Plant costs controller limitations

Limited Data First, plant data are limited. Unfortunately, those easiest to obtain are not necessarily the most useful. In many cases, the measurements that are absolutely required for accurate model development are unavailable. For those that are available, the sensitivity of the parameter estimate, model evaluation, and/or subsequent conclusion to a particiilar measurement may be very low. Design or control engineers seldom look at model development as the primaiy reason for placing sensors. Further, because equipment is frequently not operated in the intended region, the sensitive locations in space and time have shifted. Finally, because the cost-effectiveness of measurements can be difficult to justify, many plants are underinstru-mented. 

The advantages of thermal incineration are that it is simple in concept, has a wide application, and results in almost complete destruction of pollutants with no liquid or solid residue. Thermal incineration provides an opportunity for heat recovery and has low maintenance requirements and low capital cost. Thermal incineration units for small or moderate exhaust streams are generally compact and light. Such units can be installed on a roof when the plant area is limited. = The main disadvantage is the auxiliary fuel cost, which is partly offset with an efficient heat-recovery system. The formation of nitric oxides during the combustion processes must be reduced by control of excess air temperature, fuel supply, and combustion air distribution at the burner inlet, The formation of thermal NO increases dramatically above 980 Table 13.10)... 

An important reason for preparing study reports is to determine the capital expenditure (investment) of a project [Muthmann, 1984, Prinzing 1985, DAdda 1997]. This is made up of the location-independent ISBL (inside battery limits) process plant costs such as investment in the production plant and the associated control rooms, laboratories, employee facilities, tank farms, and loading and unloading stations, as well as the location-dependent OSBL (outside battery limits) costs of all the service facilities. 

Once a distillation column or train for a continuous chemical process has been designed and built, the three best opportunities for profitable operation via process control are usually (1) maximum-capacity operation if the plant is production limited, (2) minimum-cost operation if the plant is market limited, and (3) increased annual availability (sometimes called utility). The last of these is greatly aided by constraint controls, commonly called overrides, to keep the plant operating safely and smoothly when it mi t otherwise be shut down by interlodu or operator decision. Maximum-capacity or minimum-cost operation is facilitated by the use of on-line models, usually implemented by a digital computer. Fxmctionally these will be, in most cases, sophisticated feedforward compensators, or constraint controls. With these two limited exceptions, optimization is not considered in this book. Since the on-line models to be employed require calibration, some degree of on-line identification is highly desirable. 

Three types of computer control systems are commonly used for pilot-plant instmmentation. The first is a centralized system, usually based on a minicomputer or occasionally a mainframe. These systems have large storage capacities, substantial memories, and much associated equipment. They typically control all the pilot plants in an area or faciUty. Centralized systems are economical if a large number of units are involved but are becoming less common due to their high installation and maintenance costs as well as the limitation that any failure of the central system shuts down all pilot plants involved. 

Stand-alone computer systems, usually based on a personal computer (PC) or programmable logic controller (PLC), provide a separate computer system for each pilot plant. This allows for economical expansion for new units, separates pilot plants completely for maintenance and troubleshooting, and often has the lowest initial cost. Standardization can be a problem and software control, data gathering, and storage packages can be limited in size, scope, and capabiUty these are usually acceptable trade-offs. 

A factor which previously limited installation of automatic corrosion monitoring systems was the cost of cabling between sensors and control room instrumentation-this was particularly relevant to the electrical resistance (ER) systems. Developments to overcome this have included transmitter units at the probe location providing the standard 4-20 mA output (allowing use of standard cable) for onward transmission to data systems or the use of radio linkage which has been successfully used for other process-plant instrumentation. 

Despite their flaws, batch processes have stood the test of time for a number of reasons, the most important of which is the flexibihty it brings to the manufacturer in terms of the range of products that the plant can produce, the feedstocks used to produce them, and the speed at which they can be brought to market with very limited information on physical properties, reaction kinetics, and so on (very few, if any, Michelin-starred chefs have ever measured the rheology or kinetics of their latest culinary creation). This flexibility, however, has a price which comes in the form of lower efficiencies in terms of production, energy, labor, and so on, and ultimately efficiency equates to cost However, one should never underestimate the pull of flexibility particularly, as discussed earlier in the examples of fermentation, where control of important parameters is difficult to achieve. 

While insect control is more often a limiting factor in tropical bean production, there are several diseases that are serious in certain seasons and locations. Bean rust is one of the most widespread diseases, and it can be controlled effectively with protective fungicides, such as elemental sulfur and certain of the carbamic acid derivatives. The economics of bean production usually preclude any costly applications, however, and the problem has generally been turned over to the plant breeder to solve with resistant varieties. At present, the most practical control of bean anthracnose and the bean blights is through the use of clean seed and resistant varieties. Control with fungicides has always proved difficult and of doubtful value. 

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