Gas cooling processes, as well as refrigerating machines and units, which provide these processes, are essential elements of gas industry manufacturing. The specific features of the refrigeration equipment, applied in gas industry, are the following: large unit capacity, a wide application range of compressors (gas-turbine or gas reciprocating ones) gas drives, prior use of air-cooling units in the function of condensers for the refrigerant; an application of cheap refrigerants (basic products and by-products in this industry); a high reliability degree, sufficiently long operational life, and automation maximum level.
The main technological processes of the gas industry, which involve the use of refrigeration systems, are as follows:
– gas field processing before its supply into a gas pipeline for delivery to consumers. The gas processing is performed with the purpose of its dehydration to the desired value of water and hydrocarbon dew point. This process is applied at gas processing plants of gas and gas-condensate fields. The gas is to be cooled to the temperature range of 0…–20 0С.
– gas cooling before its supply into a gas pipeline to the temperatures similar to the multi-year frozen ground temperature. This technological process is applied in northern fields, located in a permafrost zone, to prevent melting of frozen ground. The gas is to be cooled by using a gas cooling station to the temperature range of –2…–40С.
– natural and petroleum gas decomposition with obtaining ethane fraction, propane, butane and others in the form of commercial products. This process is realized at gas processing plants (GPPs) and units. Depending on types of the products obtained and the degree of valuable components recovery, the gas may be cooled up to –100 0С.
– natural gas liquefaction for its further transportation by various modes of transport (LNG technology). The gas is liquefied both at large liquefied natural gas (LNG) plants and small units placed on gas distribution stations (GDS) and NGV-refuelling compressor stations. Depending on the working pressure of the LNG technological process, the gas is to be cooled up to the temperature of – 162 0С (the methane condensing temperature at atmospheric pressure).
Refrigeration at any specified temperature level is carried out in systems implementing one or another cooling cycle. The following process flow diagrams are currently used in low-temperature processes of gas industry processing stations:
– with an external refrigeration cycle based on refrigerating machines of various types. At that, both the single-component refrigerants (propane, ethane, ammonia, etc.) and the multi-component, mixed (hydrocarbon mixture) ones may be applied as a working medium. The cascade refrigeration cycles are used for deep cooling.
– with an internal refrigeration cycle, when the direct cooling of process flows is used via their isenthalpic expansion in a throttle and isentropic expansion in an expander.
– with a combined refrigeration cycle, when, for example, at the early stage the flow is cooled by using a refrigerating machine with its further throttling and expanding.
Let us consider in more detail the design, technical characteristics and operation features of vapour-compression refrigerating machines used in various technological processes of the gas industry.
The refrigerants being the natural gas components and their mixtures (propane, ethane, propane/butane mixture and others) are preferred in the gas industry. Being behind the аmmonia in the evaporation heat value, they are low-cost and accessible refrigerants which production from the natural gas may be organized in a processing area.
The main thermodynamic properties of the most widely used refrigerants are presented in Table 1.
Table 1. The refrigerants thermodynamic characteristics.
|Formula||Molecular weight μ, kg/kmol||Normal boiling temperature ts, 0С||Critical temperature tcr, 0С||Critical pressure Рcr, MPa||
Evaporation heat r, kJ/kg
In addition to the specified refrigerants, the ammonia and various freons are also widely used.
It is typical for cold consumption systems to create centralized cooling stations with an evaporating unit brought, as a rule, out of a machinery hall, closer to production facilities.
The direct cooling circuit is more efficient, but may prove to be inapplicable because of sufficient pressure losses in suction lines or due to the refrigerant incompatibility with a technological product cooled in the evaporator. In this case it makes sense to use a circuit with an intermediate cooling medium. The aqueous solutions of glycols (for example, ethylene glycol and propylene glycol) are mainly used as cooling media.