WO1990005015A1

WO1990005015A1 - Method and apparatus for removal of scrubber liquid with separation of predominantly solid impurities

Info

Publication number: WO1990005015A1
Application number: PCT/SE1989/000628
Authority: WO
Inventors: Lennart Eriksson; Frank Schouten; Henricus Johannes Lanters
Original assignee: Flaekt AB
Current assignee: ABB Technology FLB AB
Priority date: 1988-11-07
Filing date: 1989-11-03
Publication date: 1990-05-17
Anticipated expiration: 1991-05-07
Also published as: DE3991314T; SE464220B; SE8804025L; AU4502089A; SE8804025D0

Abstract

A method and an apparatus for extracting scrubber liquid with a removal of predominantly solid impurities, wherein a gas stream containing such impurities is cleaned in a scrubber by means of a recirculated scrubber liquid supplied via cleaning ramps (2), with spray nozzles (3), at the top of the scrubber. Surplus liquid together with captured impurities are drained from the scrubber via a drainage outlet (9) with a drainage valve (10). The scrubber liquid together with the impurities captured from the gas are collected in a collecting device (11) and taken down through a passage (12) into the bottom area of the scrubber, below a separation device (13), through which the scrubber liquid is made to rise to a higher extraction level (6) for recirculation. The separation device (13) is cleansed by a back-flow of scrubber liquid towards the drainage outlet (9) by periodical opening of the drainage valve (10).

Description

METHOD AND APPARATUS FOR REMOVAL OF SCRUBBER LIQUID WITH SEPARATION OF PREDOMINANTLY SOLID IMPURITIES

The present invention relates to a method for removing predominantly solid impurities from a scrubber liquid as described in the preamble of Claim 1. The invention also relates to an apparatus for carrying out the method according to the preamble of the first apparatus claim.

The invention is mainly, but not solely, intended for use in processes and plants for manufacturing of fibre board, particle board and similar products. In such a manufacturing process, wood particles, usually in the form of fibres, and a binder, usually a resin, mixed with the particles, together with a drying transport gas and steam, evaporated from the particles during the drying process, are brought to a cyclone. In the cyclone most of the particles are separated from the transport gas. The transport gas leaving the cyclone is still containing fumes of the binder and some particles with binder and should not be released into the air. Therefore the transport gas is then led to a scrubber where it is cleaned. The thus cleaned gas may be reused in the process, partly or wholly.

The concentration of impurities in the scrubber liquid is increased by the removal of particles and fumes from the transport gas. The increasing concentration, mainly of solid particles, causes problems by clogging in the scrubber, especially as usually most of the scrubber liquid is recirculated and only the surplus, due to condensation of steam in the gas, is removed.

The objective of the invention is therefore to create the conditions for taking charge of, first of all, the solid impurities in the gas which is to be cleaned, so that at least most of the particles are prevented from being recirculated in the scrubber and moreover are being removed there¬ from easily and efficiently. Another objective of the invention is to improve the technique in various ways within this area.

This objective is achieved by a method according to the preamble of Claim 1 characterized by the elements set forth in the characterizing section of Claim 1. This objective is also achieved by an apparatus as specified in the first apparatus claim. Other characteristics and benefits of the invention are explained in the following description, with reference being made to the attached drawings, which show, in a simplified form, some non-limiting embodiments of the invention.

Figure 1 shows a vertical section of a scrubber comprising a first embodiment of the invention and figure 2 shows in a similar perspective the lower part of a scrubber comprising a second embodiment of the invention.

In figure 1, a scrubber 1, known per se, comprises two cleaning ramps 2 at the top, with spray nozzles 3, the cleaning ramps being fed with cleaning or scrubber liquid via feeders 4, with a pump 5 sucking liquid from the lower part of the scrubber at an extraction level 6.

To the scrubber 1, preferably in the lower part, is connected an inlet pipe 7, for the gas that is to be cleaned. At the top the scrubber has an outlet pipe 8 for cleaned gas. At the bottom, which preferably is tapered, the scrubber is provided with a drainage outlet 9, at the lowest level, comprising a drainage valve 10.

In accordance with the main characteristics of the invention, there is a collecting device 11, below the inlet pipe 7 but above the extraction level 6. The collecting device 11 opens into a passage 12, which runs centrally downwards to a level above the drainage outlet 9 but below a separation device 13, which is located on a level below the extraction level 6. The separation device 13 can be a strainer screen, a suitable sieve or a perforated plate. The holes can, in a practical design, have a diameter of 0.1 to 3 mm, preferably about 0.5 mm. On its level, the separation .device 13, covers the whole cross section of the scrubber. The separation device is replaceable and possibly vertically dirigible, so that for example a pressure shock may be absorbed without any damage to the apparatus. It should though be in a fixed position at the time of back flow cleansing.

The collecting device 11 is preferably designed to follow the contour of the inside of the scrubber so that there is a surrounding gap 14 between the inner wall of the scrubber and the collecting device. This gap 14 works as an overflow means and as a pressure balance means.

Just below the gap 14 there is a high-level control sensor 15 and further down there is a low-level control sensor 16. The mentioned sensors 15, 16 are connected to a control unit 18 which is regulating the drainage valve 10. Between the levels of the separation device 13 and the low-level control sensor there is a differential pressure sensor 17.

In figure 2, the corresponding numbers refer to the same parts as they do in figure 1. The main difference between the figures is that the embodiment in figure 2 has -an asymmetrical passage 12 running out of the scrubber. This passage is provided with a control valve 19, type throttle or shut-off. A control valve can of course also be located in the passage 12 in figure 1 but it is far more accessible with the design according to figure 2.

The control valve 19 should preferably be controlled by the same control unit 18 as the drainage valve 10.

At the bottom of the scrubber there is a flow divider 20 or guide vanes to assist the flow from the passage 12 towards the separation device 13.

The function of the apparatus is as follows. Polluted gas, e.g. air at a temperature of 60-90 deg. C, containing fibres with binder, dust and fumes of phenol and formaldehyde, is fed through the inlet pipe 7 and moves upwards through the scrubber 1.Through the feeders 4, cleaning liquid is supplied to the cleaning ramps 2 from the pump 5. The cleaning scrubber liquid is sprayed through the moving polluted gas by the nozzles 3 and thereby absorbing solid, liquid and gaseous impurities. The cleaned gas leaves the scrubber through the outlet pipe 8.

The scrubber liquid together with captured impurities is collected in the collecting device 11 and led through the passage 12 to the bottom area 21 of the scrubber, below the separation device 13, e.g. a sieve. The polluted liquid then rises through the separation device, where most of the solid impurities remains, to a middle chamber 22 filled with clean scrubber liquid around the extraction level 6 where liquid continuously is sucked to the pump 5 for recirculation. The liquid level in the middle chamber 22 is kept between the level of the high-level control sensor 15 and the level of the low-level control sensor 16._. When the level of the liquid rises to the high-level, the control unit 18 will open the drainage valve 10 for a predetermined period of time or until the liquid level reaches the low-level. Thus liquid is flowing downwards through the separation device 13 to the drainage outlet 9 removing the solid impurities, which have gathered underneath the separation device and carrying these solid impurities out through the drainage outlet.

Surplus fluid is normally derived from condensation of the humidity in the incoming polluted gas if this has a high water vapour content and if it has a temperature significantly higher than the temperature of the scrubber liquid, which should be kept at 25-40 deg. C. In cases, where the amount of surplus liquid is insufficient, fresh water is added regularly and the draining interval is determined by experience or by measuring of a differential pressure by the sensor 17.

An improved removal of the solid impurities which have gathered under¬ neath the separation device 13 can be achieved by the embodiment of the invention shown in figure 2. The control valve 19 in the passage 12 is then kept closed during the same time period as the drainage valve 10 is kept open. Thus all liquid flowing out through the drainage outlet 9 has to pass through the separating device 13 from above and thereby assist with the removal of the solid impurities. The risk, that liquid from the collecting device is running straight from the passage 12 to the outlet 10, is el minated.

The invention is not restricted to the described and illustrated embodi¬ ments. Several modifications can be made within the scope of the inven¬ tive idea and the following claims.

Claims

1. A method for extracting scrubber liquid with a removal of predominantly solid impurities, wherein a gas stream containing such impurities is cleaned in a scrubber by means of a recirculated scrubber liquid supplied via cleaning ramps (2), with spray nozzles (3), at the top of the scrubber, and surplus liquid together with captured impurities are drained from the scrubber via a drainage outlet (9) with a drainage valve (10), c h a r a c t e r i z e d in that the scrubber liquid together with the impurities captured from the gas are collected in a collecting device (11) and taken down through a passage (12) into the bottom area of the scrubber, below a separation device (13), through which the scrubber liquid is made to rise to a higher extraction level (6) for recirculation, and that, preferably, the separation device (13) is cleansed by a back-flow of scrubber liquid towards the drainage outlet (9) by periodical opening of the drainage valve (10).

2. A method according to Claim 1, c h a r a c t e r i z e d in that the liquid level in the scrubber is kept between a controllable high level (15) and a controllable low level (16), both higher than the extraction level (6), through the agency of the periodical opening of the drainage valve (10).

3. A method according to Claim 1 or 2, c h a r a c t e r i z e d in that the flow from the collecting device (11), through the passage (12) to the scrubber's bottom area (21), is controlled, preferably shut off, when the drainage valve (10) is opened to effect said back-flow cleansing of the separation device (13).

4. A method according to Claim 1 or 2, c h a r a c t e r i z e d in that the occurrence of pressure differences between the lower and the upper side of the collecting device (13) is prevented by a gap between the collecting device and the inner wall of the scrubber.

5. Apparatus for carrying out the method according to Claim 1, wherein a scrubber is provided with an inlet pipe (7) for feeding a gas stream, containing impurities, to be cleaned, into the scrubber, an outlet pipe (8) through which cleaned gas is leaving the scrubber, and cleaning ramps (2) with spray nozzles (3) at the top of the scrubber for feeding scrubber liquid into contact with the gas, c h a r a c t e r z e d by a drainage outlet (9), with a drainage valve (10), in the bottom of the scrubber a collecting device (11), for collecting scrubber liquid together with captured impurities, positioned below the inlet pipe (7) and the outlet pipe (8), with a passage (12) leading the collected polluted liquid to the bottom area of the scrubber, a separation device (13) above the bottom area of the scrubber, and means to extract scrubber liquid for recirculation positioned below the collecting device (11) but above the separation device (13).

6. Apparatus according to Claim 5, c h a r a c t e r i z e d in that the collecting device (11) is funnel shaped with a central passage (12) through the separation device (13).

7. Apparatus according to Claim 5, c h a r a c t e r i z e d in that the collecting device (11) is shaped like a bowl or similar, with a passage (12) that runs out through the wall of the scrubber (1) above the separation device (13) and is connected to an opening in the bottom of the scrubber.

8. Apparatus according to Claim 7, c h a r a c t e r i z e d in that the passage (12) is provided with a control valve (19) outside the scrubber.

9. Apparatus according to Claim 5, c h a r a c t e r i z e d by a gap (14) between the collecting device (11) and the inner wall of the scrubber.

10. Apparatus according to Claim 5, c h a r a c t e r i z e d in that a high level control sensor (15) and a low level control sensor (16) are connected to a control unit (18) controlling the drainage valve (10) and a possible control valve (19) in the passage (12).