Gas Absorption

 GAS ABSORPTION

Gas Absorption-Gas absorption is a unit operation in which gas mixture is contacted with liquid for preferentially dissolving one or more constituents of gas providing a solution of them in liquid.

Example Gas from by product coke ovens is washed with H2O to remove NH3 and again with an oil to remove benzene and toluene vapours.

 Comparison of absorption with distillation

Absorption		Distillation
1	Gas absorption is separation of constituents of gas mixture		It is related with separation of constituents of liquid mixture.
2	The constituents of gas mixture are separated by using liquid solvent		The constituents of liquid mixture are separated by using thermal energy.
3	Liquid solvent used in gas absorption is below its boiling point.		Liquid in related with the distillation operation is at its bubble point.
4	Gas absorption heat effects are attributed to heat of solution of the dissolved gas.		Distillation gives almost pure product in single processing step.
5	Gas absorption operation is well below its dew point		Vapour in distillation is always at its dew point

Concept of equilibrium

                        ►The degree to which gas is absorbed from gaseous mixture by liquid solvent is determined by its partial pressure. Each dissolved gas exerts a definite partial a pressure at a given temperature and concentration. The gas is said to be slightly soluble when the partial pressure exerted by the dissolved gas from the solution of given concentration is

                        ►A given quantity of liquid will dissolve any amount of slightly or more soluble gas depending upon the partial pressure of dissolved gas in phase in contact with the liquid. Slightly soluble gas requires higher partial pressure in contact with liquid yield solution of desired concentration. While very soluble gas requires very low partial pressure to give the solution of same concentration.

                        ►Equilibrium condition will be reached when the partial pressure of solute gas in gas phase is equal to its partial pressure from liquid phase. The equilibrium condition thus fixes the upper limit for operation of absorption unit.

For dilute solutions of most gases the equilibrium relationship is given by Henry’s law:

                        pA = H.x

                            xA – mole fraction of solute gas in liquid phase

              pA- partial pressure of solute gas in the gas phase

                             H- Henry’s constant

 Minimum liquid gas ratio

                      ►Consider that the values of the terms V,Y1,Y2 and X2 are fixed by process requirement, and the value of L’ is decreased through the absorption tower

                        ►The operating line X-Y coordinates passes through the point( X2,Y2) and has a slope equal to L’/V’.

                        ►As liquid flow rate is decreased, the slope of operating line decreases and exist liquid composition increases from X1’ to X1 as indicated by line.

                        ►A Reduction in liquid flow rate decreases the driving force for mass transfer and absorption becomes more and more

Mechanically agitated Vessel       ►It is usually used in small scale industry. This unit gives performance equal to one theoretical stage.      ►It consists of baffled vessel usually incorporating open to disc flat bla►Gas is introduced below the impeller through a ring shaped sparger under agitation.      ►Adequate free board is provided over liquid level and ultimately unabsorbed gas leaves the top of vessel.     ►The diameter of sparger is equal to or slightly less than that of impeller and is arranged with holes (3 to 6.5 mm) on top.        ►Deep vessels with usually two impellers are used when time of contact required to relatively large.     ►Deep vessels with usually two impellers are used when time of contact required to     relatively large wherein large interfacial area is maintained by re dispersing the gas bubble coalesced.      Packed Column  Ø    Packed columns are most frequently used for gas absorption. Ø    The liquid is dispersed in the form of film and gas flows as continuous contact equipment’s generally operated in a counter current fashion.  Ø    Packed column essentially consists of vertical cylindrical shell. Filled with suitable packing’s which offer large out of interfacial area for gas liquid contact for mass between phases. Ø    The liquid is introduced from top through liquid distributor, which irrigates the packing uniformly and liquid trickles down the bed.  Ø    Liquid flow rate should be sufficient for good wetting of packing. Ø    The gas is introduced from the bottom of the tower and rise upward. Ø    The lean gas leaves the tower from top of the tower. Ø    In case of tall towers ,liquid redistributors are used. Advantages of packed columns

                        ►Minimum structure.

                        ►Low Pressure drop.

                        ►Low liquid Hold up.

                        ►Handle corrosive liquids that tend to foam.

                        ►Disadvantages of packed column.

                        ►Relatively inflexible.

                        ►Cannot operate over wide ranges of either vapour or liquid rates per unit cross section.

                        ►Distribution of liquid is difficult.

                        ►Cannot handle dirty fluids that tend to deposit a sediment.

                        ►Cannot be used where large temperatures are encountered.

 Applications of packed column

              ►Small diameter.

               ►Handling highly corrosive fluids.

               ►Handling liquids that tend to foam.

             ►Operations where it is important to have low pressure drop and low liquid hold up.

 Characteristics of tower packing

Ø It should provide a large interfacial area for phase contacting.

Ø It should possess good wetting characteristics.

Ø It should have a high corrosive resistance .

Ø It should be relatively cheap.

Ø It should possess enough structural strength.

Ø It should be chemically inert to the fluids handled in the tower.

Types of Packing

For obtaining efficient gas liquid contact ,many different types of packing’s are available

They are generally classified as random packings and regular packings.

If the packings are simply dumped into the tower during installation and the individual pieces are not arranged in any particular pattern, They are known as random packings.

The packings arranged in a particular pattern are called stacked packings.

Most common packings are :

 1) Raschig rings

2) Pall rings

 3) Hy-pak

4) Berl saddles

5) Intalox saddles

 6) Super intalox saddles

 7) Lessing ring

 

 Most common random packings used in industrial towers are raschig rings, pall ringsHy-pak,berl saddles, Intalox saddles super intalox saddles etc.

    As pall rings and intalox saddles permit a more economical tower design.

li    Pquall rings are made out of metal or plastic and have same general form as raschig rings

    Berl saddles and intalox saddles are available in range of 6 to 50 mm and are formed out of plastic or ceramics They provide more gas liquid contacting area than raschig rings.

    Rashig rings are hollow cylinders with their length equal to outside diameter .It is made up of of ceramic ,carbon and metal. They are simple in construction and relatively cheap as compared to other.

 

Selection criteria for solvent

              1) Gas solubility: the solubility of solute gas in a solvent should be high . the solvent selected should have a high solubility for the solute to be absorbed

                        2) Volatility: As the gas leaving an absorption unit is generally saturated with the solvent, there will be a loss of the solvent with the gas leaving the unit operation, hence to minimize the solvent loss , the solvent should be less volatile.

           3) Corrosive nature : The solvent should not be corrosive towards common materials of construction so that the construction material for an absorption equipment will not be too expensive

            4) Viscosity : the solvent should have a low viscosity for rapid absorption rates, low pumping cost and better heat transfer. The solvent should be non viscous.

                        5) Cost and availability : the solvent should be cheap and readily available

                        6) Miscellaneous : the solvent should be non-toxic, non-flammable, nonfoaming, and chemically stable from a handling and storage point of vie

 Hydrodynamics of Packed column

                        ►The variation of pressure drop with gas velocity is plotted on log-log graph

        ►In case of dry packing, the relationship between pressure drop and gas velocity is represented by a straight line indicating that pressure drop is proportional to G 1.8-2. For wet packing, the relationship is indicated by straight line, but for a given velocity, pressure drop will be more than that for d   ►At point X the gas flow begins to impede the down flow of liquid and local accumulation of liquid appears here and there in packing’s. As the gas velocity increases further liquid hold up progressively increases due to which free area for gas flow becomes smaller and pressure drop rises much more quickly.

                        ►At gas flow rates beyond Y, pressure Drop rises very steeply. At point Y, entrainment of liquid by gas leaving the top of tower increases and tower is then said to be flooded. The gas velocity corresponding to the flooding conditions is called as flooding velocity.

Loading and flooding velocity in packed column

                        ►Loading velocity- Loading point of a column is when the gas velocity is high enough to restrict the flow of liquid. After this point, the pressure drops at a much faster rate till another point, known as the flooding point, when all the liquid is carried away by the gas. It marks the start of entrainment regime in columns.

                        ►Flooding velocity –After loading point the pressure drops at a much faster rate till another point, known as the flooding point, at which this effect occurs the velocities of which a column is operated is called as flooding velocities .

                        ►Loading point-It is a point where liquid hold up starts to increase and caused a change in the slope of the pressure drop.

             

                        ►Flooding point –It is a point where the gas velocity result in all packing to be wetted at this point.The contact area between liquid and gas will be maximised.

 Height equivalent to theoretical plate(HETP)-

 Ø    Height equivalent to theoretical plate (HETP) Height equivalent to theoretical plate (HETP)-It is a height of section of packing that will gives the same separation as that achieved with one theoretical plate.

Ø    The HETP must be an experimentally determined quantity and is characters tics of each packing It depends upon the type and size of packing, flow rates of fluid and concentration.

Ø    he total height of column to effect the separation is calculated by multiplying the number of theoretical plates required for a given degree of separation by the HETP.