Scrubber, maximum flow

Chlorine is to be removed from a vent stream by scrubbing with a 5 per cent w/w aqueous solution of sodium hydroxide. The vent stream is essential nitrogen, with a maximum concentration of 5.5 per cent w/w chlorine. The concentration of chlorine leaving the scrubber must be less than 50 ppm by weight. The maximum flow-rate of the vent stream to the scrubber will be 4500 kg/h. Design a suitable packed column for this duty. The column will operate at 1.1 bar and ambient temperature. If necessary, the aqueous stream may be recirculated to maintain a suitable wetting rate. 

A fixed roof tank contains liquid with a vapor space above it, as illustrated in Figure 12.1. When the tank is being filled, vapors are discharged through the vent. If the vapors are hazardous or detrimental to the environment, they will be discharged to a scrubber system and/or a flare. The vent must be big enough to handle the maximum flow of liquid into the tank, as demonstrated by two Process Safety Beacon reports CCPS (2002) and CCPS (2003) in which the vents were not big enough and tank rupture occurred. 

You are required to design an aerocyclone to remove as much dust as possible from the exhaust coming from a rotary drier. The gas is air at 100°C and 1 atm and flows at a rate of 40,000 m3/hr. The effluent from the cyclone will go to a scrubber for final cleanup. The maximum loading to the scrubber should be 10 g/m3, although 8 g/m3 or less is preferable. Measurements on the stack gas indicate tha tthe solids loading from the drier is 50 g/m3. The pressure drop... 

The collection of particles is achieved in a countercurrent flow between the water droplets and the particulates. In a cyclonic scrubber, water is injected into the cyclone chamber from sprayers located along the central axis, as shown in Fig. 7.19. The water droplets capture particles mainly in the cross-flow motion and are thrown to the wall by centrifugal force, forming a layer of slurry flow moving downward to the outlet at the bottom of the cyclone. Another type of scrubber employs a venturi, as shown in Fig. 7.20. The velocity of the gas-solid suspension flow is accelerated to a maximum value at the venturi throat. The inlet of the water spray is located just before the venturi throat so that the maximum difference in velocity between droplets and particles is obtained to achieve higher collection efficiency by inertial impaction. A venturi scrubber is usually operated with a particle collector such as a settling chamber or cyclone for slurry collection. 

Venturi scrubbers. In a venturi scmbber, a liquid jet issues from a nozzle. The jet induces cocurrent gas flow into the throat of the jet. Mass transfer takes place between the gas and the atomized liquid downstream of the nozzle. Mass transfer is usually poor and depends on the throat velocity or pressure drop, the liquid/gas ratio, and the liquid atomization pattern. Because of the cocurrent nature of contacting, the maximum solute removal does not exceed a single theoretical stage. Venturi scmbbers are used primarily for separation of fine particulate matter or... 

The gas-side mass-transfer coefficients kefl and ko increase with liquid feed rate or with gas velocity at each given position in the venturi scrubber and decrease at constant liquid rate and gas velocity with increasing distance from the point of liquid injection (J7, VI1). The values ofkifl generally increase with increasing liquid flow rate or gas velocity (often referred to as the velocity at the throat). However, ki,a will sometimes exhibit a maximum when the gas velocity increases the explanation is that, at higher gas velocities, an increase in turbulence in the throat of the venturi results in the formation of droplets smaller than the thin filaments first formed at lower gas velocities. Internal circulation is reduced in these smaller droplets, and there is also a reduction in the size of the zone of intense turbulence. These two phenomena lead to a maximum for the values of/cL. as found experimentally by Kuznetsov and Oratovskii (K15) and Virkar and Sharma (VI1). The values of the effective interfacial area a increase with both gas and liquid flow rates. 

Other points made by Euro Chlor include recommendations for the maximum concentrations of the various scrubbing solutions. NaOH should be prepared at a maximum strength of 20-22% in order to avoid formation of solid salt as the NaOH is replaced by NaCl and NaOCl. When a substantial amount of CO2 is present or when the scrubber operates with a continuous flow of air, the solution should be more dilute. Diaphragmcell liquor should be limited to 10% NaOH. In the presence of CO2, this should be reduced to 6-8%. Milk of lime should contain 130-2()0 gpl CaO. 

Vent absorbers are designed to receive gas, not liquid. The presence of liquid chlorine in a vent stream can disrupt flow in the scrubber, overload the system, and cause an early breakthrough. When appropriate, design should therefore include a gas/liquid separator to prevent carryover of liquid droplets. The separator should have the capacity to trap the maximum amount of liquid likely to be present and should include a temperature or level alarm to signify its presence. 

This gas rate represents only 53% of the maximum operational capacity at a flow parameter of 0.0400. Therefore, the replacement of the 1-in. Pall Ring packing with Intalox structured packing 2T easily should permit an increase of 50% in the air rate through this scrubber. 

In a horizontal flow concurrent scrubber, the gas velocity carries scrubbing liquid into the packed bed and the device actually operates as a wetted entrainment separator. Normally, superficial gas velocity is limited to a maximum of 9.6 ft/s due to liquid reentrainment at higher velocities. Packed bed thickness is restricted because the depth of liquid penetration into the tower packing is limited by the allowable gas velocity. 

Mass transfer driving forces are intermediate between vertical concurrent scrubbers and countercurrent scrubbers [1]. If the absorbed solute obeys Henry s Law in the liquid phase, the mass transfer driving force will limit maximum solute removal efficiency to about 90% of that obtained in a countercurrent scrubber for typical chemical fumes—assuming scrubbing water flow is limited. However, if the absorption of solute is followed by a rapid chemical reaction in the liquid phase, so that there is no appreciable vapor pressure of solute above the solution, the mass transfer driving force will be the same as for a countercurrent scrubber. 

The most widely used type of scrubber operates with gas and liquid in countercurrent flow as the liquid flows vertically downward under the influence of gravity. Maximum gas flow rate is limited by liquid entrain-... 

An industrial wastewater treatment unit is ventilated at an air flow of 25,800 cfm at atmospheric pressure and a temperature of 65The inlet air stream contains 50 ppm by vol HCHO that is to be absorbed into water. The exhaust air is to contain a time-weighted average of 2 ppm by vol maximum HCHO concentration. A scrubber design is needed that does not require more than 4-in. H2O total pressure drop. 

Heat batch until acetone refluxes (56°C) 10) Putting too much heat into tiie batch can cause vq)ors to pass the condenser, reach scrubber blower motor, and cause a fire. 10) Use 70°C hot water in reactor jacket to heat batch, using maximum w flow. Watch vapor condensation level in condenser and adjust hot water temperature to make level about half to two-thirds up the condenser. 

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