WO2001049393A1

WO2001049393A1 - Apparatus and method for purifying gases

Info

Publication number: WO2001049393A1
Application number: PCT/SE2000/002398
Authority: WO
Inventors: Kenneth Andersson; Peter Arvidsson
Original assignee: Recyclean Ab
Priority date: 2000-01-04
Filing date: 2000-12-01
Publication date: 2001-07-12
Also published as: SE0000014D0; AU1911501A; SE515625C2; SE0000014L

Abstract

The invention concerns an apparatus for purifying gases, comprising a column (2), inlet and outlet means (3 and 4, respectively) for introduction of and removal of gases to and from the column (2), a group of trays (5a-5e) arranged sequentially inside the column (2) and so as to be filled with a fluid (10) by way of feeder pipes, and a fan (6) for conveyance of gas through said column (2). The fan (6) is adjustable as to its rotational speed and that parts of said group of trays (5a-5e) are provided with individually controllable fluid feeder pipes (7a-7e), preferably intended for supply of water, to allow essentially individual filling of said trays (5a-5e). The invention likewise concerns a method for optimising a gas purification appartus (1) comprising a column (2), inlet and outlet means (3 and 4, respectively) for introduction of and removal of gases to and from the column (2), a group of trays (5a-5e) arranged sequentially inside the column (2) and so as to be filled with a fluid (10) by way of feeder pipes (7a-7e), and a fan (6) for conveyance of gas through said column (2), said method comprising the steps of establishing (101) a flow of gas by way of the inlet (3), regulating (102) the rotational speed of the fan (6) to ensure that the flow volume of the fan essentially coincides with said gas flow, and individually regulating (103) the intake of fluid, preferably water, to parts of said group of trays (5a-5e).

Description

APPARATUS AMD METHOD FOR PURIFYING GASES

Technical Field

The present invention relates to an apparatus for purifying gases . The apparatus is of a kind comprising a column, inlet and outlet means for introduction of and removal of gases to and from the column, a group of trays arranged sequentially inside the column and so as to be filled with a fluid by way of feeder pipes, and a fan for conveyance of gas through said column. The invention likewise concerns a method for optimising said gas puri- fication apparatus.

Background of the Art

Apparatuses of the above kind, preferably so called scrubber plants, are already known in various fields. One example are tray-column plants, which are well known for purification of polluted gases emanating for example from zinc plating plants, but also for example for use in distillation plants. A tray column for purification purposes comprises a column having a number of superposed trays. In the lower part of the column, there is an inlet for gas and in the upper part a corresponding outlet . In the upper part of the column, there is also an inlet for intake of water or the like. The water then flows gradually downwards, from tray to tray, and it is collected at the lower part of the column. In addition, the trays are designed to allow gas to permeate the tray and thus to rise upwards. The fluid and the gas thus flow inside the device in essentially opposite directions, and upon their contact the gas and the water inter-react, whereby the polluted substances are dissolved in the water. In order to additionally facilitate and accelerate the reaction, a fan may be provided at the column outlet to suck the gas through the trays .

In order to achieve optimum purification results, the dimensions of the fan and the number of trays are determined in the very erection of the plant . The dimensions are determined in consideration of the desired gas purification capacity, the incoming flow volumes, and so on . In zinc plating plants, for example, such dimensio - ing might cause problems. In order to obtain efficient purification when the plant is used to its full capacity and consequently maximum amounts of pollution substances are generated, the plant must be dimensioned to handle maximum flows. This means that the suction efficiency of the fan must be sufficient to cope with large amounts of gas and that the number of trays in the column must be sufficient to provide a gas/water contact area of a size that ensures purification of such large amounts of pollution substances. In this type of plants the purification need does, however, reach its maximum for brief periods only, for example during immersion in a zinc bath of an article to be zinc plated. In the intervals between such immersions, the amount of polluted substances present in the gas is considerably smaller, although not negligible. There is therefore a need for purification of also such less polluted gases. To do so is, however very expensive, since on account of the dimensions of the plant the fan must be run at full capacity in order to be able to draw the polluted gas through the tray column. If the fan is allowed to run at a lower rotational speed, the purification process thus comes to a standstill. Object of the Invention

An object of the present invention thus is to provide a purification plant that is capable of purifying pollution-laden gases upon small as well as large flows of gas .

Summary of the Invention

This and other objects are achieved in accordance with the invention by means of an apparatus of the kind defined in the introduction and possessing the characteristics set forth in the appended claim 1 and the subsequent claims 2-6 defining preferred embodiments. The object is achieved also by means of a method in accordance with the subsequent claims 7-9. The apparatus in accordance with claim 1 is characterised in that the fan is adjustable as to its rotational speed and that parts of said group of trays are provided with individually controllable fluid feeder pipes, preferably intended for supply of water, to allow essentially individual filling of said trays. This possibility of filling the trays individually in conjunction with the adjustability regarding the rotational speed of the fan makes it possible to adapt the number of active trays in response to momentary needs. Upon maximum flows, all trays are filled to ensure maximum purification. Upon small flows, the rotational speed of the fan is reduced and the supply to a number of trays is interrupted, whereby the resistance that the flow encounters upon its passage through these trays is very small . The total flow resistance of the apparatus thus is reduced, thus enabling the fan to sustain the flow. Also when the flow is of the same magnitude, but containing less pollution substances, the number of active trays may be reduced, owing to the smaller purification need. This control function could be designed such that each tray is formed with an individual fluid feeder pipe. In accordance with a preferred embodiment of the invention, the amount of gas en- tering the column through the inlet is likewise variable. Preferably, two or more separate gas conduits are connected to the inlet and the flows from these gas conduits to the inlet are also individually controllable by means of dampers, valves or the like, located in each one of the gas conduits. Preferably, the gas conduits are arranged to carry polluted gases from two or more separate sources of gas, up to the inlet for purification in a common gas purification apparatus . Such gases could emanate for instance from pre-treatment processes or zinc immersion processes in a zinc plating plant. In this manner, the apparatus may be used both to purify the highly pollution-laden gases generated upon zinc immersion as the less polluted gases generated in conjunction with pre-treatments . Owing to the provision of dampers, the flow from e.g. the pre-treatment process may be closed off during the zinc immersion process, in order to achieve maximum purification of the highly polluted gas generated during the immersion. The positions of the dampers may be made use of to determine the rotational speed of the fan, and thus the number of active trays. In accordance with a further embodiment, a pump is provided to pump fluid via a fluid conduit that is common to two or more fluid feeder pipes, to said fluid feeder pipes, and the capacity of the pump is adjustable. There- fore, the pump will pump only exactly the amount of fluid that is needed to fill the trays active at that moment, with consequential reduced operational costs. Preferably, the trays are of the type known as bell- shaped trays, wherein the gas passes freely past a tray to which intake of fluid is prevented, but is forced to pass through a layer of fluid at a tray to which intake of fluid is in progress, whereby resistance of the apparatus to gas flow is adjustable.

The method defined in claim 7 for optimising a gas purification apparatus comprises the steps of determining the incoming flow of gas through the inlet, regulating the rotational speed of the fan such that the suction volume of the fan essentially coincides with said incoming flow of gas, and individually regulate the intake of fluid, preferably water, to parts of said group of trays. This method provides the advantages referred to previ- ously. Additional preferred embodiments of this method are defined in the appended claims 8-9.

Brief Description of the Drawings

The invention will be described in more detail in the following with reference to the accompanying drawings, which for exemplifying purposes show a presently preferred embodiment of the invention. In the drawing figures :

Fig 1 is a schematic view of a scrubber system im- plementing the apparatus and the method in accordance with the invention.

Fig 2 is a flow chart illustrating a method for optimising a gas purification apparatus in accordance with the invention. Fig 3 is a view showing details of the flow chart of

Fig 2 in accordance with the present invention.

Fig 4 is a view showing further details of the flow chart of Fig 2 in accordance with the present invention. Detailed Description of Preferred Embodiments

The system illustrated in Fig 1, designed for implementation in a zinc plating plant, comprises a gas puri- fication apparatus 1 of scrubber type, three sources of gas 13a-13c, in the subject case a first zinc pot 13a, a second zinc pot 13c and a pre-treatment line, in Fig 1 shown as a pre-treatment vessel 13b, and gas conduits 11a-lie, which are arranged to carry polluted gases from the sources of gas 13a-13c to the gas purification apparatus 1. A damper 12a-12c is located in each one of the gas conduits lla-llc. The gas purification apparatus 1 further comprises a column 2 having an inlet 3 and an outlet 4. The inlet 3 is connected to said gas conduits lla-llc. The inlet debouches into the lower part of the column, however spaced a predetermined distance above the bottom of the column. The outlet 4 is located at the upper part of the column 4, and the column 4 preferably is mounted in a vertical position. In addition, a fan 6 is located in the outlet. Inside the column, five trays 5a- 5e are provided, each one of said trays 5a-5e covering the entire cross-sectional area of the column 2. Furthermore, the trays 5a-5e are arranged in sequence in spaced- apart relationship in the longitudinal direction of the column. In accordance with this embodiment, the trays 5a- 5e are of so called bell-bottom type. In the vicinity of each tray 5a-5e debouch feeder pipes 7a-7e, respectively, each one of which is controlled by a valve 14a-14e. The feeder pipes 7a-7e are derived from a common supply line carrying a fluid 10, in the present case water, from the lower part of the column 2 to the respective valve 14a- 14e of the feeder pipes 7a-7e, respectively, via a pump unit 8. In addition, the gas purification apparatus has a bottom outlet 15 for removal of sludge, a water supply line 16 for replacement of evaporated water, and a droplet separator 17, but these components are of no real significance to the invention as defined in the appended claims.

In operation of the system, the amount of flow of polluted gases to be purified therein is controlled by opening and closing, respectively, the dampers 12a-12c. The flow originates from one of the sources of gas 13a- 13c or from a combination of these sources. For example, the first zinc pot 13a could form the principal zinc plating plant in which are immersed large-size objects to be galvanised, in which process large amounts of pollution substances are produced in this pot during a limited period of time. In accordance with this embodiment, the other two sources, i.e. the second zinc pot 13c and the pre-treatment line/pre-treatment vessel 13b, produce considerably smaller amounts of pollution substances over longer periods of time. Owing to the provision of the dampers 12a-12c mentioned previously it thus becomes possible to control the plant so as to purify the fluids where most needed at any one time. Downstream from the dampers 12a-12c, the gas conduits lla-llc debouch into a common conduit leading to the inlet 3 of the gas purifi- cation apparatus 1, the gases being carried from the relevant source/sources of gas into the column 2. The rotational speed of the fan 6 is regulated in dependence on the inflow into the column and the degree of pollution of that flow, in order to optimise the capacity of the fan 6 to suck the flow through the apparatus relative to the power consumption of the fan. In conjunction with the regulation of the rotational speed of the fan 6, the number of active trays is regulated also. The total number of trays N is dimensioned in adaptation to the maximum suction capacity of the fan 6 and when operating at a lower rotational speed, the fan 6 cannot draw the air through all trays. Regulation is effected by stopping the supply of water via the feeder pipes 7a-7e with the aid of those of the valves 14a-14e that are associated with the X uppermost trays, X depending on the rotational speed of the fan, whereby the level of the water of these

X uppermost trays sinks to such a low level that air may pass through the tray essentially without having to be forced through a resistive layer of water. In this manner, the number of active trays is reduced from N to N-X, and the gas is allowed to penetrate through the column, also when the flows are small. This means that the appa- ratus is useful for purifying large, highly polluted flows of gas as well as small flows of gas containing low quantities of polluted substances. The consumption of energy of the apparatus could be lowered further by providing a common pump 8 arranged to pump water to the feeder pipes 7a-7e. In this manner it becomes possible to regulate the pumping effect of the pump 8 , ensuring that the fluid line 9, which is connected to the pump 8 and which leads to the feeder pipes 7a-7e delivers only the amount of water required to fill the N-X active trays, a feature that provides further power economy.

Fig 2 shows a block diagram illustrating the method of optimising a gas purification apparatus. The method comprises essentially three steps, viz. step 101 for establishing the inflow volume of gas through the inlet to the column, step 102 for regulation of the rotational speed of the fan to ensure that its extraction volume corresponds to the inflow volume, and step 103 for regulation of the intake of fluid, in this case water, indi- vidually to each one of the trays. It is particularly noteworthy that these three steps need not necessarily be performed in the order indicated, but that the method steps indicated in Fig 2 and the subsequent Figs 3 and 4 are given for exemplifying purposes only. Step 101 for establishing the inflow, for example could consist of step 201 for regulating the inflow by means of individual control of dampers located one in each gas conduit, and of step 202 for establishing gas inflow via inlets by means of ascertaining which dampers are open, as shown in Fig 3. In addition, step 103 for regulating the intake of fluid individually to each one of the trays could comprise step 301 for supplying a plurality of fluid feeder pipes via a common fluid line, step 302 for pumping 302 fluid through the fluid line by means of a pump, step 303 for controlling the individual feeder pipe to each one of the trays by opening and closing, respectively, a valve associated with the respective one of the trays, and step 304 for controlling the capacity of the pump to pump the amount of fluid required to feed the currently open fluid feeder pipes, as shown in Fig 4. Preferably, the different method steps, when applied in the above apparatus, are controlled from a central computer unit or equivalent equipment (not shown) . For example, an operator may de- cide that immersion in the first (large-size) zinc pot

13a should take place, for which reason the purification activity inside the apparatus should be concentrated to the polluted gases emanating from this process. By pushing a button on a control panel, for example, the opera- tor transmits this information to the control unit, whereupon said unit forwards a signal to the damper 12a to open and/or signals to the dampers 12b-12c to close. Consequently, only the gas emanating from the first zinc pot 13a will be carried into the purification apparatus.

This means that also the flow through the apparatus is established, and the control unit emits a control signal to the fan 6 to adapt its rotational speed to the actual flow. In addition, the control unit establishes the number of active trays N-X required in view of that flow, and emits signals to the valves 14 associated with the X uppermost trays to close. The control unit also estimates the water supply required to fill the active trays, and then emits a signal for regulation of the pumping capacity of the pump 8.

It should be understood that many modi ications of the above embodiments of the invention are possible within the scope of protection of the invention such as the latter is defined in the appended claims. For example, the pump 8 could be non-regulated, and constantly pump the same amount of flow to the feeder pipes 7a-7e. Further, the number of gas sources 13 and thereto associated gas conduits 11 and dampers 12 may be varied based on the configuration of the plant. In addition, the total number of trays may vary for adaptation to the maximum flow through the apparatus, for example when all dampers 12 are open. Furthermore, the supply of fluid to the trays could be regulated individually to each trays, or individually to part groups of said group of trays. As previously mentioned, the method steps may be effected in a different sequence and to some extent concurrently, for example as regards regulation of the rotational speed of the fan and the number of trays .

Claims

1. An apparatus for purifying gases, comprising a column (2), inlet and outlet means (3 and 4, respec- tively) for introduction of and removal of gases to and from the column (2) , a group of trays (5a-5e) arranged sequentially inside the column (2) and so as to be filled with a fluid (10) by way of feeder pipes, and a fan (6) for conveyance of gas through said column (2) , c h a r a c t e r i s e d in that said fan (6) is adjustable as to its rotational speed and that parts of said group of trays (5a-5e) are provided with individually controllable fluid feeder pipes (7a-7e) , preferably intended for supply of water, to allow essentially individual filling of said trays (5a-5e) .

2. An apparatus as claimed in claim 1, c h a r a c t e r i s e d in that the amount of gas entering the column (2) via the inlet (3) is variable.

3. An apparatus as claimed in any one of the pre- ceding claims, c h a r a c t e r i s e d in that two or more separate gas conduits (lla-llc) are connected to said inlet (3) and in that the flows from these gas conduits (lla-llc) to the inlet (3) are individually controllable by means of dampers (12a-12c) , valves or the like, located in each one of the gas conduits (lla-llc) .

4. An apparatus as claimed in claim 3 , c h a r a c t e r i s e d in that said gas conduits (lla-llc) are arranged to carry polluted gases from two or more separate sources of gas (13a-13c) , up to said inlet (3) for purification in a common gas purification apparatus (1) , such gases emanating for instance from different treatment steps in zinc plating processes.

5. An apparatus as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that a pump (8) is provided to pump fluid (1) via a fluid conduit (9) that is common to two or more fluid feeder pipes (7a-7e) , to said fluid feeder pipes (7a-7e) , and in that the flow of the pump (8) is adjustable.

6. An apparatus as claimed in any one of the preceding claims, c h a r a c t e r i s e d in that said trays (5a-5e) are of the type known as bell-shaped trays, wherein the gas passes freely past a tray (5a-5e) , to which intake of fluid is prevented, but is forced to pass through a layer of fluid at a tray to which intake of fluid is in progress, whereby the resistance of the apparatus (1) to gas flow is adjustable.

7. A method for optimising a gas purification apparatus (1) comprising a column (2) , inlet and outlet means (3 and 4, respectively) for introduction of and removal of gases to and from the column (2) , a group of trays (5a-5e) arranged sequentially inside the column (2) and so as to be filled with a fluid (10) by way of feeder pipes (7a-7e) , and a fan (6) for conveyance of gas through said column (2) , said method comprising the steps of: establishing (101) a flow of gas by way of the in- let, regulating (102) the rotational speed of the fan to ensure that the flow volume of the fan essentially coincides with said gas flow, and individually regulating (103) the intake of fluid, preferably water, to parts of said group of trays.

8. A method as claimed in claim 7 for optimising a gas purification apparatus (1) , wherein the step of es- tablishing (101) a flow of gas by way of the inlet further comprises the steps of: regulating (201) the gas flow through the inlet by individually opening and closing, respectively, dampers or valves located in two or more gas conduits that debouch into said inlet, and establishing (202) the inflow of gas via the inlet by establishing which dampers are open.

9. A method as claimed in any one of claims 7 or 8 for optimising a gas purification apparatus, wherein the step of individually regulating (103) the intake of fluid, preferably water, to parts of said group of trays further comprises the steps of : supplying (301) a plurality of fluid feeder pipes via a common fluid line, pumping (302) fluid through said fluid line by means of a pump, controlling (303) the individual fluid feeder pipe to each part of said group of trays by opening or closing a valve associated with the respective fluid feeder pipe, and controlling (304) the capacity of the pump to pump essentially the amount of fluid required to feed the currently open fluid feeder pipes.