External recycling

In cases where a large reactor operates similarly to a CSTR, fluid dynamics sometimes can be estabflshed in a smaller reactor by external recycle of product. For example, the extent of soflds back-mixing and Hquid recirculation increases with reactor diameter in a gas—Hquid—soflds reactor. Consequently, if gas and Hquid velocities are maintained constant when scaling and the same space velocities are used, then the smaller pilot unit should be of the same overall height. The net result is that the large-diameter reactor is well mixed and no temperature gradients occur even with a highly exothermic reaction. 

The smaller reactor approaches plug-flow behavior and exhibits a large temperature gradient. In this case, external recycle provides the same degree of back-mixing as is provided by internal circulation in the larger diameter reactor. 

FIG. 24-4 Air-lift fermenters (a) Concentric cylinder (h) external recycle. 

While it is easy to add materials to a fermentation, removal is difficult. Membrane devices have been placed in the fermenter or in external recycle loops to dialyze away a soluble component. Cells release wastes or metabolites that can be inhibitory these are sometimes referred to as staling factors. Their removal bv dialysis has allowed cell concentrations to reach ten to one hundred times that of control cultures. 

Jankowski et al (1978) discuss in detail the great variety of gradientless reactors proposed by several authors with a pictorial overview in their paper. All of these reactors can be placed in a few general categories (1) moving catalyst basket reactors, (2) external recycle reactors, and (3) internal recycle reactors. 

In summary, external recycle reactors are expensive and their usefulness is limited. They can be practical for simple chemical systems where no condensation can occur and neither high pressure nor high temperature is needed. For example Carberry et al (1980) preferred an external recycle reactor over a spinning basket reactor for the study of CO oxidation in dry air at atmospheric pressure. 

Another view is given in Figure 3.1.2 (Berty 1979), to understand the inner workings of recycle reactors. Here the recycle reactor is represented as an ideal, isothermal, plug-flow, tubular reactor with external recycle. This view justifies the frequently used name loop reactor. As is customary for the calculation of performance for tubular reactors, the rate equations are integrated from initial to final conditions within the inner balance limit. This calculation represents an implicit problem since the initial conditions depend on the result because of the recycle stream. Therefore, repeated trial and error calculations are needed for recycle... 

NOTE LP coil boilers with integral recycle are designed with the drain from the water-steam separator recycled within the boiler feed circuits. LP coil boilers with external recycle are designed with the drain from the water-steam separator either being dumped or recycled to the FW tank. 

Recycling of partially reacted feed streams is usually carried out after the product is separated and recovered. Unreacted feedstock can be separated and recycled to (ultimate) extinction. Figure 4.2 shows a different situation. It is a loop reactor where some of the reaction mass is returned to the inlet without separation. Internal recycle exists in every stirred tank reactor. An external recycle loop as shown in Figure 4.2 is less common, but is used, particularly in large plants where a conventional stirred tank would have heat transfer limitations. The net throughput for the system is Q = but an amount q is recycled back to the reactor inlet so that the flow through the reactor is Qin + q- Performance of this loop reactor system depends on the recycle ratio qlQin and on the type of reactor that is in the loop. Fast external recycle has... 

Microreactors Low conversion, catalytic reactions Simple design, transport rates can be increased by external recycling Limited ease of variation of parameters, maldistribution of flow can be prohibitive... 

External recycle reactor Polymerizations, catalytic reactions Very useful for viscous mixtures Equipment cost can be high (for viscous systems and for high pressure operations)... 

If substrate inhibition exists, a well-mixed bioreactor is desirable. Mixing in three-phase fluidized bed bioreactors can be increased by adding an external recycle loop, by inserting a draft tube in the reactor, or by decreasing the height to diameter ratio. 

The disc holder was machined to fit into the stainless steel flange in such a way that it directs the gas to consecutively sweep both faces of the catalyst disc, with an expansion volume in-between. This configuration provided good gas-phase mixing in the cell, thus allowing the reactor to be characterized as a CSTR. This mode of internal mixing eliminates the need for internal moving parts or external recycle loops and pumps. 

A conmercial catalyst frcm Harshaw was used, a 3 1 mixture of molybdenum trioxide and ferric molybdate, as well as the two separate phases. Kinetic experiments were done previously in a differential reactor with external recycle using these same catalysts as well as several other preparations of molybdenun trioxide, including supported samples. Hie steady state kinetic experiments were done in the temperature range 180-300 C, and besides formaldehyde, the following products were observed, dimethylether, dimethoxymethane, methyl formate, and carbon-monoxide. Usually very little carbon dioxide was obtained, and under certain conditions, hydrogen and methane can be produced. 

Various materials are added for maintaining an optimal temperature, melt viscosity, reaction conditions and internal and external recycling ratio of copper. 

Arrangement External — recycling External/submerged External / submerged... 

The feed to the reactor element within the loop is amix. The flow rate entering the reactor element is Qin + q and the exit concentration is aout. The relationship between amix and aout can be calculated without direct consideration of the external recycle. In the general case, this single-pass solution must be obtained numerically. Then the overall solution is iterative. One guesses amix and solves numerically for aout. Equation (4.21) is then used to calculate amix for comparison with the original guess. Any good root finder will work. The function to be zeroed is... 

Recycle. A large number of manufacturing facilities, especially chemical plants, have internal recycle streams that are considered part of the process. In this case, recycle refers to the external recycle of materials, such as polyester film and bottles, Tyvek envelopes, paper, and spent solvents. 

A continuous stirred tank reactor (CSTR) is a vessel to which reactants are added and products removed while the contents within the vessel are vigorously stirred using internal agitation or by internally (or externally) recycling the contents. CSTRs may be employed in series or in parallel. An approach to employing CSTRs in series is to have a large... 

Slurry Reactors Slurry reactors are akin to fluidized beds except the fluidizing medium is a liquid. In some cases (e.g., for hydrogenation), a limited amount of hydrogen may be dissolved in the liquid feed. The solid material is maintained in a fluidized state by agitation, internal or external recycle of the liquid using pipe spargers or distributor plates with perforated holes at the bottom of the reactor. Most industrial processes with slurry reactors also use a gas in reactions such as chlorination, hydrogenation, and oxidation, so the discussion will be deferred to the multiphase reactor section of slurry reactors. 

FIG. 19-32 Some examples of fermenters. (1) Conventional batch fermenter. (2) Air lift fermenters (a) Concentric cylinder or bubble column with draft tube (b) external recycle. (3) Rotating fermenter. (4) Horizontal fermenter. (5) Deep-shaft fermenter. (6) Flash-pot fermenter. 

Figure 2.3 displays the input/output structure, which includes inlet and outlet streams, as well as large external recycles that requires separate plants and storage facilities. Design decisions formulated as heuristics, are given in Table 2.1, mostly... 

Operation of the various reactors may be in integral or differential mode, and the latter may involve cither a single pass or a recycle system using internal or external recycle. 

Truly differential reactors of this type provide the most useful kinetic data. However, as indicated above they suffer from problems of analysis, etc. The addition of a recycle stage permits one to overcome these problems since the conversion per pass remains differential but the overall conversion becomes large enough to be measured and reflects accurately the influence of products as well as that of the reactants. A drawback, however, concerns the design of experiments. At the beginning of the investigation, trial and error is necessary for obtaining the desired concentrations at the exit. In external recycle reactors the recycle stream... 

Temkin has proposed the fixed bed reactor with external recycle [42]. Various constructions proposed afterwards differ mainly in the type of circulating system. 

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