Reactor design rate controlling steps

C designates the non-reactive elemental carbon contained in the char product The chemical reaction is assumed to be the rate controlling step. This assumption is justified in the Discussion of Results. The reactions are considered to be first order with respect to fraction of carbon remaining in coal as well as converted to hydrocarbons and m 1 order with respect to H2 partial pressure. The details of the development of the model is reported elsewhere ( ). The experimental data correlated was obtained from dilute phase operation in an excess of hydrogen atmosphere, so the partial pressure of hydrogen was considered to be approximately equal to the total system pressure and was assumed constant along the length of the reactor. 

Because the main alkylation reactions occur at the interface, both isobutane and olefins in the dispersed droplets are transferred to the interface, and the resulting C5-C16 isoparaffins are transferred from the interface back into the dropletJ Experimental data indicate that such transfer steps are in part at least rate controlling steps. In any case, each droplet acts as a different reaction zone (basically a separate minireactor). As droplets of different compositions and sizes occur in all commercial reactors, the alkylation results differ in various droplets, i.e., different alkylates, RONs, yields, amounts of by-products, etc. Improved results would occur if alkylation reactors could be designed and operated so that all the alkylate was produced only at optimal conditions. 

A brief discussion of the rate-controlling step in coal oxydesulfurization is given on the basis of the shrinking core model. A diree-phase slurry reactor is designed by including die nonideal behavior of die solid phase. 

For sound process design, we need values of numerous design parameters such as fractional phase holdups, pressure drop, dispersion coefficients (the extent of axial mixing) of all the compounds, heat and mass transfer coefficients across a variety of fluid-fluid and fluid-solid interfaces depending on the type of multiphase system, type of reactor, and the rate-controlling steps. To clarify the scope of the case studies selected, their salient features are next listed. 

Obviously, many steps are involved, and any step can be the rate-determining one. In addition, if the reaction is highly endothermic or exothermic (typical of oxidation, hydrogenation reactions), then heat has to be supplied or removed from the reactor. Sometimes the rate of heat transfer may control the overall rate of the reaction. In gas-liquid reactions catalyzed by solid particles, the suspension of catalyst particles can sometimes control the overall rate of reaction. As a first step in the process design portfolio, the rate-controlling step has to be determined, as described below. 

The most important step in design of a commercial multiphase reactor is identification of the rate-controlling step on the large scale. Further, it requires a rate expression for this rate-controlling step in terms of known parameters. 

B.15.1 Conventional Batch Stirred Reactor with Air Sparging for Microcarrier-Supported Cells A Simple Design Methodology for Discerning the Rate-Controlling Step... 

In this review we will limit our scope to the parameters of type (a), only. Below, we summarize new information on each parameter as collected from open literature published since 1980. However, we start with a short discussion of the product of kj and the specific contact area, a, (kj a) because this often is the overall rate controlling step. Fortunately, this parameter is also relatively easy to measure easier than kQa and kj and a separately. Therefore it is always essential to estimate as early as possible in the progress of the reactor design which resistance(s) might be rate controlling because that dictates the information necessary for a sound design and the type of experiments to be done if the available information turns out to be insufficient. 

Also, the conversion is strongly temperature-sensitive suggesting that the reaction step is rate-controlling. Design a commercial sized fluidized bed reactor (find W) to treat 4 metric tons/hr of solid feed of size R = 0.3 mm to 98% conversion. 

Even though unit operation control is not addressed in our plantwide control design procedure until Step 8, it is important to understand up front what all the dominant variables are and their relationship with potential manipulators. This is particularly true if appropriate manipulators are unavailable, in which case design changes must be made. The dominant variables influence several steps in the design procedure, in particular our choice of controlling reactor temperature, production rate, and recycle stream compositions. 

Two types of disturbances are used to test the response of the system a step change in toluene recycle flowrate and a step change in the setpoint of the reactor inlet temperature controller. These two variables are the primary manipulators for production rate. In the results presented below we will explore which of the two is better. In addition we will see how several design parameters (FEHE area and heat-exchanger bypassing) impact the load response of the process. 

This case study is an example of how a common reaction can provide the basis for modeling a novel reaction system a gas-liquid-solid reaction performed in the batch mode the solid in this case is first dissolved followed by chemical reaction with a product of the reactive absorption of the solute gas. Unlike Case Study 11.9, where all steps were in series, here some steps occur in parallel. Moreover, the rate-controlling mechanisms often change with time and process conditions. These facets of the problem are dealt with to determine the maximum production capacity of a reactor, which can often be a cost-determining issue. The lesson here is that maximizing the use of an existing reactor is sometimes preferable to designing a new one. 

Special consideration needs to be given to heterogeneous reactors, in which interaction of the phases is required for the reactions to proceed. In these situations, the rate of reaction may not be the deciding factor in the reactor design. The rate of transport of reactants and products from one phase to another can limit the rate at which products are obtained. For example, if reactants cannot get to the surface of a soHd catalyst faster than they would react at the surface, then the overall (observed) rate of the process is controlled by this mass transfer step. To improve the rate, the mass transfer must be increased. It would be useless to make changes that would affect only the surface reaction rate. Furthermore, if products do not leave the surface rapidly, they may block reaction sites and thus limit the overall rate. Efficient contacting patterns need to be utilized. Hence, fluidized bed reactors (two-phase backmixed emulator), trickle-bed systems (three-phase packed bed emulator), and slurry reactors (three-phase backmixed emulator) have... 

This process is very tricky to control in practice, because it is accompanied by a number of side reactions, which are also very fast for example, formation of nitrogen and water. These are minimized by careful reactor design and very fine temperature and flow rate control. Incredibly, the actual contact time in the reactor is a mere 10 s — a very fast reaction indeed The gases enter at 300 °C and leave at 900 °C due to the highly exothermic nature of the reaction. The catalyst for this step is a platinum-rhodium gauze. ... 

The real power of the model developed in this work lies in the transient or dynamic simulations such as those necessary for control system design. The model we have developed can be used to simulate the effects on the reactor of various process disturbances and input changes. Under normal reactor operating conditions, step or pulse changes in inlet gas temperatures, concentrations, or velocity or changes in cooling rates can significantly affect... 

Step 1. In this process we want to achieve the desired production rate and control the impurity of normal butane in the isobutane product at 2 mol %. Reactor pressure cannot exceed the design operating pressure... 

Figure 11.4 shows what happens when the production rate handle (reactor exit temperature) is changed. The starting conditions are the base-case design where reactor exit temperature is 159CC. The reactor temperature controller is tuned at this operating point. Step changes of 8°C at time 5 minutes and 120 minutes are made in the setpoint of the reactor temperature controller (Figure 11.4a). Decreasing the...

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