Simultaneous Design and Control of Pressure Swing Adsorption Processes

Simultaneous Design and Control of Pressure Swing Adsorption Processes

In the last few decades, pressure swing adsorption (PSA) has evidenced substantial growth in terms of size, versatility and complexity. In addition to handling multi-component separation and purification, it offers tremendous flexibility at the design stage, requiring careful selection of key decision variables including number of beds, time duration of process steps, bed length and diameter. Further challenges are posed by the fact that the PSA operation is periodic in nature and never attains a true steady state. Traditional approach for the “design” of such process systems usually employs a two step, first-design-and-then-control, sequential treatment. In contrast, integrated design and control (IDC) approach takes into account the fact that it is the design of the system which determines its dynamic controllability. Its potential for significant design improvements in terms of superior real time operability has been realized from quite some time now. However, early contributions to this field mostly performed analysis on the linear dynamic representation of the original system. The application of IDC approach to inherently dynamic and highly nonlinear system like PSA remains a challenging task and has never been attempted before.

In this work, a systematic and rigorous methodology for the simultaneous design and control of PSA systems, incorporating a comprehensive treatment of its design and operational constraints is presented. Towards this goal, a detailed first principle based model is first developed for a double bed, 6 step PSA system (Khajuria and Pistikopoulos, 2010). In the next step, the full scale integrated design and control dynamic optimization problem is formulated and solved incorporating all operational constraints (Bansal, et.al., 2000; Mohideen, et.al., 1996; Sakizlis, et.al., 2004). The design objective for the PSA system separating 70 % H₂ - 30 % CH₄ mixture into high purity hydrogen, is to maximize the H₂ recovery, while the controller objective is to minimize the integral square error (ISE) in purity from a set point value of 99.99 %, for time varying disturbances in feed temperature and feed rate. Furthermore, detailed studies are also conducted to compare the optimal design obtained from integrated design and control case in terms of hydrogen recovery and disturbance rejection, as compared to sequential design and control case.

References:

Bansal, V.; Perkins, J.; Pistikopoulos, E.; Ross, R. & van Schijndel, J. (2000), 'Simultaneous design and control optimisation under uncertainty', Computers & Chemical Engineering 24(2-7), 261 - 266.

Khajuria, H. & Pistikopoulos, E. N. (2011), 'Dynamic modeling and explicit/multi-parametric MPC control of pressure swing adsorption systems', Journal of Process Control 21(1), 151 - 163.

Mohideen, M. J.; Perkins, J. D. & Pistikopoulos, E. N. (1996), 'Optimal Design of Dynamic Systems under Uncertainty', AlChE Journal 42(8), 2251-2272.

Sakizlis, V.; Perkins, J. D. & Pistikopoulos, E. N. (2004), 'Recent advances in optimization-based simultaneous process and control design', Computers & Chemical Engineering 28(10), 2069 - 2086.