Claims
- 1. In a method for the cryogenic cooling of a normally gaseous feed which includes at least two compression cycles, each having at least a low pressure stage of compression and a turbine driver, and said compression cycles are adapted to compress a normally gaseous fluid selected from the group consisting of a refrigerant for cooling said normally gaseous feed and a portion of the normally gaseous feed, the improvement, comprising:
- preventing overloading of said turbine drivers, due to changes in compressor limiting operating conditions, by:
- (a) measuring the suction pressures to said low pressure stages of each of said compression cycles;
- (b) establishing set point signals for each of said low pressure stages of each of said compression cycles, wherein said set point signals are representative of the maximum desired suction pressure;
- (c) comparing the measured suction pressures and the set point suction pressures for the low pressure stages of each of said compression cycles and establishing control signals in response to such comparison, wherein each of said control signals is responsive to the difference between the particular suction pressure and the particular set point compared and wherein each of said control signals is representative of the flow rate of said normally gaseous feed required to prevent the actual suction pressure for any particular low pressure stage of each of said compression cycles from exceeding the set point suction pressure for that particular low pressure stage of each of said compression cycles;
- (d) selecting the one of the thus generated control signals which is representative of the lowest flow rate of said normally gaseous feed; and
- (e) adjusting the flow rate of said normally gaseous feed in response to the selected control signal.
- 2. A method in accordance with claim 1 wherein the normally gaseous feed is a natural gas.
- 3. A method in accordance with claim 1 wherein the speed of each of the turbine drivers is also regulated in response to the suction pressure to the low pressure stage of the compressors driven by said turbine driver.
- 4. A method in accordance claim 1 wherein the normally gaseous feed is at an elevated pressure and the compression cycles include at least one refrigerant compression cycle adapted to compress a refrigerant for cooling said normally gaseous feed and a normally gaseous feed compression cycle adapted to compress a portion of the normally gaseous feed.
- 5. A method in accordance with claim 4 wherein the normally gaseous feed is cooled to a temperature sufficient to liquefy the same, the thus liquefied normally gaseous feed is further cooled by expanding the same in an expansion cycle, having at least a low pressure expansion stage, thus concomitantly evaporating a portion of said normally gaseous feed at said low pressure and the thus evaporated low pressure, normally gaseous feed is the portion of the normally gaseous feed thus compressed.
- 6. A method in accordance with claim 5 wherein the liquefied normally gaseous feed is expanded in the expansion cycle to at least three successively lower pressures, thus concomitantly evaporating high pressure, intermediate pressure and low pressure portions, respectively, of the normally gaseous feed and said low pressure, intermediate pressure and high pressure portions of said normally gaseous feed are compressed in a low pressure stage, an intermediate pressure stage and a high pressure stage of the normally gaseous feed compression cycle.
- 7. A method in accordance with claim 4 wherein the refrigerant is liquefied and the thus liquefied refrigerant is expanded to at least three successively lower pressures, thus concomitantly evaporating high pressure, intermediate pressure and low pressure refrigerant streams, respectively, and said low pressure, intermediate pressure and high pressure refrigerant streams are compressed in a low pressure stage, an intermediate pressure stage and a high pressure stage of the refrigerant compression cycle.
- 8. A method in accordance with claim 7 wherein the compression cycles include two like refrigerant compression cycles, utilizing two different refrigerants.
- 9. A method in accordance with claim 8 wherein the normally gaseous feed is a natural gas, one of the refrigerants is propane and the other of the refrigerants is selected from the group consisting of ethane and ethylene.
- 10. A method in accordance with claim 4 wherein the speed of each of the turbine drivers is also regulated in response to the suction pressure to the low pressure stage of the compressors driven by said turbine driver.
- 11. A method in accordance with claim 1 wherein the compression cycles include two like refrigerant compression cycles, utilizing two different refrigerants, the refrigerant from each refrigerant cycle is liquefied, the thus liquefied refrigerant is expanded to at least three successively lower pressures, thus concomitantly evaporating high pressure, intermediate pressure, and low pressure refrigerant streams, respectively, and said low pressure, intermediate pressure and low pressure streams are compressed in a low pressure stage, an intermediate pressure stage and a high pressure stage, respectively, of the refrigerant compression cycle.
- 12. A method in accordance with claim 11 wherein the normally gaseous feed is a natural gas, one of the refrigerants is propane and the other of the refrigerants is selected from the group consisting of ethane and ethylene.
- 13. A method in accordance with claim 11 wherein the speed of each of the turbine drivers is also regulated in response to the suction pressure to the low pressure stage of the compressors driven by said turbine drivers.
BACKGROUND
This application is a continuation-in-part of application Ser. No. 621,336, filed June 15, 1984 by the same inventors, now abandoned.
The present invention relates to a method of turbine drivers in a cryogenic gas plant and in a more specific aspect, the present invention relates to a method of controlling turbine drives in a liquefied natural gas plant.
Cryogenic liquefaction of normally gaseous materials is utilized for purposes of separation of mixtures, purification of the component gases, storage and transportation in an economic and convenient form, etc. Most such liquefaction systems have many operations in common, regardless of the gases involved, and, consequently, have many of the same problems. One common operation and its attendant problems is the compression of refrigerants and/or components of the gas. Accordingly, the present invention will be described with specific reference to processing natural gas but is applicable to other gas systems.
It is common practice in the art of processing natural gas to subject the gas to cryogenic treatment to separate hydrocarbons having a molecular weight higher than methane (C.sub.2 +) from the natural gas to thereby produce a pipeline gas predominating in methane and a C.sub.2 + stream for other uses, usually involving first separating this fraction into individual components, for example, C.sub.2, C.sub.3, C.sub.5 and C.sub.5 +. It is also common practice to cryogenically treat natural gas to liquefy the same for transport and storage.
Such cryogenic plants have a variety of forms, the most efficient and effective being a cascade-type operation and this type in combination with expansion-type cooling. Also, since methods for the production of liquefied natural gas (LNG) include the separation of hydrocarbons of higher molecular weight than methane as a first part thereof, a description of a plant for the cryogenic production of LNG effectively describes a similar plant for removing C.sub.2 + hydrocarbons from a natural gas stream.
In the cascade-type of cryogenic production of LNG, the natural gas is first subjected to preliminary treatments to remove acid gases and moisture. The natural gas at an elevated pressure, either as produced from the wells or after compression and at approximately atmospheric temperature, is cooled in a plurality of multistage (for example, three) cycles by indirect heat exchange with a plurality of refrigerants. For example, the gas is sequentially passed through a plurality of stages of a first cycle utilizing a relatively high boiling refrigerant, such as propane, and thereafter through a plurality of stages of a second cycle in heat exchange with a refrigerant having a lower boiling point, for example, ethane or ethylene. This sequential cooling of the natural gas is also controlled in a manner to remove as much as possible of the C.sub.2 and higher molecular weight hydrocarbons from the gas to produce a gas predominating in methane and containing small amounts of ethane. The C.sub.2 + hydrocarbons are then usually further processed, as by fractionation in one or more fractionation zones, to produce individual components such as C.sub.2, C.sub.3, C.sub.4 and C.sub.5 +. In the last stage of the second cooling cycle the main gas stream predominating in methane will generally be liquefied at essentially the pressure of the original feed gas. The liquefied main gas stream is then further cooled in indirect heat exchange with flashed gases, hereinafter described. Following this third cooling step nitrogen, if significant amounts thereof are present in a natural gas, is separated from the liquefied gas by fractionation or expansion and separation of the flashed gases to separate a gaseous methane stream containing most of the nitrogen. In a combined operation, after the nitrogen removal, the liquefied gas is further cooled in a fourth step or cycle comprising multiple stages of expansion and separation of flashed gas. Gases flashed or fractionated in the nitrogen separation step and those flashed in the expansion-flash step are utilized in the third cooling step referred to above. In each stage of the first and second cooling stages the gas is cooled by compressing the refrigerant to a pressure at which it can be liquefied by cooling. The liquefied refrigerant is then expanded to flash a portion thereof and the mixture of gas and liquid is passed to a chiller through which the feed gas stream passes in indirect heat exchange. The chiller often also functions as a separator for separating the flashed gas from the remaining liquid. The remaining liquid is then further expanded to flash a second portion thereof in the second stage of the cooling cycle, again the liquid and gas are separated and the liquid is further expanded to flash the remainder thereof in the third stage of the refrigeration cycle. These stages for convenience are referred to as a high stage, an intermediate stage and a low stage. The flashed gas from the high stage are at the highest pressure and highest temperature, the flashed gas from the second chiller is at an intermediate pressure and temperature and the flashed gas from the third stage is at the lowest pressure and lowest temperature. The flashed gases are then sequentially fed to an appropriate compressor or compressors. The gas from the third stage of the refrigeration cycle, which is at the lowest pressure and lowest temperature, is compressed. This compressed gas is then combined with the gas from the second stage and further compressed and the thus compressed third and second stage gases are mixed with the first stage gas and further compressed. The compressed refrigerant is then reused in the refrigeration cycle. The gases flashed in the expansion-separation cycle or fourth cooling cycle are compressed and recycled to the main feed gas stream. This compression of the flashed gases follows substantially the same procedure as that utilized in the compression of the refrigerants in the first two cycles. Specifically, if three stages of expansion-separation are utilized, the gas flashed in the first stage has the highest pressure and highest temperature, that flashed in the second stage an intermediate pressure and temperature and that flashed in the third or last stage has the lowest pressure and lowest temperature. Consequently, in compression of the flashed gases the gas flashed in the third stage, having the lowest pressure and temperature, is first compressed, then combined with the gas from the second stage, further compressed, and finally these two are combined with the gas from the first stage and still further compressed. In each of the separate compression cycles, i.e., the first refrigerant compression cycle, the second refrigerant compression cycle and the flashed gas compression cycle, a single turbine is utilized to drive one or more compressors.
Obviously, the refrigerant and flashed gas compressors have a design limit which should not be exceeded. Obviously, overloading the compressors will result in undue wear or damage to the compressors. Unfortunately, there are a number of compressor limiting operating conditions which fluctuate and as a result tend to overload one or more of the compressors. Such flucutations include changes in inlet gas composition, changes in climate that affect turbine horsepower, changes in boil-off rates resulting from ship loading or non-ship loading operations, shutdown of a turbine (either planned or unplanned), if more than one is used in parallel operation, and changes in the operation of a fractionating unit or the like. While an individual turbine can be protected, as by a speed control or the like, this is not a complete answer since changes in the operation of one turbine will change the operation of the entire cryogenic system resulting in possible overloading of other compressors as well as failure to maintain balanced operating conditions through out the cryogenic system.
It is, therefore, an object of the present invention to provide an improved method of compressor control which overcomes the above and other problems of the prior art. A further object of the present invention is to provide an improved method for controlling the feed rate of a gas through a cryogenic cooling system which overcomes the above and other problems of the prior art. Another and further object of the present invention is to provide an improved method for preventing the overloading of turbine drivers driving compressors in a plurality of compression cycles. Yet another object of the present invention is to provide an improved method of preventing overloading of turbine drivers driving compressors in a plurality of compression cycles in the cryogenic cooling of gas. Another object of the present invention is to provide an improved method of preventing overloading of turbine drivers driving compressors in a plurality of compression cycles utilized to compress refrigerant and a portion of the feed gas in a cryogenic gas cooling process. A further object of the present invention is to provide an improved method for controlling the feed rate in a cryogenic gas cooling process. A still further object of the present invention is to provide an improved method for controlling the feed rate of a gas to a cryogenic gas cooling process and the speed of turbine drivers utilized to compress refrigerant and/or a portion of the gas. Yet another object of the present invention is to provide an improved method of controlling the feed rate of a gas to a cryogenic gas cooling process and thereby prevent overloading of turbine drivers utilized to drive compressors in a plurality of compression cycles utilized in the process. Another and further object of the present invention is to provide an improved method for controlling the feed rate of a gas to a cryogenic gas cooling process wherein operation of the turbine drivers driving compressors in a plurality of compression cycles in the process are prevented from overloading and the feed gas rate is maintained below a predetermined maximum. These and other objects of the present invention will be apparent from the following description.
Overloading of turbine drivers, due to changes in compressor limiting operating conditions, driving compressors in at least two compression cycles utilized in a process for cryogenically cooling a normally gaseous feed, is prevented by adjusting the flow rate of the normally gaseous feed in response to the highest one of the suction pressures of the low pressure stages of the compression cycles. In a further embodiment, the feed gas flow rate is also maintained below a predetermined maximum. In yet another embodiment, the speed of the turbine drivers is also controlled in response to the suction pressure to the low pressure stage of the compression cycle driven by the turbine driver in question.
US Referenced Citations (3)
Continuation in Parts (1)
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Number |
Date |
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621336 |
Jun 1984 |
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Patent Grant
4698080
