AU2005224308B2

AU2005224308B2 - Method for liquefying a hydrocarbon-rich flow

Info

Publication number: AU2005224308B2
Application number: AU2005224308A
Authority: AU
Inventors: Heinz Bauer; Hubert Franke; Rainer Sapper; Marc Schier
Original assignee: Linde GmbH
Current assignee: Linde GmbH
Priority date: 2004-03-09
Filing date: 2005-02-25
Publication date: 2010-12-16
Anticipated expiration: 2025-02-25
Also published as: WO2005090886A1; AU2005224308A1; EG24721A; RU2358213C2; DE102004011483A1; RU2006129467A; NO20064557L

Description

WO 2005/090886 - 1 - PCT/EP2005/002019 Description Process for liquefying a hydrocarbon-rich flow 5 The invention relates to a process for liquefying a hydrocarbon-rich flow, in particular a natural gas stream, the liquefaction of the hydrocarbon-rich flow taking place against a mixed refrigerant cycle cascade comprising two mixed refrigerant cycles, the first 10 mixed refrigerant cycle being used for precooling and the second mixed refrigerant cycle being used for liquefaction and supercooling of the hydrocarbon-rich flow to be liquefied, and each mixed refrigerant cycle having at least one single-stage or multi-stage 15 compressor driven by at least one gas turbine, the gas turbines being assigned starters which can be used to assist the gas turbines during normal operation. In the following text, the term "precooling" is to be 20 understood to mean cooling the hydrocarbon-rich flow to be liquefied down to a temperature at which the separation of heavy or higher boiling-point hydrocarbons is carried out. The following, further cooling of the hydrocarbon-rich flow to be liquefied 25 falls under the term "liquefaction" in the following text. Natural gas liquefaction processes of the generic type - generally designated the dual-flow LNG process - are 30 sufficiently well known from the prior art to those skilled therein; by way of example, US patent 6,105,389 may be mentioned. If heavy hydrocarbons are contained in the natural gas 35 stream to be liquefied, these are separated out between the precooling and liquefaction and drawn off as what are known as the NGL (Natural Gas Liquid) fraction and, if appropriate, supplied to further processing. Those components of the hydrocarbon-rich flow or natural gas -2 to be liquefied which are designated heavy or higher boiling-point hydrocarbons are those components which would freeze out during the subsequent cooling and liquefaction - that is to say C 5 .- hydrocarbons and 5 aromatics. It is often the case that those hydrocarbons which would undesirably increase the calorific value of the liquefied natural gas - in this case propane and butane, in particular, are meant - are also separated out before the liquefaction. 10 This separation of higher boiling-point hydrocarbons is normally done by an HHC (Heavy HydroCarbon) column or scrub column being provided, which is used to separate out the heavy hydrocarbons and benzene from the 15 hydrocarbon-rich flow to be liquefied. A process management system of this type is described, for example, in DE-A 197 16 415. In dual-flow LNG plants, the cycle compressor is 20 normally driven by gas turbines. These are in turn normally started up by electrical or steam-operated starters. Since such starters often have to provide significant power - 20 to 40% of the gas turbine output - they are used as "helpers" to assist the gas turbines 25 during normal operation. Larger gas turbines are available on the market only in discrete output stages with comparatively large steps in output. The power of the starters or helpers is limited in relation to the gas turbine output, in order to avoid synchronization 30 problems. On account of a large number of process boundary conditions, such as the composition and pressure of the hydrocarbon-rich flow to be liquefied, ambient 35 temperature, etc., and the requirements on the separation of heavy hydrocarbons which may be required, an optimal division of power between the compressor drives of the two mixed refrigerant cycles cannot be reached or can be reached only by accident. Typically, C:\NRPorthlDCCWAMU775777_.DOC- 7 11/210 -3 the first or precooling cycle needs about 40 to 55% of the total energy. The power demand of the precooling cycle is additionally often smaller than that of the second or liquefaction cycle. 5 This asymmetry can be compensated for by means of different utilization of the helpers. For instance, if the power distribution between the first and the second mixed refrigerant cycle is 45 to 55% and if both mixed refrigerant 10 cycles each have a gas turbine with an output of 70 MW and a helper with an output of 20 MW, the helper of the first cooling cycle is operated with only 4 instead of with the possible 20 MW. A large part of the investment in this helper thus remains unused during the normal liquefaction 15 operation. The present invention seeks to specify a generic process for liquefying a hydrocarbon-rich flow in which the installed power of the gas turbines and starters/helpers may be used 20 fully during normal operation. Furthermore, the investment and operating costs of the gas turbines and starters/helpers used may be reduced and/or optimized, in particular the use of identical gas turbines and starters/helpers may be made possible. 25 The present invention provides a process for liquefying a hydrocarbon-rich flow, in particular a natural gas stream, the liquefaction of the hydrocarbon-rich flow taking place against a mixed refrigerant cycle cascade comprising two 30 mixed refrigerant cycles, the first mixed refrigerant cycle being used for precooling the hydrocarbon-rich flow to condense any heavy or higher boiling-point hydrocarbons for C:\NRP b\DC WAM\2775777_ .DOC-17/11 /201 -3A separation from the hydrocarbon-rich flow to form a precooled hydrocarbon-rich flow and the second mixed refrigerant cycle being used for liquefaction and supercooling of the precooled hydrocarbon-rich flow; 5 wherein: a) the second mixed refrigerant cycle has a cold intake compressor with a pressure ratio of at least 10, and b) the first mixed refrigerant cycle is used at least 10 partly for intermediate cooling of at least a partial stream of a partially compressed mixed refrigerant stream of the second mixed refrigerant cycle, wherein the temperature of the intermediate cooling of the at least partial stream of the 15 partially compressed mixed refrigerant stream of the second mixed refrigerant cycle is influenced by drawing the intermediately cooled partial stream off from the intermediate cooling at different temperature levels and/or supplying any 20 partial stream of the partially compressed mixed refrigerant stream which is not supplied to the intermediate cooling to the following compressor stage or stages. 25 The process according to the invention and further embodiments of the same are explained in -4 more detail in the following text by using the exemplary embodiment illustrated in the figure. As the figure shows, the hydrocarbon-rich flow to be 5 liquefied is supplied via line a to a heat exchanger El. In the latter, the hydrocarbon-rich flow to be liquefied is cooled down to such an extent that the heavy or higher boiling-point hydrocarbons contained therein condense and can be separated out from the 10 hydrocarbon-rich flow in the separation unit H, which is supplied with the cooled process stream via line b. The separated hydrocarbons are drawn off via line c and, if appropriate, supplied to further use. 15 It should be emphasized that the process according to the invention can be combined with all known separation methods for higher boiling-point hydrocarbons counting as prior art. 20 Via line d, the hydrocarbon-rich flow now freed of higher boiling-point hydrocarbons is supplied to a second heat exchanger E2 and, in the latter, is liquefied and supercooled against the refrigerant mixture of the second mixed refrigerant cycle. The 25 liquefied and supercooled hydrocarbon-rich flow is drawn off from the heat exchanger E2 via line e, optionally expanded in an expansion turbine T1 and then, via valve f and line g, supplied directly to further use or (intermediate) storage. 30 In the procedure illustrated in the figure, the refrigerant mixture compressed in the compressor V1 is supplied via line 10 to a condenser E3 and then via line 11 to the heat exchanger El and supercooled in the 35 latter. In the heat exchanger El, separation into three mixed refrigerant partial streams 12, 15 and 18 takes place. In the valves 13, 16 and 19, these partial streams are expanded to different pressure levels and, after renewed passage through and evaporation in the -5 heat exchanger El, are supplied via the lines 14, 17 and 20 to the compressor V1 at different pressure levels. 5 The compressor V1 is driven by a gas turbine Gl. The starters required for the operation of the gas turbines G1 and G2, as has already been explained at the beginning, are not illustrated in the figure. 10 In a procedure analogous to that described by using the first mixed refrigerant cycle, the compressed refrigerant mixture of the second mixed refrigerant cycle is first supplied via line 30 to a recooler E4 and then via line 31 to the heat exchanger El and 15 cooled down and condensed in the latter. The liquefied mixed refrigerant stream is then supplied via line 32 to the heat exchanger E2, supercooled further in the latter, expanded in the optional expansion turbine T2 after passing through the heat exchanger E2 and then 20 supplied via line 33 to an expansion valve 34 and expanded in the latter. The second mixed refrigerant partial stream, following evaporation in the heat exchanger E2, is than supplied via line 35 to the input stage of the cycle compressor V2. 25 The heat exchanger E2 can be constructed as a spiral heat exchanger or as a plate exchanger. If the liquefaction and supercooling of the hydrocarbon-rich flow to be liquefied takes place in a plate exchanger, 30 then - according to an advantageous refinement of the process according to the invention - the refrigerant mixture 28 of the second mixed refrigerant cycle can be evaporated while rising or falling. 35 The aforementioned cycle compressor V2, which, according to the invention, is a cold-intake compressor which has a pressure ratio of at least 10, is likewise driven by a gas turbine G2, which is assigned a starter/helper not illustrated in the figure.

-6 According to the invention, a partially compressed mixed refrigerant stream is now drawn off from an intermediate stage of the cycle compressor V2 via line 5 36, subjected to recooling E5 and then at least partly supplied via line 39 to the heat exchanger El and cooled intermediately in the latter against the first cooling cycle. The intermediately cooled, partly compressed mixed refrigerant stream is then again 10 supplied via line 40 to a suitable intermediate pressure stage of the compressor V2 and compressed to the desired final pressure. Using the first cooling cycle for the intermediate 15 cooling of the the second cooling cycle relieves the latter at the cost of the first cooling cycle, since the compressor power of the compressor V2 in its high pressure part falls in proportion with the now lowered intake temperature of the intermediately cooled 20 refrigerant stream in line 40. According to the invention, a displacement of the compressor powers as far as power equality between the two compressors V1 and V2 and their associated starters/helpers can now be realized. 25 The optimum choice of the abovedescribed intermediate cooling is determined by the dew point of the refrigerant mixture chosen for the second cooling cycle at the selected intermediate pressure at which the 30 refrigerant mixture is drawn off. Ideally, the entire refrigerant mixture of the second cooling cycle is cooled down by means of the first cooling cycle until power equality of the two cycle drives V1 and V2 is achieved. 35 The fact that the first mixed refrigerant cycle is now used for intermediate cooling of the second mixed refrigerant cycle means that the installed power of C:\NRPonbKlDCC\WAM\2775777_l DOC-111/ 12011$ -7 identical gas turbines and starters/helpers can be used fully. In view of the already mentioned limiting of the starter or 5 helper power in relation to the gas turbine output, it is obvious that the full utilization of the two helpers which is now achieved leads to maximization of the plant capacity. This will be explained by using the following example. 10 If, on account of the process according to the invention, a power distribution of 50% to 50% between the first and the second mixed refrigerant cycle is now reached, then assuming identical gas turbines and starters/helpers for the two refrigerant cycles - these and their investments can be 15 used fully. To return to the example given above, the starter/helper of the second cooling cycle can now also be operated with a power of 20 MW. As compared with the initial stage mentioned at the beginning, the useable installed power is increased from 164 MW to 180 MW by the 20 process according to the invention. With a given drive concept, the plant output can therefore be increased by about 10%. As already explained, the precooling of the hydrocarbon-rich 25 flow to be liquefied is carried out at three different temperature levels (mixed refrigerant streams 12/14, 15/17 and 18/20). In the process according to the invention, the temperature of the intermediate cooling El of at least one partial stream of the partially compressed mixed refrigerant 30 stream 36, 39 of the second mixed refrigerant cycle is influenced by the fact that the intermediately cooled partial stream is drawn off from the intermediate cooling El C:WRPortbPDCCWAM\2775777_ .DOC-17/11/2011 -8 at different temperature levels - illustrated in the figure by the line 21 shown dotted - and/or the partial stream of the partially compressed mixed refrigerant stream 37 which is not supplied to the intermediate cooling El, which is 5 expanded to the inlet pressure in the valve 38, is supplied to the following compressor stage or stages. The desired intake temperature of the high-pressure part of the compressor V2 can now be set by means of this procedure. 10 The reference in this specification to any prior publication (or information derived from it) , or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter 15 forms part of the common general knowledge in the field of endeavour to which this specification relates. Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", 20 and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

Claims

1. A process for liquefying a hydrocarbon-rich flow, the liquefaction of the hydrocarbon-rich flow taking place 5 against a mixed refrigerant cycle cascade comprising two mixed refrigerant cycles, the first mixed refrigerant cycle being used for precooling the hydrocarbon-rich flow to condense any heavy or higher boiling-point hydrocarbons for separation from the 10 hydrocarbon-rich flow to form a precooled hydrocarbon rich flow and the second mixed refrigerant cycle being used for liquefaction and supercooling of the precooled hydrocarbon-rich flow; wherein: a) the second mixed refrigerant cycle has a cold 15 intake compressor with a pressure ratio of at least 10, and b) the first mixed refrigerant cycle is used at least partly for intermediate cooling of at least a partial stream of a partially compressed mixed 20 refrigerant stream of the second mixed refrigerant cycle, wherein the temperature of the intermediate cooling of the at least partial stream of the partially compressed mixed refrigerant stream of the second mixed refrigerant cycle is influenced 25 by drawing the intermediately cooled partial stream off from the intermediate cooling at different temperature levels and/or supplying any partial stream of the partially compressed mixed refrigerant stream which is not supplied to the 30 intermediate cooling to the following compressor stage or stages. C:NRPonb\DCCwAM\77577 OC. 17/11/2010 -10

2. The process as claimed in claim 1, wherein the liquefaction and supercooling of the precooled hydrocarbon rich flow takes place in a spiral heat exchanger or in a plate exchanger. 5

3. The process as claimed in claim 2, wherein the liquefaction and supercooling of the precooled hydrocarbon rich flow takes place in a plate exchanger and the mixed refrigerant of the second mixed refrigerant cycle evaporates 10 while rising or falling.

4. A process according to claim 1, 2 or 3, wherein the hydrocarbon-rich flow is a natural gas stream. 15

5. A process for liquefying a hydrocarbon-rich flow, substantially as hereinbefore described with reference to the accompanying drawing.