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Control of water and hydrocarbon dew points of natural gas streams is required for both safe transportation and safe use of natural gases. Depending on market specifications, the typical natural gas dew points range from -5 °C to -20 °C as water dew point and from 0 °C to -10 °C as hydrocarbon dew point while lower values can be required for subsea pipeline transportation.
To meet these specifications, the removal of both water and C6+ hydrocarbon fraction that contribute most to hydrocarbon dew point is required. Several technologies are available to control gas dew points. The water dew point is achieved by means of dehydration processes (as described in the Gas Dehydration and Hydrates Control process) while the hydrocarbon dew point is achieved by means of gas cooling processes which are based both on gas expansion or on direct gas cooling to condensate the heavy hydrocarbons fraction. Alternatively, it is possible to use the Silica Gel adsorption process that allows both water and heavy hydrocarbons to be removed in a single step.
These processes are described in more detail below:
Gas Cooling Processes for Hydrocarbon Dew Point Control: gas cooling is achieved in different ways, by means of gas expansion in a Joule Thomson (JT) Valve or by use of the more efficient Turbo-Expander. Alternatively, the feed gas is cooled by direct refrigeration by use of a Chilling Unit which is normally based on propane refrigerant. The lowest temperatures achieved in the cold section are normally in the range of -20 °C to -40 °C to meet the required hydrocarbon dew point in the full pressure range and therefore all cooling processes require an upstream gas dehydration to avoid hydrate deposition in cold section.
Cooling by expansion, by JT Expansion or by Turbo-Expander, is normally used for high pressure natural gas streams and often requires gas re-compression to deliver the treated gas to the transport pipeline; while the direct refrigeration by means of propane Chiller Unit is more suited for treating a low or medium pressure associated gas stream.
Finally, the gas cooling processes used for the hydrocarbon dew point control are the same as used for LPG or NGL recovery (refer to NGL/LPG Recovery and Fractionation) but the minimum operating temperatures are normally higher than for LPG or NGL recovery.
Silica Gel Adsorption for combined Water & Hydrocarbon Dew Point Control: Silica gel is porous solid in form of beads able to adsorb both water & heavy hydrocarbons (C6+) from natural gas. It is suitable for treating high pressure natural gas streams allowing the control of both water and hydrocarbon dew points in a single process unit and with minimum pressure drops.
Adsorption conditions for hydrocarbons are higher pressure (> 35 bar) and lower temperatures (< 40 °C). Adsorption is not influenced by the treated gas vapour-liquid equilibrium and it is effective even above Critical Pressure (limit of conventional cooling technologies); a high activity silica gel will be used to adsorb hydrocarbons.
Like for mol. sieves, the operation of unit is cyclical. After a silica gel bed is saturated, it is regenerated by means of heating through a slip stream of feed gas. Heating temperature is 280 °C and after heating the bed is cooled to recover adsorption capacity. Regeneration cycles are short and typical regeneration time is 100 - 120 minutes (much shorter than mol. sieves).