Refining Process

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Crude Oil

Crude oils are complex mixtures containing many different hydrocarbon compounds that vary in appearance and composition from one oil field to another. Crude oils range in consistency from water to tar-like solids, and in color from clear to black. An average crude oil contains about 84% carbon, 14% hydrogen, 1 - 3% sulfur, and less than 1% each of nitrogen, oxygen, metals, and salts.

Crude oils are generally classified as paraffinic, naphthenic, or aromatic, based on the predominant proportion of similar hydrocarbon molecules. Mixed - base crudes have varying amounts of each type of hydrocarbon. Refinery crude base stocks usually consist of mixtures of two or more different crude oils.

Refining of Petroleum Crude oil

Petroleum refining begins with the distillation, or fractionation of crude oils into separate hydrocarbon groups. The resultant products are directly related to the characteristics of the crude processed. Most distillation products are further converted into more usable products by changing the size and structure of the hydrocarbon molecules through cracking, reforming, and other conversion processes as discussed below.

These converted products are then subjected to various treatment and separation processes such as extraction, hydro treating, and sweetening to remove undesirable constituents and improve product quality. Integrated refineries incorporate fractionation, conversion, treatment, and blending operations and may also include petrochemical processing.

The first step in the refining process is the separation of crude oil into various fractions or straight run cuts by distillation in atmospheric and vacuum towers. The main fractions or cuts obtained have specific boiling-point ranges and can be classified in order of decreasing volatility into gases, light distillates, middle distillates, gas oils & residues.

Fractional Distillation

At the refinery, the de-salted crude feedstock is preheated using recovered process heat. The feedstock then flows to a direct-fired crude charge heater and then it is fed into the vertical distillation column just above the bottom, at pressures slightly above atmospheric and at a temperatures ranging from 650 to 700 Deg F (heating crude oil above this temperature may cause undesirable thermal cracking). All but the heaviest fractions flash into vapor. As the hot vapor rises in the tower, its temperature is reduced by the circulating refluxes. Reduced Crude Oil (RCO) is taken from the bottom. At successive higher points on the tower, various major products including Naphtha, Gasoline, Kerosene, Diesel and uncondensed gasses (which condense at lower temperature) are drawn off.

Vacuum Distillation

In order further to distill the residuum or topped crude from the atmospheric tower at higher temperatures, reduced pressure is required to prevent thermal cracking. The process takes place in one or more vacuum distillation towers. The principles of vacuum distillation resemble those of fractional distillation and, except that larger diameter column is used to maintain comparable vapor velocities at the reduced pressures, the equipment is also similar. The internal designs of some vacuum towers are different from atmospheric towers in that random packing and demister pads are used instead of trays. A typical vacuum tower may produce gas oils, lubricating - oil base stocks, and heavy residual for propane de-asphalting. Vacuum towers are typically used to separate catalytic cracking feed-stocks from surplus residuum.

Cracking

Because the simple distillation of crude oil produces amounts and types of products that are not consistent with those required by the marketplace, subsequent refinery processes change the product mix by altering the molecular structure of the hydrocarbons. One of the ways of accomplishing this change is through cracking, a process that breaks or cracks the heavier, higher boiling - point petroleum fractions into more valuable products such as gasoline, fuel oil, and gas oils. The most common type of cracking is catalytic cracking.

Catalytic cracking breaks complex hydrocarbons into simpler molecules in order to increase the quality and quantity of lighter, more desirable products and decrease the amount of residuals. This process rearranges the molecular structure of hydrocarbon compounds to convert heavy hydrocarbon feed stocks into lighter fractions such as kerosene, gasoline, LPG, heating oil, and petrochemical feed stocks. Typical temperatures are from 850 - 950 degrees F at much lower pressures of 10 - 20 psi. The catalysts used in refinery cracking units are typically solid materials (zeolite, aluminum hydrosilicate, treated bentonite clay, fullers earth, bauxite, and silica - alumina) that come in the form of powders, beads, pellets or shaped materials called extrudites.

The three types of catalytic cracking processes are:

  • Fluid catalytic cracking (FCC)
  • Moving - bed catalytic cracking and
  • Thermofor catalytic cracking (TCC)

The catalytic cracking process is very flexible, and operating parameters can be adjusted to meet changing product demand. In addition to cracking, catalytic activities include dehydrogenation, hydrogenation, and isomerization.

The most common process is Fluid Catalytic Cracking (FCC), in which the oil is cracked in the presence of a finely divided catalyst which is maintained in an aerated or fluidized state by the oil vapors. The fluid cracker consists of a catalyst section and a fractionating section that operate together as an integrated processing unit. The catalyst section contains the reactor and regenerator, which with the standpipe and riser forms the catalyst circulation unit. The fluid catalyst is continuously circulated between the reactor and the regenerator using air, oil vapors, and steam as the conveying media.

A typical FCC process involves mixing a preheated hydrocarbon charge with hot, regenerated catalyst as it enters the riser leading to the reactor. The charge is combined with a recycle stream within the riser, vaporized, and raised to reactor temperature (900 - 1,000 degrees F) by the hot catalyst. As the mixture travels up the riser, the charge is cracked at 10 - 30 psi.

In the more modern FCC units, all cracking takes place in the riser. The reactor no longer functions as a reactor; it merely serves as a holding vessel for the cyclones. This cracking continues until the oil vapors are separated from the catalyst in the reactor cyclones. The resultant product stream (cracked product) is then charged to a fractionating column where it is separated into fractions, and some of the heavy oil is recycled to the riser.

Spent catalyst is regenerated to get rid of coke that collects on the catalyst during the process. Spent catalyst flows through the catalyst stripper to the regenerator, where most of the coke deposits burn off at the bottom where preheated air and spent catalyst are mixed. Fresh catalyst is added and worn - out catalyst removed to optimize the cracking process.

Solvent treating is a widely used method of refining lubricating oils as well as a host of other Refinery stocks. Since distillation (fractionation) separates petroleum products into groups only by their boiling - point ranges, impurities may remain. These include organic compounds containing sulfur, nitrogen, and oxygen; inorganic salts and dissolved metals; and soluble salts that were present in the crude feedstock.

The purpose of solvent extraction is to improve finished products by removing unsaturated, aromatic hydrocarbons from lubricant and grease stocks. The solvent extraction process separates aromatics, naphthenes, and impurities from the product stream by dissolving them in solvent. The solvent is separated from the product stream by heating, evaporation, or fractionation, and residual trace amounts are subsequently removed from the raffinate by steam stripping. The solvent thus regenerated may be used again in the process.

The most widely used extraction solvents are phenol, furfural, and NMP (N - Methyl Pyrledene). Other solvents less frequently used are liquid sulfur dioxide, nitrobenzene, and 2,2 dichloroethyl ether. The selection of specific processes and chemical agents depends on the nature of the feedstock being treated, the contaminants present, and the finished product requirements.

Solvent dewaxing is used to remove wax from either distillate or residual base stocks at any stage in the refining process. There are several processes in use for solvent dewaxing, But all have the same general steps, which are:

  • Mixing the feedstock with a solvent
  • Precipitating the wax from the mixture by chilling, and
  • Recovering the solvent from the wax and dewaxed oil for recycling by distillation and steam stripping

Usually solvents used are Propane & methyl ethyl ketone (MEK), Other solvents that are sometimes used include benzene, Toulene, methyl isobutyl ketone, petroleum naphtha, ethylene dichloride, methylene chloride, and sulfur dioxide. In addition, there is a catalytic process used as an alternate to solvent dewaxing.