Title : Optimization of Experiments for Heterogeneous Reactor Processes
The study of chemical kinetics is concerned with the measurement and interpretation of the rates of chemical reactions in the so-called process window. It tracks how quickly, the transition from an initial to the final states of reaction occurs and what compounds are formed. This know-how is necessary to commercialize any reactor process, it helps to determine proper temperature and concentration of substances, amounts of desirable products and by-products produced that is helps designing of chemical reactors and optimization experimental conditions. The necessary condition of correct design and optimization is precision of mapping of the influence of process variables (usually: pressure, temperature, concentrations, etc) on the rate of the process. Kinetic studies can link with determination of reaction mechanism, so they can give a deeper insight into process nature. Further analysis will be presented for two heterogeneous hydrogenation processes namely hydrogenation of propylene and hydrogenation of carbon dioxide on nickel catalysts.
Let's accept, that expenditure of money, time, and other resources can be treated as a total cost of the kinetic research. Kinetic studies are tedious and resulting expensive. For heterogeneous reactor processes, many authors recommended a full research plan (FRP) as a method of determination of the kinetic dependences. FRP assumes that for all process variables the process window is divided on the grids and for each combination of the variables the rate of the process is determined experimentally. This idea is costly, especially for many process variables and the wide process window. For instance, for the first considered here process there are three process variables (temperature, total flow and propene concentration) and number of necessary measurements (NoM) is equal to 49. Designing of experiments (DoE) is one of the most powerful quality improvement techniques for reducing process variation and enhancing investigation effectiveness. It helps to reduce number of experiments that is a cost reduction without or with small precision reduction. To show how simple and efficient this manner of planning experiments is, we propose the application of factorial-design method (FP). This technique speeds up the problem solution by permitting evaluations to be made before completing all experiments. Additionally, it also indicates the relative importance of process variables and possible interactions. The most often DoE method applied in heterogeneous catalysis basis on the FP. Since heterogeneous processes are usually highly nonlinear full factorial plan (FFP) or 3k factorial plan (3kFP) are used, however the 3k design predominates. 2k factorial plan are not used. The goal of the work is presentation of steps taken to determine a kinetic equation of the mentioned hydrogenation processes, application of DoE methods and verification of results. We show that application of factorials of mixed levels - factors at two and three levels - (MFP) helps to the further reduction of costs comparing with the FFP and the 3kFP. For instance, the FFP for the first considered here process includes 27 measurement points, while the MFP – only 8. Experimental verification confirmed the correctness of the concept.
- The results of work help to reduce costs of experimental work by reduction of the number of experiments by application of Design of Experiments methods. In the area of heterogeneous catalysis, we show that remarkable reduction of costs is possible in spite of the fact that those processes are highly non-linear.
- The research results can be easily expanded to other areas and can help to reduce numbers of experiments, and, of course, costs. The specific characters of other processes should be considered.
- The method presented help make the job more efficient.