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Simplified cooling method Disclosure Number: IPCOM000242074D
Publication Date: 2015-Jun-17
Document File: 2 page(s) / 17K

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This paper covers methodologies for cooldown of the main liquefaction exchanger without the pre-cooling step in a precooled liquefaction facility for LNG production.

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Simplified cooling method

Coil wound heat exchangers have operating limitations that must be observed to protect the mechanical integrity of the equipment.  One of the main limitations is that differential temperatures within the exchanger at any horizontal plane should not exceed about 28°C to avoid creating stresses that might lead to physical damage.  In precooled liquefaction cycles, such as propane precooled mixed refrigerant process, dual mixed refrigerant process, etc, the bottom temperature of the main exchanger operates at the temperature of the precooling system.  For a propane precooled system, this temperature is typically about -36°C.  To start this exchanger it has been necessary to bring the temperature of the exchanger down to about ‑36°C before any flow is admitted to the exchanger tubes.  This is typically accomplished by using a shell side flow from the bottom to the top that is gradually reduced in temperature from ambient to about -36°C, and cooling the entire exchanger to this temperature.  For large exchangers this might require about 12 hours of vented, typically flared, cooling gas.

In order to eliminate the need to flare a large quantity of gas that is required to cool the main exchanger, a different start up method needs to be developed.  It is possible to start the main exchanger with the normal mixed refrigerant refrigeration loop, as long as it is not precooled with the precooling refrigeration system.  To assure that there is no refrigeration of the mixed refrigerant before it enters the main exchanger, the precooling system must be prevented from cooling the mixed refrigerant.  In a propane precooled process, this can be done by bypassing the propane coolers, which would require special piping or by isolating the propane flow in all of the mixed refrigerant propane coolers by either removing the propane from the coolers or allowing the pressure of any propane within the coolers to rise above the bubble point so that there is no evaporative cooling provided.  Once the mixed refrigerant cooling flow has been established for the main exchanger, it is now permissible to slowly start precooling of the mixed refrigerant stream.  The precooling refrigerant is slowly started by either slowly admitting the refrigerant flow to each cooler individually, only decreasing the temperature of the mixed refrigerant at the rate allowable for the ma...