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Catalytic dewaxing is a cost effective and flexible alternative to solvent dewaxing. Unlike solvent dewaxing, which physically separates wax crystals, catalytic dewaxing transforms the wax into either non-waxy isoparaffinic lube molecules or light-fuel by-products, depending on the dewaxing catalyst.
Mobil first commercialized the catalytic dewaxing process in 1979 with the Mobil lube dewaxing process (MLDW) based on the shape-selective cracking of its proprietary ZSM catalyst. MLDW charge stocks can include solvent-refined raffinates spanning the entire viscosity range from spindle oil through bright stock and stocks from high pressure hydroheaters. In recent years, Mobil has developed a second generation process and catalyst known as Mobil selective dewaxing (MSDW) targeted for hydro-cracked or severely hydrotreated stocks. The improved selectivity of MSDW for the highly desirable isoparaffinic-lube components results in higher lube yields and product VIs than either MLDW or solvent dewaxing. MSDW-charged stocks, however, must have lower sulfur and nitrogen contents than used for MLDW. This typically requires the feed to be treated via hydrocracking or moderate-to-high-pressure hydrotreating and interstage-gas separation to reduce H2S and NH3 in the de-waxer unit treat gas. MSDW charge stocks can include lube hydrocrackates, fuels-hydrocracker bottoms, and hydrotreated raffinates.
The flow scheme and equipment requirements for either MLDW or MSDW are similar to those of a middle-distillate desulfurizer. The reaction section includes a two-reactor system. The first reactor contains the proprietary Mobil dewaxing catalyst and the second reactor contains a hydrotreating catalyst to impart product stability. Both reactors include a unique distribution system to ensure good oil, gas, and catalyst contacting. After leaving the reactors, the reaction mixture is passed through conventional exchange, pressure reduction, and separation to provide a hydrogen-rich recycle gas, byproducts, and a lube base oil which meets specification pour and flash points.
A detailed economic comparison of solvent and catalytic dewaxing, technologies completed by a third-party engeneering company has shown an advantage of about $5/bbl of product for catalytic dewaxing over solvent dewaxing based on lower operating and investment costs.
Commercial applications of catalytic dewaxing include:
Debottlenecking lube plants by taking slow-filtering heavy stocks off solvent dewaxing units and dewaxing them more efficiently catalytically;
Improving the efficiency of existing lube plants by replacing old, inefficient solvent dewaxing units;
Improving economics of grassroot solvent or hydrocracking-based lube plants;
Making very low-pour specialty oils, which are not attainable by solvent dewaxing.
To date, 12 MLDW units have been streamed, with two more expected to be completed by early 1998. Three of these units are located in Mobil refineries and the rest are licensed.
WHAT IS A REFINERY
A refinery is a factory. Just as a paper mill turns lumber into legal pads or a glassworks turns silica into stemware, a refinery takes a raw material - crude oil - and transforms it into gasoline and hundreds of other useful products.
A typical large refinery costs billions of dollars to build and millions more to maintain and upgrade. It runs aroun the clock 365 days a year, employs between 1,000 and 2,000 people and occupies as much land as several hundred football fields. It so big and sprawling, in fact, that workers ride bicycles from one station to another. Chevron has five gasoline-producing factories in the United States and another in British Columbia. Chevron Texaco has refining capacities worldwide of over two million barrels per day. These world-class operations had surprisingly low origins. In 1876, company pioneers used wagons and mules to haul two primitive stills to a spot near Pico Canyon, California, the site of California’s first producing oil wells. The stills, each about the size of a garage, were used to heat oil at the prodigious rate of 25 to40 barrels a day. This «oil boiling» produced kerosene, lubricants, waxes and gasoline - a clear, lightweight liquid that generally was discarded as a useless byproduct. Gasoline’s low status rose quickly after 1892, when Charles Duryea built first US gas-powered automobile. From then on, the light stuff from crude oil became the right stuff.
Today, some refineries can turn more than half of every 42-gallon barrel of crude oil into gasoline. That’s a remarkable technological improvement from 70 years ago, when only 11 gallons of gasoline could be produced. How does this transformation take place? Essentially, refining breaks crude oil down into its various components, which then are selectively reconfigured into new products. This process takes place inside a maze of hardware that one observer has likened to « a metal spaghetty factory». Employees regulate refinery operations within highly automated control rooms. Because so much activity happens out of sight, refineries are surprisingly quiet places. The only sound most visitors hear is the constant, low hum of heavy equipment.
The complexity of this equipment varies from one refinery to the next. In general, the more sophisticated a refinery, the better its ability to upgrade crude oil into high-value products.
SEPARATION: HEAVY ON THE BOTTOM, LIGHT ON THE TOP
All refineries perform three basic steps: separation, conversion and treatment. Modern separation involves piping oil through hot furnaces. The resulting liquids and vapors are discharged into distillation towers, tall narrow columns that give refineries their distinctive skylines.
Inside the towers, the liquids and vapors separate into components or fractions according to weight and boiling point. The lightest fractions, including gasoline and liquid petroleum gas (LPG), vaporize and rise to the top of the tower, where they condense to liquids. Medium weight liquids, including kerosene and diesel oil distillates, stay in the middle. Heavier liquids, called gas oils, separate lower down, while the heaviest fractions with the highest boiling points settle at the bottom. These tarlike fractions, called residuum, are literally the bottom of the barrel.
The fractions are now ready for piping to the next station or plant within the refinery. Some components require relatively little additional processing to become asphalt base or jet fuel. However, most molecules that are destined to become high-value products require much more processing.
CONVERSION:
CRACKING AND REARRANGING MOLECULES
TO ADD VALUE
At the stage of conversion fractions from the distillation towers are transformed into streams (intermediate components) that eventually become finished products. This also is where a refinery makes money, because only through conversion can most low-value fractions become gasoline.
The most widely used conversion method is called cracking because it used heat and pressure to crack heavy hydrocarbon molecules into lighter ones. A cracking unit consists of one or more tall, thick-walled, bullet-shaped reactors and a network of furnaces, heat exchangers and other vessels. Fluid catalytic cracking or cat cracking is the basic gasoline-making process. Using intense heat (about 1,000 degrees Fahrenheit), low pressure and a powdered catalyst , the cat cracker can convert most relatively heavy fractions into smaller gasoline molecules.
Hydrocracking applies the same principles but uses a different catalyst, slightly lower temperatures, much greater pressure and hydrogen to obtain chemical reactions. Although not all refineries employ hydrocracking, Chevron is an industry leader in using this technology to cost-effectively convert medium- to heavyweight gas oils into high-value streams. Some Chevron refineries also have cockers, which use heat and moderate pressure to turn residuum into lighter products and a hard, coal-like substance that is used as an industrial fuel. Cockers are among the more peculiar-looking refinery structures. They resemble a series of giant drums with metal derricks on top.
Other refinery processes , instead of splitting molecules, rearrange them to add value. Alkylation makes gasoline components by combining some of the gaseous byproducts of cracking. This process takes place in a series of large, horizontal vessels and tall, skinny towers. Reforming uses heat, moderate pressure and catalysts to turn naphtha, a light, relatively low-value fraction into high-octane gasoline components.
TREATMENT
Today a major proportion of refining involves blending, purifying, fine-tuning and otherwise improving products to meet modern requirements. Refinery technicians carefully combine a variety of streams from the processing units. Among the variables that determine the blend and the octane level, vapor pressure ratings and special considerations, such as whether the gasoline will be used at high altitudes. They add some performance additives and dyes that distinguish the various grades of fuel. Refining has come a long way since the oil boiling days of Pico Canyon. By the time a gallon of gasoline is pumped into a car’s tank, it contains more that 200 hydrocarbons and additives. All that changing of molecules pays off in a product that ensures smooth, high performance driving.
REFRIGERATING EQUIPMENT OF SHIPS
Тематический словарь:
Refrigerating plant - холодильная установка
navigation area - район плавания
ships of unrestricted service - суда неограниченного района плавания
design conditions - расчетный условия
cargo spaces - грузовые помещения
intercascade heat exchanger - межкаскадный теплообменник
intermediate vessel - промежуточный сосуд
standby pump - резервный насос
secondary refrigerant - холодоноситель
ammonia - аммиак