Software helps to save water

More commonly known as 'pinch technology', process integration has become a mainstream process engineering technique for optimising a plant's heat and energy demands. More recently, it has also been applied to water flow and consumption, with encouraging results. Much of this 'water pinch' development has been done by leading process integration specialists such as Linnhoff March and Aspen Technology, as part of their consultancy services. But a new software package from the Engineering Sciences Data Unit aims to make the technology more accessible to end-users in industry.

The Aquaint software forms part of ESDU's Guide to Industrial Water Conservation and, says Helen Polley, 'uses process integration methods to identify opportunities for switching between freshwater and reusing dirty water'. Dr Polley is with Technology Transfer Services (www.pinchtechnology.com), and provides ESDU with technical consulting for process engineering projects. 'A key practical method for water conservation,' she says, 'is the re-use of spent water. Economic factors driving re-use include capital costs, the unit cost of raw water, the cost of water disposal, and the environmental liability and cost of non-compliance with discharge limits.'

Increasing pressure

Add to these factors new cooling water intake regulations, and the pressures on industry to improve its water usage become clear. In response to these pressures, a working party of industry experts contributed to and guided Aquaint's development. 'The software', says Polley, 'explains the technology in a way that removes the 'mystery', and it is available at a price designed to make the option of an in-house water re-use project feasible for many companies. It offers a 'what if' capability that explores the implications of water re-use, based on constraints set by the user.'

As with other process engineering operations, the first step is to do a mass balance — or here, a 'water balance'. And to do this, you need data. Aquaint's design helps the user to understand the water system and to prioritise the data collection operations that are critical to setting the freshwater demand of the system. Menu bars help users navigate through record entry sheets for operations, water supplies and water demands.

The goal is to allow engineers to find the minimum amount of water needed by a system and to design a network that uses the minimum amount of freshwater. The user guide and software make the process simpler by helping to identify where to focus the data collection.

Care is taken to identify small or low volume flows with high contaminant levels', says Polley, 'and streams that can be re-used are identified, along with any constraints that would prevent stream mixing.' Again, the software is designed to keep this analysis as simple as possible, while still including everything critical to the design problem.The next stage is almost classical pinch technology. To calculate the minimum freshwater 'target', operations that use water can be represented on a plot of contaminant concentration vs contamination

load — for efficient operation, contaminant concentration in the process stream must be higher than that in the water stream at all points. The size of the difference depends on the process, with the maximum inlet and outlet concentrations defining the 'limiting water line'.

Composite curves

Now most opportunities for water re-use occur in systems with more than one operation. So the 'limiting water line' or profile of the whole system has to be considered. This profile forms a composite line that shows the constraints on the system design. Any water line falling below the composite is acceptable for use, but where the supply line touches the composite curve a 'pinch' is created — the value of this pinch concentration being the important focus of the water network design.

From the data collected, Aquaint generates these graphs automatically to calculate the system's minimum freshwater target. To minimise water consumption, all water fed to the process must be taken to the pinch concentration; and water with a concentration below the pinch must not be fed directly to any operation above the pinch.

What this means in practice is that all spent water with a concentration below the pinch value should be re-used within the system. And spent water of concentration equal to or greater than the pinch value would generally not be re-used within the system — although it could possibly be used elsewhere on the site.