REFINERY OPERATIONS
The function of the refinery is to convert crude oil into the finished products required by the market in the most efficient, and hence most profitable manner. The methods employed necessarily vary widely from one refinery to another, depending on the crude processed, the nature and location of the market, the type of equipment available, and many other factors. However, for simplification, it may be considered that all refining processes fall into one of four basic categories.
The first category is fractionation or distillation. This method of physically separating a mixture of compounds was the earliest process used in petroleum refining, and today is still one of the most important. However, since it is not generally possible to separate the complex petroleum mixtures into individual compounds, such mixtures are segregated into fractions or "cuts", each of which is characterized by a carefully controlled boiling range. These cuts are then further processed or utilized in the refinery operations.
The second basic type of process, essentially chemical in nature, consists of converting or chemically transforming certain of these "cuts" into products of higher commercial value. There are many ways of doing this, but all consist fundamentally of altering the molecular structure of the components. In the case of a heavy oil, the molecules may be cracked to form lighter, more valuable products, as for instance in catalytic cracking and coking. On the other hand, gaseous products may be polymerized or otherwise combined to form liquid products which may be blended into gasoline. With certain processes, e.g. catalytic reforming, both cracking and polymerization take place concurrently with the more desirable de-hydrogenation, hydrogenation, and isomerization reactions. The net result of all these transformations is the production of mixtures containing new arrays of hydrocarbons of higher value than the starting materials. Nearly all the fractions produced by the processes mentioned above contain certain objectionable constituents or impurities.
The third basic category is, therefore, treating. This group of processes includes the removal of the unwanted components, or their conversion to innocuous or less undesirable compounds. Removal of the impurities is sometimes accomplished by physical treating, as exemplified by the process for manufacturing kerosene, wherein sulfur and certain undesirable hydrocarbons are removed by extraction with liquid sulfur dioxide. Alternatively, the removal may be carried out by converting the unwanted compounds to a form more readily removed as is done in the hydrodesulfurization of diesel fuel. Here the sulfur compounds are cracked and hydrogenated. The sulfur is converted to hydrogen sulfide which can be readily separated from the heavier diesel oil by fractionation. An example of the conversion of undesirable components to innocuous compounds which remain in the product is found in the gasoline sweetening processes. There the mercaptans present give the product a foul, objectionable odor. The sweetening process Introductionto Petroleum Refinery Operations merely transforms the mercaptans to organic disulfides which are less objectionable. Although sulfur is perhaps the commonest and most troublesome of the impurities found in petroleum, it is certainly not the only one. Substances such as nickel, vanadium, and nitrogen may also be present in the crude oil. These impurities are undesirable because of the difficulty they cause during processing in the refinery or because of some detrimental effect during consumer use of the product. Furthermore, presence of certain hydrocarbons or certain types of hydrocarbons may lower the quality of a specific product. It was mentioned that aromatics are removed from kerosene by SO, extraction. The aromatics have undesirable burning characteristics and hence the product quality is improved if these "impurities" are removed. Lube oil treating process such as dewaxing, deasphalting, and phenol treating also fall into this category.
The fourth basic category is blending of the finished cuts into commercially saleable products such as motor gasoline, kerosene, lubricating oils, and bunker fuel oil, according to their specifications.
These four basic categories encompass the fundamental operation of a refinery. All other activities are carried out to implement them. The specifications for a given product are established to insure a satisfactory level of product performance. Specifications can be altered from time to time, but a product normally must meet the then existing product specifications. Various crudes on the other hand yield fractions with significantly different properties. At first glance, it might appear reasonable to select crudes to best match the product needs of each refinery. Many times, however, this is not economical as the money saved in eliminating various conversion and treating processes is offset by other factors. These might include crude availability, price, and transportation or specialty product requirements. A refinery is a sophisticated multi-component process operated in overall balance. The balance is set by economic considerations with the major variables being crude oil, process costs, and final products. It is thus easier to see why (1) no two refineries are exactly alike, (2) various conversion and purification processes are required, and (3) crude selection is important.
TYPES OF REFINERIES
Each refinery is designed to manufacture products as economically as possible based on the best knowledge available with regard to end product needs, future expansion plans, crude availability and other pertinent factors. A basic modern refinery which does not produce lubricating oils or chemicals is commonly referred to as a fuel products refinery. It is designed to produce primarily motor gasoline, distillate fuels (diesel oil, jet fuel, and heating oil), and bunker (residual) fuel oil. The fuel products refineries can be considered basic and minimum as regards refinery product and processing requirements. Hydroskimming and conversion are the two major variations of this type refinery. There is a wide range of conversion levels. The term maximum conversion type has no precise definition but is often used to describe a level of conversion ,where there is no net fuel oil manufacture. A fuel products refinery with specialities may manufacture lubricating oils, asphalts, greases, solvents, waxes and chemical feed stocks in addition to the primary fuel products. The number and diversity of products will naturally vary from one refinery to another.
Refineries produce chemical feed stocks for sale to the chemical affiliates and do not have responsibility for the manufacture of chemical products directly. Both operations may be carried out at the same physical location but the corporate product responsibilities are usually separate.
FUEL PRODUCTS REFINERY
Hydroskimmer
A hydroskimming refinery lends itself to locations where the market demands for the major fuel products (gasoline, gas oil, and residual fuel oil) approximate the quantities of these products obtainable by distillation from the available crudes. A typical hydroskimming refinery would include the following:
- Atmospheric Pipestill
- Powerforming (Catalytic Naphtha Reforming)
- Light Ends Recovery -Fractionation
- Treating and Blending
The atmospheric pipestill performs the initial distillation of crude oil into gas, naphtha, distillates, and residuum. The naphtha may be separated into gasoline blending stock, solvents, and Powerformer feed. The distillates include kerosene, jet fuel, heating oil and diesel oil. The residuum is blended for use as bunker fuel oil. The Powerforming unit is required to upgrade virgin naphtha to produce high octane gasoline. Powerforming is a fixed bed catalytic reforming process employing a regenerable platinum catalyst. In the process, a series of reactions takes place. The most important of these is aromatization; other reactions include isomerization, cracking, hydrogenation, and polymerization. The desired product is of approximately the same boiling range as the feed, but the molecules have been rearranged or reformed into higher octane compounds.
Light ends recovery and fractionating equipment is necessary after the Powerformer and on the pipestill overhead stream to separate the effluent mixtures into the desired boiling range cuts. Hydrofining is used to reduce sulfur and/or other impurities and to improve odor, color, and stability of the pipestill fractions. Hydrofining is a fixed-bed catalytic process using a regenerable cobalt molybdate catalyst in a hydrogen atmosphere. The hydrogen is produced by the Powerformer with supplemental hydrogen manufactured if necessary. The difficulty of hydrofining (desulfurization) increases with increase in the hydrocarbon boiling point. Naphthas are generally desulfurized up to 99+ % by hydrofining while the maximum desulfurization of distillates is usually 90 %. The components produced by the process sequence outlined above are blended as required to meet final product rates and qualities.
Conversion
The hydroskimming type refinery is used where the gasoline demand is substantially lower and hence the final product demand is close to that yielded by single stage distillation. In areas where the demand for gasoline is relatively high, conversion processing is required. The minimum processes for a fuel products refinery designed would typically include:
- Atmospheric and Vacuum Crude Distillation
- Catalytic Gas Oil Cracking
- Powerforming
- Light Ends Recovery -Fractionation
- Treating and Blending
The atmospheric P/Sresiduum can be fed to a vacuum pipestill. The vacuum tower enables the refiner to cut deeper into the crude, at the same time avoiding high temperatures (above about 750 OF) which cause thermal cracking with resultant deposition of coke and tarry residues in the equipment. The vacuum gas oil produced by vacuum distillation is fed to a catalytic crackmg unit for conversion into high octane gasoline blending stock. Byproducts are gas, distillate, cycle gas oil, and fractionator bottoms. The process uses a fluidized catalyst system. The catalyst is circulated continuously between the reactor where cracking takes place and the regenerator where the coke deposited on the catalyst is burned off. The major competing process is hydrocracking which offers greater conversion and flexibility but usually requires a higher investment.
Hydrocracking is a fixed bed catalytic process which cracks and hydrogenates hydrocarbon feeds. The process consumes large quantities of hydrogen and a hydrogen plant is usually necessary to support the operation. Practically any stock can be hydrocracked, including refractory feeds which resist conversion by other processes. In general, the very heavy residuum from the vacuum pipestill does not make good quality feed for catalytic cracking. In the refinery shown it is blended into residual fuel oil. Many times, however, the market for large volumes of residual fuel oil does not exist. When this is the case, additional conversion units are added to further process the vacuum pipestill bottoms. In other words, the higher the conversion of the refinery the more lighter fractions are produced. The relative levels of conversion vary from refinery to refinery.
A typical maximum conversion type refinery is shown in Figure 3. The higher conversion levels are obtained by ad&tional processing of the bottoms and/or light ends. To increase conversion of the bottoms the amount and/or severity of processing is increased. The resulting fuel oil levels may decrease to zero. Included here in addition to the basic components of a conversion refinery may be fluid coking, delayed coking, and/or visbreaking. These processes are basically thermal cracking processes for reducing the volume and viscosity of the vacuum residuum while producing appreciable quantities of lighter products.
Each of the three processes is commercially used with selection based on particular needs at a given refinery. Some of the various characteristics include:
1. Coking-Delayed Coking and Fluid Coking are the two major variations of this process. Fluid coking produces less coke as compared with delayed coking and hence yields a better product distribution. That is, for a given product slate less crude is converted into coke. The coke produced by fluid coking, however, is of little value as it consists of fine hard particles in contrast to large pieces for delayed coke. This difference in size and texture is important to electrode manufacturers who historically have used delayed coke.
2. Visbreaking is the least expensive of the cracking processes but is limited to the lowest conversion of perhaps 20 to 25% of the feed to 680 "F material. To obtain light ends conversion, alkylation and polymerization are used to increase the relative amounts of liquid fuel products manufactured. Alkylation converts olefins, (propylene, butylenes, amylenes, etc.), into high octane gasoline by reacting them with isobutane. Polymerization involves reaction of propylene and/or butylenes to produce an unsaturated hydrocarbon mixture in the motor gasoline boiling range.
An old variation of the conversion type is a catalytic combination unit. Development of this scheme was necessitated by the rising cost of refinery construction after World War I1 and by the great demand for capital for postwar expansion. The scheme reduced the investment and operating costs for refining equipment. The basic feature of the combination unit lies in the integration of the fractionation facilities of the reduced crude distillation and catalytic cracking sections.
A FUEL PRODUCTS REFINERY WITH SPECIALTIES
A fuel products refinery with specialties may manufacture products such as lubricating oils, asphalts, greases, solvents, waxes and chemical feed stocks in addition to the primary fuel products. The number and diversity of products will naturally vary from one refinery to another, but for purposes of discussion a fuel products refinery with specialties may include many of the following processes.
1. Two-Stage Crude Distillation (Atmospheric and Vacuum) -The vacuum stage can be used alternately to produce heavy gas oil for catalytic cracking feed or raw lube distillate cuts for lubricating oil manufacture.
2. Virgin Naphtha Catalytic Reforming (Powerforming) -This technique is used for the production of high octane motor gasoline, or as a source of aromatic compounds.
3. Light Ends Recovery, Fractionation, and Conversion -Propylenes and butylenes may be recovered for feed to a polymerization plant for production of high octane gasoline; or chemicals. Butylenes and isobutane may be desired for use in an alkylation plant where they are combined to make aviation gasoline and motor gasoline blendstocks. Propanes and butanes may be recovered in essentially pure form for sale as liquefied petroleum gases. It may be profitable to recover ethylene for chemical production. Certain of the light ends components, particularly ethylene, propylene, and butadiene are so in demand that processes such as steam cracking are employed specifically for their
4. Fuel Products Treating
a. Sweetening -This is a process for improving odor of gasolines, kerosenes, and heating oils. The foul smelling mercaptans are converted into bisulfides whose odor is much less objectionable. Among the types in use are copper chloride, hypochlorite, Merox, Mercapfining, and air inhibitor sweetening.
b. Hydroprocessing -The nomenclature system with regard to hydrogen processing is quite confusing with an array of labels involving trade names, terms such as mild, medium, and severe, high and low pressure. Choice of terminology varies widely from company to company. A wide variety of petroleum fractions may be treated at elevated temperature and pressure with hydrogen in the presence of a catalyst to reduce sulfur, improve stability, odor, combustion characteristics, appearance, and to convert heavy fractions to lighter more valuable products. The most severe form of hydroprocessing as discussed previously is hydrocracking. For fuel products treating, however, two less severe hydroprocessing operations are used, hydrofining and hydrotreating. Hydrofining usually involves only minor molecular changes of the feed with hydrogen consumption in the range of about 100 to 1,0oO cu.ft./bbl. Typical applications include desulfurization of a wide range of feeds (naphtha, light and heavy distillates, and certain residua) and occasional pretreatment of cat cracker feeds. Hydrotreating essentially involves no reduction in molecular size with hydrogen consumption less than about 100 cu. ft./bbl. Primary application is to remove small amounts of impurities with typical uses including naphtha and kerosene hydrosweetening.
c. SO, Extraction -This is a method of solvent extraction with liquid SO, to remove aromatic hydrocarbons and cyclic sulfur compounds. It is used to improve the burning qualities of kerosene and diesel fuels, and to reduce sulfur. This process has practically been supplanted by other solvent extraction or by hydrotreating.
5. Fluid catalytic Cracking.
6. Hydrocracking.
7. Residuum Conversion -Included here may be fluid coking, delayed coking, visbreaking, and residuum hydroprocessing.
8. Solvent Deasphalting -This is the solvent extraction of virgin residuum to remove asphaltenes or other tarry constituents. The deasphalted oil may be further processed into lubricating oils and greases, or used as cat cracking feed.
9. Lubricating Oil Manufacture -This will usually consist of the following processes: &. Solvent Deasphalting.
Phenol Treating -An extraction process for removal of aromatic asphaltic and sulfur compounds from the lube cut.
Solvent Dewaxing -Waxy lube is diluted with a solvent such as propane or methyl ethyl ketone (MEK), and cooled to crystallize the wax which is then removed by filtration.
10. Grease Manufacture -Selected lube oil fractions 2::: blended with various metallic soaps to produce high viscosity lubricating greases.
11. Wax Manufacture -A waxy distillate cut frox i(ude or the wax byproduct from lube oil dewaxing is first deoiled. Resulting low oil content wax is hydrofined for color improvement and fractionated into appropriate melting point grades.
12. Asphalt Manufacture -Saleable asphalts are produced from the residua of selected crudes. The residuum itself may be sold as straight reduced cuts to make it easier to handle, producing the so called cut-back asphalts. Another variation is air blown or oxidized asphalts for improved tenacity, greater resistance to weathering, and decreased brittleness. Emulsified asphalts are made for application at relatively low temperatures.
13. Chemical and Other Specialty Manufacture -A wide variety of products may be derived from petroleum feed stocks, including such diverse materials as alcohols, butyl rubber, sulfur, additives, and resins. Other specialties such as solvent naphthas, white oils, Isopars, Varsol, may also be produced. As indicated previously the respective chemical affiliate usually has responsibility for products broadly classified as petrochemicals.
There are many other processes used in refineries not mentioned here. The list above is intended only to emphasize the wide diversity of processing which is common to petroleum refining and to introduce in a very general way some of the more important of these processes. Also it must be emphasized that only fundamental principles of refinery operations have been discussed and modem manufacturing techniques vary widely from company to company.
