Electrostatic treater

The electrostatic treater electrodes can short out due to the rise 1n water level caused by the There are two considerations for vessel location ... 

An electrostatic treater would process this crude at about 75 bpd, per square fool ol electrode coalescing area. Required treating temperature would l>e about 120 F. Notice that a given size electrostatic treater will process about twice as much crude oil as the same size mechanical treater. Also, it will accomplish this at a 30 F lower temperature. Significance of this will be dealt with later, (. cist of electricity to treat this 1.000 barrels of oil would be about 75 cents based on 4.5 cents pet kWh. 

Gravitation A r, SEPARATORS, described in Parts I and 2 of this series, remove almost all of the water in produced fluids. However, contract specifications may call for almost complete water lemoval and it is sometimes necessary to include a heater or electrostatic treater unit as part of wellsite process equipment. 

Fig. 3—Except lor electrodes in coalescing section, electrostatic treaters are simriar to other horizontal-type treaters.

Electrostatic treaters. Fig. 3 illustrates a typical horizontal elrrirostatir treater. The flow path in an electrostatic treater di- -am- - in a hi-i i/unt il hcan-r-trc.in-r. th- n il - difference is that an AC and/or DC electrostatic field is used to promote coalescence of water droplets. 

There are two types of dehydration equipment that can make a 0.5% or less water cut. These are the electrostatic treater and the heater-... 

The electrostatic separator shown in Fig. 4.2 is normally placed in stream 6, shown in Fig. 4.1. A heater should be installed upstream of this electrostatic treater to control temperature. Here heating temperatures should normally range from 100 to 200°F for light crudes 22°API and above. For heavier crudes, use higher temperatures, such as 250°F for a 14°API crude oil. Some heavier crudes, below 14°API, may require temperatures up to 300°F for adequate electrostatic dehydration. This temperature is to achieve a gravity difference between the crude oil and water of 0.001 or more. The electric power required by these electrogrids is normally 0.05 to 0.10 kVA/ft2. The ft2 area is the horizontal cross-section area in the electric grid section of the electrostatic dehydrator. 

It is important to note that you should consider removing all free water before attempting to size the electrostatic water separation section. The dL factor calculated is only for this electrostatic section. You should therefore make a good estimate of how much water will pass into the electrostatic section and input this value in the Prod + Desalting Water, BPD input block. Inputting all of your production water will seriously err sizing results. Free water should be removed in free water knockout (KO) tanks or vessels upstream. True and needed electrostatic treater sizing may thereby be determined. 

Free water is that water which will freely separate from oil in accordance with Eq. (4.1). Time, chemical emulsion breakers, electronic fields, and temperature are the factors that will break an oil-water emulsion. It is prudent to heat the emulsion prior to entering the electrostatic section. Such heating lowers the viscosity, which not only allows more free water removal, but also will enhance the efficiency of the electrostatic-treater section. Hereby, even a 0.5% water cut in treated oil is easily and commonly achieved. 

Figure 5. Dual-polarity electrostatic treater, simplified internal structure.

Electrostatic Treater A vessel used to break emulsions by promoting coalescence through the application of an electric field. See also Treater. 

Some horizontal heater-treaters add an electrostatic grid in the coalescing section. Figure 1.25 illustrates a simplified schematic of a typical horizontal electrostatic treater. The flow path in an electrostatic heater-treater is basically the same as in a horizontal heater-treater, except that an electrostatic grid is included in the coalescing-settling section, which helps to promote coalescence of the water droplets. 

Field experience tends to indicate that electrostatic treaters are effective at reducing water content in the crude to the 0.2-0.5% level. This makes them particularly attractive for oil desalting operations. However, for normal crude treating, where 0.5-1.0% BShorizontal heater-treater, neglecting any contribution from the electrostatic grids. By trial and error after installation, the electric grids may be able to allow treating to occur at lower temperatures. 

Using the same procedure, the following correlation for droplet size was developed for electrostatic treaters ... 

For viscosities below 3 cp. Equations (1.8a) and (1.8b) should be used. The two equations intersect at 3 cp, and electrostatic treaters would not be expected to operate less efficiently in this range. Additionally, the data from which the electrostatic treater droplet size correlation was developed did not include oil viscosities less than 7 cp. 

The same authors also investigated the effect of water cut on minimum droplet size. Data from both conventional and electrostatic treaters over a range of water cuts were used to back-calculate an imputed droplet size as a function of water cut, resulting in the following equation ... 

FIGURE 1.43. Flow rate versus treating temperature for electrostatic treaters. 

Since the salt content is directly related to the amount of residual water, the best desalters remove as much water as possible. Any device that removes water from oil can be used as a desalter. However, the majority of desalters employed are horizontal electrostatic treaters. These treaters will produce the lowest residual water level of all treaters. Figure 1.28 illustrates a horizontal electrostatic treater of the type typically used in desalting operations. Because very low water contents are required, the cmde is usually pumped through the desalter at pressures above its bubble-point. In addition, the temperature of the crude to be desalted is determined by upstream heat exchangers. Thus, there is need for an inlet degassing and heating section as shown in the typical oil field horizontal electrostatic treater discussed earlier. 

In this example, an electrostatic treater might be all that is required to achieve an oil outlet that contains less than 0.3 % water. This example assumed a low salt content. If the water had a high salt content, say 200,000 ppm NaCl, there would be approximately 70 lb of salt per barrel of water (Ib/bbl). In this case, even dehydrating to 0.1 % leaves 70 PTB. To reach the required 10 PTB, desalting would be required. 

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