PSA in Petrochemical Industry

Pressure Swing adsorption technology is playing a significant role in the petrochemical industries. The unique capacity of this technology to selectively adsorp gases makes it a preferred choice for the Petrochemical Industries. Hydrogen production and purification plays an important role in the Petrochemical industries. Predominantly the consumers of purified hydrogen are the petrochemical industries that requires pure hydrogen in the processes of methanol and ammonia synthesis, MTBE processes etc.

The PSA process eliminates the impurities from a feed gas that is rich in hydrogen by directing the gas through a fixed bed that is filled with different adsorbents. This is done at high pressures. Then at low pressures impurities are subsequently desorbed as an off-gas stream. The end products can become 99.999% pure.

Apart from Hydrogen purifications, Pressure Swing Adsorption techniques play a significant role in the production of polyethylene. This is described here in detail. The olyethylene unit generally uses a combination of Pressure Swing adsorption (PSA) recovery process and partial condensation. This enables total recovery and re use of nitrogen gas. In fact, Nitrogen is used for removal of hydrocarbons from the polyethylene. This results in considerable savings in expenditure and eliminates the harmful emissions generally associated with partial or non recovery of the nitrogen and hydrocarbons. The Nitrogen produced with the technology of partial condensation with PSA is over 99% pure.

Working of the PSA system:
Pressure Swing Adsorption has enabled marked improved in recovery of hydrocarbons from a mixture of gases. Consequently, these hydrocarbons get recycled into the polymerization process. Essentially the PSA unit has an adsorbent material which extracts selectively the hydrocarbons from the nitrogen stream. Nitrogen do not get adsorbed here, leaving the unit as a pure stream. As the adsorbent bed approaches saturation with hydrocarbons, the bed is regenerated. This gets done by lowering the pressure in the bed which releases the hydrocarbons. After this the hydrocarbons are recovered by way of low-pressure tail gas and is recycled back to the compressor. In the way it again passes through the partial Condensation and PSA units.

The cycle is allowed to operate until the hydrocarbons gets condensed in the Partial condensation unit and leaves the recovery process. Each adsorption bed operates alternating between the adsorption and desorption phases. For a continuous flow process generally multiple beds are operated in staggered cycles.

Diagram showing a Polyethylene Plant using PSA technology to optimum use

Benefits of using PSA system in polyethylene plant: The main benefits emanating from installation of PSA units can be shown in the form of a table

The table gives the data for a Polyethylene Plant with 320,000 MT of production per year. The table clearly shows the direct costs of the recovery system are considerably lesser than the cases with either no recovery or incomplete recovery cases.

	No Recovery	Incomplete Recovery	Recovery System with PSA
Lost Propylene ($1,000)	2,034	610	0
Purchased Nitrogen ($1,000)	366	363	2
Added Fuel to Flare ($1,000)	0	551	0
Electric Power ($1,000)	-0	136	256
Total ($1,000)	2,400	1,661	258
Cost Per Metric Ton Polypropylene	$9. 60	$6. 64	$1. 03

Courtesy: US department of energy.