The hydrocarbon processing industry needs reliable and economic sources of water for present and future operations. Ensuring a sustainable water supply requires a focused effort to evaluate the quality and quantity of alternative water sources, reuse of individual water streams or the combined outfall stream, and/or changing regulatory requirements.
The most common methodologies to analyze water circuits are pinch technology and mass balance/solution modeling. Application of classic pinch technology for water systems, as shown in Fig. 1, evaluates only the hydraulic demands for water.
Fig. 1. Sample water-pinch diagram.1
A better solution is to construct a sophisticated computerized model of the facilitys water systems that incorporates hydraulic information, along with ionic equilibria of soluble contaminants. Like pinch technology, this approach requires an accurate water balance.
Fig. 2 is a sample flow diagram with color-coded streams: steamsred, waterblue, recycled watergreen and wastewaterbrown. This approach models the ionic equilibria of the soluble contaminants, providing information about water quality for each unit operation.
Fig. 2. Refinery water flow schematic.2
Modeling a water system also requires creating a salt or contaminant balance. The optimal approach is to profile water quality throughout the system by analyzing numerous samples at every location, and then compare the actual water quality to the predicted value in the model. When the actual water quality and flowrates closely match the predicted values, the model is considered validated for the present plant conditionsthe baseline case.
A validated model allows the facility to hypothetically reprocess water to meet the specification limits for individual process units and to identify candidate water streams for reuse or retreatment (recycling). The model also provides insight into the hydraulic and chemical impacts on the unit and the total system balances for mass and salt concentrations.
This computerized modeling provides an accurate assessment of options: different configurations and/or operating scenarios to improve system operability, justification capital improvement projects, optimization system reliability and minimization of the risk of off-spec or lost production. Embedded within this analysis is a projection of the chemistry change for the cooling water. The only remaining analysis for this unit operation is a separate modeling task using a different software program to design an appropriate chemical-treatment program to control the corrosion, deposition and microbiological populations within the cooling water circuit.
As the quantity and quality of water decreases, industrial users will need to increase their efforts to conserve, recycle water and conform to even stricter regulatory requirements for withdrawal and discharge. Software tools can provide methods for plant personnel to quickly and economically analyze numerous system configurations providing a high level of confidence about the option that best meets their objectives.
1 EPRI document TA-114453, Electric Power Research Institute, Inc., 1999.
2 Data Mobility Systems, a business of Nalco, www.datamobility.com, 2011.
Loraine A. Huchler is president of MarTech Systems, Inc., a consulting firm that provides technical advisory services to manage risk and optimize energy- and water-related systems including steam, cooling and wastewater in refineries and petrochemical plants. She holds a BS degree in chemical engineering, along with professional engineering licenses in New Jersey and Maryland, and is a certified management consultant.