Refrigerant Expanders Improve LNG Liquefaction Process Chen-Hwa Chiu TEXACO Bellaire, TX 77401, USA Hans E. Kimmel EBARA INTERNATIONAL CORPORATION Sparks, Nevada 89434, USA In conventional natural gas liquefaction plants the gas is condensed under high pressure and then expanded in an isenthalpic process across a Joule-Thomson valve. Current LNG liquefaction plants expand the high pressure of liquefied gas across liquid turbine expanders, reducing the enthalpy of the liquefied gas and improving the process. The natural gas liquefaction process offers two different locations to install cryogenic liquid turbines: the expansion of the final LNG condensate and the expansion of the liquid coolant, the mixed refrigerant MR in the upstream cooling cycle. The first LNG plant using LNG expanders and MR cooling cycle expanders are successfully operating in Oman and in Malaysia. A practical model is presented to define the process improvements of mixed refrigerant and LNG expanders in natural gas liquefaction plants. The model is also applicable to multiple refrigerant expanders in cascade processes. INTRODUCTION The East Ohio Gas Co. in Cleveland/Ohio built the first commercial liquefaction plant for natural gas [1] in the early 1940 s. The plant produced 76 tons of LNG per day with a specific power consumption of 3010 kj/kg LNG. In 1956 a barge-mounted liquefaction plant built by Conch Methane Services Ltd. produced 150 tons LNG per day with a power consumption of 3000 kj/kg LNG. During this initial phase, attention was focused more on solving the difficult technological problems of liquefying larger quantities of LNG, rather than on reducing the liquefaction costs represented by the specific power consumption. This aspect has significantly changed in recent years. Current LNG liquefaction plants produce up to 7200 tons LNG per day per liquefaction module and operate with a specific power consumption of 1000 kj/kg LNG [2]. Increasing the mass flow and operating the plant at higher pressures achieved this substantial reduction in specific power consumption, and most recently by replacing the Joule-Thomson expansion valves with LNG and mixed refrigerant turbine expanders [5]. REFRIGERANT EXPANDERS Refrigerant expanders are cooling cycle expanders operated by refrigerants, which are liquefied gases such as propane, ethane, or hydrocarbon mixtures, used as cooling medium in the liquefaction process for natural gas. Refrigerant expanders are installed upstream the LNG condensation. The temperature of the liquefied refrigerant is higher than the temperature of LNG and the refrigerant is condensed under high pressure and expanded across the liquid expander. Similar to the LNG expander [2], the cooling cycle expander reduces the enthalpy of the refrigerant fluid. Different from the LNG expander the enthalpy reduction of the refrigerant is not directly reducing the LNG boil-off because the expansion occurs above the boiling temperature of LNG. A model is presented to calculate the effect of cooling cycle refrigerant expanders on the improvement of LNG liquefaction process through reduction of LNG boil-off. The model is also applicable to multiple refrigerant expanders used in cascade processes.
EXPANDER THERMODYNAMICS The reduction of LNG boil-off gas m depends on the LNG enthalpy reduction h across the LNG turbine expander at the LNG temperature T L over the specific heat of vaporization C P. m = h / C P To remove the same amount of enthalpy through an ideal Carnot refrigeration process using seawater cooling at the temperature T S would require a minimum work input w C of w C = h (T S T L )/ T L The ratio COP = T L (T S T L ) is the Carnot Coefficient of Performance for refrigeration. Figure 1 compares the LNG enthalpy reduction directly achieved by the LNG expander for an ideal Carnot refrigeration process [3]. The actual refrigeration process is less efficient and requires a work input of w A. Typically the value of w A is 3.75( h). Figure 2 compares the LNG enthalpy reduction directly achieved by the LNG expander for an actual refrigeration process. Figure 1: Carnot Process and LNG Expander The ratio of Carnot to actual work input defines A, the actual process efficiency, which is approximately constant across the entire refrigeration process. A w C w A Assuming that the refrigerant expander for an ideal Carnot process reduces the enthalpy of the refrigerant by the same amount of h as the LNG expander, but at a higher refrigerant temperature T M, then an additional amount of work w MC is necessary to achieve the same effect of enthalpy reduction h at the lower LNG temperature T L. w MC = h (T M T L ) T L Figure 3 compares the LNG enthalpy reduction h achieved by the refrigerant expander for an ideal Carnot process, by reducing the refrigerant enthalpy with ( h + w MC ) at the higher temperature T M. To achieve the same effect with an actual refrigeration process the reduction of the refrigerant enthalpy at the temperature T M using refrigerant expanders has to be equal to h MA : h MA = h + w MA = h + w A (T M T L ) (T S T L ) = h [ 1+ (T M T L ) ( A T L )] Figure 4 compares the LNG enthalpy reduction h achieved by the refrigerant expander for an actual refrigeration process with w MA as the additional amount of work [4].
Figure 2: Actual Process and LNG Expander COOLING CYCLE ZERO POINT The total enthalpy reduction h MA is a linear function of T M and for h MA = 0 the line intersects with the temperature scale at T Z. h MA h = (T M T Z ) (T L T Z ) = 1+ (T M T L ) ( A T L ) This equation simplifies to the linear relation: T Z = T L (1 A ) The interpretation of T Z is that a refrigerant cooling cycle expander at T M would have no effect on enthalpy reduction at the temperature T Z. The definition of the temperature T Z is an expression unique to refrigerant cooling cycle expanders in gas liquefaction processes and is defined as Cooling Cycle Zero Point [3]. In the case of an ideal Carnot refrigeration process with an efficiency A = 1, the temperature T Z is equal to the absolute zero point temperature T Z = 0. The maximum theoretical reduction of LNG boil-off for a Carnot refrigeration process using cooling cycle expanders is proportional to: h = h MA T L T M With A = 0 for a completely inefficient refrigeration process the temperature T Z is equal to T L and the necessary additional work w MA becomes infinite large. In this case the refrigerant cooling cycle expander would have no effect on the reduction of LNG boil-off. The Cooling Cycle Zero Point T Z describes the process efficiency of gas liquefaction using liquid expanders. The efficiency is higher for smaller T Z. LNG BOIL-OFF REDUCTION The actual refrigeration process efficiency A is within the range 0 A 1 and the effect of the enthalpy reduction h MA of cooling cycle expanders to the reduction h of LNG boil-off can be calculated by h = h MA A T L / [T M (1 A )T L ] Also h can be calculated by using T Z. h = h MA (T L T Z ) (T M T Z ) Both formulas include the case of the LNG expander itself with the temperature of the refrigerant equal to the LNG temperature T M = T L and calculating the expected result of h = h MA for LNG expanders. The effect of refrigerant expanders at higher temperature T M on the reduction of LNG boil-off gas m is equal to: m = h C P = [ h MA (T L T Z )]/[C P (T M T Z )] with h MA as the enthalpy reduction across the refrigerant cooling cycle expander.
Figure 3: Carnot Process and Refrigerant Expander Figure 4: Actual Process and Refrigerant Expander CASCADE PROCESS The above-described model is a practical method to calculate the process improvements of refrigerant cooling cycle and LNG expanders in natural gas liquefaction plants. The model is also applicable to multiple refrigerant expanders used in cascade processes. In cascade processes with N multiple cooling cycles each cycle R operates with a different refrigerant at different temperatures T MR (R = 1,2,3 N). Each cooling cycle expands the high-pressure liquid refrigerant across an expander reducing the enthalpy of the refrigerant R by the amount of h MAR. The effect of the refrigerant expander R on the LNG enthalpy reduction is equal to h R and on the LNG boiloff reduction is equal to m R.
h MAR h R = (T MR T Z ) (T L T Z ) = 1+ (T MR T L ) ( A T L ) 1 Habets and Kimmel [2] presented the formula for the additional LNG mass output m using only LNG turbine expanders. m R = h R C P = [ h MAR (T L T Z )]/[C P (T MR T Z )] These formulas can be extended to calculate the additional LNG mass output m R achieved by each refrigerant cooling cycle expanders R. MR m MR H MR g (T L T Z ) m R = C P (T MR T Z ) MR m MR H MR isentropic efficiency of refrigerant expander R mass flow through refrigerant expander R differential head across refrigerant expander R g gravity g = 9.81 m/s 2 To calculate the total LNG boil-off reduction m TOTAL using one LNG expander and N multiple refrigerant expanders R in cascade processes the effect of each individual expander has to be added. m TOTAL = m LNG + m R with R= 1,2,3 N m LNG = m R for T MR = T L and with the refrigerant values for LNG Figure 5 demonstrates the total effect of multiple refrigerant expanders in a cascade process on the total LNG enthalpy and boil-off reduction h TOTAL. The example uses one LNG expander with an LNG enthalpy reduction of h and three refrigerant expanders R=1,2,3 with refrigerant temperatures of T M1,T M2,T M3 and refrigerant enthalpy reductions of h M1, h M2, h M3. The total LNG enthalpy reduction h TOTAL is the sum of h and the effective enthalpy reductions h R of each refrigerant expander R. h TOTAL = h + h R To achieve the same effect in LNG enthalpy reduction without LNG and refrigerant expanders a total work input of ( h TOTAL + w TOTAL ) would be required at the temperature T S of the seawater coolant. Figure 5: Effect of Multiple Refrigerant Expanders in Cascade Processes
CONCLUDING REMARKS Refrigerant cooling cycle expanders significantly improve the LNG liquefaction process by increasing the LNG output. The total effect of multiple refrigerant expanders on the LNG output is calculated by adding up the effects of all individual refrigerant expanders to the effect of the LNG expander. The effect of refrigerant expanders depends on the temperature of the refrigerant. As lower the temperature of the refrigerant, as higher is the effect on LNG boil-off reduction. REFERENCES 1. Turner, C.F., (1944) Liquefying and Storing Natural Gas for Peak Loads, American Gas Association Monthly Vol.26, pg.243-6, June 1944 2. Habets, G.L.G.M. and Kimmel, H.E., (1999) Economics of Cryogenic Turbine Expanders, The International Journal of Hydrocarbon Engineering, December/January 1998/99 3. Song, M.C.K. and Kimmel, H.E., (2000) Cooling Cycle Expanders Improve LNG Liquefaction Process CUChE-3, Third Joint China/USA Chemical Engineering Conference, September 2000, Beijing, China 4. Chiu, Chen-Hwa and Kimmel, H.E. (2001) Process Thermodynamics of Heavy Mixed Refrigerant Liquid Expanders Natural Gas Utilization Topical Conference, 2001 AIChE Spring National Meeting, April 2001, Houston, Texas, USA 5. Chiu, Chen-Hwa and Kimmel, H.E. (2001) Turbo-Expander Technology Development for LNG Plants Thirteenth International Conference on Liquefied Natural Gas (LNG 13), May 2001, Seoul, Korea