Low-temperature separation of gas is an on-site natural gas treatment process aimed at gas condensate recovery, moisture removal, and gas preparation for subsequent transportation or supply to consumers as fuel gas.
Chilling is extensively used for gas dehydration and condensate separation from the gas of gas condensate fields, using low-temperature separation units, as well as for the recovery of individual components from the gas.
The low-temperature method of gas separation enables, depending on the chilling range, to recover from 80 to 100 percent of heavy hydrocarbons and perform gas dehydration during the transportation of a single-phase component to a required moisture and hydrocarbons dew point.
This gas treatment method allows concurrent performance of the following:
- gas dehydration to a water low dew point and increasing the methane number by the reduction of heavy hydrocarbons content;
- recovery of gas condensate and individual components or production of natural gas liquids.
The basic treatment process is performed in a low-temperature separator at low temperature. Depending on the feed gas parameters, different ways to decrease the gas temperature can be applied:
- Lowering temperature by using a Joule-Thompson (JT) valve.
This technique is used when the feed gas is supplied at high pressure, and the pressure at the low-temperature separation unit outlet allows achieving a pressure drop which during throttling gives the required for the treatment process separator inlet temperature.
This is the most economical and easy technique as it makes possible the utilization of the inlet gas potentially high pressure and requires minimum equipment within the configuration of separation units. Normally, this unit is supplied skid-mounted and fully prefabricated.
- Lowering temperature by using a turboexpander.
By its operating principle, this technique practically does not differ from the gas flow throttling technique. In this case, a highly efficient turboexpander is used as a throttling device.
It is applied when it is required to achieve low temperatures at a minimal pressure drop between the inlet and outlet and, also, to achieve low temperatures in fractionation units.
A turboexpander has a higher efficiency compared with a throttling device, and it can be used at low inlet gas pressures, though there is a requirement for the use of complex centrifugal equipment. A turboexpander unit has a relatively compact structure, and its installation in a modular enclosure is possible. This unit has a wide range of performance regulation.
- Chilling in refrigeration units
This technique is used if it is required to provide gas preconditioning with a minimal pressure loss, i.e. the preconditioned gas outlet pressure is the same as the inlet flow pressure.
Chilling is performed in a refrigeration unit that operates using different types of refrigerants. Depending on a specific case and the temperature required for gas preconditioning, a specific refrigerant may be selected (Freon, propane, ammonia, and others).
The refrigeration unit configuration includes refrigerant compressors which enable the closed-circuit refrigerant circulation. In the refrigeration cycle, the refrigerant condenses with subsequent evaporation. The transfer of chill is performed by the evaporation/boiling of the refrigerant in the evaporator.
Gas chilling to a required temperature can be achieved both directly in the evaporator by the boiling refrigerant and in the refrigerant circuit by supplying it with circulation pumps. The refrigeration units have a fairly compact design, and therefore can be delivered to their customers in maximum shop-assembled form.