Separator / Desalter Level Control Systems
Emulsions in a vessel or tank are a common problem during many separation processes. Emulsions are caused by the physical properties of different fluids or gasses, such as differences in density, viscosity but can also be caused due to turbulence of the process itself e.g. pumping or mixing.
Traditional level measurements have difficulties in distinguishing the hydrocarbon phase and water phase from the emulsion phase. Furthermore, information on the actual concentration being present can not be provided.
KPS offers an unique Turn-Key solution using the Agar Interface Level Measurements, which tackles the problems related to most emulsion rich processes. The Agar Interface Level Measurements are capable of determining the size and concentration of the emulsion present and can control the process accordingly. A typical application where emulsions are present; is the Desalting process:
It is all too easy to forget that the primary function of the desalting system is the removal of inorganic chlorides and other water-soluble compounds from crude oil. One need not be a corrosion specialist to realize that the acids that form from these compounds can do tremendous, long-term damage in the downstream processes of the refinery (as the inspection of crude tower overhead condensers can prove). However, the desalting process has become the focus of attention in areas that are no longer limited to the simple removal of salts and water. Most critical of these recent areas of concern has been the degree to which the desalting system contributes to the load of volatile organic contaminants (VOC’s) at the wastewater system. In fact, the condition of the brine from a modern desalter is frequently under greater scrutiny than the condition of the effluent crude.
There can be no doubt that the operation of the desalting system is an exercise in compromise. A constant balance must be maintained between mixing intensity, wash water quality, chemical demulsifier feed and control of other parameters that can provide optimal salt removal without forming an emulsion so tenacious that it compromises the system’s dehydration capabilities. Adding to this balancing act the new legislative demands placed on effluent water quality present the operator with a difficult challenge.
Optimizing the desalting process is a matter of optimizing the individual components and maximizing the efficiency of the electrostatic dehydration stage. By “pushing” the electrostatic process, one seeks to obtain the greatest amount of electrical work possible. The work performed near the grids can provide the dual benefits of enhanced salt removal (secondary mixing) and optimal coalescence. The question therefore becomes how to maximize this electrical work.
In the vessel, the structural parameters such as vessel size, grid elevations and feed discharge points are all fixed. The most critical remaining variables then become interface condition and position. In fact, optimal interface control has been proven to have significant impact on both the oil and water quality resulting from the dehydration process. Yet, in spite of the obvious need for such control, the traditional methods of control have operated on a fundamentally flawed assumption: Level. The very term “level control” indicates the presumption that the interface between oil and water in the desalter exists at a single point (such as that observed between gasoline and water). Any review of the internal conditions in the desalter vessel via the try-lines or swing-arm will dispel this notion. There is no level, rather the interface consists of a transition zone from oil to water in a continuous change of volume percent. Understanding the true nature of the interface leads to the conclusion that efficient control comes from controlling these percentages and not an imaginary level.
The Agar Interface Level Measurements represents the first and only interface control system to utilize this understanding of the oil/water interface as the basis for control. The system consists of a minimum of two or a maximum of four Agar energy absorption instruments: three designated for service in the vessel and one installed well upstream of the unit in the feed line. In the system control scheme, these probes provide continuous 4 to 20 mA output signals that are proportional to the water concentrations at their locations.
Animation on Agar Desalter Control System
Animation on Agar Interface Level Measurement
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Desalting at the MIRO refinery
The MIRO Refinery in Karlsruhe is Germany's largest refinery and one of the most modern and productive refineries in Europe. MIRO has four partners ConocoPhillips, Esso, Ruhr Oel and Shell, 1,000 workers and refine crude oil into valuable petroleum products such as gas or petrol, diesel, heating oil, propylene and bitumen - around 16 Million tons a year.
With KPS having the know how on Desalting Processes, and an installed base of similar Turn-Key projects at many refineries throughout Europe. KPS was selected to design and supply the Agar Desalter control system. A requirement of the system was that the amount of salt heading to the distillation columns needed to be as low as 1-2 ppm's. This to prevent any salt desposits in the top section of the distillation columns, hence corrosion. Furthermore, as the crude oil coming into the refinery is becoming heavier throughout time, larger emulsion layers tend to build up. Any water upsets causing short circuits of the electrical grids and any water upsets heading to the destilation columns should be prevented.
The Desalter Control System provided by KPS was able to push the performance of the desalters to their limits. Together with the chemical vendor Baker Petrolite (Cameron), the amount of demulsifiers being injected were further optimized.
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