The processing unit

As previously noted, the processing unit lies at the heart of the control system, processing the signals received from the input devices and providing actuating signals to the output devices. In its very simplest form, it may be a single circuit, as shown in Fig. 13.20, that directly connects an input device, a switch, to an output device, a contactor coil, whose contacts switch a motor 

For many years, processing units in electrotechnical control systems were implemented using electromechanical devices such as relays, switches, push buttons and so on. This discrete component technology remains in widespread use today. One of the disadvantages of this type of technology, particularly in more complex applications, is that as the component count increases so does the amount of cabling needed to interconnect them all, with concomitant increases in cost and unreliabihty, and problems with maintainability. 

One of the main benefits of the standard is that it allows multiple languages to be used within the same PLC. This allows the program developer to select the language best suited to each particular task. It also facilitates the production of modular and structured programs which, in turn, reduces errors and increases programming efficiency. These are significant advantages where software is used in safety applications. 

Devices such as PLCs are used to control the operational functions of 

An example of a design incorporating hardwired safety circuits is shown in Fig. 13.24. This shows how the outputs of the interlocking switch and the emergency stop actuator are routed not only to the input of the PLC 

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Flow-sheet models are used at all stages in the life cycle of a process plant during process development, for process design and retrofits, and for plant operations. Input to the model consists of information normally contained in the process flow sheet. Output from the model is a complete representation of the performance of the plant, including the composition, flow, and properties of all intermediate and product streams and the performance of the process units. 

Item of Equipment An item of equipment is a hardware item that performs a specific purpose. Examples are pumps, heat exchangers, agitators, and the like. A process unit could consist of a single item of equipment, but most process units consist of several items of equipment that must be operated in harmony in order to achieve the function expec ted of the process unit. 

Most refinery process units and equipment are manifolded into a collection unit, called the blowdown system. Blowdown systems provide for the safe handling and disposal of liquids and gases that are either automatically vented from the process units through pressure relief valves, or that are manually drawn from units. Recirculated process streams and cooling water streams are often manually purged to prevent the continued buildup of contaminants in the stream. Part or all of the contents of equipment can also be purged to the blowdown system prior to shutdown before normal or emergency shutdowns. 

Perfect tracer Tracer molecules behave identically with the process fluid molecules within the process unit. 

There are a variety of ways of accomplishing a particular unit operation. Alternative types of process equipment have different inherently safer characteristics such as inventory, operating conditions, operating techniques, mechanical complexity, and forgiveness (i.e., the process/unit operation is inclined to move itself toward a safe region, rather than unsafe). For example, to complete a reaction step, the designer could select a continuous stirred tank reactor (CSTR), a small tubular reactor, or a distillation tower to process the reaction. 

As far as refrigeration is concerned, this system produces ammonia liquid as the refrigerant that is evaporated in the process unit requiring the refrigeration. In principle this is the same operation at the evaporator as if the system were one of compression. The method of removal of the heat of the ammonia vapor and then the reliquelaction are the points of difference between the systems. 

A small engineering building is located 350 ft (107 m) from the process unit discussed in Example 8. It has an occupancy load of 500 person-hours, which exceeds the company s occupancy criteria. The building is constructed of unreinforced concrete and contains several windows. Earlier calculations estimated the incident side-on overpressure to be 0.5 psi at 350 ft (0.069 bar at 105 m). 

As discussed in Chapter 1, nonessential personnel, including secretarial, laboratory, and engineering and management work groups, are sometimes located near process areas. Buildings that support other activities, such as maintenance shops, instrument and electrical shops, and purchasing and stores, may also be located close enough to the process units to be possibly affected by an event of concern. 

Figure 6.2 shows two process units. Process Unit 1 handles propane vapor at relatively low pressure, while Process Unit 2 periodically handles large volumes of nitrogen gas at elevated pressures. Six buildings are situated in or near the process units. These are ... 

Finally, the facility is considering constructing new engineering offices near the process units. Guidance on the location of the new offices is also desired as part of the study. 

The company had no consistent design and spacing standards that applied to the process units being evaluated. Therefore, the decision was made to perform consequence modeling as a next step in the screening. 

Denver, Colorado (Refs. 16 and 19) 3 (0 in buildings) A propane release at a polymerization unit in a process plant resulted in a blast that destroyed the process unit. The blast-resistant control house, located only 98 ft (30 m) from the blast center, sustained little damage. 

Signal transmission limitations of pneumatic control systems made it necessary to limit the distance between the control house and the transmitter or control valve. As a result, early control houses were located within or at the periphery of the process unit. 

Building 4—Wood trailers, housing various contractor personnel. The trailers are permanently located near the process units, housing seven occupants for 40 hours a week. 

Batch processes are designed to operate intermittently. Some, or all, the process units being frequently shut down and started up. 

The values of the split-fraction coefficients will depend on the function of the processing unit and the constraints on the stream flow-rates and compositions. Listed below are suggested first trial values, and the basis for selecting the particular value for each component. 

The split-fraction coefficients can be estimated by considering the function of the process unit, and by making use of any constraints on the stream flows and compositions that arise from considerations of product quality, safety, phase equilibria, other thermodynamic relationships and general process and mechanical design considerations. The procedure is similar to the techniques used for the manual calculation of material balances discussed in Section 4.3. 

The basis of the F El is a Material Factor (MF). The MF is then multiplied by a Unit Hazard Factor, F3, to determine the F El for the process unit. The Unit Hazard factor is the product of two factors which take account of the hazards inherent in the operation of the particular process unit the general and special process hazards, see Figure 9.2. 

The first step is to calculate the Damage factor for the unit. The Damage factor depends on the value of the Material factor and the Process unit hazards factor (F3 in Figure 2). It is determined using Figure 8 in the Dow Guide. 

The individual fire and explosion indexes are combined to give an overall index for the process unit. The overall index is the most important in assessing the potential hazard. 

The process units and ancillary buildings should be laid out to give the most economical flow of materials and personnel around the site. Hazardous processes must be located at a safe distance from other buildings. Consideration must also be given to the future expansion of the site. The ancillary buildings and services required on a site, in addition to the main processing units (buildings), will include ... 

The location of the principal ancillary buildings should then be decided. They should be arranged so as to minimise the time spent by personnel in travelling between buildings. Administration offices and laboratories, in which a relatively large number of people will be working, should be located well away from potentially hazardous processes. Control rooms will normally be located adjacent to the processing units, but with potentially hazardous processes may have to be sited at a safer distance. 

Utility buildings should be sited to give the most economical run of pipes to and from the process units. 

The main storage areas should be placed between the loading and unloading facilities and the process units they serve. Storage tanks containing hazardous materials should be sited at least 70 m (200 ft) from the site boundary. 

The processing units of most large chemical plants today are not located inside buildings. This is true as far north as Michigan. The only equipment enclosed in buildings is that which must be protected from the weather, or batch equipment that requires constant attention from operators. Much of the batch equipment used today does not fit this category. It is highly automated and does not need to be enclosed. 

The networks that interconnect various process units and vessels to the discharge zones or flares occur widely in refineries and chemical plants. Figure 11 shows a typical configuration in which the root represents the flare, the terminal vertices represent the relief valves, and the edge (each labeled with an arabic numeral) represents a pipe section between two physical junctions (valves, flare, or pipe joints). The configuration of such a network is dictated by the layout of the process unit. In this discussion both the lengths of the pipe sections and the interconnections will be treated as specified variables. 

The design of the network calls for the selection of pipe diameters such that the discharge through each valve attains the maximum (sonic) velocity for an initial transitory period. Since the flare pressure and the process unit pressures are specified, this requirement amounts to the stipulation of a maximum allowable pressure drop over each path Sj (labeled with a roman numeral) from the valve to the flare. The optimal design in this case may be formulated as the following constrained minimization problem ... 

Continuous benzene alkylation was conducted in a reactive distillation column of the type illustrated in Figure 1. The process unit comprises the following principal elements a double column of solid catalyst 32, packing columns above and below the catalyst bed, a liquid reboiler 42 fitted with a liquid bottoms product takeoff 44, a condenser 21 fitted with a water collection and takeoff, and a feed inlet... 

Reuse of this nature has a number of advantages. Firstly, wastewater produced is reduced since it is reused in product. In situations where the amount of water used in product is more than the amount of wastewater produced, it is then possible that all the water is reused and there is no effluent from a cleaning operation. Furthermore, the reuse allows for the capturing of the product residue that is left in the processing unit. This could account for substantial economic gain. Lastly, the amount of water that is used in product is reduced and water that would normally be discarded is now used in product, which also allows for some economic gain. 

The first minor change to the mass balance constraints from the scheduling formulation is found in constraint (8.2), which defines the size of a batch. In the synthesis formulation, the batch size is determined by the optimal size of the processing unit. Due to this being a variable, constraint (8.2) is reformulated to reflect this and is given in constraint (8.59). The nonlinearity present in constraint (8.59) is linearised exactly using Glover transformation (1975) as presented in Chapter 4. 

The amount of water used for a cleaning operation is also now variable and is dependent on the size of the processing unit. Constraint (8.6) is reformulated to account for this and takes the form of constraint (8.60). Constraint (8.60) shows that the amount of water used is related to the size of the processing unit through a proportionality factor. Once again there is a nonlinear term present in constraint... 

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