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 . 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 . 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 . 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 . 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 . 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 : 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 . 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 . Figure 3 shows the process flow of a conventional distillation process . 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 . 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 . 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 , 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 . 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 . Figure 5 shows the plan and cross-section view of each plate type . 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 . Selection of plate type is based on the following factors : (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 , 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 . 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 . 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 . 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 . 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 . 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 : (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 . 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 . 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 . 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 . 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 . 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 . 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 , treatment of wastewater polluted with acetic acid . 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 : (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 . (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 . 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 . 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 . (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 . 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 . 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 . (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 . (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 . 6.3 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 . 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 . 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 : (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 . 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 . 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 . (ii) Film boiling - In kettle reboilers, the effect of fluid velocity is much smaller and film boiling may be a severe problem . 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 . 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 ). 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 . 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 . 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 . 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 . 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 ). 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) . 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 . 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 . 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 ) to remove sulfur and a catalytic reformer to reform its hydrocarbon molecules with a higher octane rating value . 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 : (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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