Absorbing the costs of air pollution

Scrubbing up: removing chemicals from gas streams prior to discharge can be a simple, or a highly complex, operation. Dr Aubrey Arrowsmith of Parsons Engineering offers some advice on selecting the right equipment at the right cost

The two-stage spray scrubber

The two-stage spray scrubber

The effective solution to any problem involving the cleaning of a gas stream prior to discharge into the environment involves recognition of the nature of the offending gases and then the selection of the most appropriate equipment to remove them. The range of chemicals which are removed is very extensive, varying from simple inorganic gases to complex compounds. There may be a single component in low concentration or a variety of compounds at elevated temperature with a particulate present also. The wrong choice of scrubber design may not only result in ineffective operation but may lead to more serious problems than if there was no unit at all.

The two-stage spray scrubber


A correspondingly wide range of scrubbers have been developed to cover the range of applications. Many designs however are minor modifications to the basic designs which will be described in the following sections. Designs vary in order to accommodate different operating requirements, for example the presence of particulates, or provide high efficiency, usually attracting a comparable increase in cost. In some instances efficiency may be extremely important but in other instances, typically where the concentration of pollutants is very low, it may be possible to use a low efficiency, low cost unit. In some designs it may be possible to increase the efficiency of the unit by increasing the size of the mass transfer system; in others the efficiency is an inherent feature which cannot be improved significantly and multi-stage units must be used.

Overall efficiency of removal is a misleading concept when comparing the performance of wet scrubbers. Expressed as the amount of material removed divided by the original amount, efficiency is a linear relationship and it is very easy to obtain values in excess of 90%. Theoretically it can be shown that the number of transfer units required to attain a given removal is a logarithmic relationship of the driving force. It therefore becomes more and more difficult to remove the lower concentrations of material. If we consider a packed column to reduce the concentration of a component from 20,000 to 1,000 ppm, the efficiency would be 95% and this may require three transfer units. To reduce the concentration from 1,000 to 50 ppm would require a further three transfer units and the overall efficiency would increase to 99.75%. The efficiency now appears to be very high but at 50 ppm the component may still be above the OEL (Occupational Exposure Limit). A further three transfer units would bring the concentration down to 2.5 ppm, which may approach the OEL, and gives an efficiency of 99.99%. If we consider each of these three transfer units as a stage then it is obvious how the mass of material removed falls dramatically as the concentration driving force for mass transfer falls.

Therefore it is most important when considering scrubber performance to note the concentration change and to appreciate the problem of attaining low concentration.

The main objective of all scrubbers is to contact the gas and the liquid phases with as high an interfacial area as possible, and under turbulent conditions which promote the transfer of material across the interface.

The following sections consider some of the basic designs of scrubbing equipment, comparing their size, performance and cost, based on a gas flow of 10,000 m3/hr. Equipment costs depend very much on the standard of manufacture which in turn can depend upon the location, support, the nature of the chemicals passing through the system and the degree of instrumentation and control required. As many scrubbing operations involve the use of mineral acids the prices will be given for self supporting units, suitable for outside installation, fabricated from polypropylene/GRP and with no instrumentation and control.

Spray scrubbers

Preformed spray scrubbers are perhaps the simplest of all scrubbers. Mass transfer is enhanced by the large surface area of the droplets which are produced by the atomizing nozzle. The size and velocity of the droplets are determined by the nozzle design, the properties of the liquid and the atomizing pressure. The spray is directed into a chamber and the gas flows counter-current through it, usually in a vertical orientation with the droplets travelling in a downwards direction. Ultimately the droplets will travel at their terminal velocity in the medium and as this is relatively small, there is limited turbulence around them. The majority of mass transfer therefore takes place where the spray is formed and where there is a continuously renewed surface produced under highly turbulent conditions.

Typical gas velocities are 2m/s; at values above this small droplets will be conveyed with the gas flow and pressure drops are very low, perhaps 25mm wg. To operate at all effectively, the spray must be distributed uniformly across the chamber with little hitting the walls of the unit. For a duty of 10,000m3/hr this would result in a diameter of 1300mm and an overall height, including the recirculating liquor sump, of 4000mm.

The limited turbulent contact between the phases means that the operating efficiencies are low -approximately 85%. They are therefore of limited use, generally in areas where there is a low contaminant gas concentration, the gas is extremely soluble, there are particulates present or where low pressure drop is important. The effectiveness and pressure drop will depend to some extent on the ratio of the gas and liquid loadings and these can be varied over considerable ranges to accommodate either of these operating requirements. Droplets are very effective at removing particulates from the air flow by inertial impaction but non-clogging nozzles must then be used to maintain an effective spray distribution.

The basic performance of the spray chamber can be improved by putting a number of stages together or by increasing turbulence as in the throat of a venturi.

Cost - £4,000

Spray bank scrubbers


Some of the limitations of a single spray tower can be overcome by horizontal chambers containing a series of spray banks interspersed with sections of de-entrainment vanes.

The spray banks distribute the droplets in counter-current flow to the gas stream and are then turned back by the flow to impinge on the vanes. Mass transfer is improved by the higher velocities possible with the gas flow, up to 6 m/s, and the increased turbulence thereby generated. By constructing several sections, the number of transfer units is increased and the amount of material absorbed is increased.

To process a 10,000m3/hr gas stream a single unit would typically be 850x2000x1500mm. The mass transfer sections are rectangular in shape to accommodate the demisting blades, incorporate a sump to store the recirculating liquor and have a final mist elimination section to prevent mist carryover.

The total pressure drop in such a system obviously depends upon the arrangement and the number of sections making up the total unit but per section would be less than 50mm wg. There is less flexibility with such systems than with a simple spray chamber as the two loadings and their ratio will affect the operation of the vane sections. It is also very important to ensure that the inlet gas flow is uniform in order to present a uniform velocity through the system, particularly through the de-entrainment vanes. Excessive local velocities will result in re-entrainment with the subsequent flow of liquor into downstream sections.

The multi-unit spray systems suffers the same limitations as the previously mentioned single unit but does have increased efficiency, which depends upon the number of stages.

Cost

a) single stage unit £7,500

b) three stage unit £12,500


Venturi Scrubbers

There are two types of venturi scrubbers: spray and jet systems.

The spray scrubber leads to improved gas absorption compared to a simple spray by increasing the turbulence between the two phases. The gas stream, normally entering at 10m/s, is increased in the covergent section of a venturi to 20-100m/s. The liquid phase is introduced to the venturi by nozzle atomisation directly into the gas stream.

Mass transfer is limited by the co-current nature of the gas and liquid flow and the ensuing short residence time. The high turbulence generated to promote absorption attracts a high pressure drop, up to 1500mm wg. The units are ideally suited to handle gas flows containing solid particulates and can operate at a range of liquid loadings, typically 1-10 litres/m3 gas. As the gas and liquid rates increase so does the pressure drop, proportional to the square of the gas velocity.

Typically for the 10,000m3/hr gas flow, the inlet and outlet diameters would be 600mm and the overall length 1200mm. The venturi would then be positioned on a sump to house the recirculating liquor and demisting section to prevent droplet carry-over.

The jet system is essentially an educator, using a high liquid flow through the venturi to induce the gas flow. There is then no requirement for a fan but the mass transfer tends to be less efficient that the spray design.

Cost - £9,000

Packed columns


Packed columns are perhaps the most important and widely used gas scrubbers. The diameter of the column is fixed by the gas and liquid flows and the height can be easily varied for the required concentration change.

A wide variety of random packings exist to accommodate a range of operating conditions of pressure drop and efficiency. Regular packing may also be used, preformed before insertion into the column or by uniformly stacking random packings such as rings. As with all units, the efficiency of operation is a function of the interfacial area between the gas and the liquid, the design of the packing, the turbulence at the interface and the pour points. The packing area must not only provide a high interfacial area per unit volume but must promote liquid flow over it which is a function of both design and material of construction.

Over a limited range of flows as the gas and liquid loadings in the packing increases, so does the mass transfer coefficient and the pressure drop. An optimum operating condition has to be chosen which depend upon the pressure drop limitation of the fan drawing the gas through the packing and the flexibility required, and therefore the closeness to which flooding can be approached.

For the 10,000m3/hr gas flow a typical unit operating with a high efficiency packing would be 1400mm diameter with a packed height of 3000mm and an overall height of 7000mm.

In the design of packed columns it is important that the liquid and the gas are distributed uniformly over the packing at the top and base of the column respectively and it may be necessary to redistribute the liquid at intervals throughout the height of the packing. The packing density is generally less near the walls of the column and this promotes a flow of the liquid to the walls. The effect is reduced by ensuring that the diameter of the packing is sufficiently smaller than that of the column - for example, less than 1:10. Even then it may be necessary to redistribute the liquid, say, every five tower diameters to remove the wall flow. The flow is obviously affected greatly by the orientation of the column, if the packing is to be effectively utilised it must be vertical.

One of the major limitations to the use of packed columns is that they cannot handle gas streams heavily laden with solids. Under these circumstances, it is essential to pretreat with a unit such as a venturi to remove them. The do have a large advantage over some of the other designs in that effectively with designed distributors they can operate with considerable turndown in flow rates.

Cost - £30,000

Plate and fluidised bed scrubbers

Both of these designs are similar to the packed columns in basic design but have different internals and are very efficient in handling particulate laden gas streams.

The plate columns consist of vertical towers with a series of plates or trays over which the liquid flows and through holes in which the gas is dispersed.

The diameter of the column is determined by the liquid and gas flowrates and the height is determined by the number of trays required to effect the necessary concentration change in the gas stream. In order to promote transfer and increase tray efficiency, it is important again to increase the surface area of the gas phase, by reducing bubble size, increase the turbulence and increase the contact time between the two phases. Deep pools of liquid and high gas velocities produce the effect but are limited by pressure drop and entrainment considerations.

A variety of tray designs exist with differing operating characteristics for particular operations. In general, place columns can be used with low solids concentrations. They are relatively expensive, particularly in the smaller sizes, when compared to packed columns. The fluidised bed units use a bed of hollow plastic spheroids which are fluidised by the ascending gas. This motion generates high levels of turbulence in the liquid and gas phases and this is claimed to create significant increases in the mass transfer coefficient when compared to static or fixed beds of packing.

A limitation in both of these design is flexibility in flowrate variation that can be achieved without loss of mass transfer performance when compared to conventional packed columns.


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