Gas absorptionThe application of gas absorption techniques in chemical engineering process operations has expanded rapidly in recent years, particularly in the fields of exhaust gas scrubbers and the recovery of hydrogen chloride, hydrogen bromide and oxides of sulphur. In many instances the efficient recovery of these materials is dictated by economic operating requirements.
The absorption process relies on the ability of a liquid to hold within its body relatively large quantities of a gaseous or vapor component. This ability makes it possible to remove one or more selected components from a gas stream by contacting with and absorbing into a suitable liquid.
Whatever type of absorption process is being considered, in order to achieve efficient gas absorption, the gas must be brought into intimate contact with the liquid phase. The equipment must therefore provide a large interfacial area for this contact to occur. In order to achieve this, several basic designs have evolved.
These are very similar in design to distillation columns and can be fitted with a range of gas liquid contractors i.e. perforated plates, bubble caps and turbo grids. Columns efficiencies range from 20-80%. However, plate columns are not particularly suited for absorption process especially where corrosive materials are handled.
Owing to their design these units are ideally suited for handling corrosive materials. They have high efficiencies and are easy to construct and operate.
Falling Film Absorbers
Falling Film absorption units are very similar in construction to vertically mounted shell and tube heat exchangers. The absorbent flows down the walls of the tubes co-currently with the gas stream. Liquid distribution is provided by a weir arrangement at the top of the tube bundle. The shell side of the unit contains the cooling water.
These units are particularly suited for handling of corrosive materials because of their deign and ease of operation. They offer the further advantage of being able to handle a wide range of loadings with a flow rates as low as 24 litres/min/m to maintain full tube wetting.
All three types of units can be supplied by us, constructed from Borosilicate Glass.
The main construction material of all these process plants is borosilicate glass which therefore has the following particular advantages in the absorption operations.
- It has an exceptional corrosion resistance to all chemicals, excluding hydrofluoric acid and hot strong caustic solutions.- Because of the inertness of the glass, contamination does not occur.- It is very transparent, therefore any visible malfunctions can be seen as soon as they occur, Remedial action can be taken without waiting for spoiled product, or damaged plant to indicate fault.- Borosilicate glass has very good surface properties, therefore, fouling within the column on the packing and walls is unlikely.- All units although designed exactly to customer requirements are of modular construction. There is therefore minimum outlay in equipment and plant construction costs.- Modular construction enables modifications to be carried out easily to suit changes in operating requirements.
Exhaust Gas Scrubber
The exhaust gas scrubber represents one of the simplest forms of the absorption process and is used where concentrations in the feed gas to the column are low and the component cannot be economically recovered. As the liquid to gas ratio is normally high, any heat of solution generated in the process is taken up by the liquid phase. Heat exchangers to cool the product stream are therefore not normally required. Absorption is this type of operation can also be combined with a neutralization process, for example, the use of an alkaline solution in an acid gas scrubber as shown in the fig.
Pure Gas Absorber (Constant Feed Rate)
As the feed gas is pure, or contaminated with air and water vapour only, any vapours generated by the heat of solution during the absorption process are condensed within the column itself and are returned with the make up water to the packed section. Any non-condensibles leave the column by a vent at the top. The product is cooled by the lower heat exchanger before it leaves the column. Note : - The acid strength control unit is only applicable in the case of HCl absorption.
Impure Gas Absorber (Constant Feed)
As the feed gas is impure, any vapours generated by the heat of solution of the absorption process are condensed externally to the column. The condensate which is usually water and organic phases are thus removed from the system, ensuring product purity. The condensate can be treated further to reclaim any valuable material. The heat exchanger in the base of the column cools the product before it leaves the unit.
Gas Absorber (Varying feed rates)
In certain situations the feed rate of the gas entering the absorption unit can vary widely, as in a batch process. For this application a recirculatory system incorporating a holding vessel is the best solution, where the proportion of feed gas can be reduced to zero. For this reason the gas loading can be varied between 0-100%, whilst maintaining a fully wetted packing.
The tank is precharged with a calculated mass of absorbant which is circulated through the column. The gas is introduced at a point between the two heat exchangers, and rises through the packing against the falling stream of absorbant. The upper exchanger removes the heat of solution, the lower one cools the product. An alternative arrangement, mounting the condenser externally to the column can be used for contaminated gas feeds.
Once the product in the recirculation tank has reached the desired concentration it can be discharges and refilled with absorbant ready for the next operation.
Falling Film Absorber
The system consists essentially of a vertical shell and tube exchanger known as the cooler-absorber, a packed tail gas scrubber and interconnecting pipe. The gas to be absorbed enters the system through an inlet at the upper end of the heat exchanger and flows down inside the parallel tubes with the flow of absorbing liquid. Unabsorbed gas passes through a riser to the bottom of the tail gas scrubber. The absorbing liquid is fed through an inlet at the top of the tail gas scrubber and falls over the packing counter current to the rising gas. The acid leaving the tower is fed to the top tube sheet of the heat exchanger. In this way, the heat exchanger works as a number of water cooled, wet-wall columns in parallel. The falling film absorber is provided with weirs and an elaborate distribution systems to effect an equal flow of liquid and gas in each tube. The greatest virtue of the falling film absorber is its capability to produce strong acid without detectable vent losses. The Falling Film Absorber can also be used for the absorption of HCl, HBr, SO2, NH3 etc.
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