Turboexpander-Compressor Technology for Ethylene Plants

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 turboexpander)Lower Hydrogen recovery

(more flow through turboexpander)

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


Mole weight13.9914.5213.2


Temperature ( oC)-3-3-4

Flow rate (Am3/min)214194250

Re-compressor Outlet


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


Mole weight4.947.66


Temperature( oC)-114-118

HP Expander outlet


HP Compressor Inlet


Mole weight4.727.17


Temperature ( oC)6360


HP Compressor Outlet



Turbo-expander RPM20,00016,240



Table 3B: Overview light (ethane, ethane/propane) feedstock cracking (Low-Pressure Machine)

100% Ethane feed cracking50/50 Ethane/propane feed cracking

LP Expander Inlet


Mole weight4.917.42


Temperature( oC)-135-134

LP Expander outlet


LP Compressor Inlet


Mole weight4.727.17


Temperature ( oC)4343


LP Compressor Outlet



Turbo-expander RPM20,00016,340



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:

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EXP

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

To Hydrogen Purification

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HydrogenRich Stream

Methane Rich Stream

To Fuel gas

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Turboexpander-Compressor Technology for Ethylene Plants

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