Optimum plant operation

After the base case is digested and accepted by the designer as valid, various modification cases can be obtained. Because the base case and each modification case is presented in its best light (at the optimum plant operation for that case), bias between cases is eliminated. Therefore, the designer can compare cases on the same basis. 

Improve Chemical Engineering. Improvements in catalyst performance inevitably mean that the optimum plant operating condition will be different from that for the unimproved catalyst. Design changes may be needed to obtain the maximum benefit from improved performance. The cost of such changes must be taken into account when assessing the value of catalyst improvement. 

Patents claiming specific catalysts and processes for thek use in each of the two reactions have been assigned to Japan Catalytic (45,47—49), Sohio (50), Toyo Soda (51), Rohm and Haas (52), Sumitomo (53), BASF (54), Mitsubishi Petrochemical (56,57), Celanese (55), and others. The catalysts used for these reactions remain based on bismuth molybdate for the first stage and molybdenum vanadium oxides for the second stage, but improvements in minor component composition and catalyst preparation have resulted in yields that can reach the 85—90% range and lifetimes of several years under optimum conditions. Since plants operate under more productive conditions than those optimum for yield and life, the economically most attractive yields and productive lifetimes maybe somewhat lower. 

A real-time optimization (RTO) system determines set point changes and implements them via the computer control system without intervention from unit operators. The RTO system completes all data transfer, optimization c culations, and set point implementation before unit conditions change and invahdate the computed optimum. In addition, the RTO system should perform all tasks without upsetting plant operations. Several steps are necessaiy for implementation of RTO, including determination of the plant steady state, data gathering and vahdation, updating of model parameters (if necessaiy) to match current operations, calculation of the new (optimized) set points, and the implementation of these set points. 

Most ethylene plants operate continuously with the expander functioning at or near design point. However, by using inlet guide vanes, the expander can still provide optimum performance at off-design conditions. Also, the expansion process generates power, which is used by the compressor. The ethylene enters the expanders at approximately 26 bar (377 psia) and exits at approximately 6 bar (87 psia). The expanders generate over 2,000 hp for gas compression. 

The selection of lean oil for an absorber is an economic study. A light lean oil sustains relatively high lean oil loss, but has the advantage of high mols/gal compared to a heavier lean oil. The availability of a suitable material has a large influence on the choice. A lean oil 3 carbon numbers heavier than the lightest component absorbed is close to optimum for some applications. In natural gas plant operations, however, the author generally sees a lean oil heavier by about 10-14 carbon numbers. 

Not all plants produce one major product. Some plants, called multipurpose plants, may use the same equipment to produce a number of different products. This might be true for insecticides or herbicides. The major market for these chemicals is in the spring and early summer. If another product could be produced for fall and winter consumption, such as a de-icer additive for gasolines, then the plant could be operated all year with a minimum amount of storage. Such a plant would probably not be the optimum plant for either process, but the company s over-all profits would be greater than if two separate plants were built. 

Marlin and Hrymak (1997) reviewed a number of industrial applications of RTO, mostly in the petrochemical area. They reported that in practice a maximum change in plant operating variables is allowable with each RTO step. If the computed optimum falls outside these limits, you must implement any changes over several steps, each one using an RTO cycle. Typically, more manipulated variables than controlled variables exist, so some degrees of freedom exist to carry out both economic optimization as well as establish priorities in adjusting manipulated variables while simultaneously carrying out feedback control. 

One important aspect of pilot plant operations relating to the successful operation of a commercial process is the training of operators, supervisors, and analytical personnel. Expertise and knowledge gained can mean several months saved in getting the plant operation to optimum production. 

With many complex reactions, it is useful to have some indication of the maximum value of which can be obtained, even though this may not be the optimum value to achieve in plant operation. If a reaction mechanism is well understood and the flow pattern in a prospective reactor is defined clearly, then it may be possible to obtain an analytical expression for the relative yield of a particular product. In other cases, graphical or numerical procedures may be used. 

The paper discusses a two-step approach to guide an ammonia plant operator in the design, selection, and implementation of the optimum cooling water management program for this cooling system. 

Results are shown graphically in Figure 4 for a brine temperature of 220°F., condenser tube velocity of 5 feet per second, blowdown temperature of 90°F., and brine concentration of twice sea water. As can be seen, a minimum water cost for these conditions is obtained with a 50-stage plant operating with a terminal temperature difference of about 4°F. Similar calculations were made for a blowdown concentration of 1.5 times sea water and for a once-through system. By cross plotting, it was then possible to determine the optimum blowdown salt concentration for the plant. It was about 1.7 times sea water. However, the curve is almost flat in the range of 1.5 to 2.0 times sea water. 

Table I shows a detailed breakdown of the operating cost for this plant. The cost of steam represents about half of the water cost for the optimum plant. The capital charges for the evaporator plant, which includes amortization, interest on working capital, and real estate, represent about 30%. The remaining 15 to 20% is equally divided between the cost of chemicals for scale control and all the other costs. The converted water is estimated to cost approximately 42 cents per thousand gallons. This water cost represents a realistic figure for a large-capacity multistage flash evaporator that could be built today when the energy in the form of steam costs between 35 and 40 cents per million B.t.u.

Optimization can be performed on many different time scales and levels, from production planning over the next year to determining optimal setpoints for a chemical process unit operation every minute. Typical optimization levels in the petrochemical industry include management decisions, process design, and plant operations. In these cases, the solution to the optimization problem will be the one that maximizes some measure of profit. An example of optimization applied to process design is determination of the optimum thickness of insulation for a given steam pipe installation, as shown in Fig. 3. 

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. 

Many of these elements are strongly interrelated with each other and may affect different sections of the plant concept. It is thus a demanding engineering task to arrive at an optimum plant concept, which can only defined by the conditions set by the feedstock price, the site influences, and the economic premises of the customer. An evaluation of the individual merits of the described measures in terms of investment and operational cost in a generalized form is not possible and can be done only from case to case in real project studies. 

Figure 9 shows an experimental set-up for continuous foam separation experiment. Continuous pilot plant operations allow the engineers to determine not only the optimum chemical dosages but also the optimum operational conditions in terms of flows, feed locations, chentical dosages, and so on. 

By varying the blast volume, oxygen concentration, preheating temperature and other factors, the optimum quantity of refuse charge, coke requirement and other conditions for stable plant operation have been studied. 

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