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The Refrigerant is the Key

Many peak shaving LNG liquefaction plants use a mixed refrigerant to liquefy the natural gas along a smooth condensing curve from ambient temperatures to cryogenic storage temperatures (typically –250 degrees Fahrenheit). The refrigerant consists of several components, often in a proprietary mixture selected by the original designers of the plant to best characterize the condensing curve of the incoming natural gas.

The refrigerant mixture can be the source of inefficiency and higher operating costs. Since refrigerant compression is often the largest expense at these facilities, it is important to maintain the proper amount and composition of refrigerant in the closed loop cycle. Failure to maintain the refrigerant load will cause inefficient heat transfer and require more compression horsepower for the same LNG production.

It’s All in the Mix

Refrigerant make-up must be done on occasion due to normal losses and leaks. However, it is imperative that proper filtering and desiccation systems are used to ensure the refrigerant entering the system is clean and bone dry. After all, the refrigerant will be letdown to as cold or colder temperatures as the LNG, in the –250 degree F range. Always assume the refrigerant make-up is dirty and wet. Many of the problems at facilities we support can be traced to a gas cylinder of wet isopentane or a bad load of slightly polymerized ethylene.

Refrigerator Raider

Often, some refrigerant components are substituted for lower cost species. An example of this is the use of propane instead of propylene. In one plant we have supported, using the lower cost component did not adversely affect performance. However, ethane should not be substituted for ethylene; both modeling and field data support this conclusion. The main benefit to ethylene is its heat capacity, which helps smooth out the heat curve at the point where methane begins to condense.

Colder Isn’t Always Better

Methane and especially nitrogen provide the coldest constituents of the refrigerant. However, at the moderate pressure of the refrigeration loop, neither are significantly condensed with air or cooling water and provide refrigeration primarily with sensible heat, which is much less efficient than the latent heat available from the ethylene and heavier components.

Often, the impression among operators is “colder is better” and the refrigerant is loaded preferentially with nitrogen or methane. Often the perceived solution to high LNG temperature going to storage is to load up excess amounts of nitrogen into the refrigerant loop. Because the nitrogen never condenses at these conditions, once it is overloaded, the refrigerant tends to “stall out” and the plant is left with no choice but to vent the refrigerant and start over.

Convenience = Trouble

Another concern is methane makeup directly from warm feed gas to the plant. Although bone dry and CO2 free by this point, methane make up directly from the feed gas will contain other constituents (primarily ethane) which affect the composition and therefore the efficiency of the refrigerant. This makeup is quick and easy (open a valve instead of hookup a gas cylinder, change out desiccant, etc.) and may be overused, adversely affecting the refrigerant mixture.

Ice, Ice Baby

Another common problem is water or CO2 build up in the brazed heat exchangers used to liquefy LNG. This can happen when treating facilities upstream (often outside the control of the LNG Plant) go off spec for even the shortest amount of time. Ice of either sort will plug up passages in the exchangers and severely limit the capacity of the plant. Without a significant shutdown to thaw out the exchangers, this problem doesn’t go away. There are two operations often tried “on the fly” that can have disastrous effects on equipment: “puffing” the cold box and methanol injection.

Puff, Puff, Pass

“Puffing” cold box refers to a short shutdown where the brazed aluminum liquefaction exchanger(s) are isolated and filled with warm nitrogen. This operation itself, if done too fast, can induce thermal stress on the exchanger in the portions that were running cryogenically. In one instance, it was even suggested that the nitrogen be heated, risking melting the aluminum parting sheets and ruining the exchanger.

Perhaps the greatest danger of “puffing” is the puff procedure chosen. Unless special blowdown connections were installed originally, the puff must be done manually. Once the pressure of the inflowing nitrogen is raised to about 50-100 psig, a blind or other connection is “broken”, resulting in a high velocity outflow of nitrogen in hopes of “puffing” the ice block to the storage vessel.

Raise the Roof

Since there is often a Demethanizer in the system that is integrally coupled by piping to the cold box, there is every chance that this high velocity “puff” may puff the trays or packing, perhaps damaging these expensive internals. And often in the haste to accomplish this puff, safety is shortcut in the manner of how the blind is inserted, or, if no blinds are available, we have seen a case where a manway flange was “broken” or unbolted on the fly against full pressure in the cold box.

Methanol: The Big Lie

Methanol injection can also be a disaster if done incorrectly, with the methanol itself freezing and making the ice block that much bigger. Also, methanol in the LNG product, where it will eventually go, is usually strictly forbidden by the product specs because end users demand extremely clean, steady fuel.

Domain Engineering Inc. 406 South Boulder, Ste 234 Tulsa, OK 74103 USA

(918)-582-4280 (918)-582-4283 fax