ADVANCE SEPARATION TECHNIQUES

Department Of Chemical Engineering

G.H. Patel College of Engineering & Technology Vallabh Vidhyanagar –388120

Topic: 4-STEP AND 6-STEP OF PRESSURE SWING ADSORPTION SYSTEM AND ITS APPLICATION

Academic Year – 2015-16, 2th Semester

Submitted By:Patel Rishi (130110105035)Patel Ronak (130110105036)Patel Vrushang (130110105037)

GUIDED BYDr. Tejal M. PatelProf. Arpita Patel

CONTENT

1. Introduction

1.1 Process

1.2 adsorbent

1.3 Applications

2. 4-step PSA system

3. 6-step PSA system

4. Advantages & Disadvantages

1. Introduction

• 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 adsorbent material.

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

• Specific adsorptive materials (e.g., zeolites, activated carbon, molecular sieves, etc.) are used as a trap, preferentially adsorbing the target gas species at high pressure.

• The process then swings to low pressure to desorb the adsorbed material.

1.1 Process

Fig1.1 PSA System

• Pressure swing adsorption processes rely on the fact that under high pressure, gases tend to be attracted to solid surfaces, or "adsorbed".

• The higher the pressure, the more gas is adsorbed; when the pressure is reduced, the gas is released, or desorbed.

• PSA processes can be used to separate gases in a mixture because different gases tend to be attracted to different solid surfaces more or less strongly.

• If a gas mixture such as air, for example, is passed under pressure through a vessel containing an adsorbent bed of zeolite that attracts nitrogen more strongly than it does oxygen, part or all of the nitrogen will stay in the bed, and the gas coming out of the vessel will be enriched in oxygen.

• When the bed reaches the end of its capacity to adsorb nitrogen, it can be regenerated by reducing the pressure, thereby releasing the adsorbed nitrogen.

• It is then ready for another cycle of producing oxygen-enriched air.

• This is the process used in medical oxygen concentrators used by emphysema patients and others who require oxygen-enriched air to breathe.

• Using two adsorbent vessels allows near-continuous production of the target gas.

• It also permits so-called pressure equalisation, where the gas leaving the vessel being depressurised is used to partially pressurise the second vessel.

• This results in significant energy savings, and is common industrial practice.

1.2 Adsorbent

• Aside from their ability to discriminate between different gases, adsorbents for PSA systems are usually very porous materials chosen because of their large specific surface areas.

• Typical adsorbents are activated carbon, silica gel, alumina and zeolite. Though the gas adsorbed on these surfaces may consist of a layer only one or at most a few molecules thick, surface areas of several hundred square meters per gram enable the adsorption of a significant portion of the adsorbent's weight in gas.

• In addition to their selectivity for different gases, zeolites and some types of activated carbon called carbon molecular sieves may utilize their molecular sieve characteristics to exclude some gas molecules from their structure based on the size of the molecules, thereby restricting the ability of the larger molecules to be adsorbed.

1.3 Applications

• Aside from its use to supply medical oxygen, or as a substitute for bulk cryogenic or compressed-cylinder storage, which is the primary oxygen source for any hospital, PSA has numerous other uses.

• One of the primary applications of PSA is in the removal of carbon dioxide (CO2) as the final step in the large-scale commercial synthesis of hydrogen (H2) for use in oil refineries and in the production of ammonia (NH3).

• Refineries often use PSA technology in the removal of hydrogen sulphide (H2S) from hydrogen feed and recycle streams of hydro treating and hydrocracking units.

• Another application of PSA is the separation of carbon dioxide from biogas to increase the methane (CH4) ratio.

• Through PSA the biogas can be upgraded to a quality similar to natural gas.

2. 4-Step Pressure Swing Adsorption System:

• In 4-step pressure swing adsorption system, mainly four prosses are occuring.

1. Pressurization

2. Product removal

3. Depressurization

4. Purging

Fig 1.2 𝐻2 production by 4-step PSA system

• Step (1): [Pressurization]

In the pressure swing adsorption system, when the adsorption column which is filled with feed is pressurized at that time product line valve is kept closed.

• Step (2): [Product removal]

Adsorption at high pressure with the withdrawal of the product at the constant rate. Doing the stage, Feed is continuously fed to the to the adsorption column and the product is continuously withdrawal from the column. In pressure swing adsorption system, some components are selectively adsorbed via. Vander waal force.

• Step (3): [Depressurization]

In depressurization step, the column is depressurized and the product line valve is kept close. During depressurization, gases compounds which are attached via low Vander waal forces are removed. But components which are attached via strong Vander waal binding force are not removed in this stage.

• Step (4): [Purging]

The purging occur at low pressure or under vacuumed. Purging is done in counter current direction or in reverse direction of feed. As a purg gas in most of the small fraction of product gas is use in the step components which are attached via strong binding force are also remove and adsorption bed is regenerated. Partial pressure of the impurities in purg gas should be less than the same in the feed gas.

3. 6-Step Pressure Swing Adsorption System:

Fig 1.3 6-Step Pressure Swing Adsorption System

• 6 step system containing 6 number of adsorber. Each adsorber contains adsorbent in two section; bottom section is activated carbon and upper section is molecular sieve.

• Adsorption of pressurize raw gas which produce H2 in 1st adsorber. At the end of this step column 1st is becoming saturated, at the same time following steps are also carried out simultaneosly in other adsorber.

• 2nd absorber, which has just complete purging is pressurised from 0.2 Mpa to 0.6 Mpa by feed gas. Then it is pressurised further from 1.6 Mpa to 2.2 Mpa via equalization with the column 6. Pressure is increased further in column 2 from 2.2 Mpa to 2.8 Mpa by feed gas.

• Countercurrent purging is carried out in column 3. Purge gas is obtained via depressurization of column 5 and 6. Purging is carried out at lower pressure 0.2 Mpa. Tail gas from column 3 is sent to buffer drum.

• Countercurrent depressurization of column 4 is carried out and blown down or tail gas is sent to pressure in column 4 is decreased from 0.4 Mpa to 0.2 Mpa.

4. Advantages & Disadvantages :

Advantages:

1. Complete automation and simplicity of operation

2. No manned attendence required during operation

3. Enhanced failure safety and realiability

4. Quick start and stop

5. Moderate dimensions and light weight

6. Low noise level

7. Extended operational life

8. Low operating costs

9. No special workshop requiremmts

10. Easy installation and integration into an exiting air system.

Disadvantages:

1. Relatively low oxygen purity – 93-95% for adsorption.

2. Limited capacity

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