(186i) Integrated Design and Control of Intensified Membrane-Based Hydrogen Production Via Methane Steam Reforming

Significant effort has been invested in order to improve chemical process economic efficiency through process design that aims to optimize economic criteria (e.g., total annualized cost). Chemical process design aims at determining the design point, both structural and operational, at which the process can fulfil the product specifications and meet safety requirements, whereas minimizing both equipment and operational cost. Process synthesis, flowsheet configuration, unit interconnections followed by the calculation of equipment sizing and nominal operating conditions are among the steps of the traditional chemical process design procedure. Usually, the evaluation of alternative process flowsheets is performed within an optimisation framework subject to safety regulations and environmental constraints, which utilizes engineering knowledge and experience in order to define a meaningful design space by eliminating bad possible choices.

Nevertheless, process systems under real operating environments are affected by exogenous disturbances, intrinsic process uncertainties, and switching between different operating conditions. As a result, a control system is employed that compensates the effects of disturbances and uncertainties in order to satisfy product specifications. However, in order to compensate for those disturbances additional resources are needed resulting to increased actual dynamic operational cost of the process. The traditional approach is to design the chemical process followed by the design of an efficient automatic control system, which is essential to achieve an acceptable dynamic performance. In chemical processes there are cases, where the achievable dynamic performance cannot be improved unless design limitations are waived, despite the employment of sophisticated control strategies. It is therefore, quite important to incorporate process operability as an additional criterion during process design in order to ensure the achievement of the desirable dynamic performance. Integrated process and control system design approaches have reached a certain degree of maturity in terms of methods and employed techniques [1-3]. However, the current work investigates a process of significant degree of complexity representative of actual process systems and an increased degree of intensification.

Compared to the conventional method of hydrogen production via methane steam reforming, the employment of a palladium-based membrane reactor allows for the simultaneous separation of hydrogen from the produced synthesis gas mixture. Hence, in integrated membrane steam reforming reactors a higher methane conversion can be achieved at much lower reactor temperature than in conventional reactors. The intensified process therefore results to a highly complex system. The interactions among the reforming reactions mechanism, the convective and molecular diffusion of the reacting mixture species, the diffusion of hydrogen through the membrane, and the thermal effects due to reaction and heat exchange with an external heat source must be then taken into consideration for the optimal design of a highly performing system [4].

This work focuses on the development and implementation of an integrated process design and control framework for the highly intensified process of hydrogen production in a membrane-based reactor via methane steam reforming. More specifically, the framework accounts for the optimal selection of process system design variables (material, process units and their interconnections, and operating conditions) along with the control structure configuration (selection of manipulated and control variables) for a given control algorithm. The design problem is stated as a mixed integer constrained non-linear dynamic optimization problem with an objective function that incorporates both investment and operational costs along with closed-loop dynamic performance criteria. Rigorous dynamic, one-dimensional, nonlinear, pseudo-homogeneous mathematical models of the tubular membrane reactor equipped flowsheets, which consists of mass, energy and momentum balances, validated utilizing experimental data, are used for the modelling of the system. Additionally, a linear model predictive control scheme (MPC) [5] is employed for each alternative flowsheet and in order to evaluate the closed-loop dynamic performance of the process under the presence of multiple disturbances. The disturbance scenario to be simulated consists of a regulatory problem, where the controller has to counteract the effect of multiple simultaneous disturbances, and a servo problem, where the controller has to keep the output close to a given reference input. Simultaneous disturbances associated with the direction of maximum variability for the system are used for the evaluation of the control performance. The direction of maximum variability is estimated by performing a singular value decomposition of the sensitivity matrix, which incorporates variation of process states and eigenvalues with respect to disturbance perturbation around its nominal values. The utilization of the information included in this matrix is essential towards the definition of the direction of maximum variability which is utilized in order to effectively determine the dynamic performance of each flowsheet under consideration by minimizing computational cost (evaluation of the dynamic performance under the worst case instead of every possible disturbance scenario). Given the set of possible design variables, control problem and algorithm and predefined possible disturbance scenarios, a simulated annealing algorithm is utilized for the solution of this highly complex non-linear dynamic optimization problem. After the initialization of the problem, at every iteration of the optimization algorithm, where a new set of design variable is properly chosen, a steady state problem for the calculation of the equipment and operation cost is solved. Successively, the dynamic simulation along with the solution of the control algorithm is performed in order to evaluate the dynamic performance at each design point. This step includes the linearization of the non-linear dynamic model (to be utilized in the MPC) and the calculation of the direction of maximum variability for the determination of the disturbances.

The proposed developments are illustrated in case studies considering the comparative assessment of three alternative flowsheeet design configurations of hydrogen production process via methane steam reforming utilizing membrane-based hydrogen separation. The first flowsheet utilizes an integrated multi-tubular palladium-based membrane reactor which results to a highly intensified process. The second flowsheet utilizes a common tubular reactor connected in series with a tubular palladium based separation module. Several successive reaction-separation steps are used to fully describe the process. The resulting process is less intensified compared to the first flowsheet configuration but its performance (both dynamic and steady state) is limited due to thermodynamic limitations. The last flowsheet under consideration utilizes the successive reactor-separator configuration, as described in the second flowsheet, but with two reaction-separation modules connected in series, where recycle of several streams is also taken into account. In all flowsheets additional units (heat exchangers, flowrate splitters and mixers) are utilized. Optimal designs ensure efficient operation while being able to perform better under dynamic transition compared to the designs obtained by the traditional successive approach of process design followed by the controller design. Additionally, the integrated membrane reactor scheme exhibits superior performance than the cascaded systems due to its ability to better handle the investigated disturbance scenarios. This ability is attributed to the simultaneous hydrogen separation from the traction zone which makes the membrane reactor design more flexible compared to the cascaded designs.

Acknowledgements

Cited References

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