Desalting Crude Oil
Desalting is a water-washing operation performed initially at the production field and thereafter at the refinery site for additional crude oil cleanup. Salt and water content specifications are even more rigid because of their negative effect in downstream processes due to corrosion, and catalyst deactivation. An optimum formulation concept is presented to describe emulsion breaking in desalting process. In the stabilization mechanism is accepted that water droplets are stabilized by the formation of a mechanically strong and viscoelastic film at the interface composed of asphaltenes. In the case of water-in-crude-oil emulsions, a balanced optimum formulation is attained by adding to the lipophilic natural surfactants contained in the crude oil, demulsifiers which are hydrophilic. The aim is to relate the nature and concentration of the added demulsifier products to the amphiphilic mixture at the interface. All formulation parameters, such as solvent, alcohols, kind and concentration of demulsifier, amoung others, can be explained for proportional and saturation regimens.
Oil produced in most oil fields is accompanied by water in the form of an emulsion that must be treated. In addition, this water normally contains dissolved salts, principally chlorides of sodium, calcium, and magnesium. If crude oil is left untreated, when it is processed in a refinery the salt can cause various operating problems. This paper investigates experimentally the effect of five factors (gravity settling, chemical treatment, freshwater injection, heating, and mixing) on the efficiency of the dehydration/desalting process for a Kuwaiti crude oil and a commercial demulsifier (Servo CC 3408). These factors are systematically varied and efficiency is analyzed. Two efficiencies are defined: a Salt Removal (S/R) efficiency and a Water Cut (W/C) dehydration efficiency. The investigation was carried out through changes made to a single factor at a time as well as multiple variations of factors. Plots, based on experimental data, showing the variation of the two efficiencies as a function of the various factors are given. Two main conclusions are drawn for the system studied. First, excessive amounts of a demulsifying agent had adverse effects on the desalting/dehydration process. Secondly, the most important factor that improved both efficiencies (S/R and W/C) was found to be the settling time. Efficiencies up to 75% were obtained at settling times of 5min. This factor was simulated in the experimental runs through the use of a centrifuge. The implication of this finding is that future desalting/dehydration systems for the oil and demulsifier studied should be based on centrifugal techniques.
• SALT REMOVAL AT HIGH TEMPERATURE, PRESSURE AND ADDITION OF WATER
Salts in crude oil are mainly in the form of magnesium, calcium, and sodium chlorides, sodium chloride being the most abundant. These salts can be found in two forms: dissolved in emulsified water droplets in the crude oil, as a water-in-oil emulsion, or crystallized and suspended solids.
The negative effect of these salts in downstream processes can be summarized as follows: salt deposit formation as scales where water-to-steam phase change takes place and corrosion by hydrochloric acid formation. Hydrochloric acid is formed by magnesium and calcium chlorides’ decomposition at high temperatures (about 350 °C) as follows :
CaCl2 + 2H2O → Ca(OH)2 + 2HCl
MgCl2 + 2H2O → Mg(OH)2 + 2HCl
The objective of desalting process is to remove chloride salts and other minerals from the crude oil by water-washing. Depending on the desired salt content in the desalted crude oil, a one- or two-step process could be applied. For refining purposes, a salt concentration of maximum 1, 5 PTB (pound of salt measured as NaCl per thousand barrels) is desired. By desalting, a considerable percentage of suspended solids (sand, clay, or soil particles, or even particles product from corrosion of pipelines and other upstream equipment) are removed. Figure 1 shows a general process flow diagram for one- and two-step desalting process.
Simplified desalting process flow diagrams for different configurations: (a) one-step and (b) two-step.
The process starts by mixing the raw crude oil with an appropriate amount of washing water also known as dilution water. The washing water as a volume percent of the crude oil processed could oscillate between 3 and 10%, depending on the API gravity of the crude oil – the heavier the crude oil, the more the water required . Demulsifiers are added to the crude oil in this process step.
Here it was explained the approach from the optimum formulation concept to assist emulsion breaking in desalting process. As part of the stabilization mechanism is accepted that the water.