Intelligent Networking
of Process Wastewater Streams in the Chemical Industry
During the last 15 years, process integration techniques based on pinch analysis have been successfully applied to improving thermal efficiency in the chemical and process industries. Analogous techniques have now been developed for water conservation and wastewater minimisation.
Initial applications of WaterPinch in oil refining and petrochemical processes have shown promising results with "easy" water savings of 15-25% from simple piping and control changes. From so-called process modifications and selective wastewater regeneration savings are greater, often exceeding 50%. Furthermore, WaterPinch analysis has been used for the design of distributed effluent treatment systems which meet effluent water standards at lower capital cost than centralised wastewater treatment plant.
This article gives an overview of the concepts underlying WaterPinch analysis and its approach to water conservation and wastewater minimisation. It describes applications in four different industries - oil refining, chemicals, polymers and pulp and paper.
The fundamental theoretical formulations of the application of Pinch Analysis principles to (waste)water problems were pioneered by El-Halwagi and co-workers and by Smith and co-workers. Since then, two major design approaches have been developed - by UMIST and by Linnhoff March. Linnhoff March’s approach has been patented and has the trade name WaterPinch. It has been successfully applied to a broad range of industries with freshwater savings of 15-40% and wastewater savings of 20-50%. A listing of typical results is given in Table 1.
METHODOLOGY
The basic concepts are analogous to those for heat recovery. Figure 1 shows the source and sink curves for a single contaminant representing water sources and water sinks. Water flowrate is represented on the horizontal axis and water purity is represented on the vertical axis. Thus, the source - sink curves are also referred to as "purity profiles".
The overlap between the source curve and the sink curve indicates the scope for water re-use and is limited by the pinch point where the two curves touch. The open parts of the profiles represent targets for minimum freshwater consumption (on the right) and wastewater (on the left).
In principle, the purity profiles need to be developed for each contaminant. Each contaminant will have an ideal design that meets its specific flowrate target. These targets will all be different and so will the designs. As a practical matter, therefore, the designs have to be merged into a common piping network that performs well for all components. At that stage, graphical approaches become iterative and tedious. A mathematical programming formulation is therefore used that optimises trade-offs and provides a single piping network design that will re-use water subject to minimised objective cost.
The practical application of WaterPinch can be broken down into four steps:
1) Draw a flowsheet of the entire water system, showing all places where water is used (including utility services), and all points where (waste)water is generated. Develop a water balance accurate to within 10% of the metered amounts of the larger streams. Define the appropriate data for the WaterPinch analysis, i.e. determine water "sources" and "sinks".
2) Select key contaminants - e.g. BOD, salts, suspended solids. A contaminant is defined as any property that prevents the direct re-use of that wastewater stream, including heat content (temp) and acidity (pH). Choose design concentrations - maximum allowable for sinks and minimum practical for sources. This requires input from experts in the relevant process technologies.
3) Develop the multi-dimensional pinch analysis to determine optimum matches between sources and sinks using appropriate software. The first round of results will probably not be a practical design, as it represents an unconstrained solution. It is usually necessary at this point to evaluate the theoretical design to determine which additional contaminants should be considered, which matches should be forbidden, and which matches (if any) should be required. The procedure includes identifying pinches and a consideration of process modifications and regeneration options that would result in lower targets.
4) Repeat step 3 until a practical design has been evolved.
At present there is no commercial grade software available for performing these calculations. Together with a consortium of companies bringing together operating and contracting companies and equipment manufacturers, LM is developing windows-based software. This software has been used to date in over thirty applications and is estimated to be ready for full commercial release in 1999.
CASE STUDY NO.1
CHEMICALS PLANT
Unilever’s Vinamul plant in Warrington, England produces about 200 different specialty chemicals, principally paints and adhesives, using batch processing. Because of wide variations in processing requirements and tight quality specifications the plant had a strong preference for the use of utilities rather than recovered process water - steam for heating, cooling tower water for cooling and freshwater for washing. Changing environmental perceptions and rising costs for raw water and effluent treatment caused the management to re-evaluate this philosophy.
The WaterPinch project (using the early UMIST methods) resulted in a simple segregation, collection, and re-use strategy as shown schematically in Fig. 2. The design reduced freshwater demand by 50% and wastewater effluent flow by 65%. Together these reductions were estimated to be worth US$100K per year. However, the real savings lie in the future, when on-site treatment will be required prior to discharge. The reduced wastewater flow is expected to save about 50% of the capital cost of any future treatment plant. Furthermore, at the higher concentrations resulting from lower flow, it becomes feasible to introduce new treatment technology for total recovery of product species from the effluent water, thus virtually eliminating all pollution.
CASE STUDY NO. 2
POLYMERS PLANT
A chemicals and fibres plant in the South Eastern US was planning a capacity expansion spanning 5 years. Wastewater flow to the treatment plant was expected to increase by 20%, which was beyond the treatment plant’s hydraulic limit. It was recognised that if wastewater flow rate could be reduced by 20% the capital cost of expanding the wastewater treatment plant, along with the associated permitting requirements, could be avoided.
The engineering staff had looked at a number of options for reducing water consumption, such as two-stage filtration, reducing filtration temperature, etc., but none of them were found to be economically feasible.
Ultimately, a WaterPinchh analysis was undertaken. The key projects that emerged are highlighted in Figure 3. Net wastewater flow was reduced by 21% and freshwater intake was reduced by 16%. The capital cost of implementing the conservation measures was less than 1/3 of the projected cost of expanding the wastewater treatment facility. The permitting procedure was avoided entirely.
CASE STUDY NO. 3
OIL REFINERY
Amoco’s oil refinery in Yorktown, Virginia, USA negotiated a special arrangement with the E.P.A. (Environmental Protection Agency) to pilot test a co-operative approach to environmental compliance. The concept was to achieve air and water emissions reduction through process integration rather than conventional end-of-pipe treatment.
Data from the project was subsequently analysed using WaterPinch. The essential results are shown in Figure 4.
Freshwater consumption was reduced by 14% and wastewater effluent flow was reduced by 24%. The stripper reboiler project was not strictly necessary for water conservation as the stripper bottoms stream was being re-used in the cooling tower anyway. However, it was recommended because it shifts the cooling tower make-up mix towards freshwater which helps to mitigate odour emissions (due to trace amounts of phenolics) from the cooling tower.
CASE STUDY NO. 4
PAPER MILL
The Parenco paper mill in Holland produces newsprint from recycled waste paper. Freshwater is obtained from on-site wells, at 12,8°C, and used for once-through cooling before it is sent to the process. The existing system is shown schematically in Figure 5. The main WaterPinch project involved re-routing relatively clean DAF effluent from the de-inking pulper (which can tolerate lower quality water) to the high-pressure water sprays in the paper machine. The water balance was maintained using more white water in the de-inking pulper, and reducing (almost eliminating) the white water overflow to sewer, see Figure 6. Total potential savings were 111 tons/hour of fresh water (23%).
The fact that well water was used as a cooling medium posed some unique problems, as reduced freshwater make-up meant reduced cooling capability. One option was to provide supplemental cooling using river water from the Rhine for selected duties. The WaterPinch solution was to reconfigure the heat exchange network so as to make better use of driving forces, and use the lower water flow to absorb the same quantity of heat from the process. This means that the water going to intermediate warm water storage will be hotter, yielding the corollary benefit of reduced steam requirement. Of course, some capital expenditure was necessary for additional heat exchanger surface area and re-piping.
CONCLUSIONS
The WaterPinch approach to water conservation and wastewater minimisation was outlined, and the results of four industrial case studies were presented. The approach is recommended as a powerful and practical way to help solve increasingly severe problems faced by industry with respect to availability and cost of freshwater and the cost of wastewater effluent treatment.
The major applications are as follows:
1) Avoid production cutbacks under limiting water supply conditions
2) Reduce water supply costs
3) Reduce capital cost of new water supply facilities (e.g. new wells, pipelines, etc.)
4) Reduce capital cost of water treatment facilities
5) Reduce wastewater treatment costs
6) Reduced sewer charges
7) Reduce capital cost of expanding on-site wastewater treatment facilities to meet increased production loads or more stringent emissions regulations
8) Help comply with environmental regulations
