AIChE T4 174869
Turboexpander-Compressor
Technology for Ethylene PlantsRadjen Krishnasing
Senior Lead Process Engineer
The Shaw GroupGabriele Mariotti
Engineering Manager
GE Oil & Gas
Florence, Italy
Kara Byrne
Applications Engineer & Commercial Manager
GE Oil & Gas
Houston, TX, USA
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 PlantsRadjen Krishnasing
Senior lead process Engineer
The Shaw Group
Gabriele MariottiEngineering Manager
GE Oil & Gas
Florence, Italy
Kara Byrne
Applications Engineer & Commercial Manager
GE Oil & Gas
Abstract
Todays 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 ARadjen
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 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
typeEthanePropaneNaphthaGasoil
Cracked Gas H2 / C2H41.070.630.440.30
Cracked Gas CH4 / C2H40.231.240.830.62
Cracked Gas (C4 & C5) / C2H40.020.070.220.24
Cracked Gas Pygas / C2H40.010.050.190.14
Tail gas H2 / CH4 ratio4.150.510.530.48
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.
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 (0.0145 psi / kPA)
kW
Hp = 0.746 kilowatts
Naphtha feed crackingHigher Hydrogen recovery (less flow through
turbo- expander)Lower Hydrogen recovery
(more flow through turbo- expander)
Expander Inlet
Flow rate (kmol/hr)240021762850
Molecular weight14.014.513.2
Pressure (kPA)305030503050
Temperature( oC)-100-97-103
Expander outlet
Flow rate (Actual m3/min)696383
Re-compressor Inlet
Flow rate (kmol/hr)240021762850
Mole weight13.9914.5213.2
Pressure (kPA)354356357
Temperature ( oC)-3-3-4
Flow rate (Am3/min)214194250
Re-compressor Outlet
Pressure (kPA)604604604
Expander dHs (kJ/kg)111105118
Turbo-expander RPM28,63027,64030,000
Expander power (kW)9708701140
Expander efficiency (%)868685
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.
Table 3A: Overview light (ethane, ethane/propane) feedstock
cracking (High-Pressure Machine)
Key Item Clarifications: Refer to Table 2
100%
C2 feed cracking50/50
C2/C3 feed cracking
HP Expander Inlet
Flow rate (kmol/hr)79357883
Mole weight4.947.66
Pressure (kPA)21312131
Temperature( oC)-114-118
HP Expander outlet
Flow rate (Am3/min)127121
HP Compressor Inlet
Flow rate (kmol/hr)78377567
Mole weight4.727.17
Pressure (kPA)630636
Temperature ( oC)6360
Flow rate (Am3/min)580550
HP Compressor Outlet
Pressure (kPA)740740
Expander dHs (kJ/kg)13080
Turbo-expander RPM20,00016,240
Expander power (kW)13601255
Expander efficiency (%)8685
Table 3B: Overview light (ethane, ethane/propane) feedstock
cracking (Low-Pressure Machine)
100% Ethane feed cracking50/50 Ethane/propane feed cracking
LP Expander Inlet
Flow rate (kmol/hr)79237742
Mole weight4.917.42
Pressure (kPA)11651175
Temperature( oC)-135-134
LP Expander outlet
Flow rate (Am3/min)215207
LP Compressor Inlet
Flow rate (kmol/hr)78367567
Mole weight4.727.17
Pressure (kPA)532541
Temperature ( oC)4343
Flow rate (Am3/min)647612
LP Compressor Outlet
Pressure (kPA)630636
Expander dHs (kJ/kg)12482
Turbo-expander RPM20,00016,340
Expander power (kW)13551265
Expander efficiency (%)8986
Further evaluation/observations
An important turbo-expander design parameter is the isentropic
enthalpy drop (dHs) 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 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 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-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 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 todays 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
ArrangementTurboexpander-CompressorMechanical 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-DynoMechanical 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-DynoOften 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
PROSCONS
TURBOEXPANDER- GENERATOR Very high efficiencies can be achieved.
The wheel can be optimized to achieve the best aerodynamics by
freely changing the RPM without other machinery constraints.
Recompressor is designed independently from the turboexpander,
merging more stages into a single machine with higher
efficiency.
Simpler plant layout: reduced number of piping
interconnections.
Simpler machine control can easily be set up for a fully
automatic control system.
A fixed speed machine can typically perform better in off-design
condition when the enthalpy drop is maintained constant with
process controls.
The machine has a tendency to speed up in case of electric load
rejection. This limits the maximum tip speed of the wheel and
tripping devices need to be redundant for safety reasons.
The machine is typically more complex than a
Turboexpander-Compressor due to the presence of a gearbox,
generator, and other auxiliaries.
Cost per unit is higher and oil free solutions are not yet
economically feasible.
TURBOEXPANDER-COMPRESSOR Very robust and simple machine. Perfect
for oil free applications with the use of active magnetic bearings
(AMB). The stiff shaft design improves the operating range and the
capability to withstand very high imbalances. Labyrinth, or
similar, seals and the pressurized auxiliaries system makes it very
difficult for gas to escape from the machine in case of
failure.
For a well-balanced machine, the turboexpander flow and
re-compressor flow are linked. This reduces the size of required
anti-surge systems to manage unbalances in flow between the
turboexpander and compressor.
Efficiencies are sometimes lower than turboexpander-generator
due to the balancing of the turboexpander and compressor
performance and limitations. If the plant throughput (flow) is
decreased while the pressure ratio is kept constant, the machine
speed will reduce with a significant loss in efficiency.
Units may be arranged in series, increasing the complexity and
tuning of the control system.
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 LineThe 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
FRAME #TURBOEXPANDER RATING ACCORDING TO ANSI (PSI)OUTLET FLOW
(ACMH)
1503006009001500
10xxxx450
15xxxx1000
20xxxx4000
25xxxx5500
30xxxxx9000
40xxxx16000
50xxxx25000
60xxxx36000
80xxxx45000
100xxx65000
130xx100000
160xx160000
180x200000
XTURBOEXPANDER 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
BearingThe 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.
Figure-6: Hirth Serration
Turboexpander PerformanceTurboexpander Selection
The turboexpander performance is computed as a function of a
non-dimensional factor called specific speed (Ns) defined as:
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 u1/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.
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:
Flow coefficient:
Compressor Peripheral Mach Number:
Work Coefficient:
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 < < 0.100): 150 kJ/kg (63.5
BTU/lbm)
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.
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 todays 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 DescriptionBASE H2 RECOVERYLOWER H2 RECOVERYHIGHER H2
RECOVERY
UNITExpCompExpCompExpComp
ConditionDesignOff-DesignOff-Design
RPM35,00035,00033,80033,80036,63036,630
Ns1,5003,200
Diameter(mm)200230
Efficiency (%)84-88%72-76%84-88%72-76%82-86%71-74%
Wheel Power(hp)1039103593693312231219
Weight Liquid(%)15.314.715.7
Frame sizeEC0201
GAS CRACKERGas 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
Case Description100% Ethane BASE100% Ethane BASE50/50
Ethane/Propane50/50 Ethane/Propane
UNITExp_HPComp_HPExp_LPComp_LPExp_HPComp_HPExp_LPComp_LP
ConditionDesignDesignOff-DesignOff-Design
RPM20,00020,00020,00020,00016,27016,27016,36016,360
Ns1,1003,0001,4003,200
Diameter(mm)325425350425
Efficiency
(%)83-87%73-77%86-89%72-76%83-87%70-74%84-88%70-74%
Wheel Power(hp)16291626163016271509150715281526
Weight Liquid(%)0.64.84.95.5
Frame sizeEC0401EC0401
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 Description100% Ethane BASE50/50 Ethane/Propane
UNITExpComp_LPComp_HPExpComp_LPComp_HP
ConditionDesignOff-Design
RPM23,00023,00023,00018,89018,89018,890
Ns1,0003,9003,700
Diameter(mm)350350350
Efficiency (%)78-82%74-78%74-78%77-81%71-75%71-75%
Wheel Power(hp)2396146614662724413861386
Weight Liquid(%)0.64.9
Frame sizeECC401
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.
Figure-13: Turboexpander-Compressor-Compressor (ECC)
Arrangement
ConclusionsThis 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. SPOUTING VELOCITY:
EMBED Equation.3
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EXP
Figure 1: Turbo-Expander in an Ethylene Plant based on Liquid
Feedstock
To Hydrogen Purification
COM
HydrogenRich Stream
Methane Rich Stream
To Fuel gas
PC
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