Controllability chemical reactor

The concept of an automatic control system is illustrated in Fig. 2.28, based on a temperature-controlled chemical reactor. 

We suggest that consideration of Nature s ways of running and controlling chemical reactors provides lessons of the ideal reactor of which we can create only primitive copies from our education as chemical engineers. As the Bible says, we are fearfully and wonderfully made. ... 

Most chemically reacting systems tliat we encounter are not tliennodynamically controlled since reactions are often carried out under non-equilibrium conditions where flows of matter or energy prevent tire system from relaxing to equilibrium. Almost all biochemical reactions in living systems are of tliis type as are industrial processes carried out in open chemical reactors. In addition, tire transient dynamics of closed systems may occur on long time scales and resemble tire sustained behaviour of systems in non-equilibrium conditions. A reacting system may behave in unusual ways tliere may be more tlian one stable steady state, tire system may oscillate, sometimes witli a complicated pattern of oscillations, or even show chaotic variations of chemical concentrations. 

Figure 11 shows a system for controlling the water dow to a chemical reactor. The dow is measured by a differential pressure (DP) device. The controller decides on an appropriate control strategy and the control valve manipulates the dow of coolant. The procedure to determine the overall failure rate, the failure probabiUty, and the reUabiUty of the system, assuming a one-year operating period, is outlined hereia. 

Ratio and Multiplicative Feedforward Control. In many physical and chemical processes and portions thereof, it is important to maintain a desired ratio between certain input (independent) variables in order to control certain output (dependent) variables (1,3,6). For example, it is important to maintain the ratio of reactants in certain chemical reactors to control conversion and selectivity the ratio of energy input to material input in a distillation column to control separation the ratio of energy input to material flow in a process heater to control the outlet temperature the fuel—air ratio to ensure proper combustion in a furnace and the ratio of blending components in a blending process. Indeed, the value of maintaining the ratio of independent variables in order more easily to control an output variable occurs in virtually every class of unit operation. 

One such approach is called cascade control, which is routinely used in most modern computer control systems. Consider a chemical reactor, where reac tor temperature is to be controlled by coolant flow to the jacket of the reac tor (Fig. 8-34). The reac tor temperature can be influenced by changes in disturbance variables such as feed rate or feed temperature a feedback controller could be employed to compensate for such disturbances by adjusting a valve on me coolant flow to the reac tor jacket. However, suppose an increase occurs in the... 

Other types of selective systems employ multiple final control elements or multiple controllers. In some applications, several manipulated variables are used to control a single process variable (also called split-range control). Typical examples include the adjustment of both inflow and outflow from a chemic reactor in order to control reactor pressure or the use of both acid and base to control pH in waste-water treatment. In this approach, the selector chooses from several controller outputs which final control element should be adjusted (Marlin, Process Control, McGraw-Hill, New York, 1995). 

Composition Control The first requirement for successflil control of a chemical reactor is to establish the proper stoichiometry, that is, to control the flow rates of the reac tants in the proportions needed... 

The SIMULAR, developed by Hazard Evaluation Laboratory Ltd., is a chemical reactor control and data acquisition system. It can also perform calorimetry measurements and be employed to investigate chemical reaction and unit operations such as mixing, blending, crystallization, and distillation. Ligure 12-24 shows a schematic detail of the SIMULAR, and Ligure 12-25 illustrates the SIMULAR reaction calorimeter with computer controlled solids addition. 

Refining and applying chemical engineering principles to the design and control of the chemical reactors in which devices are fabricated. 

A semiconductor microcircuit is a series of electrically intercoimected films that are laid down by chemical reactions. The successful growth and manipulation of these films depend heavily on proper design of the chemical reactors in which they are laid down, the choice of chemical reagents, separation and purification steps, and the design and operation of sophisticated control systems. Microelectronics based on microcircuits are commonly used in such consumer items as calculators, digital watches, personal computers, and microwave ovens and in information processing units that are used in communication, defense, space exploration, medicine, and education. 

The earth itself is the reaction vessel and chemical plant. The complicated reaction chemistry and thermodynantics involve ntixers, reactors, heat exchangers, separators, and flnid flow pathways that are a scrambled design by nature. Only the sketchiest of flowsheets can be drawn. The chemical reactor has complex and ill-defined geometry and must be operated in intrinsically transient modes by remote control. Overcoming these difficulties is a trae frontier for chemical engineering research. 

Why are the CSTRs worth considering at all They are more expensive per unit volume and less efficient as chemical reactors (except for autocatalysis). In fact, CSTRs are useful for some multiphase reactions, but that is not the situation here. Their potential justification in this example is temperature control. BoiUng (autorefrigerated) reactors can be kept precisely at the desired temperature. The shell-and-tube reactors cost less but offer less effective temperature control. Adiabatic reactors have no control at all, except that can be set. 

The general material balance of Section 1.1 contains an accumulation term that enables its use for unsteady-state reactors. This term is used to solve steady-state design problems by the method of false transients. We turn now to solving real transients. The great majority of chemical reactors are designed for steady-state operation. However, even steady-state reactors must occasionally start up and shut down. Also, an understanding of process dynamics is necessary to design the control systems needed to handle upsets and to enable operation at steady states that would otherwise be unstable. 

CONFINED SPACE A boiler, chamber, pipe, tank, chemical reactor or storage vessel, sewer, vat, flue or similar space into which entry must be controlled by a permit-to-work. 

Chapter 3 concerns the dynamic characteristics of stagewise types of equipment, based on the concept of the well-stirred tank. In this, the various types of stirred-tank chemical reactor operation are considered, together with allowance for heat effects, non-ideal flow, control and safety. Also included is the modelling of stagewise mass transfer applications, based on liquid-liquid extraction, gas absorption and distillation. 

In the processing industry, controllers play a crucial role in keeping our plants running—virtually everything from simply filling up a storage tank to complex separation processes, and to chemical reactors. 

This section is a review of the properties of a first order differential equation model. Our Chapter 2 examples of mixed vessels, stined-tank heater, and homework problems of isothermal stirred-tank chemical reactors all fall into this category. Furthermore, the differential equation may represent either a process or a control system. What we cover here applies to any problem or situation as long as it can be described by a linear first order differential equation. 

We certainly want to respond very differently if the temperature of a chemical reactor is changing at a rate of 100°C/s as opposed to l°C/s. In a way, we want to "project" the error and make corrections accordingly. In contrast, proportional and integral controls are based on the present and the past. Derivative controller action is based on how fast the error is changing with time (rate action control). We can write... 

Continuous flow stirred tank reactors are normally just what the name implies—tanks into which reactants flow and from which a product stream is removed on a continuous basis. CFSTR, CSTR, C-star, and back-mix reactor are only a few of the names applied to the idealized stirred tank flow reactor. We will use the letters CSTR as a shorthand notation in this textbook. The virtues of a stirred tank reactor lie in its simplicity of construction and the relative ease with which it may be controlled. These reactors are used primarily for carrying out liquid phase reactions in the organic chemicals... 

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