method and apparatus for cooling a process stream

Abstract

The invention provides a method of cooling a process stream, the method comprising at least the steps of: (a) heat exchanging a first coolant supply stream in a first cooling circuit against a process stream at a first process temperature to produce a first coolant return stream and a cooled process stream; (b) passing the first coolant return stream to a first coolant return tank to provide warmed first coolant; (c) withdrawing a portion of the warmed first coolant from the first coolant return tank as a warmed first coolant stream; (d) heat exchanging a cooled second coolant stream in a second cooling circuit against the warmed first coolant stream to produce a cooled first coolant stream, (e) passing the cooled first coolant stream to a first coolant supply tank to provide cooled first coolant; (f) withdrawing a portion of the cooled first coolant from the first coolant supply tank as the first coolant supply stream; wherein the rate of flow of the warmed first coolant stream in step (c) is controlled in response to the cooling duty available from the second cooling circuit and the flow from the supply to the return buffer is controlled by the required cooling of the process stream and wherein the difference between the minimum and maximum cooling duty of the process stream over a time period is larger than the difference between the minimum and maximum cooling duty of the second cooling circuit over said time period.

1 . A method of cooling a process stream, the method comprising at least the steps of:
 (a) heat exchanging a first coolant supply stream in a first cooling circuit against a process stream at a first process temperature to produce a first coolant return stream and a cooled process stream;   (b) passing the first coolant return stream to a first coolant return tank to provide warmed first coolant;   (c) withdrawing a portion of the warmed first coolant from the first coolant return tank as a warmed first coolant stream;   (d) heat exchanging a cooled second coolant stream in a second cooling circuit against the warmed first coolant stream to produce a cooled first coolant stream,   (e) passing the cooled first coolant stream to a first coolant supply tank to provide cooled first coolant; and   (f) withdrawing a portion of the cooled first coolant from the first coolant supply tank as the first coolant supply stream; wherein the rate of flow of the warmed first coolant stream in step (c) is controlled in response to the cooling duty available from the second cooling circuit and the flow from the supply to the return buffer is controlled by the required cooling of the process stream and wherein the difference between the minimum and maximum cooling duty of the process stream over a time period is larger than the difference between the minimum and maximum cooling duty of the second cooling circuit over said time period.   
     
     
         2 . A method according to  claim 1  wherein the second cooling circuit is any chiller system, such as compression refrigeration systems with refrigerants such as propane, ammonia, R-134a or absorption chilling systems based on Lithium Bromide. 
     
     
         3 . A method according to  claim 2  wherein the first coolant comprises water. 
     
     
         4 . A method according to  claim 3 , wherein the first coolant is a refrigerant, and wherein the second coolant is cooling water or ambient air. 
     
     
         5 . A method according to  claim 4  wherein the sum of the amount of warmed first coolant in the first coolant return tank and the amount of cooled first coolant in the first coolant supply tank is held at a constant value. 
     
     
         6 . A method according to  claim 5  wherein steps (a) to (g) are repeated over at least one day and night cycle. 
     
     
         7 . An apparatus for cooling a process stream, such as a stream derived from natural gas, the apparatus at least comprising:
 a first cooling circuit comprising a first heat exchanger, a first coolant return tank, a second heat exchanger and a first coolant supply tank, said first heat exchanger having a first inlet which is connected to a process stream line, a first outlet which is connected to a cooled process stream line, a second inlet which is connected to the outlet of the first coolant supply tank and a second outlet which is connected to the inlet of the first coolant return tank, said second heat exchanger having a first inlet which is connected to the outlet of the first coolant return tank and a first outlet which is connected to the inlet of the first coolant supply tank; and   a second cooling circuit comprising the second heat exchanger and a cooling system, said second heat exchanger having a second inlet connected to the outlet of the cooling system and a second outlet connected to the inlet of the cooling system, wherein the cooling system of the second cooling circuit comprises a compressor, a condenser, and an expansion device, said compressor having an inlet connected to the second outlet of second heat exchanger, an outlet connected to an inlet of the condenser, said condenser having an outlet connected to an inlet of the expansion device and said expansion device having an outlet connected to the second inlet of the second heat exchanger and wherein the first inlet of first heat exchanger is connected to the outlet of an air cooler.   
     
     
         8 . An apparatus according to  claim 7  wherein a first pump is provided between the outlet of first coolant return tank and the first inlet of second heat exchanger and a second pump is provided between the outlet of first coolant supply tank and the second inlet of the first heat exchanger, wherein a temperature sensor is provided in stream line at the outlet of heat exchanger to detect the temperature in process stream line, said temperature sensor being connected to a first processor which determines the net supply of first coolant via the first pump to first heat exchanger and wherein a temperature sensor is provided between the outlet of second heat exchanger and the inlet of first coolant supply tank, said sensor being connected to a second processor which determines the supply of first coolant via the second pump to second heat exchanger and wherein a level sensor is provided in first coolant supply tank, said level sensor being connected to a third processor which determines the duty of the cooling system.