Simant R. Upreti

A new robust technique for optimal control of chemical engineering processes

Abstract A new optimal control technique is presented to provide good quality, robust solutions for chemical engineering problems, which are generally non-linear, and highly constrained. The technique neither uses any input of feasible control solution, nor any auxiliary condition. The technique generates optimal control by applying the genetic operations of selection, crossover, and mutation on an initial population of random, binary-coded deviation vectors. Each element of a deviation vector corresponds to a control stage, and is a deviation from some “mean” control value randomized initially for that stage. The deviation, and the mean control value map on to the actual discrete step value of control at that stage. The mapping is logarithmic in beginning, but is later allowed to alternate with a linear one. The genetic operations are periodically followed by the replacement of mean control values by a newly available optimal control solution, and by the size-variation of control domain between its limits. The optimal control technique is successfully tested on four challenging problems of chemical engineering.

Effect of ozone pretreatment on hydrogen production from barley straw

Application of ozone technology to lignocellulosic biohydrogen production was explored with a barley straw. Ozone pretreatment effectively degraded the straw lignin and increased reducing sugar yield. A simultaneous enzyme hydrolysis and dark fermentation experiment was conducted using a mixed anaerobic consortium together with saccharification enzymes. Both untreated and ozonated samples produced hydrogen. Compared to the untreated group, hydrogen produced by the groups ozonated for 15, 30, 45 and 90 min increased 99%, 133%, 166% and 94%, respectively. Some inhibitory effect on hydrogen production was observed with the samples ozonated for 90 min, and the inhibition was on the fermentative microorganisms, not the saccharification enzymes. These results demonstrate that production of biohydrogen from barley straw, a lignocellulosic biomass, can be significantly enhanced by ozone pretreatment.

Modeling, Simulation and Optimal Control of Ethylene Polymerization in Non-Isothermal, High-Pressure Tubular Reactors

Low-density polyethylene (LDPE) is a very important polymer, which is usually produced through a free radical polymerization process in high-pressure tubular reactors. In this study, the mathematical model of this process is developed on the basis of a comprehensive reaction mechanism to accurately determine polymerization rate, and polymer properties under extreme temperature conditions. The model comprises of (i) the mass balances of monomer, initiator, solvent, live radicals and dead polymer chains, and (ii) the energy balances of a tubular LDPE reactor, and its jacket under steady state condition. The model considers the variations in the density and viscosity of reactants along reactor length. Computer simulation of the model is successfully carried out to accurately describe the performance of a non-isothermal, high-pressure, industrial LDPE reactor with multiple initiator injections. Based on the developed non-linear model, the optimal control of the industrial reactor is carried out using genetic algorithms to maximize monomer conversion using jacket temperature as a control function of reactor length. The application of optimal control leads to significant improvements in the final monomer conversion, and capacity utilization of the reactor.

EXPERIMENTAL AND NUMERICAL STUDIES ON MIXING OF YIELD-PSEUDOPLASTIC FLUIDS WITH A COAXIAL MIXER

An anchor coaxial mixer is a combination of an anchor impeller and a central impeller. The central and anchor impellers provide the shear and bulk flows within the mixing tank, respectively. The agitation efficiency is governed by the proper design of this mixing system. In this study, the effects of the central impeller speed, anchor impeller speed, operation mode, and the speed ratio of an anchor-Scaba coaxial impeller were evaluated on its power consumption, mixing time, and flow pattern using electrical resistance tomography (ERT) and computational fluid dynamics (CFD). An ERT system with a five-plane assembly of peripheral sensing rings, each containing 16 stainless steel electrodes, was utilized to measure the mixing time for this coaxial mixer. The sliding mesh (SM) technique in the CFD analysis with the modified Herschel-Bulkley model was applied to simulate the impeller rotation and the rheological behavior of the xanthan gum solution (a pseudoplastic fluid possessing yield stress). To validate the model, the CFD results for power consumption were compared to the experimental data. The mixing times were correlated using the specific power consumption model.

Permeability effect on the concentration-dependent propane dispersion coefficient in vapex

In this paper, the effect of the packed medium permeability on concentration-dependent dispersion coefficient of propane in heavy oil during vapex process is investigated. The experiments are carried out at 21°C using propane at 0.689 MPa pressure. Heavy oil packed with glass beads of different permeabilities is used as a porous medium in a cylindrical geometry. The concentration-dependent dispersion coefficient of propane is optimally determined for three medium permeabilities by matching the experimental live oil and the model predicted production rates. The results show that propane concentration-dependent dispersion coefficient is a unimodal function. The dispersion coefficient of propane (at all concentrations) as well as its solubility in heavy oil increases with the decrease of the packed medium permeability. The properties determined in this work would enable engineers to optimise oil recovery, and perform more accurate simulations and implementations.

Optimal Control of Continuous Infrared Dryers

In this work, the optimal control policies of radiant temperature versus dryer length are determined for the continuous, steady-state operation of infrared dryers. The optimal control objective is to minimize the product humidity subject to a mathematical model of infrared dryers as well as a set of process inequality constraints. A robust optimal control method based on genetic algorithms is applied. Multiple air injections are also considered. The optimal results show considerable reduction in product humidity. Comparisons with corresponding base cases (using maximum possible uniform radiant temperatures) indicate improvements greater than 50% with the application of optimal control policies.

Dispersion of Butane in Vapex: The Effect of Drainage Height~!2009-10-13~!2010-03-25~!2010-06-08~!

In this work, the dispersion of butane in heavy oil and bitumen is experimentally determined during the Vapex process with varying drainage heights. The experiments were performed at room temperature (22 o C) using butane as solvent at the dew point pressure. We used cylindrical models with different heights varying from 15 to 45 cm, packed with uniform mixture of heavy oil and glass beads. From the experiments, production rate dependency towards the drainage height is evaluated along with determination of butane gas solubility, live oil density, and viscosity. Conjugate gradient method is used to optimally determine concentration dependent dispersion coefficient as well as the solvent concentration at the solvent-oil interface by matching up experimental live oil production rates with ones predicted by model. A computational algorithm is developed to simultaneously solve process models of different heights, and optimally determine the corresponding dispersion coefficients.