Gas Dehydration

Dehydration is important in three areas:

Gas gathering

Product dehydration: Maximum water in sales gas at 4 to 7 lb/MMscf For liquids, the water content is 10 to 20 ppmw

Hydrocarbon recovery

Water content is stated in a number of ways:

Mass of water per volume of gas, lb/MMscf Concentration, parts per million by volume (ppmv)

In any mixture, where both the gas and liquid phases are in equilibrium, each component, i, in the mixture obeys the relationship

Water Content Of Hydrocarbons

Water Content Of Hydrocarbon Gases as a Function of Temperature and Pressure

Calculate the water content of the sweet natural gas shown in Table 6.1at 300 psia and 80°F

The MW of the gas mixture is 18.41Specific gravity : Sp. Gr.= 18.41/28.96 = 0.636

Two processes, absorption and adsorption, are the most common

ABSORPTION PROCESSES: Water levels in natural gas can be reduced to the 10 pmmv range in a physical

absorption process.

The solvent used for the absorption should have the following properties:

A high affinity for water and a low affinity for hydrocarbons

A low volatility at the absorption temperature to reduce vaporization Losses

A low viscosity for ease of pumping and good contact between the gas and liquid phases

A low potential for corrosion

Gas Dehydration Processes

Glycol Dehydrator Unit

The two types of adsorption are physical adsorption and chemisorption.

In physical adsorption, the bonding between the adsorbed species and the solid phase is called van der Waals forces.

Physical adsorption is an equilibrium process like vapor−liquid equilibria

Two steps are involved in adsorbing a trace gas component.

The first step is to have the component contact the surface The second step is to have it travel through the pathways

inside the adsorbent

Adsorption Processes

Water Loading on Adsorbent

Vapor-phase Concentration Profile Of An Adsorbate In The Three Zones Of An

Adsorption Bed

When used as a purification process, adsorption has two major disadvantages:

o It is a fixed-bed process that requires two or more adsorption beds for continuous operation.

o It has limited capacity and is usually impractical for removing large amounts of impurity.

Three types of commercial adsorbents are in common use in gas processing plants:

Silica gel, which is made of pure SiO2

Activated alumina, which is made of Al2O3

Molecular sieves

Two-bed Adsorption Unit

To minimize MTZ thickness, the bed diameter should be kept small.

This feature also reduces the wall thickness of the high-pressure vessels and increases the superficial velocity, which improves mass transfer in the gas phase.

An existing 4A molecular sieve bed has been processing 80 MMscfd on a 12-hour cycle with two beds. Exit gas goes to a cryogenic turboexpander section. Gas flow is increased to 100 Mscfd. Estimate the increased pressure drop and determine whether the bed capacity allows continued operation on a 12-hour cycle or the cycle time should be changed. The gas enters the bed at 120°F and 950 psig. Water content is 60% of saturation at 120°F. The molar mass of the gas is 18.5, with a viscosity of 0.014 cP and a compressibility factor of 0.84. The adsorption bed contains 41,000 lbs of 1/8-inch diameter beads with a bulk density of 44 lb/ft3. The inside wall diameter of the bed is 7.5 ft. The absorbent was installed 2 years ago.

Desiccant Processes:

Can reduce the water content down to 20 ppmv

Membrane Processes Refrigeration Processes Twister Technology Vortex Tube Technology

Other Processes