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Oct-2007

Oil/water interface control for desalters

Profiling instrument provides vertical real-time phase density measurement in desalter vessels. Benefits include reduced process upsets from interface excursions, optimisation of crude blends, minimisation of chemical usage and reduction in chloride carry-through with washed oil

Paul Hewitt
Tracerco
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Article Summary
Within many refineries there is a growing tendency to process heavier crudes or blend these with lighter feedstocks. During crude oil refining, it is advantageous to optimise the blend to reduce overall feedstock costs, while at the same time ensuring the final mix can be adequately processed with minimal upset.

One fundamental need within a refinery process is the efficient separation of wastewater from crude feedstock using a desalter vessel after water washing the material to reduce chloride levels. In order to optimise this separation requirement, appropriate vessel and internals design must be used alongside reliable level and interface instrumentation.

When applied to the relatively “dirty” service experienced within a desalter, a number of existing level and interface control instruments have been found to demonstrate many shortcomings. Upon a request by a major oil refiner, the proprietary Tracerco Profiler instrument was redesigned. This instrument is commonly used in three-phase separation devices within the upstream oil and gas industry to withstand the elevated operating temperatures typically experienced in a desalter vessel.

Common instruments used in desalter vessels
There are many types of level and interface instruments available that have been used to measure critical levels within desalter vessels. The most common include displacement systems, differential pressure, capacitance probes, microwave, guided wave radar, ultrasonic, thermal conductivity and nuclear. Out of this list, only four have shown reasonable reliability for interface measurement. These include displacement, capacitance, guided wave radar and nuclear. However, there are distinct limitations for these instrument types when a rag layer/emulsion band occurs in the desalter. The principles of measurement of these instruments are examined in more detail, including:
- Capacitance probes
- Displacement interface gauges
- Nucleonic gauges
- Guided wave radar.

Capacitance probes
Capacitance probes operate by measuring the electrical characteristics of fluids that they contact. As an interface level rises on the probe, the measured capacitance changes as the probe goes from complete immersion in oil (0% on the interface level range) to complete immersion in water (100% on the interface level range). One problem with this method of measurement is that if the electrical characteristics of either fluid change significantly from the expected values, as will happen when different feedstocks are introduced to the vessel or when emulsions/rag layers are formed, the capacitance measured may change. This in turn can be interpreted incorrectly as a change in level to operators and inappropriate control actions taken. In addition, capacitance probes only measure a very small distance into a particular fluid. If the process material being measured is dirty and prone to surface coating, it can be rendered inoperable. As the capacitance measurement requires contact with the process fluid, any failure requires a shutdown for repair.

Therefore, this type of instrument is most suited to measurement within fluids that are relatively clean and not prone to solids deposition. It is also advantageous to have a well-defined step change in density. In practice, this is rarely the case in a desalter.

Displacement interface gauges
Displacement instruments have been the most commonly used instrument within desalters for interface-level control for many years. The instrument consists of a submerged body (displacer) suspended in the fluids, the weight of which is balanced by an upward force exerted on it by a particular density of fluid. As the level changes, the force increases or decreases, and this results in a vertical movement of the displacer, indicating a change in interface level. The force exerted depends on the density of each fluid; therefore, if this characteristic changes significantly, the density change will be interpreted as a change in level. If the displacer is immersed in an emulsion, it will give an output corresponding to the elevation of a particular emulsion density, but will not allow the extremities of the interface emulsion layer to be determined.

Within desalters, an internal displacer is normally installed in the vessel using a stilling well. The stilling well provides protection for the suspended displacer tube and allows the measurement of “in vessel” conditions to be made, as opposed to an external bridle system, which may not necessarily mimic true fluid levels. The major drawback when using an internal displacer as opposed to a bridle-mounted displacer is maintenance. If a problem occurs with an internal stilling well instrument, the only way to get access is to shut down, depressurise the vessel and remove the instrument. Obviously, this is undesirable in a continuous refinery operation. One other common issue that has occurred in past applications concerns the use of moving parts in dirty service conditions. At times, displacement systems have experienced solids build-up, resulting in them sticking at a specific point within their measurement range. 

Nucleonic instruments
Nucleonic interface gauges use a radioactive source, which is mounted internally using a sealed 1in pipe that looks similar to a thermowell. The radioactive source is located at the end of the pipe inside the vessel. An external radiation detector is on the outside of the vessel close to the internal pipe. Figure 1 shows a typical arrangement.

When the radiation passes through the phase of lower density, the signal intensity at the external detector is high. When the denser phase increases in height, the radiation intensity at the detector is reduced. Through calibration of the system using the densities of both fluids involved, the interface position within the vessel can be measured and controlled.

However, the instrument essentially measures the elevation of fluid having a density that is the average density of the upper and lower phases. It therefore provides an accurate indication of the elevation of both a clean interface and the midpoint of a diffuse emulsion band, but it does not provide information about the depth of an emulsion layer.

Guided wave radar
Guided wave radar works in much the same way as conventional radar. An electromagnetic energy signal is transmitted from an antenna. When it reaches a definitive interface point, some of this energy is reflected back. Time domain reflectometry measures the transit time to this point and back, allowing calculation of the interface position. The system utilises a guide that is in contact with the liquids. This is used to transmit the electromagnetic radiation pulses to reduce signal degradation through the liquid. The technology relies upon back-reflection of the signal from the interface position. Unless there is a clearly defined interface, there will be no point at which this will occur. Unlike displacement, capacitance and nuclear instruments, which will measure an approximate or average interface position when a rag layer is present, the signal from guided wave radar technology may continue to the bottom of the vessel and no interface measurement will be observed.

Density profiling technology
Density profiling technology can define all fluids inside a separation vessel to a greater degree than conventional instruments. It measures the density and extent of different phases within a vessel. It maps different densities of materials such as gases, liquids and interface spans between phases. These materials can be separated into different user variable-density bands or phases. The interface of the various phases can then be calculated with respect to the vessel height.

In place of traditional single-point measurements for interface and bulk level, an operator can measure the position of each phase in real-time and determine the quality of each interface. This level of measurement gives the operator confidence to increase fluid throughput and minimise the use of various separation-enhancement chemicals. The information provided can also be used to automatically control interface levels in a DCS system and influence the injection of effect chemicals. 

A density profiler is housed in dip-pipes (sealed pockets similar to thermowells) installed within the vessel through a single flange, as shown in Figure 2.

A narrow dip-pipe holds an array of Americium-241 sources, Americium-241 being a low-energy gamma emitter commonly used in domestic smoke detectors. The other dip-pipes hold radiation detectors made up of a vertical array of up to 150 Geiger Muller (GM) tubes, each one 28mm in height. Each tube is matched to the radiation source on the same plane, as shown in Figure 3. On the screen, the fluid density at each 28mm channel is depicted.
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