Abstract
Performing a pollution prevention assessment of a chemical process design is typically resource intensive. Time constraints, a lack of resources and limited in-house expertise, particularly in developing nations, have resulted in inconsistent and non-routine identification of pollution prevention opportunities. To address these problems, computer based tools may be developed to enable designers to rapidly understand environmental implications and provide suggestions of process modifications. A software prototype, P2TCP (Pollution Prevention Tool for Continuous Processes) is presented. P2TCP provides support and helps facilitate pollution prevention assessments of continuous chemical processes at all stages of design.

There is a significant need for quantitative methods for comparison of process alternatives. Although chemical and safety hazard rating schemes have a long history, there is no universally accepted method for comparison in terms of potential environmental impacts. Full health and environmental risk analysis is not suitable for conceptual process design decision making but models or indicators can provide a suitable approach. A number of methods are incorporated in P2TCP.

The benefits of heat and mass integration tools for pollution prevention (P2) have been widely demonstrated. However no tool has been developed specifically to assist in the intuitive process of systematically identifying alternatives for continuous chemical processes which may reduce waste generation and energy consumption. P2TCP incorporates an expert system for P2 design analysis which is complementary to existing integration and simulation tools. The key features of the analysis system are presented.

Introduction
The chemical process industries face continually increasing pressure from society and legislation to reduce releases to the environment. Over the past 20 years, waste treatment methods have provided sufficient control against environmental impact for compliance with media specific legislation. As legislation strengthens and impact to all media is simultaneously considered, treatment processes alone no longer provide a cost effective solution. Good house keeping and elimination of some fugitive emissions have demonstrated impressive reduction results with good returns. However, further reductions of wastes and reliance on treatment may be achieved through modification of designs. The identification of modifications and comparison of alternatives is termed a pollution prevention (P2) assessment.

Performing a P2 assessment is typically resource intensive and particularly not suitable to provide decision making during conceptual process design. An informal survey (Fromm 1992) of several process design and project engineers indicated that "P2 is practised in design but not consistently, routinely or in the form of specific design criteria" but typically as a result of reducing material loss, toxic release and avoidance of costly treatment, reflected by an engineer's personal awareness and experience. To address these problems, computer based tools may be developed which enable designers to rapidly understand environmental implications and to suggest alternatives.

Process design simulation tools facilitate the creation of process flow diagrams and provide estimates of waste quantities. However the use of these tools for pollution prevention assessment are limited because they do not incorporate systems for environmental comparison or identification of potential modifications.

Development of an appropriate environmental methodology to date is limited. (Brennan 1992, SERC 1993). Available methods to identify design alternatives still require expert knowledge, a significant number of "skilled" man hours and applicability of these methods, particularly during conceptual process design, is limited. (Fromm 1992, Hethcoat 1990)

P2TCP (Pollution Prevention Tool for Continuous Processes) is a computer based prototype which was developed to assist in the comparison of process alternatives and the identification of potential modifications to reduce environmental impacts. P2TCP is presented in this paper.

Conclusion
A prototype of P2TCP (Pollution Prevention Tool for Continuous Processes) has been developed in response to the demand for a computer based pollution prevention assessment systems which compliments existing mass and heat integration tools. P2TCP comprises of modules for environmental comparison and P2 design analysis of continuous chemical processes. P2TCP is suitable for structured conceptual and retrofit design. The prototype has been validated using a number of case studies.

A number of environmental methodologies have been presented in literature which may be used for computer based comparison of process alternatives. However, no single methodology is appropriate for all situations and uncertainty is typically un-quantified. Several regional and global comparison methodologies which reflect the current state-of-the-art are incorporated into the environmental comparison module of P2TCP.

In P2TCP, three modules represent the consecutive processes required for P2 design analysis; characterisation, diagnosis and prescription. Three process sub-systems are considered which correspond to levels 2 to 4 of hierarchical design. Conflicts within and between sub-systems are identified, including between energy consumption and waste reduction.