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ABSTRACT

The Desalter Control System commonly referred to as System # 3, uses multiple probes to control the desalter and warn the operators of possible problems---giving operations time to take corrective actions before upsets occur. This paper reviews two different types of Early Warning Instruments.

One of the major causes of desalter upsets is grid over-load. The first indications of Emulsion Build-Up in the desalter are the decrease in Volts and increase in Amps on the Grids. By detecting the presence of emulsions building upwards before they enter the grids will allow operators to take corrections before the grids begin to overload.

Another major causes of desalter upsets is high water content crude being delivered to the desalter from the tank farm. The detection of high water content crude before it arrives at the desalter allows operations to make adjustments to minimize the adverse effects of the wet crude.

It is all too easy to forget that the primary function of the desalting system is the removal of the 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 (VOCs) 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 desalted 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 are the new legislative demands placed on effluent water quality. This presents 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, crude 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 a 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: That a Level Exists. 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 clear-cut level; rather the interface consists of an emulsion transition zone from oil to water with a continuously changing water percentage by volume. Understanding the true nature of the interface leads to the conclusion that efficient control comes from controlling these water percentages and not an imaginary level.

The System #3 (Desalter Control System) represents the first and only interface control system to utilize this understanding of oil/water interface as the basis for control. The system consists of the minimum of two or a maximum of four energy absorption 4 instruments: three designated for service in the vessel and one installed well upstream of the unit in the crude feed line. In the System #3 control scheme, these probes provide continuous 4-20mA output signals that are proportional to the water concentrations (% water) at their physical locations.

Probe # 1 controls the brine outlet valve, using its ability to measure small amounts of oil in water to maintain a very high (and unstable) percentage of water several feet above the bottom of the vessel. This allows suspended oil in the water phase to separate, thus inhibiting oil undercarry as a primary control function. While Probe # 1 establishes this lower limit or bottom for the emulsion layer, Probe # 2 monitors the water content from its position in the oil phase just below the lower grid. This provides real time detection of the rate and extent of the emulsion growth (which must, by Probe # 1's control, occur in the upward direction). Probe # 2's monitoring function allows the operator to detect emulsions building towards the grids, avoid downstream upsets by providing an advance warning of such growth, and allows time for corrective measures to prevent grid overload (transformer "trip"). Probe # 3 is an in-line monitor of the water content of the incoming crude feed and is typically located as far as possible upstream of the desalter (generally near the tank farm). This probe provides the advance warning of contaminated crude feed resulting from "tank switching" and/or the introduction of slop oil. The final element of the System # 3 is the water phase alarm probe designated # 4. It monitors the condition of the water phase below the control probe (12"-18" off bottom), alarming on the presence of suspended oil that does not readily separate or solids build-up that threatens the condition of the brine effluent. This is of particular value when low-quality sources of wash water (e.g. stripped or straight sour water) are utilized that can upset the separation process and form stable oil-in-water mixtures (reverse emulsions).

The benefits of this control scheme can be summarized as follows:

• Routine oil undercarry is either significantly reduced or completely eliminated, providing both environmental and economic benefits.
• The operator is provided with a real time "barometer" of the conditions in both the oil and water phases of the vessel as well as the crude feed line.
• Upsets are detected long before their effects can be felt in the unit, and the alarm outputs offer the option of automating upset response assures such as increasing or initiating chemical feeds.
• Control action that constantly forces emulsions growth toward the electrical grids maximizes electrical work, allowing for reduction or, in come cases, the elimination of chemical feed.
• By considering location of an alarm signal, more information is provided as to the source of the upset (wash water quality, wet feed, etc.).

The System 3 provides extensive information and control capabilities far beyond simple level control. Through measurement of percent water at specific points in the vessel and feed line, the overall behavior of the desalting process can be observed and the results optimized.

Below are examples of outputs of the System 3's early warning capabilities.

The Desalter is operating "steady state". The Output of the Upper/Emulsion Probe is between 4-5% water, indicating that the Emulsion Layer is stable. This is confirmed by the measurement of the AMPs from the Lower Grid.

The Upper/Emulsion Probe began to detect a growth in the Emulsion Layer (increase water content) at approximately 11:10AM. Twenty minutes later at approximately 11:30AM, the AMPs of the Lower Grid began to react (increase in AMPs) as the wet emulsion began to enter the area around the Lower Grid.

The Upper/Emulsion Probe began to detect a growth in the Emulsion Layer (increase water content) at approximately 11:10AM. If additional Emulsion Breaking Chemicals were added immediately, the problem could have been resolved before the AMPs of the Lower Grid increased in reaction to the Emulsions.

The Upper/Emulsion Probe began to detect a growth in the Emulsion Layer (increase water content) at approximately 11:10AM. If additional Emulsion Breaking Chemicals were added immediately, the problem could have been resolved before the AMPs of the Lower Grid increased in reaction to the Emulsions.

The Crude Feed Monitor began to detect Water/Emulsion in the Crude Charge Line from the Tank Farm at approximately 10:35AM. Approximately 30 minutes later at 11:05AM, the AMPS of the Lower Grid began to react to the Wet Crude Feed/Emulsions.

The Crude Feed Monitor began to detect Water/Emulsion in the Crude Charge Line from the Tank Farm at approximately 10:35AM. Approximately 30 minutes later at 11:05AM, the AMPS of the Lower Grid began to react to the Wet Crude Feed/Emulsions. With a 30 minute advance warning, the operators can take corrective actions---both in the Tank Farm and Desalter Operations.