REFRIGERATION FROM EXPANSION OF TRANSMISSION PIPELINE GAS

Abstract: Refrigeration is produced while delivering transmission pipeline gas (20) of elevated pressure to a branch pipeline of lower pressure by expanding the gas in a turbo-expander (29) to a pressure below the branch pipeline (47) pressure and recovering refrigeration from the expanded gas which is then compressed by a centrifugal compressor (41) directly driven by the turbo-expander (29) to the pressure required for delivery into the branch pipeline (47). The gas (20) is dehydrated by adding methanol through pump (24), and separating aqueous methanol condensate from the expanded gas in separator (31), and the condensate is distilled in column (55) with compression heat in reboiler (64) to recover methanol for reuse.

Description: REFRIGERATION FROM EXPANSION OF TRANSMISSION PIPELINE GAS BACKGROUND OF THE INVENTION This invention relates to the production of refrigeration during the reduction of pressure of gas with¬ drawn from a transmission pipeline. More specifically, the invention produces refrigeration with energy recovered when gas at elevated pressure in a transmission pipeline is supplied to a branch or regional pipeline maintained at a lower high pressure. The gas pressure of transmission or trunk pipe¬ lines is reduced at many letdown stations or city gates simply by isenthalpic expansion, i.e., by passage through a reducing valve. Such pressure reduction is a waste of valuable energy. Two schemes for utilizing the energy available in pipeline gas at letdown control stations are the generation of electrical energy and the liquefaction of natural gas. To produce electrical energy, the pipeline gas is passed through an expansion turbine which drives an electric generator. U.S. Patent 3,360,944 illustrates a process wherein pipeline natural gas is expanded with the performance of work to produce refrigeration utilized to liquefy a portion of the natural gas. Depending on the location of each letdown control station, the generation of electric energy or the production of liquified natural gas may not be economically attractive. In such case, the conversion of the energy available in the transmission pipeline gas reaching the letdown station to bulk, low-cost refrigeration may be a preferred and valuable alternative particularly where local industries require refrigeration. The frozen food industry, suppliers of ice and manufacturers of dry ice are examples of industries 5 which consume large quantities of refrigeration and thus represent potential customers of commercially saleable refrigeration. • Accordingly, a principal object of this invention is to convert energy derived from reducing the pressure of ID J transmission pipeline gas to low-cost refrigeration. Another important object is to maximize the generation of refrigeration by isentropic expansion of the transmission pipeline gas, i.e., expansion with the perform¬ ance of work. 15 These and other objects and advantages of the invention will be evident from the description which follows: SUMMARY OF THE- INVENTION ' In accordance with this invention, transmission pipeline gas reaching a letdown station generally at a 20 pressure in the range of about 400 to 1000 psia.more fre¬ quently in the range of about 500' to 800 psia(pounds per square inch absolute) , is expanded in a turbo-expander to a reduced pressure which is in most cases at least 50 psi below the desired delivery pressure but is not reduced 25 enough to drop the temperature of the expanded gas below ahαut -100°F, recovering refrigeration from the expanded gas,, and thereafter compressing the expanded gas to the de_Livery pressure in a centrifugal compressor driven by the t rbo-expander. 30 The gas pressure is often reduced by the expander at least 250 psi to a reduced pressure which is below the ultimate delivery pressure but which does not drop the temperature of the expanded gas below about -100°F. Limiting the reduction of pressure so that the temperature does not 35 fall below about -100°F avoids excessive condensation of hydrocarbons usually present in natural gas. Furthermore, there is no significant commercial market for the very low temperature refrigeration that is recoverable from expanded gas having a temperature below -100°F. Low level refrigera¬ tion is generally recovered from the expanded gas at a temperature below about -40°F, and if desired, additional high level refrigeration is recoverable at a temperature below about 20°F. Prior to entering the turbo-expander, the transmission pipeline gas is desirably cooled, usually to a temperature below about 45°F, by heat exchange with the expanded gas after refrigeration has been recovered there¬ from so that the recovered refrigeration is at a lower temperature than it would be without such prior cooling. The warmed expanded gas is passed through the centrifugal compressor which increases the pressure so that the gas can be delivered at the desired pressure in the range of about 200 to 450 psia, frequently in the range of about 250 to 350 psia. Transmission pipeline gas is herein used to mean natural gas or synthetic natural gas having a very high methane content and a heating value of at least about 950 British Thermal Units per standard cubic foot. Transmission pipeline gas reaching letdown stations invariably contains moisture which would freeze during the expansion of the gas and cause plugging of the equipment with possible damage thereto. A simple and inexpensive method of removing moisture from the pipeline gas involves the injection of a small, quantity of methanol into the gas so that the moisture merely condenses during expansion of the gas and is separated from the expanded gas as a water-methanol solution. This method has been integrated with the novel generation of refrigeration according to this invention so that some of the energy derived from reducing the pressure of the pipeline gas is utilized to separate methanol from the water-methanol solution. Thus, regenerated methanol can be recycled for injection into transmission pipeline gas to be work expanded according to this invention. BRIEF DESCRIPTION OF THE DRAWINGS For further clarification of the invention, the ensuing description will refer to the appended drawings of which: Fig. 1 is a flow diagram of a preferred embodiment of the invention in which the dehydration of the pipeline gas is integrated with the production of low level refriger¬ ation; and Fig. 2 is a partial flow diagram showing a modifi- cation of the portion of Fig. 1 indicated thereon within an encircling dotted line. Fig. 1 as modified by Fig. 2 is the flow diagram of another preferred embodiment of the invention yielding both low level and high level refrigera¬ tion. DESCRIPTION OF PREFERRED EMBODIMENTS Fig. 1 is a flow diagram of the basic process of the invention incorporating the method of eliminating the moisture usually present in pipeline gas through the use of methanol. The description of Fig. 1 will include a specific example of a transmission pipeline feeding a branch pipeline with gas which is substantially pure methane containing small amounts of moisture, carbon dioxide and higher hydro¬ carbons. The transmission pipeline gas at a pressure of 615 psia and a temperature of 60°F in line 20 enters the bottom of tower 21 and flows upwardly through a spray of methanol supplied from tank 22 by line 23 and pump 24. Methanol is injected into tower 21 at the rate of about 200 pounds per million standard cubic feet of gas passing through tower 21. The gas with vaporized methanol leaves tower 21 through line 25, passes through heat exchanger 26 and dis¬ charges into separator 27 at 610 psia and 37°F wherein condensed methanol is removed from the gas before it enters turbo-expander 29 via line 28. The gas expanded with very little liquefaction of hydrocarbons such as propane and butane leaves expander 29 at 230 psia and -60°F, discharging from line 30 into separator 31 wherein an aqueous methanol condensate is removed from the expanded cold gas which flows through line 32 to refrigeration recovery exchanger 33. The gas exits from exchanger 33 at 227 psia and -10°F, flowing 5 through line 34 to exchanger 26. Antifreeze or other suitable fluid is passed through refrigeration exchanger 33 to convey the recovered refrigeration to one or more opera¬ tions requiring refrigeration, such as the commercial feezing of fish and meat. 10 . The gas issues from exchanger 26 at 224 psia and L8°F by way of line 35 which conveys the gas to heat exchanger 36. Control valve 37 in line 35 is used to divert about 6.5% of the gas leaving exchanger 26 into branch line 38 which directs this minor stream through reflux condenser 15 39 and thence at 221 psia and 100°F back into line 35 before it reaches exchanger 36. The recombined stream at 221 psia and 23°F enters exchanger 36 and flows therefrom at 215 psia and 155°F via line 40 to centrifugal compressor 41 which is directly driven by turbo-expander 29. The gas 20 compressed to 316 psia and 230°F passes through line 42 to air cooler 43. Control valve 44 in line 42 is used to divert about 30% of the gas in line 42 through branch line 45 and reboiler 64 whence at 313 psia and 215°F it returns to line 42. The recombined stream of compressed gas flows 25 from air cooler 43 at 310 psia and 195°F through line 46 and exchanger 36 from which it discharges into branch pipe¬ line 47 at its normal operating conditions of 305 psia and 70°F. The aqueous methanol condensate removed from the 30 expanded gas in separator 31 at 230 psia and -60°F flows through line 48 and exchanger 26 whence it issues at 225 psia and 18°F via line 49 provided with reducing valve 50. The condensate passing through valve 50 discharges into separator 51 at about 23 psia. Traces of gases released 35 from the condensate are vented from separator 51 through valved line 52. The condensate flows from separator 51 through line 53 into mixing tank 54. Warm water at 205°F drawn from the bottom of distillation column 55 through line 56 is passed by line 57 and pump 58 for admixture with the aqueous methanol of line 53 discharged into tank 54. The warm mixture causes the small quantity of hydro¬ carbons that may have been carried in solution by the aqueous methanol into tank 54 to separate out as a super¬ natant layer on the aqueous methanol. The separated hydro¬ carbon layer is removed from tank 54. by line 59 and pump 60 and in moat cases is injected into the compressed gas before it is delivered to branch pipeline 47. The bottom layer of aqueous methanol passes from tank 54 through line 61 and control valve 62 into an intermediate section of column 55. The distillation separates the aqueous methanol into methanol vapor which rises to the top of column 55 and into water bottoms. Water in the bottom of column 55 circulates through line 56, line 63 and reboiler 64, discharging back into column 55 to supply the heat required for the distillation. Valved line 74 is used to drain water from column 55 as required. Methanol vapor passes from the top of column 55 via line 65 to reflux condenser 39, discharging therefrom as liquid methanol into line 66. Thence pump 67 returns part of the liquid methanol through line 68 as reflux into the top of column 55 while part is returned via line 69 to storage tank 22. The division of the liquid flow from pump 67 into lines 68 and 69 is controlled by valve 70. The methanol recovered by distillation in column 55 and returned by line 69 to tank 22 together with condensed methanol discharged from separator 27 through line 71 and reducing valve 72 into tank 22 for recycling through line 23 and pump 24 to tower 21 amounts to almost 99.5% of the methanol injected into tower 21. In other words, slightly more than one pound of methanol is lost, almost entirely with the water drained from column 55 by valved line 65, for each million standard cubic feet of gas passing through tower 21. < rt M TJ co P. rt •n P- Φ Φ rt P- φ Φ rt P. ►P < o o tr T) Φ Φ P Φ o P *<! M CO P « M φ Q. Φ tr tr rt co TI rt P Φ Φ o ω P' P P- B P Qt Hi TJ rt co Φ rt o φ P> tr rt tr P Hi M O tr *-i rt * cr P- >~i P rt P P- o o >τ| P o tr P OQ P. P P> φ rt o Φ o • O P- rt OQ Hi M O CO . OQ Φ < to M rt 1 P P rt Φ P- P P- Ui I- 1 co o g P 4 rt CO 01 M Ul tr Φ vj tr p M 0 TJ 01 o P ^i VO rt g rt 01 P 3 Ul rt Ul o P- rt g TJ 01 Ul P- P o H- 1 e. P TI o rt rt Φ M P M M P Hi P Φ O ω P- CO M P > H P 01 O g Φ g o o rt co Φ P P- ►P Qt o g o Φ P- co P P. M o o co co P P o Hi TJ r fϋ cr P- t P- P ■ <! Hi P <! Φ P o rt P- p φ rt rt P P P- M o Φ rt φ P. rt P- P X Ul Φ Hi tf P Tf V! P rt rt Ui P. rt o rt P> φ P- Φ • P- 0 P* TJ O O H cr OQ rt φ Hi ■ -J P P j Φ o M CO P 2 rt P P P- g o M P P o o 4 P Φ rt Hi P P rt o P- φ rt O 01 01 rt P- o o tr rt o * • P- o rt OQ P rt tr P Hi P P. Hi P P P Φ H rt rt P- Φ 01 o P O tr φ P P Φ . M t Hi Φ Φ φ rt OQ rt M N M ro O P « P Φ P rt o O ro z CO φ O Hi 01 φ Hi rt rt rt Qt o P. M Φ rt rt o P « cr o g P 4 rt P- P <! rt S3 TJ Φ o P- φ K P OQ P- P H P P rt o rt P rt TJ OQ P g tr B Φ rt rt P- Φ P. o P co 1 rt 1 Φ of Fig. 1 which permits the recovery of refrigeration at two temperature levels. Reference numerals appearing in Fig. 1 are applied to corresponding elements of Fig. 2. The only new element in Fig. 2 is a high level refrigeration recovery exchanger with reference numeral 75 which is placed in line 34 between refrigeration recovery exchanger 33 and exchanger 26. In the specific example that has been presented in connection with the flow diagram of Fig. 1,- the modifica- tion of: Fig:. 2 does not change any of the pressure and temperature conditions previously mentioned, starting at line 20 and proceeding to the discharge from refrigeration exchanger 33 into line 34 where, as previously stated, the gas is at 227 psia and -10°F. Now that refrigeration recovery exchanger 75 is interposed in line 34 the gas gives up additional refrigeration and leaves exchanger 75 at 224 psia and 25°F. The additional refrigeration recovered through exchanger 75 is at a higher temperature level of about -5°F and amounts to 68 tons of refrigeration per million standard cubic feet of transmission pipeline gas processed hourly in accordance with the flow diagram of Fig. 1 as modified by Fig. 2. The additional refrigeration obtained with exchanger 75 does not diminish the 95 tons of low level refrigeration recovered through exchanger 33. Downstream of exchanger 75 some conditions change but none of the changes are of any importance. Thus, the ■ gas leaves exchanger 26 at 53°F instead of 18°F when exchanger 75 was not present. Because the gas in line 35 is now warmer, the portion diverted through line 38 and reflux condenser 39 is increased to 11% of the gas issuing from exchanger 26. The recombined gas stream entering exchanger 36 is at 58°F. The aqueous methanol from separator 31 leaves exchanger 26 at 53°F. The gas entering exchanger 36 is at 162°F but is still delivered to branch pipeline 47 at 70°F and 305 psia as in the example of Fig. 1. Hence, Fig. 2 yields 70% more refrigeration than -s. CO to to 1— ' 1— > o Ul O Ui o Ul o rt rt rt o Φ Hi φ Φ rt rt o 0 Φ Φ P- S3 P tf rt P- Ml Hi Hi S3 rt Φ J P P- Φ Φ o X o X X o Φ o u 0 X P P- tr p. P- P rt o Φ tr Φ tr X P- o Hi Hi P o rt o o τι M Φ rt P OQ M O OQ O φ rt rt Φ g Φ a OQ rt P rt rt < tr tr tr P M P. 04 φ g M P tr P P φ Φ P ■TJ rt P 4 . P 4 • P- P- Φ P Hi P P P rt TJ rt N φ P Φ Φ P Φ OQ OQ V! P M P P tr o 0Q P- CO M Hi cr M P- o φ X rt rt rt P P M φ φ OQ o OQ OQ Φ P- P P φ rt φ ro Φ CO o P tr P CO 0Q P- t> rt rt rt Φ 3 φ Φ P P CO P o o < < M Hi g rt Φ rt Φ cr P o P P φ rt rt rt rt 0Q TJ P- g P o ro rt P- P- TJ Φ rt P rt rt P < o rt rt Hi • rt P- M rt TJ TJ TJ M Φ 01 M g Φ rt tr rt φ o P- P- rt o S3 P rt Φ P P P rt TJ S . o 01 >• tf Φ TJ φ Φ ^ P rt o o P- tr P rt X o a g tr P P rt o rt ** Φ P π rt rt p. P P OQ H Φ P. P P M φ o TJ Φ rt P φ <J o £2 rt rt o M tr P" P- φ tr P n P tr P- o Φ j Hi Φ rt P P P Φ Hi o P' Φ O P rt P rt Φ P Φ Φ co P P o M OΛ u P rt rt P 04 rt S O P OQ O P P P* X Hi P φ Hi Φ o TJ rt <! P- Φ OQ o P o P P M P. rt Hi P P- φ OQ rt P- P 0Q P. φ P P rt P cr M P P. P VJ rt P- P- S3 P rt rt φ CO P g rt rt rt Φ oi Φ co o Φ rt >• P- oi tr rt o rt P- o P « rt to P" 4 P P- O P- rt P S3 o rt rt P < P- φ tr P O rt rt P Hi CO rt CO V! rt P P rt P P Q M o rt Φ P rt P O Φ rt tr tr OQ M g . rt rt rt rt CO g Φ g M P- rt P o rt P. 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Φ M S3 n rt P> Φ P- rt P « P 01 rt rt P P P M P- rt P M M tr Φ rt •• 01 M O tr o OQ Hi Φ rt o Hi rt 3 P P φ P __; • tr rt P o VI P- M CO Φ P Φ P Φ ►α P o M Φ P rt CO rt X φ P Φ P- rt S3 > * ø___ Φ TJ Hi Tl P o CO M rt P P M P rt Φ P- M o < o tr <! Φ rt P- < Hi P O P- 01 Q. P- TJ g rt P V! tr •P- Φ P P P P M φ o P- 0Q P P TJ O S3 Hi rt Hi P. o O OQ " • £2 ro P n CO σ 4 o M M rt OQ • P- P TJ tr M P- Φ Φ rt ffi P M P rt rt rt o s3 Φ P- Φ M cr Φ Φ o M P P rt ti s. rt P rt Φ P- 0Q P- o Φ P < rt < rt to P o P P Φ P- OQ t o P- P. rt P Φ P o £ Hi g rt M Φ φ φ P cr P P P- M <! P. Hi P o Φ Φ rt 01 P P- TJ Φ M Hi rt P- M rt < Φ - φ tr M o M P o P X TI rt • rt rt φ 1 rt o P- O φ P P. M 3 P- P Hi P* Hi rt o P- o tr tr rt ^ Φ rt P- TJ o 1 P- cr o OQ P- o tr OQ o cr o Φ P CD M φ 0Q φ P l_ι. P Ul M o Φ rt P tr P< rt Φ Hi rt rt P • P Φ TI P rt o Ml φ rt P rt o P M Φ M tr P tr Φ P M Hi o rt P- P TI rt rt rt P P 01 TI cr P P- Hi P. M S3 φ rt rt Φ M OQ ro P- o rt P rt Φ P- P rt rt rt P M P « g rt Φ o Φ tr P Φ rt P* <! φ P Hi OQ rt P- P- Φ g TJ P- O < P rt Φ rt rt rt cr Φ Φ Φ P rt Φ P- O P. Hi CO o OQ o Φ M P- tr Φ T) P o 4 P- P- rt o P P- P- cr CO φ P M P* o Φ P- P rt φ OQ P P oi Hi Φ vj Φ rt < P rt rt P- M Φ rt 1 P. P P φ rt o rt P- rt t vj 1 P i P O o 1 P

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Inventors: SCHORR HANS P, WEISS IRVING, SCHORR, HANS, P., WEISS, IRVING
Applicants: BROOKLYN UNION GAS CO, KRYOS ENERGY INC, KRYOS ENERGY INC., THE BROOKLYN UNION GAS COMPANY
Application year: 1985
Main classification: F25J3
IPC codes: F17D1, F25B9