Other Unit Operations

In the last several chapters we have discussed the control of the important unit operations of reactors, columns, and heat exchangers. In this chapter we briefly explore typical control structures and plantwide dynamic considerations for a variety of other unit operations. The treatment is at best sketch and at worst superficial, but we hope it is sufficient to provide some appreciation of the types of control systems that are used for these processes. More material of this type can be found in Shinskey (1988). 

Fired furnaces are frequently used in chemical plants to provide energy at high temperatures. If a column, a reactor, or some other unit requires energy at a temperature level above that attainable by steam at reasonable pressure levels, a fired reboiler or heater is used. Steam in a chemical plant is seldom available for process use at pressures above 300 psia. The saturation temperature of 300 psia steam is 417°F. so if the column base temperature is above about 350°F, steam cannot be used. 

Furnaces usually have to deal with on-demand load changes, i.e., a customer instantaneously needs more steam or more heat input to a unit. The control system on the furnace must be set up to respond quickly to these load changes. The process shown in Fig. 7.1 show s a furnace in which a stream is being heated in a furnace. The outlet temperature of the process stream is controlled by adjusting the fuel flowrate. The air flow is ratioed to the fuel flow. This ratio is adjusted by the output signal from an excess oxygen controller that looks at the composition of the stack gas. The use of too much air increases energy consumption, but too little air can lead to air pollution problems due to incomplete combustion. 

An interesting aspect of furnace control is the need to be always on the air-rich side, never on the fuel-rich side. If the furnace became filled with uncombusted fuel and then air was added, the resulting rapid combustion could blow the furnace apart. The same concern makes it important that the start-up of a furnace follow a very carefully thought-out procedure. The control system shown in Fig. 7.1 accomplishes this air-rich operation by the use of several selectors and a lag unit. When the temperature controller calls for more fuel, the air wall increase first before the fuel increases because the low selector on the fuel passes the low signal from the lag to the fuel flow controller while the high selector on the air passes the high signal to the air flow controller. The reverse operation occurs when the temperature controller calls for less fuel The fuel flow decreases first and then the air flow- decreases. 

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Advances in fundamental knowledge of adsorption equihbrium and mass transfer will enable further optimization of the performance of existing adsorbent types. Continuing discoveries of new molecular sieve materials will also provide adsorbents with new combinations of useflil properties. New adsorbents and adsorption processes will be developed to provide needed improvements in pollution control, energy conservation, and the separation of high value chemicals. New process cycles and new hybrid processes linking adsorption with other unit operations will continue to be developed. 

Separation Efficiency. Similarly to other unit operations in chemical engineering, filtration is never complete. Some soflds may leave in the hquid stream, and some Hquid will be entrained with the separated soHds. As emphasis on the separation efficiency of soHds or Hquid varies with application, the two are usually measured separately. Separation of solids is measured by total or fractional recovery, ie, how much of the incoming solids is coUected by the filter. Separation of Hquid usually is measured in how much of it has been left in the filtration cake for a surface filter, ie, moisture content, or in the concentrated slurry for a filter-thickener, ie, solids concentration. 

More shortcut design methods and rules of thumb have been developed for fractionation than probably any other unit operation. For example the paper reprinted in Appendix 5 on development of shortcut equipment design methods contains 18 references for fractionation shortcut methods out of 37 total. Both the process and mechanical aspects of fractionation design have useful rules of thumb. Many of the mechanical design rules of thumb become included in checklists of do s and don ts. 

While working in a plant, a troubleshooter read a pressure gauge daily for several weeks and only realized it was inaccurate when one day the blower was down. The gauge still read about normal operating pressure. Had this have been a distillation unit, it could have been more serious. In distillation service, pressure is a more important variable than in many other unit operations. Relative volatility is a function of pressure. Pressure, or more accurately delta-P, is the best indication of the tower hydraulics. 

Ultrafiltration equipment are combined with other unit operations. The unique combination of unit operations depends on the wastewater characteristics and desired effluent quality, and cost considerations. 

Numerous other unit operations and subsystems can be found in fuel cell systems. It is not however the intent of this handbook to review all of these operations and subsystems that are well documented in many other references [e g., (2,8,9,10)]. For convenience, the unit operations that are commonly found within fuel cell power system are listed below ... 

The ability of the process to interface with other unit operations both upstream (leaching) and downstream (winning) in the overall extraction flow sheet... 

See other UNIT OPERATION OR UNIT PROCESS INCIDENTS See also CATALYTIC HYDROGENATION AUTOCLAVES... 

Naturally, there exist a variety of membrane separation processes depending on the particular separation task [1]. The successful introduction of a membrane process into the production line therefore relies on understanding the basic separation principles as well as on the knowledge of the application limits. As is the case with any other unit operation, the optimum configuration needs to be found in view of the overall production process, and combination with other separation techniques (hybrid processes) often proves advantageous for large-scale applications. 

Micro-organisms present in feed water may adsorb to carbon beds, deionizer resins, filter membranes, and other unit operation surfaces and initiate the formation of a biofilm [2,8],... 

Reactions can be combined with other unit operations, as in the example of reactive stripping in the production of hypochlorous acid (HOC1). An RPB was... 

Airhandlers today are frequently controlled the same way as they were 20 or 30 years ago. For this reason, optimization can often cut their energy consumption in half—a savings that can seldom be achieved in any other unit operation. The optimization goals include the following ... 

Here, some examples of real applications of 03/UV/H202 systems are given. Applications may involve other unit operations such as coagulation, biological carbon filtration, biological oxidation in industrial wastewater treatment, or control of trihalomethane compounds. Examples taken here focus on pollutant degradation or by-product disinfection control. 

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