Reactor selectivity definition

The two commercial coil designs just discussed have identical pyrolysis selectivity despite their differences in geometrical and process characteristics. The axial gas temperature and partial pressure profiles constitute the major differences. The effect of axial temperature and axial partial pressure profiles on pyrolysis reactor selectivity are taken into account in the definitions of average residence time and hydrocarbon partial pressure. Therefore, when pyrolysis coils of different geometries and thus different temperature and partial pressure profiles are com-... 

Problem Definition InteUigent selection of a separator requires a careful and complete statement of the nature of the separation problem. Focusing narrowly on the specific problem, however, is not sufficient, especi ly if the separation is to be one of the steps in a new process. Instead, the problem must be defined as broadly as possible, beginning with the chemical reactor or other source of material to be separated and ending with the separated materials in their desired final form. In this way the influence of preceding and subsequent process steps on the separation step will be iUuminated. Sometimes, of course, the new separator is proposed to replace an existing unit the new separator must then fit into the current process and accept feed materials of more or less fixed characteristics. At other times the separator is only one item in a train of new equipment, all parts of which must work in harmony if the separator is to be effective. 

The main aims of MicroChemTec are the development of a unit construction kit for micro reactors, definition of standardized interfaces, investigations of modules on the market for their suitability for affiliation in the unit construction kit, documentation for this purpose, and demonstration of functioning of the concept with the example of selected unit operations or processes. 

In order to exemplify the potential of micro-channel reactors for thermal control, consider the oxidation of citraconic anhydride, which, for a specific catalyst material, has a pseudo-homogeneous reaction rate of 1.62 s at a temperature of 300 °C, corresponding to a reaction time-scale of 0.61 s. In a micro channel of 300 pm diameter filled with a mixture composed of N2/02/anhydride (79.9 20 0.1), the characteristic time-scale for heat exchange is 1.4 lO" s. In spite of an adiabatic temperature rise of 60 K related to such a reaction, the temperature increases by less than 0.5 K in the micro channel. Examples such as this show that micro reactors allow one to define temperature conditions very precisely due to fast removal and, in the case of endothermic reactions, addition of heat. On the one hand, this results in an increase in process safety, as discussed above. On the other hand, it allows a better definition of reaction conditions than with macroscopic equipment, thus allowing for a higher selectivity in chemical processes. 

To give a thorough, rational review of the field of chemical micro-process technology itself, one ideally would like to follow a deductive analysis route, pursuing a bottom-up approach. First, one may provide a definition of micro reactors, then search for the impacts on the engineering of chemical processes, and try to propose routes for exploitation, i.e. applications. Alternatively, for a less comprehensive, but more in-depth description, one could use a top-dovm approach starting with a selected application and try to design an ideal micro reactor for this. 

In continuous processes the reactants are fed to the reactor and the products withdrawn continuously the reactor operates under steady-state conditions. Continuous production will normally give lower production costs than batch production, but lacks the flexibility of batch production. Continuous reactors will usually be selected for large-scale production. Processes that do not fit the definition of batch or continuous are often referred to as... 

It can be seen that complex reactions often produce more than one product. In most industrial processes, one particular product (or group of products) is usually considered more desirable than the rest. Efforts will be made to choose reaction conditions and reactor types which favour the production of the desired material. Also, if more than one reactant is involved, attempts will be made to reduce the relative consumption of the most expensive reactant. In order to make quantitative comparisons between various courses of action, it is convenient to have some way of expressing relative product yields. This may be achieved by defining a reaction selectivity which refers to the comparitive formation rates of reaction products or by relating the appearance of a particular product to the consumption of a specified reactant. Various definitions have appeared in the literature the choice of terms is arbitrary. The use of terms in this chapter can be illustrated by an example. Consider the reactions... 

We need to complicate these definitions further by noting that the above definitions are the per pass yield of a reactor. If unreacted A can be recovered and recycled back into the feed, then the overall yield of the reactor plus the separation system becomes the single-pass selectivity of the reactor because no unreacted A leaves the reactor system. 

The product selectivity is strongly affected by the flow rate, reactor geometry (i.e., internal diameter and "heated zone) and weight of catalyst. On this account, the space time yield to HCHO - or HCHO productivity -( HCHO Scat h ) appears to be the more definite parameter to evaluate the reactivity of the partial oxi tion catalysts. 

There is a definite need, therefore, for systems that combine the advantages of high activity and selectivity of homogeneous catalysts with the facile recovery and recycling characteristic of their heterogeneous counterparts. This can be achieved by employing a different type of heterogeneous system, namely liquid-liquid biphasic catalysis, whereby the catalyst is dissolved in one liquid phase and the reactants and product(s) are in a second liquid phase. The catalyst is recovered and recycled by simple phase separation. Preferably, the catalyst solution remains in the reactor and is reused with a fresh batch of reactants without further treatment or, ideally, it is adapted to continuous operation. 

Concentration of active sites increases with the S/V-factor, all other parameters being equal. As a consequence, the target product yield also increases. Therefore, all following experiments were implemented in a reactor filled with quartz granules [95], A temperature increase above 640 °C abruptly reduces 4-VP yield and the process selectivity, and increases the amounts of side products (pyridine, in particular), which indicates that under definite conditions in the presence of hydrogen peroxide alkylpyridine dealkylation reaction may be implemented. 

In general, the procedure for designing a bubble column reactor (BCR) (1 ) should start with an exact definition of the requirements, i.e. the required production level, the yields and selectivities. These quantities and the special type of reaction under consideration permits a first choice of the so-called adjustable operational conditions which include phase velocities, temperature, pressure, direction of the flows, i.e. cocurrent or countercurrent operation, etc. In addition, process data are needed. They comprise physical properties of the reaction mixture and its components (densities, viscosities, heat and mass diffusivities, surface tension), phase equilibrium data (above all solubilities) as well as the chemical parameters. The latter are particularly important, as they include all the kinetic and thermodynamic (heat of reaction) information. It is understood that these first level quantities (see Fig. 3) are interrelated in various ways. 

For the simple network 5.26 and a reaction with no fluid-density variation, the magnitude of the effect is easily calculated The cumulative selectivity of conversion to P (moles of A converted to P per mole of A consumed, see definition 1.11) in batch and continuous stirred-tank reactors as a function of fractional conversion,/A, is... 

The (cumulative) selectivity for K (fraction of reacted A that is converted to K, see definition 1.11) in a batch reactor is... 

Another definition of selectivity used in the cunent literature, 5ou, is given in terms of the flow rates leaving the reactor. 5du is the overall selectivity. 

As a consequence of the different definitions for selectivity and yield, when reading literature dealing with multiple reactions, check careftilJy to ascertain the definition intended by the autoor. Prom an economic standpoint it is the overall selectivities, S, and yields, Y, that are important in determining profits. However, the rate-based selectivities give insists in choosing reactors and reaction schemes that will help maximize the profit. However, many times there is a conflict between selectivity and conversion (yield) because you want to make a lot of your desired product (D) and at the same lime minimize the undesired product (U). However, in many instances the greater conversion you achieve, not only do you make more D, you also form more U. 

The three kinds of reactors already described in this section are all traditional cross-flow reactors with permeable plates or membranes. The electrochemical filter-press cell reactors used, e.g., for electrosynthesis, are equipped with cation-selective membranes to prevent mixing of the anolyte and the catholyte. These cell reactors are therefore good examples of the extended type of cross-flow reactors according to the definition transferred from the filtration field. The application of the electrochemical filter-press cell reactor technique... 

The process uses two radial reactors in series with one preheater and one interstage heater. Steam is used as an energy carrier (adiabatic reactor) and diluent [43,50,51]. Reactor temperatures and pressures are 570-630°C and 1.5 bar, respectively. Total hydrocarbon mass flow (96 wt% ethylbenzene) is 95,000 kg/h. The steam/hydrocarbon ratio is 2. Typical conversion, selectivity and yield numbers are 71, 92 and 66%, respectively. Definitions are given in the appendix. Reaction equations and kinetics are taken from literature [43,51]. 

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