Identifying the most appropriate structural modification is not straightforward, but mathematical programming guided by thermodynamic insights can be used. Commercial software that is now becoming available allows a retrofit to be carried out one step at a time, leaving the designer in control to accept or reject options at each stage.
Reaching a watershed
The developments in the 1980s have extended beyond heat integration. El-Halwagi and Manousiouthakis  created a new area of mass exchange networks from some of the fundamental ideas for the thermodynamic analysis of heat exchanger networks. Later work focused on the particular problem of water networks [17-19]. The design objective in water-using networks is to minimize consumption by maximizing water reuse. Introducing processes for partial treatment of wastewater, termed regeneration, allowing further reuse or recycling of water, can provide further reduction in water use.
Figure 3 illustrates how various water-using processes can be combined to give a composite curve for the entire system, allowing targets to be set for minimal water supply. Extending the methods of energy integration to water integration is, in principle, straightforward. It is possible, although very difficult, to deal with multiple contaminants rather than a single contaminant  and to handle flow-rate constraints that are often encountered with water-using operations. Add to this the desire to introduce cost optimization and forbidden matches, etc., and the simplistic extension of the energy integration approach to water minimization rapidly runs into problems.
Figure 3. Water system integration
Composite curves for water use allows targets to be set for minimal water consumption.
However, formulating the problem using mathematical programming allows all of the complexities of designing a water system to be included . Multiple contaminants and water sources, flow-rate constraints and forbidden matches, as well as water, effluent-treatment and piping costs all can be included in the problem. Commercial software is available for this new area of process integration.
Keeping refineries gassed up
Changing specifications for gasoline and diesel are causing fundamental alterations in petroleum refining throughout the world. When changing the product specifications at a refinery or optimizing or debottlenecking it, the hydrogen supply to a process is often a limiting factor, particularly when making low-sulfur, low-aromatics products. Sources of hydrogen include recovery from fuel gas and tail gas, dehydrogenation processes, heavy-end gasification, steam reforming and outside purchase. The recovery of hydrogen from off gases is analogous to the recovery of process heat. Developing integrated designs that make better use of available resources can provide similar benefits and allow the best overall operation of the refinery. This requires systematic methods of identifying hydrogen needs and available production to optimize site performance and maintain the flexibility to run different crude-oil feedstocks while maintaining maximum equipment utilization.
Graphical targeting can be used to analyze the hydrogen resources available in the refinery and suggest which off-gases should be sent to hydrogen recovery processes . It also can pinpoint targets for required hydrogen production or off-site purchase (Figure 4).
Figure 4. Refinery hydrogen integration
A hydrogen-surplus diagram provides insights on realistic target for consumption of the gas.
Mathematical programming can handle the details of the pipe work, compressors and the capital cost associated with hydrogen recovery processes. Although the methodology is new, it has already seen many successful applications, and commercial software is available. However, to make the most of opportunities from hydrogen network design, process changes for the hydrogen consumers needs to be included in the methodology. For example, exploiting hydrogen network interactions to increase the feed purity to units, such as a hydrocracker, might bring considerable benefits from increased process yields.
Advancing in columns
Distillation system design also is attracting increasing attention. Before we get into this, it is useful to distinguish between simple and complex columns. A simple column has a reboiler and a condenser, and splits a single feed into two products. Although different configurations are possible when using simple columns, the design problem still is a manageable size. However, separations often require more complex arrangements, including thermal coupling, side strippers, side rectifiers, prefractionators and dividing-wall columns (Figure 5). As the number of products increases, the number of possible complex column configurations explodes. In addition to this problem, we need to choose the most appropriate pressure for each separation in a complex arrangement and to consider heat integration simultaneously.
Figure 5. Complex distillation
Opting for more complex arrangements than a series of simple distillation
columns may offer advantages.
Whereas noteworthy advances have been achieved in the development of process integration design methods for the synthesis of heat-integrated distillation systems, much work remains before the full potential can be exploited commercially. Methods now can synthesize complex column systems, but not those that are heat-integrated. Commercial software can handle certain aspects of the problem. Some progress has been made for a particular case of heat-integrated refinery distillation, but further development work is required .