Optimum economic design

Exploration for an acceptable or optimum design of a new reaction process may need to consider reactor types, several catalysts, specifications of feed and product, operating conditions, and economic evaluations. Modifications to an existing process hkewise may need to consider many cases. These efforts can oe eased by commercial kinetics services. A typical one can handle up to 20 reactions in CSTRs or... 

Economic optimum design usually implies high circulation rates, although not high enough to give mist flow. 

Clearly, when considering chemicals and/or catalyst cost alone there is an economic optimum design that is a hybrid process involving catalytic treatment for the high... 

We follow a three-step procedure First, we must find how equilibrium composition, rate of reaction, and product distribution are affected by changes in operating temperatures and pressures. This will allow us to determine the optimum temperature progression, and it is this that we strive to approximate with a real design. Second, chemical reactions are usually accompanied by heat effects, and we must know how these will change the temperature of the reacting mixture. With this information we are able to propose a number of favorable reactor and heat exchange systems—those which closely approach the optimum. Finally, economic considerations will select one of these favorable systems as the best. 

Economic optimum design of piping will be touched on later, but the rules of Table 6.2 of typical linear velocities and pressure drops provide a rough guide for many situations. 

This chapter presents a comparison of the steady-state economics of four alternative tubular reactor systems. The entire process will be considered, not just the reactor in isolation, because the optimum economic steady-state design can be determined only for the entire plant. The type of recycle, the phase of the reaction, and the heat transfer configuration all affect the optimum design. 

Table 6.8 shows the optimum economic steady-state design for the hot reactor system when the catalyst cost is 100/kg. The important steady-state design parameters for this hot reaction system are a total catalyst weight of 11,880 kg, a recycle flow of 0.27kmol/s, a tube diameter of 0.0592 m, and a heat transfer area of 401 m2. The design optimization variables used are the same as discussed in Chapter 5. The TAC of the optimum design is 770,000 per year. 

The optimum steady-state economic design was determined with these new kinetic parameters, and the parameters are given in Table 7.4. The FS2 flowsheet is used with a ratio (2p,/2totai = 0.1. The impact of the kinetic parameters on the optimum design is striking. The hotter reaction requires a much larger recycle flowrate and a higher reactor inlet temperature for the same reactor exit temperature 7 ollt = 500 K. These lead... 

OPTIMUM REACTOR DESIGN ECONOMIC EVALUATION FLEXIBILITY STUDY PROCESS OPTIMIZATION/CONTROL... 

The previous section assumed that product composition (or product flow) requirements are fixed. In this very common situation, the optimum design minimizes the costs of achieving these requirements. Often, product specs are not fixed, but depend on economics. Even when a product must obey a "less than" purity spec, better purity may fetch a better price. The better price may justify additional investment in equipment and/or a higher operating cost. Here, a design must optimize product purity value versus distillation cost. This optimization is also important in an operating column and is commonly performed by on-line computer control. It is outlined below, and discussed in detail elsewhere (1,2). 

An optimum design is based on the best or most favorable conditions. In almost every case, these optimum conditions can ultimately be reduced to a consideration of costs or profits. Thus, an optimum economic design could be based on conditions giving the least cost per unit of time or the maximum profit per unit of production. When one design variable is changed, it is often found that some costs increase and others decrease. Under these conditions, the total cost may go through a minimum at one value of the particular design variable, and this value would be considered as an optimum. 

Although cost considerations and economic balances are the basis of most optimum designs, there are times when factors other than cost can determine the most favorable conditions. For example, in the operation of a catalytic reactor, an optimum operation temperature may exist for each reactor size because of equilibrium and reaction-rate limitations. This particular temperature could be based on the maximum percentage conversion or on the maximum amount of final product per unit of time. Ultimately, however, cost variables... 

The same principles used for developing an optimum design can be applied when determining the most favorable conditions in the operation of a manufacturing plant. One of the most important variables in any plant operation is the amount of product produced per unit of time. The production rate depends on many factors, such as the number of hours in operation per day, per week, or per month the load placed on the equipment and the sales market available. From an analysis of the costs involved under different situations and consideration of other factors affecting the particular plant, it is possible to determine an optimum rate of production or a so-called economic lot size. 

The derivation of equations for determining optimum economic pipe diameters is presented in Chap. 11 (Optimum Design and Design Strategy). The following simplified equations [Eqs. (45) and (47) from Chap. 11] can be used for making design estimates ... 

The impact of several design parameters has been explored. Control performance worsens when the steady-state economic optimum design, consisting of a large feed-effluent heat exchanger and a small furnace, is used. The most robust control is obtained when a small FEHE and a large furnace are employed. 

Tphis work explores the important variables which must be considered - to design an extractive distillation process. The discussion identifies the economic effects of these variables and their possible interactions. Some of the design variables may have synergistic effects in terms of separation cost while others may not. As a result, the optimum design for an economic extractive distillation process must be a compromise set of values for the different process variables. These compromises are discussed and are illustrated for a particular case—i.e., separation of propane-propylene mixtures. For this commercially important separation fractional distillation is most often used, regardless of the low relative volatility (about 1.13-1.19 at 200 psia). 

The task of the piping designer is to size the line so that the flow of fluid will create a pressure gradient that is near the economical optimum. The economic optimum conditions take into account both the capital cost of equipment and operating costs of pumping, energy, etc. 

Optimum design basic principles for, 6-9, 341-342 break-even chart for, 349-350 comparison of graphical and analytical methods for, 348-349 economic, 7-8... 

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