The cryogenic liquefaction of natural gas is routinely practiced as a means of converting natural gas into a more convenient form for transportation and storage. Such liquefaction reduces the volume of the natural gas by about 600-fold and results in a product which can be stored and transported at or near atmospheric pressure.
Natural gas is frequently transported by pipeline from the supply source to a distant market. While it is desirable to operate the pipeline under a substantially constant and high load factor, often the deliverability or capacity of the pipeline will exceed demand while at other times the demand may exceed the deliverability of the pipeline. In order to shave off the peaks where demand exceeds supply or the valleys when supply exceeds demand, it is desirable to store the excess gas in such a manner that it can be delivered when demand exceeds supply. Such practice allows future demand peaks to be met with material from storage. One practical means for doing this is to convert the gas to a liquefied state for storage and to then vaporize the liquid as demand requires.
The liquefaction of natural gas is of even greater importance when transporting gas from a supply source which is separated by great distances from the candidate market and a pipeline either is not available or is impractical. This is particularly true where transport must be made by ocean-going vessel. Ship transportation in the gaseous state is generally not practical because appreciable pressurization is required to significantly reduce the specific volume of the gas. Such pressurization requires the use of more expensive storage containers.
In order to store and transport natural gas in the liquid state, the natural gas is preferably cooled to −240° F. to −260° F. where the liquefied natural gas (LNG) possesses a near-atmospheric vapor pressure. Numerous systems exist in the prior art for the liquefaction of natural gas in which the gas is liquefied by sequentially passing the gas at an elevated pressure through a plurality of cooling stages whereupon the gas is cooled to successively lower temperatures until the liquefaction temperature is reached. Cooling is generally accomplished by indirect heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, methane, nitrogen, carbon dioxide, or combinations of the preceding refrigerants (e.g., mixed refrigerant systems).
In any natural gas liquefaction process, there will be progressive accumulation of buildup upon the interior surfaces of the cryogenic heat exchanger. Such buildup can be caused by water in the form of ice or relatively heavy hydrocarbons present in the gas feed in solid form. The various sections of the cryogenic heat exchanger operate at different temperatures depending upon what stream is passing through a particular section. For example, one section of the cryogenic heat exchanger can operate at an inlet temperature of −35° F. and an outlet temperature of −50° F., while a nearby or contiguous section can operate at an inlet temperature of −147° F. and an outlet temperature of a temperature colder than −147° F. while yet another nearby or contiguous section in the cryogenic heat exchanger can operate at an inlet temperature of −147° F. and an outlet temperature of −204° F. Thus, it can be seen that a specific stream containing materials having various freeze points may pass through one or more sections of the unit without forming a buildup, but the same stream may encounter a separate section operating at a lower temperature than the other section(s), and buildup can ultimately result thus adversely affecting the overall heat transfer efficiency of the unit. Build-up of solids in these cryogenic heat exchangers, control valves and other associated equipment can lead to reduced heat transfer, high pressure drop and/or reduced flow resulting in a decrease in LNG production.
Of special concern, is the need for the removal, or deriming (or “defrosting”), of heavy hydrocarbons which precipitate, wax or freeze in small passageways of cryogenic equipment. Build-up of solids in these systems can lead to reduced heat transfer, high pressure drop and/or reduced flow resulting in a decrease of LNG production. Liquid petroleum gas (LPG) as a deriming solvent, as well as other liquid solvents, have been utilized in efforts to remove such buildup while the equipment remains in operation. However, for many applications in high pressure portions of the plant, motor-driven metering pumps for injection of liquid solvents into the process can be difficult to acquire and may have limitations on the flow rate and/or the pressure head required for the service.
Therefore, a need exists for improved equipment and methods to facilitate the removal, or de-riming, of heavy hydrocarbons that precipitate, wax up or freeze in the passages of cryogenic equipment, such as cryogenic heat exchangers, control valves and other associated equipment.