In this work, a reactive distillation process for the production of a fuel additive has been modelled, simulated and controlled using proportional-integral-derivative (PID) control method. The fuel additive considered was isopropyl alcohol that was produced from the reaction occurring between propylene and water, with diisopropyl ether as a side product. In accomplishing the work, the ChemCAD model of the process was first developed using SCDS Distillation Column #1 and the fluid package employed was UNIFAC property model. The ChemCAD column had 15 stages where the feed stream for water was the 6th stage and the one propylene was the 10th stage; the section of the column between the two feed streams was the reaction section of the column. After simulating the developed ChemCAD model to convergence, it was converted to dynamic type from which the dynamic responses of the system were generated and used with the aid of MATLAB to develop transfer function model having the reboiler duty, the reflux ratio and the temperature of the bottom product as the input variable, the disturbance and the output variables of the process, respectively. The obtained transfer function of the model was used to develop both open-loop and closed-loop Simulink models for the process that were used to carry out the open-loop and the closed-loop simulations of the process. The closed-loop simulation was carried out with the desire of achieving a fuel additive product with a mole fraction of 0.97. This was accomplished using a PID controller applied inferentially via the product temperature and tuned by trial-and-error technique. It was observed from the results obtained that isopropyl alcohol could be produced successfully from a reaction between propylene and water using reactive distillation suppressing the associated side reaction. It was also found out that it is possible to control the mole fraction of isopropyl alcohol inferentially using bottom temperature because temperature and mole fraction have been found to be dependent on each other. Finally, it has has been shown that a reactive distillation has been controlled to give high purity of isopropyl alcohol of approximately 0.97 mole fraction as the bottom product in the developed reactive distillation column using the PID controller with trial-and-error tuning technique.
Keywords: Fuel additive, reactive distillation, ChemCAD, MATLAB/Simulink, proportional-integral-derivative.
1.1 Background of Study
Agreeing to statistics that has been provided from U.S Energy Information Administration in 2007 annual report on rapid increase of demand for petroleum and gas production. World demand for oil is projected to increase by 37% over 2006 levels by 2030. It is because the oil is widely used in many industries such as transportation, manufacturing, polymers, shipment and others. Transportation consumes major amount of the energy and increase year by year. This growth has largely come from new demand for personal-use vehicles powered by internal combustion engines. There is endless need to reduce carbon emissions and problems encountered with biodiesel blends, such as fuel system corrosion, increased fuel foaming and water separation. Fuel additives are compounds put together to increase the quality and efficiency of the fuels used in motor vehicles through treatments. Cars and trucks are predicted to cause the highest demand in the transportation approaching to 75%. In other to reduce the consumption of fuel as well as improvement of gas produced during combustion, isopropyl alcohol (IPA) is used as an additive in the fuel.
IPA is used in gasoline blending as an octane enhancer to improve hydrocarbon combustion efficiency. It is primarily produced by combining water and propene in a hydration reaction. It is also produced by hydrogenating acetone. In the conventional process, separate system between reactor and separation units are used. This technology features a two-stage reactor system of which the first reactor is operated in a recycle mode. With this method, a slight expansion of the catalyst bed is achieved which ensures very uniform concentration profiles within the reactor and can avoid hot spot formation. Undesired side reactions, such as the formation of diisopropyl ether (DIPE) also can be minimized.
Nowadays, the search for a novel method to replace the conventional one has been a major interest both in industry and academia. This novel method is learnt to give high conversion of fuel additives economically (Giwa and Giwa, S.O., 2013), and it is know as “reactive distillation”. Reactive distillation is a process in which the chemical reactor is also the still (an apparatus used to distill liquid mixtures by heating to selectively boil and then cooling to condense the vapor). Separation of the product from the reaction mixture does not need a separate distillation step, which saves energy (for heating) and materials.
Furthermore, reactive distillation is a process that combines chemical reactions and physical separations into a single unit operation. This process, as a whole, is not a new concept as the first patent dates back to 1920. The initial publications on this process dealt with homogeneous self-catalyzed reactions such as esterifications and hydrolysis, but heterogeneous catalysis in reactive distillation is a more recent development. While the concept existed much earlier, the first real- world of the system implementation of reactive distillation took place only in the 1980s.
The relatively large amount of new interest in reactive distillation is due to the numerous advantages it has over typical distillation. It can enhance reaction rates, increased conversion, enhanced reaction selectivity. Also, heat integration benefits and reduced operating costs are part of the benefits associated with reactive distillation. All these factors contribute to the growing commercial importance of reactive distillation.
However, since heat transfer, mass transfer, and reactions are all occurring simultaneously, the dynamics which can be exhibited by catalytic distillation columns can be considerably more complex than found in regular columns. These results in an increase in the complexity of process operations and the control structure installed to regulate the process.
1.2 Problem statement
The conventional method of isopropyl alcohol (a fuel additive) production is not only ineffective in handling the side reaction involved in the process but also very costly because many pieces of equipment (reactors, separators, etc) are required by it. The inefficiency of this process to suppress those side reactions as well as its high cost are the major problems identified it and to which solutions must to be proffered. One approach of solving this problem is by developing a control algorithm that will be able to make the process behave as desired.
1.3 Aim and Objectives
The aim of this project to carry out proportional-derivative-integral (PID) control of a reactive distillation process for fuel additive (isopropyl alcohol) production. In order to achieve this aim, the following objectives are set:
developing ChemCAD model of the process,
simulating the developed ChemCAD process model for both steady-state and dynamics to generate dynamic response data,
developing the process transfer functions with the aid of MATLAB using the generated data,
developing the Simulink mode of the process using the developed transfer function,
carrying out the open-loop simulation of the transfer function in Simulink environment, and
applying an appropriate method to tune the controller and simulating the control system of the process for both servo and regulatory cases.
1.5 Scope of Study
This work is limited to applying ChemCAD and MATLAB/Simulink to model, simulate and control a reactive distillation process for fuel additive production.
1.6 Significant of Study
The successful completion of this work will provide the parameters required to be inputed into a PID controller used for the control of a reactive distillation process in order to obtain a very high purity of isopropyl alcohol, which is a fuel additive.