7 Adsorbents, PSA, adsorptioncycle, rotary bed/fluidized bed

Dr. Geeta

epgp books

 

 

 

    Introduction

 

Adsorption: It is the process of accumulation of substance on the surface of another substance. It may be defined as surface phenomenon which deals with the utilization of surface force and concentration of materials on the surface of solid bodies. Adsorption is most commonly used to recover product and purify the gas stream having high concentration of organic compounds.

 

Adsorbent: It is defined as material that has the ability to extract certain chemical substances from gases, liquids and solids on adhere on their surface without any change in their physical properties. Adsorbent play an important role in various industries, cleaning of environment and in processing of fuel. Various kinds of adsorbents are used in separation, emission of gas and in controlling environmental reaction.

 

24.1 Types of adsorbents: Depending upon the structure, pore size, distribution of microspore size there found significant differences in the adsorptive properties. Based upon type of material the adsorbents are classified into two types i.e. inorganic adsorbent and organic adsorbent

 

Inorganic adsorbent: Most minerals and synthetic inorganic material are used as adsorbents like metal chlorides, silicates (MgSiO3), oxides (MgO), diatomaceous earth, limestone, sodium bicarbonate. Commonly used inorganic adsorbent are silica, zeolites and alumina.

 

Organic adsorbent: Adsorbents based upon organic material (synthetic or natural) like carbon and charcoal, cellulose, collagen, wool, chitin, crop residue, corn polysaccharide etc are used as adsorbent as well as absorbent. Commercial available adsorbent are activated carbon and polymers (polymethacrylate, vinylpyridine).

  1. Silica gel: It is made up of sodium silicate. It can be prepared in two ways. In one method sodium silicate is mixed with sulfuric acid, jelly like precipitant are formed .These precipitant are dried and roasted. Chemical formula is SiO2.nH2O.In another method silicic acid is polymerize and condenses in the aqueous solution to form a chain of spherical particles of 20-200 Ao diameters. When drying, the particle agglomerate to form a micro porous structure.Silica gel is useful desiccant where high capacity is required at low temperature and moderate pressure. It is ineffective at high temperature (above 500oF).
  2. Activated alumina: It is a porous high area form of aluminum oxide, prepared either directly from bauxite (Al2O3.3H2O) or from monohydrate by dehydration and recrystallization at elevated temperature. It is amphoteric in nature showing both acid and basic character. It surface is strongly polar. The capacity of activated alumina is higher at elevated temperature so it can use as a desiccant for drying warm air of gas stream. It also used as support material in catalytic reaction.
  3. Activated carbon: It is made by thermal decomposition of carbonaceous material such as coal, wood, coconut, nutshell and petroleum based products, then activation (removal of tarry carbonization product so opening old pores) with steam or carbon dioxide at elevated temperature. These gases attack the carbon and increase the pore structure. Surface of carbon is nonpolar in nature, showing slight polarity due to surface oxidation and used to control emission of toxic gases and wide variety of organic solvents. They are hydrophobic and oragnophilic therefore used in water purification, solvent recovery, in automobiles, air purification and decolorizing sugar.
  4. Carbon molecular sieves: Carbon sieves prepared from decomposition of polyvinylidene dichloride (saran) at earliest but now variety of material are used in there preparation. Most commonly used material is anthracite or hard coal. Carbon sieve is prepared by their controlled oxidation and thermal treatment. The pore size is depending up on condition during their preparation. Theses carbon sieves are used in air separation, production of hydrogen gas and clean-up of off-gases from nuclear facilities.
  5. Zeolites(molecular sieves): Zeolites are porous crystalline aluminosilicate obtained naturally or formed synthetically. It consist of SiO4 and AlO4 terrahedra joined together in various arrangement by sharing oxygen atoms, forming a crystal lattice containing pores where other molecule can easily penetrate. Two crystal types are common: type A (having opening formed by 4 sodalite cages) and type X or Y (having opening formed by 6 sodalite cages) as shown in Fig 1.It can be used to capture or separate gases on the basis of their molecular shape and size. Molecular sieves are used to remove moisture from exhaust streams, to remove nitrogen oxides compounds and to separate hydrocarbon in refining processes. MCM-41 is a zeolite material.It has uniform and large pore size; surface is acidic in nature and having large surface area .It help in removal and recovery of VOCs and reduction of nitrogen oxide with hydrocarbons. Zeolite HM is an effective in separating CO2 and NO2 gases in low concentration in a mixture due to its cationic and geometry of pore.

 

Fig 1: Showing structure of zeolites

 

The adsorption/Desorption cycle with Zeolite

 

Adsorption: Zeolite is a crystalline mineral which has the property of adsorbing water vapors because of its porous structure and high amount of heat is released out in the process.

 

Desorption: In this process water is baked out from the zeolite in the form of vapors by heating the zeolite and liquefies in the evaporation vessel.

 

Table 1: Showing physical properties of major adsorbents

 

6.  Synthetic polymer: These adsorbents are made by cross linking long chain polymers having variety of functional groups. They have rigid microporous structure. They have high adsorption capacities for selected organic compounds and desorbed more rapidly (in hot air, hot nitrogen, heating and in microwave) as compared to activated carbon. They are used in gas stream containing large amount of water vapors. Synthetic polymers are used to control organic compound like ketones, reactive monomers and aldehydes. Major limitation for using them is there high cost.

 

7. Activated coconut charcoal: It is used to adsorbs volatile organic compounds

 

The use of solid wastes like waste tyres,coal reject and oil palm fibers are use to develop adsorbents which are used to the removal of VOCs,NOx and phenolic compounds.

 

The selection of suitable adsorbents is based upon their properties like having high internal surface and high porosity, high adsorption efficiency,regenerability,kinetics thermal stability, mechanical intergrity,low pressure drop and low cost . Adsorption occurs at gas-solid interface so effectiveness of any adsorbent is depending upon the available surface area. Larger the surface area higher is the adsorption capacity. Dubinin classified the pores in activated carbon as micropore (10-100Ao), transitional (100-1000 Ao) and macropores (above 1000 Ao).Most of the gaseous pollutants is in 400-900 Ao in size. So larger pore act as only passageway and smaller one serve as the best having strongest adsorption force.

 

24.2 Adsorption system

 

Non regenerative adsorption systems: It consist of thin adsorbent beds ranging in thickness from 0.5 to 4 inches (1 to 10 cm).It is mainly used in air purification devices for small air flow streams like laboratory exhaust and offices. The thin bed absorber available in different shapes like flat, cylindrical and pleated. The activated carbon granules are retained by porous supporting material as perforated metal sheet.

 

Regenerative adsorption system: It consists of fixed, moving and fluidized beds. These system are used to control variety of organic vapors and are regenerated by low pressure steam. Fixed carbon beds are used for controlling the air pollutants. The organic compound may be recovered by condensing the exhaust from the regeneration step and separating the water and the solvent.

 

Regeneration is done by reversing the adsorption process by increasing the temperature and decreasing the pressure. Four methods are usually used for regeneration.

 

a) Pressure swing: In this pressure is lowered at constant temperature to reduce the adsorbent binding capacity.

b) Thermal swing: The bed is heated to reduce the adsorption capacity. Steam regeneration is a best example of thermal swing regeneration.

c) Inert purge gas stripping: Stripping is caused by inert gas which reduces the partial pressure of the contaminant. In this molecule migrates from surface into the gas stream.

d) Displacement cycle: This method is used where the adsorbate is heat sensitive and valuable. The adsorbate is displaced by preferentially adsorbed material.

 

24.3 PRESSURE SWING ADSORPTION (PSA)

 

Adsorption is a separation process in which essential requirement is an adsorbent that adsorbs one component from the mixture of components. The selectivity may be depends on difference in kinetic (sorption rates) or on difference in adsorption equilibrium. PSA technology is defined as the selective adsorption of one or more components of a feed gas mixture on a solid adsorbent so that the adsorbed phase with composition different from the feed mixture is formed when the feed is contacted with the adsorbent. The adsorbed components are then desorbed by reducing the partial pressure and the clean adsorbent can be reused again. In short we can say that PSA process is consist of cyclic sequence of adsorption at high partial pressure and desorption at low partial pressure

 

There are two main principles in the separation processes in all adsorption

1.Adsorption: here the preferentially adsorbed species are picked from the mixture.

2.Desorption: here the species are removed from the adsorbent. This process is also known as regenerating.

 

In PSA process the adsorbs species are removed in the regeneration step by reducing the total pressure under isothermal condition. The adsorption taking place at high pressure and desorption occur at low pressure. This principle is proposed by skarstrom. A simple process consists of four cycles. In the first step high pressure feed flow through bed here a part of its effluent is expanded to the lower pressure, and then bed undergoes blowdown through the feed end in the second step. In the third step the heavy component is desorbed from the fixed bed and other are purged out at low pressure from the storage tank. The gas from the storage tank is utilized in fourth step for pressurize.

 

Application of PSA

  1. Separation of air: Removal of H2O and CO2 from air feed to distillation columns.
  2. Production of CO: From blast furnace and steel industry.
  3. Hydrogen production: from steam-methane reformer (SMR) off gas. Production and upgrading of hydrogen from different hydrogen containing gases.
  4. Production of ammonia: from SMR off gas.
  5. Production of CO2: from methane separation and upgrading crude natural gas.
  6. Alcohol dehydration, helium recovery from diving gas, solvent vapor removal and recovery, separation of normal and iso-parffins,ozone enrichment

We can say that it can provide an energy efficient, low cost and very flexible method of gas separation as compared to other conventional methods like distillation and adsorption.

 

Fig 2 :(a) Two bed PSA System and 2(b) The movement of the adsorbed phase concentration profile for the more strongly adsorbed species in two bed PSA system.

 

24. 4 Adsorption cycle

 

Adsorption is a reversible process where the molecule is fixed onto a solid matrix consists of porous material. Adsorption is exothermic reaction because energy is lost when molecules are fixed on the adsorbent.

 

Adsorption cycle of refrigeration consists of four periods:

 

  1. Heating and pressurization: in this period the absorber receives heat while being closed. The temperature of adsorbent increase resulting in increase in pressure and it is known as compression cycle.

 

2. Heating and desorption + condensation: Here absorbers receiving heat continually which induces desorption of vapour.The desorbed vapor is liquefied in the condenser. This period is called condensation in compression cycles.

Fig 3: Showing four steps in adsorption process in refrigeration

 

3. Cooling and Depressurization: In this cycle absorber release heat while being closed. The temperature of adsorbent is decrease, which induce to decrease in pressure .This is called expansion in compression cycles.(Fig 3)

 

4. Cooling and adsorption + Evaporation: the Absorber continues releasing heat while being connected to the evaporator. The temperature of adsorbent decreasing, which induce adsorption of vapour.This is known as evaporation in compression cycles.

 

24.5 ROTARY BED/FLUIDISED BED

 

24.5.1 Rotary bed:

 

In this solid and vessel move together, relative to fixed inlet for the feed and fixed outlet for the product. It is used in recovery from air onto activated carbons.

 

 

Fig4: Rotary-bed absorber

 

It consist of following components:Steam,solvent,rotor drive, air filter, solvent laden air, motor, water, solvent vapor, fan, air cooler, storage drain(Figure 4).

 

The activated carbon is contained in a thick annular layer and radial partition is done. Air enters through drum circumference and passes through the carbon layer to emerge free of solvent. When drum rotate, carbon enter a section where it exposed to steam. Stem and solvent pass to condenser and solvent is recovered by decanting or by distillation process. There is no separate provision for cooling the regenerated adsorbent. They allowed cooling when in contact with vapor laden air so adsorption capacity becomes lower. Adsorption and desorption are performed simultaneously on different sector of rotor. The system is attractive for the retreatment of dilute solvent –laden air streams prior to incineration.

 

24.5.2 Fluidized bed: As the rotary bed absorber has not being successfully used as commercial level due to high steam requirements in cyclic heating and cooling of adsorbents. Another problem associated with fixed bed system is poor steam, gas distribution and complex valving requirements. To achieve the target in which solid moves with respect to the containing vessel, the fluidized bed absorber do better. Solids is easily added as well as removed from fluidized bed and the absence of cyclic heating. Fluidized beds are design that use activated carbon, activated alumina and polymeric adsorbents. There are large number of units was designed for SO2 adsorption. The reinluft and alkalized alumina processes are two example of fluidized bed SO2 adsorbs. They adsorbed SO2 along with conversion of adsorbed gas to SO3.

 

Fig 5: Fluidized bed Absorber

 

The commercial fluidized bed is Purasive HR unit, in which adsorption and regeneration occur continuously occur in separate zone as shown in figure5. Inlet gas enters the fluidized top section and, flow upward through section and exit of the unit. The desorbed adsorbent is cooled by heat exchange and lifted by air back to the top of the vessel.

 

It consist of two section

a.       Adsorption section

b.      Desorption section

 

Adsorption section: The sieve trays are used into which raw gas is passing through the sieves of the tray and thus fluidizing the solid adsorbents particles. The fluidized solids flow across the tray.

 

Desorption section: The solid from adsorption section pass into desorption section .The solid enter into preheating tube and then through desorption tube. Moving bed is used in this section because the stripping steam flow rate is insufficient for fluidizing the solids.

 

At the bottom the regenerated solids are picked up by a carrier gas, which then flow upward by gas lift-line in the vessel on top tray to repeat the adsorption cycle.

 

Summary:

  • Various types of adsorbents are ued to make the high adsorption capacity.
  • Most commonly used adsorbents are activated carbon, zeolite, silica gel and alumina. The selection of best adsorbent is depending upon their physical properties like pore size, surface area, sorption capacity etc.
  • Pressure swing adsorption provides selective absorption and it is a cost effective method.
  • Two types of adsorption system are mostly used i.e. non-generative and regenerative. Among all the fluidized bed provides the continuous operation and most efficient utilization of adsorbent.

    Glossary

 

Absorption– Penetration of one substance into the inner structure of another .Occurs between a gas or vapor and a liquid. The process of movement of a drug from the site of application into the extracellular compartment of the body.

 

Adsorbent-A substance which has the ability of condensing or holding other substances on its surface. Active carbon, activated alumina, and silica gel are examples

 

Adsorption -to gather (a gas, liquid, or dissolved substance) on a surface in a condensed layer

 

Desorption– The release of materials that have been absorbed or adsorbed in or onto a formation.

 

Non polar-containing no permanently dipolar molecules; lacking a dipole.

 

Partial pressure-the pressure that a gas in a mixture of gases would exert if it occupied the same volume as the mixture as the same temperature

    References

  • Douglas M Ruthven, Farooq S and Knaebel Kent S. Pressure swing adsorption, 1994
  • Douglas M Ruthven. Principle of adsorption and adsorption processes, Wiley-interscienc e, 1984

Web links:

  1. http://learnindustry.blogspot.com
  2. http://onlinelibrary.wiley.com/doi/10.1002/aic.690410122/abstract
  3. http://perso/ims.fr/mpons/pricyc.htm
  4. https://books.google.co.in/books/about/Principles_of_Adsorption_and_Adsorption.html
  5. http://separationprocesses.com/adsorption/AD_Chp03.htm
  6. http://www.sssj.org/ejssnt/duan-small.jpg
you can view video on Adsorbents, PSA, adsorptioncycle, rotary bed/fluidized bed