Level of analysis: Micro (processes and products)
Assessed aspects of sustainability: Environmental, Economic and Social
Main purpose of the assessment: To aid sustainable process design on a life cycle basis, integrating technical, economic, environmental and social criteria.
Description of the methodology: This approach enables identification of relevant sustainability criteria and indicators, comparison of alternatives, sustainability assessment of the overall process design and identification of ‘hot spots’ in the life cycle of the system.
Conventional process design is based on technical and economic criteria, with the system boundary drawn around the process itself. This often leads to designs which are optimised economically around the plant but under-optimised upstream and downstream of the process. Recognising this deficiency in process design, Azapagic et al. (2004 & 2006) have proposed a general methodology for Sustainable Process Design (SPD) to enable consideration on a life cycle basis of economic, environmental and social criteria, alongside the usual technical criteria.
The methodology consists of the three following steps:
Identification of sustainability design criteria and stakeholders
The starting point in SPD is identification of sustainability criteria on which the alternatives, and later the whole design, will be assessed. At the initial stage of design when little information on the whole system may be available, it is proposed to use generic sustainability criteria that apply to any process; some examples are listed in Table 1. Further examples are provided by IChemE (2003), who have developed a set of sustainability indicators specifically for the chemical and process industry. Identification of specific sustainability criteria and their prioritisation for a particular process is then carried out in consultation with stakeholders, who may include employees, investors, neighbouring communities and citizens, non-governmental organizations (NGOs) and government.
Table 1: Examples of sustainability design criteria and indicators
– Capital costs
– Operating costs
– Investments (e.g. decommissioning, health and safety and so on).
– Value added
– Taxes paid
– Other investments (e.g. ethical)
– Environmental liabilities
– Air, water, and land.
Environmental impacts, e.g.
– Global warming
– Ozone depletion
– Human toxicity
– Summer smog
|Provision of employment
Health and safety of:
– Visual Impact
5. Identification and evaluation of alternatives
Evaluation of process alternatives is first carried out on a qualitative basis by identifying advantages and disadvantages of different alternatives with respect to the sustainability criteria identified in the previous step. Ideally, the outcome of this stage should be identification of the most promising processing route, raw materials, energy sources and so on; however, in practice, it is more likely that there will be several feasible and potentially sustainable alternatives. In that case, further, quantitative, assessment of the alternatives is necessary; this is carried out as part of step 3.
6. Sustainability assessment and design optimisation
The qualitative sustainability criteria identified in step i) are translated into the appropriate economic, environmental and social indicators or metrics. The indicators are largely quantitative, although some are expressed as qualitative statements. They are then used to assess economic, environmental and social sustainability of the design on a life cycle basis. In addition to the traditional microeconomic indicators, such as net present value, discounted cash flow analysis, returns on capital invested and so on, it is important to consider the additional economic indicators, as shown in Table 1, including value added and investments in, for example, pollution prevention and decommissioning. LCA is used as a tool for assessing environmental sustainability. Both the life cycles of the plant and the product are considered.
Social sustainability criteria (see Table 1) can be translated into both quantitative and qualitative indicators. For example, provision of employment can be expressed in quantitative terms as ‘number of employees’. The design is then optimised to address the sustainability ‘hot spots’ identified during the assessment.
Sustainable Process Design (SPD) methodology shows how sustainability considerations could be integrated into process design on a life cycle basis.
It enables identification of relevant sustainability criteria and indicators, comparison of alternatives, sustainability assessment of the overall design and identification of ‘hot spots’ in the life cycle of the system. In this way, it is possible to arrive at a design configuration that would ensure the most sustainable performance of the plant and product over their whole life cycles especially by identifying and choosing the alternatives that would not normally be considered in conventional design.
Application of SPD principles at a process design stage would help to improve the level of sustainability of future processes and products.
SPD helps in identifying and addressing the important environmental or social issues at an early stage that otherwise would have remained hidden until the plant is in operation.
Currently, there is little experience in applying SPD in practice. For instance, commercially available flowsheeting packages have yet to incorporate sustainability considerations.
The methodology is quite complex.
Interpretation of results can be difficult, due to numerous criteria that are considered simultaneously.
Opportunities for broadening and deepening LCA
The tool proposes quantitative indicators for process-relevant social and economic sustainability criteria. Social indicators include number of employees, occupation exposure limits, potential safety risks from fire and explosion. Economic criteria include the usual micro-economic aspects (capital and operating costs, NPV etc.) and macroeconomic issues (e.g. contribution to GDP, value added, etc.). These indicators could also be used to broaden LCA for sustainability analysis.
The drive for broader corporate social responsibility also demands consideration of integrating sustainability criteria at the early stage of product formulation i.e. into process design.
Threats for broadening and deepening LCA
Some of SPD’s weakness, such as complexity could act as barriers for wider use of the tool.
8. This section is mainly based on Azapagic et al. (2004 and 2006).
Azapagic, A., Millington, A. and Collett, A. (2004) Process design for sustainability: the case of vinyl chloride monomer, in Azapagic, A., Perdan, S. and Clift, R. (eds). ‘Sustainable Development in Practice: Case Studies for Engineers and Scientists’, 201–249 (John Wiley & Sons, Chichester, UK).
Azapagic, A., Millington, A. and Collett, A. (2006) A Methodology for Integrating Sustainability Considerations into Process Design, Trans IChemE, Part A ChemicalEngineering Research and Design, 84(A6): 439–452.
Institution of Chemical Engineers (IChemE) (2003) The Sustainability Metrics: Sustainable Development Progress Metrics Recommended for the Use in Process Industries (Institution of Chemical Engineers, Rugby, UK).