Operational Variables

It is necessary to understand the operational variables to obtain a good SEM image with desired depth of field and resolution. This section discusses how to control the depth of field and resolution by adjusting operation variables such as the working distance, final aperture size, acceleration voltage, probe current and astigmatism. 

Within certain limits, the majority of operational variables do not determine the results of gel chromatographic separations in a decisive way. Nevertheless, it is advisable to test all variables for each particular system and application in order to obtain optimal and reproducible data. The most common operational variables in gel chromatography are the volume, Uj, and the concentration, C, of the sample applied, both influencing the retention volumes and the separation efficiency. In analytical separations, it is necessary to work with the lowest v, and C allowed by the particular detection system (cf., section 4.6.3.6). The allowed sample volume depends primarily on the volume of the column and is usually several millilitres in the case of 

The composition of the eluent, its pH and its ionic strength are very important operational variables for numerous gel chromatographic systems, mainly for the sensitive biological samples and for systems containing ionizable groups (cf., section 

2) while temperature and mobile phase flow rate show less influence on the results of separation. 

Detection is understood to be the determination of the concentration, and sometimes also the particular properties of the sample components in the effluent. The separated substances can be detected by discontinuous characterization of the collected fractions, by continuous measurement at the column outlet and — in special cases — also by direct column scanning in the course of elution. From the physical point of view, one can measure either the properties of the separated substances in the effluent (solute property detectors like photometers) or the properties of the effluent as a whole (bulk property detectors, e.g. differential refractome-ters). Many biological substances can be detected on the basis of measurement of their biological activity. 

According to another classification, the detectors can be divided into specific, universal and mixed. For the choice of the detection method, the properties of sample and mobile phase as well as experimental requirements are decisive however, sometimes several types of instruments based on the same detection principle are marketed. When judging the detectors one considers primarily (i) linearity, dynamic range and sensitivity, (ii) erroneous responses like noise and drift, caused in the former by the instability of operational variables such as temperature variations, pulsating eluent flow, etc. (iii) response distortion due to hydraulic broadening and skewing of the sample zone as well as response delay. 

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Data for the several flame methods assume an acetylene-nitrous oxide flame residing on a 5- or 10-cm slot burner. The sample is nebulized into a spray chamber placed immediately ahead of the burner. Detection limits are quite dependent on instrument and operating variables, particularly the detector, the fuel and oxidant gases, the slit width, and the method used for background correction and data smoothing. 

Temperature and pressure are not considered as primary operating variables temperature is set sufficiendy high to achieve rapid mass-transfer rates, and pressure is sufficiendy high to avoid vaporization. In Hquid-phase operation, as contrasted to vapor-phase operation, the required bed temperature bears no relation to the boiling range of the feed, an advantage when heat-sensitive stocks are being treated. 

Two main operational variables that differentiate the flotation of finely dispersed coUoids and precipitates in water treatment from the flotation of minerals is the need for quiescent pulp conditions (low turbulence) and the need for very fine bubble sizes in the former. This is accompHshed by the use of electroflotation and dissolved air flotation instead of mechanically generated bubbles which is common in mineral flotation practice. Electroflotation is a technique where fine gas bubbles (hydrogen and oxygen) are generated in the pulp by the appHcation of electricity to electrodes. These very fine bubbles are more suited to the flotation of very fine particles encountered in water treatment. Its industrial usage is not widespread. Dissolved air flotation is similar to vacuum flotation. Air-saturated slurries are subjected to vacuum for the generation of bubbles. The process finds limited appHcation in water treatment and in paper pulp effluent purification. The need to mn it batchwise renders it less versatile. 

The operating variables for a dmm or roHer dryer iaclude coadeasatioa of incoming product ia an evaporator, temperature of incoming product, steam pressure (temperature) ia dmm, speed of dmm, and height of product over dmm. The capacity of the dryer is iacreased by increa sing the steam pressure, the temperature of the milk feed, the height of milk over the dmms, the gap between dmms (double), and the speed of rotation of the dmms. Increasing the capacity is limited by the effect on the product quaHty. 

Reaction and Transport Interactions. The importance of the various design and operating variables largely depends on relative rates of reaction and transport of reactants to the reaction sites. If transport rates to and from reaction sites are substantially greater than the specific reaction rate at meso-scale reactant concentrations, the overall reaction rate is uncoupled from the transport rates and increasing reactor size has no effect on the apparent reaction rate, the macro-scale reaction rate. When these rates are comparable, they are coupled, that is they affect each other. In these situations, increasing reactor size alters mass- and heat-transport rates and changes the apparent reaction rate. Conversions are underestimated in small reactors and selectivity is affected. Selectivity does not exhibit such consistent impacts and any effects of size on selectivity must be deterrnined experimentally. 

For fixed-bed reactors containing rapidly deactivating catalysts, the scheduled changes ia operating variables to accommodate activity loss can have a marked effect on mn length. This is exemplified by acetylene hydrochiorination to produce vinyl chloride ia tubular fixed-bed reactors. Steel reactors,... 

AU processed material is screened to return the coarse fraction for a second pass through the system. Process feed rates are matched to operating variables such as rpm speed and internal clearances, thus minimizing the level of excess fines (—200 mesh (<0.075 mm mm)). At one installation (3) the foUowing product size gradation of total smaller than mesh size (cumulative minus) was obtained ... 

Using this simplified model, CP simulations can be performed easily as a function of solution and such operating variables as pressure, temperature, and flow rate, usiag software packages such as Mathcad. Solution of the CP equation (eq. 8) along with the solution—diffusion transport equations (eqs. 5 and 6) allow the prediction of CP, rejection, and permeate flux as a function of the Reynolds number, Ke. To faciUtate these calculations, the foUowiag data and correlations can be used (/) for mass-transfer correlation, the Sherwood number, Sb, is defined as Sh = 0.04 S c , where Sc is the Schmidt... 

J. R. Murphy and Y. L. Cheng, "The Interaction of Heat Balance and Operating Variables in ZeoUtic Catalyst Operations," presented at Katalistiks 5th Annual FCC Symposium, Vieima, Austria, May 1984. 

Along with operating variables of the crystallizer, nucleation and growth determine such crystal characteristics as size distribution, purity, and shape or habit. 

Correlations of nucleation rates with crystallizer variables have been developed for a variety of systems. Although the correlations are empirical, a mechanistic hypothesis regarding nucleation can be helpful in selecting operating variables for inclusion in the model. Two examples are (/) the effect of slurry circulation rate on nucleation has been used to develop a correlation for nucleation rate based on the tip speed of the impeller (16) and (2) the scaleup of nucleation kinetics for sodium chloride crystalliza tion provided an analysis of the role of mixing and mixer characteristics in contact nucleation (17). Pubhshed kinetic correlations have been reviewed through about 1979 (18). In a later section on population balances, simple power-law expressions are used to correlate nucleation rate data and describe the effect of nucleation on crystal size distribution. 

The production rate is 2—4 t/h, depending on the feed rate, monomer concentration in the feed, and conversion. The conversion of isobutylene and isoprene typically ranges from 75—95% and 45—85%, respectively, depending on the grade of butyl mbber being produced. The composition and mol wt of the polymer formed depend on the concentration of the monomers in the reactor Hquid phase and the amount of chain transfer and terminating species present. The Hquid-phase composition is a function of the feed composition and the extent of monomer conversion. In practice, the principal operating variable is the flow rate of the initiator/coinitiator solution to the reactor residence time is normally 30—60 minutes. 

Constrained Optimization When constraints exist and cannot be eliminated in an optimization problem, more general methods must be employed than those described above, since the unconstrained optimum may correspond to unrealistic values of the operating variables. The general form of a nonhuear programming problem allows for a nonlinear objec tive function and nonlinear constraints, or... 

Results of diying tests can be correlated empirically in terms of overall heat-transfer coefficient or length of a transfer unit as a function of operating variables. The former is generally apphcable to all types of dryers, while the latter applies only in the case of continuous diyers. The relationship between these quantities is as follows. 

In the references cited previously, the authors discuss the influence of operating variables upon the performance of plate towers when large heat effects are involved. Some general observations are as follows ... 

Figure 14-12 illustrates the influence of system composition and degree of reaetant eonversion upon the numerical values of for the absorption of CO9 into sodium hydroxide solutions at constant conditions of temperature, pressure, and type of packing. An excellent experimental study of the influence of operating variables upon overall values is that of Field et al. (Pilot-Plant Studie.s of the Hot Carbonate Proce.s.s for Removing Carbon Dioxide and Hydrogen Sulfide, U.S. Bureau of Mines Bulletin 597, 1962). 

The evidence is that the coefficients kc and ko and the interfacial area depend differently upon operating variables. For purposes of design, therefore, it is ultimately necessary to have separate information on the quantities kc, ko, and a, .. The role of an additional surface resistance is emphasized by the studies of Kishinevsld and Moehalova [Zh. Prikl Khim.,. 3.3, 2049 (I960)]. 

Primarily because of the lack of adequate experimental techniques (particularly, the produc tion of appropriate monodisperse aerosols), there has been no comprehensive experimental test of any of the venturi-scrubber models over wide ranges of design and operating variables. The models for other types of scrubbers appear to be essentially untested. 

Another basic consideration is whether crystallization is best carried out on a batch basis or on a continuous basis. The present tendency in most processing plants is to use continuous equipment whenever possible. Continuous equipment permits adjusting of the operating variables to a relatively fine degree in order to achieve the best results in terms of energy usage and product characteristics. It allows the use of a smaller labor force and results in a continuous util-... 

Granulation Rate Processes and Effect of Operating Variables. 20-75... 

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