Reactors, chemicals

Reactors are at the heart of a chemical process plant They are to be designed, fabricated, tested, erected, commissioned, operated and maintained with utmost care so that the process plant can run efficiently and safely. 

Small-sized reactors are generally fabricated in vendor s workshop and then transported to the purchaser site. 

Large-sized reactors are sometimes fabricated and assembled on-site if it is difficult to transport the entire unit and then erect at site. Internal components such as brick lining, grid supports, heat exchange coils are then assembled or fabricated at the site. 

Rubber, glass and Fibreglass Reinforced Plastic (FRP) lining can be provided in the fabrication shop itself or carried out in another workshop (sub-contracting by the main vendor). The lined reactor should be carefully transported to the site. 

However, the purchaser should put the responsibility of the entire job on one party only (the main vendor) to avoid controversy (e.g., if the lining fails). Protective brick lining is, however, done at site only when (i) the reactor may become too heavy to transport or (ii) there is possibility of the lining getting disturbed during transport. 

Quantitative models of reactor performance are based on the principles of conservation of mass and energy. This chapter focuses on the conservation of mass, but modeling the energy budget of reactors uses the same approach. The rate of accumulation of a substance in the reactor equals the rate of addition minus the rate of removal plus the rate of generation. 

Ideal chemical reactors can be linked in many different ways so that the output of one reactor becomes the input of the next so that the reactions occur in stages. Staged reactor models can be adapted to simulate most, if not all, real-world scenarios. This means that the behavior of complex interacting systems can be simplified to combinations of simple reactors, each of which is relatively easy to model. Ideal chemical reactor models can be applied as a first approximation to most natural situations and the concept easily leads to mathematical descriptions of those situations. For example. 

Tracer behavior is different in each of the ideal chemical reactors, so the first step in developing a tracer kinetics model is to decide which reactor type best simulates the real situation and which kind of tracer dosing has occurred. If the amount and timing of tracer introduction is known, a forward model can be developed. If the amount and timing of tracer detected is known, an inverse model can be produced. The equations derived in the following section use concentration imits of mg/L because these units are typical of field studies. For laboratory experiments the models would use molal concentration units. The mass (M) variables would be replaced by mole quantities ( ). The volume variables (V) would be replaced by mass of water (M) and the flow rate (0 would have units of kg/sec. 

Adding a tracer spike to a batch reactor produces a step increase in the tracer concentration in the reactor and the concentration remains constant thereafter. No tracer is discharged from the reactor and no tracer is generated in the reactor. 

The concentration in the reactor after the spike is added (C, mg/L) is the sum of the initial mass of tracer M mg) and the mass of tracer added in the spike (M, mg) divided by the total volume of solution (T, L). 

When it is necessary to include these effects - slow reaction rates, catalysts, heat transfer, and mass transfer - it can make an engineering problem extremely difficult to solve. Numerical methods are a must, but even numerical methods may stumble at times. This chapter considers only relatively simple chemical reactors, but to work with these you must leam to solve ordinary differential equations as initial value problems. 

Introduction to Chemical Engineering Computing, by Bruce A. Finlayson Copyright 2006 John Wiley Sons, Inc. 

Reactor for production of ethylene by steam-cracking in Burghausen, Germany. Courtesy of Linde Engineering, Germany. 

In this chapter we discuss chemical reactor types and show how, despite the variety of reactions and the almost unlimited variety and possibilities for reactor design, only a few equations are needed to describe the reaction progress. Reactor modeling is, therefore, in many cases comparatively straightforward, and is facilitated by the classification into ideal reactors and real reactors. 

The reactor is either a concentrated system like a well-mixed stirred tank reactor, that is, parameters such as Tand c do not vary within the whole reactor, or a distributed system, that is, the conditions depend on the local position, for example, a tubular reactor with axial gradients of c and often also of T. 

For a basic understanding of chemical reactor design, start with Sections 4.10.1 and 4.10.2, where different ideal and isothermal reactor types are introduced and the respective performance equations are derived. You should then study the behavior of real reactors (non-ideal flow and residence time distribution, Section 4.10.4) and the simplest model to account for deviations of real systems from ideal reactors, the tanks-in-series model (Section 4.10.5). 

Intermediate learners should then also read Section 4.10.3, where non-isothermal ideal reactors and criteria for prevention of thermal runaway are discussed. 

---

Denbigh, K. G., and Turner, J. C. R., Chemical Reactor Theory, 3d ed., Cambridge University Press, Cambridge, England, 1984. 

Rase, H. F., Chemical Reactor Design for Process Plants, vol. 1, Wiley, New York, 1977. 

The search for Turing patterns led to the introduction of several new types of chemical reactor for studying reaction-diffusion events in feedback systems. Coupled with huge advances in imaging and data analysis capabilities, it is now possible to make detailed quantitative measurements on complex spatiotemporal behaviour. A few of the reactor configurations of interest will be mentioned here. 

Modelling plasma chemical systems is a complex task, because these system are far from thennodynamical equilibrium. A complete model includes the external electric circuit, the various physical volume and surface reactions, the space charges and the internal electric fields, the electron kinetics, the homogeneous chemical reactions in the plasma volume as well as the heterogeneous reactions at the walls or electrodes. These reactions are initiated primarily by the electrons. In most cases, plasma chemical reactors work with a flowing gas so that the flow conditions, laminar or turbulent, must be taken into account. As discussed before, the electron gas is not in thennodynamic equilibrium... 

The concept of macroscopic kinetics avoids the difficulties of microscopic kinetics [46, 47] This method allows a very compact description of different non-thennal plasma chemical reactors working with continuous gas flows or closed reactor systems. The state of the plasma chemical reaction is investigated, not in the active plasma zone, but... 

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. 

The most effective phosphoms production technology uses a submerged arc furnace. The submerged arc furnace performs three functions chemical reactor, heat-exchanger, and gas—soHd filter, respectively, each of which requires a significant amount of preparation for the soHd furnace feed materials. 

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. 

H. Kramers and K. R. Westerterp, Elements of Chemical Reactor Design and Operation, Academic Press, Inc., New York, 1963, p. 228. 

Product Recovery. Comparison of the electrochemical cell to a chemical reactor shows the electrochemical cell to have two general features that impact product recovery. CeU product is usuaUy Uquid, can be aqueous, and is likely to contain electrolyte. In addition, there is a second product from the counter electrode, even if this is only a gas. Electrolyte conservation and purity are usual requirements. Because product separation from the starting material may be difficult, use of reaction to completion is desirable ceUs would be mn batch or plug flow. The water balance over the whole flow sheet needs to be considered, especiaUy for divided ceUs where membranes transport a number of moles of water per Earaday. At the inception of a proposed electroorganic process, the product recovery and refining should be included in the evaluation to determine tme viabUity. Thus early ceU work needs to be carried out with the preferred electrolyte/solvent and conversion. The economic aspects of product recovery strategies have been discussed (89). Some process flow sheets are also available (61). 

A thermal oxidizer is a chemical reactor in which the reaction is activated by heat and is characterized by a specific rate of reactant consumption. There are at least two chemical reactants, an oxidizing agent and a reducing agent. The rate of reaction is related both to the nature and to the concentration of reactants, and to the conditions of activation, ie, the temperature (activation), turbulence (mixing of reactants), and time of interaction. 

Stanley M. Walas, Ph.D., Professor Emeritus, Department of Chemical and Petroleum Engineering, University of Kansas Fellow, American Institute of Chemical Engineers (Section 7, Reaction Kinetics Section 23, Chemical Reactors)... 

Perlmutter, D. Stability of Chemical Reactors, Prentice Hall, Englewood Chffs, NJ (1972). 

Packages exist that use various discretizations in the spatial direction and an integration routine in the time variable. PDECOL uses B-sphnes for the spatial direction and various GEAR methods in time (Ref. 247). PDEPACK and DSS (Ref. 247) use finite differences in the spatial direction and GEARB in time (Ref. 66). REACOL (Ref. 106) uses orthogonal collocation in the radial direction and LSODE in the axial direction, while REACFD uses finite difference in the radial direction both codes are restricted to modeling chemical reactors. 

Next consider the case that uses randomized blocking to eliminate the effect of some variable whose effect is of no interest, such as the batch-to-batch variation of the catalysts in the chemical reactor example. Suppose there are k treatments and n experiments in each treatment. The results from nk experiments can be arranged as shown in the block design table within each block, the various treatments are applied in a random order. Compute the block average, the treatment average, as well as the grand average as before. 

Aris, Elementary Chemical Reactor Analysis, Prentice-Hall, 1969. 

Denbigh and Turner, Chemical Reactor Theory, Cambridge, 1971. 

Froment and Bischoff, Chemical Reactor Analysis and Design, Wiley, 1990. 

Horak and Pasek, Design of Industtial Chemical Reactors from Lahota-tory Data, Heyden, 1978. 

Rase, Chemical Reactor Design for Frocess Flants Ftinciples and Case Studies, Wiley, 1977. 

Rose, Chemical Reactor Design in Fractice, Elsevier, 1981. 

Westerterp, van Swaaij, and Beenackers, Chemical Reactor Design and Operation, Wiley, 1984. 

Variables It is possible to identify a large number of variables that influence the design and performance of a chemical reactor with heat transfer, from the vessel size and type catalyst distribution among the beds catalyst type, size, and porosity to the geometry of the heat-transfer surface, such as tube diameter, length, pitch, and so on. Experience has shown, however, that the reactor temperature, and often also the pressure, are the primary variables feed compositions and velocities are of secondary importance and the geometric characteristics of the catalyst and heat-exchange provisions are tertiary factors. Tertiary factors are usually set by standard plant practice. Many of the major optimization studies cited by Westerterp et al. (1984), for instance, are devoted to reactor temperature as a means of optimization. 

Of the 23 studies hsted under Modeling of Chemical Reactors in Sec. 23, a number are optimization oriented. Added to them... 

Westerterp, van Swaaij, and Beenackers (Chemical Reactor Design and Operation, Wiley, 1984, pp. 674—746) also supply many references to other problems in the literature ... 

A number of factors limit the accuracy with which parameters for the design of commercial equipment can be determined. The parameters may depend on transport properties for heat and mass transfer that have been determined under nonreacting conditions. Inevitably, subtle differences exist between large and small scale. Experimental uncertainty is also a factor, so that under good conditions with modern equipment kinetic parameters can never be determined more precisely than 5 to 10 percent (Hofmann, in de Lasa, Chemical Reactor Design and Technology, Martinus Nijhoff, 1986, p. 72). 

Many configurations of laboratory reactors have been employed. Rase (Chemical Reactor Design for Proce.s.s Plants, Wiley, 1977) and Shah (Ga.s-Liquid-Solid Reactor Design, McGraw-Hill, 1979) each have about 25 sketches, and Shah s bibliography has 145 items classified into 22 categories of reactor types. Jankowski et al. (Chemlsche Tech-nik, 30, 441 46 [1978]) illustrate 25 different lands of gradientless laboratory reactors for use with solid catalysts. 

Hofmann, Tndustrial process kinetics and parameter estimation , in ACS Advances in Chemlstiy, 109, 519-534 (1972) "Kinetic data analysis and parameter estimation , in de Lasa, ed.. Chemical Reactor De.sign and Technology, Martinus Nijhoff, 1986, pp. 69-105. 

---