Acknowledgement. To get an opportunity opportunity to do my internship internship program in Attock Attock Refinery Limited, HBU-I, is truly an immense time to gain a lot of new operatio tional nal and prac ractical knowl owledg edge and experiences. I’m very fortunate to be here as I am positioned in Attock Refinery, Operation Department, HBU-I plan plant, t, whic which h is posi positi tive vely ly relat related ed to my majo major, r, Chem Chemic ical al Engineering. I want to also convey lofty gratitude to ARL, especially Mr. Malik lik Muham uhamm mad Yousa ousaf, f, (In (Inchar charge ge of HBU-I) -I) and Mr. Mumtaz Jafferi (Senior Operator), for giving me this 3 weeks internship opportunity and many other opportunities to learn a lot of thin thing gs rega regard rdin ing g worki orking ng in an orga organi niza zati tion ona al envi enviro ronm nmen entt whos whose e core core acti activi vity ty is refi refini ning ng crud crude e and and production of JP-1fuel. Since, I had been working with an international organization that is Attock ock Refinery ery Limi imited ted with small prac ractic tical know knowle ledge dge,, but but certa certain inly ly I had had wide wide opport opportun unit itie ies s to look look closer the Distillation process.
Acknowledgement. To get an opportunity opportunity to do my internship internship program in Attock Attock Refinery Limited, HBU-I, is truly an immense time to gain a lot of new operatio tional nal and prac ractical knowl owledg edge and experiences. I’m very fortunate to be here as I am positioned in Attock Refinery, Operation Department, HBU-I plan plant, t, whic which h is posi positi tive vely ly relat related ed to my majo major, r, Chem Chemic ical al Engineering. I want to also convey lofty gratitude to ARL, especially Mr. Malik lik Muham uhamm mad Yousa ousaf, f, (In (Inchar charge ge of HBU-I) -I) and Mr. Mumtaz Jafferi (Senior Operator), for giving me this 3 weeks internship opportunity and many other opportunities to learn a lot of thin thing gs rega regard rdin ing g worki orking ng in an orga organi niza zati tion ona al envi enviro ronm nmen entt whos whose e core core acti activi vity ty is refi refini ning ng crud crude e and and production of JP-1fuel. Since, I had been working with an international organization that is Attock ock Refinery ery Limi imited ted with small prac ractic tical know knowle ledge dge,, but but certa certain inly ly I had had wide wide opport opportun unit itie ies s to look look closer the Distillation process.
Events at HBU-I precisely offered me broad horizon to see the sights of Distillation Distillation Process from a very distinct distinct point of view. view. Vast amount of events regarding regarding different different equipments equipments at HBU-I plant sight obviously provided me with new practical knowledge about the Distillation Process. I am very grateful to be part of the events Attock Refinery Limited, HBU-I Plant and for the full supports and the high enco encoura uragem gement ents s of all all HB HBU-I U-I staf staffs fs durin during g my inter interns nshi hip p time, especially Mr. Kwaja Adil (trainee Engineer) for sharing his practica practicall and profes professio sional nal knowle knowledge dge with with me and for guiding me at each step during my internship.
Oil Refineries and Refining Operation. What
is an Oil Refinery?
An oil refinery is an industrial process plant where crude oil is processed and refined into more useful petroleum products, such as gasoline, diesel fuel, and asphalt base, heating oil, kerosene, and liquefied petroleum gas. Oil refineries are typical typically ly large large sprawl sprawling ing industr industrial ial comple complexes xes with with extensi extensive ve piping piping running running throughout, carrying streams of fluids between large chemical processing units.
What is Oil Refining Process? Crude oil is separated into fractions by fractional fractional distillation. The fractions at the top of the fractionating column have have lowe lowerr boili boiling ng point pointss than than the the frac fracti tions ons at the the botto bottom. m. The The heavy heavy bott bottom om fract fractio ions ns are are often often crac cracke ked d into lighter, more useful products. All of the fractions are processed further in other refining units. Raw or unprocessed ("crude") oil is not useful in the form it comes in out of the ground. Although "light, sweet" (low (low visco viscosi sity ty,, low low sulf sulfur ur)) oil oil has has been used directly as a burner fuel for steam steam vessel vessel propul propulsio sion, n, the lighter lighter elements elements form explosive explosive vapors in the fuel tanks and so it is quite dangerous, especially so in warships. For this and many other uses, the oil needs to be separated into parts and refined before use in fuels and lubricants, and before some of the byproducts could be used in petrochemical processes to form materials such as plas plasti tics cs,, dete deterg rgent ents, s, solv solvent ents, s, elast elastome omers rs,, and fiber fiberss such such as nylon nylon and polyesters. Petroleum fossil fuels are used in ship, automobile and aircraft engines. These different hydrocarbons have different boiling points, which mean they can be separated by distillation. Since the lighter liquid elements are in great demand for use in internal combustion engines, a modern refinery will convert heavy hydrocarbons and lighter gaseous elements into these higher value products.
Major
Refinery Operational Diagram.
Howe Baker Unit (HBU) at Attock Refinery Limited. Howe-Baker
Unit (HBU)
An Oil Refinery acts as a backbone of a country. Focusing Pakistan, the oldest refinery is Attock Refinery Limited installed back in 1920. ARL is giving a big hand to make economy of Pakistan strong enough. In 1979, ARL was upgraded by adding New Distillation Units. NDU-20000 (HBU-1) bpsd and NDU-5000 bpsd (HBU-2) is designed by Howe-Baker Engineers, INC. to control automatically or remote processing of 20000 bpsd and 5000 bpsd of sweet or sour crude oil to produce following products:
Stabilized Naphtha Light Weight Kerosene (LWK) Jet Petrol (JP-I) High Speed Diesel (HSD) Diesel Fuel Oil (DFO) Furnace Fuel Oil (FFO) LP Gas HP Gas Liquid Petroleum Gas (LPG)
Howe-Baker
Unit average % yield
Crude distillation units NDU-20000 is designed to process light sweet crude oils to produce light petroleum gases, LPG, stabilized naphtha, light weight kerosene, light gas oil, heavy gas oil and residue. The tables show average yield % of product, while processing light sweet crude at HBU-1 plant. HBU -1 (Feed 20000 bpsd)
Crude charge, bpd.
LPG
Naphtha
LWK, JP-1
HSD
DFO
FFO
20000
0.70%
39.0%
13.5 %
18.3%
2.8%
25.7 %
Crude Oil Feed Stock at Attock Refinery Limited.
Definition of Crude Oil. Crude oil is a naturally occurring, flammable liquid found in rock formations in the Earth consisting of a complex mixture of hydrocarbons of various molecular weights, plus other organic compound.
Crude
Oil Classification.
The petroleum industry generally classifies crude oil by:
The geographic location it is produced. Its API gravity (an oil industry measure of density) and By its sulfur content.
The two main characteristics, the API gravity and the sulfur content , are significant factors in explaining the price level and trade pattern of a particular crude oil. On the basis of API gravity and sulfur contents there are following types of crude:
Sour crude Oil
Sour crude oil is crude oil containing the impurity sulfur. It is common to find crude oil containing some impurities. When the total sulfur level in the oil is > 1 % the oil is called "sour"
Sweet crude Oil
Sweet crude oil is a type of petroleum. Petroleum is considered "sweet" if it contains less than 0.5% sulfur, compared to a higher level of sulfur in sour crude oil. Sweet crude oil contains small amounts of hydrogen sulfide and carbon dioxide. High quality, low sulfur crude oil is commonly used for processing into gasoline and is in high demand
Heavy crude oil or Extra Heavy Oil.
Heavy crude oil or Extra Heavy oil is any type of crude oil which does not flow easily. It is referred to as "Heavy" because its density or specific gravity is higher than of light crude oil. Heavy crude oil has been defined as
any liquid petroleum with an API gravity less than 20° , meaning that its specific gravity is greater than 0.933 . Light Crude Oil. Light crude oil is any type of crude oil which does flow easily. It is referred to as "Light" because its density or specific gravity is in range of 0.79 to 0.84. Light crude oil has been defined as any liquid petroleum with an API gravity grater than or equal to 40°.
Crude
at ARL
Crude oils at ARL are not all alike. They differ considerably in their physical properties of particularly their viscosity, sulfur content, metals content and the proportions of the various hydrocarbon fractions that can be turned into the different end products . These properties affect the ease with which the refinery can process various crude oils into the different products required by the consumers. ARL is interested in a crude oil for the value of the products it yields. The aim is to turn the crude oil into as much of the lighter, higher priced products and as little of the heavier, lower priced products as is cost-effectively possible. Thus the crude oils that are naturally ‘light' have higher prices than the crude oils which are naturally ‘heavy'. The basic raw for refinery is crude oil. The chemical compositions of crude oils are uniform, but their physical characteristics vary widely. The elementary composition of crude oil usually falls within the following ranges.
Carbon Hydrogen Sulfur Nitrogen
84-87% 11-14% 0-3% 0-0.6%
Equipments and Processes at HBU-1 Distillation. Distillation is the physical separation of the components by means of their relative volatility (boiling point) achieved through the contact between the rising vapors and down flowing liquid. OR
Distillation is the physical separation of the components by their partial vaporization and partial condensation . Thus the mixture is separated into different fractions of different compositions and properties.
Industrial
Distillation Process.
In most cases, the distillation is operated at a continuous steady state. New feed is always being added to the distillation column and products are always being removed. Unless the process is disturbed due to changes in feed, heat, ambient temperature, or condensing, the amount of feed being added and the amount of product being removed are normally equal. This is known as continuous, steady-state fractional distillation. Industrial distillation is typically performed in large, vertical cylindrical columns known as "distillation or fractionation towers" or "distillation columns" with diameters ranging from about 65 centimeters to 6 meters and heights ranging from about 6 meters to 60 meters or more. The distillation towers have liquid outlets at intervals up the column which allow for the withdrawal of different fractions or products having different boiling points or boiling ranges. The "lightest" products (those with the lowest boiling point) exit from the top of the columns and the "heaviest" products (those with the highest boiling point) exit from the bottom of the column. Large-scale industrial towers use reflux to achieve a more complete separation of products. Reflux refers to the portion of the condensed overhead liquid product from a distillation or fractionation tower that is returned to the upper part of the tower large-scale industrial distillation tower . Inside the tower, the reflux liquid flowing downwards provides the cooling needed to
condense the vapors flowing upwards, thereby increasing the effectiveness of the distillation tower. The more reflux is provided for a given number of theoretical plates, the better the tower's separation of lower boiling materials from higher boiling materials. Alternatively, the more reflux provided for a given desired separation, the fewer theoretical plates are required.
Design
of industrial distillation columns Design and operation of a distillation column depends on the feed and desired products. McCabe-Thiele method or the Fenske equations are used for multi-component feed. Moreover, the efficiencies of the vapor-liquid contact devices (referred to as plates or trays) used in distillation columns are typically lower than that of a theoretical 100% efficient equilibrium stage. Hence, a distillation column needs more plates than the number of theoretical vapor-liquid equilibrium stages. The reflux ratio is the ratio of the amount of moles returned as refluxed liquid to the fractionating column and the amount of moles of final product, both per unit time.
Distillation Tower and Process at HBU-1 Crude distillation is the first major processing units at ARL. They are used to separate the crude oils by distillation into fractions according to boiling point. At HBU 1 Atmospheric distillation column is installed with 40 Valve Trays. Crude distillation is processed to break up crude into the basic raw products of crude oil. In crude distillation process, crude oil is pumped through a heat exchange system, Pre-Heat Trains and Post-Heat Trains, and its temperature rises up to 400 oF. Then crude is further heated about 700 to 750 oF in furnace and charged into the flash zone of a multiple-draw distillation tower . For such processes, reboiler is not feasible to use. Several trays are generally incorporated below the flash zone and steam is provided below the bottom tray to strip any remaining gas oil into the flash zone . The steam reduces the partial pressure of hydrocarbons and this effect lowers the required vaporization.
Flash zone temperature must be high enough to cause vaporization of all the products. Flash zone temperature must be maintained below 700 oF because thermal cracking of oil will occur somewhere around this temperature. To achieve perfect distillation results the crude oil should be partially vaporized before entering the tower and this will suppress the flash zone temperature. A small amount of over flash is taken, so that about 20% of the bottoms stock is also vaporized. This action provides better fractionation on the plates. Without excess vaporization, small amount of reflux will exist at these plates and no reflux will flow from the plates above the vaporizer into the flash zone. The quantity of material vaporized depends upon temperature and pressure of the distillation tower. Operating pressure is maintained as low as reasonably consistent with enabling the off gas to flow from the over head accumulator to flare or to any other system. Decrease in pressure allows more vapors to go up in inside tower at constant flash zone temperature. The vaporized hydrocarbon rises through the rectifying plates. And intimately mixed with descending liquid on each tray. On each tray, components that are more volatile are vaporized and less volatile components are condensed. Thus, a concentration gradient exists throughout the tower. High volatile components are concentrated in the top and low volatile in bottom. Fractionation depends upon the quantity of plates and the reflux. Reflux is provided be condensing the tower top vapors and returning some portion to the tower top. The remainder is pumped as un-stabilized naphtha as feed to stabilizer. Uncondensed vapors flows to flare or to any other system. As mentioned above that distillation tower is multiple-draw fractionating tower. Thus several products are withdrawn from the trays, but these products contain some high volatile components. These components are stripped in the stripper tower by stripping stream.
Reflux
and Pump around:
Since distillation is basically a contact between the rising vapors and down flowing liquid, down flowing liquid condenses the heavier vapors coming from down and in this process it is vaporized. So partial vaporization and partial condensation occurs on every tray. Column V-301 uses two types of reflux – cold reflux and hot reflux. Cold reflux liquid is supplied to the top of the crude column at a temperature below the temperature at the corresponding addition point in the tower. This reflux
prevents components that are desired in the lower products from passing overhead. It flows down through the column absorbing the heat required to condense the heavy components, and the heat absorbed causes the reflux to be vaporized. The vaporized reflux is again condensed in either the overhead condenser or the distillation tower. Heat is removed from the system before the reflux is returned to the tower. Reflux is provided by condensing the pentanes, butanes and heavier fraction of the vapors that passes out from the top of the tower. Reflux is used to keep the low volatile components down. In multiple draw distillation towers, different kinds of reflux are used such as cold and hot reflux. The above-discussed reflux, which is provided on the top of the tower, is called cold reflux. It flows down through each plate and absorbs heat and that condenses the high boiling point components. And the absorbed heat allows reflux to be vaporized again. The cold reflux liquid supplied to the side of the tower is known as circulating reflux . It differs from the above discussed reflux because it’s not vaporized before pumping out from tower. This reflux is used to remove heat from tower at high temperature. The overflow of liquid from plate to plate inside the tower is known as hot reflux. Hot reflux overflows the kerosene draw plate to supply reflux to heavy gas oil section of tower, and HGO draw plate supplies reflux to wash oil section The cold reflux liquid supplied to the side of the tower is called circulating reflux (Pump around). Circulating reflux differs from the top reflux because it is not vaporized. It is able to remove only sensible heat, the heat required to heat a liquid from a lower temperature to a higher temperature without vaporizing. This type of reflux may be conveniently used to remove heat at higher levels than that achievable at the top of the tower, also it unloads the top section of the tower and reduce load on the overhead condensing system.. Circulating reflux is frequently used as a tool to remove heat from the tower at a high temperature level in order to allow the exchange of heat with the crude charge. At HBU-I two pump around are used. Naphtha pump around and H.S.D pump around. Naphtha pump around is at the top to middle portion of the tower and H.S.D pump around controls tower temperature and pressure from middle to downward section of the tower.
Another type of reflux is Hot reflux. It is admitted to the tower at the same temperature that is in the corresponding section of the tower.
Some Important Relations In Distillation.
Tower Top Temperature α End Point of Product.
Tower Top Temperature α IBP of Product.
Tower Top Temperature α 1/Internal Reflux (Hot Reflux)
Tower Top Temperature α 1/RVP of Product.
Tower Top Temperature α 1/External Reflux (Cold Reflux)
Cold Reflux (Internal Reflux) α External Reflux (Cold Reflux)
Reflux α 1/End Point.
Reflux α RVP.
Reflux α 1/IBP.
With Draw Flow Rate of Product α End Point.
De-Salter. Electrostatic desalting, whether employed for oil field production dehydration and desalting or at refineries, is used to facilitate the removal of inorganic chlorides and water-soluble contaminants from crude oil. In refinery applications, the removal of these water-soluble compounds is necessary to prevent or minimize long-term damage to downstream distillation process. Desalters are the first line of defense against corrosion at the costly downstream refinery equipment and units. Desalters provide more protection against corrosion than any other single piece of equipment.
Types
of Desalters
The two general types of electrical Desalters in use today are the low velocity and high velocity kind.
Low Velocity Desalter.
The low velocity type of desalter is used in oil field production where the incoming wet oil may contain up to 50% of a dispersed phase, which is usually salt water, in a continuous phase of crude oil. The wet oil flows through the distributor and then upward at a relatively low velocity through the electrodes. Initial coalescence takes place between the oil-water interface and the lower electrode. Finally coalescence takes place between the electrodes where the dispersed particles are coalesced until they are large enough to drop to the bottom of the vessel by gravity against the upward flow of oil.
High Velocity Desalter.
The high velocity desalter contains the same elements as the low velocity desalter except that the wet oil containing the dispersed phase enters the electric field through a distributor that is placed in between the electrodes. This arrangement is particularly suitable for certain types of very stable emulsions.
The high velocity is necessary in some cases to prevent the emulsion particles from forming a chain between the electrodes causing a short circuit.
Process
Description
A simple flow diagram of an electrical desalting is shown below. Crude oil is heated in the range of 120oC to 140oC by heat exchange with hot streams from crude fractionators. It is heated to reduce its viscosity such that the improved flow conditions thus obtained permit more efficient mixing and separation. Water (condensate / demineralized water) is added to the crude stream ahead of the desalter and mixture is then passed through mixing valve across which a controlled pressure drop is maintained. The mixing valve arrangement creates proper emulsification of the crude oil and water. In the processing of slop oils or crude oils having abnormally high concentrations of suspended solids, a demulsifier is also injected to the crude at a point upstream of water injection to break the hard emulsions of crude oil and water.
Desalted Crude Oil Electric Power
Process Water
Alternate
DESALTER
Crude Oil Effluent Water Heat Exchanger
Mixing Valve
The emulsified crude oil is fed into the bottom of the desalter vessel through an inlet distributor. This device is designed to meter the mixture of water and crude oil uniformly over the entire cross-section of the area below the electrodes. The flow enters the vessel under conditions of low velocity and this essentially laminar. The emulsified crude oil is subjected to high voltage electrical field in the desalter where droplets of water coalesce and separate from the emulsion together with the salt from the crude. The desalted crude overflows while water along with salt and crude sludge is withdrawn from the bottom of the desalter vessel. An interface level controller working in conjection with draw-off valve in the effluent water line maintains interface level within the desalter vessel. Additional stages can be used to get additional reduction in salt content of the crude oil before the oil enters the atmospheric distillation tower.
Desalting of Crude Oils Feed at HBU-1. As mentioned above that crude oil contains salt contents, which can cause corrosion by salt deposition on heat transfer surfaces and acids formed by decomposition of the chloride salts. The salts in the crude are in the form of dissolved or suspended salt crystals in water emulsified with the crude oil. The basic principal is to wash the salts from the crude oil with water. Water is carried in the crude as small droplets so widely dispersed that its impossible, or at least uneconomical, to remove it by ordinary gravity settings . In addition to water and chlorides, all crude oils contain suspension of varying amounts of solid materials, such as silt, iron oxides, sand, crystalline salt, carbon and sulfur. By desalting, a portion of them will be removed. Desalting is carried out by mixing the crude oil with 3-10 vol. % water at temperature from 200 to 300 oF (90 to 150 oC). Both the ratio of the water to oil and the temperature of operation are functions of the density of the oil.
API
Water wash, vol. %
Temperature oF (oC)
API>40
3-4
240-260 (115-125)
30>API<40
4-7
260-280 (125-140)
API<30
7-10
280-300 (140-150)
The salts are dissolved in the wash water and the oil and water phases separated in settling vessel either by adding chemicals to assist in breaking the emulsion or by developing the high potential electric field across the settling vessel to coalesce the droplets of salty water more rapidly. Either AC or DC fields are used with potential from 16,000 to 35,000 volts are used to promote coalescence. Basically in desalting process crude is preheated as specified, and water is mixed in the crude by passing the stream through mixing valve. The crude oil enters the desalter, which is maintained with high voltage electrical field between pair of electrodes and a slightly less intense field between the lower electrode and the water phase as well as the vessel itself. The two emulsified liquids, one electrically conductive and the other non-conductive are passed through the electrical fields, and the conductive liquid is caused to coalesce. The electrical forces form large drops and these droplets have sufficient mass to fall through the crude and settle in the bottom of the vessel. The water-soluble salts and impurities in the crude separated in desalting process are continuously drained.
Stabilizer. Stabilizer is 22 valve plate column . It operates under high pressure of 900 KPa. The major function of stabilizer is to control the RVP of the naptha. Stabilized naphtha has RVP in range of 7-11.
RVP
(Reid Vapor Pressure)
Reid vapor pressure is the absolute vapor pressure exerted by a liquid at 100°F. The higher this value, the more volatile the sample and the more readily it will evaporate. Unlike distillation data, vapor pressure provides a single value that reflects the combined effect of the individual vapor pressure of the different petroleum fractions in accordance with their mole ratios. It is thus possible for two wholly different products to exhibit the same vapor pressure at the same temperature - provided the cumulative pressures exerted by the fractions are the same. A narrow-cut distillate, for example, may exhibit the same vapor pressure as that of a dumbbell blend, where the effect of heavy fractions is counterbalanced by that of the lighter ones. Reid vapor pressure plays a role in the prediction of gasoline performance .
Stabilizer
Section at HBU-I
Stabilizer section at HBU-I is used to separate pentanes-butanes (LPG) and heavier fraction from un-stabilized naphtha . In this process heat is provided to a system by reboiler E-403 from circulating HSD. Comparing with distillation tower, stabilizer V-403 is kept under high pressure ok 900 KPa so that the eng point of the LPG is in range of atmospheric pressure and temperature. An effective separation of pentanes requires large number of fractionating plates and high reflux ratio. In this process un-stabilized naphtha is pumped into stabilizer as feed. Naphtha flows down from plate to plate and from bottom of stabilizer it flows to reboiler. In reboiler naphtha is heated, and high volatile components vaporize and naphtha vapors (C3,C4) enter to stabilizer again. Stabilized naphtha is pumped from reboiler. The vapors go up from plate to plate and exits from the top of the stabilizer. These vapors are condensed in over head accumulator. Vapors that don’t condense under pressure are sent to flare or to any other system. Part of condensed liquid is returned to stabilizer as reflux. And the remaining is sent for storage to LPG treating section for separation of sulfur components.
Reboiler. Reboilers are heat exchangers typically used to provide heat to the bottom of industrial distillation columns. They boil the liquid from the bottom of a distillation column to generate vapors which are returned to the column to drive the distillation separation. Proper reboiler operation is vital to effective distillation. In a typical classical distillation column, all the vapor driving the separation comes from the reboiler. The reboiler receives a liquid stream from the column bottom and may partially or completely vaporize that stream. Steam usually provides the heat required for the vaporization.
Types
of Reboilers
There are following types of Reboilers used in industry:
Boiler
Kettle Reboilers Forced Circulation Reboilers Fired Reboiler Thermosyphon Reboilers
at HBU-I
A typical kettle reboiler E-403 is used at HBU-I. Kettle Reboilers are very simple and reliable. They may require pumping of the column bottoms liquid into the kettle, or there may be sufficient liquid head to deliver the liquid into the reboiler. In this reboiler type, HSD flows through the tube bundle . The liquid from the bottom of the stabilizer V-403 , commonly called the stabilized Naphtha, flows through the shell side. There is a retaining wall or overflow weir separating the tube bundle from the reboiler section where the residual reboiled liquid (called the stabilized naphtha product) is withdrawn, so that the tube bundle is kept covered with liquid.
Heater. A direct fired heater is equipment used to provide heat for a process or can serve as reactor which provides heats of reaction. Furnace designs vary as to its function, heating duty, type of fuel and method of introducing combustion air. However, most process furnaces have some common features.
Working. Fuel flows into the burner and is burnt with air provided from an air blower or louvers . There can be more than one burner in a particular furnace which can be arranged in cells which heat a particular set of tubes. Burners can also be floor mounted, wall mounted or roof mounted depending on design. The flames heat up the tubes, which in turn heat the fluid inside in the first part of the furnace known as the radiant section or firebox . In this chamber where combustion takes place, the heat is transferred mainly by radiation to tubes around the fire in the chamber. The tubes in radiation section may be vertical or horizontal tubes depending upon the design. The heating fluid passes through the tubes and is thus heated to the desired temperature. The gases from the combustion are known as flue gas. After the flue gas leaves the firebox, most furnace designs include a convection section where more heat is recovered before venting to the atmosphere through the flue gas stack.
Parts of a Heater. The major parts of a heater are as follows:
Radiant section Convection section Bridge Zone Burner Soot blower Stack
Radiant section
The radiant section is where the tubes receive almost all its heat by radiation from the flame. In a vertical, cylindrical furnace, the tubes are vertical (mostly) or horizontal. Tubes can be vertical or horizontal, placed along the refractory wall, in the middle , etc., or arranged in cells. Studs are used to hold the insulation together and on the wall of the furnace. They are placed about 1 ft (300 mm) apart. the tubes, shown, are carbon steel tubes and run the height of the radiant section. The tubes are a distance away from the insulation so radiation can be reflected to the back of the tubes to maintain a uniform tube wall temperature. Tube guides at the top, middle and bottom hold the tubes in place. Convection section
The convection section is located above the radiant section where it is hotter to recover additional heat. Heat transfer takes place by convection here, and the tubes are finned to increase heat transfer . The first two tube rows in the bottom of the convection section and at the top of the radiant section is an area of bare tubes (without fins) and are known as the shield section, so named because they are still exposed to plenty of radiation from the firebox and they also act to shield the convection section tubes, which are normally of less resistant material from the high temperatures in the firebox. Bridge-zone
The area of the radiant section just before flue gas enters the shield section and into the convection section called the bridge-zone. Crossover is the
term used to describe the tube that connects from the convection section outlet to the radiant section inlet. The crossover piping is normally located outside so that the temperature can be monitored and the efficiency of the convection section can be calculated. The sight-glass at the top allows personnel to see the flame shape and pattern from above and visually inspect if flame impingement is occurring.
Flame impingement.
Flame impingement happens when the flame touches the tubes and causes small isolated spots of very high temperature. Burner
In a heater there are two types of burners. o o
Pilot Burners (6-10 in number per each main burner) Main Burner. (3-4 in number)
The burner in the vertical, in cylindrical furnace, is located in the floor and fires upward. Some furnaces have side fired burners, e.g. LUMMUS heater’s burners . The burner tile is made of high temperature refractory and is where the flame is contained in. Air registers located below the burner and at the outlet of the air blower are devices with movable flaps or vanes that control the shape and pattern of the flame, whether it spreads out or even swirls around. Flames should not spread out too much, as this will cause flame impingement. Air registers can be classified as primary, secondary and if applicable, tertiary, depending on when their air is introduced. The primary air register supplies primary air, which is the first to be introduced in the burner. Secondary air is added to supplement primary air. Burners may include a pre-mixer to mix the air and fuel for better combustion before introducing into the burner. Some burners even use steam as premix to preheat the air and create better mixing of the fuel and heated air. The floor of the furnace is mostly made of a different material from that of the wall, typically hard castable refractory to allow technicians to walk on its floor during maintenance.
Soot-blower Soot blowers are found in the convection section . As this section is above the radiant section and air movement is slower because of the fins, soot tends to accumulate here. Soot blowing is normally done when the efficiency of the convection section is decreased. This can be calculated by looking at the temperature change from the crossover piping and at the convection section exit. Soot blowers utilize flowing media such as water, air or steam to remove deposits from the tubes. This is typically done during maintenance with the air blower turned on. The lances are connected to a steam source with holes drilled into it at intervals along its length. When it is turned on, it rotates and blows the soot off the tubes and out through the stack. Stack
The flue gas stack is a cylindrical structure at the top of all the heat transfer chambers. The breeching directly below it collects the flue gas and brings it up high into the atmosphere where it will not endanger personnel. Stack damper
The stack damper contained within works like a butterfly valve and regulates draft (pressure difference between air intake and air exit) in the furnace, which is what pulls the flue gas through the convection section. The stack damper also regulates the heat lost through the stack. As the damper closes, the amount of heat escaping the furnace through the stack decreases, but the pressure or draft in the furnace increases which poses risks to those working around it if there are air leakages in the furnace, the flames can then escape out of the firebox or even explode if the pressure is too great.
Heater
at HBU-I
A direct fired heater E-301 is used at HBU-I for providing heat to crude, preheating thus vaporizing the crude before entering the column V-301. Before entry into the convection section of the heater crude is divided into four passes. In the convection section there are 56 finned tubes. Here crude receives heat from the flue gases and finned tubes are used to increase contact time and heat transfer rate. Total no of tubes in the radiation section are 88 . H-301 is rectangular furnace which operates on natural draft. Different fuel options are provided on H-301 where L.P gas, Field gas and F.F.O can be burnt in the nine burners. Tube strength of the tubes is up to 865 degree Fahrenheit . To maintain this temperature following things usually are observed:
Heater
Excess oxygen Draft at different location of the heater like top and bottom draft Stack temperatures. Skin temperatures
Start-Up
In starting a heater few steps are taken which are as follows:
Louvers are closed first of all and steam is injected in heater The damper is fully opened so as the flue gases remaining leave the heater The steam injected creates a negative draft in the heater Then pilot burners are operated and damper is closed such that 20-35 % is open. Then air is given through the blowers and main burner is ignited. The partially opened damper and burner create the negative draft in the heater.
Crude Section.
Crude Specifications. Crude processed at HBU is Light Sweet Crude having sulfur less then 0.5% and gravity in the range of 0.79 – 0.81. Crude is stored in tanks 1A, 2A, 4A. Crude flows by gravity to the plant at the west side battery limit. At this point the pressure is in the range of 75 – 150 KPa and temperature in the range of 25-35 0C.
Auto Cleaner, Stainer and Bi-Rotometer. Crude is first passed through Auto cleaner. Auto cleaner works on the principle of centrifugal force and it removes suspended particles and sludge from the crude. It is drained once in every shift. A Bucket filter is used if the Auto-Cleaner is out of order. Then crude passes through Stainer having a mesh which retains the larger particles. Steam is also injected into the Stainer so as to decrease the viscosity of the crude. It is cleaned on weekly basis and during winter steam is applied around it to avoid choking of the Stainer. Then crude passes through birotometer for flow measurement in gallons per day and into the suction of Crude Charge Pump P-301 A/B. At the suction of the Pumps, naphtha and water is injected in line to:
Pre-Heat
To decrease the viscosity of crude To remove the salts in crude charge dissolved in fed water
Train Exchangers.
The discharge pressure of Pump 301 A/B is in the range of 1550-1700 KPa. This head is sufficient to move crude through the Pre-Heat Train exchangers E-301, E-302, E-303 A&B and through the Pre-heat train exchangers E-321, E-322, E325, and E-326. Then both streams of crude combine. The combined streams are at a temperature range of 80-90 0C.
De-Salter (V-306) In these combined five streams of hot water are added into it to make good emulsion. Mixing valve arrangement is used for making emulsion across which pressure drop should be in the range of 5-15 psid. Water is added 3 vol %. Other addition points of water are also provided. Then crude enters into the De-Salter V-306 from bottom whose pressure should be at least 965KPa to always maintain hydraulic head and prevent any possible
vaporization. The temperature of emulsion is around 100 0C, so that there should be a mark able difference between the API gravities of water and crude. The difference in gravities is basis of de salting process. Normal De-Salter pressure maintained at HBU-1 is 1040 KPa. This pressure is maintained through the Cascade control PIC-401.
De-Salter is supplied with a voltage of 400 V which is stepped up to 12kv, 16kv, 20kv. Normal operating voltage is 12kv . In the De-Salter voltage produces turbulence by moving the plate at a frequency of 50 Hz and emulsion of crudewater is agitated. Water molecules along with salts combine with each other and they coalesce, bringing salts with them and settle to the bottom of the De-Salter due to gravity.
Post-Heat Train Exchangers. The crude from De-Salter flows into the suction of Crude Booster pump P-313 A/B which provides the pressure head of 2200-2400 KPa to move crude through the Post-heat train exchangers E-304,E-305,E-306, E-307 B/A, E-308 and through the Post-Heat Train exchangers E-324,E-318,E-317,E-327,E-328and E329.
Heater. (H-301) Then both streams combine and travel towards Heater H-301.Before entry into the convection section of the Heater a FFO stream joins crude through FCV-104. This recycle stream is used when yield of the furnace is less then 25%. The function of this recycle stream is
To maintain satisfactory flow of crude to the heater Prevent excessive vaporization and Usually sued at the start up.
Before entry into the convection section of the heater crude is divided into four passes. In the convection section there are 56 finned tubes. Here crude receives heat from the flue gases and finned tubes are used to increase contact time and heat transfer rate. The heated crude leaves the heater fro Radiation section . Total no of tubes in the radiation section are 88. Heater H-301 is rectangular furnace which operates on natural draft. Different fuel options are provided on H-301 where L.P gas, Field gas and F.F.O can be burnt in the nine burners. Four outlets of the heater combine into the common header where pressure is usually 150 kilo Pascal .Then crude enters into the flash zone of the tower V-301 for Distillation.
Crude Section Process Flow Diagram.
Naphtha Product and Pump Around. Naphtha
Product.
Naphtha vapors exits from the top of the Distillation Column V-301 and passes through the overhead fan condenser E-309 by which the overhead vapor is essentially totally condensed. Condensed vapors plus any non-condensable vapors flow into the Overhead Accumulator V-305 . Sufficient pressure is maintained between 70 and 140 KPa .
Pressure in the vessel can be controlled by PCV-403 A&B. Condensed naphtha collected in the crude tower overhead accumulator V-305 is pumped through the naphtha reflux pump P-302 A/B with a discharge pressure of approx. 550-600 KPa. This stream divides into two parts. One stream of that naphtha flows through flow control valve FCV-106 and returns back into the distillation tower on the 40th plate. Crude tower overhead vapor temperature is controlled by this reflux stream. The reflux rate is reset by the crude tower overhead vapor temperature controller.
Stabilization of Naphtha. For the separation of pentanes-butanes and heavier fraction (LPG) from unstabilized naphtha, It is further processed in the stabilizer section. The second stream of Un-stabilized Naphtha (naphtha from V-305) , which was pumped by P-302 A/B flows into the suction of the stabilizer feed pump P-303 A/B. This pump provides the necessary discharge pressure approx. 15002000 KPa to move naphtha through the system. Un-stabilized naphtha flows through stabilizer feed flow control valve FCV107 and from the tube side of the stabilizer feed-bottoms exchanger E201A/B and enters the naphtha stabilizer V-401 , on tray number 15. Un-stabilized naphtha flows to the bottom of tower. Un-stabilized naphtha level, in the bottom of the stabilizer rises and that provides enough pressure, which moves un-stabilized naphtha from the bottom of the stabilizer to the shell side of kettle reboiler E-403 . HSD pump-around flows through the tube side of this kettle reboiler. From reboiler naphtha vapors (propane-butane fraction) flows back to the stabilizer below the bottom plate. The reboiler vapor return temperature to the stabilizer is maintained be resetting the high speed diesel flow rate through the reboiler.
Stabilized Naphtha Product. Stabilized naphtha flows from the stabilizer reboiler E-403 , through the shell side of stabilizer feed-bottoms exchanger E-401 A/B and then through the naphtha fan cooler E-404 1&2. The stream then enters the naphtha trim cooler E-205 and the stabilizer reboiler level control valve LCV-308 . Stabilized naphtha flows for storage at approx. 100 oF (38 oC) and 150 KPa . A by-pass is provided around the shell side of the stabilizer feed-bottoms exchanger E-201A/B to allow control of the stabilizer feed temperature if required. A line of stabilized naphtha before LCV-308 is sent to P-301 A/B Crude Pumps. After LCV-308 a line is also taken to slop used at start up.
Naphtha
Pump Around
Naphtha pump around is taken from 37th Plate of the Distillation Column V-301. The Pump-around Naphtha is pumped via P-312 A/B at the pressure of 650-800 KPa. The stream is divided into two streams. One stream is sent to Heat Exchangers E303 A&B and then through FCV-102 is refluxed to the 40th plate of the column. The second stream is sent to Heat Exchangers E-326 and E-325 then the stream via FCV-129 joins the first stream. Naphtha from FCV-106 is also added to the stream and is refluxed to the top plate of Column V-301 .
Naphtha
Product
and
Pump
Around.
Light Weight Kerosene (LWK) Light weight kerosene is withdrawn from 27th plate at (180-190 oC) of the Distillation tower V-301 , as a side-cut and is enters the light weight kerosene stripper (V-302) . Plates in the LWK stripper is 4.
The level in the stripper is controlled by level control valve, (LCV-305) . The light weight kerosene is steam stripped for flash point correction. The steam enters the stripper via FT-133 at a rate of 10-15 Kg/hr. The low boiling point components in the stripper rise through the plates and exit from the top of the stripper and enters the distillation tower above the 28th plate. Light weight kerosene product is pumped by LWK Pump P-304 A/B, with a discharge pressure of approx. 50-850 KPa, to move the product to the storage tank. However, before sending product for storage it must be cooled down. Light weight kerosene flows through the shell side of Heat Exchanger E-301 and Heat Exchanger E-321 . Then the two streams enters the TDV 214, the function of TDV is to make uniform the temperature of two incoming streams. The stream coming out is at temperature range of 50-55 oC is then sent to the kerosene trim cooler E-310. From the cooler the product flows through the flow control valve FCV-108 at approx. 100 oF (38 oC), 20-25 m3/hr and 1050 KPa to the storage tank
Light Weight Kerosene Product (LWK)
High Speed Diesel Product and Pump around (HSD) HSD
Product.
High Speed Diesel (HSD) is withdrawn from 15th plate of at approx. (446 oF) of the Distillation Column V-301 , as a side-cut divides into two streams. Part of HSD withdrawn enters the High Speed Diesel stripper V-303 quantity of plates in HSD stripper V-303 is 4. The level in the stripper is controlled by level control valve LCV-304. The HSD is steam stripped for flash point correction. The low boiling point components in the stripper rise through the plates and exit from the top of the stripper and enters the distillation tower above the 18th plate. HSD product is pumped by High Speed Diesel Pump P-305 A/B with a discharge pressure of approx. 600-800 KPa, to move the product to the storage tank. However, before sending product for storage it must be cooled down. High Speed Diesel stream is splitted into two streams. One flows through the shell side of Heat Exchanger E-302 and the Heat Exchanger E-306 . The second stream is sent to the shell side of Heat Exchanger E-317 and the Heat Exchanger E-322. The two streams are then mixed at TDV212. From here the HSD is sent to Fan cooler E-311. From the fan cooler the product flows through the flow control valve FCV-110 at approx. 35-40 oC and 55 KPa to the storage tank.
HSD
Pump Around.
The other part of the HSD withdrawn from the Multiple Draw TowerV-301 is pumped by Pump-around Pump P-307A/B with a discharge pressure of approx. 700-900 KPa. This pump-around loop unloads the top section of the crude tower and reduces the load on overhead system. Three trays in the main tower server as the HSD pump-around section. This stream is further divided into two streams. One stream of pump-around is further splitted into two streams. One flow through the shell side of Heat Exchangers E-307 A&B and then through the flow control valve FCV-105. the other stream flow through the shell side of Heat Exchangers E-328and E-327 and then through the flow control valve FCV-131. The streams then returns to the tower on the 18th plate, allowing high level heat recovery in the crude pre-heat train. The other part of pump-around flows through the flow control valve FCV-111 and the tube side of R eboiler E-403, and join the other pump-around stream to tower. This stream is circulated as pump-around stream, supplying all of the heat required
by the stabilizer reboiler E-403 . The combined pump-around stream returns to the main tower on tray 18.
FCV-131
High Speed Diesel Product and Pump around (HSD)
Liquid Petroleum Gas (LPG) Propane-Butane vapors (LPG) exits from the top of the stabilizer V-401 and flows to the stabilizer overhead fan condenserE-402 , which condenses the C3/C4 fraction.
Condensed liquid and non-condensed vapors flow to the stabilizer overhead accumulator V-402 . Stabilizer accumulator is pressure controlled at 900 KPa by pressure control valve PCV-404 allowing flow of the non-condensed vapors to the gas-gathering vessels. Condensed propane-butane fraction (liquefied Petroleum Gas) is pumped by reflux pump P-401 A/B with a discharge pressure of approx. 800 KPa through flow control valve FCV-112 . Before going to the top of stabilizer the reflux enters a Double Pipe Heat Exchanger E-406 , and then is refluxed to the top of stabilizer. Almost all of the condensed liquid is sent back as reflux at the rate of 78 m3/hr Small amount water collects in the accumulator water boot, which is observed and drained periodically. A part of LPG from reflux pump P-401 A/B is sent back to the over head accumulator V-402. The second stream is sent to Pump P-501 A/B from here the LPG is sent to LPG treatment section.
