Selection of the factors

As a first step one selects a number of factors to examine. The selected factors should be chosen from the description of the analytical procedure or from environmental parameters which are not necessarily specified explicitly in the analytical method. The factors can be quantitative (continuous, numerical) or qualitative (discrete). The factors to be tested should represent those that are most likely to be changed when a method is transferred, for instance, between different laboratories, different devices, or over time, and that potentially could influence the response of the method. However it is not always obvious which factors will influence a response and which will not. This is one of the reasons why screening designs are used (see Section 3.4.4). They allow to screen a large number of factors in a relatively small number of experiments. 

In Table 3.3 a list of different factors investigated in different publications is given [1,4,13,16,17], The list is not exhaustive and is only shown to give the reader an idea. 

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The selection of the factor type of acid in a ruggedness test could be accepted when only the pH is specified by the method rather than the acid used to bring the solution or the buffer up to the desired pH. Clearly, however, in such a case the method is poorly defined. 

McWatters and Holmes ( ) developed multiple regression models of the effects of pH and salt concentration on functional properties of soy flour. Design of the experiment and selection of the factors to be Included were based. In part, on earlier findings that emulsion capacity of defatted peanut meal was Inhibited around the Isoelectric point (ca pH 4.0)... 

Two limitations are imposed on the selection of the factor-variation interval lower (it may not be smaller than the error of fixing a factor) and upper (upper and lower levels cannot be outside the factor space). In trying to find the optimum in an experimental region, a subregion should be such as to permit the procedure of moving towards the optimum step by step. When determining a model or interpolation, variation intervals include the complete factor space. 

The selection of the operating principle and the design of the calorimeter depends upon the nature of the process to be studied and on the experimental procedures required. Flowever, the type of calorimeter necessary to study a particular process is not unique and can depend upon subjective factors such as teclmical restrictions, resources, traditions of the laboratory and the inclinations of the researcher. 

The described method can generate a first-order backward or a first-order forward difference scheme depending whether 0 = 0 or 0 = 1 is used. For 9 = 0.5, the method yields a second order accurate central difference scheme, however, other considerations such as the stability of numerical calculations should be taken into account. Stability analysis for this class of time stepping methods can only be carried out for simple cases where the coefficient matrix in Equation (2.106) is symmetric and positive-definite (i.e. self-adjoint problems Zienkiewicz and Taylor, 1994). Obviously, this will not be the case in most types of engineering flow problems. In practice, therefore, selection of appropriate values of 6 and time increment At is usually based on trial and error. Factors such as the nature of non-linearity of physical parameters and the type of elements used in the spatial discretization usually influence the selection of the values of 0 and At in a problem. 

In order to obtain an improved yield of the desired product, an excess over the proportion required by the chemical equation of one (or more) of the reactants is often used. In a given preparation, the selection of the reagent to be employed in excess will depend upon a number of factors these include its relative cost and ease of removal after the reaction, and... 

Prepare a log-log plot of rx versus X and evaluate the slope as a test of the Rayleigh theory applied to air. The factor M/pN in Eq. (10.36) becomes 6.55 X 10 /No, where Nq is the number of gas molecules per cubic centimeter at STP and the numerical factor is the thickness of the atmosphere corrected to STP conditions. Use a selection of the above data to determine several estimates of Nq, and from the average, calculate Avogadro s number. The average value of n - 1 is 2.97 X 10" over the range of wavelengths which are most useful for the evaluation of N. ... 

The functional requirements of the ablative heatshield must be well understood before selection of the proper material can occur. Ablative heatshield materials not only protect a vehicle from excessive heating, they also act as an aerodynamic body and sometimes as a stmctural component (2,3). Intensity and duration of heating, thermostmctural requirements and shape stabiUty (4,5), potential for particle erosion (6), weight limitations (7—10), and reusabiUty (11) are some of the factors which must be considered in selection of an ablative material. 

For an equiUbrium-based separation, a convenient measure of the intrinsic selectivity of the adsorbent is provided by the separation factor which is defined by analogy with the relative volatility as... 

Gasohol in the United States. Over 90% of the fuel ethanol in the United States is produced from com. Typically, 0.035 m (1 bushel) of com yields 9.5 L (2.5 gal) of ethanol. Ethanol is produced by either dry or wet milling (87). Selection of the process depends on market demand for the by-products of the two processes. More than two-thirds of the ethanol in the United States is produced by wet milling. Depending on the process used, the full cost of ethanol after by-product credits has been estimated to be between 0.25—0.53/L ( 1—2/gal) for new plants (88). Eeedstock costs are a significant factor in the production of fuel ethanol. A change in com price of 0.29/m ( 1.00/bushel) affects the costs of ethanol by 0.08/L ( 0.30/gal). 

There are a large number of standard methods suitable for each stage in the hazard analysis and risk assessment procedure. The selection of the proper method depends on several factors. Some of these are the type of process, the stage in the lifetime of the process, the experience and capabiUties of the participants, and the step in the procedure that is being examined. Information regarding the selection of the proper procedure is available in an excellent and comprehensive reference (1). 

Liquid—Liquid Extraction. The tiquid—tiquid extraction process for the rare-earth separation was discovered by Fischer (14). Extraction of REE using an alcohol, ether, or ketone gives separation factors of up to 1.5. The selectivity of the distribution of two rare-earth elements, REI and RE2, between two nonmiscible tiquid phases is given by the ratio of the distribution coefficients DI and D2 ... 

Process Measurements. The most commonly measured process variables are pressures, flows, levels, and temperatures (see Flow LffiASURELffiNT Liquid-levell asurel nt PressureLffiASURELffiNT Temperaturel asurel nt). When appropriate, other physical properties, chemical properties, and chemical compositions are also measured. The selection of the proper instmmentation for a particular appHcation is dependent on factors such as the type and nature of the fluid or soHd involved relevant process conditions rangeabiHty, accuracy, and repeatabiHty requited response time installed cost and maintainabiHty and reHabiHty. Various handbooks are available that can assist in selecting sensors (qv) for particular appHcations (14—16). 

Selection of the most suitable machine for a given requirement is an extremely complex process. Added to variations in the properties of the different materials, many of the machines involved have been specifically developed or adapted to perform only particular tasks. The principal factors which must be addressed are toughness/britdeness, hardness, abrasiveness, feed size, cohesity, particle shape and stmcture, heat sensitivity, toxicity, explodability, and specific surface. 

The most common method for screening potential extractive solvents is to use gas—hquid chromatography (qv) to determine the infinite-dilution selectivity of the components to be separated in the presence of the various solvent candidates (71,72). The selectivity or separation factor is the relative volatihty of the components to be separated (see eq. 3) in the presence of a solvent divided by the relative volatihty of the same components at the same composition without the solvent present. A potential solvent can be examined in as htfle as 1—2 hours using this method. The tested solvents are then ranked in order of infinite-dilution selectivities, the larger values signify the better solvents. Eavorable solvents selected by this method may in fact form azeotropes that render the desired separation infeasible. 

Electrodes. At least three factors need to be considered ia electrode selection as the technical development of an electroorganic reaction moves from the laboratory cell to the commercial system. First is the selection of the lowest cost form of the conductive material that both produces the desired electrode reactions and possesses stmctural iategrity. Second is the preservation of the active life of the electrodes. The final factor is the conductivity of the electrode material within the context of cell design. An ia-depth discussion of electrode materials for electroorganic synthesis as well as a detailed discussion of the influence of electrode materials on reaction path (electrocatalysis) are available (25,26). A general account of electrodes for iadustrial processes is also available (27). 

Thus the factors vaiy with the type of processing, although the boundaries between the classifications are not clear-cut and considerable judgment is required in selection of the correct factor. 

Many factors enter into the selection of the proper design for bearings. Some of these factors are ... 

All filters require a filter medium to retain solids, whether the filter is for cake filtration or for filter-medium or depth filtration. Specification of a medium is based on retention of some minimum parficle size at good removal efficiency and on acceptable hfe of the medium in the environment of the filter. The selection of the type of filter medium is often the most important decision in success of the operation. For cake filtration, medium selection involves an optimization of the following factors ... 

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