Concurrent flow operation

In those systems where there is practically no vapor pressure of solute above the liquid phase, concurrent flow operation should be considered. Such applications include ... 

Influence of Packing Shape, Irrigated Packed Beds, High Liquid Rate Performance, Liquid Holdup in Packed Beds, Pressure Drop Calculation, Effects of Surface Tension and Foaming, Concurrent Flow Operation, Cross-Flow Operation, Example Problem, Notation, References... 

Depending on the gas and liquid residence times required, the reactor could be operated horizontally or vertically with either downflow or upflow. Weikard (in Ullmann, Enzyklopaedie, 4th ed., vol. 3, Verlag Chemie, 1973, p. 381) discusses possible reasons for operating an upflow concurrent flow tubular reactor for the production of adipic acid nitrile (from adipic acid and ammonia). The reactor has a liquid holdup of 20 to 30 percent and a residence time of 1.0 s for gas and 3 to 5 min for liquid. 

In recent years, the process has been modified to increase the yield of lower olefines, too. Continually improved since then, especially in the mid-1960s with the replacement of the original silica-alumina catalyst by a zeolite. The catalyst is now typically a zeolite Y, bound in a clay matrix. The feed is vaporized and contacted in a pipeline reactor with concurrently flowing microspheroidal catalyst particles. The catalyst is then separated from the hydrocarbon products and is continuously regenerated by burning off the coke in a fluidized bed. The process is licensed by UOP several hundred units are in operation worldwide. See also HS-FCC. 

Or the cell may be operated in concurrent flow as shown in Figure 19.9, or in countercurrent flow as shown in Figure 19.10. 

Equipment changes for concurrent flow. The concurrent-flow design and provision for operation on partially liquid charge involved a number of equipment changes in addition to the reversal of external piping to introduce feed at the top and withdraw products from the bottom. These changes were as follows ... 

A simplified flow diagram of a concurrent-flow unit operating on vapor feed is shown in Figure 21. Details of the reactor for operation on partially liquid feed are shown in Figures 22 and 23. The latter operation was typical when revamping an existing countercurrent-flow unit, where it was simpler to leave the tar separator in the circuit. For operation on heavy gas oil, the partially-vaporized effluent from the primary furnace was flashed in the tar separator in the usual manner. The vapors were superheated in the secondary furnace and introduced into the reactor via the vapor-inlet nozzle near the top of the reactor, while the tar-separator bottoms were pumped into the reactor through a liquid-injection nozzle. 

Reactors of air-lift units are generally similar to previous concurrent-flow TCC design. Steam is used in the seal leg above the reactor. The reactors are operated at pressures from 5 to 15 p.s.i.g. (107,333). Spent catalyst is stripped with steam, in the purge zone in the bottom of the reactor, and then flows through external pipes to the regenerator. These pipes are equipped with shutoff safety valves which are normally wide open. No adjustment can be made at this point to control catalyst flow. 

Another result, which is interesting for model simplification, concerns dispersion. In catalytic vapor phase reaction the effect of backmixing on reactor efficiency generally proves to be negligible except for cases of high conversion and short beds. For concurrent trickle-flow operation Hears [47] has developed on the basis of a perturbation solution of the one-dimensional plugflow dispersion (PD-)model, a criterion for negligible (< 5 %) influence of axial dispersion in case of first order reactions ... 

Of further interest and concern is the operation of a membrane cell as a continuum. Such a view may be referred to as differential permeation. The permeate may be withdrawn at points along the membrane, as illustrated in Figure 1.8. Or the cell may be operated in concurrent flow, as shown in Figure 1.9, or countercurrent flow, as shown in Figure 1.10. There is the possibility, even, of producing two permeate products if two different membrane materials are employed separately in the same unit or module. This is indicated in Figure 1.11. 

A packed bed in concurrent flow has no conventional flooding limitation because liquid holdup tends to decrease with an increasing gas rate, as shown in Figure 1-17 [21]. The curve at zero gas flow shows the usual effect of liquid flow rate on operating holdup. As gas flows downward through the bed it accelerates the liquid velocity, thus reducing the volumetric holdup. 

As can be seen from Figures 1-18 and 1-19 the pressure drop in concurrent flow can be 1.0, 2.0, or even 4.0 in. H20/ft of packed depth. Because of the higher allowable pressure drop, the capacity of a given diameter column is much greater in concurrent than in countercurrent flow operation. Gas and liquid contact intensity is greatly increased at higher flow rates thus, mass transfer rates can be elevated in concurrent flow. 

Owing to the fact that the oxidation-reduction potential varies considerably throughout the FPS removal column, it may be preferable to operate it with concurrent flow within each stage and countercurrent flow between stages. Alternatively, two separate concurrent columns could be used. The U recovery column, on the other hand, would clearly be operated with countercurrent flow because, chemically, conditions are reductive throughout the column. 

Figure 6.64 Water vapor and hydrogen mole fraction distributions for a concurrent flowing serpentine isothermal fuel ceU with 100% RH anode inlet, 0% RH cathode inlet at 0.7 V operation. Even though current is being produced and hydrogen consumed, the hydrogen mole fraction along-the-channel is increased near the inlet due to dehumidification of the anode. (Adapted from Ref. [11].)...

Three basic fluid contacting patterns describe the majority of gas-liquid mixing operations. These are (1) mixed gas/mixed liquid - a stirred tank with continuous in and out gas and liquid flow (2) mixed gas/batch mixed liquid - a stirred tank with continuous in and out gas flow only (3) concurrent plug flow of gas and liquid - an inline mixer with continuous in and out flow. For these cases the material balance/rate expressions and resulting performance equations can be formalized as ... 

In planning a control system, a flow diagram is needed to indicate what may influence each item of plant. In many diagrams it will be seen that complexity arises and two items work in conflict. A typical instance is the cooling and dehumidifying of air, to a room condition lower than design, with concurrent operation of a humidifier. 

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