Unit operations, control chemical reactor

We conclude that most reaction systems in the chemical industries are exothermic. This has some immediate consequences in terms of unit operation control. For instance, the control system must ensure that the reaction heat is removed from the reactor to maintain a steady state. Failure to remove the heat of reaction would lead to an.accumulation of heat within the system and raise the temperature. Forreversible reactions this would cause a lack of conversion of the reactants into products and would be uneconomical. For irreversible reactions the consequences are more drastic. Due to the rapid escalation in reaction rate with temperature we will have reaction runaway leading to excessive by-product formation, catalyst deactivation, or in the worst case a complete failure of the reactor possibly leading to an environmental release, fire, or explosion. 

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. 

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. 

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. 

Chemical engineering is no longer confined to purely physical processes and the unit operations, and a number of important new topics, including reactor design, automatic control of plants, biochemical engineering, and the use of computers for both process design and control of chemical plant will be covered in a forthcoming Volume 3 which is in course of preparation. 

The key component in any process is the chemical reactor if it can handle impure raw materials or not produce impurities in the product, the savings in a process can be far greater than if we simply build better separation units. In typical chemical processes the capital and operating costs of the reactor may be only 10 to 25% of the total, with separation units dominating the size and cost of the process. Yet the performance of the chemical reactor totally controls the costs and modes of operation of these expensive separation units, and thus the chemical reactor largely controls the overall economics of most processes. Improvements in the reactor usually have enormous impact on upstream and downstream separation processes. 

The area of reactor design has been widely studied, and there are many excellent textbooks that cover this subject. Most of the emphasis in these books is on steady-state operation. Dynamics are also considered, but mostly from the mathematical standpoint (openloop instability, multiple steady states, and bifurcation analysis). The subject of developing effective stable closedloop control systems for chemical reactors is treated only very lightly in these textbooks. The important practical issues involved in providing reactor control systems that achieve safe, economic, and consistent operation of these complex units are seldom understood by both students and practicing chemical engineers. 

The field of chemical process miniaturization is growing at a rapid pace with promising improvements in process control, product quality, and safety, (1,2). Microreactors typically have fluidic conduits or channels on the order of tens to hundreds of micrometers. With large surface area-to-volume ratios, rapid heat and mass transfer can be accomplished with accompanying improvements in yield and selectivity in reactive systems. Microscale devices are also being examined as a platform for traditional unit operations such as membrane reactors in which a rapid removal of reaction-inhibiting products can significantly boost product yields (3-6). 

The book is divided into three parts. Part I surveys concepts for heat-integrated chemical reactors, with special focus on coupling reactions and heat transfer in fixed beds and in fuel cells. Part II is dedicated to the conceptual design, control and analysis of chemical processes with integrated separation steps, whilst Part III focuses on how mechanical unit operations can be integrated into chemical reactors. 

The dynamic simulation file prepared in Aspen Plus is exported in Aspen Dynamics [10]. We select the flow-driven simulation mode. Aspen Dynamics files have already implemented the basic control loops for levels and pressures. Units with fast dynamics, such as the evaporator or some heat exchangers, may be handled as steady state. The implementation of control loops for the key operational units, chemical reactor and distillation columns, take into account some specific issues from the plantwide perspective, which are developed in detail in Luyben et al. [8]. 

Many effective control schemes have been established over the years for individual chemical units (Shinskey, 1988), For example, a tubular reactor usually requires control of inlet temperature. High-temperature endothermic reactions typically have a control system to adjust the fuel flowrate to a furnace supplying energy to the reactor. Crystallizers require manipulation of refrigeration load to control temperatui e. Oxygen concentration in the stack gas from a furnace is controlled to prevent excess fuel usage. Liquid solvent feed flow to an absorber is controlled as some ratio to the gas feed. We deal with the control of various unit operations in Chaps. 4 through 7. 

Most importantly, we introduced the ideas of dominance, effective degrees of freedom, and partial control for chemical reactors. In essence, dominant variables are controlled by manipulators with a fast response and the setpoints are adjusted to achieve the economic objectives. These notions are useful in this context, but they can be utilized more widely for other unit operations. 

Step 3 of our plantwide control design procedure involves two activities. The first is to design the control loops for the removal of heat from exothermic chemical reactors. We dealt with this problem in Chap. 4, where we showed various methods to remove heat from exothermic reactors and how to control the temperature in such reactors. At that point we assumed that the heat was removed directly and permanently from the process (e.g., by cooling water). How-ever. it is wasteful to discard the reactor heat to plant utilities when we need to add heat in other unit operations within the process. Instead, a more efficient alternative is to heat-integrate various parts of the plant by the use of process-to-process heat exchangers. 

The goal of this book is to help chemical engineering students and practicing engineers develop effective control structures for chemical and petroleum plants. Our focus is on the entire plant, not just the individual unit operations. An apparently appropriate control scheme for a single reactor or distillation column may actually lead to an inoperable plant when that reactor or column is connected to other unit operations in a process with recycle streams and energy integration. 

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