GB2158731A - Gas scrubber - Google Patents

Abstract

A gas scrubber (10) comprises a duct (12) in which are supported a large number of baffles (14) extending across the duct (12) and spaced apart along the duct (12). The baffles (14) are shaped and arranged so that a liquid flowing down the duct (12) from nozzles (22) follows a sinuous path (S) over the baffles (14), and such that the gap through which the liquid flows between the edge of one baffle (14) and the next, is at least 20% wider in the vertical direction (V) than in the horizontal direction (H). The scrubber (10) may be used effectively to remove aerosols from a gas stream flowing up the duct (12) in countercurrent to the liquid, and its efficiency can be controlled by adjusting the liquid flow rate. <IMAGE>

Description

SPECIFICATION Gas scrubber This invention relates to a gas scrubber, particularly but not exclusively for removing aerosols from a gas stream.

According to the present invention there is provided a gas scrubber suitable for removing aerosols from a gas stream comprising a duct which in use is arranged with its longitudinal axis substantially vertical, and a set of baffles spaced apart along the duct and each extending between opposite sides of the duct, the baffles being shaped and arranged such that a liquid flowing down the duct follows a sinuous path over the baffles, and such that the narrowest gap between one baffle and the next in a direction parallel with the longitudinal axis of the duct is at least 20% greater than the narrowest gap between one baffle and the next in a direction transverse to the longitudinal axis of the duct.

It will be appreciated that although the path of the liquid as a whole is sinuous, due to splashing and turbulence the path of any individual droplet of liquid cannot be predicted.

Preferably the scrubber includes a plurality of sets of baffles arranged across the width of the duct.

When gas is passed up the duct and a low pressure liquid stream passed down the duct in countercurrent, stable vortices are created between the baffles. The control graph (i.e. the graph of gas pressure drop against liquid flow rate for any given gas flowrate) for the scrubber does not show hysteresis, and so the gas pressure drop across the scrubber can be readily controlled by controlling the liquid flow rate.

The invention will now be described by way of example only and with reference to the accompanyiong drawings, in which: Figure 1 shows a sectional view of part of a gas scrubber and Figure 2 is a graphical representation of the variation of gas pressure drop with liquid flow rate, at a constant gas flow rate, for the gas scrubber of Figure 1.

Referring to Figure 1, a gas scrubber 10 comprises a rectangular duct 12 up which a gas is caused to flow. A large number of identical members 14 (only eighteen are shown), of inverted Yshape in cross-section, extend across the duct 12 parallel to each other between opposite faces of the duct 12. The members 14 are arranged in horizontal rows, those in alternate rows being directly underneath each other, while adjacent rows are in staggered relationship. Each member 14 consists of a vertical plate 16 and two sloping plates 18, and the vertical plates 16 in one row are midway between the lower edges of two sloping plates 18 of the row above.

Above the uppermost row of the members 14 is an array of pipes 20 (only one of which is shown) with nozzles 22 arranged to spray water at low pressure over the members 14. The water thus flows downwardly in countercurrent to the gas, and the members 14 are close enough together that the water follows, at least approximately, a sinuous path (illustrated by arrow S), flowing over the lower edge of one sloping plate 18 onto an oppositely sloping plate 18 of a member 14 of the row below. Adjacent to the walls 24 of the duct 12 are sloping plates 28 which extend parallel to the members 14 between the opposite faces of the duct 12, and which ensure that the water flow pattern is the same near the walls 24 as elsewhere in the duct 12. For the plates 28, the wall 24 serves the same function as do the vertical plates 16 of the members 14.

The members 14 are arranged at such a spacing that the vertical distance V, between the lower edge of a sloping plate 18 (or 28) and the surface of the oppositely sloping plate 18 of the member 14 of the row below, is at least 20% greater than the horizontal distance H between the lower edge of the sloping plate 18 (or 28) and the vertical plate 16 (or the wall 24) of the said member 14 of the row below. Thus the gap through which the water flows, as it flows over the lower edge of a sloping plate 18, is at least 20% greater in the vertical direction than in the horizontal direction.

In operation of the gas scrubber 10 a gas containing solid andlor liquid aerosols is passed up the duct 12 while water flows downwardly from the nozzles 22. As the water passes through the gaps between consecutive members 14 it breaks up into a highly turbulent two-phase mixture which serves effectively to remove aerosols from the gas stream. The efficiency of the scrubber 10 at removing aerosol particles depends upon the gas pressure drop, AP, across the scrubber 10, which, as shown for a constant gas flow rate in Figure 2, is a single-valued function of the water flow rate, W.

The graph of the gas pressure drop, AP, across the scrubber 10 against the gas flow rate, for a constant water flow rate, is of similar shape. Thus if the aerosol size range or concentration, or the gas flow rate entering the scrubber 10 varies, a substantially constant degree of purity of the outgoing gas stream can readily be achieved by adjusting the water flow rate W to obtain a desired gas pressure drop AP and hence a desired efficiency.

It will also be appreciated that the scrubber 10 is also effective at removing water-soluble gases from the gas stream.

It will also be appreciated that the spacing between successive members 14 must be chosen in accordance with the expected flowrates and the corresponding Reynolds' Number. For example in a particular case the horizontal spacing might be 10 cm.

Claims (4)

1. A gas scrubber suitable for removing aerosols from a gas stream comprising a duct which in use is arranged with its longitudinal axis substantially vertical, and a set of baffles spaced apart along the duct and each extending between opposite sides of the duct, the baffles being shaped and arranged such that a liquid flowing down the duct follows a sinuous path over the baffles, and such that the narrowest gap between one baffle and the next in a direction parallel with the longitudinal axis of the duct is at least 20% greater than the narrowest gap between one baffle and the next in a direction transverse to the longitudinal axis of the duct.

2. A gas scrubber as claimed in Claim 1 including a plurality of sets of baffles arranged across the width of the duct.

3. A gas scrubber as claimed in claim 2 wherein at least some of the baffles are of inverted Y-shape in cross section.

4. A gas scrubber substantially as herinbefore described with reference to, and as shown in, Figure 1 of the accompanying drawings.