Your company is planning an expansionbuilding a new process unit or constructing an entire new plant. Regardless of the scope, the facility staff will specify project requirements, complete the process design, and, ultimately, operate and maintain the equipment. As the project takes shape, project engineers will talk with process engineers about the facilitys water systems. But project engineers and process engineers have different responsibilities, and sometimes they dont speak the same language.
Bridging the gap
Here are some ways to bridge the gap between these two groups:
Define the design basis. In water treatment, the design basis describes the range of sustained operating conditions; these are not startup, shutdown or transient conditions. The design basis should include:
Worst case raw water quality; it should reflect seasonal, drought and flood conditions
Range of flowrates, pressures and water temperatures throughout the pretreatment system
Predicted effluent quality from each pretreatment unit that reflect the effects of aging of consumables, e.g., the projected permeate quality from three-year-old reverse osmosis (RO) membranes (per the manufacturers specification).
The best information sources to construct the design basis are plant personnel who have experience operating the utility water systems. Current and former utility process engineers and service representatives from the water-treatment vendor can also provide good information. The water authority for the local watershed, or the industrial or potable water supply, is an excellent source of historical raw water quality.
For example, the Delaware River Basin Authority, a joint state agency, maintains a historical record of selected water quality parameters from numerous monitoring sites on its website. Obtaining water quality information for well water is more difficult. Your water-treatment supplier may have information from wells at nearby plants that can serve as a first approximation for your design basis.
Understand the total cost of ownership. Project engineers are responsible for accurately estimating the capital costs for equipment. But typically they do not evaluate or assign a relative ranking for the operating cost or complexity. Process engineers can assist with the qualitative analysis of the cost and site-specific complexity.
A good example is choosing between a batch-process technology (ion exchange) and a continuous-process technology (RO) to meet varying demands for treated water. Ion-exchange units operate in a batch mode and require frequent idle time for regeneration. Conversely, RO units must run continuously for reliable operation. Choosing RO units for a highly variable flow demand requires either a treated-water storage tank with a sufficiently large working capacity or a complex operating scheme to sequence units for in-service and idle conditions. Idling RO units is not a best practices, it irreversibly compromises the performance and service life of the membranes.
Properly analyze trade-offs. Selection of alternative technologies always involves a judgment of intangible attributes. For example, cold-lime softening creates a larger waste stream than polymer clarification, but the reduction in total hardness in cold-lime-softened water has benefits: smaller downstream ion-exchange equipment and/or a reduction of wastewater (concentrate) from the downstream RO unit. Project engineers should identify and analyze cases for several alternative technologies to understand these trade-offs.
Dont forget about integration issues. Project engineers in existing plants sometimes focus too narrowly on the plot planhow to fit the new equipment into an existing facility, ignoring other issues such as integration of controls, dual-train operating considerations, and man-machine interface issues.
For example, installing a cold-lime softener in parallel with a polymer clarifier and mixing their effluent streams will require pretreatment of the lime-softened water with acid or carbon dioxide prior to mixing to prevent scaling of the blended-water transfer line from post-precipitation.1
Environmental issues. Most environmental issues create constraints for production and wastewater quality; however, occasionally, there are exceptions. Some plants on the lower Mississippi River are allowed to discharge blowdown from cold-lime softeners directly into their outfall to the river, eliminating the cost and constraint of managing this waste stream and changing the balance of trade-offs with alternative technologies.
The discussion continues. Project and process engineerings vet operability issues and new technology before finalizing a water treatment system designs. HP
1 Post-precipitation is the result of excess lime reacting with the calcium hardness and precipitating calcium carbonate in situ.
|The author |
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. She can be reached at firstname.lastname@example.org.