Bench scale

An example of a size-exclusion chromatogram is given in Figure 7 for both a bench-scale (23.5 mL column) separation and a large-scale (86,000 mL column) mn. The stationary phase is Sepharose CL-6B, a cross-linked agarose with a nominal molecular weight range of 5000-2 x 10 (see Fig. 6) (31). 

Bench scales are small and light enough to be placed on a bench or table. They generally have a platform on which small quantities can be placed by hand. Alternatively, some have a roUer conveyor platform on which the item is moved across the scale. Capacities range from 5 kg to 250 kg. 

Portable scales generally have a platform near the floor with the indicator mounted on a column for ease of operation. These scales may be on wheels, and many are battery-powered. Capacities are generally less than 600 kg. Bench scales can be placed on a wheeled table for similar use. 

Precision bench scales are designed for use in industrial settings they typically have capacities below 50 kg and 30,000 or more displayed divisions. Typical apphcations are counting very small parts or weighing hand-add materials for proportioning operations. 

Precision balances are similar to precision bench scales but are typically used in laboratory settings with up to 500,000 displayed divisions. 

Until recently most industrial scale, and even bench scale, bioreactors of this type were agitated by a set of Rushton turbines having about one-thind the diameter of the bioreactor (43) (Fig. 3). In this system, the air enters into the lower agitator and is dispersed from the back of the impeller blades by gas-fiUed or ventilated cavities (44). The presence of these cavities causes the power drawn by the agitator, ie, the power requited to drive it through the broth, to fall and this has important consequences for the performance of the bioreactor with respect to aeration (35). k a has been related to the power per unit volume, P/ U, in W/m and to the superficial air velocity, in m/s (20), where is the air flow rate per cross-sectional area of bioreactor. This relationship in water is... 

V. Babrauskas, Development of the Cone Calorimeter. A Bench-Scale EHR Apparatus Based on Occggen Consumption, NSBIR 82-2611, U.S. Dept, of Commerce, Gaithersburg, Md., 1982. 

If equation 7 holds, then the soHd is exclusively in the aqueous phase equation 8 defines the condition at which the soHd resides in the oil phase whereas if equation 9 is satisfied then the soHd collects at the water—oil interfacial region. Figure 16 is the flow sheet of a bench-scale study that demonstrates the concept of two-Hquid flotation (40). 

After years of bench-scale and pilot-plant studies, constmction was begun on a gca 1600 m /d (10,000 bbl/d) unit in Sarawak, Malaysia, by Shell in a... 

R. L. Patel and co-workers, "Reactivity Characteri2ation of SoHd Fuels in an Atmospheric Bench-Scale Fluidi2ed-Bed Combustor," presented at the 1988 Joint AS ME/IEEE Power Generation Conference, Philadelphia, Sept. 25—29, 1988 also as Combustion Engineering Tpuhlic tion TlS-8391. 

Acetylene traditionally has been made from coal (coke) via the calcium carbide process. However, laboratory and bench-scale experiments have demonstrated the technical feasibiUty of producing the acetylene by the direct pyrolysis of coal. Researchers in Great Britain (24,28), India (25), and Japan (27) reported appreciable yields of acetylene from the pyrolysis of coal in a hydrogen-enhanced argon plasma. In subsequent work (29), it was shown that the yields could be dramatically increased through the use of a pure hydrogen plasma. 

Based on the bench-scale data, two coal-to-acetylene processes were taken to the pilot-plant level. These were the AVCO and Hbls arc-coal processes. The Avco process development centered on identifying fundamental process relationships (29). Preliminary data analysis was simplified by first combining two of three independent variables, power and gas flow, into a single enthalpy term. The variation of the important criteria, specific energy requirements (SER), concentration, and yield with enthalpy are indicated in Figure 12. As the plots show, minimum SER is achieved at an enthalpy of about 5300 kW/(m /s) (2.5 kW/cfm), whereas maximum acetylene concentrations and yield are obtained at about 7400 kW/(m /s) (3.5 kW/cfm). An operating enthalpy between these two values should, therefore, be optimum. Based on the results of this work and the need to demonstrate the process at... 

The demonstration unit was later transported to the CECOS faciHty at Niagara Falls, New York. In tests performed in 1985, approximately 3400 L of a mixed waste containing 2-chlorophenol [95-57-8] nitrobenzene [98-95-3] and 1,1,2-trichloroethane [79-00-5] were processed over 145 operating hours 2-propanol was used as a supplemental fuel the temperature was maintained at 615 to 635°C. Another 95-h test was conducted on a PCB containing transformer waste. Very high destmction efficiencies were achieved for all compounds studied (17). A later bench-scale study, conducted at Smith Kline and French Laboratories in conjunction with Modar (18), showed that simulated chemical and biological wastes, a fermentation broth, and extreme thermophilic bacteria were all completely destroyed within detection limits. 

The proposed mechanism by which chlorinated dioxins and furans form has shifted from one of incomplete destmction of the waste to one of low temperature, downstream formation on fly ash particles (33). Two mechanisms are proposed, a de novo synthesis, in which PCDD and PCDF are formed from organic carbon sources and Cl in the presence of metal catalysts, and a more direct synthesis from chlorinated organic precursors, again involving heterogeneous catalysis. Bench-scale tests suggest that the optimum temperature for PCDD and PCDF formation in the presence of fly ash is roughly 300°C. 

One goal of catalyst designers is to constmct bench-scale reactors that allow determination of performance data truly indicative of performance in a full-scale commercial reactor. This has been accompHshed in a number of areas, but in general, larger pilot-scale reactors are preferred because they can be more fully instmmented and can provide better engineering data for ultimate scale-up. In reactor selection thought must be given to parameters such as space velocity, linear velocity, and the number of catalyst bodies per reactor diameter in order to properly model heat- and mass-transfer effects. 

Other variations of catalytic and noncatalytic coal Hquefaction schemes were also developed (27,28). Additionally, bench-scale and semiworks systems have been operated in Germany by researchers at Bergbau-Forschung in Essen (29). A 2.5 ton per day pilot plant is being operated by the National Coal Board in the United Kingdom at Point of Ayr in Wales (30). This facdity is notable for the use of semibatch or candle filters for removal of mineral matter and unreacted coal from the primary Hquefaction products. 

Reports of bench-scale electroorganic reactions date back to the nineteenth century (18). Although the Nalco and Monsanto processes are the only two really large-scale operations, there has been significant growth in processes on a small scale. At least 60 processes appear to be commercial woddwide (4). A listing of some processes can be found in Table 5 further details and a listing of more processes are also available (4,5,25,88, and 99—104). 

Over 25 years ago the coking factor of the radiant coil was empirically correlated to operating conditions (48). It has been assumed that the mass transfer of coke precursors from the bulk of the gas to the walls was controlling the rate of deposition (39). Kinetic models (24,49,50) were developed based on the chemical reaction at the wall as a controlling step. Bench-scale data (51—53) appear to indicate that a chemical reaction controls. However, flow regimes of bench-scale reactors are so different from the commercial furnaces that scale-up of bench-scale results caimot be confidently appHed to commercial furnaces. For example. Figure 3 shows the coke deposited on a controlled cylindrical specimen in a continuous stirred tank reactor (CSTR) and the rate of coke deposition. The deposition rate decreases with time and attains a pseudo steady value. Though this is achieved in a matter of rninutes in bench-scale reactors, it takes a few days in a commercial furnace. 

As an example the use of ceramic membranes for ethane dehydrogenation has been discussed (91). The constmction of a commercial reactor, however, is difficult, and a sweep gas is requited to shift the product composition away from equiUbrium values. The achievable conversion also depends on the permeabihty of the membrane. Figure 7 shows the equiUbrium conversion and the conversion that can be obtained from a membrane reactor by selectively removing 80% of the hydrogen produced. Another way to use membranes is only for separation and not for reaction. In this method, a conventional, multiple, fixed-bed catalytic reactor is used for the dehydrogenation. After each bed, the hydrogen is partially separated using membranes to shift the equihbrium. Since separation is independent of reaction, reaction temperature can be optimized for superior performance. Both concepts have been proven in bench-scale units, but are yet to be demonstrated in commercial reactors. 

Methanol to Ethylene. Methanol to ethylene economics track the economics of methane to ethylene. Methanol to gasoline has been flilly developed and, during this development, specific catalysts to produce ethylene were discovered. The economics of this process have been discussed, and a catalyst (Ni/SAPO 34) with almost 95% selectivity to ethylene has been claimed (99). Methanol is converted to dimethyl ether, which decomposes to ethylene and water the method of preparation of the catalyst rather than the active ingredient of the catalyst has made the significant improvement in yield (100). By optimizing the catalyst and process conditions, it is claimed that yields of ethylene, propylene, or both are maximized. This is still in the bench-scale stage. 

Consider the possibility of scahng up the design of a new system from experimental data obtained in a laboratory-bench scale or a small pilot-plant unit. 

Use of Literature for Specific Systems A large body of experimental data obtained in bench-scale laboratoiy units and in small-diameter packed towers has been published since the early 1940s. One might wish to consider using such data for a particular chemically... 

There are many uses of fluidized beds. A number of applications have become commercial successes others are in the pilot-plant stage, and others in bench-scale stage. Generally, the fluidized bed is used for gas-solids contac ting however, in some instances the presence of the gas or sohd is used only to provide a fluidized bed to accomplish the end result. Uses or special (maracteristics follow ... 

Apparatus There are several variations of the bench-scale test leaf that may be used, but they all have features similar to the one discussed below. 

Precoat Procedure Precoat filtration tests are run in exactly the same manner as bottom-feed tests except that the leaf must first be precoated with a bed of diatomaceous earth, perhte, or other shave-able inert sohds. Some trial and error is involved in selecting a grade of precoat material which will retain the filtered solids to be removed on the surface of the bed without any significant penetration. During this selection process, relatively thin precoat beds of I to 2 cm are satisfactory. After a grade has been selected, bench-scale tests should be... 

The depth of cut involved in precoat filtration is a veiy important economic factor. There is some disagreement as to the method required to accurately predic t the minimum permissible depth of cut. Some investigators maintain that the depth of cut can be evaluated only in a quah-tative manner during bench-scale tests by judging whether the process solids remain on the surface of the precoat beck This being so, they indicate that it is necessaiy to run a continuous pilot-plant test to determine the minimum permissible depth of cut. The use of a continuous pilot-plant filter is a veiy desirable approach and will provide accurate information under a variety of operating conditions. 

The overall scale-up faclor used to convert a rate calculated from bench-scale data to a design rate for a commercial installation must incorporate separate factors for each of the following ... 

Scale-Up on Rate Filtration rates calculated from bench-scale data shouldbe multiplied by a factor of 0.8 for all types of commercial units which do not employ continuous washing of the filter medium and on which there is a possibility of filter-medium bhnding. For those units which employ continuous filter-medium washing, belt-type drum and horizontal units, the scale-up fac tor maybe increased to 0.9. The use of this scale-up fac tor assumes the following ... 

Overall Scale-Up Factor The final design filtration rate is determined by multiplying the bench-scale filtration rate by each of... 

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