DEVELOPMENT OF PRESSURE SWING ADSORPTION

BY: AZYYATI BINTI JOHARI

ADVANCE SEPARATION SYSTEM

OVERVIEW OF PSA UNIT

Pressure swing adsorption (PSA) is a technology used to separate some gas species from a mixture of gases under pressure according to the species' molecular characteristics and affinity for an adsorbant material.

It operates at near-ambient temperatures and so differs from cryogenic distillation techniques of gas separation.

Special adsorptive materials (e.g., zeolites) are used as a molecular sieve, preferentially adsorbing the target gas species at high pressure.

The process then swings to low pressure to desorb the adsorbent material.

Development of PSA unit

1930 : D.Finlayson and A.J. Sharp

introduce a single column with a

pressurization step and a sequence of depressurizations.

1960: C. W. Skarstrom proposed operation

consists of four distinct steps: Pressurization, adsorption, counter-

current depressurization and desorption

1985: P. L. Cen, W. N. Chen, and R. T. Yang

introduced the vacuum desorption using H2, change

counter-current pressurization to co-current

pressurization and used feed blower to supply air

1987: S. J. Doong and R. T. Yang

created PSA model for zeolite sorbent

which both micropore and

macropore diffusions are considered.

2001: B.K.,Na, K.K.,Koo, H.M.,Eum,

H.,Lee and H.K.,Song constructed PSA unit with 3 bed with 8-

step process including pressure equalization and

product purge step was designed.

2008: M.Zahra, T.Jafar, and

M.Masoud studied of a four bed pressure swing adsorption

process using zeolite13X to

decrease the power consumption of the

system

2011: Z.Liu, C.A.Grande, P.Li, J.Yu, and A.E. Rodrigues, studied for Multi-bed

Vacuum Pressure Swing Adsorption and introduced 7 steps of operation

cycle and using the zeolite as adsorbent

Development of PSA unit

HISTORICAL DEVELOPMENT OF PSA PROCESS

1930 : U.K. Patent 365092, D.Finlayson and A.J.Sharp (British Celanese Corp).

Process with a single column with a pressurization step and a sequence of depressurizations.

Co-current flow Concentration of vapours by a pressure swing process

over a charcoal adsorbent. Problems:

Not efficient Not get high purity of the separated gases Not effective Limited only to 2 mixture of gaseous. Small surface area of adsorbent Low product recovery Need high energy to operate Energy consumption : >100%

2 beds, 4 steps. Used to separate dry air. Operation consists of four distinct steps:

Pressurization Adsorption Counter-current depressurization or blow down

(reduce energy consumption to 90%) Light reflux or purge (or desorption)

Problems: This cycle can only separate 2 mixture of

gaseous only and the component that want to be separated have similar properties.

Use high energy to purge the gas to second bed(use air compressor)

Product recovery too low (want high purity of heavy product)

Energy requirement too high to be economical

1960:C. W. Skarstrom, Method and Apparatus for Fractionating Gaseous Mixtures by Adsorption, US Patent 2944627

Separate H2/CH4/H2S mixture into 3 useful products. Adsorbent used is activated carbon. 

They introduced the vacuum desorption using H2 or weak adsorbate for purposes ranging from air separation to separation of multicomponent hydrocarbon.

Change counter-current pressurization to co-current pressurization to increase the concentration of the most strongly adsorbed component in the bed.

Feed blower is used to supply air to the system instead of an air compressor and then, the purified gas is collected using vacuum blower to desorb the adsorber vessel.

Energy consumption reduce to 85%

1985:P. L. Cen, W. N. Chen, and R. T. Yang, Ternary Gas Mixture Separation by Pressure Swing Adsorption: a Combined Hydrogen-Methane Separation and Acid Gas Removal Process

Steps: Pressurization Feed Co-current depressurization/blowdown (increase product

recovery:recovering medium product, CH4)

Counter-current blow down (recovering heavy product,H2S and cleaning the bed for high purity of H2 product)

Problems: Instantaneous equilibrium is assumed between the gas and

adsorbed phases or the diffusion type considering only a monodisperse pore structure.

High energy needed as the pressure energy contained by the gas in the high pressure bed cannot be shared with the next bed which has been subjected to blow down.

2 3 4 1 Bed 1

3 1 2 3 Bed 2

43 1 2 Bed 3

They come out with PSA model for zeolite sorbent which both micropore and macropore diffusions are considered. This model is enough to be applied to bulk, multicomponent separations using any PSA cycle. They discusses about the bulk separation of a hydrogen-methane mixture using 5A zeolite.

5 Steps cycle: Repressurization with the light product High pressure feed Co-current depressurization Counter current blowdown Low pressure purge

Problems: 2 bed meaning separating 2 mixture of gaseous only High energy needed to perform this unit as we need energy to

purge the feed gas from 1st bed to 2nd bed.

1987:S. J. Doong and R. T. Yang, Bidisperse Pore Diffusion Model for Zeolite Pressure Swing Adsorption, AIChE J. 33 (1987), no. 6, 1045-1049.

2001:B.K.,Na, K.K.,Koo, H.M.,Eum, H.,Lee and H.K.,Song, CO2 Recovery from Flue Gas by PSA Process using Activated Carbon, Korean J. Chem. Eng., 18(2), 220-227 (2001)

In this study, a PSA unit with 3 bed was constructed. 8-step process including pressure equalization and product purge step was designed.

Activated carbon was used as an adsorbent .The feed gas mixture was 17% CO2, 79% N2, and 4% O2.

They introduced vacuum desorption. The bed was then connected to the vacuum pump to decrease the pressure to 0.1 atm and to obtain pure CO2. This vacuum desorption can give better power efficiency and lower energy expenses.

They also introduced pressure equalization step where beds are connected each other. So that bed with high pressure can be shared with the bed that want to be blown down.

Energy consumption reduce to 73%

3 Bed-8 Steps

1) Pressurization

2) Adsorption

3) Co-current blowdown

4) Co-current pressure equalization (the gas leaving the vessel being depressured is used to partially pressurise the second vessel. This results in significant energy savings, and is common industrial

practice)

5&6) Product purge

7) Vacuum desorption

8) Counter-current pressure equalization (converse mechanical energy)

Problems: Still use activated carbon as adsorbent although knew that

zeolite is better than activated carbon. As they are consider about pressure equalization, zeolite must

be used as zeolite has a bidisperse pore structure.

Study of a four-bed pressure swing adsorption process using zeolite13X to provide oxygen-enriched air.

The PSA cycle is consist of 6 steps. These Steps are as follows: 1- Pressurization with feed (RF) 2- Adsorption (AD) 3- Depressurization equalization (ED) 4- Blow down (BD) 5- Purge (PG) 6- Pressurization equalization (EP)

They successful decrease the power consumption of this system as the system can be operated in short cycle time.

Energy consumption reduce to 71% Problems:

Not considered vacuum blow down/vacuum desorption. Want better product purity up to 90%

2008: M.Zahra, T.Jafar, and M.Masoud, Study of a Four-Bed Pressure Swing Adsorption for Oxygen Separation from Air International Journal of Chemical and Biological Engineering 1:3 2008

They developed CO2 capture from flue gases of coal-fired power stations using zeolite 5A as selective adsorbent.

4 beds-7 steps Energy consumption reduce to 46%. Operation steps:

1-Pressurization 2-Feed 3-Co-current Depressurization 4-Counter-current Blow down 5-Counter-current Purge 6-Rinse 7-Pressure equalization

2011: Z.Liu, C.A.Grande, P.Li, J.Yu, and A.E. Rodrigues, Multi-bed Vacuum Pressure Swing Adsorption for Carbon Dioxide Capture from Flue Gas, Separation and Purification Technology 81 (2011) 307–317

Results:In this unit, result higher performance of separation which VPSA unit where CO2 purity increases to 96%. The overall unit productivity of this process is 0.0146 kgCO2/(kgads h) with an energy consumption of 645.7 kJ/kgCO2.

CONCLUSION

Many improvements have been made to the basic cycles to increase product purity, product recovery, adsorbent productivity and energy efficiency.

Among these modifications are: Use of 3, 4 or more beds co-current pressurization pressure equalization purge with a strongly adsorbing gas use of extremely short cycle time to approach isothermal

operation.

RECOMMENDATION

Pre-treatment feed gas first to remove strongly adsorbed components that might interfere with adsorption of other components.

For example, zeolite easily adsorb water vapor and carbon dioxide. So, we need remove water vapor first.

Problem if we not separate first is these gases will accumulate in the bed as they are not easily desorbed.

Solution: 1) Make separate pre treatment outside PSA system 2) Include pre treatment bed inside PSA system so the

integrated system can be regenerated as a whole.