Achieve the Benefits of Intensification through Process Control

Better understanding of process engineering fundamentals and process dynamics will enable the design of improved process control structures for intensified processes.

Process integration and intensification (PII), a means to improve productivity and minimize costs, is an area of active research in academia and development in industry. Lutze et al. (1) define PII as a process design and development option that aims to improve a whole process by integrating unit operations, functions, and phenomena, and/or by adding or enhancing phenomena within an operation.

The chemical process industries (CPI) are experiencing a surge of interest in the development and application of intensified and multifunctional processes (2). PII concepts have been applied in the design of petrochemical processes (3–5). Process intensification objectives, such as reducing capital and operating costs by combining unit operations, have also been applied to pharmaceutical manufacturing (6) and biotechnology-based production processes (7).

PII has been implemented in numerous large-scale systems such as industrial petrochemical distillation units and less-common applications such as microreactors. Its principles have inspired the creation of several technologies, including rotating packed bed systems, side-draw distillation columns (SDCs), dividing-wall columns (DWCs), and reactive distillation columns (RDCs).

From a process design point of view, PII enables multiple operations or phenomena to be combined in a single unit operation and provides opportunities for identifying novel, more sustainable process alternatives. From an economic point of view, intensified unit operations offer multiple benefits including, but not limited to:

• lower capital costs by reducing the equipment required to carry out product synthesis and refinement (8)
• lower energy and operating costs due to better utilization of heat and materials flows (e.g., baffled reactors) (9)
• higher production rates due to the continuous in situ removal of inert or inhibitory material when reaction and separation are combined in one unit operation (e.g., reactive distillation) (7).

However, despite the potential benefits, design and control of these intensified and integrated processes can be complicated; combining operations, tasks, or phenomena reduces degrees of freedom. For example, in a multitower distillation train, each tower has at least four degrees of freedom — reflux, distillate, bottoms flow, and heat input. Therefore, a train with two columns would have at least eight degrees of freedom. However, if that train were combined into a single DWC, it would have only six degrees of freedom — two fewer than the original design.

The complexities brought on by PII contribute to a lack of process understanding, which could be to blame for the general poor control of intensified processes. This article describes some of the complexities of PII and uses case studies to demonstrate practical solutions. The conclusion outlines the lessons learned from each case study and describes ways to tackle potential control pitfalls with respect to PII.