Dew point control

The gas processing options described in the previous section were designed primarily to meet on-site usage or evacuation specifications. Before delivery to the customer further processing would normally be carried out at dedicated gas processing plants, which may receive gas from many different gas and oil fields. Gas piped to such plants is normally treated to prevent liquid drop out under pipeline conditions (dew point control) but may still contain considerable volumes of natural gas liquids (NGL) and also contaminants. 

Facilities for the treatment and compression of gas have already been described in earlier sections. However, there are a number of differences in the specifications for injected gas that differ from those of export gas. Generally there are no technical reasons for specifications on hydrocarbon dew point control (injected gas will get hotter not cooler) although it may be attractive to remove heavy hydrocarbons for economic reasons. Basic liquid separation will normally be performed, and due to the high pressures involved it will nearly always be necessary to dehydrate the gas to avoid water drop out. 

Absorber oil units offer the advantage that Hquids can be removed at the expense of only a small (34—69 kPa (4.9—10.0 psi)) pressure loss in the absorption column. If the feed gas is available at pipeline pressure, then Httle if any recompression is required to introduce the processed natural gas into the transmission system. However, the absorption and subsequent absorber-oil regeneration process tends to be complex, favoring the simpler, more efficient expander plants. Separations using soHd desiccants are energy-intensive because of the bed regeneration requirements. This process option is generally considered only in special situations such as hydrocarbon dew point control in remote locations. 

Turboexpander compressor Dew point control duty AMOCO Empress Plant, Canada 99.80... 

Sihca gel Drying of gases, refrigerants, organic solvents, transformer oils desiccant in packings and double glazing dew point control of natural gas. 

Ideally, all sizes of the fluid bed equipment should be equipped with inlet air dew point control. In the northern climates, this would include dehumidification in the summer months and humidification during the cold, dry winter months. In this manner, the inlet air dew point becomes a recipe... 

A second use of modeling is to evaluate the quality of the inlet air. Most coating installations have conditioned air with constant dew point control. However, there are cases where the moisture content of the inlet air can result in overwetting of the tablet bed. Using Eq. (9), the effect of inlet humidity on exhaust temperature can be predicted as shown below in Table 13. 

Commercial applications of the Selexol solvent for simultaneous hydrocarbon dew-point control and natural gas dehydration are de.scribed by Epps (1994). A plant design used in several European installations pretreats natural gas before it enters a molecular sieve unit. The design is intended to meet a treated gas specification of a maximum of 0.50 mole% CO2 and a maximum of 6.5 mole% ethane and heavier components. A plant is de.signed to treat 26 MMsefd of gas at 32"F and 603 psia. Operating data for this plant, given in Table 14-12, show that it meets the CO2 and ethane-plus removal specifications. The plant also reduces the water content of the gas from 75 ppmv to 12 ppmv, decreasing the load on the molecular sieve unit, and removes a major fraction of the sulfur components. 

---