Industrial reactors space time

By an industrial investigation of a gas-phase reaction, the chlorination of alkanes, thermal management (faster temperature ramping, avoidance of overshoots) was improved and, hence, control over radical formation was exerted. As a result, a significant increase in space-time yield to about 430 g h 1 was achieved using a hybrid micro-reactor plant compared with the conventional performance of 240 g h [127, 161]. 

However, these investigations also point out that we need a proper definition of space-time yields for micro reactors. This refers to defining what essentially the reaction volume of a micro reactor is. Here, different definitions lead to varying values of the respective space-time yields. Following another definition of this parameter for ethylene oxide formation, a value of only 0.13 t h m is obtained -still within the industrial window [159, 162, 163]. 

Quite new ideas for the reactor design of aqueous multiphase fluid/fluid reactions have been reported by researchers from Oxeno. In packed tubular reactors and under unconventional reaction conditions they observed very high space-time yields which increased the rate compared with conventional operation by a factor of 10 due to a combination of mass transfer area and kinetics [29]. Thus the old question of aqueous-biphase hydroformylation "Where does the reaction takes place " - i.e., at the interphase or the bulk of the liquid phase [23,56h] - is again questionable, at least under the conditions (packed tubular reactors, other hydrodynamic conditions, in mini plants, and in the unusual,and costly presence of ethylene glycol) and not in harsh industrial operation. The considerable reduction of the laminar boundary layer in highly loaded packed tubular reactors increases the mass transfer coefficients, thus Oxeno claim the successful hydroformylation of 1-octene [25a,26,29c,49a,49e,58d,58f], The search for a new reactor design may also include operation in microreactors [59]. 

Substrate and product inhibition. Few academic researchers are familiar with this phenomenon as they usually mn their hydrogenations at low substrate concentrations and low SCR. However, for industrial applications the space-time yield of a reaction - the amount of product per unit reactor volume per time unit - is quite important. Clearly, the higher the substrate concentration the higher the space-time yield and the more economic the process. More often than not, either substrate or product inhibition becomes a problem when the substrate concentration is increased to 10 wt% or more. 

Hence the dimension ("the order") of the reaction is different, even in the simplest case, and hence a comparison of the two rate constants has little meaning. Comparisons of rates are meaningful only if the catalysts follow the same mechanism and if the product formation can be expressed by the same rate equation. In this instance we can talk about rate enhancements of catalysts relative to another. If an uncatalysed reaction and a catalysed one occur simultaneously in a system we may determine what part of the product is made via the catalytic route and what part isn t. In enzyme catalysis and enzyme mimics one often compares the k, of the uncatalysed reaction with k2 of the catalysed reaction if the mechanisms of the two reactions are the same this may be a useful comparison. A practical yardstick of catalyst performance in industry is the space-time-yield mentioned above, that is to say the yield of kg of product per reactor volume per unit of time (e.g. kg product/m3.h), assuming that other factors such as catalyst costs, including recycling, and work-up costs remain the same. 

Figure 3-7 Plot of nominal space times (or reactor residence times) required for several important industrial reactors versus the nominal reactor temperatiwes. Times go from days (for fermentation) down to milliseconds (for ammonia oxidation to form nihic acid). The low-temperature, long-time processes involve liquids, while the high-temperature, short-time processes involve gases, usually at high pressures.

Experiments performed with the reactor, in that case made of titanium, by Worz at BASF in a proprietary reaction revealed 60% yield for the desired product at residence times as low as 40 ms in the micro reactor. This performance was superior to that of the experiments performed in an aluminum capillary, which corresponds well with the reactor design of the industrial process (Figure 3.24). A 2000% gain in space-time yield was found for the porous coated micro structures compared with the aluminum capillaries. 

It is useful to have a measure of time for a flow reactor even though the major design variable is reactor or fluid volume. A commonly used quantity in industrial reactor design is space time. Space time is defined as the time required to process one reactor volume of feed, measured at some set of specified conditions. The normal conditions chosen are the inlet concentration of a reactant and inlet molar or volumetric flow rate. 

A catalytic process is commercially viable if the catalyst transformation is achieved within definite, practical limits of space and time. To quantify this aspect, one can determine the so-called space-time yield. This measure of activity is simply the amount of product obtained per unit time and per unit reaction space (where reaction space is usually the reactor volume). Weisz (79) pointed out that in industry the useful space-time yield is rarely less than 10"6 g/mol of reactant per cubic centimeter of volume of reactor space per second. This has been called the Weisz window on reality. Figure 9 (79) shows the Weisz window and other windows of chemical activity that apply to biochemistry and petroleum geochemistry (79). 

Shiraishi et al. [49,50] immobilized glucoamylase of Rhizopus delemar in monolith structures and used them for saccharification of soluble starch. The process was studied at first in a batch reactor at SOX and 4.5 bar. The simplified kinetic model was developed. A continuous process was realized in a monolith reactor consisting of 10 pieces stacked on top of each other, where the blocks were rotated by ir/4 on their axes. The reaction rate at a glucose concentration of 460 g dm" was approximately two times higher than in a conventional industrial process. Conversion of 47% was reached at a space time of 12 hr. The half-life of enzyme was 79 days. 

For industrial reactors of large volume, the structured packing of the LFR and PPR is for practical reasons best made up from standard modules ( unit cells ) that are arranged in the reactor space by stacking. These modules are shop fabricated and filled before being installed in the reactor. Thus time can be saved in the startup of a plant and during catalyst replacement. 

Electrochemical reaction engineering deals with modeling, computation, and prediction of production rates of electrochemical processes under real technical conditions in a way that technical processes can reach their optimum performance at the industrial scale. As in chemical engineering, it centers on the appropriate choice of the electrochemical reactor, its size and geometry, mode of operation, and the operation conditions. This includes calculation of performance parameters, such as space-time yield,... 

Mitra et al. (1998) employed NSGA (Srinivas and Deb, 1994) to optimize the operation of an industrial nylon 6 semibatch reactor. The two objectives considered in this study were the minimization of the total reaction time and the concentration of the undesirable cyclic dimer in the polymer produced. The problem involves two equality constraints one to ensure a desired degree of polymerization in the product and the other, to ensure a desired value of the monomer conversion. The former was handled using a penalty function approach whereas the latter was used as a stopping criterion for the integration of the model equations. The decision variables were the vapor release rate history from the semibatch reactor and the jacket fluid temperature. It is important to note that the former variable is a function of time. Therefore, to encode it properly as a sequence of variables, the continuous rate history was discretized into several equally-spaced time points, with the first of these selected randomly between the two (original) bounds, and the rest selected randomly over smaller bounds around the previous generated value (so as... 

There are many other approaches to industrial applications of flash chemistry, although available information is limited. Let us briefly touch on some examples. The Kolbe-Schmitt synthesis serves as a useful standard method to introduce a carboxyl group into phenols (Scheme 10.6). The Kolbe-Schmitt synthesis has been widely used in industry, and there are many variants of this transformation. Microflow systems can be used for conducting the Kolbe-Schmitt synthesis under aqueous high-pressure conditions.A decrease in reaction times by an order of magnitude (a few tens of seconds instead of minutes) and increase in space-time yields by orders of magnitude can be attained using a microflow system. For example, a microflow system composed of five parallel capillaries (inner volume 9 ml) has a productivity of 555 g/h, whereas the productivity of a macrobatch reactor (IL flask) is 28 g/h. 

In the absence of dispersion, which is discussed in Chapter 14, the space time is equal to the mean residence time in the reactor, This time is the average time the molecules spend in the reactor. A range of typical processing times in terms of the space time (residence time) for industrial reactors is shown in Table 2-4. 

Table 2-4 Tyhcal Space Time tor Industrial Reactors-...

Table 2-5 shows space times for six industrial reactions and reactors. Table 2-5 Sample Industrial Space Times ...

For industrially relevant process the relaxation time is ca. 1-100 s. For construction of a kinetic model for nonstationary conditions, knowledge about the evolution of the concentrations of adsorbed species on the catalyst surface is needed. Under nonstationary conditions the changes of concentration fields in time, reactor space and catalyst surface (for heterogeneous catalysis) are interrelated by complex dependencies. Therefore, for experimental investigation under nonstationary conditions, knowledge about the gas and surface composition is required. 

Thus, it is seen that the most direct measure of the reactor s capability for carrying out the conversion is the space time, which is the result of making a rigorous mass balance in the steady-state plug flow system. In industrial practice, the reciprocal is commonly used—termed space velocity. Specifically, using... 

Diaphragm cells used in chlor-alkali production are also effectively a parallel-plate flow reactor but they are constructed in a very different way they will be discussed in the next chapter. While the potential distribution in a parallel-plate cell is good and the mixing conditions can be made to meet most requirements, the space time yield leaves much to be desired and it is often difficult to reduce the inter-electrode gap sufficiently to give the required space time yield and energy efficiency. These problems have led to the development of many novel cell designs at the present time they remain laboratory or pilot-plant concepts but it is to be expected that some will eventually have an impact on the industrial scene. 

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