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Oct-2008

Refinery water management

Waste minimisation and loss prevention for improving effluent water quality and rendering more water suitable for reuse are discussed

Gert-Jan Fien, KBC Energy Services

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Article Summary

More refiners are finding reason to rationalise their water consumption. However, so many issues impact the amount and quality of water required that most are unaware of their own performance and of the opportunities to improve. A 360-degree approach to improving water management in oil refineries has therefore been implemented, ranging from benchmarking issues to typical inefficiencies encountered in water usage and incorporating best practice improvements. This approach has been successfully applied to several refineries.

Changing conditions
Compared to other industries, oil refiners have had relatively few concerns about water usage. However, the situation is changing. Factors contributing to a growing need for better water management are:
• Processing dirtier crudes With specific gravities closer to water, (slop) oil is harder to separate from (waste) water and desalting is more difficult. Sour crudes cause more waste water contamination
• Clean fuels production More desulphurisation can overload sour water strippers and sulphur recovery units, thus impacting waste water qualities. Often, deeper conversion requires site expansion and increased water consumption
• Stricter emissions regulations There are more gas flows to scrub and more waste water contaminants to remove
• Climatic changes Increased rainfall can reduce residence time in waste water treatment (WWT) and even overflows, whereas reduced rainfall can mean less dilution of final effluent
• Less rainfall, falling ground water levels and higher municipal and/or agricultural consumption These factors can also reduce fresh water availability
• Greater public awareness of environmental issues Often, public sentiment urges the reduction of water consumption and effluent.

The availability of fresh water and the licence to discharge effluent directly both affect sustainable operation and are, therefore, worth much more than the annual cost of water import or disposal.

Benefits
Better water management may enable expansion or higher throughput by freeing up fresh water. It can also improve effluent quality due to reduced contamination, better waste water segregation and longer residence time in treatment units. It is also likely to reduce operating costs associated with water purchase, pumping, treatment (chemicals, resin, air compression) and heating (steam use, boiler blow-down).

Possible additional benefits include less fouling and corrosion in process equipment, better desalting of crude oil, colder cooling water (better vacuums, higher LPG recovery) and an improved public image.

Challenges

With the proprietary WaterPinch methodology and software, KBC Energy Services carried out water minimisation projects in many branches of industry, including oil refining, which presents particular challenges due to:
• Distribution of water consumption Typically, cooling towers and steam generation account for 80–85% of water consumption. Most other users are relatively small and widespread
• Complex water chemistry Refining processes release a great variety of contaminants, each of which can determine the reusability of water for certain applications and cause problems with effluent discharge. Contaminant interactions pose threats of VOC emissions, scale formation, corrosion, bio-fouling, sludge build-up and more, making the application of quantitative tools very difficult
• Poor metering Usually caused by relative neglect of water-related systems, this also hinders numerical analysis
• Relatively low cost of water This can make it hard to justify water-saving projects
• Generally conservative attitudes “We’ve always used this amount/type of water, so we must need it.”

With water becoming a bigger issue, a good methodology is needed to benchmark water efficiency and analyse where improvements can realistically be made.

Benchmarking
Quantitative benchmarking of a refinery’s water consumption implies the calculation of objective water efficiency and compares this to a database of results from other refineries, such as the KBC Best Technology (BT) analysis for energy usage. Ideally, as in the case of BT analysis, it should also provide an objective refinery-specific best practice target in terms of tonnes of water imported per barrel of crude processed. To do this, the calculations need to account for differences in:
• Cooling water systems Once-through cooling water, as opposed to make-up for a circulating system, is normally not counted in the water consumption per tonne of crude oil and should therefore lead to a lower target
• Refinery configuration Clean fuels production requires more contaminant removal. Coking processes are dirty and cause swings in waste water production, while downstream (chemical) processes add their own complications
• Quality of imported water (for example, saline or potable) The process of preparing water for use in boilers and processes gives reject flows that depend in size on the quality of imported water.
Water usage is further affected by:
• Crude types processed (sweet or sour, light or heavy) This determines how much contamination ends up in waste water and how hard it is to treat that water
• Climatic conditions (temperature and precipitation) Effective handling (and reuse) of rain water can offer opportunities, while optimal air cooling can reduce make-up water for cooling towers.

There are also minor influences from on-site or off-site power generation, economies of scale and site layout. All of this makes objective benchmarking of a refinery’s water efficiency a challenge. A benchmark that only considers a certain combination of common process units would fail to identify the (generally much larger) inefficiencies and losses that exist in the utility systems (power plant, demin plant, cooling towers).


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