prp unit - 3

UNIT 3: PRP by M.A.KHAN @AHCET Page 1

UNIT -3: FRACTIONATION OF PETROLEUM

The petroleum refining industry converts crude oil into more than 2500 refined products, including liquefied petroleum gas, gasoline, kerosene, aviation fuel, diesel fuel, fuel oils, lubricating oils, and feedstocks for the petrochemical industry. Petroleum refinery activities start with receipt of crude for storage at the refinery, include all petroleum handling and refining operations, and they terminate with storage preparatory to shipping the refined products from the refinery. The petroleum refining industry employs a wide variety of processes. A refinerys processing flow scheme is largely determined by the composition of the crude oil feedstock and the chosen slate of petroleum products. The example refinery flow scheme presented in Figure shows the general processing arrangement used by refineries in general. The arrangement of these processes will vary among refineries, and few, if any, employ all of these processes. The refinery processes are classified into the following categories.

1. Separation processes a. Atmospheric distillation b. Vacuum distillation c. Light ends recovery (gas processing)

The first phase in petroleum refining operations is the separation of crude oil into its major constituents using 3 petroleum separation processes: atmospheric distillation, vacuum distillation, and light ends recovery (gas processing). Crude oil consists of a mixture of hydrocarbon compounds including paraffinic, naphthenic, and aromatic hydrocarbons with small amounts of impurities including sulfur, nitrogen, oxygen, and metals. Refinery separation processes separate these crude oil constituents into common boiling-point fractions.

2. Petroleum conversion processes

a. Cracking (thermal and catalytic) b. Reforming c. Alkylation d. Polymerization

e. Isomerisation f. Coking g. Visbreaking

To meet the demands for high-octane gasoline, jet fuel, and diesel fuel, components such as residual oils, fuel oils, and light ends are converted to gasoline and other light fractions.


Cracking, coking, and visbreaking processes are used to break large petroleum molecules into smaller ones. Polymerization and alkylation processes are used to combine small petroleum molecules into larger ones. Isomerisation and reforming processes are applied to rearrange the structure of petroleum molecules to produce higher-value molecules of a similar molecular size.

3. Petroleum treating processes

a. Hydro desulfurization b. Hydro treating c. Chemical sweetening

d. Acid gas removal e. Deasphalting

Petroleum treating processes stabilize and upgrade petroleum products by separating them from less desirable products and by removing objectionable elements. Undesirable elements such as sulfur, nitrogen, and oxygen are removed by hydro desulfurization, hydro treating, chemical sweetening, and acid gas removal. Treating processes, employed primarily for the separation of petroleum products, include such processes as deasphalting. Desalting is used to remove salt, minerals, grit, and water from crude oil feedstocks before refining. Asphalt blowing is used for polymerizing and stabilizing asphalt to improve its weathering characteristics

4. Feedstock and product handling

a. Storage b. Blending

c. Loading d. Unloading

5. Auxiliary facilities a. Boilers b. Waste water treatment c. Hydrogen production d. Sulfur recovery plant

e. Cooling towers f. Blow down system g. Compressor engines


A wide assortment of processes and equipment not directly involved in the refining of crude oil is used in functions vital to the operation of the refinery. Examples are boilers, waste water treatment facilities, hydrogen plants, cooling towers, and sulfur recovery units. Products from auxiliary facilities (clean water, steam, and process heat) are required by most process units throughout the refinery.

ATMOSPHERIC DISTILLATION UNIT :

Process description

The first process encountered in any conventional Refinery is the Atmospheric Crude Distillation Unit. In this unit the crude oil is distilled to produce distillate streams which will be the basic streams for the refinery product slate. These streams will either be subject to further treating downstream or become feed stock for conversion units that may be in the Refinery Configuration. A schematic flow diagram of an atmospheric crude unit is shown in Figure

Crude oil is pumped from storage to be heated by exchange against hot overhead and product

side streams in the Crude Unit. At a preheat temperature of about 200250F water is injected into the crude to dissolve salt that is usually present. The mixture enters a desalter drum usually containing an electrostatic precipitator. The salt water contained in the crude is separated by means of this electrostatic precipitation. The water phase from the drum is sent to a sourwater stripper to be cleaned before disposal to the oily water sewer. It must be

understood however that this de-salting does not remove the organic chlorides which may be present in the feed. This will be discussed later when dealing with the towers overhead system. The crude oil leaves the desalter drum and enters a surge drum. Some of the light ends and any entrained water are flashed off in this drum and routed directly to the distillation tower flash zone (they do not pass through to the heater). The crude distillation booster pump takes suction from this drum and delivers the desalted crude under flow control to the fired heater via the remaining heat exchange train. On leaving heat exchanger train, the crude oil is heated in a fired heater to a temperature that will vaporize the distillate products


in the crude tower. Some additional heat is added to the crude to vaporize about 5% more


than required for the distillate streams. This is called over flash and is used to ensure good


reflux streams in the tower. The heated crude enters the fractionation tower in a lower section


called the flash zone. The unvaporized portion of the crude leaves the bottom of the tower via


a steam stripper section, while the distillate vapors move up the tower counter current to a


cooler liquid reflux stream. Heat and mass transfer take place on the fractionating trays


contained in this section of the tower above the flash zone. Distillate products are removed


from selected trays (draw-off trays) in this section of the tower. These streams are stream

UNIT 3: PRP by M.A.KHAN @AHCET

stripped and sent to storage. The full naphtha vapor is allowedbe condensed and collected in the overhead drum. Areflux while the remainder is delivered to the lightdistillation.


stripped and sent to storage. The full naphtha vapor is allowed to leave the top of the tower to be condensed and collected in the overhead drum. A portion of this stream is returned as reflux while the remainder is delivered to the light end processes for stabilizing and further

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to leave the top of the tower to

stream is returned as end processes for stabilizing and further


A Pump around section is included at the light gas oil draw off. This is simply an internal condenser which takes heat out of that section of the tower. This in turn ensures a continued reflux stream flow below that section. The product side streams are stripped free of entrained light ends in separate stripping towers. These towers also contain fractionation trays (usually four but sometimes as many as six) and the side stream drawn off the main tower enters the top tray of its respective stripper. Steam is injected below the bottom tray and moves up the tower to leave at the top, together with the light ends strip out, and is returned to the main fractionator at a point directly above the side stream draw-off tray. These side stream stripper towers are usually stacked one above the other in a single column in such a way as to allow free flow from the side stream draw-off tray to its stripper tower. On a few occasions, where the particular side stream specification requires it, the stripping may be effected by reboiling instead of using steam. One such requirement maybe in the Kero side stream if this stream is to be routed directly into jet fuel blending and therefore must be dry.

The residue (unvaporized portion of the crude) leaves the flash zone to flow over four stripping trays counter current to the flow of stripping steam. This stripping steam enters the

tower below the bottom stripping tray. Its purpose primarily is to strip the residue free of entrained light ends. The fact that this steam enters the flash zone it also enhances the flashing of the crude in this zone by creating a reduced partial pressure for the liquid/vapor separation. This becomes an important factor in the design and operation of the atmospheric crude distillation unit. The stripped residue leaves the bottom of the unit to be routed either

through the units heat exchanger system and the to product storage or hot to some down stream processing unit such as a vacuum distillation unit or a thermal cracker.

Vacuum Distillation Unit (VDU)

The atmospheric residue is further distilled to provide the heavy distillate streams used for producing lube oil or as feed to conversion units. This distillation however has to be conducted under sub atmospheric pressure conditions. The temperature required for vaporising the residue at atmospheric pressure would be too high and the crude would crack. The process follows very much the same pattern as the atmospheric distillation. Should the cold feed be pumped from storage, it is heat exchanged against hot product and pump around streams before being vaporised in the distillation unit heater. Normally though the feed is


pumped hot directly from the CDUs residue stripper to the vacuum units heater. Thereafter the distillate vapours are condensed in the tower by heat and mass transfer with the cold reflux streams moving down the tower in the same way as the side streams Atmospheric unit. The products are taken off at the appropriate sections are cooled either by heat exchange with colder streams in the atmospheric unit, by air coolers or, in some cases as heating mediums to light end residue that leaves the bottom of the tower in this process nor thestripped. The vacuum condition is produced by steam ejectorsthe tower. These ejectors remove inert andabout 5 mm HG absolute. The tower internals are

pressure drop such that the flash zone


pumped hot directly from the CDUs residue stripper to the vacuum units heater. Thereafter the distillate vapours are condensed in the tower by heat and mass transfer with the cold reflux streams moving down the tower in the same way as the side streams Atmospheric unit. The products are taken off at the appropriate sections are cooled either by heat exchange with colder streams in the atmospheric unit, by air coolers or, in some cases as heating mediums to light end reboiler. They are then pumped to storage. Neither the vacuum residue that leaves the bottom of the tower in this process nor the sidestripped. The vacuum condition is produced by steam ejectors taking suction from the top of the tower. These ejectors remove inert and other vapour that may exist and pull a vacuum of

HG absolute. The tower internals are usually expanded grid type which offer low

pressure drop such that the flash zone pressure is about 2530 mmHg absolute

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pumped hot directly from the CDUs residue stripper to the vacuum units heater. Thereafter the distillate vapours are condensed in the tower by heat and mass transfer with the cold reflux streams moving down the tower in the same way as the side streams in the Atmospheric unit. The products are taken off at the appropriate sections are cooled either by heat exchange with colder streams in the atmospheric unit, by air coolers or, in some cases as

Neither the vacuum

-streams are steam

taking suction from the top of

that may exist and pull a vacuum of usually expanded grid type which offer low

30 mmHg absolute


BLENDING

Gasoline or diesel blending is a complex refining process as operating personnel are required to meet fuel quality and legislative targets while operating at the lowest possible cost. To meet these operating targets, typical properties that are measured and controlled include RON, MON, RVP, aromatics, benzene, olefins, ASTM-D86 distillation points, and oxygenates for gasoline, and for diesel, cetane index, cloud point, pour point and ASTM-D86 distillation or recovery points. Traditionally, these measurements have been obtained by periodically stopping the blend and removing samples to the laboratory, or have been provided by a host of classical on-line analytical techniques, e.g. Octane Engines and Gas Chromatographs. There are, however, a number of problems associated with these approaches. These include the high capital and operating costs of multiple techniques, slow response time and, in many cases, poor analytical repeatability. These disadvantages are especially evident in the utilization of octane engines. These performance issues can lead to significantly higher blending costs due to unavoidable property giveaway, as well as reduced blender throughput, coupled with increased inventory and demurrage costs. The financial incentive to blend continuously and faster is therefore very significant, which means that rapid on-line analysis of key product properties is highly desirable.

GASOLINE ADDITIVES

Additives are gasoline-soluble chemicals mixed with gasoline to enhance certain performance characteristics or to provide characteristics not inherent in the gasoline. Typically, they are derived from petroleum-based raw materials and their function and chemistry are highly specialized. They produce the desired effect at the parts-per-million (ppm) concentration range. (One ppm is 0.0001 mass percent or 1mg/kg.)

Oxidation inhibitors, including aromatic amines and hindered phenols, are also called antioxidants. They prevent gasoline components from reacting with oxygen in the air to form peroxides or gums. They are needed in virtually all gasolines but especially in those with high olefin content. Peroxides can degrade antiknock quality, cause fuel pump wear, and attack plastic or elastomeric fuel system parts. Soluble gums can lead to engine deposits, and insoluble gums can plug fuel filters. Inhibiting oxidation is particularly important for fuel used in modern fuel-injected vehicles because those with fuel recirculation design may subject the fuel to more temperature and oxygen-exposure stress.


Corrosion inhibitors are carboxylic acids and carboxylates. The tank and pipeline facilities of gasoline distribution and marketing systems are constructed primarily of uncoated steel. Corrosion inhibitors help prevent free water in gasoline from rusting or corroding these facilities. Corrosion inhibitors are less important once gasoline is in a vehicle. The metal parts in the fuel systems of todays vehicles are made of corrosion-resistant alloys or of steel that is covered with a corrosion-resistant coating. More plastic and elastomeric parts are replacing metal parts in fuel systems. In addition, service station systems and operations are designed to help prevent free water from being delivered to vehicle fuel tanks.

Silver corrosion inhibitors are substituted thiadiazole. Combinations of trace amounts of elemental sulfur, hydrogen sulfide, and mercaptans can cause the silver used in vehicle fuel gauge sender units to corrode and fail. Silver corrosion inhibitors, also referred to as filmers, inhibit the corrosion caused by active sulfur compounds.

Metal deactivators are chelating agents, that is, chemical compounds that capture specific metal ions. The more active metals such as copper and zinc effectively catalyze the oxidation of gasoline. These metals are not used in most gasoline distribution and vehicle fuel systems, but when they are present, metal deactivators inhibit their catalytic activity.

Demulsifiers are polyglycol derivatives. An emulsion is a stable mixture of two mutually insoluble materials. A gasoline/water emulsion can be formed when gasoline passes through the high-shear field of a centrifugal pump if the gasoline is contaminated with free water. Demulsifiers improve the water-separating characteristics of gasoline by preventing the formation of stable emulsions.

Antiknock compounds increase the antiknock quality of gasoline. They include materials based on:

Lead alkyls, such as tetraethyl lead (TEL) and tetramethyl lead (TML)

Manganese, called methylcyclopentadienyl manganese tricarbonyl (MMT)

Iron, called ferrocene

Because only a small amount of additive is needed, using antiknock compounds is a lower-cost method to increase octane number than changing gasoline chemistry. Gasoline containing TEL was first marketed in 1923. The average concentration of lead in gasoline gradually was increased until it reached a maximum of about 660 milligrams per liter (mg/L)


or 2.5 grams per gallon (g/gal) in the late 1960s. After that, a series of events resulted in the use of less lead. First, new refining processes produced higher-octane gasoline components. Then, the population of vehicles that require unleaded gasoline those with catalytic exhaust emission controls steadily grew. Last, U.S. EPA regulations required the reduction of the lead content of gasoline in phased steps beginning in 1979. The U.S. EPA completely banned the addition of lead additives to on-road gasoline in 1996. Currently, the amount of incidental lead may not exceed 13.2 mg/L (0.05 g/gal) and cannot be deliberately added.

Anti-icing additives are surfactants, alcohols, and glycols. They prevent ice formation in the carburetor and fuel system (see page 4). The need for this additive is disappearing as oldermodel vehicles with carburetors are replaced by vehicles with fuel injection systems.

Dyes are oil-soluble solids and liquids used to visually distinguish batches, grades, or applications of gasoline products. For example, gasoline for general aviation, which is manufactured to unique and exacting requirements, is dyed blue to distinguish it from motor gasoline.

Markers are a means of distinguishing specific batches of gasoline without providing an obvious visual clue. A refiner may add a marker to its gasoline so it can be identified as it moves through the distribution system.

Drag reducers are high-molecular-weight polymers that improve the fluid flow characteristics of low-viscosity petroleum products. As energy costs have increased, pipelines have sought more efficient ways to ship products. Drag reducers lower pumping costs by reducing friction between flowing gasoline and pipe walls

prp unit - 3

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