Choice of Operating Conditions

The feed composition and flowrate to the distillation are usually specified. Also, the specifications of the products are usually known, although there may be some uncertainty in product specifications. The product specifications may be expressed in terms of product purities or recoveries of certain components. The operating parameters to be selected by the designer include  

As pressure is lowered, these effects reverse. The lower limit is often set by the desire to avoid  

Both vacuum operation and the use of refrigeration incur capital and operating cost penalties and increase the complexity of the design. They should be avoided if possible. 

For a first pass through the design, it is usually adequate, if process constraints permit, to set distillation pressure to as low a pressure above ambient as allows cooling water or air-cooling to be used in the condenser. If a total condenser is to be used, and a liquid top product taken, the pressure should be fixed such that  

If a partial condenser is to be used and a vapor top product taken, then the above criteria should be applied to the dew point of the vapor top product, rather than the bubble point of the liquid top product. Also, if a vapor top product is to be taken, then the operating pressure of the destination for the product might determine the column pressure (e.g. overhead top product being sent to the fuel gas system). There are two major exceptions to these guidelines  

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The lowest cost corresponds to F= 58 m and Tout = 364K, but the minimum is very flat so that there is essentially no dilference in cost over a wide range of reactor volumes and operating temperatures. The good news is that an error in determining the minimum will have little elfect on plant economics or the choice of operating conditions. The bad news is that perfectionists will need to use very precise numerical methods to find the true minimum. 

For two-dimensional TLC under capillary flow controlled. conditions it should be possible to achieve a spot capacity, in theory, on the order of 100 to 250, but difficult to reach 400 and nearly impossible to exceed 500 [52,140]. Theoretical calculations indicate that by forced-flow development it should be relatively > easy to generate spot capacities well in excess of 500 with an upper bound of several thousand, depending on the choice of operating conditions. -fE... 

Different separation mechanisms, which determine selectivity, can be exploited in HPCE by appropriate choice of operating conditions. There are four principal modes of operation (Table 4.22) and it should be noted that in only one, micellar electrokinetic capillary chromatography (MEKC), is it possible to separate neutral species from one another. 

The use of cyclone separators for the removal of suspended dust particles from gases is discussed in Section 1.6. By suitable choice of operating conditions, it is also possible to... 

Choice of operating conditions favoring the adsorption of the reactant molecules and the desorption of the product molecules fixed-bed reactor instead of batch reactor, high molar substrate/acylating agent ratio, low conversion, high temperature, use of solvents with adequate polarities. 

The optimum choice of operating conditions are around a steam to methanol ratio of 1.5 and a temperature range of 250°C to 399°C. Pressure does not influence the reaction rate, but very high pressures limit the equilibrium conversion, which otherwise is better than 99% at the preferred range of 5 to 15 bars. The CU/Zn/Al and Cu/Zn/Cr based catalysts have been used in large units in industry for many years (12). 

To sum up, the choice of operating conditions for a specific FFF application is made in a way that recalls the general criteria used in chromatography. An accurate search of literature addressed to similar samples that have been already analyzed by FFF techniques is very useful. A number of specific reviews have been published concerning, for example, enviromnental, pharmaceutical, and biological samples (see Section 12.5). As previously mentioned above, one of the most important factors is the stability of the considered colloidal system, for which a great deal of information can be obtained from specialized literature, such as colloid, polymer, and latex handbooks [33], For example, the use of the proper surfactant (e.g., Fl-70) is common for SdFFF applications. Polymer analysis with ThFFF requires solvent types similar to those employed in size exclusion chromatography. 

At present few hospital laboratories use gas chromatographic methods because colorimetric, fluorometric, and other spectro-photometric methods can give data more rapidly, are less expensive, and are often of sufficient accuracy and precision for clinical evaluation of the patient. Radioimmunoassay procedures are available, but these are limited because of availability of the necessary antibodies and a sufficient demand to justify the expense of setting up the laboratory. For versatility and adaptability, many different analyses can be done on a single gas chromatograph. The only requirement is the proper column and choice of operating conditions. Furthermore, the techniques, in many cases, are now automated. The utility of GC coupled with... 

If more than one chemical reaction can take place in a reaction mixture, the type of reactor used may have a quite considerable effect on the products formed. The choice of operating conditions is also important, especially the temperature and the degree of conversion of the reactants. The economic importance of choosing the... 

Enzymes are frequently used as catalysts to promote specific reactions in free solution. They are typically required in small amounts and are attractive in that they obviate both the need to provide the nutritional support which would be required for micro-organisms to perform the same conversion, and the possible subsequent removal of those microbes. Furthermore, the enzyme need not necessarily be of microbial origin so that a wider choice of operating conditions and characteristics may be available. 

Finally a reminder is made that the flows of the mobile phase generally are rather modest, about 0.6-0.7 ml/min, and they rarely go over 1 ml/min. In fact, peak broadening may be observed on certain stationary ion-exchange phases, due to the slowness of the mass transfer. This means that very low flows must be maintained. Despite this, most separations may be completed in times that can vary between about 20 min and 40 min. A useful paper to consult, both because of the number of analytes considered, 63, 15 of which are alcohols, and for the choice of operative conditions applied to a cation exchange column is Ref. 13. Table 1 enumerates the alcohols considered Fig. 1 shows the separation of a standard solution. 

Test evaluation of the catalytic performances, again involving the expertise of the available robots and the proper choice of operating conditions (tested reactions, stationary or non-stationary regimes, T P, contact time, etc.). 

By focussing on the above points, cat cracking experiments can be performed in laboratory equipment with residue-containing feedstocks. With proper choice of operating conditions, the resulting product yields and selectivity simulate those in commercial units quite well. 

Many industrial reactions are not carried to equilibrium. In this circumstance the reactor design is based primarily on reaction rate. However, the choice of operating conditions may still be determined by equilibrium considerations as already illustrated with respect to the oxidation of sulfur dioxide. In addition, the equilibrium conversion of a reaction provides a goal by which to measure improvements in the process. Similarly, it may determine whether or not an experimental investigation of a new process is worthwhile. For example, if the thermodynamic analysis indicates that a yield of only 20 percent is possible at equilibrium and a 50 percent yield is necessary for the process to be economically attractive, there is no purpose to an experimental study. On the other hand, if the equilibrium yield is 80 percent, an experimental program to determine the reaction rate for various conditions of operation (catalyst, temperature, pressure, etc.) may be warranted. 

An operating regime plot can then be employed to provide further insight into the choice of operating conditions for a displacement process. Use of this plot is particularly important for a selective displacement process which typically has narrower operating regimes for successful operation. 

Efficiencies In actual operation the trays of a distillation column rarely, if ever, operate at equilibrium despite attempts to approach this condition by proper design and choice of operating conditions. The usual way of dealing with departures from equilibrium in multistage towers is through the use of stage and/or overall efficiencies. 

As demonstrated elsewhere [34], it is possible to utilize such a different thermal dependence of the enzymes involved in the cascade system for process control. The appropriate choice of operational conditions allowed the enzyme activities to be controlled and directed the selectively of the process to the first reaction product, the amide or to the second reaction product, the acid. A higher process temperature and residence time (20 °C and r = lOh) favor production of the acid (amidase activity prevails), while a lower temperature and residence time (5 °C and t = 5 h) allow the production of the intermediate amide (amidase activity negligible) [34]. 

Therefore, it is easy to understand how some experimental evidences, such as the catalytic activity decay, have been attributed to diffusion limitations w-196,199,200) Some of these limitations, connected to mass transfer of the monomer from the gas to the liquid phase and from the liquid phase to the polymer surface, can be generally eliminated or at least minimized by means of an appropriate choice of operating conditions... 

Flexible Process - Ihe selectivity of SCE can be adjusted by proper choice of operating conditions and procedures. The process can be fine tuned to achieve a desired flavor or fragrance profile. 

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