Vent Gas System

Moderate Water hose Water used to wash flame arrester on vent gas system entered furnace... 

Figure 21.2 Two principal designs for membrane vent-gas systems (a) vapor retained by a glassy membrane and (b) vapor permeated by a rubbery membrane.

Vapor Treatment. The vapors from the tank space can be sent to a treatment system (condenser, absorption, etc.) before venting. The system shown in Fig. 9.1 uses a vacuum-pressure relief valve which allows air in from the atmosphere when the liquid level falls (Fig. 9.1a) but forces the vapor through a treatment system when the tank is filled (Fig. 9.16). If inert gas blanketing is required, because of the flammable nature of the material, then a similar system can be adopted which draws inert gas rather than air when the liquid level falls. 

Both air and oxygen processes can be designed to be comparable in the following areas product quaUty, process flexibiUty for operation at reduced rates, and on-stream rehabiUty (97,182). For both processes, an on-stream value of 8000 h/yr is typical (196). The rehabiUty of the oxygen-based system is closely linked to the rehabiUty of the air-separation plant, and in the air process, operation of the multistage air compressor and power recovery from the vent gas is cmcial (97). 

The pump gas can be used to fuel the reboiler. The amount of pump gas is normally close to balancing the reboiler fuel gas requirements. The pump gas can also be routed to the facility fuel gas system or to a low-pressure system for compression and sales. If it is not recovered in one of these ways and is just vented locally, the cost of using this type of pump can be very high. 

As vent collection systems normally contain vapor/air mixtures, they are inherently unsafe. They normally operate outside the flammable range, and precautions are taken to prevent them from entering it, but it is difficult to think of everything that might go wrong. For example, an explosion occurred in a system that collected flarmnable vapor and air from the vents on a number of tanks and fed the mixture into a furnace. The system was designed to run at 10% of the lower explosion limit, but when the system was isolated in error, the vapor concentration rose. When the flow was restored, a plug of rich gas was fed into the furnace, where it mixed with air and exploded [17]. Reference 34 describes ten other incidents. 

The explosion would not have occurred if the recommendations made after the first explosion had been followed—if there had been a flow of inert gas into the vent collection system and if the atmosphere inside had been tested regularly for oxygen. 

An existing lO-in. I.D. packed tower using 1-inch Berl saddles is to absorb a vent gas in water at 85°F. Laboratory data show the Henry s Law expression for solubility to be y = 1.5x, where y is the equilibrium mol fraction of the gas over water at compositions of x mol fraction of gas dissolved in the liquid phase. Past experience indicates that the Hog for air-water system will be acceptable. The conditions are (refer to Figure 9-68). 

Pressure control will be necessary for most systems handling vapour or gas. The method of control will depend on the nature of the process. Typical schemes are shown in Figures 5.17a, b, c, d (see p. 230). The scheme shown in Figure 5.17a would not be used where the vented gas was toxic, or valuable. In these circumstances the vent should be taken to a vent recovery system, such as a scrubber. 

Figure 26.30 shows details of a gas vent pipe system. The two details at the top left of the illustration show close-ups of the boot seal and flange seals located directly at the interface of the SWCR system with the FMC. To keep the vent operating properly, the slope of the closure system should... 

A second gas inlet system (M, N, P) serves to set starting conditions and to vent the system after a test. 

The vent gas from the ERH flows through an electrically heat-traced duct (to prevent condensation) into an electric heat exchanger that raises its temperature to 260°C (500°F) and then into a CATOX unit to destroy residual organic contaminants. The gas is then cooled to between 12°C and 15°C (60°F) in a scrubber/con-denser that removes additional contaminants. The gas then flows through an induced draft fan, discharging into the plant s HVAC system. 

Sulfur It is now well established that sulfur compounds in low ppm (parts per million) concentrations in fuel gas are detrimental to MCFCs (74,75,76,77,78). The tolerance of MCFCs to sulfur compounds (74) is strongly dependent on temperature, pressure, gas composition, cell components, and system operation (i.e., recycle, venting, gas cleanup). The principal sulfur compound that has an adverse effect on cell performance is H2S. At atmospheric pressure and high gas utilization (-75%), <10 ppm H2S in the fuel can be tolerated at the anode (tolerance level depends on anode gas composition and partial pressure of H2), and <1 ppm SO2 is acceptable in the oxidant (74). These concentration limits increase when the temperature increases, but they decrease at increasing pressures. 

The outside surfaces of refrigerated storage vessels are thermally insulated to assist in maintaining the desired storage temperature. Thermal excursions increase product vaporization, raise vessel pressure, and increase the amount of gas relieved to atmosphere orto a relief vent collection system. Fire exposure of a refrigerated vessel can increase product temperature and vessel pressure, possibly exceeding the capacity of relief valves or relief vent collection system, and could result in vessel rupture with major fire or explosion consequences. 

L Friedel J Schmidt, "Design of Long Safety Valve Vent Line Systems for Gas Relief, J Loss Prev Process Ind, 6, 293-301,1993... 

The Vortex Treating System s design requirements include complete separation of inlet gas from the liquid and removal of all liquid surges. Vent-gas volumes at this production facility were never expected to exceed 150 Mcf/D [4250 m3/d[, even with an upcoming waterflood expansion. Because of these assessments, the new vortex system was installed with a flume-type vertical separator, as indicated in Fig. 1. 

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