Overall Process Optimization

This example comes from Loe and Pults (2001) and is reproduced, with permission from AIChE. This example explains the operational improvements for a single deisopentanizer fractionation tower. To do so, the current operation is simulated and assessed. Then improvement opportunities are identified and the limiting factors are determined. Optimal solutions are obtained by optimizing tower ISBL conditions and OSBL conditions. The improvements on this single tower have generated over 500,000 as compared with historical operation over the first six months. 

The DIP tower has 50 trays in comparison with 70- - trays used in a typical deisopentanizer. As a consequence, the DIP tower often has a difficult time making a good split between isopentane and normal pentane components. 

FIGURE 14.8. Deisopentanizer flow scheme. (From Loe and Pulls (2001), reprinted with 

Historically, the DIP tower had been operated with a target of 10% nCs in the overhead, and 20-30% /C5 in the bottom product. The tower was reported to be limited by reboiler or condenser duty. One of the two steam reboUers had been out of service for some time, and 20-30% of the condenser fin fan motors were not operating and in need of repair. The DIP equipment had not been a maintenance priority, in part because no economic penalty had been calculated for having areboUer or condenser out of service. There was also a concern that the tower could flood if both reboUers were placed in service. 

The DIP process control was accomplished with a Distributed Control Stystem (DCS) system equipped with an APC algorithm. The controller was set to target 10% nCs in the overhead and 10% /C5 in the product, and would increase reboiler steam and reflux rate until reaching the maximum limits for these flows. The tower pressure was also controlled within a specified range by the APC, and this could indirectly limit the reboiler duty as well, if the tower pressure increased beyond its maximum limit. Inferential estimates for product tCs and Cs qualities were calculated based on tower temperatures and pressures, and a bias for these values was continually updated based on daily laboratory (lab) data. 

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Finally, optimization means dealing with time and other improvements spanning the overall process. Optimizing the speed of the analysis is obvious, but optimizing resolution can improve the process as well (as we will see later). An economic optimization of individual analyses will result in time improvements throughout the process because it will liberate resources for other tasks. 

For variables that do not lie within a recycle loop, optimization may be sinplified. An exanple is a distillation column in a separation sequence in which two products are purified and sent to storage. The operation of such a column does not impact any part of the process upstream and can therefore be considered independently after the upstream process has been optimized. First, single- and two-variable optimizations for single pieces or small groups of equipment are considered. Then overall process optimization strategies are studied. 

No attempt should be made to optimize pressure, reflux ratio, or feed condition of distillation in the early stages of design. The optimal values almost certainly will change later once heat integration with the overall process is considered. 

In practical applications, gas-surface etching reactions are carried out in plasma reactors over the approximate pressure range 10 -1 Torr, and deposition reactions are carried out by molecular beam epitaxy (MBE) in ultrahigh vacuum (UHV below 10 Torr) or by chemical vapour deposition (CVD) in the approximate range 10 -10 Torr. These applied processes can be quite complex, and key individual reaction rate constants are needed as input for modelling and simulation studies—and ultimately for optimization—of the overall processes. 

A thinking process optimizing system performance. It examines the system and focuses on the constraints that limit overall system performance. It looks for the weakest link in the chain of processes that produce organizational performance and seeks to eliminate it and optimize system performance. 

Overall, catalytic processes in industry are more commonly described by simple power rate law kinetics, as discussed in Chapter 2. However, power rate laws are simply a parameterization of experimental data and provide little insight into the underlying processes. A micro-kinetic model may be less accurate as a description, but it enables the researcher to focus on those steps in the reaction that are critical for process optimization. 

At the beginning of this chapter, several points were made about general metrics principles that are particularly applicable within the context of equipment and operability. It is worth to revisit them for just a moment to say that good process metrics for these categories are especially dependent on an understanding of the overall process, and optimization of a process should be done from a multivariate perspective. Metrics in these categories should be seen as having considerable dependencies on each other and on the materials and chemicals used in the process. 

The overall cost optimization criterion (5.4-137) to be minimized is composed of two terms. The first, which is called proportional, is related to the yield of both C and E one needs to maximize the yield of C, Yc, while minimising the ratio of yields, Y c. Yc and Kg are nc.f/nB.o and nE./nBA, respectively, n is number of moles, and y is a factor expressing the relative weight of the two terms (y was assumed to be one). The second term, called the non-proportional or fixed cost of operation, is the reciprocal of the ratio of yield of C per batch time, s, and as such it should be minimized, p is the weighing factor (equal to 174 in the process under consideration) in this term. 

Using the colloidal Pt(i t ) + RU c/C catalysts described above, the optimal atomic ratio depends upon methanol concentration, cell temperature, and applied potential, as shown by the Tafel plots recorded with methanol concentrations of 1.0 and 0.1 M at T = 298K (Fig. 11.4) and 318K (Fig. 11.5). Some authors have stated that for potentials between 0.35 and 0.6 V vs. RHE, the slow reaction rate between adsorbed CO and adsorbed OH species must be responsible for the rate of the overall process [Iwasita et al., 2000]. From these results, it can be underlined that, at a given constant potential lower than 0.45-0.5 V vs. RHE, an increase in temperature requires an increase in Ru content to enhance the rate of methanol oxidation, and that, at a given constant potential greater than 0.5 V vs. RHE, an increase in temperature requites a decrease in Ru content to enhance the rate of methanol oxidation. 

Process optimization. The settings of some process variables can have a major influence on the decision-making in developing the flowsheet and on the overall profitability of the process. Optimization of such variables is usually required. 

Process economics is required to evaluate design options, carry out process optimization and evaluate overall project profitability. Two simple criteria can be used ... 

Once the structure of the recycle and separation has been established, some important degrees of freedom can be optimized that can have a very significant effect on the overall process economics. Start by considering the optimization of reactor conversion. 

Microwave heating is often applied to already known conventional thermal reactions in order to accelerate the reaction and therefore to reduce the overall process time. When developing completely new reactions, the initial experiments should preferably be performed only on a small scale applying moderately enhanced temperatures to avoid exceeding the operational limits of the instrument (temperature, pressure). Thus, single-mode reactors are highly applicable for method development and reaction optimization. 

The end customer s main benefits after commissioning of the solution described can be summarized as follows optimal production schedules with optimal batch recipes are available on demand within seconds, better overall process coordination and visibility of the process and faster recovery from disturbances through efficient scheduling. This leads to a significant increase in plant throughput and revenues. 

Phase 4 Optimizing and scale-up of the catalyhc step as well as of the overall process. 

Fig. 13.5 The compound acquisition scheme. The overall process depicted in the figure provides an efficient procedure for the selection and acquisition of compounds that optimally populate the PRCC with diverse, druglike molecules for suitable screening.

The dynamic behavior of the cell metabolism initiated by different external effects (addition of substrates or inhibiting reagents) can be followed via this instantaneous method. These effects can be used to control the overall process and optimize the bioprocess. Meyer and Beyeler [50] developed a control system for a continuous yeast cultivation process. Here the increase up to the optimal dilution rate was controlled via fluorescence monitoring. The dilution rate was only increased when no negative effect on the metabolic state of the cells was observed. During the cultivation of Candida utilis the fluorescence signal was used for the addition of substrate ethanol. The addition was started when... 

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