; AIChE GE Energy
Documents
Resources
Learning Center
Upload
Plans & pricing Sign in
Sign Out
Your Federal Quarterly Tax Payments are due April 15th Get Help Now >>

AIChE GE Energy

VIEWS: 3 PAGES: 27

  • pg 1
									                               AIChE T4 174869

                        Turboexpander-Compressor
                      Technology for Ethylene Plants

                               Radjen Krishnasing
                            Senior Lead Process Engineer
                                  The Shaw Group


                                 Gabriele Mariotti
                                Engineering Manager
                                    GE Oil & Gas
                                   Florence, Italy


                                    Kara Byrne
                    Applications Engineer & Commercial Manager
                                    GE Oil & Gas
                                 Houston, TX, USA


                               Radjen Krishnasing
                            Senior lead process Engineer
                                  The Shaw Group




           Prepared for Presentation at the 2010 Spring National Meeting
                        San Antonio, TX, March 21-25, 2010


AIChE and EPC shall not be responsible for statements or opinions contained in papers
                            or printed in its publications
                        Turboexpander-Compressor
                      Technology for Ethylene Plants
                                Radjen Krishnasing
                            Senior lead process Engineer
                                  The Shaw Group                                         Formatted

                                 Gabriele Mariotti
                                Engineering Manager
                                    GE Oil & Gas
                                    Florence, Italy
                                    Kara Byrne
                    Applications Engineer & Commercial Manager
                                    GE Oil & Gas
                                Radjen Krishnasing
                            Senior lead process Engineer
                                  The Shaw Group



                                      Abstract

Today’s ethylene plants incorporate Turboexpander Systems to optimize cryogenic
recovery and reduce the energy demand. The molecular weight and flow rate of the
residue gas depend directly on the selected upstream feedstock gas composition,
conversion, and feedrates. Various recent ethylene units have generated residue gas
volumetric flow ranges from approximately 100-200%. Hence, the Turboexpander
system is designed and manufactured accordingly.

As we are aware, the typical naphtha cracker produces a methane rich residue gas
(bulk hydrogen is recovered, treated, and delivered as a high pressure co-product). On
the other hand, the typical ethane or E/P cracker produces a very high hydrogen
content residue gas. Current designs and revamps require a wider range of feedstocks,
and hence, a correspondingly wide range of residue gas composition and quantity.

In order to meet the above demands, the Turboexpander solution must be flexible. As
an overview, we will discuss the typical performance of one- and two-stage
Turboexpander solutions for the expansion and recompression of the residue gas. Key
mechanical design recommendations (e.g., magnetic bearings, variable nozzles,
multistage control, high head wheels) will be outlined. Based on the demand from the
different feedstocks and the industry requirements for feedstock flexibility, we will then
discuss the technology and mechanical solutions. This presentation will also include
related design improvements that have been successfully utilized in other
Turboexpander applications.

                                         Part A
                                   Radjen Krishnasing


Introduction
Turbo-expanders/re-compressors play a crucial role in the recovery of both ethylene
and hydrogen from cracked gas in steam cracking units. A turbo-expander converts
energy that has been incorporated into the cracked gas, by the cracked gas compressor
and by the ethylene/propylene refrigeration systems, back to refrigeration at the lowest
temperature levels, to further enhance the recovery of ethylene and hydrogen. Turbo-
expanders are, therefore, integrated into the cold fractionation cryogenic section of an
ethylene unit.
Turbo-expanders take the tail gas (mixture of hydrogen and methane) at high pressure
and low temperature and drop the pressure over the expander with isentropic
efficiencies of well more than 80%, producing a cryogenic stream that can be 40oC to
50oC lower than the lowest level of ethylene refrigerant. These cryogenic streams are
then used for refrigeration to retrieve the last minor portion of ethylene from the tail
gas that otherwise would have been lost. After providing refrigeration, the warmed up
tail gas is compressed by the re-compressor to fuel gas pressure level. The driver of the
re-compressor is the expander that conveys the energy liberated by the expansion
through a common shaft.
Effects ethylene plant feedstock
A critical parameter in the integration and design of turbo-expanders is the composition
of the tail gas (mixture of hydrogen and methane). Depending on the plant fresh
feedstock and the potential hydrogen pre-recovery, the tail gas can be very rich in
methane for one feed or very rich in hydrogen for another. Most ethylene units are
designed to crack either a light feedstock, such as ethane/propane, or a heavy
feedstock, such as naphtha or heavier liquid feedstock. However, there are units with a
much wider range of feedstock. Cracking a light feedstock, in particular ethane,
produces a high ratio of hydrogen to methane. However, a typical ethylene complex
based on ethane (or ethane/propane) needs very little hydrogen as product. The need
is limited to the hydrogenation of acetylenes and small quantities of high purity
hydrogen product, for use by downstream polymer units. To the contrary, an ethylene
unit cracking naphtha or heavy liquid feedstock produces a lower ratio of hydrogen to
methane but demands much more hydrogen co-product for the hydrogenation of
unsaturated by-products that have been produced.
Table 1 below demonstrates the yield patterns of different feedstock, expressed in
component molar ratio with respect to ethylene. It shows a noticeable difference
between ethane feed and any other feedstock:
    - Ethane as feed produces the highest ratio of hydrogen to ethylene, while the           Formatted: Bullets and Numbering

       ratio of heavier byproducts to ethylene is the lowest. It produces very low ratio
       of methane leaving a tail gas high in hydrogen.

   -   Naphtha and gasoil as feed produce a relatively low ratio of hydrogen to
       ethylene, but a very high ratio of heavy byproducts to ethylene, therefore
       requiring very high recovery of hydrogen as product.

   -   Propane as feedstock has a very interesting mid-position. It produces a tail gas
       that has a close resemblance to naphtha or gasoil. Propane can act as a buffer
       for the heavy feedstock in ethylene plants designed with a broad range of feed
       slate such as a unit to crack a combination of ethane and heavy feeds.

Table 1: The molar ratio of key components / ethylene in cracker effluent for
          typically used feedstocks.
Feedstock type                 Ethane  Propane     Naphtha Gasoil
Cracked Gas H2 / C2H4          1.07    0.63        0.44        0.30
Cracked Gas CH4 / C2H4         0.23    1.24        0.83        0.62
Cracked Gas (C4 & C5) / C2H4   0.02    0.07        0.22        0.24
Cracked Gas Pygas / C2H4       0.01    0.05        0.19        0.14
Tail gas H2 / CH4 ratio        4.15    0.51        0.53        0.48

Ethylene plants cracking primarily liquid feedstock produce relatively high ratios of
unsaturated C4 and heavier fractions. These fractions often require hydrogenation to
either serve as recycle feed to the cracking furnaces or as finished product of the
ethylene plant. A typical ethylene unit cracking liquid feed is therefore characterized by
a very high recovery of hydrogen to balance this need. Recovery of hydrogen as
product can be as high as 90%. Hydrogen is recovered at high pressure (3000 kPa),
which means that the recovered hydrogen can no longer be part of the tail gas that
feeds the turbo-expander. The challenge in the integration and design of the turbo-
expander is to find the optimal balance between maximizing hydrogen recovery while
maintaining a reasonable flow to the turbo expander to minimize loss of ethylene.
On the contrary, an ethylene plant cracking ethane or a combination of ethane/propane
is characterized by a very high ratio of hydrogen to ethylene, a low ratio of methane
and an insignificant amount of C4 and heavier fractions. As a result, the recovery of
hydrogen as a product is little to none, meaning that virtually all of the tail gas is
available as feed to the turbo-expander. However, as a lighter tail gas will have a richer
ethylene content, maximizing the available tail gas for the turbo-expander is a critical
parameter in reducing the loss of ethylene.
Case study
The following two cases are presented to further emphasize the design challenges when
specifying and selecting a turbo-expander.
The first case is for an ethylene plant where the predominant feedstock is naphtha,
producing a nominal product rate of 1,000 KTA ethylene (1 million metric tons per
year). This case will demonstrate that with the integration of a turbo-expander, only a
single stage is needed. Hydrogen recovery is maximized while minimizing the loss of
ethylene in the tail (or residue) gas. The variations in composition, and frequently the
flow rate of the tail gas to the turbo-expander, are not affected if the feedstock cracked
by the ethylene unit does not vary over a wide range from heavy to light naphtha. It is
also not very sensitive to the cracking severity because the high hydrogen recovery
results in a residue gas feeding the turbo-expander that is very rich in methane. A
minimal variation of composition and flow rate to the turbo-expander is then often
caused by the extent of hydrogen recovery, or the overall plant capacity.
    To Fuel gas



                                                                            To Hydrogen Purification




          COM               EXP                             Methane
                                                           Rich Stream
                                                                          Hydrogen
                                                                         Rich Stream


                                                      PC




                                                                COLD
                                                            FRACTIONATION
                                                               SYSTEM




        Figure 1: Turbo-Expander in an Ethylene Plant based on Liquid Feedstock


Table 2: Overview liquid (Naphtha) feedstock cracking.
Key item clarifications:
Am3 / min             Actual cubic meters / minute
dHs                   Isentropic enthalpy difference between inlet & outlet
kmol / hr             1000 moles per hour
kPA                   kilo pressure atmospheric Ξ (0.0145 psi / kPA)
kW                    Hp = 0.746 kilowatts




                        Naphtha           Higher Hydrogen               Lower Hydrogen
                        feed             recovery (less flow                recovery
                        cracking           through turbo-              (more flow through
                                             expander)                  turbo- expander)
Expander Inlet
Flow rate (kmol/hr)     2400           2176                       2850
Molecular weight        14.0           14.5                       13.2
Pressure (kPA)          3050           3050                       3050
Temperature( oC)        -100           -97                        -103

Expander outlet
Flow rate (Actual 69                   63                         83
m3/min)

Re-compressor
Inlet
Flow rate (kmol/hr)     2400           2176                       2850
Mole weight             13.99          14.52                      13.2
Pressure (kPA)          354            356                        357
Temperature ( oC)       -3             -3                         -4
Flow rate (Am3/min)     214            194                        250

Re-compressor
Outlet
Pressure (kPA)          604            604                        604

Expander        dHs     111            105                        118
(kJ/kg)
Turbo-expander          28,630         27,640                     30,000
RPM
Expander     power      970            870                        1140
(kW)
Expander efficiency     86             86                         85
(%)
The second case (Tables 3A and 3B) is for an ethylene unit cracking light feedstock,
ethane or ethane/propane. It is based on 1,500 KTA ethylene production rate (1.5
million metric tons per year).
As can be seen from Table 1, that when ethane is cracked, it produces a high ratio of
hydrogen and a low ratio of methane. The opposite is true if propane is cracked,
resulting in a low ratio of hydrogen and a high ratio of methane. A turbo-expander
designed for a hydrogen rich feed will, in general, require two single-stage expanders in
series. The limitation is imposed by the re-compressor section as is discussed in the
second part of this paper.
                                                                        To Hydrogen Purification

    To Fuel gas




                                                                                     Tail Gas
           COM               EXP
           (HP)              (HP)
                                                                               PC




                                                                   PC


           COM               EXP
           (LP)              (LP)



                                                                             COLD
                                                                         FRACTIONATION
                                                                            SYSTEM




            Figure 2: Turbo-Expander in an Ethylene Plant based on Ethane Feedstock
 Table 3A: Overview light (ethane,                  Table 3B: Overview light (ethane,
     ethane/propane) feedstock                     ethane/propane) feedstock cracking
 cracking (High-Pressure Machine)                       (Low-Pressure Machine)
Key Item Clarifications: Refer to Table 2
                          100%       50/50                               100%          50/50
                          C2 feed    C2/C3 feed                          Ethane feed   Ethane/propan
                          cracking   cracking                            cracking      e feed cracking
HP Expander Inlet                                 LP Expander Inlet
Flow rate (kmol/hr)       7935       7883         Flow rate (kmol/hr)    7923          7742
Mole weight               4.94       7.66         Mole weight            4.91          7.42
Pressure (kPA)            2131       2131         Pressure (kPA)         1165          1175
Temperature( oC)          -114       -118         Temperature( oC)       -135          -134

HP Expander outlet                                LP Expander outlet
Flow rate (Am3/min)       127        121          Flow rate (Am3/min)    215           207

HP       Compressor                               LP Compressor
Inlet                                             Inlet
Flow rate (kmol/hr)       7837       7567         Flow rate (kmol/hr)    7836          7567
Mole weight               4.72       7.17         Mole weight            4.72          7.17
Pressure (kPA)            630        636          Pressure (kPA)         532           541
Temperature ( oC)         63         60           Temperature ( oC)      43            43
Flow rate (Am3/min)       580        550          Flow rate (Am3/min)    647           612

HP Compressor                                     LP Compressor
Outlet                                            Outlet
Pressure (kPA)            740        740          Pressure (kPA)         630           636
Expander dHs (kJ/kg)      130        80           Expander dHs (kJ/kg)   124           82
Turbo-expander RPM        20,000     16,240       Turbo-expander RPM     20,000        16,340
Expander power (kW)       1360       1255         Expander power (kW)    1355          1265
Expander     efficiency   86         85           Expander efficiency    89            86
(%)                                               (%)
Further evaluation/observations
      An important turbo-expander design parameter is the isentropic enthalpy drop (dHs)          Formatted: Bullets and Numbering

       across the expander. As discussed in the second part of this paper, this number is
       indicative of the expander or re-compressor wheel tip speed. As a general guideline, an
       enthalpy drop of up to 180kJ/kg is considered to set an optimal basis for the turbo-
       expander design. For our naphtha case, the isentropic enthalpy drop is in the order of
       110kJ/kg – a number that falls in this range and does not provide unusual constraints
       to the design of the turbo-expander. A single-stage design is therefore very common
       for naphtha (or other liquid/LPG feedstock) based ethylene plants.



      For our ethane cracking case, a two-stage turbo-expander/re-compressor design is            Formatted: Bullets and Numbering

       used. The isentropic enthalpy drop across each expander stage is kept around
       125kJ/kg. Although using a single stage expander is not impossible, the overall
       isentropic drop in that case would be 250 kJ/kg. In general, the constraint is not the
       expander side but the compressor side. As can be seen from the tables, the volumetric
       flow of gas flowing into the re-compressor is nearly five times higher than the expander
       outlet flowrate. The re-compressor rotor is therefore the larger of the two wheels,
       becoming the limiting factor in the design.



      The naphtha case demonstrates the effects of higher or lower hydrogen recovery than         Formatted: Bullets and Numbering

       the design recovery of the turbo-expander. A higher recovery of hydrogen can be
       desired in plant operations as a way to produce more product hydrogen. This will
       reduce the total flow through the expander, while at the same time increasing the
       molecular weight. As can be seen in the second column in Table 2, the turbo-expander
       is still within its operable range, but it will provide less refrigeration because of the
       reduced flow rate through the turbo-expander. This will have to be taken into
       consideration       when    deciding    on      increasing   recovery      of   hydrogen.


      As the demand for raw C4 and perhaps also raw C5 as finished co-products without
       hydrogenation increase, an ethylene plant cracking liquid feedstock can end up with
       excess hydrogen product. If there is no other output for product hydrogen, it is
       ultimately letdown to the fuel gas header and combusted in the cracking furnaces.
       Instead of letting the product hydrogen across a control valve (isenthalpic), it would be
       more beneficial to pass this excess of hydrogen through the expander. The third column
       of Table 2 (the naphtha case) demonstrates the effects this will have. More hydrogen
       across the expander will result in more cryogenic duty from the turbo-expander, and as
    an overall effect, it will reduce the refrigeration demand from ethylene/propylene
    refrigeration systems. Table 2 shows that the increased flow rate combined with a
    reduced molecular weight will increase the RPM of the turbo-expander. How much
    hydrogen can be diverted to the turbo-expander is a function of how much room is
    available in the design of the turbo-expander. A typical design comfortably will
    accommodate an increase such as demonstrated in the table.



   The gas cracker case evaluation demonstrates the simple fact that in case a turbo-          Formatted: Bullets and Numbering

    expander is designed for the tail gas of an ethylene plant cracking ethane (tail gas very
    rich in hydrogen), a mixed feed case of ethane and propane is less stringent to the
    operation of the turbo-expander. The first column of Table 3A and Table 3B are for
    pure ethane feedstock, while the second column of each is for a 50/50 ethane/propane
    case.



   In these days of mega-size steam cracking units, serious challenges are presented to        Formatted: Bullets and Numbering

    the sizes of major compressors and other equipment, such as separation columns.
    When it comes to turbo-expanders however, the sizes are far from reaching their
    maximum. While the naphtha case turbo-expanders use a 225mm expander wheel and
    the gas case a 350mm wheel; these are by far not the largest sizes used in other
    branches of the industry for turbo-expanders. It is also interesting to note that the
    scale-up, which has been seen since the early use of turbo-expanders, from small
    ethylene units to today’s mega-size plants, hardly has affected the high (isentropic)
    efficiencies the industry has relied upon. This feature continues to make turbo-
    expanders a very important choice in maximizing the economics of ethylene plants.




                                         Part B

                                   Gabriele Mariotti
                                      Kara Byrne


                                       Foreward
       The importance of turboexpanders has increased significantly over the past few
decades since the first application of a turboexpander in the oil and gas industry by the
founder of Rotoflow, Dr. Judson Swearingen. Typically, turboexpanders were used to replace a
Joule-Thompson (JT) valve in order to increase the overall efficiency of air separation plants.
Driven by increased competition in the oil and gas market, it is increasingly common to find a
turboexpander as a key component for the overall production in a hydrocarbon gas separation
plant. This is especially important for designing a more efficient and competitive ethylene
plant.
       While the turboexpander alone can easily reach isentropic efficiencies of up to 90%,
when it is directly coupled to a compressor the interaction of the two machines must be taken
into account. The turboexpander efficiency is limited by the compressor (and vice versa) and,
therefore, cannot be optimized beyond the mechanical limitations of each machine.
       This paper, after a brief discussion of current technologies and the characteristics of GE
Oil & Gas Turboexpanders, will focus on some typical turboexpander compressor selections
showing the interaction between the selection of the turboexpander and re-compressor.

                                   Turboexpander History

       The turboexpander is a reaction type radial turbine originally developed to replace the
Joule-Thompson (JT) valve in air separation plants.
       The French Engineer, George Claude, utilized the first radial turbine for air liquefaction
in the early 1900s. German engineers, including Dr. Carl von Linde, further developed and
improved the turbines for many other applications, such as refrigeration and jet propulsion
engines.
       After World War II, Dr. Judson Swearingen began to develop the turboexpander for
natural gas processing applications (Photo-1). He realized the overall cooling capacity of the
plant and, therefore, the cost and performance, is greatly improved by replacing the JT Valve
with a simple and reliable machine that expands a single-phase stream in a nearly isentropic
method. The fact that the radial inflow turbine could handle two-phase flow at the discharge
made the machine perfect for heavy hydrocarbon removal.
      The turboexpander continues to date to develop in the natural gas industry. In the
1960s, turboexpanders were used in ethylene projects and then naturally progressed into
several other markets such as liquefied natural gas, geothermal, and gas-to-liquids.
                              Turboexpander Applications

        Turboexpanders are predominantly used in refrigeration/liquefaction processes and
power generation applications.
        The refrigeration/liquefaction process utilizes the Turboexpander for cooling fluids
through nearly isentropic expansion from a higher pressure to a lower one. This is able to
achieve much lower temperatures than throttling the fluid through a JT valve by isenthalpic
expansion. The lower temperatures considerably increase the overall refrigeration cycle
efficiency.
        Typical applications covered by GE Oil & Gas Turboexpanders are: Natural Gas
Processing/Dew Point Control Plants, Pressure Let Down Energy Recovery, and
Geothermal/Waste Heat Energy Recovery.
        Depending on the service required, mechanical power produced by expansion of flow in
the radial turbine can be recovered or dissipated through three main machine configurations:

Turboexpander-Generator
      Mechanical power is converted into electrical power through a reduction gear and a
generator (Photo-2).




              Photo-2: Turboexpander-Generator General Arrangement

Turboexpander-Compressor
      Mechanical power drives a compressor impeller either coupled to the same shaft as the
turboexpander or driven via a gearbox (Photo-3).
             Photo-3: Turboexpander-Compressor General Arrangement

Turboexpander-Dyno
       Mechanical power is dissipated through an oil brake if it is not economical to convert
the excess power into another form of energy (Photo-4).




                              Photo-4: Turboexpander-Dyno

      Often it is not clear which turboexpander configuration is suitable for an ethylene plant,
since the same service can be covered through either a Turboexpander-Generator or a
Turboexpander-Compressor. Table-1 lists the pros and cons of both solutions.
          Table-1: Comparison of Various Turboexpander Machinery Configurations
                            PROS                               CONS
                   Very high efficiencies can be achieved. The             The machine has a tendency to speed up
                    wheel can be optimized to achieve the best               in case of electric load rejection. This
                    aerodynamics by freely changing the RPM                  limits the maximum tip speed of the wheel
                    without other machinery constraints.                     and tripping devices need to be redundant
 TURBOEXPANDER-




                   Recompressor is designed independently from              for safety reasons.
   GENERATOR




                    the turboexpander, merging more stages into a           The machine is typically more complex
                    single machine with higher efficiency.                   than a Turboexpander-Compressor due to
                   Simpler plant layout: reduced number of piping           the presence of a gearbox, generator, and
                    interconnections.                                        other auxiliaries.
                   Simpler machine control can easily be set up            Cost per unit is higher and oil free
                    for a fully automatic control system.                    solutions are not yet economically feasible.
                   A fixed speed machine can typically perform
                    better in off-design condition when the
                    enthalpy drop is maintained constant with
                    process controls.

                   Very robust and simple machine.                         Efficiencies are sometimes lower than
                   Perfect for oil free applications with the use of        turboexpander-generator due to the
                    active magnetic bearings (AMB).                          balancing of the turboexpander and
                   The stiff shaft design improves the operating            compressor performance and limitations.
 TURBOEXPANDER-




                    range and the capability to withstand very high         If the plant throughput (flow) is decreased
   COMPRESSOR




                    imbalances.                                              while the pressure ratio is kept constant,
                   Labyrinth, or similar, seals and the pressurized         the machine speed will reduce with a
                    auxiliaries system makes it very difficult for gas       significant loss in efficiency.
                    to escape from the machine in case of failure.          Units may be arranged in series, increasing
                   For a well-balanced machine, the                         the complexity and tuning of the control
                    turboexpander flow and re-compressor flow                system.
                    are linked. This reduces the size of required
                    anti-surge systems to manage unbalances in
                    flow between the turboexpander and
                    compressor.



       It should be noted that dyno, pump, and blower configurations have not been included
in the comparison table because they are not typically applied to medium and large sized
machines that are commonly found in ethylene plants.

                                               GE Oil & Gas Product Line

       The GE Oil & Gas Turboexpanders product line is standardized so that most of the
components are pre-designed. Parts that normally need to be customized for each project are
the wheels (both turboexpander and compressor), shaft, nozzle assembly, diffuser cone,
compressor follower, gear, auxiliaries and controls.
       The naming convention for machine standardization is the “Frame” size. The frame size
is directly linked to the casing and, therefore, the overall dimension of the machine. Each
standard frame can accommodate a specific diameter range of turboexpander wheels. Frame
sizes are also distinguished by the design pressure and flow rate. The design pressure sets the
flange ratings. Each of the Frame Sizes are clarified further in Table-2.
             Table-2: GE Oil & Gas Frame Size vs. Flange Ratings & Flow
                  TURBOEXPANDER RATING ACCORDING TO ANSI (PSI)
        FRAME #                                                    OUTLET FLOW (ACMH)
                    150      300      600      900        1500
           10                 x        x        x           x              450
           15                 x        x        x           x             1000
           20                 x        x        x           x             4000
           25                 x        x        x           x             5500
           30        x        x        x        x           x             9000
           40        x        x        x        x                         16000
           50        x        x        x        x                         25000
           60        x        x        x        x                         36000
           80        x        x        x        x                         45000
          100        x        x        x                                  65000
          130        x        x                                          100000
          160        x        x                                          160000
          180        x                                                   200000
                     X    TURBOEXPANDER GENERATOR FRAME SIZE AVAILABLE
                          TURBOEXPANDER COMPRESSOR FRAME SIZE AVAILABLE

               Table-2 is applicable to turboexpander-compressors (EC), turboexpander-
multistage compressors (ECC), and turboexpander-generators (EG) single stage or multistage
integrally geared types.
             Typical design limitations are as follows:
             Power up to 35 MW
             Wheel diameter up to 1800mm
             Design temperature from –270oC to +315oC
             Mechanical design in accordance with API 617 Chapter 4
             Lube oil system in accordance with API 614 Chapters 1, 2, and 4
             Turbine operability in accordance with IEC45 or API 612 Chapter 12
        As with most turbomachinery designs, there are standard comments and exceptions to
all of the industry specifications listed above.
        The design temperatures typically set the materials of construction for the components.
For cryogenic applications the turboexpander casing is typically stainless steel, but if warm
enough low temperature carbon steel can be used. The compressor casing and bearing
housing are typically carbon steel due to the warmer temperatures. Other components are
also affected mechanically. For example, by using a fixed nozzle instead of a variable nozzle,
the design temperature limitations can exceed the values given above.
        While there are no size limitations for turboexpander-generators and turboexpander-
compressors with traditional oil bearings, the active magnetic bearing (AMB) units need to be
checked versus the standard bearing size from AMB suppliers.
        GE Oil & Gas has additional experience with special “canned type” magnetic bearings
that are suitable for aggressive and sour gases typically not tolerated by standard electrical
devices. This design encapsulates traditional electrical components of the AMB within a metal
“can” made of Inconel material that prevents any contact with process gas. This design,
mainly used in natural gas applications, allows the AMB to operate without being contaminated
or harmed by the aggressive gas. Photo-1 shows a machine currently installed with this
technology.




       Photo-3: Turboexpander-Compressor with “Canned” Active Magnetic Bearing

       The GE Oil & Gas product line offers a fabricated casing design, as shown in Figure-1, in
addition to the traditional Rotoflow cast casing design. This recently applied technology is able
to ensure the highest quality pressure-containing components while also minimizing any
potential defects during the manufacturing of the unit.
       Moreover, the use of a fabricated casing ensures the flexibility to design for a wide
range of applications, ratings, and nozzle loads. The internal parts made by castings can now
be aerodynamically shaped for the best efficiency. In particular, the re-compressor discharge
volute can be manufactured with a variable section scroll and a tangential nozzle to provide
the best efficiency and range.




           Figure-1: Turboexpander-Compressor Cross-Sectional Drawing
      The control of the turboexpander is primarily accomplished by means of adjustable
guide vanes (nozzles). GE Oil & Gas can provide patented solutions with a traditional Rotoflow
slot and pin mechanism, shown in Figure-2, which is very effective on turboexpander-
compressors. Also available is a newly patented multilink mechanism, shown in Figure-3,
which adjusts the guide vanes using a “progressive” opening law for precision flow control and
minimal actuating forces.




              Figure-2: Slot and Pin Inlet Guide Vane (Nozzle) Assembly




      Precise flow regulation is useful in turboexpander-generators in order to minimize the
speed fluctuations at low load and synchronize the generator to the grid without using an
external control valve.
      The improved mechanical design of the nozzle mechanism is associated with increased
aerodynamic performance design. Antifriction and anti-wear coatings on the nozzle segments
minimize the losses during the first isenthalpic expansion.
      Nozzle segments are subjected to severe working conditions as shown in the Finite
Element Analysis of Figure-3. These conditions are due to the high velocities of the gas at this
location (similar to the wheel tip speed) and because of the presence of solid particles and
liquid droplets passing through the turboexpander. For this reason, tungsten carbide coatings
or surface induction hardening are typically applied to the nozzles to minimize erosion
problems.
        Another key component of the turboexpander-compressor is the wheel. To ensure the
reliability of the machine, the turboexpander and compressor wheels need to be carefully
designed in order to avoid excessive stresses, harmful resonances, and erosion by liquid
droplets. The wheel and wheel attachment has a strong influence on the rotor dynamics of the
machine.
        As shown in Figure-4, GE Oil & Gas designs and manufactures open and closed wheel
designs up to 1800 mm diameters in various materials.




        In general, the most common material in ethylene plants is 7050 Aluminum. This
material has a very good weight to strength ratio, which is required to reach very high tip
speeds. Titanium with superior properties is not typically used when there is hydrogen in the
tail gas, but is commonly used in many other turboexpander applications.
        Each wheel is analyzed by means of a finite element analysis (FEA) tool to assess the
stress and modal behavior. The modal behavior is assessed to avoid possible resonances
between the stimuli from the nozzle segments and natural modes of the wheel.




              Figure-5: Finite Element Analysis of a Compressor Wheel

       In ethylene plants, where the compressor head requirements are very severe (Figure-
5), the maximum head is determined by a compromise between the mechanical aspects (tip
speed) and aero design (blade loading). GE Oil & Gas uses hirth serration (Figure-6), a splined
fit, to attach the wheel to the shaft. This solution minimizes the centrifugal stresses on the
wheel and, therefore, improves the maximum tip speed and head capability.


                                                                  HIRT
                                    TIE ROD
       KEYS




                                 Figure-6: Hirth Serration

                               Turboexpander Performance

Turboexpander Selection
       The turboexpander performance is computed as a function of a non-dimensional factor
called specific speed (Ns) defined as:
                                               N Q2
                                        Ns 
                                               his
                                                      3/ 4


where Q2 is volumetric flow at the discharge, his is the isentropic enthalpy drop through the
turboexpander, and N is the rotating speed of the machine selected. The specific speed is the
key parameter for the assessment of the efficiency of a radial turbine at the design point. The
optimal range of specific speed for turboexpander design, as shown in Figure-7, is from ~1800
to ~2000.




          Figure-7: Normalized Efficiency vs. Turboexpander Specific Speed
       The specific speed is related to the maximum enthalpy drop that one stage can handle.
Typical numbers for the maximum enthalpy drop are:
             Low Specific Speed (500 < Ns < 1000): 350 kJ/kg (148.2 BTU/lbm)
             High Specific Speed (2000 < Ns < 2500): 180 kJ/kg (76.2 BTU/lbm)
       A second important parameter to consider is the u 1/Co factor. This is a non-dimensional
parameter where u1 is the tip speed of the wheel and Co is the spouting velocity. The spouting
velocity is the fluid speed that would be achieved if the entire isentropic enthalpy drop were to
be converted into speed. In other words, it is the speed that is created from putting work into
the system. This is similar to converting the potential energy in a water tower into a velocity at
the exit of the tower. Figure-8 further explains this idea pictorially, with H being the potential
energy and w being the speed at the water tower exit.

                                                         SPOUTING VELOCITY:

                                                           Co  hts ,is


                   Figure-8: Spouting Velocity Pictorially Represented


       The u1/Co factor determines the degree of reaction of the turboexpander stage and is
selected during the design phase (Figure-9). The optimum u1/Co is around 0.7, corresponding
to approximately a 50% degree of reaction. In this configuration, the inlet of the
turboexpander wheel is radial, improving the ability to withstand liquid at the inlet.




       The u1/Co factor becomes important during the testing of a turboexpander. Current API
617 practices call for it to be one of the measured values in the machine final testing.
       In an ethylene plant, the gas conditions are never constant. It is important to predict
the behavior of the turboexpander in off-design conditions. The turboexpander efficiency is
affected by the change in two main parameters: u1/Co and Q2/N (the flow coefficient).
       The efficiency of the machine in off-design conditions considers the effect of variation
of flow rate and u1/Co ratio. After the calculations have been completed, formula correction
factors are provided in correlation curves, based on experience (Figure-10).
       Figure-10: Sample Correlation Curves for Efficiency Correction Factors

      The overall plant control and machine selection should take into account the
turboexpander behavior during off-design conditions. Here is a typical range for u1/Co and
Q2/N turboexpander off design conditions:
          % Q2/N: 30 to 140% of design case
          % u1/Co: from 30 to 135% of design case

Compressor Selection
       The compressor is used as a brake for the turboexpander. The absorbed power
determines the operating speed of the turboexpander-compressor. The compressor selection is
very important in ethylene applications, where very often the compressor is required to
produce very high head. Recent developments in ethylene plant design also impose more
importance on the re-compressor performance. The compressor is no longer seen as a “by
product”, but rather an important plant component that is required to operate with good
polytropic efficiency, turndown, and head rise.
       The compressor load influences the turboexpander efficiency. Compressors with
controllable power absorption characteristics can be supplied to provide more flexibility to the
turboexpander.
       The compressor selection is made using three main parameters:
                                        4Q1
             Flow coefficient:   
                                       D22u 2
                                                            u2
             Compressor Peripheral Mach Number:     Mu 
                                                            a 0t
                                         h
             Work Coefficient:
                                        2
                                         u2
where Q1 is the volumetric flow at the inlet, D2 is the impeller diameter and u2 is the wheel
peripheral speed.
       The capability for a given wheel to produce power depends on both  and u2 squared
and the mass flow rate that is handled by the compressor wheel.
       The Work Coefficient is limited by the aerodynamic design of the wheel and the
peripheral speed affects the static stress on the impeller. In ethylene applications, the Mach
number is normally not an issue because of the low molecular weight gas.
       Typical numbers for the maximum enthalpy change on the compressor wheel are as
follows:
          Low flow coefficient (0.025 <                                  m)

          High flow coefficient (0.180 < < 0.280): 120 kJ/kg (50.8 BTU/lbm)
      A well-balanced turboexpander and compressor wheel depends on the process design.
The turboexpander wheel power (including mechanical losses) should be the same as the
compressor absorbed power.

                            Gexphis is  Gcomphis
       It should be noted that the capability for the compressor to act as a load for the
turboexpander does not depend on the polytropic efficiency. For this reason, an optional hot
bypass around the compressors can be used to artificially increase the absorbed power, also
reducing the turboexpander speed. As a consequence, the efficiency of the compressor will
drop because of the “internal” recirculation.

Turboexpander and Compressor Interaction
        As seen earlier, the specific speed (Ns) is one of the main parameters to determine the
efficiency of the expander. The efficiency vs. Ns curve has a flat peak portion ranging from
~1800 to ~2000 (Graph-2).
        Targeting a minimum value of Ns (i.e. Ns > 800), it is possible to determine the
minimum rotational speed of the machine. This is important in order to stay within an
acceptable efficiency range as a function of the ratio h to the expander volumetric flow at the
outlet (Figure-11).




                Figure-11: Minimum Rotational Speed of Turboexpander
         (Assuming Similar Mass Flow Rate Between Turboexpander Compressor)
        On the other hand, the rotational speed affects the compressor flow coefficient. The
rotational speed must be limited below a given value in order to limit the compressor flow
coefficient and also to increase the capability to produce head and power. This behavior is
exactly the opposite of the turboexpander.
        The following graph (Figure-12) represents the change of compressor flow coefficient
as a function of the rotational speed for two density ratios. This ratio is between the density at
the expander outlet and the density at the compressor inlet. The warmer gas at lower density
on the compressor side tends to increase the flow coefficient. This needs to be kept under a
given value by reducing the speed, which has an impact on the expander efficiency as seen in
Figure-11.




             Figure-12: Compressor Rotational Speed vs. Flow Coefficient

       In summary, the turboexpander and the compressor selection have to be balanced. In
order to do so, the turboexpander efficiency may be negatively affected. This could occur for
several reasons, but the major issue that affects this “balance” is the density ratio imbalance
between the turboexpander discharge and the compressor suction.

                                         Case Studies

       Two case studies where analyzed, to provide examples of the trends in today’s ethylene
plants: a naphtha cracker producing a methane-rich residue gas and a typical ethane or
ethane/propane (EP) cracker producing a light hydrogen-rich residue gas were analyzed. The
focus was on the turboexpander-compressor configuration since this is more complex than a
turboexpander-generator in conjunction with a stand-alone re-compressor.

Liquid Cracker
      The liquid cracker evaluation was made considering the following scenarios:
          Base Case: high percentage of hydrogen recovery.
          Lower Hydrogen Recovery: reduced rate of hydrogen recovery and,
             therefore, a larger percentage of ethylene recovery. This case reduces the C2
             and C3 refrigeration to a certain extent.
              Higher Hydrogen Recovery: increased rate of hydrogen recovery with
               decreased flow. With the margins available in cold boxes, this increased rate of
               hydrogen does not affect the ethylene recovery or the refrigeration.
        The machine selection for this service does not have any issues related to specific
speed at the higher range of efficiencies. The turboexpander-compressor is at the lower end
of GE Oil & Gas production capabilities, corresponding to a Frame 20 (EC201). This service
can be satisfied either with oil bearings or active magnetic bearings.
        The selection based on compressor efficiency can be further optimized to improve the
efficiency. However, based on all parameters, the initial selection fits into a very standard unit,
and both the mechanical and aerodynamic characteristics are well within proven experience.
        The same case study was analyzed by increasing the flow rate by 25%. Since the gas
conditions remain unchanged, the machine selection resulted in a similar unit design, scaled
up to the Frame 25 (EC251).
        Table-2 provides a summary of the machinery sizing for the Liquid Cracker case to
highlight the important turboexpander factors, such as specific speed (Ns).

      Table-2: Liquid Cracker Turboexpander-Compressor Sizing at 100% Flow
       Case Description           BASE H2 RECOVERY    LOWER H2 RECOVERY HIGHER H2 RECOVERY
              UNIT                 Exp        Comp      Exp        Comp     Exp        Comp
       Condition                        Design            Off-Design           Off-Design
       RPM                        35,000     35,000    33,800     33,800   36,630     36,630
       Ns                          1,500      3,200
       Diameter           (mm)      200        230
       Efficiency          (%)    84-88%     72-76%    84-88%    72-76%    82-86%    71-74%
       Wheel Power         (hp)    1039       1035       936       933      1223      1219
       Weight Liquid       (%)     15.3                 14.7                15.7
       Frame size                 EC0201

GAS CRACKER
       Gas crackers produce a very large residue gas stream with high concentrations of
hydrogen. The gas does not vary with hydrogen product demand. In fact, the demand of
hydrogen product is very low. Variation occurs due to co-cracking of propane or other
feedstock.
       This reference is based on 100% ethane cracking (the base case) with the option of
50/50 Ethane/Propane cracking.
       From a machinery design point-of-view, this service is considered to be more difficult
due to the high enthalpy change involved. A first selection was made with a 2-stage expander
compressor, a standard configuration for the 100% and 111% flows. Both units are sized into
a Frame 40 (EC401) with good efficiencies and with well-referenced mechanical and
aerodynamic parameters. Table-3 shows an overview of the machine performance.
        Table-3: Gas Cracker Turboexpander-Compressor Sizing at 100% Flow
                                                                           50/50                50/50
Case Description            100% Ethane BASE     100% Ethane BASE      Ethane/Propane       Ethane/Propane
       UNIT                 Exp_HP Comp_HP Exp_LP Comp_LP             Exp_HP Comp_HP      Exp_LP Comp_LP
Condition                          Design            Design                Off-Design          Off-Design
RPM                         20,000     20,000 20,000     20,000       16,270     16,270   16,360     16,360
Ns                           1,100      3,000  1,400      3,200
Diameter            (mm)      325        425    350        425
Efficiency           (%)    83-87% 73-77% 86-89% 72-76%               83-87%     70-74%   84-88%      70-74%
Wheel Power          (hp)    1629       1626   1630       1627         1509       1507     1528        1526
Weight Liquid        (%)      0.6               4.8                     4.9                 5.5
Frame size                         EC0401            EC0401

        If the flow is increased by 11%, the design remains basically the same. However, the
selected wheels are larger in terms of flow capability (larger flow coefficient). The flow
capacity of a turboexpander can be increased by either using a wheel design with a higher
flow coefficient/specific speed, or by increasing the diameter and reducing the rotational speed
to keep the same peripheral speed. The second option is required to handle the different
enthalpy change.
        With the intent of simplifying the plant layout and reducing cost, GE Oil & Gas has
selected for this service a single Frame 40 (ECC401) machine, with two-stage compressors
directly coupled to a single expander wheel. This type of unit is referenced with oil bearings
and can also be developed with AMB.

 Table-4: Gas Cracker Turboexpander-Multistage Compressor Sizing at 100% Flow
    Case Description                       100% Ethane BASE                    50/50 Ethane/Propane
           UNIT                      Exp       Comp_LP Comp_HP           Exp       Comp_LP       Comp_HP
    Condition                                    Design                             Off-Design
    RPM                               23,000       23,000    23,000       18,890        18,890        18,890
    Ns                                 1,000        3,900     3,700
    Diameter                (mm)         350          350       350
    Efficiency               (%)    78-82%      74-78%    74-78%       77-81%       71-75%       71-75%
    Wheel Power              (hp)       2396         1466      1466       27244          1386         1386
    Weight Liquid            (%)         0.6                                4.9
    Frame size                                  ECC401

       Due to the very high enthalpy drop across the expander stage, the efficiency is highly
penalized with respect to the traditional design at nearly the same specific speed.
       The turboexpander-compressor-compressor solution (Figure-13) could be considered as
a low cost alternative solution. This arrangement would also be considered if the
turboexpander enthalpy drop per stage were lower.
       The rotor dynamics of this arrangement needs to be analyzed carefully to ensure a
robust design without harmful expander wheel-overhung modes throughout the operating
range.
                                                          BRG




                                               Expander
                                                                Compressor

      Figure-13: Turboexpander-Compressor-Compressor (ECC) Arrangement

                                        Conclusions

       This paper presents an overview of current turboexpander technology to provide
information for the selection of the best machine configuration and thermodynamic design for
ethylene plant applications. GE Oil & Gas has analyzed potential selections for turboexpander-
compressors for large ethylene plants. The results show that there are no issues with
increasing the machine capacity, due to the scalability of the unit frame sizes. However, large
enthalpy drops per stage and optimization trade-offs between the expander and compressor
wheels need to be carefully evaluated to find the best compromise between cost and
performance.

								
To top