Gas bypassing

Quench Converter. The quench converter (Fig. 7a) was the basis for the initial ICl low pressure methanol flow sheet. A portion of the mixed synthesis and recycle gas bypasses the loop interchanger, which provides the quench fractions for the iatermediate catalyst beds. The remaining feed gas is heated to the inlet temperature of the first bed. Because the beds are adiabatic, the feed gas temperature increases as the exothermic synthesis reactions proceed. The injection of quench gas between the beds serves to cool the reacting mixture and add more reactants prior to entering the next catalyst bed. Quench converters typically contain three to six catalyst beds with a gas distributor in between each bed for injecting the quench gas. A variety of gas mixing and distribution devices are employed which characterize the proprietary converter designs. 

Gas bypassing—returning discharge gas to suction (within the compressor or outside the compressor). 

Another pressure control method is a hot gas bypass. The author has seen a retrofitted hot gas bypass system solve an unsatisfactory pressure control scheme. In the faulty system, a pressure control valve was placed between the overhead condenser and the lower reflux drum. This produced en atic reflux drum pressures. A pressure safety valve (psv) vented the reflux drum to the flare to protect against over-pressure. A psv saver control valve, set slightly below the psv relieving... 

The fix for the erratic reflux drum pressure problem was to provide for separate pressure control of the fractionator column and the reflux drum. A new pressure control valve was installed upstream of the condenser and the old condenser outlet control valve was removed. A hot gas bypass, designed for 20% vapor flow, was installed around the pressure control valve and condenser. A control valve was installed in the hot gas bypass line. The column pressure was then maintained by throttling the new control valve upstream of the condenser. The reflux drum pressure w as controlled by the hot gas bypass control valve and the psv saver working in split range. The new system is shown in the figure below. 

Chin states that a scheme having only a control valve in the hot gas bypass line manipulated by the column... 

For a total condenser, accumulator level is typically set by varying distillate draw. For a partial condenser, it can be controlled with a condenser hot gas bypass. 

With shell boilers, economizers will generally only be fitted to boilers using natural gas as the main fuel and then only on larger units. It would be unlikely that a reasonable economic case could be made for boilers of less than 4000kg/hF and A100°C evaporative capacity. The economizer will incorporate a flue gas bypass with isolating dampers to cover for periods when oil is used and for maintenance. The dampers require electric interlocks to the selected fuel. 

Hot gas bypass valve, to keep the evaporating temperature up above freezing point. 

At the heart of an automotive catalytic converter is a catalyzed monolith which consists of a large number of parallel channels in the flow direction whose walls are coated with a thin layer of catalyzed washcoat. The monolith catalyst brick is wrapped with mat, steel shell and insulation to minimize exhaust gas bypassing and heat loss to the surroundings. 

Avidan and Edwards (1986) successfully scaled up from bench scale to demonstration plant from 0.04 m to 0.6 m diameter while maintaining nearly 100% conversion for a fluid bed methanol to gasoline process. In this case, they ran at a superficial gas velocity which was high enough to be in the turbulent flow regime suppressing bubbles. By this technique they eliminated the losses associated with gas bypassing in bubbles. 

The important design parameters for a recirculating fluidized bed with a draft tube were identified by Yang and Keaims (1978a) as the gas bypassing characteristics of the distributor plate, the area ratio between the downcomer and the draft tube, the diameter ratio between the draft tube and the draft tube gas supply, the distance between the distributor plate and the draft tube inlet, and the area ratio of the draft tube gas supply and the concentric solids feeder. The design and operation of a recirculating fluidized bed with a draft tube are discussed below. 

Figure 3. Important design parameters that affect gas bypassing...

The gas bypassing results obtained from tracer gas injection studies for a flat and a conical distributor plate are shown in Fig. 4. Theflow ratio, FR, is defined as the total gas flow supplied through the draft tube gas supply and the concentric solids feeder divided by the total gas flow supplied through the downcomer gas supply. The A and Y are the actual amounts of gas passing up the draft tube and the downcomer, respectively, determined from the tracer gas injection studies. If FR equals A Y. there is no gas bypassing. If FR is less than A Y. some of the flow supplied through the downcomer gas supply passes into the draft tube. If FR is larger than A/7, the reverse is true. 

Figure 4. Summary of gas bypassing data for a conical and a flat distributor plate.

Effect of Draft Tube and Downcomer Area Ratio. When a draft tube of 9.55 cm I.D. (downcomer/draft tube area ratio = 7.8) was changed to a draft tube of 5 cm I.D. (downcomer/draft tube area ratio = 30) with other design parameters being the same, the gas bypassing reversed direction, as shown in Fig. 4. With the smaller draft tube (D/dD = 1), the gas bypasses from the draft tube side into the downcomer side for most experimental conditions, except for jet velocities in excess of 76 m/s at the concentric solids feeder with the larger draft tube (D/dD = 1.9), the gas bypasses from the downcomer side into the draft tube side in most experiments. 

Effect of Distance between the Distributor Plate and the Draft Tube Inlet Figure 4 clearly indicates that the gas bypassing phenomenon depends not only on the design parameters but also on the operating conditions. For the conical plate at a distance from the draft tube inlet of L = 21.7 cm, gas bypasses from the draft tube side to the downcomer side at a high flow ratio and reverses the direction at a low flow ratio. When the conical plate was moved closer to the draft tube inlet atL = 14.1 cm, the gas bypassing direction was exclusively from the downcomer side to the draft tube side. 

Figure 5. Gas bypassing characteristics of conical distributor plates of different design configurations (No. 3 and No. 7 flows).

A straight line relationship between LJpr and Gr as shown in Fig. 8 implies that the volumetric solid loading (j) is approximately constant because Ar is constant and er can be assumed to be approximately constant when the downcomer is not fluidized. More than 85% of the gas supplied through the central No. 7 and No. 8 flows in those experiments ends up in the draft tube as can be seen from the gas bypassing data presented in Fig. 7. 

Effect of Distance Between Distributor Plate and Draft Tube Inlet. As expected, the closer the distance between the distributor plate and the draft tube inlet the lower the solids circulation rate as shown in Figs. 8 and 9. This is not only because of the physical constriction created by locating the distributor plate too close to the draft tube inlet but also because of the different gas bypassing characteristics observed at different distributor plate locations as discussed earlier. When the distance between the distributor plate and the draft tube inlet becomes large, it can create start-up problems discussed in Yang et al. (1978). 

Yang, W. C., and Keaims, D. L., Studies on the Solid Circulation Rate and Gas Bypassing in Spouted Fluid-Bed with a Draft Tube, Can. J. Chem. Eng., 61 349 (1983)... 

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