Optimize Plant Water Use from Your Desk

Water pinch analysis and mathematical programming can be carried out using only a spreadsheet to determine the optimal water system for your plant. Examples, including a shale gas to ethylene process, illustrate the concepts.

Water scarcity and poor water quality impact virtually everyone, making water management a critical global issue. For example, Cape Town, South Africa, recently imposed severe water-use restrictions in order to avert a crisis that would have required it to shut off the municipal water supply. Lack of water is not the only problem. A recent United Nations (UN) report states that more than 80% of global wastewater is released to the environment without treatment, and that more than 95% of that wastewater is discharged in the least-developed countries (1).

Water management strategies attempt to minimize water use. A common water management strategy involves the 4Rs (1):

• reduce pollution at the source
• remove contaminants from the wastewater
• reuse wastewater
• recover byproducts.

With many chemical process industries (CPI) facilities facing ever-tightening water discharge limits, as well as the rising cost of wastewater treatment, the chemical engineering community has focused efforts on the first R, i.e., reduction. The emphasis has been on designing more-sustainable chemical processes that produce less wastewater.

Process integration, which aims to create a more-unified process (2), is one of the most promising techniques for improving process designs. It has been applied for water minimization in water-intensive processes since the mid-1990s (3). The technique is similar to pinch analysis for energy management, and is commonly known as a water pinch analysis. The targeting step of water pinch analysis uses graphical and algebraic techniques to determine the minimum freshwater and wastewater flowrates prior to the design of the water network. Once the flowrate targets have been established, a systematic procedure can be used to design a water network that meets the flowrate targets. This maximizes the water recovery in the plant.

Water pinch analysis is limited to a single contaminant and does not address cost optimization. Mathematical programming is an alternative technique that overcomes these limitations, but it does not provide much insight into the process (e.g., how much water can be recycled). It is important to understand the strengths and limitations of these techniques to decide which to use for a given design problem.

This article describes how to use standard spreadsheet software to carry out both water pinch analysis and mathematical programming.