One of the most significant developments in distillation in recent years has been the application of partition, or dividing wall, columns [22-25] (Figure 6). Such a column can yield three essentially pure products with efficiency greater than that possible with conventional designs. These columns allow energy reductions on the order of 30%, together with capital cost reductions, when compared with a series of simple distillation columns. Although the basic idea has been around since 1947, only in recent years has the arrangement been exploited commercially [22, 24, 25]. Commercial software is now available that can simulate such columns, but software that identifies appropriate use in an integrated context isn’;t.
Figure 6. Dividing-wall column
This column provides three essentially pure products, as well as energy
and capital cost savings.
So far, we have considered non-azeotropic systems. With azeotropes, a new set of problems emerges. Graphical techniques have been developed to help designers visualize the difficulties associated with azeotropic and extractive distillation . Unfortunately, these tools are restricted largely to ternary systems. With the use of elaborate graphics, the principles can even be extended to quaternary systems. However, such simple graphical tools cannot handle the most complex cases. Even though much progress has been made in recent years toward a better understanding of azeotropic and extractive distillation, much work still needs to be done. Fundamental knowledge of heterogeneous systems needs to be improved before the potential for simple phase separations can be fully exploited.
Reactive distillation is even more complex . Again, graphical tools can provide insights about simple systems. However, much work remains to be done to fully understand how to synthesize reactive distillation systems for the most complex cases. The development of better simulation tools for this problem has been welcome, but process integration of these systems is at an early stage.
Tackling reactions and beyond
Obstacles certainly need to be overcome in established areas of process integration — and new areas pose enormous challenges. Little work has taken place in new areas to develop methods that can be exploited commercially.
For instance, process integration of reactions systems is still in its infancy. Reactor design generally considers the reactor to be in isolation even though, in most cases, the design interacts closely with the rest of the process. Taking this a step further, catalyst design often occurs earlier and without too much consideration of reactor design. Yet, the design of the catalyst, reactor and separation system should be carried out simultaneously and in an integrated fashion. This is particularly important when the separation system is extremely complex and costly, as is the case with many azeotropic systems. Biochemical reaction systems present similar and, in many ways, more daunting challenges due to the difficulties in modeling such systems at a level of sophistication appropriate for process integration methods.
The process integration of separation systems other than distillation has hardly been considered. Membranes, crystallization and solids-processing operations in general all require further development.
In addition, more attention needs to be paid to batch processes. Whereas heat integration of batch processes usually is not worthwhile, there still is significant potential for improvement through better process integration. The time element in batch processes provides flexibility in design and operation. However, this flexibility is often taken as an excuse for not doing a thorough and optimized design. In these applications, data are often the biggest problem. How can we design and optimize such a process if we don’;t have basic information such as reaction kinetic data? Such problems require experimental investigation (and the design of the experiments) to be conducted in parallel with the design of the process and its integration. The experimental design and the process design must be integrated in a coordinated activity.
The trend in process integration has been moving away from using graphical systems and moving toward employing automated optimization techniques. Graphical techniques still have a role to play, but are limited. We need the power of mathematics combined with graphical displays for better understanding. Lack of software for the newer areas of process integration is a bottleneck. There is plenty of commercial software for energy integration. More recently, software has come on the market for water- and hydrogen-system integration and the design of distillation schemes.
However, software on its own is not enough. To take the fullest advantage of process integration, engineers need to be educated about newer techniques. Although university curricula are already crowded, this education must start at school and continue through professional training. Otherwise, process engineers will miss significant opportunities to improve designs.
Dr. Robin Smith is a professor in the School of Chemical Engineering and Analytical Science at the University of Manchester. A Chartered Engineer, Fellow of the Institution of Chemical Engineers and Fellow of the Royal Academy of Engineering, he is the author of the upcoming book, “Chemical Process Design and Integration.” E-mail him at firstname.lastname@example.org or email@example.com.
1. Hohman, E.C., “Optimum Networks of Heat Exchange,” Ph.D. Thesis, University of Southern California, Los Angeles (1971).