An educational laboratory experimental system for teaching chemical reaction process dynamics and control

Abstract

Chemical processes are governed by reaction rates that convert one set of chemical species into others. While these reactions are at the heart of all chemical engineering industrial processes, it is surprising that there are few laboratory experiments that introduce students to the time-domain evolution of chemical reactions. The vast majority of commercially available process dynamics and control educational equipment encountered by undergraduate chemical engineering students considers the dynamics of heat and mass transport. Students are able to study the time rate of change of temperature in a process, or the rate of flow in a process, but not the rate of reaction. This paper presents the concept and the design for an inexpensive experimental apparatus that makes the dynamic study of chemical reactions accessible to any undergraduate chemical engineer. The proposed laboratory system allows for an easy connection between the theoretical differential equations used to model such a system and the actual behavior observed experimentally. The experiment uses opacity of a liquid solution as a surrogate for chemical concentration of a reactant thereby allowing a change in concentration to be relatively easily transduced with a light source and photodetector. The experimental hardware consists of a stand in which a transparent beaker with two reactants is placed. A light source, such as a laser pointer, is on one side and a photodetector is on the other. The stand is mounted on a Peltier device allowing for thermal regulation of the reactants. In this paper we use a crystal violet bleaching reaction as the example but others can be substituted. The crystal violet bleaching reaction consists of combining a purple crystal violet solution and a clear sodium hydroxide solution together to form an initially purple solution which turns clear as the crystal violet concentration decreases to zero. The photodetector measures the percent transmittance of light through the solution which is converted to the concentration of crystal violet. The top of the beaker is unobstructed, which allows more crystal violet solution to be added in order to test the dynamic system response. This open setup provides increased flexibility in that liquid can be added at multiple times, something unavailable with a spectrophotometer. Additionally, the Peltier device allows both cooling and heating, demonstrating the direct influence of temperature on reaction rate. In addition to the experimental hardware, this paper describes a simulation software tool developed in Matlab Simulink that is designed to simulate these types of reactive systems. The environment is a graphical programming one, with easy drag-and-drop icons already created so that relatively little programming is required. The simulations provide an intermediate link between the mathematical representation and the physical experiment thereby increasing student understanding. Overall, the simulation and experimental package provide an excellent foundation for understanding reaction rates and chemical processes with first order dynamics. Examples will be given of laboratory exercises that can be performed with the setup. Additionally, there is a discussion of process feedback control opportunities that could be available for advanced undergraduate courses.