Vent streams

If regulations governing specific emission limit VOC concentrations to the low ppm range then, of course, vapor fractions such as those illustrated by the above tabulation will not be acceptable. It may, however, still be justified to consider VOC condensation as a precursor to a final abatement device such as an adsorption bed. Removing most of the solvent from a vent stream by condensation, can drastically reduce the size and cost of a downstream cleanup system. 

AH other organic waste-process and vent streams are burned in a dare, in an incinerator, or in a furnace where fuel value is recovered. Wastewater streams are handled in the plant biological treatment area. 

Calcium C rbon te. Calcium carbonate, like R2O2, affects sulfuric and oleum consumption in the HF process. Sulfuric acid loss is approximately 0.98% H2SO4 for each percentage of CaCO. The carbon dioxide evolved by the reaction increases the noncondensable gas flow, and because it carries HF, contributes to yield losses in the vent stream. 

In all HF processes, the HF leaves the furnace as a gas, contaminated with small amounts of impurities such as water, sulfuric acid, SO2, or SiF. Various manufacturers utilize different gas handling operations, which generally include scmbbing and cooling. Cmde HF is condensed with refrigerant, and is further purified by distillation (qv). Plant vent gases are scmbbed with the incoming sulfuric acid stream to remove the bulk of the HF. The sulfuric acid is then fed to the furnace. Water or alkah scmbbers remove the remainder of the HF from the plant vent stream. 

Deposits which are forming are frequentiy characterized by venting streams of hot (300°C) mineralized fluid known as smokers. These result in the local formation of metalliferous mud, rock chimneys, or mounds rich in sulfides. In the upper fractured zone or deep in the rock mass beneath the vents, vein or massive sulfide deposits may be formed by the ckculating fluids and preserved as the cmstal plates move across the oceans. These off-axis deposits are potentially the most significant resources of hydrothermal deposits, even though none has yet been located. 

Inerts present in chlorine feed are emitted in this vent stream. 

Gaseous vent streams from the different unit operations may contain traces (or more) of HCl, CO, methane, ethylene, chlorine, and vinyl chloride. These can sometimes be treated chemically, or a specific chemical value can be recovered by scmbbing, sorption, or other method when economically justified. Eor objectionable components in the vent streams, however, the common treatment method is either incineration or catalytic combustion, followed by removal of HCl from the effluent gas. 

Constant Flow into Protected Equipment For the steady-state design scenario with a constant, steady flow of fluid from a pressure source that is above the maximum aUowed pressure in the protected equipment, volume is being generated within the equipment at a rate RV = F/ f. Substituting into Eq. (26-21) and noting that the specific volume of the vent stream is l/p, gives the required mass flow rate ... 

The time-dependent nature of the emergency pressure relieving event is obtained by the simultaneous solution of Eqs. (26-27) and (26-28). Generally, the only unknown parameters in these two equations are the venting rate W and the vent stream quahty (Xo). The vent rate W at any instant is a func tion of the upstream conditions and the relief system geometry. 

Introduction In determining the disposal of an effluent vent stream from an emergency relief device (safety valve or rupture disk), a number of factors must be considered, such as ... 

Some vent streams, such as light hydrocarbons, can be discharged directly to the atmosphere even though they are flammable and explosive. This can be done because the high-velocity discharge entrains sufficient air to lower the hydrocarbon concentration below the lower explosive limit (API RP 521, 1997). Toxic vapors must be sent to a flare or scrubber to render them harmless. Multiphase streams, such as those discharged as a result of a runaway reaction, for example, must first be routed to separation or containment equipment before final discharge to a flare or scrubber. 

For partial condenser systems, the pressure can be controlled by manipulating vapor product or a noncondensible vent stream. This gives excellent pressure control. To have a constant top vapor product composition, the condenser outlet temperature also needs to be controlled. For a total condenser system, a butterfly valve in the column overhead vapor line to the condenser has been used. Varying the condenser cooling by various means such as manipulation of coolant flow is also common. 

Figure 1-6A. Typical flow scheme for separation and purification of vent streams.

An inert gas process vent stream contains 91 Ib/hr (5.35 mol/hr) ammonia. This is 5.7% (volume) of the total. The absorber is to operate at 150 psig and recover 99% of the inlet ammonia as aqua using 90°F water. Average vapor... 

Example 9-13. Desdgn a Packed Tower Usii Caustic to Remove Carbon Dioxide from a Vent Stream... 

Unit 3, Scrubber. To give a high plant yield, the scrubber would be designed to recover most of the alcohol in the vent stream. Assume 99 per cent recovery, allowing for the small loss that must theoretically occur, 0 431 = 0.99. 

Around 98 per cent of the MEK and alcohol can be considered to be absorbed in this unit, giving a 10 per cent w/w solution of MEK. The water feed to the absorber is recycled from the next unit, the extractor. The vent stream from the absorber, containing mainly hydrogen, is sent to a flare stack. 

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. 

Flare stacks. Flares use an open combustion process with oxygen for the combustion provided by air around the flame. Good combustion depends on the flame temperature, residence time in the combustion zone and good mixing to complete the combustion. The flare stack can be dedicated to a specific vent stream or for a combination of streams via a header. The flare can be used for normal process releases or emergency upsets. Flares can be categorized by the ... 

Supplementary fuel is required for start-up and required if the feed concentration of organic material is low or the feed concentration varies. Supplementary fuel is usually required for vent streams, as processed vents are normally designed... 

Hui C-W and Smith R (2001) Targeting and Design for Minimum Treatment Flowrate for Vent Streams, Trans IChemE, 79A 13. 

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