Reactor micro-mixed

Mixing Models. The assumption of perfect or micro-mixing is frequently made for continuous stirred tank reactors and the ensuing reactor model used for design and optimization studies. For well-agitated reactors with moderate reaction rates and for reaction media which are not too viscous, this model is often justified. Micro-mixed reactors are characterized by uniform concentrations throughout the reactor and an exponential residence time distribution function. 

Calculated Molecular Weight Distributions. The calculated weight fraction distributions for the micro-mixed, segregated, and micro-mixed reactor with dead-polymer models for Runs 2, 5,... 

Evaluation of Mixing Models. The micro-mixed reactor will produce polymer disttibutions with increasing amounts of high molecular weight tail as the degree of polymerization of the polymer product increases over that of the original seed polymer. 

This trend is illustrated by the curves for the micro-mixed reactor in Figures 8 through 14. Also characteristic of the seeded, micro-mixed reactor is the convergence of the polydispersity index to 2 for a high degree of polymerization. This trend is illustrated to some extent in Table VI which presents the calculated degrees of polymerizations. 

The micro-mixed reactor with dead-polymer model was developed to account for the large values of the polydispersity index observed experimentally. The effect of increasing the fraction of dead-polymer in the reactor feed while maintaining the same monomer conversion is to broaden the product polymer distribution and therefore to increase the polydispersity index. As illustrated in Table V, this model, with its adjustable parameter, can exactly match experiment average molecular weights and easily account for values of the polydispersity index significantly greater than 2. 

To differentiate between the micro-mixed reactor with dead-polymer and the by-pass reactor models in this investigation, the effect of mixing speed on the value of "( )" was observed. As illustrated in Table V, the value (j>" is not observed to increase with decreasing mixing speed as would be expected for a by-pass reactor. This rules out the possibility of a by-pass model and further substantiates the dead-polymer model. 

The effects of micro-mixing on the molecular weight distribution are much more pronounced than those of segregation. According to Patterson (33) only a small increase in micro-mixing over that of total segregation will yield a pol)nner distribution very similar to that of micro-mixed reactor. 

A micro-mixed, seeded reactor will produce a broad polymer distribution with a high molecular weight tail and polydispersity index that approaches 2 at large degrees of polymerization. 

The effect of dead-polymer and by-passing on the micro-mixed reactor for the same degree of monomer conversion is to broaden the product polymer distribution and thus allow values of the polydispersity index much larger than 2. 

The micro-mixed reactor with dead-polymer model simulated the product of the laboratory reactor well within experimental accuracy. 

In spite of visual indications of at least partial segregation, the concept of micro-mixing proved to be most useful in modeling the laboratory reactor. 

Suppose is a function of a alone and that neither dSt Ajda nor d Alda change sign over the range of concentrations encountered in the reactor. Then, for a system having a fixed residence time distribution. Equations (15.48) and (15.49) provide absolute bounds on the conversion of component A, the conversion in a real system necessarily falling within the bounds. If d S A/dc > 0, conversion is maximized by maximum mixedness and minimized by complete segregation. If d 0i A/da < 0, the converse is true. If cf- A/da = 0, micro-mixing has no effect on conversion. 

The experimental results in Fig. 27 show the influence of the reactor system (see Fig. 28) on the disintegration of enzyme activity. It was found that the low-stress bladed impeller results in less activity loss than the propeller stirrer which causes much higher maximum energy dissipation ,. The gentle motion the blade impeller produces means that stress is so low that its disadvantage of worse micro mixing in NaOH (in comparison with the propeller) is more than compensated. 

A detailed characterization of micro mixing and reaction performance (combined mixing and heat transfer) for various small-scale compact heat exchanger chemical reactors has been reported [27]. The superior performance, i.e. the process intensification, of these devices is evidenced and the devices themselves are benchmarked to each other. 

As a second example, several Hantzsch syntheses using diverse ring-substituted 2-bromoacetophenones and 1-substituted-2-thioureas are given. For these reactions, comparative and better yields were achieved when using a micro-mixing tee chip reactor as compared with conventional laboratory batch technology. The increase in yield amounted to about 10-20% [156, 157]. 

P 33] The catalyst bed was manually positioned in the micro channel (300 pm wide 115 pm deep) at room temperature using a 10% (v/v) solution offormamide and potassium silicate [6, 7]. Micro reactor bottom and top plates were thermally bonded thereafter. Then, 75% THF (aqueous) solutions of 4-bromobenzonitrile and phenylboronic acid having equimolar concentrations were placed in the two reservoirs of a micro-mixing tee chip. In the collection reservoir, 30 pi of the THF solution was placed. Voltages ranging from 100 to 400 V were used, but kept constant only for one reservoir. The other one was switched on and off at 200 V for given time periods. 

OS 52[ [OS 53[ [OS 54[ [OS 55[ [R 4b[ [P 38[ In a two-micro-mixing tee chip reactor, substrates with diketone moieties of known different reactivity, such as 2,4-pentanedione, benzoylacetone and diethyl malonate, were processed, each with the same acceptor ethyl propiolate [8]. Also, a reaction with the less alkynic Michael acceptor methyl vinyl ketone was carried out. 

The Hantzsch synthesis was primarily chosen to evaluate the potential of a micro-mixing-tee chip reactor for carrying out reactions above room temperature (e.g. 

P 54] A 50 gl volume of a 0.3 M solution of cyclohexanone in anhydrous methanol with about 1 mg of DCC is placed in one reservoir of a micro-mixing tee chip reactor [11] 50 gl of a 0.3 M solution of pyrrolidine is added to the other reservoir and anhydrous methanol is filled in the third, the collection reservoir. Voltages ranging from 300 to 1000 V are applied for a period of 40 min to transport the reaction species. The reaction is carried out at room temperature. 

The rotary speed of the propellers exhibits a turning influence on the average size of the product this is similar to the results obtained by Chen et al. [165] in their investigation on an analogous problem in a stirred tank reactor and also similar to the results on the preparation of nano copper described in the previous chapter. It results from the mutual effect between macro- and micro-mixing, as mentioned before. 

The two extremes of the state of mixedness arc represented by the plug flow reactor (PFR, no mixing) and by the perfectly stirred reactor (PSR, perfectly mixed). The reactant flow in the PFR is neither macro nor micro mixed, whereas in the PSR mixing occurs down to the molecular level, thus both macro and micro mixing take place (see Figure 6). A variety of real flows can be characterised by series, parallel or loop connections of PFR and PSR. Additionally there exist other models such as the dispersion model (dispersed plug flow) which allows to model mixing conditions between the two extremes of PFR and PSR. 

These mechanisms of gas formation are probably valid for many other experimental devices where solid biomass particles react inside a colder medium. Hence, it seems difficult to completely avoid gas formation in most pyrolysis reactors because of the bad micro-mixing efficiencies existing in the vicinity of the particles. 

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