Continuous-flow systems reactor time

Normally, conversion increases with the time the reactants spend in the reactor. For continuous-flow systems, this time usually increases with increasing reactor volume consequently, the conversion X is a function, of reactor volume V. If is the molar flow rate of species A fed to a system operated at steady state, the molar cate at which species A is reacting within the entire system will be... 

For a batch reactor, we saw that conversion increases with time spent in the reactor. For continuous-flow systems, this time usually increases with increasing... 

The time that a molecule spends in a reactive system will affect its probability of reacting and the measurement, interpretation, and modeling of residence time distributions are important aspects of chemical reaction engineering. Part of the inspiration for residence time theory came from the black box analysis techniques used by electrical engineers to study circuits. These are stimulus-response or input-output methods where a system is disturbed and its response to the disturbance is measured. The measured response, when properly interpreted, is used to predict the response of the system to other inputs. For residence time measurements, an inert tracer is injected at the inlet to the reactor, and the tracer concentration is measured at the outlet. The injection is carried out in a standardized way to allow easy interpretation of the results, which can then be used to make predictions. Predictions include the dynamic response of the system to arbitrary tracer inputs. More important, however, are the predictions of the steady-state yield of reactions in continuous-flow systems. All this can be done without opening the black box. 

Washout experiments can be used to measure the residence time distribution in continuous-flow systems. A good step change must be made at the reactor inlet. The concentration of tracer molecules leaving the system must be accurately measured at the outlet. If the tracer has a background concentration, it is subtracted from the experimental measurements. The flow properties of the tracer molecules must be similar to those of the reactant molecules. It is usually possible to meet these requirements in practice. The major theoretical requirement is that the inlet and outlet streams have unidirectional flows so that molecules that once enter the system stay in until they exit, never to return. Systems with unidirectional inlet and outlet streams are closed in the sense of the axial dispersion model i.e., Di = D ut = 0- See Sections 9.3.1 and 15.2.2. Most systems of chemical engineering importance are closed to a reasonable approximation. 

Metal-catalyzed cross-couplings are key transformations for carbon-carbon bond formation. The applicability of continuous-flow systems to this important reaction type has been shown by a Heck reaction carried out in a stainless steel microreactor system (Snyder et al. 2005). A solution of phenyliodide 5 and ethyl acrylate 6 was passed through a solid-phase cartridge reactor loaded with 10% palladium on charcoal (Scheme 2). The process was conducted with a residence time of 30 min at 130°C, giving the desired ethyl cinnamate 7 in 95% isolated yield. The batch process resulted in 100% conversion after 30 min at 140°C using a preconditioned catalyst. 

This correlation corresponds to an exponential decay model, k = koe aY. This expression differs from the conventional exponential model often used in continuous-flow systems 22, 23), k = koe at, in that the analog to time in a pulsed reactor is pulse number or its equivalent, cumulative feed introduced. In our case the correlating quantity is cumulative feed converted, Y. If one assumes that deactivation is caused by coke, the amount of which is proportional to hexane actually converted, this... 

When we speak of a cold wall CVD reactor, we refer to a continuous flow system where the wafer is kept at the required high temperature, but all other surfaces bounding on the reacting gases are cold. The objective here is to cause the desired reaction only on the hot wafer and keep all other surfaces free of deposits. In practice this is a goal that can only be partially attained. Although reactions will proceed more slowly on colder surfaces, they will proceed-and films will build up. At the same time the films that form on the colder surfaces may be more porous than the normal film and may spall off more easily. All of which says that in spite of our best efforts, cold walled reactors may have their cold walls an undesirable source of particulates which may end up on the hot substrate. The occurence of such particulates can be minimized by frequent cleaning of the chamber walls to remove deposits. 

From the above mole balance equation we can develop the design equation for various reactor types. By solving the design equation we can then determine the time required for a batch reactor system or a reactor volume for a continuous flow system to reach a specific conversion of the reactant to products. 

For a series reaction network the most important variable is either time in batch systems or residence time in continuous flow systems. For the reaction system A - B - C the concentration profiles with respect to time in a batch reactor (or residence time in a PFR) are given in Figure 6. 

High labor and handling costs as well as the start-up and shutdown times required to fill and empty the reactor are important drawbacks in a batch operation. Continuous flow systems are nearly always more cost-effective than batch reactors, especially when large volumes are to be treated, i.e., the main application of this reactor configuration is wastewater treatment. The removal of phenolic compounds from waters has been performed using SBP and HRP in continuous stirred tank reactor (CSTR) [49, 75, 76, 81, 83, 84],... 

When such a stirring is absolutely absent in a continuous flow system, as it takes place in the piston reactor (PR), regularities of the batch processes with the same residence time 0 are realized. This implies that in order to describe copolymerization in continuous PR one can apply all theoretical equations known for a common batch process having replaced the current time t for 0. As for the equations presented in Sect. 5.1, which do not involve t al all, they remain unchanged, and one can employ them directly to calculate statistical characteristics of the products of continuous copolymerization in PR. It is worth mentioning that instead of the initial monomer feed composition x° for the batch reactor one should now use the vector of monomer feed composition xin at the input of PR. In those cases where copolymer is being synthesized in CSTR a number of specific peculiarities inherent to the theoretical description of copolymerization processes arises. 

According to this scheme of plasma polymerization of TMS in a closed system, it is anticipated that the atomic composition of the plasma polymer should continuously change with the plasma polymerization time. Figure 13.21 depicts comparison of XPS cross-section profile of C/Si ratios for plasma polymers deposited in a flow system reactor and that in a closed system reactor. The results clearly show that a closed system plasma polymerization of TMS indeed produces a... 

Another way of looking at the segregation model for a continuous-flow system is the PFR shown in Figures 13-15(a) and (b). Because the fluid flows down the reactor in plug flow, each exit stream corresponds to a specific residence time in the reactor. Batches of molecules are removed from the reactor at different locations along the reactor in such a manner so as to duplicate the RTD function, (/). The molecules removed near the entrance to the reactor... 

The following configuration demonstrates a general cell model of a continuous flow system described in ref. [77]. There are two possibilities to arrive at state 4, i.e. directly and via the upper plug flow reactor. However, in order to materialize these possibilities it was necessary to add state 3-a perfectly mixed reactor 3. The residence time in this reactor is controlled by the quantity [13. 

In design development, reactants are charged into the batch-type reactor one at a time at the beginning of electrolysis while products are removed at the end of the run. A continuous flow system can be evaluated as a natural extension of the batch system. 

A perfect mixer has an exponential distribution of residence times W t) = exp(—r/7). Can any other continuous flow system have this distribution Perhaps, surprisingly, the answer to this question is a definite yes. To construct an example, suppose the feed to a reactor is encapsulated. The size of the capsules is not critical. They must be large enough to contain many molecules but must remain small compared to the dimensions of the reactor. Imagine them as small ping-pong balls as in Figure 15.1 la. 

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