Crude oil distillation is the separation of different components of crude oil in a distillation column at different temperatures. Crude oil is consist of hydrocarbon compounds that vary in temperature range (boiling points, from methane to asphalt). The main objective is to separate lighter hydrocarbon and heavy hydrocarbons. Then cracking can take place to reduce the heavy hydrocarbon to lower ones. Lighter hydrocarbon are found within the range of C1 –C4 , gasoline and middle distillate like kerosene(AFT), gas oil.
Impurities in crude oil
The impurities contained in crude oil are classified as follows:
- Oleophobic: these include; salts of sulphates, sodium, calcium, magnesium. Sediments such as drilling mud, sand and many more. Soluble water and emulsified water. All these are insoluble in crude oil.
- Oleophilic: they include, organo-metallic compounds like Nickel(Ni), iron and sulphur and Nitrogen compounds and naphthenic acid. It is necessary to remove these impurities before distillation. Impurities can cause any of the following:
- Trace metals in distillate which acts as a catalyst poison.
- Plugging of equipment and fouling of heat transfer surfaces
- Product degradation
- Increase in the level of ammonia consumption to neutralize hydrochloric acid.
Petroleum emulsion is a hydrophilic and hydrophobic. They have varying colors. For emulsion to stabilize, an emulsifying agent is needed because the viscosity of emulsion increases with water content. (up to 60%-80%). It is worthy of note crude oil containing emulsion cannot be processed without de-emulsification. This could be carried out in the following ways:
Mechanical method: the stages involved in this method are; settling, centrifuging and filtering.
Thermal method: this method is centered on heating processes. An emulsifying agent is added; the film of the emulsifying agent expands and breaks, letting water droplet coalesce. It is observed the water and petroleum separate with water at the bottom part while petroleum on the upper surface. Then the petroleum is heated upto 70˚c. despite the method , some emulsion are difficult to separate even at higher temperatures.
Chemical method: here, the added emulsifying agent destroys the emulsion by dissolution or substitution. Non ionogenic surfactant can be employed example ethylene and propylene.
Thermo-chemical method: this method is effective if a high emulsifying agent is used. Emulsifying agents use the method of dehydration at 0.9MPa air tight
Electrical method: the formation of electric field at high voltage separate the emulsion by destroying it’s film.
Process description of desalting
The procedure of electrical desalting of crude oil has been improved in recent times.
From the diagram above, crude oil is heated to a temperature of about 120˚c to 130˚c by heat exchanger. In addition with hot stream from crude fractionators, the heating is to reduce the viscosity of the crude oil. This is to improve the effective flow, mixing and separation. After the crude is passed through the heat exchanger, demineralized water is added and passes over the mixing valve. Also, an emulsifying agent is added to destroy the difficult emulsion in the crude. Then the mixture is passed to a desalter vessel. The vessel helps in homogenous mixing of the components through the passage of electrical current. After this process, the crude overflow while water containing salt moves along with the crude oil withdrawn. For firm with larger financial capacity can employ installation of the following units to further remove salts: reduced corrosion, plugging and catalyst poisoning in downstream equipment.