Gas separation: processes – Liquid contacting – And deflection
Reexamination Certificate
2001-03-12
2003-12-02
Smith, Duane S. (Department: 1724)
Gas separation: processes
Liquid contacting
And deflection
C096S275000, C096S323000, C261SDIG003
Reexamination Certificate
active
06656250
ABSTRACT:
BACKGROUND OF THE INVENTION
Method and apparatus for wet-cleaning crude gas streams, the method involving the crude gas stream flowing through one venturi throat which is sprayed with a scrubbing liquid via one periodically pulsating hybrid nozzle.
To wet-clean crude gas streams, use is often made of venturi scrubbers (U.S. Pat. No. 4,152,126 and U.S. Pat. No. 4,193,778). These consist of a flow tube with a constriction, the venturi throat, and, disposed above or in the throat, a supply means for the scrubbing liquid in the form of a pressure nozzle.
Dust can thus be bound up to particle sizes of 0.1 &mgr;m. Dust removal takes place in three phases: 1. The particles impinge on the liquid surface, 2. they adhere to the liquid surface, 3. the drops of the liquid are removed.
The present invention relates to an improvement in dust removal by an improvement in phase 1. Phase 2 is unproblematic, since capture can always be assumed once the drop of liquid comes into contact with the dust grain. Phase 3 is carried out in a separate liquid separator, e.g. in a cyclone.
The collection of the grains of dust in phase 1 by the drops of liquid is effected primarily by inertial separation on high-velocity drops. The inertial separation improves with increasing relative velocity between drop and grain of dust and decreasing drop diameter.
In a venturi equipped with conventional nozzling, drops are generated which, depending on the type of nozzle (single-fluid nozzle, two-fluid nozzle or swell nozzle) and nozzle inlet pressure used, have a size range of 30-2000 &mgr;m. Close to the nozzle, these drops all have the same initial velocity which, depending on the nozzle pressure, is in the range of 3-50 m/sec. In the venturi throat, these relatively large drops are broken down into extremely fine droplets, owing to the high gas acceleration and the shear flow in the venturi throat, and are turbulated by turbulence. The droplets, now small, and the large velocity scatter of the droplets, in terms of magnitude and direction relative to the gas stream, permit many dust particles to encounter a liquid surface, leading to a high collection efficiency of dust on the liquid surface. The separation of the dust-laden liquid droplets as a condensate is then carried out in a liquid separator. The collection efficiency is measured as a ratio of the level of the constituents to be collected in the clean gas after wet cleaning and in the crude gas prior to wet cleaning.
In venturi scrubbers with conventional nozzling, the collection efficiency depends on the extent to which the drops of the scrubbing liquid are atomized in the venturi throat and are turbulated with the crude gas, so that as many dust particles as possible impinge on the liquid surfaces and are collected. The shear forces and the degree of turbulation of the crude-gas/liquid mixture in the venturi throat decreases with increasing size of the venturi throat. The size of the venturi throat and the velocity of the crude gas stream define the venturi pressure drop of the crude gas stream. The pressure drop increases as the venturi throat becomes smaller and the crude gas stream becomes larger. The collection efficiency increases with increasing venturi pressure drop.
A drawback of all known venturi scrubbers is that only a high venturi pressure drop will lead to good collection efficiency. Typical venturi scrubbers are operated from pressure drops of 20-30 mbar up to 150 mbar. A high pressure drop means high energy demands to achieve the required pumping capacity for the crude gas stream.
Another drawback is that in the event of a change in the crude gas flow rate while the venturi throat is a fixed size, the change in the velocity of the crude gas will cause a change in the venturi pressure drop. DE 43 31 301 describes a tube-gap venturi scrubber which consequently has two adjustable venturi throats. The tube-gap venturi scrubber has a tube gap of approximately rectangular cross-section. Disposed downstream behind said tube gap above the scrubber sump is a displacer which extends over the entire length of the gap and is mounted so as to be translatable towards the tube gap and away from it. Between the walls of the tube gap and the displacer wall, two venturi throats running parallel are formed. The cross-sections of these two venturi throats can be adjusted by sliding the displacer. Proposed as feeder means for the scrubbing liquid are swell nozzles.
A drawback of this solution for controlling the pressure drop is that it is subject to mechanical wear and the adjustment means must be run, in particular, through the sump below the venturi throats, which results in sealing problems.
SUMMARY OF THE INVENTION
It is an object of the invention to achieve high collection efficiencies in a venturi scrubber without pressure drops or with low pressure drops and to provide a simple option for controlling the collection efficiency. Such control is necessary, in particular, in the event of retrofitting measures which lead to a higher gas flow rate.
The object of the invention is achieved by a method and an apparatus for cleaning a crude gas stream with the aid of an atomized scrubbing liquid.
In accordance with the method according to the invention, the crude gas stream is sprayed, by means of a hybrid nozzle, with the atomized scrubbing liquid and is then passed, without a pressure drop or with a low pressure drop up to 30 mbar, preferably up to 20 mbar, through one or more venturi throats. A hybrid nozzle is known per se from DE 43 15 385.
A hybrid nozzle is constantly supplied with a scrubbing liquid and a gas as an atomization aid. The liquid inlets and gas inlets are connected to a first resonance chamber, connected to which on its downstream side, via a restrictor, there is at least one further resonance chamber. The last resonance chamber, seen in the flow direction, is connected to the outlet orifice of the hybrid nozzle.
The atomization aid used can, for example, be air or inert gas.
The hybrid nozzle can adopt both the operating mode of a pressure nozzle and the operating mode of a two-fluid nozzle. The characterizing feature of the hybrid nozzle is that, if constantly supplied with a specific amount of liquid and a specific amount of air, it will not atomize these amounts of liquid and air uniformly, but instead, in a pulsating manner, continuously change its mode of operation.
In the pressure nozzle mode, drops having a relatively large mean drop diameter are produced continuously. The mean drop diameter is essentially determined by the nozzle outlet orifice size. The effective range of a drop is determined by its initial momentum. The initial velocity of the drops is the same for all drops. Owing to their higher mass, the large drops have a higher initial momentum and consequently a higher effective range. 99% of the atomized amount of liquid is formed by drops whose diameters differ from one another by a ratio of up to 1:20.
A two-fluid nozzle differs from a pressure nozzle in that it is additionally supplied with air, continuously generating drops having a small mean drop diameter, compared with the pressure nozzle. The mean drop diameter is determined by the mass flow ratio of atomizing air to liquid in the nozzle and decreases with increasing atomizing air flow rate. The effective range of a drop is determined by the momentum of the atomizing air and the transfer of this momentum to an entire drop cluster. As with the pressure nozzle, 99% of the atomized amount of liquid are formed by drops whose diameters differ from one another by a ratio of up to 1:20.
The pulsating change in the mode of operation in the case of the hybrid nozzle can occur, depending on the pulsation frequency, between the pressure nozzle mode and a two-fluid mode or between different two-fluid modes which differ in terms of the flow rate of the atomizing air supplied.
The pulsating change in mode of operation if the supply of the hybrid nozzle with compressed air and liquid is constant over time is generated in the hybrid nozzle itself owing to periodic start-up phenomen
Listner Uwe
Schweitzer Martin
Bayer Aktiengesellschaft
Smith Duane S.
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