Minimize Wastes from Specialty Chemical Processes

Chemical Engineering Progress, Jan 2008 by Mueller, Jeffrey, Cipullo, Michael

A set of tools put forth by Britest Ltd. known as "Best Process Design" (2) can be very useful in developing the most cost-effective and lowest-waste processes. This approach can be combined with the stage-gate process development format (3) to ensure that a product does not pass to the next stage of development unless key goals are met - such as producing less than 1 lb of waste per pound of product from the final scaled-up process.

The Britest tools allow scientists and engineers to structure their thinking to make the best use of existing knowledge, while identifying knowledge gaps that can be targeted for experimentation. This method (Figure 1) is broken into six phases:

1. project definition and evaluation

2. process structure analysis

3. duty definition and equipment selection

4. experimental plan

5. risk appraisal

6. project definition statement.

These tools, particularly the process structure analysis phase, have proven effective in the development of new processes. To complete a process structure analysis, the process reactions, proven or theorized, are systematically mapped out to show the path to the desired molecule and the many side reactions that may occur. Then, a driving force analysis (DFA) table is constructed to compare which reactions are expected to be favored, relative reaction rates and heats of reaction, how the reactions are influenced in general by temperature, pH, etc. A subsequent table, a process structure table, may be used to display process step and phase transition information visually. Additional tools are available, including a separation technique selection tool and a rapid process screening tool.

To demonstrate the value of process structure analysis, the following examples illustrate how some of the Britest tools were applied to specialty-chemical processes under development.

Example 1: Purification

This process involves the treatment of a commercial material (A) with a strong base (B) to remove halogenated impurities (C and H, which are present in the raw material feed). Table 1 is a simplified reaction-transformation map for the process. In this case, A is the desired target material; it is already present, and we don't want it to react. Unfortunately, in this purification process, A can react to form additional unwanted byproducts (D, F and G). The salt byproduct formed in the reactions is J. The raw-material feed impurities to be removed are C and H, which are converted to E and I so they can more easily be removed by a downstream process (e.g., extraction into water, crystallization or distillation).

Table 2 presents the driving force analysis for this process. Plus signs indicate that an increase in the concentration or parameter will increase the reaction rate, while a minus sign indicates that the parameter slows the reaction.

Missing knowledge becomes more obvious as the DFA is completed, stimulating additional investigation and experimentation. Process sensitivities should be studied. Knowledge gaps should be filled and risks should be identified when completing the DFA. Options should be evaluated and observations should be validated, with design decisions made based on firm data.