Compact liquefied gas expander technological advances Joel V. Madison President Ebara International Corporation SYNOPSIS LNG expanders are now an important part of every new LNG liquefaction plant. The paper presents a concise overview of the process and plant control benefits brought by the latest generation of Expanders which are able to expand partially into the vapour phase. Before 1996 conventional liquefaction processes for natural gas operated with the high pressure condensation phase flow of liquefied natural gas expanding across a Joule-Thomson valve. Replacing the JT-valve with a cryogenic LNG liquid turbine substantially increased the thermodynamic efficiency of the refrigeration process resulting in a total LNG output increase of 3 to 5%. The development of Two Phase Expanders improves the process yet further. Two Phase Expanders present significant mechanical design challenges in order to handle the increasing volumetric flow and change in density whilst providing flexibility of operating range. Avoiding unstable flow within the machine and resultant vibration further increases the range of problems to be solved. Improving the thermodynamic efficiency of the expander when handling both liquid and vapour raises some interesting problems. The novel concepts used to overcome these problems and mechanical design are dealt with. 1.0 INTRODUCTION In 1895 Carl von Linde, a German engineer, invented the first continuous process for gas liquefaction. Linde's liquefaction process was based on repeating cycles of gas compression, pre-cooling of the compressed gas in a heat exchanger and expansion of the compressed pre-cooled gas across a Joule-Thomson throttling valve. This process yielded the desired result but unfortunately had a high energy consumption that made it commercially unattractive. In 1902 the French engineer George Claude developed a piston expansion engine to replace the Joule-Thomson valve to extract mechanical work from the gas expansion process, thus increasing the efficiency and reducing the high energy consumption. To further reduce the cost of gas liquefaction the first turbo-gas expander was introduced in 1964 by the Elliott turbomachinery company. The comparatively high efficiency of radial inflow turbo-gas expanders compared to piston expanders allowed more work to be extracted from the compressed gas, resulting in a further reduction in power consumption and net increase in process efficiency. Whilst the inefficient expansion process across a Joule-Thomson valve was already eliminated for the gaseous phase in 1902, for many years it remained the only solution to
expand the liquefied cryogenic gas in the liquid phase. For the particular case of liquefied gases used as a fuel, like propane, ethane and methane, it was not until 1995 that the first generation of LNG liquid expanders was available to be installed at a liquefaction plant in Malaysia [1]. The engineering challenges associated with the design of engines operating at cryogenic temperatures coupled with the stringent safety and hazard rules for explosive fluids prevented the technology from being commercially available prior to this installation. The ten years following the initial concept for cryogenic LNG expanders saw many developments and technological advances that dramatically improved the performance, reliability and efficiency of the machines. In addition, the size and complexity of the devices were greatly reduced. The increase in overall process plant efficiency and resultant increase in product liquid were well established during this period. As a result every new LNG liquefaction plant that has been built around the world since 1995 has been equipped with liquid expanders in place of the Joule-Thomson valves. The liquefaction of gas is in principle a Carnot refrigeration process that was first described by the French physicist Sadi Carnot in 1824. Carnot discovered that the efficiency of a heat engine is dependant only upon its input and output temperatures. For such a refrigeration process the lower the final resultant temperature, the lower the Carnot efficiency will be as more energy input is necessary to achieve the end temperature. The energy input approaches infinity for the case of an output temperature of absolute zero. In summary this entails that more energy input is required to reduce the temperature of a fluid by one degree at a relatively lower temperature than is required to achieve the same reduction at a relatively higher temperature. When applied to a gas liquefaction process, the end result is that the Carnot efficiency of the process is proportionally lower for fluids having a lower liquefaction temperature since more energy input is required. The purpose of liquid expanders in gas liquefaction processes is to further reduce the temperature of the liquefied gas beyond the typically very low cryogenic liquefaction temperature without going through the Carnot refrigeration process. The cryogenic liquid expander directly extracts the heat energy from the liquefied gas by expanding the liquid from a high pressure level to a low pressure level, converting the static pressure energy into kinetic fluid energy and further into mechanical torque and electrical energy where it is ultimately removed from the system. By extracting work in the form of electrical energy from the cryogenic fluid, the thermodynamic internal energy, the enthalpy, is reduced and with it the temperature is reduced. The efficiency of this refrigeration process by means of direct enthalpy reduction across an expansion machine at the low liquefaction temperature is independent of the low Carnot efficiency that would normally be expected. In an ideal isentropic expansion machine the enthalpy reduction is equal to the static differential pressure energy reduction, while in actual expansion machines the achievable energy reduction is between 80% and 90% of this value. The purpose of liquid expanders is very similar to that of gas expanders: Gas expanders reduce the enthalpy of the natural gas in its gaseous state Liquid expanders reduce the enthalpy of the natural gas in its liquefied state
Reduced enthalpy corresponds to a reduction in temperature and is effectively a refrigeration of the natural gas. Liquid expanders reduce the pressure, the temperature and the enthalpy of the cryogenic liquefied gas stream. Therefore in the liquefaction process they can be modelled as a heat exchanger combined with a Joule-Thomson valve. 2.0 DESIGN OF LIQUEFIED GAS EXPANDERS Habets and Kimmel [2] described in detail the basic design of cryogenic liquid expanders in the proceedings of the IMechE 7 th European Congress on Fluid Machinery in The Hague 1999. Figure 1 shows a three stage liquid expander for LNG installed in 2002 at the LNG liquefaction plant in Ras Laffan, Qatar. Figure 1. Three-stage liquid expander for Ras Laffan LNG Figure 2 shows a cross section of the three stage liquid expander. The high pressure liquid stream at the end of the traditional liquefaction process enters on the top passing through the expander and exits under low pressure at the bottom of the engine. The hydraulic assembly consists of three stages, each with fixed geometry inlet guide vanes and a radial inflow reaction turbine runner. The inlet guide vanes convert the static pressure energy into kinetic rotational energy and the runner converts the resulting rotational energy into shaft
torque. The electric generator and the hydraulic assembly are mounted on a common shaft and the generator converts the shaft torque into electrical power. The electrical power is transported by cryogenic power cable to the external power grid. Generator Stator Generator Rotor Thrust Equalization Mechanism (TEM) Fixed Geometry Inlet Guide Vanes Runners Figure 2. Ras Laffan three stage liquid expander The expander extracts enthalpy directly from the fluid by converting the static pressure energy of the fluid into electrical energy to be exported to the power grid. This design constitutes a very efficient refrigeration engine despite the low inlet and exit temperatures. It should be noted that the cryogenic fluid temperature at the exit of the expander is lower than at the inlet. However, in this case it is not necessary to use the Carnot refrigeration process for this final cooling step. The total savings in energy input for the same resultant
output temperature are approximately five times the power output of the expander generator when an expander is used in lieu of continued Carnot refrigeration cycles. In a comparison of liquefaction plants with and without liquid expanders, the following statements describe the main differences: In a new plant for a given liquefied gas output production the liquid expander allows installation of less power generation, smaller gas compressors, smaller gas expanders and smaller heat exchangers. In an existing plant or a new plant with given sizes of power generation, gas compressors, gas expanders and heat exchangers the liquid expander increases the liquefied gas output production. The production costs for liquefied gas are invariably lower with a liquid expander than without. 3.0 TWO-PHASE LIQUEFIED GAS EXPANDERS Since the early days of gas liquefaction technology it has been known that two-phase cryogenic expanders would improve the thermodynamic efficiency of gas liquefaction processes. Carl von Linde described the advantages of two-phase expansion engines more than a century ago. Unfortunately only in recent years has the technology been made available to design and manufacture liquid-vapour cryogenic expanders suitable for reliable operation [3]. Two-phase expanders expand the static energy in the form of the available pressure differential from the liquid phase into the liquid-vapour phase across the saturation line of the fluid. The enthalpy of the liquefied gas is reduced significantly more than it would be with single phase expanders due to the vaporisation heat extracted from the liquid portion of the two-phase fluid. Figure 3 shows the design of a single stage two-phase expander. In contrast to the original expander design [2] the pressurised fluid enters radially from the side of the turbine runner and passes through the hydraulic assembly in a predominantly upward flow direction. As a result of the change in density the buoyancy forces of the liquid-vapour mixture support and stabilize the fluid mixture in the upward flow direction. Figure 4 details the hydraulic assembly for a two-phase expander. The pressurized fluid enters the fixed inlet guide vane nozzle ring in the radial direction, converting the static pressure energy into high velocity rotational kinetic energy. The guide vane channels are converging in the axial and radial directions, thus accelerating the fluid to a high angular velocity which creates a high component of angular momentum. The fluid enters the radial inflow reaction turbine runner with this high angular momentum and passes through the runner, which converts the angular momentum into mechanical torque while thereby reducing the pressure of the fluid. At the exit of the runner the pressure of the fluid reaches the saturation point and begins to partially vaporise. Immediately downstream of the runner discharge the fluid enters a two-phase exducer to further vaporize the liquid. Due to the partial vaporisation the specific volume of the twophase fluid is increasing and further acceleration of the fluid occurs. At the exit of the
Two Phase Exducer Runner Fixed Geometry Inlet Guide Vanes Generator Stator Generator Rotor Figure 3. Single stage two-phase expander exducer the two-phase fluid reaches a high exit velocity approaching the speed of sound, forming a jet-like fluid stream exiting the exducer. This jet-like fluid stream exerts a reaction force on the exducer, which causes additional torque to be generated As a result the total power extracted from the fluid is significantly increased due to this additional expansion step. Two-phase expanders utilising this principle have been operating reliably and economically since 2003 [4]. Two Phase Exducer Runner Fixed Geometry Inlet Guide Vanes
The Carnot refrigeration process requires several cooling cycles to reach the liquefaction temperature of natural gas and similar cryogenic gases. For natural gas liquefaction the cooling cycle uses heavier hydrocarbons such as propane and ethane as a mixture with methane to pre-cool the processed final methane rich natural gas. These cooling cycles include liquid expanders as well due to the advantages for the refrigeration process. Typical LNG liquefaction plants therefore make use of two liquid expanders, one for heavy hydrocarbon mixtures in the pre-cooling cycle and one for final methane rich product LNG. To optimize the refrigeration process, increasing the overall liquefaction efficiency and reducing the unit liquefaction costs, the compact two-cycle expander represents the most advanced solution in gas liquefaction process technology available today[5]. Figure 4. Two-phase hydraulic assembly 4.0 COMPACT TWO-CYCLE LIQUEFIED GAS EXPANDERS Runner Upper 2-Phase 2-Stage LNG Expander LNG Inlet Vessel Seal Arrangement Generator Single Shaft MR Inlet Vessel Lower 2-Stage MR Expander Figure 5 Compact two-cycle expander Figure 5 shows the design of the Compact Two-Cycle Expander for the application as a two-phase LNG expander combined with the mixed refrigerant (HMR) expander for the pre-cooling cycle. The Compact Two-Cycle Expander utilizes a single shaft and generator driven by two separate expanders. The upper LNG expander is separated from the generator by a seal and is directionally opposed to the lower expander. The bearings are
lubricated and the generator cooled by the HMR fluid so that the heat of the generator is sent to the MR circuit with the consequent increase in process efficiency on the LNG expander as the fluid stream is not exposed to any parasitic losses that will reduce the total heat removed. A single TEM (Thrust Equalisation Mechanism) is sufficient to balance the residual thrust from both upper and lower expanders, further reducing the internal recirculation and improving efficiency. The bypass fluid stream required to balance the thrust in this configuration is significantly lower when compared to a single machine due to the opposed orientation of the two expanders. Figure 6 shows the design of the compact two-cycle expander for the same application but with the improvement in the efficiency by completely separating the generator cooling from the hydrocarbon cooling cycles. The High-Efficiency Compact Two-Cycle Expander retains the single shaft and generator concept with the upper expander and lower expander opposed to balance the majority of the hydraulic thrust. The lower single phase HMR expander is also separated from the generator by a seal in this configuration. The generator and generator bearings are cooled by a third liquid available in the process. By removing the generator inefficiency, the heat of the generator is completely removed from the LNG/MR circuits with a resultant increase in process efficiency. Figure 7 highlights the dramatic reduction in overall size offered by the High-Efficiency Compact Two-Cycle Expander when compared to the first generation expanders. It can be seen that a single compact machine is used to replace two separate machines. The benefits related to the reduction in scale are apparent, with the associated reduction in complexity and improvements in performance making this solution even more attractive. 5.0 CONCLUSION The process efficiency, size and complexity of equipment utilised in liquefaction plants will have even more importance in the future. The concepts discussed allow for significant increases in process efficiency and dramatic reductions in the physical size and complexity of the systems. These improvements are important for all new and existing plants, and are particularly critical for applications in which space is limited, such as offshore liquefaction facilities. Retro-fitting of existing liquefaction plants that are based on older technology also benefit as lower production costs will enable such plants to remain competitive with the new installations currently operating or under construction. REFERENCES 1. Verkoehlen, J., "Initial Experience with LNG/MCR Expanders in MLNG-Dua" Proceedings GASTECH 96, Volume 2, Vienna, Austria, Dec 1996, ISBN 1874134162 2. Habets, G.L.G.M. and Kimmel, H.E., "Development of a Hydraulic Turbine in Liquefied Natural Gas" IMechE 7 th European Congress on Fluid Machinery for the Oil, Petrochemical, and Related Industries, 15-16 April 1999, The Hague, The Netherlands, ISBN 1 86058 217 6 3. Kikkawa, Y. and Kimmel, H.E., "Interaction between Liquefaction Process and LNG Expanders", Proceedings 2001 AICHE Spring National Meeting, Natural Gas Utilization Topical Conference, Houston, Texas, April 2001
4. Cholast, K.; Kociemba, A. and Heath, J., "Two-Phase Expanders Replace Joule- Thomson Valves at Nitrogen Rejection Plants", 5 th World LNG Summit, Rome, December 2004 5. Madison, J. V. Comprehensive Applications for LNG Expanders 6 th World LNG Summit, Rome, November 2005. Seal Arrangement Generator coolant flow Generator Seal Arrangement Figure 6. High-Efficiency Compact Two-Cycle Expander
Figure 7. High-Efficiency Compact Two-Cycle Expander size compared to first generation expanders