
results obtained) [36]. Many research work involving distillation
columns are on modeling, simulation and analysis in the academic
field. Plant operation for distillation columns are expensive and are
only carried out in the industry. The next section will highlight some
examples of application of distillation in the industry (worldwide view).


8.2 Applications of Distillation in the Industry

Distillation columns can be found almost in any plant in the chemical
industries. Such industries are natural gas processing, petrochemical
production, coal tar processing, liquefied air separation, brewing,
hydrocarbon solvents production and similar industries. The widest
application of distillation is in petroleum refineries [38]. Petroleum crude
oils contain hundreds or more different hydrocarbon compounds. The
crude oil processing column (fractionator) does not produce product
having a single boiling point, rather it produces fractions having boiling
ranges [39]. For example, the crude oil fractionation process produces
an overhead fraction called “naphtha”. This naphtha “cut” has very
many different hydrocarbon compounds. Therefore, it has an “initial”
boiling point of about 35°C and a “final” boiling point of about 200°C;
“boiling range” produced in fractionating columns. This naphtha will
become a gasoline component after it is further processed through a
catalytic hydrodesulfurizer (a reactive distillation column [40]) to remove
sulfur and a catalytic reformer to reform its hydrocarbon molecules
with a higher octane rating value [41].
38 / Professorial Inaugural Lecture Series 31



An American chemical company, Koch Modular Process Systems
(KPMS), LLC., has applied distillation intensively in its operations. Below
are some of the past projects carried out by this company [42]:

(i)     Pharmaceutical Solvent Recovery - this modular system is
        designed to quote in a semi-continuous mode to recover isopropyl
        alcohol (IPA), near its azeotropic concentration from an aqueous
        process steam generated in the production of a pharmaceutical.
        The system is subsequently operated in a continuous mode to
        dry the IPA, using azeotropic distillation.
(ii)    PEG Steam Stripper - this unit treats various organic laden
        aqueous wastes for a pharmaceutical producer in order to bring
        them into compliance with the pharmaceutical effluent guidelines.
        The column has anti-fouling stripping trays and a short-trayed
        rectification section.
(iii)   High Vacuum Aroma Distillation - this distillation system separates
        two very close boiling compounds under reduced pressure for
        an aroma chemical producer. The column was packed with 5
        beds of wire gauze structured packing (80 theoretical stages).
(iv)    Benzene Stripper -benzene (VOC) contaminated wastewater is
        stripped from 30 to 0.5 ppm with a fuel gas that is subsequently
        sent to flare. The de-contaminated stream is then sent to a
        wastewater treatment plant.


9.0 CONCLUSION

Developments in distillation techniques have been largely inspired by
increasing operation costs and complex chemical separation
processes. Modeling, simulation and analytical studies on distillation
have been carried out extensively to improve distillation operations.
Industrial application of distillation is so wide that refineries and gas
processing plants base their operations on distillation. Continuous
development on technology of distillation is needed in order to maintain
stable and optimum operations. As modern day distillation operation
problems become more complex and difficult, the need to develop
better technique will never be over.

				
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