GB2024663A

GB2024663A - Gas scrubber

Info

Publication number: GB2024663A
Application number: GB7922005A
Authority: GB
Original assignee: Stork Friesland BV
Current assignee: Stork Friesland BV
Priority date: 1978-06-23
Filing date: 1979-06-25
Publication date: 1980-01-16
Also published as: NL7806795A; DE2925321C2; DE2925321A1; FR2429036A1; CA1125672A; FR2429036B1; GB2024663B; NZ190800A

Abstract

A scrubber for removing particles carried in a stream of gas comprises a housing 1 into which the gas is directed by means of an inlet 4. The housing is preferably cylindrical and the inlet is preferably tangential to the surface of the housing so that the gas swirls through the housing. Between a first part 2 and a second part 3 of the housing 1 is provided an annular gutter 7 having a plurality of holes 8. Liquid added to this gutter passes through the holes and forms a falling layer on an inner wall of the second part 3 of the housing 1. The swirling gas contacts this falling layer of liquid and is thereby cleaned. The cleaned gas leaves the housing through an eduction pipe 5. <IMAGE>

Description

SPECIFICATION Scrubber The invention relates to an apparatus suitable for removing particles especially pulverized particles.

A scrubber is known from US Patent 2.696.275 according to which a stream of gas is injected into a number of cyclones via a stationary blade rim which then passes long a moistened inner wall, after which the scrubbed stream of gas is removed upward through an eduction pipe.

It is a problem of similar scrubbers that some wall portions inside the housing are moistened now and then as a result of local turbulence, so that there is a precipitation or scruff of the particles present or of the solved matter may occur. These scruffs may be the cause of pollution or of bacteria growth, as a result of which the sanitary character of the scrubber is lost. The occurrence of these problems is particularly promoted bytheturbulantsteam of gas inside the housing.

We have now devised a scrubber which can overcome the disadvantages mentioned and which can meet high sanitary requirements. According to the invention, these can be obtained in that the means for flushing are made up of a peripheral or annular gutter which is preferably circular and which, via a large number of outlets, debounches into a second portion of the housing which widens gradually, within which portion the central eduction pipe extends.

Thus, the present invention consists in a scrubber which comprises a housing: means between a first part of the housing and a second part of the housing through which liqued can pass to form a falling layer on an inner wall of the second part; an eduction pipe which extends into the second part; and a gas inlet arranged relative to the housing to cause gas to swirl through the second part in contact with the layer.

We prefer that the first part and the second part are co-axial circular cylinders and that the second part is of larger diameter.

The means through which the liqued can pass is preferably an annular gutter which can be positioned between separable first and second parts, or it can simply be positioned around the outside of an integral housing. In this second case holes will, of course, need to be provided in the housing between the first and second parts. In the first case, the gutter preferably defines a frusto-conical surface between the first and second parts.

This type of scrubber will generally have a housing with a first part provided with a tangentially directed inletfora stream of gas, means for flushing the innerwall ofthe housing located in an annularzonewhich stands transversely on the centre-line of the housing and means for letting out this fluid, the housing being furthermore provided with an eduction pipe which debouches outside the housing in such a manner that the stream of gas to be purified flows through an axial track inside the housing.

For obtaining a lowest possible turbulent flow of the stream of gas, it is favourable if the transition between the inner wall of the first part to that of the second part of the housing runs so fluently that the stream of gas does not leave the inner wall. As a result of this gradual curve of the inner wall, no airturbulence and spatters of fluid will occur at the inlet part of the fluid.

As a result of these measures, the inner wall of the housing can be efficiently flushed without the risk of drops of fluid spraying away from the walls. The particles in the circulating stream of gas are urged outward towards the direction of the flushed inner wall, and a sharp separation occurs between the dry inner wall of the first part of the housing and the inner wall of the second part of the housing which is kept wet by complete flushing.

The flushing gutter preferably forms part of an annular intermediate element which merges with the two parts of the cylindrical housing. This simplifies the construction of the apparatus and allows, furthermore, a certain thermal isolation of the gutter and the inner wall of the housing. As a result unwanted condensation of fluid on the dry wall part is prevented. In the flushing gutter fresh fluid is supplied continually which will circulate in the gutter.

The length of the central eduction pipe inside the second (flushed) part of the housing is important since it influences the amount of particles trapped from the stream of gas. We prefer that the central eduction pipe extends a distance past the annular flushing zone which is many times larger than the distance between the pipe and the inner wall of the second part of the housing.

It is preferred that the stream of gas is directed tangentially to the first portion via a slit-shaped aperture.

Also, we prefer that this aperture has a height which is practically equal to the height of the first part ofthe housing, and has a width which is smaller than the distance between the central eduction pipe and the inner wall of the first housing portion. This contributes to the suppression ofturbulance and to the promotion of transport of particles to the inner wall of the housing. The residence time of the stream of gas in the housing may be increased by lengthening the central eduction pipe.

Bacteria growth may be reduced by reducing the temperature of the fluid collecting on the bottom of the second portion. This may be done in many applications of the scrubber of the invention, in particular when the scrubber is used with an apparatus for spray-drying milk. In this case a first portion of a flushing fluid can be supplied for flushing directly along an inner wall of the scrubber, and a second portion can be supplied, in a controlled way, to a lower portion of the flushing zone for cooling purposes. This means that bacteria growth is fought, not only on the dry surface of the eduction pipe (by means of the high temperature), but also on the wet surface by means of the low temperature.

The invention will be explained in further detail with reference to the drawings, in which: Figure 1 is a side-view in partial cross-section of a scrubber; Figure 2 is a cross-section on an enlarged scale of a part of a scrubber; Figure 3 is a top-view according in the direction of the arrow Ill of Figure 2; and Figure 4 shows schematically a complete plant which includes a scrubber.

The apparatus shown in the Figures includes a mainly cylindrical housing 1 consisting of a first (upper) portion 2 and a second (lower) portion 3 which is somewhat wider then the portion 2. The first portion 2 of the housing 1 is provided with a tangentially directed inlet 4 for the stream of gas to be scrubbed. Inside the housing there is an eduction pipe 5 for the (scrubbed) gas, which extends far into the lower part of portion 3 and debouches above the portion 2 of this housing. Between the inlet 4 and the bottom of the eduction pipe 5, there is an annular gas track A which is traversed by the stream of gas to be scrubbed. Overlapping this trackAthere is a flushing zone B. At the end near the gas inlet 4, the flushing zone B is terminated by a flushing device which is an annular zone arranged perpendicular on the centreline 6 of the housing 1.This device comprises an annular gutter 7, which is open, via a large number of apertures 8 into the widened second portion 3 of the housing 1 which defines the beginning of the flushing zone B. At its other end, the zone B is limited by the bottom 9 of the housing 1. The fluid in the bottom 9 is collected in an annular gutter 9' defined by the inward curve 10. A discharge line 11 is connected to this gutter9' as is a connection 14 which will be explained below.

The passage, shown in Figure 1, between the inner wall of the first portion 2 and that of the second portion 3 is preferably smooth in order to reduce or suppress turbulence. In the figures the passage 8 between the gutter 7 and the inner wall of the lower portion 3 of the housing are carried out as short pipes with which an efficient flushing of the inner wall without splashing can be obtained. This makes possible the outside of the eduction pipe 5 remaining dry and the inner wall of the lower portion 3 remaining moistened permanently and totally. The fluid required for this purpose may be supplied to the gutter 7 in such a way that a certain circulating stream will occur which promotes a uniform flushing ofthe inner wall ofthe housing.The gutter7 may be double-walled, or arranged isolated in another manner with respect to the upper portion 2 so that local cooling of the inner wall is prevented; and, in this manner, no drops of condensation will be precipitated from the stream of gas. The gutter 7 is preferably open and locked by a lid 12 which may be made up of one or more parts. This allows the connections 8 to be accessible from the outside for cleaning, if necessary.

The lid 12 as illustrated allows the flushing gutter 7 to form a closed space so that the fluid may be supplied under pressure, preferably by means of a positive displacement pump. This may force the stream through the passages 8.

The length of that portion of the gas eduction pipe 5 which is located inside the housing 1, and the distance C between this pipe and the inner wall of the second portion 3 of the housing 1 are important: a small distance C increases the possibility that the particles entrained in the stream of gas will touch the inner wall, and a large length A ensures a sufficient residence time. Preferably the distance A, in which the central eduction pipe 5 is located in the flushing zone B, is main times, more preferably about 10 times, greater than distance C. It is also much preferred that the tangentially directed supply of the stream of gas is effected via a slit-shaped aperture whose height D illustrated in Figure 1 is about equal to that of the first portion 2.The width E of the supply 4 is preferably smaller than the distance F between the central eduction pipe 5 and the inner wall of the first portion 2 of the housing.

Figures 2 and 3 show one possible design of the flushing device. In these Figures gutter 7 is part of an annular intermediate element 13 with which the two portions 2 and 3 of the cylindrical housing 1 are connected. Figure 3 gives an indication of a preferred number of connections 8 (in this case bores) between the gutter 7 and the inner wall of the portion 3. When the housing diameter is about 1700 mm, the radius of curvature R is about 25 mm.

The fluid leaving the outlet 11 should be uncontaminated, and may be conveyed back to the evaporation or drying apparatus. The dimensions of the passages or canals 8 with respect to the total amount of fluid supplied should be such that the inner wall of the portion 3 remains totally wet down to the bottom 9 since this prevents precipitation or growth of vegetation or bacteria. An advantage of the scrubber according to the invention is that particles emanating from a stream of gas from a drying apparatus may be trapped without contamination because the inner wall of the housing 1 is flushed continuously in the flushing zone B.

The eduction pipe 5 remains dry as a result of the centrifugal action of the rotary stream of air by which a sharp separation between the wet and the dry portion of the scrubber can be maintained. It is of particular importance that the eduction pipe 5 remains dry, because: a) no drops are carried on to the open air; b) no bacteria growth occurs on dry surfaces, especially if the temperature on the surface is sufficiently high, for example above 7000.

Any bacteria growth in the wet portion B is prevented since the flushing rate is so high that the residence time of the fluid compared with the time necessary for generation of unwanted bacteria is so small that no bacteria growth can occur. As a result, the interior of the housing 1 remains clean, and a return of the fluid to an evaporating or drying plant is possible and the loss of valuable particles is limited.

The scrubber of the invention is particularly useful for processing of dairy product, such as milk or whey.

Although the precise dimensions of the apparatus and the precise operating conditions suitable for any particular job can be easily determined in practice, the following Figures may be taken as a guide.

Diameter of the upper portion 2 of the housing 1 .... 1700mm Diameter of the portion 3 of the housing ... 1750mm Diameter of the eduction pipe 5 .. ....... ... 1100mm Length of the gas track A outside the pipe 5 - ... 3200mm Length of the flushing zone B .... 4000mm Flushing fluid . ....... .. milk Velocity of the stream of gas via inlet 4 . 25m/sec.

Amount of powdered milk by weight in this stream of gas . . 21 Omg/m3.

Temperature of the stream of gas . 950c Temperature ofthefluid . .. .5-15 c Weight ratio gas/fluid . .. abt.10 Stay of moistening fluid in the flushing zone B . ... abt.10sec.

Percentage of fluid leaving bottom 9 with respect to total amount supplied . 96% The difference between the total amount of liquid supplied and the amount leaving the bottom 9 is due, of course, to evaporation from the fluid film.

Figure 4 shows a complete plant comprising a scrubber 15 connect on one side to a spray dryer 16 for condensed milk, and a separating cyclone 17. On the other side, the scrubber 15 connected, by means of the intermediate element 13, to a stock 18 of moistening fluid, for example cool milk, via a pump 18'. The line 11 ofthe scrubber is connected to a tank 19. Via a pump 20 and optio nally vi an intermediate cooler 20' this tank is connected to an evaporating battery 21 in which the milk or otherfluid to be treated is condensed to a solids content of 50%. This condensed product is carried to the spray drier 16 which is provided with an inlet 22 for hot air, and an outlet for gas 23. This outlet 23 is connected to the inle' 4 of the scrubber 15 via the cyclone 17. Both the spray drier 16 and the cyclone 17 possess a powder oullet, 24 and 25 respectively, to a belt 26.

The moistening fluid supplied from the stock 18 to the gutter 7 will get wa -mer in the flushing zone B and obtain a temperature of, for insta nce, 40"c. This may promote bacteria growth. In order to maintain the sanitary properties of the scrubber 15, an additional line 27 may be provided between the stock of fluid 18 and the connection 14 in the bottom of the scrubber. In this manner, mixing of the warm fluid from the zone B and the cooling fluid supplied from the stock 18 occurs. a mixing temperature of 150c may be obtained in this manner.

Claims

1. A scrubber which comprises a housing; means between a first part of the housing and a second part of the housing through which liquid can pass to form a falling layer on an inner wall of the second part, an eduction pipe which extends into the second part; and a gas inlet arranged relative to the housing to cause gas to swirl through the second part in contact with the layer.

2. A scrubber according to claim 1, in which the first part and the second part are co-axial circular cylinders.

3. A scrubber according to claim 2, in which the second part is of larger c iameterthan the first part.

4. A scrubber according to claim 2 or claim 3, in which the inlet is tangen cial to the axes of the cylinders and is positioned to direct gas into the first part.

5. A scrubber according to any one of the preceding claims, in which the means is an annular gutter having a plurality of holes through which a liquid can pass to form the layer.

6. A scrubber according to claim 5, in which the gutter defines a frusto-cl)nical surface between a narrower first part and a wider second part.

7. A scrubber according to any one of the preceding claims, in which the first part and the second part are co-axial circular cylinders, and the eduction pipe is co-axial with these cylinc ers.

8. A scrubber according to claim 7, in which the distance that the eduction pipe extends into the second part is at least twice the separation between the surface of the eduction pipe and the inner wall of the second part.

9. A scrubber according to any one of the preceding claims, in which the means through which liquid can pass is an annular gutter closed such that liquid can be driven onto the wall of the second part under pressure.

10. Ascrubberas herein described with reference to the accompanying drawings.

11. A method of scrubbing a liquid, in which part of that liquid is fed to the means of a scrubber according to any one of the preceding claims, and another part of that liquid is fed to a region of the second part to which the first mentioned part of the liquid falls.

12. An apparatus for removing )powdered) particles carried on in a stream of gas, comprising a mainly cylindrical housing with a first portion provided with a tangentially directed inletforthe stream of gas, means for flushing the inner wall of the housing located in an annular zone which stands transversely on the centre-line of the housing, and means for letting off this fluid, the housing being further provided with an eduction pipe which extends centrally inside the housing and debouches outside the housing, in such a way that the stream of gas transverses an axial track inside the housing, characterized in that the flushing means are made up of an annular gutter which, via a large number of passages debouches into a gradually widening second portion of the housing, within which portion the central eduction pipe extends.

13. An apparatus according to Claim 12, characterized in that the passage between the inner wall of the first portion to the second portion of the housing runs conically with a small apex angle.

14. An apparatus according to Claim 12 or 15, characterized in that the flushing gutter forms part of an annular intermediate element merging with the two portions of the cylindrical housing.

15. An apparatus according to any one of Claims 12 to 14, characterized in that the central eduction pipe extends on a distance past the annular flushing zone which is many times greater than the distance between the pipe and the inner wall of the second portion of the housing.

16. An apparatus according to any one of Claims 12 to 15, characterized in that the tangentially directed supply of the streamof gas takes place via a slit-shaped aperture with a height which is practically equal to the height of the first portion of the housing and with a width which is smaller than the distance between the central eduction pipe and inner wall of the first portion of the housing.

17. An apparatus according to any one of Claims 12 to 16, characterized in that the flushing gutter forms a closed space so that the fluid may be let in under some overpressure.

18. A method of operating a scrubber according to any one of Claims 12 to 17 which is connected with, for instance, an apparatus for spray-drying milk, in which a first portion of a moistening fluid is utilized for flushing a zone alone the inner wall of the scrubber, characterized in that a second portion of this fluid is supplied to the lower portion of the flushing zone in a controlled manner for the purpose of cooling.

19. An apparatus according to any of Claims 12to 17, particularly for the application of the method according to Claim 18, characterized by a connection for the supply of a cooling fluid which is arranged on the gutter of the bottom of the housing.