Dealing with desalter cleaning challenges
How to overcome lengthy and potentially dangerous challenges when cleaning a crude unit desalter.
JUSTIN WEATHERFORD and MARCELLO FERRARA
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Let us play a hypothetical yet very real scenario. Like any well operated refinery, you have made many strides in the previous decade to lower operating costs. You are very aware that 85-90% of a refinery’s costs to do business is in the purchase of crude oil – feed to the refinery from which all your valuable products are derived.
The vast majority of your efforts to reduce expenses were realised by a capital project eight years ago that allowed your refinery to begin processing more ‘advantaged’ crudes. Advantaged crudes represent a lower cost for your refinery, which is worth millions of dollars in additional profits and value to your shareholders each year, but compared to your ‘normal’ crude feed slate they are typically lower in API gravity. They also tend to contain high levels of solids, unstable asphaltenes and difficult to remove chloride salts.
Just four years ago, during the previous unit maintenance turnaround, your refinery performed mandatory inspection of the desalter vessels. In the pre-turnaround planning phase, a great deal of time and energy was spent developing the plan for how to safely clean and reach safe conditions for man entry of the vessels.
As part of your due diligence, you had met with several chemical cleaning companies that promised they could help you quickly reach safe-entry conditions of the equipment throughout the unit. Time was the main driver for you. Of course you would be willing to pay a little extra for that one supplier that could deliver the unit ‘clean’ and ready to enter most quickly.
One supplier clearly stood above the rest as they were very professional in their initial presentation and they provided a fairly substantial finely bound binder with example after example of jobs they had performed with great results and happy customers.
Going into the turnaround, you had five days planned for degassing/decontamination of the equipment alone and they promised this could be achieved in only 18 hours with their chemistry. At about $2 million per day of lost profit opportunity when the unit is out of production, you were happy to pay some more for this company over the low cost suppliers because you had to go with someone who you had confidence in (and the extra represented only a fraction of the true cost of doing the job).
The plan called for a chemical injection to be performed during the steam-out of the equipment. The chemical would be injected in several locations throughout the unit and would ‘guarantee’ quick removal of all the hydrocarbon and other contaminants present so that your maintenance activities and inspection could begin a full four days ahead of schedule.
In addition to its formulation for handling H2S and a few other contaminants, the chemical essentially works by forming an emulsion with the hydrocarbon contaminants present in the unit (see Figure 1). As the steam condenses, the chemical and the hydrocarbons follow the path of the condensate. The first injection point for the degassing chemical was into the steam upstream of the first preheat exchanger. Because the chemical base is a heavy hydrocarbon based emulsifier, it effectively binds the condensate to the hydrocarbon in the unit and follows the path of the condensate, which is blown down from low points throughout the unit and collected for disposal.
In order to get the chemical dispersed throughout the unit, a high steam flow rate was specified by the chemical cleaning provider, which to meet that requirement was a large job in itself. Several additional steam out and drain lines were installed in the weeks leading up to the turnaround.
One of the first surprises was that the unit did not seem to heat up as fast as you had expected based on previous experiences. Of course the unit was to be hot (approximately steam temperature) prior to initiating the chemical injection.
You were certainly aware that the heat exchangers in the preheat train would probably be more fouled than normal due to the additional advantaged crude the unit had been running as this was evident from the lower furnace inlet temperature and hydraulic limitations that operations had been managing for the six months or so prior to the turnaround.
What you could not have predicted was that, even with more steam rate specified than normal, the unit would take over 14 hours just to heat up to steam temperature. The reason for this was clearly evident as the mechanical cleaning portion of the turnaround began a few days later and the hot preheat train exchanger heads were removed. They were packed with sludge and over half of the tubes were plugged on several exchangers, especially those just upstream of the fired heater. At least this explains the long heat-up time, but unfortunately the job was already 12 hours behind schedule and you had barely begun.
Following each condensate collection spot, an additional chemical injection was utilised. Altogether over 30 chemical injection locations were scattered throughout the unit.
Collecting the condensate, chemical and contaminants required approximately 12 full size frac tanks. This material could not be sent directly to oily water sewage because the nature of the chemical and the bound hydrocarbons from the unit (tied up in the emulsion) would overwhelm the biological process at the wastewater treatment plant (WWTP).
For this reason, a separate demulsification step was required. This step included adding an additional chemical to the frac tanks, which then had to settle for several days before the first amount of liquid could begin to be dribbled off into the water treatment system. Due to the high load placed on the WWTP, and its already fragile state following other turnaround activities, the frac tanks remained on-site for the next six months until they were finally emptied.
Other than the initial 12-hour delay, most of the unit was essentially degassed and ready for mechanical cleaning and other maintenance following 18 hours of chemical injection as promised. Most of the unit.
Due to an unknown but likely large volume of sludge in the desalter vessels, they were isolated and decontaminated independently of the remainder of the unit. Because the desalters required mandatory inspection, as previously discussed, the operators and chemical cleaning supplier made them a priority. Prior to initiating any steam-out of the unit, the vessels were floated with water to displace liquid hydrocarbon still present following unit shutdown.
Once the vessels were liquid full, they were drained of most of the free liquid water and steam-out was initiated. This step was started concurrently with steam-out of the rest of the unit.
Due to severe steam limitations during the turnaround (other steam producing units were already out of production), steam was in short supply; however, refinery management had already declared the crude unit to be the critical path to the refinery getting back online. The other downstream units also included in the turnaround had to wait until steam was available to begin decontamination efforts.
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