Consequences of Scale-up

It is evident that having made a choice of scale-up relationship, other factors will be affected in different ways there is often no way to scale up aU the significant factors together, so priorities have to be chosen. An example of this is given in Middleton (1997), to which reference should be made for full details. A summary is given here. 

In the example a gas-liquid reaction with particulate solids (e.g., a catalyst) operating in regime 11 in a stirred reactor with a Rushton turbine is to be scaled up. The primary process requirement is for the same degree of reaction conversion at each scale, which means the same number of moles of gas transferred per mole of liquid fed  

Assume for simplicity that Ca = 0 (a good approximation for regime 11) and that the degree of gas backmixing is the same at aU scales (this should be checked at the end of the calculation and reiterations performed if necessary). Given a constant feed concentration at all scales. 

Sometimes it is necessary for the outlet gas concentration to be constant (e.g., with hazardous gases) then from the mass balance this becomes 

Another constraint will then fix the design. In this example maintaining N Njs for the suspension of the catalyst particles is important, so = 

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For optimal production of hydrogen by photosynthetic bacteria, to be economically attractive, one has to take into account several parameters from an overall point of view, such as the productivity rate, the light efficiency and the yield from the carbon source. An efficient parameter to evaluate the performance of photobioreactors, in order to maximize hydrogen production is still missing. With this parameter one should also be able to forecast the consequences of scaling-up. 

Any hazard evaluation must thus include an examination of all phases of the process. As with the example above, one area which is often ignored involves the quenching, scrubbing, and disposal of reactions. It must be remembered that these reactions suffer the same consequences of scale-up as the more productive components of the process. 

Furthermore, unit operations may function in a rate-limiting manner as the scale of operation increases. When Astarita (4) decried the fact, in the mid-1980s, that there is no scale-up algorithm which permits us to rigorously predict the behavior of a large scale process based upon the behavior of a small scale process, it was presumably as a consequence of all of these problematic aspects of scale-up. 

To illustrate the similarity approach, we take an example of scaling-up of a BSCR by a factor of SF . Consequently, the following similarity rules should be respected ... 

Several workers have claimed that under the influence of microwaves, some reactions proceed faster than under conventional conditions at the same temperature because of various non-thermal microwave effects 48,53-56. Other investigators have rejected the theory of specific activation at a controlled temperature in homogeneous media57-62. A study by Stadler el al,63 on the rate enhancements observed in solid-phase reactions revealed that the significant rate enhancements were a result of direct, rapid in-core heating of the solvent by microwave energy and not a specific non-thermal microwave effect . The existence or otherwise of non-thermal microwave effects continues to be a source of great debate and if proven would have serious potential consequences for scale-up, particularly if such effects were unpredictable. 

There is no doubt that the ultimate development of process intensification leads to the novel field of microreaction technology (Figure 1) (7-9). Because of the small characteristic dimensions of microreaction devices, mass and heat transfer processes can be strongly enhanced, and, consequently, initial and boundary conditions as well as residence times can be precisely adjusted for optimizing yield and selectivity. Microreaction devices are evidently superior, due to their short response time, which simplifies the control of operation. In connection with the extremely small material holdup, nearly inherently safe plant concepts can be realized. Moreover, microreaction technology offers access to advanced approaches in plant design, like the concept of numbering-up instead of scale-up and, in particular, the possibility to utilize novel process routes not accessible with macroscopic devices. 

As can be observed in Table 9.3, cell concentrations equal to or higher than 107 cells ml-1 are needed to attain volumetric productivities higher than 50 mg L-1 d-1. Also, it is important to note that as cell concentration increases, the complexity of the process increases and, consequently, bioreactor scale-up becomes limited, for example by physical limitations of materials used in hollow-fiber cartridges. 

The questions of scale-up take us closer to industrial realities. A common problem for fixed-bed reactors is how to decrease the tendency of catalyst poisoning and deactivation. Since this seems to be unavoidable, one has to And methods to restore the catalytic activity. This problem affects all fixed-bed reactors, and consequently it will not degrade the cross-flow reactors compared to other fixed-bed reactors. 

One of the practical consequences of a basic analysis of extruder operation leads to the possibility of scale-up, namely, the prediction of operating conditions for a large extmder derived with data from a small machine, both having geometrical similarity. Assuming the following relationships prevail... 

With respect to the above-given review, it certainly can be imagined that in multiphase catalytic reactions heat and mass transfer as well as phase mixing and stagnant zones have an influence on the chemical conversion in a very different way, depending on the operating variables of the reactor. But more than that, this behavior has consequences on scale-up procedures for these types of reactors, as the most appropriate scale-up procedure depends on mode of operation, flow regimes and the relative importance of mass transfer resistances compared to the chemical... 

Cooperation between chemists and chemical engineers is vital to achieve cleaner process technology. The scale-up of processes from the laboratory bench to pilot plant, and eventually to full scale plant, is an inexact science and the parameters of scale-up are less understood than they might be. The disciplines required for process development are not well taught nor well practised in the UK chemical industry. The consequences are the less-than-optimum processes producing chemical wastes which require disposal. The overall costs of this aspect were set out earlier as were the benefits of minimising the social, political and economic costs. 

Generating all scenarios for p potential products, each one with two outcomes, results in 2 scenarios. Each individual scenario is a fairly small deterministic problem. The demand and its associated probability for the different outcomes of each product are assumed to be known. If a product fails in the clinical trials, the demand is consequently zero over all remaining time periods. The multi-site investment strategy is common to ail possible scenarios present in the second stage. However, due to the different product demand patterns, every scenario has its own characteristic production, inventory and sales profile. The operational decisions reflect the scenario-dependant decisions made upon completion of the clinical trials and resolution of the uncertainty (wait-and-see) and they include timings of scale-up and qualifications runs (binary variables), allocation of products to manufacturing suites (binary variables), detailed production plans at each production site (continuous variables), inventory profiles (continuous variables), sales profiles at each sales region (continuous variables). 

Product innovation absorbs considerable resources in the fine chemicals industry, in part because of the shorter life cycles of fine chemicals as compared to commodities. Consequently, research and development (R D) plays an important role. The main task of R D in fine chemicals is scaling-up lab processes, as described, eg, in the ORAC data bank or as provided by the customers, so that the processes can be transferred to pilot plants (see Pilot PLANTS AND microplants) and subsequently to industrial-scale production. Thus the R D department of a fine chemicals manufacturer typically is divided into a laboratory or process research section and a development section, the latter absorbing the Hon s share of the R D budget, which typically accounts for 5 to 10% of sales. Support functions include the analytical services, engineering, maintenance, and Hbrary. 

Few mechanisms of liquid/liquid reactions have been established, although some related work such as on droplet sizes and power input has been done. Small contents of surface-ac tive and other impurities in reactants of commercial quality can distort a reac tor s predicted performance. Diffusivities in liquids are comparatively low, a factor of 10 less than in gases, so it is probable in most industrial examples that they are diffusion controllech One consequence is that L/L reactions may not be as temperature sensitive as ordinary chemical reactions, although the effec t of temperature rise on viscosity and droplet size can result in substantial rate increases. L/L reac tions will exhibit behavior of homogeneous reactions only when they are very slow, nonionic reactions being the most likely ones. On the whole, in the present state of the art, the design of L/L reactors must depend on scale-up from laboratoiy or pilot plant work. 

In reaction engineering, laboratory catal54ic reactors are tools or instruments to study how catalysts behave in some desired reaction. Quantitatively, the investigator wants to know how much of the desired product can be made per unit weight of catalyst, how much raw material will be used, and what byproducts will be made. This is the basic information needed to estimate the costs and profitability of the process. The economic consequence of our estimates also forces us to clarify what the rate limiting steps are, and how much transfer processes influence the rates, i.e., everything that is needed for a secure scale-up. Making the... 

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