Question

1.1. Crude oil needs to be cleaned before it can be refined - explain what crude desalting is and why it is needed. 

1.2. Separation of the crude oil into refined products is done by fractional distillation, very shortly describe the process and explain why it is necessary to separate the residue at reduced pressure.  

Answer 1

1.1 The crude oil desalting process is carried out in the following steps :

• Dillution water injection and dispersion
• Emusification of diluted water in oil
• Distribution of the emulsion in the electrostatic field
• Electrostatic coalescence
• Water droplet settling

The Crude oil is passed into a cold preheat train and is then pumped to the Desalters by crude charge pumps. The recycled water from the desalters is injected in the crude oil containing sediments which produce salty water. This fluid enters in the static mixer, maximizing the interfacial surface area for optimal contact between both liquids.

The oil-water mixture is homogenously emulsified in the emulsifying device. The emulsifying device is used to emulsify the dilution water injected upstream in the oil. The emulsification is important for contact between the salty production water contained in the oil and the wash water.

The electrostatic coalescence is induced by the polarization effect resulting from an external electric source. Polarization of water droplets pulls them out from oil-water emulsion phase. Salt being dissolved in these water droplets, is also separated along the way.

The purpose of crude oil desalting is to remove undesirable impurities such as sand, inorganic salts, drilling mud, polymer, corrosion byproduct, etc, especially salts and water, from the crude oil which is introduced to refinery prior to fractional distillation.

1.2  The petroleum refining process is the separation of the different hydrocarbons present in the crude oil into useful fractions and the conversion of some of the hydrocarbons into products having higher quality.

Crude oil obtained from the desalter is further heated and then passed into a distillation column that allows the separation of the crude oil into different fractions depending on the difference in volatility (difference in boiling points).

The various components of crude oil have different sizes, weights and boiling temperatures; so the refining of crude oil begins with distilling the incoming crude oil by a process called fractional distillation. The steps of fractional distillation are as follows:

1. Heating of crude oil which is a mixture of two or more substances (liquids) with different boiling points to a high temperature.

2. The mixture boils, forming vapor (gases); which go into the vapor phase.

3. The vapor enters the bottom of a long column (fractional distillation column) that is filled with trays or plates. The trays have many holes or bubble caps in them to allow the vapor to pass through. They increase the contact time between the vapor and the liquids in the column and help to collect liquids that form at various heights in the column. There is a temperature difference across the column (hot at the bottom, cool at the top).

4. As the vapor rises through the trays in the column, it cools.

5. When a substance in the vapor reaches a height where the temperature of the column is equal to that of substance's boiling point, it will condense to form a liquid. (The substance with the lowest boiling point will condense at the highest point in the column; substances with higher boiling points will condense lower in the column.).

6. The trays collect the various liquid fractions.

7. The liquid fractions pass to condensers, which cool them further, and then go to storage.

Fractional distillation is useful for separating a mixture of substances with narrow differences in boiling points.

To further distill the residual oil from the atmospheric distillation column, the distillation must be performed at reduced pressures so as to limit the operating temperature.

It is important not to subject the crude oil to a higher temperatures because high molecular weight components in the crude oil will undergo thermal cracking and form petroleum coke at temperatures above that. Formation of coke would result in plugging the tubes in the furnace that heats the feed stream to the crude oil distillation column.

The process is the same as described above only that instead of atmospheric pressure, lower pressures are generated inside the whole setup.