US3504735A

US3504735A - Control of liquid level of refrigerants in serially connected indirect heat exchangers

Info

Publication number: US3504735A
Application number: US776976A
Authority: US
Inventors: James W Hobbs; Dale E Lupfer
Original assignee: Phillips Petroleum Co
Current assignee: Phillips Petroleum Co
Priority date: 1968-11-19
Filing date: 1968-11-19
Publication date: 1970-04-07
Anticipated expiration: 1987-04-07

Description

April 7, 1970 w HOBBS ET AL 3,504,735

CONTROL OF LIQUID LEVEL'OF REFRIGERANTS IN SERIALLY CONNECTED INDIRECT HEAT EXCHANGERS Filed NOV. 19, 1968 A W HHH 3 u R 0 W 1 EM m mm 0 @Q $1 3 m wzmjzim J a l 3 mww w w m 8 u mm 0 A W m mm 5 mm 5 mm 1 M24252 mm x 3 w mm a 6 mm v m mw mv a Q. 3 5 1 MZ IEE+ mm mm mm I United States Patent 3,504,735 CONTROL OF LIQUID LEVEL OF REFRIGERANTS IN SERIALLY CONNECTED INDIRECT HEAT EXCHANGERS James W. Hobbs, Bartlesville, Okla., and Dale E. Lupfer,

Sweeny, Tex., assignors to Phillips Petroleum Company, a corporation of Delaware Filed Nov. 19, 1968, Ser. No. 776,976 Int. Cl. F25j 3/08 US. Cl. 1651 8 Claims ABSTRACT OF THE DISCLOSURE A feedstream to be cooled is passed through first and second indirect heat exchangers in series. A refrigerant is passed at a variable flow rate from a limited source thereof through the second heat exchanger. The flow rate of a refrigerant to the first heat exchanger is manipulated to vary the liquid level of refrigerant in the first heat exchanger responsive to the liquid level of refrigerant in the second heat exchanger.

The invention relates to method and apparatus for controlling the cooling of a feedstream by indirect heat exchange with two refrigerants, one of the refrigerants being variable in the quantity available.

In the past, it has been common to vary the flow rate of an at least partially liquefied refrigerant to an indirect heat exchanger responsive to the liquid level of the refrigerant in the heat exchanger. However, such control is unsatisfactory when employing a refrigerant of varying and limited availability. Cooling control is lost when the required heat transfer exceeds the cooling power of the available refrigerant, and cooling power is wasted when the heat transfer requirement is less than the cooling power of the available refrigerant.

In accordance with the invention, it has been discovered that the cooling power of the refrigerant of limited availability can be employed to the fullest while maintaining the desired control of the cooling of the feedstream by passing the feedstream to be cooled through two indirect heat exchangers in series, with the refrigerant of limited availability being utilized in the second exchanger, and varying the liquid level of another refrigerant in the first exchanger responsive to the liquid level of the refrigerant of limited availability in the second exchanger.

Accordingly, it is an object of the invention to provide improved method and apparatus for controlling the cooling of a feedstream. Another object is to utilize the cooling power of a refrigerant from a variable limited source to the maximum extent possible. Another object of the invention is to maintain the desired heat transfer while employing a refrigerant having a variable flow rate from a limited source. Other objects, aspects and advantages of the invention will be a parent from a study of the specification, the drawing, and the appended claims to the invention.

The drawing is a diagrammatic representation of a process incorporating the control system of the present invention.

Referring now to the drawing in detail, a feedstream comprising hydrogen, methane, ethylene, ethane, and propylene is passed through conduit 1 into and through coil 2 in indirect heat exchanger 3, wherein the feedstream is cooled by heat exchange with a vaporizing refrigerant, for example, propylene. The at least partially liquid refrigerant is introduced into the shell of exchanger by way of conduit 4. The resulting refrigerant vapors are withdrawn from exchanger 3 by way of conduit 5. A valve 6, positioned in conduit 4, can be manipulated by liquid level controller 7 responsive to a comparison of the actual level "ice of liquid refrigerant in the shell of exchanger 3 with the desired liquid level represented by setpoint 8. Valve 9, positioned in conduit 5, is manipulated by pressure recorder controller 10 responsive to a comparison of the refrigerant vapor pressure in the shell of exchanger 3 as indicated by pressure sensor 11 and the desired vapor pressure represented by setpoint 12. The thus cooled feedstream is withdrawn from exchanger 3 and passed by way of conduit 14 into and through coil 15 of indirect heat exchanger 16. An at least partially liquid stream of ethane is passed by way of conduit 17 into the shell of exchanger 16, wherein the ethane is vaporized to further cool the feedstream in coil 15. The resulting vaporized ethane is withdrawn from the shell of exchanger 16 by way of conduit 18 and passed to further processing, utilization or storage. A valve 19, positioned in conduit 18, can be manipulated by pressure recorder controller 21 responsive to a comparison of the output signal from pressure sensor 22 which is representative of the actual refrigerant vapor pressure in the shell of exchanger 16 and a setpoint signal 23 which is representative of the desired refrigerant vapor pressure in the shell of exchanger 16. The further cooled feedstream is withdrawn from coil 15 and passed by way of conduit 26 into and through coil 27 of indirect heat exchanger 28. A refrigerant, for example ethylene, is passed through conduit 29 into the shell of exchanger 28 and is withdrawn therefrom by way of conduit 31. The still further cooled feedstream, which is partially liquefied, is then passed through conduit 32 into liquid-vapor separation tank 33.

A liquid stream, comprising ethylene, ethane and propylene, is withdrawn from the bottom of tank 33 by way of conduit 34 and passed to further processing, utilization or storage. A valve 35, located in conduit 34, can be manipulated by liquid level controller 36 to maintain the actual liquid level in tank 33 substantially at a desired value represented by setpoint 37. A vapor stream, comprising hydrogen, methane, ethylene and ethane, is withdrawn from an upper portion of tank 33 and passed by way of conduit 38, coil 39 of indirect heat exchanger 41 and conduit 42 into liquid vapor separator 43. A refrigerant, for example ethylene, is passed through conduit 44 into exchanger 41 to partially liquefy the fluid in coil 39. The resulting warmed refrigerant is withdrawn from eX- changer 41 by Way of conduit 45. A valve 46, located in conduit 38, can be manipulated by pressure recorder controller 47 responsive to a comparison of the actual pressure in tank 33 as indicated by pressure sensor 48 and the desired pressure represented by setpoint 49.

A vapor stream, comprising primarily hydrogen and methane, is withdrawn from an upper portion of separator 43 by way of conduit 51. A valve 52, positioned in conduit 51, can be manipulated by pressure recorder controller 53 responsive to a comparison of the actual pressure in separator 43 as indicated by pressure sensor 54 and the desired pressure represented by setpoint 55. A liquid stream, comprising primarily methane, ethylene and ethane, is withdrawn from a lower portion of separator 43 and passed by way of conduit 56 into demethanizing fractionator 57. A valve 58 positioned in conduit 56 can be manipulated by liquid level controller 59 to maintain a liquid level in separator 43 substantially at the value represented by setpoint 61.

A vapor stream, comprising primarily methane, is withdrawn from an upper portion of fractionator 57 through conduit 62. A liquid stream, comprising ethylene and ethane, is passed from a lower portion of fractionator 57 through conduit 63 into fractionator 64. A valve 65, positioned in conduit 63, can be manipulated by liquid level controller 66 to maintain a liquid level in fractionator 57 at a value represented by setpoint 67. A vaporous ethylene stream is withdrawn from an upper portion of fractionator 64 by way of conduit 68 while a liquid ethane stream is withdrawn from a lower portion of fractionator by way of conduit 17. Liquid level controller 71 manipulates valve 72 in conduit 17 responsive to setpoint 73 to maintain a desired liquid level in fractionator 64.

In accordance with the present invention, liquid level controller 74 compares a signal representative of the liquid ethane level in the shell of exchanger 16 with a setpoint signal 75 representative of the desired liquid ethane level in the shell of exchanger 16 and produces an output signal representative of the difference between the two signals. This output signal is applied to the setpoint input 8 of controller 7. This permits the efiicient utilization of all of the ethane available from the kettle product of fractionator 64, which varies in amount as dictated by controller 71. Moreover, the control system of the invention provides the desired cooling of the feedstream. The temperature of the ethane in the shell of exchanger 16 is maintained constant at the boiling point of ethane corresponding to the pressure represented by the setpoint 23 to pressure controller 21. As the heat transfer requirement increases due to the temperature of the feedstream in conduit 14 rising and/ or the flow rate of the feedstream through conduit 14 increasing, the liquid ethane in the shell of exchanger 16 is vaporized at a faster rate, causing a small drop in the liquid ethane level in exchanger 16 below the level represented by setpoint 75. This results in an increase in the output signal from controller 74 which is applied to the setpoint input of level controller 8, thereby calling for a higher liquid refrigerant level in exchanger 3. Controller 7 increases the opening of valve 6 to pass refrigerant through conduit 4 into the shell of exchanger 3 at a higher rate. As the liquid level rises, the heat transfer rate increases, thereby providing additional cooling to the feedstream. Similarly, a decrease in the heat transfer requirements for exchanger 16 causes a slight rise in liquid ethane level in exchanger 16, a decrease in the setpoint signal to controller 7, and a partial closing of valve 6. Thus, the control system provides for the maximum utilization of the cooling power of a variable flow rate refrigerant stream from a limited source thereof while maintaining the desired cooling of the feedstream.

While

exchangers

3, 16 28 and 41 can be shell and tube heat exchangers, one or more can be replaced with any other type of indirect heat exchanger having at least two flow paths therethrough with the liquid level in one of the flow paths being variable to vary the heat transfer rate. Pressure reduction valve can be employed in any of

conduits

12, 26, 32, 42, 56 and 63 to increase the autorefrigeration of the feedstream. While the invention has been illustrated with all of the feedstream being passed from coil 2 to coil 15, part of the feedstream can be withdrawn from conduit 14 or additional feed can be added to conduit 14.

We claim:

1. Apparatus comprising a first indirect heat exchanger having first and second flow paths therethrough,

a second indirect heat exchanger having first and second fiow paths therethrough,

first conduit means for passing a feedstream to be cooled to an inlet of said first flow path of said first indirect heat exchanger, second conduit means for passing at least a portion of said feedstream from an outlet of said first flow path of said first indirect heat exchanger to an inlet of said first flow path of said second indirect heat exchanger,

third conduit means for passing an at least partially liquefied refrigerant to an inlet of said second flow path of said first indirect heat exchanger,

fourth conduit means for withdrawing vaporized refrigerant from an outlet of said second flow path of said first indirect heat exchanger,

fifth conduit means for passing an at least partially liquefied refrigerant stream at the available flow rate from a limited source thereof to an inlet of said second flow path of said second indirect heat exchanger,

sixth conduit means for withdrawing vaporized refrigerant from an outlet of said second flow path of said second indirect heat exchanger,

means for varying the liquid level of refrigerant in said second flow path of said first indirect heat exchanger to maintain the liquid level of refrigerant in said second flow path of said second indirect heat exchanger substantially constant, and

seventh conduit means for withdrawing the thus cooled feedstream from an outlet of said first fiow path of said second indirect heat exchanger.

2. Apparatus in accordance with claim 1 further comprising means for maintaining the pressure of refrigerant in said second flow path of said first indirect heat exchanger substantially constant, and means for maintaining the pressure of refrigerant in said second flow path of said second indirect heat exchanger substantially constant.

3. Apparatus in accordance with claim 1 wherein said means for varying the liquid level comprises means for measuring the actual liquid level of refrigerant in said second flow path of said second indirect heat exchanger and establishing a first signal representative thereof, means for establishing a second signal representative of the desired liquid level of refrigerant in said second fiow path of said second indirect heat exchanger,

means responsive to said first and second signals to establish a third signal representative of the difference between said first and second signals,

means for measuring the actual liquid level of refrigerant in said second flow path of said first indirect heat exchanger and establishing a fourth signal representative thereof, means responsive to said third and fourth signals for establishing a fifth signal representative of the difference between said third and fourth signals, and

means for varying the flow rate of refrigerant through said third conduit means responsive to said fifth signal.

4. Apparatus in accordance with claim 3 further comprising means for varying the rate of withdrawal of refrigerant vapors from said second flow path of said second indirect heat exchanger to maintain the pressure in said second flow path of said second indirect heat exchanger substantially constant, and means for varying the rate of withdrawal of refrigerant vapors from said second flow path of said first indirect heat exchanger to main tain the pressure in said second flow path of said first indirect heat exchanger substantially constant.

5. Apparatus in accordance with claim 1 further comprising means for separating the feedstream passing through said seventh conduit means into at least two fractions, and means for passing one of said fractions to said fifth conduit means as the source of refrigerant for said fifth conduit means.

6. A method of cooling at feedstream which comprises passing said feedstream through a first flow path of a first indirect heat exchanging zone and then through a first flow path of a second indirect heat exchanging zone,

passing an at least partially liquefied first refrigerant through a second flow path of said first indirect heat exchanging zone in indirect heat exchanging relationship With the feedstream in said first flow path of said first indirect heat exchanging zone to cool said feedstream by vaporizing said first refrigerant,

passing an at least partially liquefied second refrigerant at a variable flow rate from a limited source thereof through a second flow path of said second indirect heat exchanging zone in indirect heat exchanging relationship with the feedstream in said first flow path of said second indirect heat exchanging zone to cool said feedstream by vaporizing said second refrigerant, and varying the liquid level of said first refrigerant in said second flow path of said first indirect heat exchanging zone to maintain the liquid level of said second refrigerant in said second flow path of said second indirect heat exchanging zone substantially constant. 7. A method in accordance with claim 6 further comprising maintaining the pressure of said first refrigerant in said second flow path of said first indirect heat exchanging zone substantially constant, and maintaining the pressure of said second refrigerant in said second flow path of said second indirect heat exchanging zone substantially constant.

8. A method in accordance with claim 7 further comprising withdrawing the thus cooled feedstream from said 6 first flow path of said second indirect heat exchanging zone, separating said thus cooled feedstream into at least two fractions, and utilizing one of said fractions as said second refrigerant.

References Cited UNITED STATES PATENTS 3,255,596 6/1966 Greco et a1. 6221 MEYER PERLIN, Primary Examiner C. SUKALO, Assistant Examiner US. Cl. X.R.