The full version of this article can be read in the June 2012 issue of Hydrocarbon Engineering. Subscribers can sign in here.
Every day, slightly over 80 million bbls of petroleum is converted into fuel. Depending on crude source and final products, it takes approximately 1 – 2 bbls of fresh water to refine each bbl of crude; worldwide consumption of water for refining purposes easily exceeds 100 million bpd. Of that, less than 1% is recycled. Jurisdictions, especially those in arid regions, are taking notice, and as a result, so are refinery operators. It is becoming increasingly evident to petroleum companies around the world that fresh water sustainability is going to become a very big issue in the near future. In the last two years this issue has become a major talking point within the industry, with many petroleum companies becoming proactive in their attempts to solve the problem.
A mix of regional and multinational companies supply water services to refineries. SUEZ, Veolia and Christ Kennicott Water Technology are prominent in Europe. Aquatech is a significant supplier of original equipment in North America. Yet two major international suppliers service approximately half of the international refinery sector; Siemens Water Technologies and GE Water & Process Technologies. Refineries understand that water represents a relatively small cost compared to the cost of crude oil and the energy required to convert it into fuel. However, the task remains important, because owner/operators know that if the water system goes down then the plant goes down.
Fresh water usage
Most refineries have two major water treatment systems: fresh water and wastewater. Fresh water is usually sourced from municipal supplies, groundwater wells or lakes and rivers. It has a host of uses, including processing, potable, utilities and fire safety, but most of the water is used for crude processing and cooling towers. Crude processing requires heat, typically delivered in the form of steam, to separate crude into phases during distillation and catalytic cracking. Cooling towers use water to remove heat from refined products.
Most jurisdictions have regulations regarding discharge quality, and refineries must clean wastewater sufficiently for discharge to surface.
Refinery wastewater arises from a number of different sources. As crude arrives, operators have to mix it with water in order to remove trace salts. Water also collects in the process units in condensers. In addition, there is storm water, cooling tower blowdown, boiler blowdown and water draws from storage tanks in the tank farm.
With this in mind, dealing with refinery wastewater is often viewed as one of the most difficult wastewater tasks. In the refining process, crude is broken down into hydrocarbons with a wide range of chemical variation. If the right pretreatment is not undertaken or if a particular system is not robust enough to respond to changes in flow or strength, it can have a profound negative impact. Most wastewater treatment systems operate in series, so if the first system fails, it can quickly shut down the later stages.
Designing a wastewater treatment facility is never done in isolation. When evaluating needs, several different factors are considered. For example, the restrictions on obtaining fresh water and the purity required under regulations for discharge water would need to be assessed. Increasingly, the most effective system is for a refinery to keep what it has and find as many ways to reuse it as possible.
Generally, the first treatment stage is oil/water separation. There are typically two steps: roughing, which removes large oil particles and solids using gravity; then polishing, which removes fine and emulsified oils. The polishing process involves a combination of chemicals that break oil emulsions and coagulate oil; then flotation, in which air bubbles are injected and the oil floats out.
The next step is equalisation. Water is retained in a large holding tank for up to 24 hours in order to even out flow and organic load. Wastewater is then sent to a biological aeration treatment system, where air is injected to aid bacteria in membrane bioreactor (MBR) systems to aerobically break down organics and nitrogen. It then goes to a settling basin where the biological sludge is removed and dewatered. When the water finally reaches regulated cleanliness, it is discharged to surface.
Newer refineries, most of which are located in developing countries where fuel demand is growing, are generally more amenable to incorporating advanced water treatment systems. Equipment manufacturers are working with them to develop components specifically designed to their needs. Biological MBRs are a major component of a refinery wastewater treatment programme’s aeration section, in which microbes break down organic matter, before water is separated using membranes. However, some organic compounds are difficult to degrade or consume, so activated carbon is added to aid the process. Problems can arise because the carbon can abrade the membranes in the MBR module, degrading its functionality.
The business of wastewater treatment at refineries is growing at more than 10% annually, but at the present moment water recycling at refineries remains in its infancy. However, industry participants are optimistic that it will become far more common over the next two decades.
Written by Gordon Cope
Read the article online at: https://www.energyglobal.com/downstream/gas-processing/12062012/waste_not_want_not_downstream_water_pressure/