Refinery feedstock

Table 5.15 gives some physical-chemical characteristics of selected main refinery streams capable of being added to the diesel fuel pool. Also shown is the weight per cent yield corresponding to each stock, that is, the quantity of product obtained from the feedstock. 

This type of study, applied over all the cuts, enables the refinery flow scheme to be defined in order to satisfy a given set of market conditions starting from one or more crude oil feedstocks. 

Feedstocks are natural gas, refinery fuel gas, LPG and paraffinic naphthas. After elimination of CO2, the last traces of contaminants are converted to methane (methanation) or eliminated by adsorption on molecular sieves (PSA process). 

Simple conventional refining is based essentially on atmospheric distillation. The residue from the distillation constitutes heavy fuel, the quantity and qualities of which are mainly determined by the crude feedstock available without many ways to improve it. Manufacture of products like asphalt and lubricant bases requires supplementary operations, in particular separation operations and is possible only with a relatively narrow selection of crudes (crudes for lube oils, crudes for asphalts). The distillates are not normally directly usable processing must be done to improve them, either mild treatment such as hydrodesulfurization of middle distillates at low pressure, or deep treatment usually with partial conversion such as catalytic reforming. The conventional refinery thereby has rather limited flexibility and makes products the quality of which is closely linked to the nature of the crude oil used. 

TURBINE fuels), are both in demand. Solvent extraction is also extensively used to meet the growing demand for the high purity aromatics such as ben2ene, toluene, and xylene (BTX) as feedstocks for the petrochemical industry (see BTX PROCESSING FEEDSTOCKS,PETROCHEMICALS). Additionally, the separation of aromatics from aUphatics is one of the largest appHcations of solvent extraction (see Petroleum, refinery processes survey). 

Approximately 50—55% of the product from a coal-tar refinery is pitch and another 30% is creosote. The remaining 15—20% is the chemical oil, about half of which is naphthalene. Creosote is used as a feedstock for production of carbon black and as a wood preservative. Because of modifications to modem coking processes, tar acids such as phenol and cresyUc acids are contained in coal tar in lower quantity than in the past. To achieve economies of scale, these tar acids are removed from cmde coal tar with a caustic wash and sent to a central processing plant where materials from a number of refiners are combined for recovery. 

Olefins are produced primarily by thermal cracking of a hydrocarbon feedstock which takes place at low residence time in the presence of steam in the tubes of a furnace. In the United States, natural gas Hquids derived from natural gas processing, primarily ethane [74-84-0] and propane [74-98-6] have been the dominant feedstock for olefins plants, accounting for about 50 to 70% of ethylene production. Most of the remainder has been based on cracking naphtha or gas oil hydrocarbon streams which are derived from cmde oil. Naphtha is a hydrocarbon fraction boiling between 40 and 170°C, whereas the gas oil fraction bods between about 310 and 490°C. These feedstocks, which have been used primarily by producers with refinery affiliations, account for most of the remainder of olefins production. In addition a substantial amount of propylene and a small amount of ethylene ate recovered from waste gases produced in petroleum refineries. 

Since the early 1980s olefin plants in the United States were designed to have substantial flexibiHty to consume a wide range of feedstocks. Most of the flexibiHty to use various feedstocks is found in plants with associated refineries, where integrated olefins plants can optimize feedstocks using either gas Hquids or heavier refinery streams. Companies whose primary business is the production of ethylene derivatives, such as thermoplastics, tend to use ethane and propane feedstocks which minimize by-product streams and maximize ethylene production for their derivative plants. 

The market value of natural gas Hquids is highly volatile and historically has been weakly related to the world price of cmde oil. During the 1980s, the market value of natural gas Hquids ranged from approximately 60% of the price of cmde to 73% (12). In this 10-year interval, several fluctuations occurred in the natural gas Hquid market. Because of the variabiHty of the natural gas Hquid market, the NGL recovery plants need to have flexibiHty. Natural gas Hquid products compete in the following markets ethane propane a Hquefted petroleum gas (LPG) a C-3/C-4 mix and / -butane all compete as petrochemical feedstocks. Propane and LPG are also used as industrial and domestic fuels, whereas 2-butane and natural gasoline, consisting of C-5 and heavier hydrocarbons, are used as refinery feedstocks. 

Used oil disposal trends include waste minimisation such as by reclaiming used fluid on site, as well as recycling of mineral oil lubricants instead of disposing by incineration. The recycling effort involves a system where spent mineral oils are collected then shipped to specialty refineries where the materials are distilled, hydrofinished, and re-refined into fresh base stocks. These re-refined materials are virtually identical to virgin feedstocks. 

Relatively small amounts of methane, ethane, and propane also are produced as by-products from petroleum processes, but these usually are consumed as process or chemical feedstock fuel within the refineries. Some propane is recovered and marketed as LPG. 

As indicated in Table 4, large-scale recovery of natural gas Hquid (NGL) occurs in relatively few countries. This recovery is almost always associated with the production of ethylene (qv) by thermal cracking. Some propane also is used for cracking, but most of it is used as LPG, which usually contains butanes as well. Propane and ethane also are produced in significant amounts as by-products, along with methane, in various refinery processes, eg, catalytic cracking, cmde distillation, etc (see Petroleum). They either are burned as refinery fuel or are processed to produce LPG and/or cracking feedstock for ethylene production. 

Petroleum refining, also called petroleum processing, is the recovery and/or generation of usable or salable fractions and products from cmde oil, either by distillation or by chemical reaction of the cmde oil constituents under the effects of heat and pressure. Synthetic cmde oil, produced from tar sand (oil sand) bitumen, and heavier oils are also used as feedstocks in some refineries. Heavy oil conversion (1), as practiced in many refineries, does not fall into the category of synthetic fuels (syncmde) production. In terms of Hquid fuels from coal and other carbonaceous feedstocks, such as oil shale (qv), the concept of a synthetic fuels industry has diminished over the past several years as being uneconomical in light of current petroleum prices. 

Fig. 1. General refinery operations (a) light petroleum refining section (b) heavy feedstock refining section.

However, simplification of the refining procedure is not always the end result. IncompatibiUty of different cmde oils, which can occur if, for example, a paraffinic cmde oil is blended with a heavy asphaltic oil, can cause sediment formation ia the unrefined feedstock or ia the products, thereby complicating the refinery process (5). 

Naphthalene (qv) from coal tar continued to be the feedstock of choice ia both the United States and Germany until the late 1950s, when a shortage of naphthalene coupled with the availabihty of xylenes from a burgeoning petrochemical industry forced many companies to use o-xylene [95-47-6] (8). Air oxidation of 90% pure o-xylene to phthaUc anhydride was commercialized ia 1946 (9,10). An advantage of o-xylene is the theoretical yield to phthaUc anhydride of 1.395 kg/kg. With naphthalene, two of the ten carbon atoms are lost to carbon oxide formation and at most a 1.157-kg/kg yield is possible. Although both are suitable feedstocks, o-xylene is overwhelmingly favored. Coal-tar naphthalene is used ia some cases, eg, where it is readily available from coke operations ia steel mills (see Steel). Naphthalene can be produced by hydrodealkylation of substituted naphthalenes from refinery operations (8), but no refinery-produced napthalene is used as feedstock. Alkyl naphthalenes can be converted directiy to phthaUc anhydride, but at low yields (11,12). 

The indirect hydration, also called the sulfuric acid process, practiced by the three U.S. domestic producers, was the only process used worldwide until ICI started up the first commercial direct hydration process in 1951. Both processes use propylene and water as raw materials. Early problems of high corrosion, high energy costs, and air pollution using the indirect process led to the development of the direct hydration process in Europe. However, a high purity propylene feedstock is required. In the indirect hydration process, C -feedstock streams from refinery off-gases containing only 40—60 wt % propylene are often used in the United States. 

Production estimates for propylene can only be approximated. Refinery propylene may be diverted captively to fuel or gasoline uses whenever recovery is uneconomic. Steam-cracker propylene production varies with feedstock and operating conditions. Moreover, because propylene is a by-product, production rates depend on gasoline and ethylene demand. 

The uses of propylene may be loosely categorized as refinery or chemical purpose. In the refinery, propylene occurs in varying concentrations in fuel-gas streams. As a refinery feedstock, propylene is alkylated by isobutane or dimerized to produce polymer gasoHne for gasoHne blending. Commercial chemical derivatives include polypropylene, acrylonitrile, propylene oxide, isopropyl alcohol, and others. In 1992, ca 64% of U.S. propylene suppHes were consumed in the production of chemicals (74). Polypropylene has been the largest consumer of propylene since the early 1970s and is likely to dominate propylene utilization for some time. 

The largest use of NMP is in extraction of aromatics from lube oils. In this appHcation, it has been replacing phenol and, to some extent, furfural. Other petrochemical uses involve separation and recovery of aromatics from mixed feedstocks recovery and purification of acetylenes, olefins, and diolefins removal of sulfur compounds from natural and refinery gases and dehydration of natural gas. 

The ethylene feedstock used in most plants is of high purity and contains 200—2000 ppm of ethane as the only significant impurity. Ethane is inert in the reactor and is rejected from the plant in the vent gas for use as fuel. Dilute gas streams, such as treated fluid-catalytic cracking (FCC) off-gas from refineries with ethylene concentrations as low as 10%, have also been used as the ethylene feedstock. The refinery FCC off-gas, which is otherwise used as fuel, can be an attractive source of ethylene even with the added costs of the treatments needed to remove undesirable impurities such as acetylene and higher olefins. Its use for ethylbenzene production, however, is limited by the quantity available. Only large refineries are capable of deUvering sufficient FCC off-gas to support an ethylbenzene—styrene plant of an economical scale. 

Butanol is produced commercially by the indirect hydration of / -butenes. However, current trends are towards the employment of inexpensive Raffinate 11 type feedstocks, ie, C-4 refinery streams containing predominandy / -butenes and saturated C-4s after removal of butadiene and isobutylene. In the traditional indirect hydration process, / -butenes are esterified with Hquid sulfuric acid and the intermediate butyl sulfate esters hydroly2ed. DEA Mineraloel (formerly Deutsche Texaco) currentiy operates a 2-butanol plant employing a direct hydration of / -butenes route (18) with their own proprietary catalyst. 

The other significant industrial route to /-butyl alcohol is the acid cataly2ed hydration of isobutylene (24), a process no longer practiced in the United States. Raffinate 1, C-4 refinery streams containing isobutylene [115-11-7], / -butenes and saturated C-4s or C-4 fluid catalytic cracker (ECC) feedstocks (23)... 

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