Desalting unit

The treated water containing sodium chloride, cyanides, phenols and traces of H2S and NH3 is recycled to the crude desalting unit and used as wash water for the hydrotreaters and FCC units. 

The first coimnercial desalting unit with a capacity of about 4000 cu. m/day has been operating in Eilat since 1965. By the end of 1979, 22 units with a combined capacity of 20,000 cu. m/day were operating or used as stand-by units at 11 sites. 

From the desalting unit, crude oil passes through a series of heat exchangers to increase the temperature to about 550°F (287,8°C). From the exchangers, oil passes... 

Desalter units generally will produce a dehydrated stream containing like amounts of BS W from each stage. Therefore, BS W can be considered pan through volume and the dilution water added is the amount of water to be recycled. The recycle pump, however, generally is oversized to com- >cnsate for difficult emulsion conditions and upsets in the system. 

Water contained in each desalter unit it an excellent source of volume ratio makeup. Fig. 4 shows the typical two stage process with recycle, but in this case note that recycle water is routed to second stage inlet as well as first stage inlet. The amount of water recycled to the first stage must be the same as the dilution water injection rate to maintain the water level in the second stage unit. Volume of water recycled to the second stage inlet can be any amount, since it immediately rejoins the controlled water volume in the lower portion of the desalter unit. This is usually referred to as internal recycle and field installations of this type have been operating successfully for several years. 

MW OAREA PTB Q Qa QAO Qo Qw RHOG RHOL RHOW S molecular weight oil cross-section area, ft2 salt in oil at outlet of desalter unit, lb/1000 bbl total flow of emulsified water and oil actual gas flow rate, ft3/s liquid rate, ft3/s crude oil flow rate, dry basis, bpd production water rate, bpd gas or vapor density, lb/ft3 liquid or oil density, lb/ft3 water-phase density lb/ft3 total salt content in the production water and dilution water, lb/day... 

A more precise study has been made from samples of another refinery site. Figure 4 displays the two sets of spectra corresponding to the inlet of the wastewater treatment plant (samples 1) and to the main flux upstream at the outlet of the desalting unit (sample 2). The raw spectra do not present a direct isosbestic point (IP), but, after normalisation (see Chapter 2), the two sets are characterised by the presence of a hidden isosbestic point (HIP) [6], more or less evident, as shown on the zoom in Fig. 4. Considering the experimental error related to the absorbance measurement, the isosbestic point may rather be considered as an isosbestic surface. 

Figure 4. Use of HIP for the variability study (example of refinery wastewater (a) inlet of treatment plant, (b) outlet of desalting unit) [4],...

Contaminated water comes from primary distillation (desalting), hydrotreating, thermal cracking and catalytic cracking units. 

While a desalter costs more to install than the flash drum system, it has the advantage of removing up to 95% of the salt from the oil permanently. Because less salt reaches the fractionating tower in a unit equipped with a desalter, a smaller quantity of corrosion products is formed due to high temperature breakdown and hydrolysis, and the salt content of the residual fuel oil is much lower. 

Figure 3-1. Flow diagram of atmospheric and vacuum distillation units (1,3) heat exchangers (2) desalter, (3,4) heater (5) distillation column, (6) overhead condenser, (7-10) pump around streams, (11) vacuum distillation heater (12) vacuum tower.

Caustic that is added downstream of the crude oil desalter. Caustic is injected downstream of the desalter to control overhead corrosion. Natural chloride salts in crude decompose to HCl at typical unit temperatures. Caustic reacts with these salts to form sodium chloride. Sodium chloride is thermally stable at the temperature found in the crude and vacuum unit heaters. This results in sodium chloride being present in either atmospheric or vacuum resids. Most refiners discontinue caustic injection when they process residue to the FCC unit. It can still be present in purchased feedstocks, however. 

Hyperfiltration (Reverse Osmosis) is a form of membrane distillation or desalination (desalting) operating with membrane pore sizes of perhaps 1 to 10 Angstrom units. The various individual RO component technologies have improved tremendously over the last 20 to 25 years, and resistance to fouling and permeate output rates have benefited. Nevertheless, all RO plants remain susceptible to the risk of fouling, and adequate pretreatment and operation is essential to minimize this problem. 

Belfort, G. In Synthetic Membrane Processes, Belfort, G. (ed.) (Academic Press, Orlando, 1984). Desalting experience by hyperfiltration (reverse osmosis) in the United States. 

As already noted (Chapter 3), petroleum oil often contains water, inorganic salts, snspended solids, and water-soluble trace metals. As a first step in the refining process, to reduce corrosion, plugging, and fouling of equipment and to prevent poisoning the catalysts in processing units, these contaminants must be removed by desalting (dehydration). 

All commercial types of processes, with the exception of freezing, namely, distillation, reverse osmosis and electrodialysis, are being applied in the above units with various kinds of distillation processes being used for seawater desalting. Two of them, horizontal tube multieffect distillation and vapor compression units were developed and manufactured locally by the Israel Desalination Engineering Ltd. Recently, two small RO units with a combined capacity of approx. 100 cu. m/day were also used to desalt seawater. The main aim of these units is to test and demonstrate the feasibility of this new technology. 

---