Designing Pollution Prevention into the Process
Consider the following recent examples of pollution prevention in chemical process plants:
An Adiponitrile plant in Orange, Texas installed an inexpensive solenoid valve on the line supplying process water to flush the seals of process pumps used in intermittent service. The solenoid, which is connected to the pump control system, shuts off flush water when the pumps are not running. As a result, the plant has reduced its waste water volume by 57 million gallons per year.
The same facility upgraded the catalyst decanting system in a petrochemical process, permitting higher recovery rates of catalyst and higher yield in the process. Solid wastes were reduced by approximately 1,000,000 pounds per year, and the investment in new equipment paid for itself in approximately 18 months.
As part of an overall effort that reduced waste generation by 98%, Dow Corning installed mechanical separators between a fluid bed reactor and a distillation column in a silane manufacture process. The separators reduced the amount of solids which had been settling in the column. The process modification reduced the frequency of maintenance and cut the amount of product lost when solids are pumped out of the column for disposal.
Examples of cost-effective process modifications such as these are becoming increasingly commonplace as pollution prevention becomes a more important component of the chemical industry’s environmental management philosophy. Indeed, through industry initiatives such as Responsible Care™ and the recently adopted ISO 14000 environmental management standards, pollution prevention has become institutionalized as a 'mainstream' environmental management practice.
Given the success of pollution prevention at the plant level, it is not surprising that the industry is increasingly looking to incorporate the concept in the earliest stages of process and product development -- that is, in process and product design.
The integration of environmental considerations in design and optimization is commonly referred to as 'Design for Environment,' or DfE. And it seems to be catching on. A recent review of Environmental Annual Reports (EAR’s) published by leading chemical companies highlighted this movement towards a design for environment perspective as one of the key trends in environmental management practices. And while chemical engineers have not developed as comprehensive a view of DfE as one might encounter in the design of manufactured goods a number of articles have been published which can help process design engineers apply DfE to process design,,
The Role of Heuristic Methods in Design
Applying pollution prevention to the design of new processes presents some interesting technical and organizational challenges. Process design engineers must often make preliminary design decisions on the basis of incomplete information and untested assumptions. As design proceeds and investment in the process is made, these decisions develop enormous 'momentum' of their own, making it increasingly difficult to revise the choices. Compared to product design, where early prototyping can permit the parallel evaluation of several design options, process design 'locks in' choices earlier and with more force, putting even more of a premium on making good choices early on.
In many aspects of process and product design, engineers rely heavily on the use of design heuristics. In the classic engineering design text by Pahl and Beitz, a heuristic is described as 'explicit knowledge [and] non-explicit knowledge…necessary in order to organize the sequence of thinking operations, including modifying operations (searching and finding) and testing operations (checking and assessing).' More commonly, a heuristic is a general procedure or rule of thumb, which is used to suggest solutions or strategies for solving a problem, often in the absence of 'deep' knowledge about a system.