Crude Oil Refinery

Crude Oil Refinery

Crude oil refinery is an organized and coordinated arrangement of manufacturing processes designed to produce physical and chemical changes in crude oil to convert it to everyday useful products such as petrol, diesel, kerosene, aviation fuel, bitumen refinery gas and sulfur. A typical crude oil refinery processes about a hundred thousand to several hundred thousand barrels of crude oil a day. Because of the high capacity, many of the refinery units operate continuously in a steady state or nearly steady state for months to years.

Actually, crude oil in its natural form contains a mixture of hydrocarbons and relative small quantities of other materials which are removed and the oil broken down to its various components. The steps involved in a typical refinery are the separation process, the conversion process and the purification process. Now, let’s briefly look at how these processes are carried out in a crude oil refinery:

The Separation Process in a Crude Oil Refinery

The crude oil is first separated by boiling points into six main grades of hydrocarbons: refinery gas (used for refinery fuel), gasoline (naphthas), kerosene, light and heavy gas oils and long residue. This initial separation is done by distillation. The long residue is further separated in the butane desaphalting unit, and some of these components (such as the refinery gas) are further separated with chemical reactions and by using solvents which dissolve one component of a mixture significantly better than another.

The Conversion Process in a Crude Oil Refinery

Of the crude oils separated out from the original crude (refinery gas, gasoline, kerosene, light and heavy gasoils and asphalt), only refinery gas can be used as is, all the others require some further treatment before they can be made into the final product. This firstly involves the removal of sulfur (as it interferes with the success of some later later processes) and then the chemical conversion of the oils into more desirable compounds. For example, naphthas are "reformed" from paraffins and naphthenes into aromatics. The chemical equilibria are also manipulated to ensure a maximum yield of the desired product.

The Purification Process in a Crude Oil Refinery

The crude oil has now been separated into refinery gas, hydrogen sulfide, naphtha, kerosene, gas oil, asphalt and bitumen. Two more processes have to be carried out, on the naphtha and the hydrogen sulfide respectively, before the hydrocarbons are ready for blending into saleable products. Some refinery plants also manufactures its own hydrogen and purifies its own effluent water. This water purification, along with gas 'scrubbing' to remove undesirable compounds from the gases to be discharged into the atmosphere, ensures that the refinery has minimal environmental impact.

Common Process Units Found in a Refinery

Desalter Unit

Removes salt from the crude oil before it enters the atmospheric distillation unit.

Atmospheric Distillation Unit

Distill crude oil into fractions.

Vacuum Distillation Unit

Further distills residual bottoms after atmospheric distillation.

Naphtha Hydrotreater Unit

Uses hydrogen to desulfurize naphtha from atmospheric distillation and must hydrotreat the naphtha before sending to a Catalytic Reformer unit.

Catalytic Reformer Unit

Used to convert the naphtha-boiling range molecules into higher octane reformate (reformer product). The reformate has higher content of aromatics and cyclic hydrocarbons). An important byproduct of a reformer is hydrogen released during the catalyst reaction. The hydrogen is used either in the hydrotreaters or the hydrocracker.

Distillate Hydrotreater Unit

Removes sulfur from distillates (such as diesel) after atmospheric distillation.

Fluid Catalytic Cracker (FCC) Unit

Upgrade heavier fractions into lighter, more valuable products.

Hydrocracker unit

Use hydrogen to upgrade heavier fractions into lighter, more valuable products.

Visbreaking unit

Upgrade heavy residual oils by thermally cracking them into lighter, more valuable reduced viscosity products.

Merox Unit

Treat LPG, kerosene or jet fuel by oxidizing mercaptans to organic disulfides.

Coking Units

Process very heavy residual oils into gasoline and diesel fuel, leaving petroleum coke as a residual product.

Alkylation unit

Produce high-octane component for gasoline blending.

Dimerization Unit

Convert olefins into higher-octane gasoline blending components.

Isomerization unit

Converts linear molecules to higher-octane branched molecules for blending into gasoline or feed to alkylation units.

Steam Reforming Unit

Produce hydrogen for the hydrotreaters or hydrocracker.

Liquified Gas Storage Vessels

Stores propane and similar gaseous fuels at pressure sufficient to maintain them in liquid form. These are usually spherical vessels or "bullets" (i.e., horizontal vessels with rounded ends).

Storage Tanks

Store crude oil and finished products, usually cylindrical, with some sort of vapor emission control and surrounded by an earthen berm to contain spills.

Claus Unit

Convert hydrogen sulfide from hydrodesulfurization into elemental sulfur.

Utility Units

Such as cooling towers circulate cooling water, boiler plants generates steam, and instrument air systems include pneumatically operated control valves and an electrical substation.

Wastewater Collection and Treating Systems

Consist of API separators, dissolved air flotation (DAF) units and further treatment units such as an activated sludge biotreater to make water suitable for reuse or for disposal.

Solvent Refining Units

Use solvent such as cresol or furfural to remove unwanted, mainly aromatics from lubricating oil stock or diesel stock.

Solvent Dewaxing Units

Remove the heavy waxy constituents petrolatum from vacuum distillation products.

Let’s look at environmental impact of crude oil refineries:

Crude Oil Refinery and the Environment

Air, water and land can all be affected by refinery operations. The refining process releases a number of different chemicals into the atmosphere and a notable odor normally accompanies the presence of a refinery. Aside from air pollution impacts there are also wastewater concerns, risks of industrial accidents such as fire and explosion, and noise health effects due to industrial noise.

Many governments worldwide have mandated restrictions on contaminants that refineries release, and most refineries have installed the equipment needed to comply with the requirements of the pertinent environmental protection regulatory agencies. Crude oil refinery environmental and safety concerns mean that oil refineries are sometimes located some distance away from major urban areas.