G. Wozny

Design and control of a reaction distillation column including the recovery system

Abstract A complex dynamic model for reactive distillation, which includes control systems and the capability of coupling additional columns is derived. On this basis, a typical reactive distillation column coupled with a recovery column is investigated and optimized. In this paper, the esterification of myristic acid in a reaction column as a typical example is optimized through simulations. To guarantee an almost complete conversion of the acid and a high purity of the reaction by product water, the reaction column is coupled with a recovery column. Both columns are designed based on steady state simulations. To develop a control structure, the reaction columns steady state and dynamic sensitivity of possible disturbances and manipulated variables are analyzed. It is shown, that in this reaction column there is no direct relationship between temperature and purity as is usual in common distillation columns. The influence of the reaction on the temperature profile within the reaction column has to be known, when the temperature is used as a reference variable for the purity. The recovery does not lead to an instability of the system. With this knowledge an efficient control structure is presented for this strongly non-linear coupled system. The purities of the product streams can be guaranteed

Integration of simulated annealing to a simulation tool for dynamic optimization of chemical processes

Dynamic optimization of processes in chemical industry became important in the last decade. Simulated annealing belongs to these optimization procedures, which allow to reach the global optimum, but are very time-consuming. In this paper, the simulated annealing algorithm has been formulated in a way to enable direct call of process simulators coded in the commercial simulation tool SPEEDUP. We apply simulated annealing for the determination of operation policies in a binary distillation. The developed approach allows the prediction of reflux flow rate and reboiler duty in order to minimize the operation time for product switchover of the column.

Theoretical and Experimental Studies on Startup Strategies for a Heat-Integrated Distillation Column System

Because of their higher efficiency of energy utilization, heat-integrated column systems have been widely used in the chemical industry. However, the heat integration leads to difficulties in startup of such columns, i.e. a long startup time, and thus considerable costs will result. In this work, a study consisting of modelling, simulation, optimization and experimental verification is carried out to develop optimal operation strategies for heat-integrated columns to reduce the startup time. A pilot two-pressure column system with bubble-cap trays is considered. More than 35% of the startup time can be reduced in comparison to conventional startup procedure. Heuristics for startup operation of such processes are suggested.

Optimal Operation of a Membrane Reactor Network

In this work, a two-dimensional model for a conventional packed-bed membrane reactor (CPBMR) is developed for the oxidative coupling of methane, which uses a nonselective porous membrane to continuously feed oxygen to the catalytic bed. The model incorporates radial diffusion and thermal conduction and assumes convective transport for the axial direction. In addition, two 10 cm long cooling segments for the CPBMR were implemented based on the idea of a fixed cooling temperature outside the reactor shell. The resulting model is discretized using two-dimensional orthogonal collocation on finite elements with a combination of Hermite polynomials for the radial and Lagrangian polynomials for the axial coordinate. The simulation study shows that it is necessary to make all transport coefficients dependent on local temperatures and compositions. This leads to a simulation with roughly 130,000 variables, which is then used to generate initial points for the optimization of the CPBMR stand-alone operation. In addition, inequality constraints and variable bounds are introduced so as to avoid potentially hazardous mixtures of methane and oxygen in both shell and tube as well as to keep the temperatures below levels stressing reactor materials (< 1,100 °C). Moreover, membrane thickness, feed compositions, temperatures at the reactor inlet and for the cooling segments, diameters of tube and shell, and finally the amount of inert packing in the reactor are considered as decision variables. The optimization procedure uses IPOPT as a solver. Afterwards, the 2D model is integrated into a membrane reactor network (MRN) proposed by H. Godin, 2010 which is simulated. Finally, attempts are made to optimize its operation.

Robust optimization of a reactive semibatch distillation process under uncertainty

Abstract Deterministic optimization has been the common approach for batch distillation operation in previous studies. Since uncertainties exist, the results obtained by deterministic approaches may cause a high risk of constraint violations. In this work, we propose to use a stochastic optimization approach under chance constraints to address this problem. A new scheme for computing the probabilities and their gradients applicable to large scale nonlinear dynamic processes has been developed and applied to a semibatch reactive distillation process. The kinetic parameters and the tray efficiency are considered to be uncertain. The product purity specifications are to be ensured with chance constraints. The comparison of the stochastic results with the deterministic results is presented to indicate the robustness of the stochastic optimization.

Design and modeling of a new periodical-steady state process for the oxidation of sulfur dioxide in the context of an emission free sulfuric acid plant

The oxidation of sulfur dioxide over vanadium pentoxide catalysts represents a basic step in the sulfuric acid production process. In conventional sulfuric acid plants the SO2 oxidation represents the limiting step with respect to the SO2 emissions. Due to the fact that the SO2 oxidation is an equilibrium reaction, sulfuric acid plants always have SO2 emissions. In this work, a new process concept is presented, which uses the transient behaviour of the reaction in two reactors operating under unsteady conditions (Saturated Metal Phase reactor). Besides several advantages, which can increase the efficiency of the whole sulfuric acid process drastically, the SMP Reactor is a key component for an efficient operation of a sulfuric acid plant which reduces the emissions down to zero while keeping the necessary conditions for the hydrogenation unit installed downstream. For this purpose, a mathematical model is used, which describes the dynamic effects of the SO2 oxidation. The model has been experimentally verified in a Miniplant, which works with commercial catalyst pellets.

Optimal production planning under uncertain market conditions

Abstract We propose to use a dynamic stochastic optimization approach to address production planning problems under uncertain market conditions. The problem is formulated as a dynamic mixed-integer chance constrained optimization problem which can be relaxed to an equivalent deterministic MILP formulation. Using this approach, a quantitative relationship between profit achievement and risk of constraints violation can be received, through which the sensitive uncertain variables can be identified. An optimal decision with a desirable trade-off can be made for the future purchase, sales and operation.

Study of a novel heat integrated hybrid pervaporation distillation process: Simulation and experiments

Abstract In the present work a new developed heat integrated hybrid pervaporation distillation process is modeled and experimental studies are carried out to analyze the effect of the heat integration in the process. With the results of the experiments, the model is validated and a comparison between industrial scale non heat integrated and heat integrated processes is done. As a result, three main advantages are presented in the approach: a) reduction of the necessary external energy supply into the process, b) improvement in the pervaporation separation performance and c) reduction in the necessary membrane surface.

Improving the Efficiency of Batch Distillation by a New Operation Mode

Abstract Batch distillation processes are well-known for their high degree of flexibility. A feature of batch distillation is that it produces not only the desired products but also off-cuts. Conventionally, off-cuts are recycled to the reboiler of the column for the next batch. In this work, we propose a new operation mode for batch distillation, namely, the off-cuts will be recycled in form of a continuous feed flow into the column. The separation effect is promoted in this way and thus economical benefits can be achieved. Simulation and optimization based on a rigorous model are carried out to study the properties of this operation mode and develop optimal operating policies. Results of applying this mode to two industrial batch columns show significant improvements of operation efficiency in comparison to the conventional recycle strategy.

Optimization of operating conditions for ferrichrome production in a membrane bioreactor using Ustilago maydis

Abstract In this work, a continous siderophore production system using the phytopathogenic fungus Ustilago maydis is considered. A hybrid process, specifically, a microfiltration membrane bioreactor is employed which is deemed to be advantageous since cells are retained in the vessel while possibly inhibitory products are continously withdrawn from the system. Accordingly, the process is operated at high cell density thereby increasing productivity. Preliminary analysis and studies for steady state optimization result in the existence of steady-state points for the maximum production rate. Thus, the optimization focus is divided into two operation stages: the startup and the steady-state operation. The former operation implies a dynamic optimization where the optimal policies are determined in order to meet the previously established optimal steady-state. Furthermore, the startup period is characterized through a switching operation mode from batch to continuous. However, the overall aim of the optimization is the maximization of the product total amount per time while minimizing the startup period. To keep the production costs at a convenient level, different constraints are included in the optimization problem such as a glucose waste limit, a lower bound for the outlet product concentration, and also technical constratiants which involve upper bounds for the biomass concentration in the reactor. For the simulation and computation of the sensitivities, we propose a new multiple-time-scaling-approach to solve the resulting optimization problem which possesses strong nonlinear properties.