Crude unit desalter, water

Wastewater reuse is a good way to reduce overall pollutant loadings. However, water quality is critical in water reuse. The contaminants present must be compatible with the reuse. For example, reuse waters with high solids content are not satisfactory for crude unit desalting. Stripped foul water containing low H2S and ammonia and high concentrations of phenols has essentially no solids. It is suitable for crude unit desalter wash water if the phenols extracted by the crude are subsequently converted by hydroprocessing units into nonphenolic compounds [36]. Some other examples include ... 

Clean steam condensate is the best source of wash make-up water. Condensate collected from fractionator reflux drums is sometimes acceptable. River, lake, or well water with high total dissolved solids (TDS) is bad. These solids can precipitate out on the mix valve and cause excessive pressure drop. In one refinery, using untreated river water in a crude-unit desalter for a few months reduced desalting efficiency from 90% to 65% salt removal. 

When sour water stripper bottoms are used in the crude unit desalter, the NH should be about 10 to 20 ppm. Higher NH levels... 

That is, the old back issues of the NPRA Q A (i.e.. The National Petroleum Refiners Associated, Question and Answer) sessions. With this authoritative source as support, 1 asked the operator to stop the NaOH addition to his sour water stripper feed. Two hours later, at the same reboiler duty, the NHj content of the stripped sour water had gone from 0 to about 5 ppm. And the emulsion layer in the crude unit desalter had been reduced enough so that brine carry-over was no longer a problem. The 5 ppm of ammonia was quite acceptable. 

The stripped water flows to the crude unit desalter and/or the hydrotreater effluent washwater makeup. 

The initial corrosion control system used in a crude unit is a desalter. Modern desalters separate oil and water electrostatically. The internals used to accomplish electrostatic separation are normally of a proprietary design. The vessel itself is usually carbon steel. The bottom is often cement lined to protect it from salt water corrosion. The payout on a desalter is difficult to establish. Desalters are normally used when the salt content of the crude exceeds 20 lb per 1,000 barrels (ptb). When high reliability of the unit is desired, crudes with salt contents of 8 to 10 ptb are desalted. When desalting is used, the target is 1 ptb or less. Fluctuations in salt content are particularly troublesome to the downstream equipment therefore, the desalter should be designed for the maximum anticipated salt content. 

A problem may occur here in that the separation of oil and water may not be complete by the time the oil exits the desalter on its way to an atmospheric fractionation unit. Any water that remains with the crude oil will have to be heated to atmospheric fractionator inlet temperature, typically 290-370 C. 

Occasionally, foaming in the preflash column can limit throughput to the crude unit. Organic surfactants or water carry-over from the desalters can be the cause of the problem. Silicones, usually of 60,000-cSt viscosity, are normally used in this process. However, sometimes the normal silicones are too soluble in the crude to be effective in controlling foam. In... 

So far, only the potential use of the compact coalescer in upstream processes has been considered. However, there is also potential for using the unit at refineries. In order to remove salt from a crude oil, fresh water is added to the oil and intimately mixed with it. In some cases, this water may stay in the oil for a long time. In order to remove this water, the oil must again be heated or treated with chemicals or both. The installation of a compact coalescer here can, therefore, provide a more effective desalting process. 

I was working on a smaller crude unit in Alabama to improve desalter efficiency. Their problem was the periodic carry-over of emulsified brine. The depth of the emulsion between the crude and water phases in their desalter was excessive. The pH of the washwa-ter from the sour water stripper was to 9, even though the NH3 content of the stripper bottoms was zero. Talking to the operators, I discovered that they added NaOH to the sour water stripper feed to diminish the NHj content of the stripped water. 

Cracking of admiralty metal (C 44300) heat-exchanger tubes has been a recurring problem in a number of refining units and petrochemical process units. For example, ammonia is often used to neutralize acidic constituents, such as hydrogen chloride or sulfur dioxide, in overhead systems of crude distillation or alkylation units, respectively. Stripped sour water containing residual ammonia is used as desalter water at some crude distillation units. This practice causes ammonia contamination of the overhead system even if no ammonia is added intentionally. 

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. 

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... 

The second alternative was the dilution with lighter crude oils or refinery cuts such as gas oil or kerosene (8, 26). Dilution would answer not only the viscosity problem but also the dehydration and desalting of these, very often, heavier-than-water crude oils, facilitating the process and enabling conventional units to be used for the purpose. This... 

This is illustrated by operation of the desalter, which is usually the first processing unit in the refinery proper. Its function is to reduce the content of bottom sediment and water (BS W) from the crude charge to the crude still. Water (generally brine) causes corrosion in units down-stream of the desalter as a result of decomposition of chlorides to hydrochloric acid at the elevated processing temperatures. Addition of alkali to the desalter reduces hydrolysis of calcium and magnesium chlorides and consequently results in less hydrochloric acid being formed in the crude still overheads, etc. 

Another common fouling problem due to inorganic deposits may occur when ammonia is used to neutralize HCl formed by hydrolysis of chlorides after crude desalting. Increasing the pH, in order to reduce corrosive potential, results in formation of the oil-insoluble salt, NH4CI. This may result in a fouling problem that can be alleviated by adding water to the affected unit, either continuously or intermittently. 

The desalting process is mostly driven by thermodynamic and hydrodynamic constraints. For the purpose of modeling the crude distillation unit, we consider the desalting operation as a simple component splitter that removes any water present in the feed crude. Desalting and dewatering processes are very effective and do not consume significant resources compared to other units, so this simple model representation is justified. 

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