PETROLEUM REFINING PROCESS

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Overview of Refinery Processes

4.1 Introduction:

Petroleum products (in contrast to petrochemicals) are those bulk fractions that are

derived from petroleum and have commercial value as a bulk product. In the strictest

sense, petrochemicals are also petroleum products but they are individual chemicals

that are used as the basic building blocks of the chemical industry. In this lecture, a

brief overview of various refinery processes is presented along with a simple sketch

of the process block diagram of a modern refinery. The sketch of the modern refinery

indicates the underlying complexity and the sketch is required to have a good

understanding of the primary processing operations in various sub-processes and

units.

4.2 Refinery flow sheet

We now present a typical refinery flow sheet for the refining of middle eastern crude

oil. There are about 22 units in the flow sheet which themselves are complex enough

to be regarded as process flow sheets. Further, all streams are numbered to summarize

their significance in various processing steps encountered in various units. However,

for the convenience of our understanding, we present them as units or blocks which

enable either distillation in sequence or reactive transformation followed by

distillation sequences to achieve the desired products. The 22 units presented in the

refinery process diagram are categorized as:

-Desalting process

- Crude distillation unit (CDU)

- Vacuum distillation unit (VDU)

- Thermal cracker

- Hydrotreaters

- Fluidized catalytic cracker

- Separators

- Naphtha splitter

- Catalytic Reformer

- Alkylation and isomerization

- Gas treating

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- Blending pools

-Stream splitters

-Claus process

A brief account of the above process units along with their functional role is presented

next with simple conceptual block diagrams representing the flows in and out of each

unit.

a) Desalting process:

Purpose of crude oil desalting:

Crude oil introduced to refinery processing contains many undesirable

impurities, such as sand, inorganic salts, drilling mud, polymer, corrosion

byproduct, etc. The salt content in the crude oil varies depending on source of

the crude oil. When a mixture from many crude oil sources is processed in

refinery, the salt content can vary greatly.

The purpose of desalting is to remove these undesirable impurities, especially

salts and water, from the crude oil prior to distillation.

The most concerns of the impurities in crude oil:

1. The Inorganic salts can be decomposed in the crude oil pre-heat exchangers

and heaters. As a result, hydrogen chloride gas is formed which condenses to

liquid hydrochloric acid at overhead system of distillation column, that may

causes serious corrosion of equipment.

2. To avoid corrosion due to salts in the crude oil, corrosion control can be used.

But the byproduct from the corrosion control of oil field equipment consists of

particulate iron sulfide and oxide. Precipitation of these materials can cause

plugging of heat exchanger trains, tower trays, heater tubes, etc. In addition,

these materials can cause corrosion to any surface they are precipitated on.

3. The sand or silt can cause significant damage due to abrasion or erosion to

pumps, pipelines, etc.

4. The calcium naphthanate compound in the crude unit residue stream, if not

removed can result in the production of lower grade coke and deactivation of

catalyst of FCC unit.

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Benefits of Crude Oil Desalting

1. Increase crude throughput.

2. Less plugging, scaling, coking of heat exchanger and furnace tubes.

3. Less corrosion in exchanger, fractionators, pipelines, etc.

4. Better corrosion control in CDU overhead

5. Less erosion by solids in control valves, exchanger, furnace, pumps.

6. Saving of oil from slops from waste oil.

Fig.1 Crude oil desalting process

The desalting process is completed in following steps:

1. Dilution water injection and dispersion.

2. Emulsification of diluted water in oil.

3. Distribution of the emulsion in the electrostatic field.

4. Electrostatic coalescence.

5. Water droplet settling.

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Crude oil passes through the cold preheat train and is then pumped to the Desalters by

crude charge pumps. The recycled water from the desalters is injected in the crude oil

containing sediments and produced salty water. This fluid enters in the static mixer

which is a crude/water disperser, maximizing the interfacial surface area for optimal

contact between both liquids.

The wash water shall be injected as near as possible emulsifying device to avoid a

first separation with crude oil. Wash water can come from various sources including

relatively high salt sea water, stripping water, etc. The static mixers are installed

upstream the emulsifying devices to improve the contact between the salt in the crude

oil and the wash water injected in the line.

The oil/water mixture is homogenously emulsified in the emulsifying device. The

emulsifying device (as a valve) is used to emulsify the dilution water injected

upstream in the oil. The emulsification is important for contact between the salty

production water contained in the oil and the wash water. Then the emulsion enters

the Desalters where it separates into two phases by electrostatic coalescence.

The electrostatic coalescence is induced by the polarization effect resulting from an

external electric source. Polarization of water droplets pulls them out from oil-water

emulsion phase. Salt being dissolved in these water droplets, is also separated along

the way.The produced water is discharged to the water treatment system (effluent

water). It can also be used as wash water for mud washing process during operation.

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b) Crude distillation unit:

The unit comprising of an atmospheric distillation column, side strippers, heat

exchanger network, feed de-salter and furnace as main process technologies

enables the separation of the crude into its various products. Usually, five products

are generated from the CDU namely gas + naphtha, kerosene, light gas oil, heavy

gas oil and atmospheric residue (Figure 4.1a). In some refinery configurations,

terminologies such as gasoline, jet fuel and diesel are used to represent the CDU

products which are usually fractions emanating as portions of naphtha, kerosene

and gas oil. Amongst the crude distillation products, naphtha, kerosene has higher

product values than gas oil and residue. On the other hand, modern refineries tend

to produce lighter components from the heavy products. Therefore, reactive

transformations (chemical processes) are inevitable to convert the heavy

intermediate refinery streams into lighter streams. Operating Conditions: The

temperature at the entrance of the furnace where the crude enters is 200 – 280oC°.

It is then further heated to about 330 – 370C° inside the furnace. The pressure

maintained is about 1 bar gauge.

Fig.2 Crude oil atmospheric distillation process

The properties of petroleum distillation products:

Product Lower C limit Upper C limit Lower B.P Upper B.P

Refinery gas C₁ C4 -100 -1

L.P.G C3 C4 -42 -1

Naphtha C5 C17 36 202

Gasoline C4 C12 40 216

Kerosene C8 C18 126 230

Diesel C10 C22 220 255

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Fuel oil C12 >C20 265 421

Lubricant oil >C20 >343

wax C17 >C20 302 >343

Asphalt >C20 >343

Coke >C50 >1000

Table 1 The properties of petroleum products

Refining end-products or the primary end-products produced in petroleum

refining may be grouped into four categories light distillates, middle distillates,

heavy distillates and others.

Light distillates products:1.

Refinery gases or off gas: density (0.20-0.38) gm/m³, draw off at temperature

(+65)C̊.

Liquid petroleum gas (LPG): density (0.52-0.58) gm/m³, draw off at

temperature (+65)C̊.

Naphtha: density (0.68-0.71) gm/m³, draw off at temperature (100-120)C̊.

Kerosene: density (0.76-0.80) gm/m³, draw off at temperature (180-220)C̊.

Diesel: density (0.82-0.84) gm/m³, draw off at temperature (230-260)Cͦ.

2. Middle distillates:

Light fuel oil: density (0.92-0.96) gm/m³, draw off at temperature (+265)C̊.

Heavy fuel oil: density (0.97-1) gm/m³, draw off at temperature (+300)C̊.

3. Heavy distillates:

Asphalt, carbon black and tar: density (2-2.36) gm/m³, draw off at temperature

(+360)C̊.

Petroleum coke: density (1-1.7) gm/m³, draw off at temperature (+450)C̊.

Lubricating oil and transformer and cable oil: density (0.83-0.89) gm/m³,

product from solvent dewaxing process and thickening process.

Waxes and greases: density (1.8-2) gm/m³, product from solvent dewaxing

process.

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Distillation tower components:

Distillation columns are made up of several components, each of which is used either

to transfer heat energy or enhance material transfer. A typical distillation contains

several major components:

Vertical shell where the separation of liquid components is carried out

column internals such as trays/plates and/or packing which are used to

enhance component separations as shown in (fig.1.4a) .

Re-boiler to provide the necessary vaporization for the distillation process.

Condenser to cool and condense the vapors leaving the top of the column.

Reflux drum to hold the condensed vapors from the top of the column so

that liquid (reflux) can be recycled back to the column.

Distillation tower tray and packing tray:

The trays or plates used in industrial distillation columns are fabricated of circular

steel plates and usually installed inside the column at intervals of about 60 to 75 cm

(24 to 30 inches) up the height of the column. That spacing is chosen primarily for

ease of installation and ease of access for future repair or maintenance .Typical bubble

cap trays used in industrial distillation columns. An example of a very simple tray is a

perforated tray. The desired contacting between vapor and liquid occurs as the vapor,

flowing upwards through the perforations, comes into contact with the liquid flowing

downwards through the perforations. In current modern practice, as shown in the

adjacent diagram, better contacting is achieved by installing bubble-caps or valve caps

at each perforation to promote the formation of vapor bubbles flowing through a thin

layer of liquid maintained by a weir on each tray.

Type of tray:

1. Bubble-cups tray.

2. Valve tray.

3. Sieve tray.

4. Packing tray: used for gases and absorption system.

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Bubble-cup tray Packing tray Sieve tray

Valve tray

Fig.3 Distillation tray section

The vertical shell houses the column internals and together with the condenser and

reboiler, constitutes a distillation column. A schematic of a typical distillation unit

with a single feed and two product streams is shown below:

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Fig.4 Schematic of distillation column

Basic Operation and Terminology:

The liquid mixture that is to be processed is known as the feed and this is

introduced usually somewhere near the middle of the column to a tray known as

the feed tray. The feed tray divides the column into a top (enriching or

rectification) section and a bottom (stripping) section. The feed flows down the

column where it is collected at the bottom in the reboiler.

Heat is supplied to the reboiler to generate vapour. The source of heat input can be

any suitable fluid, although in most chemical plants this is normally steam. In

refineries, the heating source may be the output streams of other columns. The

vapour raised in the reboiler is re- introduced into the unit at the bottom of the

column. The liquid removed from the reboiler is known as the bottoms product or

simply, bottoms.

Fig.5 Reflux section

Top section, the vapor moves up the column, and as it exits the top of the unit, it is

cooled by a condenser. The condensed liquid is stored in a holding vessel known

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as the reflux drum. Some of this liquid is recycled back to the top of the

column and this is called the reflux.

Reflux refers to the portion of the overhead liquid product from a distillation

column or fractionator that is returned to the upper part of the column as shown in

the schematic diagram of a typical industrial distillation column. Inside the

column, the down flowing reflux liquid provides cooling and condensation of the

up flowing vapors thereby increasing the efficiency of the distillation column.

The more reflux provided for a given number of theoretical plates, the better is the

column's separation of lower boiling materials from higher boiling materials.

Conversely, for a given desired separation, the more reflux is provided, the fewer

theoretical plates are required. The condensed liquid that is removed from the

system is known as the distillate or top product.

Thus, there are internal flows of vapors and liquid within the column as well as

external flows of feeds and product streams, into and out of the column.

c) Vacuum distillation unit (VDU):

Process overview:

Refineries today are facing new challenges in order to meet the requirements with

respect to environment, health and safety of the plant personnel and the quality of the

finished products.

With increasing crude oil prices, refineries are processing heavier, lower quality

crudes that set new challenges to further develop the processes and maximize the

yield of valuable distillates in an energy efficient way. Plant run-time targets are

increasing which sets more challenges for equipment reliability and process control.

Hydrocarbons should not be heated to too high temperature due to cracking reactions

that take place above about 400 °C. Coke deposits on piping and equipment increase

maintenance costs and reduce process unit run-time. Therefore crude distillation

bottom (residue) is further processed in a vacuum column to recover additional

distillates, light and heavy vacuum gasoil as feedstock to cracking units or lube-oil

processing.

Three types of vacuum towers are used:

1. Dry (no steam).

2. Wet without stripping.

3. Wet with stripping.

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Distillation is carried out with absolute pressures in the tower flash zone area of 25 to

40 mmHg. To improve vaporization, the effective pressure is lowered even further by

the addition of steam to the furnace inlet and at the bottom of the vacuum tower. The

amount of stripping steam used is a function of the boiling range of the feed and the

fraction vaporized as well as furnace outlet temperatures (380 – 420 °C).)Vacuum

towers are much larger in diameter than atmospheric towers, usually 12 – 15 meters.

The operating) pressure is maintained by using steam ejectors and condensers. The

size and number of vacuum device is determined by the vacuum needed and the

quality of vapors handled, for 25 mmHg, three ejector stages are usually required. A

few millimeters decrease in pressure drop between the vacuum-inducing device and

the flash zone will save operating costs. The capacity of the presented vacuum

distillation is 80 000 bbl. /day or 4 million tons/year) with fuel consumption of about

3200 MMBtu/day.

The atmospheric residue when processed at lower pressures does not allow

decomposition of the atmospheric residue and therefore yields LVGO, HVGO and

vacuum residue (Figure 4.1c). The LVGO and HVGO are eventually subjected to

cracking to yield even lighter products. The VDU consists of a main vacuum

distillation column supported with side strippers to produce the desired products.

Therefore, VDU is also a physical process to obtain the desired products. Operating

Conditions: The pressure maintained is about 25 – 40 mm Hg. The temperature is

kept at around 380 – 420C°.

Fig.6 Vacuum distillation unit process

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Pour point:

The pour point is the temperature at which a liquid hydrocarbon ceases to flow or

pour. This is measured by a standard method where a definite quantity of an oil

sample is taken in a test jar or tube (with a thermometer properly stoppered ,)heated to

115°F (46°C) to make all the wax dissolve in oil, and cooled to 90°F (32°C)before

testing. An ice bath containing ice and salt is made ready at a temperature of 51° F–

30°F (−9°C to −1°C) below the estimated pour point based on cloud point and the test

tube containing the sample is placed with the thermometer. At intervals of 1° F, the

test tube is removed from the ice bath and tilted to see if the oil is mobile or static. If

it is found that at a certain temperature the oil shows no movement even when the test

tube is kept horizontal for 5 sec, this temperature is reported to be the solid point. The

pour point is taken as 5°F above this solid point. Lube oils used in engines, gears,

bearings, etc., vary with the properties. For example, in reducing the bearing friction,

lube oil should be of high viscosity and high VI so that it is not squeezed out during

use and is not thinned out or thickened with frictional heat and also should be capable

of bearing load. Heavy semi-solid lubricants containing metallic soaps with base oil

classified as grease which are the common load-bearing lubricant. Lubricants used for

lubricating the surface of an engine cylinder and piston should have a very high VI

but low viscosity. Crankcase oil should also have the same property and it also serves

as a cooling medium for engine cylinders. For aviation services, lube oil should have

a high VI with a very low pour point. It has been found that the VI of paraffinic oils is

greater than naphthenic and aromatic oils, whereas viscosity is low for paraffinic oils

as compared to naphthenic and aromatic oils. Contribution toward the viscosity, VI,

and pour point from the paraffin (P), naphthene (N), and aromatic (A) hydrocarbon

groups are listed below.

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c) Thermal cracker:

Thermal cracker involves a chemical cracking process followed by the separation

using physical principles (boiling point differences) to yield the desired products.

Thermal cracking yields naphtha + gas, gasoil and thermal cracked residue (Figure

4.1a). In some petroleum refinery configurations, thermal cracking process is replaced

with delayed coking process to yield coke as one of the petroleum refinery products.

Operating Conditions: The temperature should be kept at around 450 – 500C° for the

larger hydrocarbons to become unstable and break spontaneously. A (2-3) bar

pressure must be maintained.

d) Hydrotreaters:

For many refinery crudes such as Arabic and Kuwait crudes, sulfur content in the

crude is significantly high. Therefore, the products produced from CDU and VDU

consist of significant amount of sulfur. Henceforth, for different products generated

from CDU and VDU, sulfur removal is accomplished to remove sulfur as H2S using

Hydrogen. The H2 required for the hydrotreaters is obtained from the reformer unit

where heavy naphtha is subjected to reforming to yield high octane number reformer

product and reformer H2 gas. In due course of process, H2S is produced. Therefore,

in industry, to accomplish sulfur removal from various CDU and VDU products,

various hydrotreaters are used. In due course of hydrotreating in some hydrotreaters

products lighter than the feed are produced. For instance, in the LVGO/HVGO

hydrotreater, desulfurization of LVGO & HVGO (diesel) occurs in two blocked

operations and desulfurized naphtha fraction is produced along with the desulfurized

gas oil main product (Figure 4.1d). Similarly, for LGO hydrotreating case, along with

diesel main product, naphtha and gas to C5 fraction are obtained as other products

(Figure 4.1d). Only for kerosene hydrotreater, no lighter product is produced in the

hydrotreating operation. It is further interesting to note that naphtha hydrotreater is

fed with both light and heavy naphtha as feed which is desulfurized with the reformer

off gas. In this process, light ends from the reformer gas are stripped to enhance the

purity of hydrogen to about 92 % (Figure 4.1d). Conceptually, hydrotreating is

regarded as a combination of chemical and physical processes. Operating Conditions:

The operating condition of a hydrotreater varies with the type of feed. For Naphtha

feed, the temperature may be kept at around 280-425C̊ and the pressure be maintained

at 200 – 800 psig.

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Fig.7 Hydro-desulfurization unit process

S2H + H-R heat Catalist

2SH + H-R

H2S + NaOH NaSH + H2O

Corrosion:

The presence of mercaptan sulfur may cause corrosion in the fuel pipes and the engine

cylinder and produce sulfur dioxide during combustion. In the past, merox (a catalytic

mercaptan oxidation method) treatment was done to convert corrosive mercaptans to

non-corrosive disulfi des, but this did not remove the sulfur originally present in the

fuel. However, it did give rise to the formation of sulfur dioxide during combustion.

Since emission of sulfur dioxide is prohibited by environmental protection laws,

nowadays mercaptans and other sulfur compounds are mostly removed.

by a catalytic hydrodesulfurisation unit in a refinery. The corrosive effects of other

organic compounds along with traces of sulfur-bearing compounds and additives must

be tested in the laboratory. The copper corrosion test similar to that described in the

testing of LPG is also carried out in the laboratory at standard temperature (50°C) for

3 h.

Best example for desulpherization processes is merox unit processes.

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Merox is an shortcut name for mercaptan oxidation. It is a catalytic chemical process

used in oil refineries and natural gas processing plants to remove mercaptans (light

and heavy) from LPG, propane, butanes, light naphtha, kerosene and jet fuel by

converting them to liquid hydrocarbon disulfides.

The Merox process requires an alkaline environment which, in some process versions,

is provided by an solution of sodium hydroxide (NaOH), a strong base, commonly

referred to as caustic- soda. In other versions of the process, the alkalinity is provided

by ammonia, which is a weak base.

The catalyst in some versions of the process is a water-soluble liquid. In other

versions, the catalyst is impregnated onto charcoal.

Processes within oil refineries or natural gas processing plants that remove

mercaptans and/or hydrogen sulfide (H2S) are commonly referred to as sweetening

processes because they results in products which no longer have the sour, foul odors

of mercaptans and hydrogen sulfide. The liquid hydrocarbon disulfides may remain in

the sweetened products, they may be used as part of the refinery or natural gas

processing plant fuel, or they may be processed further.

Especially when dealing with kerosene. The merox process is usually more

economical than using a catalytic hydrodesulfurization process for much the

same purpose. Indeed, it is rarely (if ever) required to reduce the sulphur content

of a straight-run kerosene to respect the sulphur specification of Jet Fuel as the

specification is 3000 ppm and very few crude oils have a kerosene cut with a

higher content of sulphur than this limit.

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Fig.8 Merox unit processes

The Merox reactor is a vertical vessel containing a bed of charcoal that have been

impregnated with the cobalt – base catalyst. The charcoal may be impregnated with

the catalyst in situ or they may be purchased from market as pre- impregnated with the

catalyst. An alkaline environment is provided by caustic being pumped into reactor on

an intermittent, as needed basis.

The jet fuel or kerosene feedstock from the top of the caustic prewash vessel is

injected with compressed air and enters the top of the Merox reactor vessel along with

any injected caustic. The mercaptan oxidation reaction takes place as the feedstock

percolates downward over the catalyst. The reactor effluent flows through a caustic

settler vessel where it forms a bottom layer of caustic solution and an upper layer of

water-insoluble sweetened product.

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The caustic solution remains in the caustic settler so that the vessel contains a

reservoir for the supply of caustic that is intermittently pumped into the reactor to

maintain the alkaline environment.

The sweetened product from the caustic settler vessel flows through a water

wash vessel to remove any entrained caustic as well as any other unwanted

water-soluble substances, followed by flowing through a salt bed vessel to

remove any entrained water and finally through a clay filter vessel. The clay

filter removes any oil-soluble substances, organometallic compounds (especially

copper) and particulate matter, which might prevent meeting jet fuel product

specifications.

The pressure maintained in the reactor is chosen so that the injected air will

completely dissolve in the feedstock at the operating temperature.

The overall oxidation reaction that takes place in converting mercaptans to

disulfides is:

4RSH + O2 → 2RSSR + 2H2O

The most common mercaptans removed are:

Methanethiol - CH3SH [m-mercaptan]]

Ethanethiol - C2H5SH [e- mercaptan]]

1-Propanethiol - C3H7SH [n-P mercaptan]]

2-Propanethiol - CH3CH(SH)CH3 [2C3 mercaptan]]

[Butanethiol - C4H9SH [n-butyl mercaptan]

e) Fluidized catalytic cracker:

The unit is one of the most important units of the modern refinery. The unit enables

the successful transformation of desulfurized HVGO to lighter products such as

unsaturated light ends, light cracked naphtha, heavy cracked naphtha, cycle oil and

slurry (Figure 4.1e). Thereby, the unit is useful to generate lighter products from a

heavier lower value intermediate product stream. Conceptually, the unit can be

regarded as a combination of chemical and physical processes. Operating Conditions:

The temperature should be maintained at 34C° with pressure ranging from 75 kPa to

180 kPa. Moreover, the process is to be carried out in a relatively wet environment.

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Aviation fuels:

The fuels used in aeroplanes are called aviation fuels. Depending on the type of

aircraft, like jet planes or turbine planes, different types of aviation fuels are used .

They are either gasoline based for jet planes or kerosene based for turbine planes.

Aviation gasoline is usually polymer gasoline or alkylated gasoline having an octane

number greater than 100, usually expressed as the performance number. Kerosene

based aviation fuel is known as aviation turbine fuel (ATF) and is mostly consumed

by passenger aeroplanes. This fuel is the hydrocarbon fraction boiling in the range of

150–250°C and is similar to the kerosene fraction. Though it resembles kerosene ,

tests are carried out under stringent conditions for the safety of the airborne people in

the flying machines. A corrosion test is carried out using the copper strip test for 2 h

at 100°C and a silver strip test is carried out for 16 h at 45°C. Distillation tests are

conducted as for kerosene while the 20% recovery should be at 200°C and the FBP

should not be more than 300°C. Besides freezing point is to be below −50°C as the

sky temperature may be very low at high altitude.

The properties of aviation fuels:

Property of ATF Specification

Final boiling: 300°C

Flash point (Abel): min 38°C

Freezing point: max −50°C

Smoke point: minimum 20 mm

Viscosity, kinematic, at –34.4°C: max 6 cst

Sulfur content, total: max 0.20% wt

Carbon residue, Ramsbottom: max 0.20% wt

Pour point: max 6°C

Ash content: max 0.01% wt

Aromatic percent vol.: max 20

Olefin percent vol.: max 5

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f) Separators:

The gas fractions from various units need consolidated separation and require stage

wise separation of the gas fraction. For instance, C4 separator separates the

desulfurized naphtha from all saturated light ends greater than or equal to C4s in

composition (Figure 4.1e). On the other hand, C3 separator separates butanes (both

iso and n-butanes) from the gas fraction (Figure 4.1e). The butanes thus produced are

of necessity in isomerization reactions, LPG and gasoline product generation.

Similarly, the C2 separator separates the saturated C3 fraction that is required for LPG

product generation (Figure 4.1e) and generates the fuel gas + H2S product as well. All

these units are conceptually regarded as physical processes. Operating Conditions:

Most oil and gas separators operate in the pressure range of 20 – 1500 psi.

:Cetane number

Cetane number or (CN) is an indicator of the combustion speed of diesel fuel. It

is an inverse of the similar octane rating for gasoline (petrol). The CN is an

important factor in determining the quality of diesel fuel, but not the only one; other

measurements of diesel's quality include (but are not limited to) energy content,

density, lubricity, cold-flow properties and sulphur content.

Cetane number or CN is an inverse function of a fuel's ignition delay, the time

period between the start of injection and the first identifiable pressure increase

during combustion of the fuel. In a particular diesel engine, higher cetane fuels

will have shorter ignition delay periods than lower Cetane fuels. Cetane numbers

are only used Cetane is a chemical compound, alkane (named hexadecane, chemical

formula n-C16H34), molecules of which are un-branched and with open chain.

Cetane ignites very easily under compression, so it was assigned a cetane number of

100, while alpha-methyl naphthalene was assigned a cetane number of 0. All other

hydrocarbons in diesel fuel are indexed to cetane as to how well they ignite under

compression. The cetane number therefore measures how quickly the fuel starts to

burn (auto- ignites) under diesel engine conditions. Since there are hundreds of

components in diesel fuel, with each having a different cetane quality, the overall

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Fig.9 Diesel cycle, cutoff point and delay time

cetane number of the diesel is the average cetane quality of all the components

(strictly speaking high-cetane components will have disproportionate influence,

hence the use of high-cetane additives).for the relatively light distillate diesel oils.

Generally, diesel engines operate well with a CN from 40 to 55. Fuels with higher

cetane number have shorter ignition delays, providing more time for the fuel

combustion process to be completed. Hence, higher speed diesel engines operate more

effectively with higher cetane number fuels.

In Europe, diesel cetane numbers were set at a minimum of 38 in 1994 and 40 in

2000. The current standard for diesel sold in European Union, Iceland, Norway and

Switzerland is set in EN 590, with a minimum cetane index of 46 and a minimum

cetane number of 51. Premium diesel fuel can have a cetane number as high as 60.

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Fig.10 Fluid catalyst cracking unit process

g) Naphtha splitter:

The naphtha splitter unit consisting of a series of distillation columns enables the

successful separation of light naphtha and heavy naphtha from the consolidated

naphtha stream obtained from several sub-units of the refinery complex (Figure 4.1f).

The naphtha splitter is regarded as a physical process for modeling purposes.

Operating Conditions: The pressure is to be maintained between 1 kg/cm2 to 4.5

kg/cm2. The operating temperature range should be 167 – 250C°.

h) Catalytic Reformer:

Catalytic reforming is a major conversion process in petroleum refinery and

petrochemical industries. The reforming process is a catalytic process which converts

low octane naphtha into higher octane reformate products for gasoline blending and

aromatic rich reformate for aromatic production. Basically, the process re-arranges or

re-structures the hydrocarbon molecules in the naphtha feed stocks as well as breaking

some of the molecules into smaller molecules. Naphtha feeds to catalytic reforming

include heavy straight run naphtha. It transforms low octane naphtha into high-octane

motor gasoline blending stock and aromatics rich in benzene, toluene, and xylene with

hydrogen and liquefied petroleum gas as a byproduct. With the fast growing demand

in aromatics and demand of high - octane numbers, catalytic reforming is likely to

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remain one of the most important unit processes in the petroleum and petrochemical

industry.As shown in Figure 4.1f, Heavy naphtha which does not have high octane

number is subjected to reforming in the reformer unit to obtain reformate product

(with high octane number), light ends and reformer gas (hydrogen). Thereby, the unit

produces high octane number product that is essential to produce premium grade

gasoline as one of the major refinery products. A reformer is regarded as a

combination of chemical and physical processes. Operating Conditions: The initial

liquid feed should be pumped at a reaction pressure of 5 – 45 atm. And the preheated

feed mixture should be heated to a reaction temperature of 495 – 520C°.

The four major catalytic reforming reactions are:

1. The dehydrogenation of naphthenes to convert them into aromatics as

exemplified in the conversion methylcyclohexane (a naphthene) to toluene (an

aromatic), as shown below:

2. The dehydrogenation and aromatization of paraffins to aromatics (commonly

called dehydrocyclization) as exemplified in the conversion of normal heptane

to toluene, as shown below:

3. The hydrocracking of paraffins into smaller molecules as exemplified by the

cracking of normal heptane into iso-pentane and ethane, as shown below:

25

Octane number or octane rating:

Octane rating or octane number is a standard measure of the performance of an engine

or aviation fuel by indicate anti-knock index or measure the rate of iso-octane in

gasoline structure. The higher the octane number, the more compression the fuel can

withstand before detonating (igniting). In broad terms, fuels with a higher octane

rating are used in high performance petrol engines that require higher compression

ratios. In contrast, fuels with lower octane numbers means higher n- heptane rates.

Petrol engines (also referred to as gasoline engines) rely on ignition of air and fuel

compressed together as a mixture without ignition, which is then ignited at the end of

the compression stroke using spark plugs. Therefore, high compressibility of the fuel

matters mainly for petrol engines. Use of petrol (gasoline) with lower octane numbers

may lead to the problem of engine knocking.

-Isooctane (upper) has an octane rating of 100.

-n-heptane (bottom) has an octane rating of 0.

26

Method of measurement:

1. Research Octane Number (RON):

The most common type of octane rating worldwide is the Research Octane Number

(RON). RON is determined by running the fuel in a test engine with a variable

compression ratio under controlled conditions, and comparing the results with those

for mixtures of iso-octane and n-heptane.

2. Motor Octane Number (MON):

Another type of octane rating, called Motor Octane Number (MON), is determined at

900 rpm engine speed instead of the 600 rpm for RON.[1] MON testing uses a similar

test engine to that used in RON testing, but with a preheated fuel mixture, higher

engine speed, and variable ignition timing to further stress the fuel's knock resistance.

Depending on the composition of the fuel, the MON of a modern pump gasoline will

be about 8 to 12 octane lower than the RON, but there is no direct link between RON

and MON. Pump gasoline specifications typically require both a minimum RON and

a minimum MON.

3. Anti-Knock Index (AKI) or (R+M)/2:

In most countries, including Australia, New Zealand and all of those in Europe, the

"headline" octane rating shown on the pump is the RON, but in Canada, the United

States, Brazil, and some other countries, the headline number is the average of the

RON and the MON, called the Anti-Knock Index (AKI), and often written on pumps

as (R+M)/2). It may also sometimes be called the Posted Octane Number (PON).

Difference between RON, MON, and AKI:

Because of the 8 to 12 octane number difference between RON and MON noted

above, the AKI shown in Canada and the United States is 4 to 6 octane numbers lower

than elsewhere in the world for the same fuel. This difference between RON and

MON is known as the fuel's Sensitivity, and is not typically published for those

countries that use the Anti-Knock Index labelling system.

27

4. Observed Road Octane Number (RdON):

Another type of octane rating, called Observed Road Octane Number (RdON), is

derived from testing gasolines in real world multi-cylinder engines, normally at wide

open throttle. It was developed in the 1920s and is still reliable today. The original

testing was done in cars on the road but as technology developed the testing was

moved to chassis dynamometers with environmental controls to improve consistency.

Fig.11 Otto cycle, 4-stroke engine cycle

28

Processes Steps in Catalytic Reforming:

Basic steps in catalytic reforming involve

Feed preparation: Naphtha Hydrotreatment.

Preheating: Temperature Control, Catalytic Reforming and Catalyst

Circulation and Regeneration in case of continuous reforming process.

Product separation: Removal of gases and Reformate by fractional

Distillation.

Separation of aromatics in case of Aromatic production.

Fig.12 Semi-continue catalytic reformer unit processes

29

Naphtha Hyderotreatment Process:

Naphtha hydrotreatment is important steps in the catalytic reforming process for

removal of the various catalyst poisons. It eliminates the impurities such as sulfur,

nitrogen, halogens, oxygen, water, olefins, di olefins, arsenic and other metals

presents in the naphtha feed stock to have longer life catalyst. Figure 4.1f illustrates

hydrotreatment of naphtha.

Sulphur: Mercaptans, disulphide, thiophenes and poison the platinum

catalyst. The sulphur content may be 500 ppm.

Maximum allowable sulphur content 0.5 ppm or less and water content <4

ppm.

Fixed bed reactor containing a nickel molybdenum where both hydro de

sulphurisation reactions and hydro de nitrification reactions take place.

The catalyst is continuously regenerated. Liquid product from the reactor

is then stripped to remove water and light hydrocarbons.

i) Alkylation & Isomerization:

The unsaturated light ends generated from the FCC process are stabilized by

alkylation process using C4 generated from the C4 separator. The process yields

alkylate product which has higher octane number than the feed streams (Figure 4.1 f).

As iso-butane generated from the separator is enough to meet the demand in the

alkylation unit, isomerization reaction is carried out in the isomerization unit (Figure

4.1f) to yield the desired make up C4.

Octane number of Hydrocarbons:

Octane number is a measurement of antiknock characteristics of fuels

Among the same carbon number compounds, the order of RON is

(Research Octane Number ) Paraffins < Naphthenes < Aromatics

Branched paraffins also have high octane. It increases with degree of

branching.

Therefore, octane number of naphtha can be improved by reforming the hydrocarbon

molecule (Molecular rearrangement).

31

Table 4.1B: Octane Number of Various Hydrocarbons:

Hydrocarbon Octane Number

n-Butane 94

i-Butane 102

n-pentane 63

i-Pentane 93

n-Heptane

Octane 100

Toluene 119

j) Gas treating:

The otherwise not useful fuel gas and H2S stream generated from the C2 separator

has significant amount of sulfur. In the gas treating process, H2S is successfully

transformed into sulfur along with the generation of fuel gas (Figure 4.1 g).

Eventually, in many refineries, some fuel gas is used for furnace applications within

the refinery along with fuel oil (another refinery product generated from the fuel oil

pool) in the furnace associated to the CDU. Operating Conditions: Gas treaters may

operate at temperatures ranging from 150 psig (low pressure units) to 3000 psig (high

pressure units).

Fig.13 H₂S Gas treater unit process

31

k) Blending pools:

All refineries need to meet tight product specifications in the form of ASTM

temperatures, viscosities, octane numbers, flash point and pour point. To achieve

desired products with minimum specifications of these important parameters,

blending is carried out. There are four blending pools in a typical refinery. While the

LPG pool allows blending of saturated C3s and C4s to generate C3 LPG and C4 LPG,

which do not allow much blending of the feed streams with one another (Figure 4.1 h).

The most important blending pool in the refinery complex is the gasoline pool where

in both premium and regular gasoline products are prepared by blending appropriate

amounts of n-butane, reformate, light naphtha, alkylate and light cracked naphtha

(Figure 4.1h). These two products are by far the most profit making products of the

modern refinery and henceforth emphasis is there to maximize their total products

while meeting the product specifications. The gasoil pool (Figure 4.1h) produces

automotive diesel and heating oil from kerosene (from CDU), LGO, LVGO and

slurry. In the fuel oil pool (Figure 4.1h), haring diesel, heavy fuel oil and bunker oil

are produced from LVGO, slurry and cracked residue.

Fig.14 Blending pools or Blending drum

32

l) Stream splitters:

To facilitate stream splitting, various stream splitters are used in the refinery

configuration. A kerosene splitter is used to split kerosene between the kerosene

product and the stream that is sent to the gas oil pool (Figure 4.1 i). Similarly, butane

splitter splits the n-butane stream into butanes entering LPG pool, gasoline pool and

isomerization unit (Figure 4.1i).

Unlike naphtha splitter, these two splitters facilitate stream distribution and do not

have any separation processes built within them. With these conceptual diagrams to

represent the refinery, the refinery block diagram with the complicated interaction of

streams is presented in (Figure 4.1i).

Fig.15 Stream splitter

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M) Claus process:

The Claus process is the most significant gas desulfurizing process, recovering

elemental sulfur from gaseous hydrogen sulfide. First patented in 1883 by the scientist

Carl Friedrich Claus, the Claus process has become the industry standard.

The multi-step Claus process recovers sulfur from the gaseous hydrogen sulfide found

in raw natural gas and from the by-product gases containing hydrogen sulfide derived

from refining crude oil and other industrial processes. The by-product gases mainly

originate from physical and chemical gas treatment units (Selexol, Rectisol, Purisol

and amine scrubbers) in refineries, natural gas processing plants and gasification or

synthesis gas plants. These by-product gases may also contain hydrogen cyanide,

hydrocarbons, sulfur dioxide or ammonia.

Gases with an H2S content of over 25% are suitable for the recovery of sulfur in

straight-through Claus plants while alternate configurations such as a split- flow set up

or feed and air preheating can be used to process leaner feeds. Hydrogen sulfide

produced, for example, in the hydro-desulfurization of refinery naphtha and other

petroleum oils, is converted to sulfur in Claus plants. The overall main reaction

equation is:

2 H2S + O2 → S2 + 2 H2O

In fact, the vast majority of the 64,000,000 metric tons of sulfur produced worldwide

in 2005 was byproduct sulfur from refineries and other hydrocarbon processing

plants. Sulfur is used for manufacturing sulfuric acid, medicine, cosmetics, fertilizers

and rubber products. Elemental sulfur is used as fertilizer and pesticide.

Fig.16 The Claus technology, process description

34

Claus Unit Description:

1. The hot combustion products from the furnace at 1000- 1300°C enter the waste

heat boiler and are partially cooled by generating steam. Any steam level from 3 to 45

bar g can be generated.

2. The combustion products are further cooled in the first sulphur condenser, usually

by generating LP steam at 3 – 5 bar g. This cools the gas enough to condense the

sulphur formed in the furnace, which is then separated from the gas and drained to a

collection pit.

3. In order to avoid sulphur condensing in the downstream catalyst bed, the gas

leaving the sulphur condenser must be heated before entering the reactor.

4. The heated stream enters the first reactor, containing a bed of sulphur conversion

catalyst. About 70% of the remaining H2S and SO2 in the gas will react to form

sulphur, which leaves the reactor with the gas as sulphur vapour.

5. The hot gas leaving the first reactor is cooled in the second sulphur condenser ,

where LP steam is again produced and the sulphur formed in the reactor is condensed.

6. A further one or two more heating, reaction, and condensing stages follow to react

most of the remaining H2S and SO2.

7. The sulphur plant tail gas is routed either to a Tail Gas treatment Unit for further

processing, or to a Thermal Oxidiser to incinerate all of the sulphur compounds in the

tail gas to SO2 before dispersing the effluent to the atmosphere.

35

Chapter Five

Lubricant production

The use of animal fats to reduce friction and wear and tear of mechanical parts has

been the practice from time immemorial. However, since the availability of petroleum

Sources, lubricants are now manufactured using petroleum stocks. Today’s

lubricating oil is mainly composed of base hydrocarbon oil, lubricating base oil stock

(LOBS), obtained from vacuum distillates after treatment in the refi nery, as discussed

in Chapter 3, with some additives to meet the requirements for its end use . Synthetic

base oils, such as polyalphaolefins, alkylated aromatics and polybutenes, are also used

as base oils.

5.1 Industrial lubricants: Industries use a large amount of lubricants, known as industrial oils, i.e., transmission

oils, turbine oils, compressor oils, seal oils, cooling oils, gear oils, bearing oils,

hydraulic oils, and cutting oils. Oils should be non-toxic and non-explosive.

Lubricating oil must satisfy the following needs:

36

1. Bearing lubricants:

Bearings used in machineries face either sliding or rolling frictions. Usually, greases

or solid lubricants are used to lubricate the small bearing surfaces.

2. Hydraulic lubricants:

Hydraulic fluids are viscous liquids used in power transmission for control, braking in

automobiles and machineries, raising or lowering loads by multiplying the transmitted

force, and so on. In addition to these activities, hydraulic fluids lubricate the mating

parts of machines and are used in a wide variety of environments, such as air, water,

gaseous, and high and low temperatures.

3. Compressor lubricant:

Gases are compressed either in reciprocating or centrifugal compressors. In the

reciprocating compressor, the piston and cylinder is lubricated by lubricating oil ,

which must have a low vapors pressure and a low carbon-forming tendency. Lube

vapour, especially petroleum base oil and carbon particles, may contaminate the

compressed gas and lead to explosion.

5.2 Definitions:

Alkylation: the reaction of the double bond in an olefin with another molecule.

Aromatic hydrocarbon: a compound of hydrogen and carbon containing one or

more benzene rings.

Base stock: the refined oil that is used to blend lubricants.

Catalyst: substance that causes or speeds up a reaction without changing itself.

Catalyst used now a day is platinum on alumina base.

Catalyst Regeneration:

Performance of the catalyst decreases with respect to time due to deactivation.

Reasons for de-activation:

Coke formation

Contamination on active sites

Agglomeration

Catalyst poisoning

37

Activity could be restored if deactivation occurred because of coke formation or

temporary poisons.

Objective of Regeneration:

Surface area should be high

Metal Pt should be highly dispersed

Acidity must be at a proper level

Regeneration changes by the severity of the operating conditions

Coke formation can be offset for a time by increasing reaction

temperatures.

Cracking: breaking large molecules into smaller ones in the presence of heat or

catalyst. Examples are thermal cracking, catalytic cracking, or hydrocracking

(cracking in the presence of hydrogen) High Temperatures.

Crude oil: a naturally occurring mixture of petroleum hydrocarbons, with small

amounts of oxygen, nitrogen, sulfur and other impurities. It was formed by the action

of bacteria, heat and pressure on ancient plant and animal remains and is usually

found in layers of porous rock capped by a layer of shale or clay that traps the oil.

Detergent: additive that prevents contaminants from contacting metal surfaces.

Detergents contain barium, calcium or magnesium, and other compounds and they

may leave an ash residue if burned.

Dewaxing: a refining step that removes wax from crude oil fractions.

Dispersant: additive that suspends very small contaminant particles harmlessly in the

oil. It prevents them from combining into large particles that could cause sludge,

varnish and wear. Dispersants generally do not contain metals and are considered to

be ash less when burned.

Distillation: a process where crude oil is heated so fractions start to boil; the fractions

are collected as they condense.

Extreme pressure (EP) additive: additive that prevents sliding metal surfaces from

seizing under conditions of extreme pressure. At high local temperatures it combines

chemically with the metal to form a surface film. EP additives are commonly made of

sulfur, phosphorus or chlorine.

Foam inhibitor: in turbulent systems it helps combine small air bubbles into large

bubbles which rise to the surface and burst. It spreads out on the bubble wall to thin it

38

out so that it pops.

Hydrocarbon: a molecule predominantly made up of hydrogens and carbons.

Hydrocracking: a process that cracks molecules in the presence of hydrogen.

Hydrotreating: a process where oil streams are treated with hydrogen at elevated

pressure and temperature in the presence of a catalyst to improve color and stability

and reduce sulfur content.

Naphthene: also called a (cycloparaffin): A hydrocarbon with the carbons arranged

in one or more rings.

Olefin: a hydrocarbon with one or more double bonds.

Oxidation: the combination of a substance with oxygen. Some products of oxidation

of oils include organic acids (which can cause corrosion), sludge and varnish.

Oxidation inhibitor: additive used to extend the life of lubricants. It can work in

three ways:

1. It combines with peroxides (the first chemicals created in the oxidation

process) to make them harmless. 2. It decomposes the peroxides. 3. It makes metal surfaces less able to promote oxidation.

Paraffin: a hydrocarbon where the carbons are arranged in a straight line or are

branched off of each other.

Rust inhibitor: additive that prevents rust.

Saponification: process of converting certain chemicals into soaps, which are the

metallic salts of organic acids. It is usually accomplished through the reaction of a fat,

fatty acid, or ester with an alkali.

Saturated hydrocarbon: a compound of hydrogen and carbon with no double bonds.

Solvent extraction: a method of removing unstable components of a refining stream

by dissolving them in solvent.

Synthetic base stock: fluid made by reacting specific chemical compounds to

produce a produce with planned and predictable properties. A number of synthetically

made oils are available that give better performance than those made from crude oil.

They are better at reducing friction and engine wear , have good detergency properties

which keep the engine cleaner, offer less resistance for moving parts, and require less

39

pumping power for distribution. With good thermal properties, they provide better

engine cooling and less variation in viscosity.

Because of this, they contribute to better cold-weather starting and can reduce fuel

consumption by as much as 15%. These oils cost several times as much as those made

from crude oil. However, they can be used longer in an engine, with 24,000 km

(15,000 miles) being the oil change period suggested by most manufacturers.

Vacuum distillation: separation of crude oil fractions by applying heat under a

vacuum.

5.2 Base Stock Refining:

Crude oil, as it comes from the ground, contains many substances other than

gasoline, diesel fuel and lubricant base stocks. The natural gas that is used to power

many city buses, or that heats burners can be dissolved in crude oil. You can find the

building blocks for plastics and chemicals in its mixture of molecules. It is also a

source of wax, asphalt, solvents, petroleum jelly and many other substances.

Fuels are the principle products that come from crude oil. Gasoline is the biggest

component, followed by diesel fuel and jet fuel. Lubricants and waxes only constitute

a little over 1% of the crude oil barrel.

Crude oil comes in many forms, depending upon its source. It can be as thin as

gasoline, or it can be so thick that it needs to be heated or pressurized to flow. It can

range from amber colored to pitch black.

The three major types of crude oil are paraffinic, naphthenic and asphaltic. Paraffinic

crudes are the primary source of “neutral” base oils, while naphthenic crudes produce

“coastal pale” oils. Asphaltic crudes are generally not acceptable for lubricant base

stocks other than some open gear compounds, highly viscous lubricants or rust

preventives.

Crude oil is sold by the barrel (42 gallons per barrel) by pipeline or in tanker ships.

From there, it is transported by barge or pipeline to a refinery, where it is processed

according to the steps described below. Lubricants are further processed in blending

plants.

Crude oil is converted to lubricating oil through a series of refining steps including

distillation, vacuum distillation, solvent extraction, hydrotreating, dewaxing and

sometimes hydrocracking and alkylation. The yield of lubricating oil from a gallon of

crude can be increased first through de-bottlenecking steps, second by a series of steps

called cracking and alkylation.

When oil comes out of the ground, the first things that are released are gases. These

include methane, ethane, propane and butane, as well as the mixture called natural

gas.

Natural gas: These gases can generally be captured without heating the oil.

41

Distillation towers, of which the atmospheric tower is the tallest in the refinery, work

on the same principle as stills worked during Prohibition. Alcohol was made by

fermenting grain or another substance and then heating it. Alcohol boils at a very low

temperature, so it would start to boil, while the rest of the mash was left in the pot.

The alcohol vapors would rise and get funneled into some coils, where they would

cool and condense back into liquid.

Gasoline is a liquid at room temperature, but it evaporates very fast, and it boils at a

very low temperature. In order to separate it from crude oil, the crude is heated until

the gasoline starts to boil. The vapors above the boiling liquid are collected and sold

as gasoline. Many of the solvents such as mineral spirits are collected at the same time

before they are further refined.

Once the gasoline fractions are removed, the crude oil is heated further. Kerosene, jet

fuel and diesel fuels are the next liquids that boil off. These fractions are also allowed

to condense and are collected and sold.

After most of the light fuels have been removed from crude oil, you would think that

they would just heat it up even more and boil off the lubricant fractions. But that is

not energy efficient, and besides, if they had to heat up lubricants to 600°F to boil

them off, they could destroy the lubricant.

Substances boil at a lower temperature if they are subject to a vacuum. Think of the

vacuum as trying to pull the molecules out of the liquid. A lot less heat is needed to

get them to boil. It is much more energy efficient to apply a vacuum to the oil to

remove lubricant fractions, and it is much less harmful to the lubricant. Vacuum

distillation prevents the cracking of lubricant fractions that would occur at higher

temperatures.

Vacuum distillation: separates lubricant fractions from crude oil. Re-refining and

reprocessing do not require this step because their feedstock, used lubricants,

should contain minimal amounts of fuels, waxes or asphalt.

The lubricant stream that is separated from crude oil still contains many impurities

and must be further refined. The next three refining steps, hydrotreating, solvent

extraction, and dewaxing, can occur in any order, or may be optional depending upon

the quality of the finished base oil.

Hydrotreating essentially bombards the stream with hydrogen to remove sulfur and

other impurities. It makes base stocks more stable. Treating base stock with hydrogen

has the following advantages.

1. Minimizes use of solvents.

2. Reduces solvent disposal.

3. Increases yield.

4. Permits use of different crude sources.

5. May reduce processing temperatures.

6. Produces base stocks with higher VI, which may increase fuel efficiency in

41

engine oils

7. Produces bases tock with lower volatility lower evaporative losses.

8. Base oils that have been severely hydrotreated don’t require a carcinogen

label.

Solvent extraction: dissolves reactive components such as aromatics to improve the

oil’s oxidation stability, viscosity index and response to additives. Sulfur and nitrogen

26 compounds are also selectively extracted. The oil and the solvent are mixed in a

tower, which results in two distinct liquid phases. The heavy components are

dissolved in the solvent. The lighter phase, which contains the clean, high quality oil,

is separated and the small amount of solvent is distilled off.

Solvent dewaxing: removes wax, lowers the pour point, and improves the low-

temperature properties of the oil. The solvent dissolves the wax and the mixture is

chilled until the wax turns solid. The wax is filtered out and stripped of solvent and

dried. The wax from this process can be used in crayons, candles, paper cups and fire

logs.

Hydrodewaxing (catalytic dewaxing): accomplishes the same result, but by a

different method. The oil is exposed to hydrogen at elevated temperature and

pressure. This cracks the normal paraffins, which are converted to light compounds

that can be used as building blocks for plastics and chemicals.

Finishing steps: can include acid treating and clay filtration to remove trace

impurities.

Refining severity is a compromise. Some of the more undesirable compounds,

asphaltenes and unsaturated, which reduce oxidation resistance, also tend to improve

boundary lubrication. An oil which has been only mildly refined may have poor

oxidation resistance but relatively good boundary lubrication. On the other hand,

severely refined oil has good oxidation resistance and a high viscosity index. The

other required properties are then obtained by the use of additives.

A lubricant formulator can specify a base stock by type, i.e. paraffinic, naphthenic,

synthetic, vegetable, and also by performance properties such as viscosity index,

viscosity, pour point, flash point, color, or sulfur level. Paraffinic base stocks

generally have higher viscosity index than naphthenic.

Base stocks also have varying natural resistance to oxidation. Here is a general guide

for oxidation resistance of petroleum base stocks.

1. Paraffin’s are most resistant. 2. Naphthenic, Aromatics, Asphaltenes and Unsaturated material are least resistant.

42

As a rule, synthetic base stocks are designed to have better oxidation stability than

petroleum oils, while vegetable oils have significantly lower resistance to oxidation.

5.3 Automotive oils:

The majority of automobiles include vehicles run on motor spirit (petrol) or diesel.

Different lubricants are used for petrol engines (which are spark ignition type) and

diesel engines (which are compression ignition type). These are known as engine oils,

which are suitable for use in high temperatures and the oxidizing environment of

engines. Load (weight to carry) and speed of the vehicles are also to be taken care of

before selecting a lube oil to apply. Usually, the temperature of an engine rises rapidly

during the startup and continues at that temperature during motion. Such a wide and

sudden change in temperature demands that the lubricant should have a high VI. In

addition to temperature fluctuations, lubricants are prone to oxidation and cracking,

leading to the formation of cokes, carbons, and gummy substances , which may

ultimately deposit on the engine, causing irreparable damage. In addition to engine

oils, different lubricants are applicable for other parts of the vehicle, such as the gears,

brake, clutch, and bearings. Gear boxes contain the gears immersed in lubricating oil

having low viscosity to reduce friction at high speed. The brake and clutch require

lubricating oils of low viscosity. Bearings are used in various parts of the automobile

from engine to wheels, and require low to high viscous lubricants. At low

temperatures and high load, bearings at wheels are lubricated by grease. Since

materials of construction and type of engines vary with the make, appropriate

lubricants are selected and prescribed by the manufacturers. No single lubricant is

therefore applicable for all makes. Finished lubes are classified according to the

Society of Automotive Engineers’ (SAE). The viscosity of the lubricants and its

variation with temperature (VI) and the pour point are the important parameters to

satisfy the compatibility of application of lubes. Winter grades are classified as SAE

numbers from 0 to 25W as typical examples of cold temperatures and from 20 to 60

SAE numbers for warming up the cranks of engines. A multigrade lubricant is a blend

of more than one type of lubricant. For example, SAE15W 50 is an example of a

blend of two grade oils. Usually, polymeric materials, such as ethylene propylene

copolymer, polymethyl acrylate, and butadiene, are added to these multigrade oils.

However, rigorous testing of appropriate lubes must be carried out on cars of different

makes in the testing laboratory or workshop for their suitability before prescribing

them for engines and other parts. Since the performance of these lubes may not be

satisfactory after a certain period of time due to degradation because of

contamination, reaction, physical and chemical changes in the property of the

ingredients or the base oils, it is inevitable that the lube must be drained out and

replaced with fresh stock. This drain out period must be specified for the prescribed

lubricants. The longer the drain out period, the more attractive the lubricant is in the

43

market.

5.4 Additives:

Lubricants are comprised primarily of base oils and additives. Grease is comprised of

base oils and additives along with thickeners.

The following is a brief description of lubricant additives and their functions :

Anti-wear additive: Zinc dialkyldithiophosphate (ZDDP) is the most common

antiwear additive, although there are many zinc-free additives based on sulfur and

phosphorus that also impart anti-wear properties. The zinc-sulfur-phosphorus end of

the molecule is attracted to the metal surface allowing the long chains of carbons and

hydrogens on the other end of the molecule to form a slippery carpet that prevents

wear.

Not a chemical reaction, rather a super-strong attraction.

Demulsified: affects the interfacial tension of contaminants so they separate out from

oil rapidly.

Dispersant: additive that suspends very small contaminant particles harmlessly in the

oil. It prevents them from combining into large particles that could cause sludge,

varnish and wear. Dispersants generally do not contain metals and are considered to

be ash less when burned.

Dye: any oil-soluble dye can be added to oils for leak detection. Water-soluble dyes

such as food coloring won’t work because oil and water don’t mix).

Emulsifier – added to some metalworking fluids, air tool oils and fire-resistance

hydraulic fluids to allow them to mix with water.

Extreme pressure (EP) additive: additive that prevents sliding metal surfaces from

seizing under conditions of extreme pressures. At high local temperatures it combines

chemically with the metal to form a surface film. EP additives are commonly made of

sulfur, phosphorus, or chlorine. They become reactive a high temp. (170°+F) and will

attack yellow surfaces. Generally, it is a good idea to stay away from extreme

pressure additives if they are not needed but when in doubt always use EP gear oils.

EP additives are generally pro-oxidants, in other words, they shorten the life of the

oil. They also can be slightly corrosive to some metals, especially at elevated

temperatures.

44

Foam inhibitor: in turbulent systems it helps combine small air bubbles into large

bubbles which rise to the surface and burst. It decreases the surface tension of the

bubble to thin and weaken it so that it pops.

Rust inhibitor: absorb onto metal surfaces to prevent attack by air and water.

Oxidation inhibitor: antioxidants act by interrupting the free radical chain reaction

that results in oxidation. Essentially, as the oil starts to decompose in the presence of

oxygen, 28 these inhibitors interrupt the reaction. They also keep metal from speeding

up the oxidation reaction by deactivating the metal.

Oiliness agent: fatty materials that have two benefits. They add extra lubricity at low

to moderate temperatures, and they help prevent water from washing oil off of

surfaces.

Pour depressant: disrupt the crystal structure of wax so that the oil will flow at lower

temperatures.

Solid additives: graphite, moly and PTFE are added to some oil and grease

formulations

Tackiness additive: polymer added to allow oils to adhere to metal surfaces.

Viscosity index improvers: polymers that change shape with temperature. At high

temperatures they are somewhat bulky and prevent the oil from thinning down as

much. Lubricating oils are generally rated using a viscosity scale established by the

Society of Automotive Engineering (SAE). Dynamic viscosity is defined from the

equation:

Ts = 1/-t(dUjdy)

where:

Ts = shear force per unit area.

1/-t = dynamic viscosity.

(dUjdy) = velocity gradient.

The higher the viscosity value, the greater is the force needed to move adjacent

surfaces or to pump oil through a passage. Viscosity is highly dependent on

temperature, increasing with decreasing temperature (Fig. 11-11). In the temperature

range of engine operation, the dynamic viscosity of the oil can change by more than

an order of magnitude. Oil viscosity also changes with shear, duj dy, decreasing with

45

increasing shear. Shear rates within an engine range from very low values to

extremely high values in the bearings and between piston and cylinder walls. The

change of viscosity over these extremes can be several orders of magnitude. Common

viscosity grades used in engines are:

SAE 5

SAE 10

SAE 20

SAE 30

SAE 40

SAE 45

SAE 50

The oils with lower numbers are less viscous and are used in cold-weather operation.

Those with higher numbers are more viscous and are used in modern high-

temperature, high-speed, close-tolerance engines.

If oil viscosity is too high, more work is required to pump it and to shear it between

moving parts. This results in greater friction work and reduced brake work and power

output. Fuel consumption can be increased by as much as 15%. Starting a cold engine

lubricated with high-viscosity oil is very difficult (e.g., an automobile at -02° C or a

lawn mower at 100e).

Multigrade oil was developed so that viscosity would be more constant over the

operating temperature range of an engine. When certain polymers are added to an oil,

the temperature dependency of the oil viscosity is reduced, as shown in Fig.

55-50 . These oils have low-number viscosity values when they are cold and higher

numbers when they are hot. A value such as SAE lOW-30 means that the oil has

properties of 10 viscosity when it is cold (W = winter) and 30 viscosity when it is

hot. This gives a more constant viscosity over the operating temperature range (Fig.

55-50 .) This is extremely important when starting a cold engine. When the engine and

oil are cold, the viscosity must be low enough so that the engine can be started

without too much difficulty. The oil flows with less resistance and the engine gets

proper lubrication. It would be very difficult to start a cold engine with high-viscosity

oil, because the oil would resist engine rotation and poor lubrication would result

because of the difficulty in pumping the oil. On the other hand, when the engine gets

up to operating temperature, it is desirable to have a higher viscosity oil. High

temperature reduces the viscosity, and oil with a low viscosity number would not give

adequate lubrication.

Some studies show that polymers added to modify viscosity do not lubricate as well

as the base hydrocarbon oils. At cold temperatures SAE 5 oil lubricates better than

SAE 5W-30, and at high temperatures SAE 30 oil lubricates better. However, if SAE

30 oil is used, starting a cold engine will be very difficult, and poor lubrication and

very high wear will result before the engine warms up.

Common oils available include:

SAE 5W-20 SAE 10W-40

SAE 5W-30 SAE lOW-50

SAE 5W-40 SAE 15W-40

46

SAE 5W-50 SAE 15W-50

SAE lOW-30 SAE 20W-50

47

48

5.5 Grease Manufacturing:

Lubricating grease is a solid or semisolid lubricant consisting of a thickening agent in

a liquid lubricant. Greases generally contain additives to enhance properties such as

oxidation stability and lubricity. Other ingredients imparting special properties may

be included.

Greases that are thickened with soap can be made according to the following steps.

The steps may vary according to manufacturing plant.

1. Charge kettle with fatty material, complexing agent and metallic hydroxide

2. Heat to dehydrate

3. Cut back with mineral oil

4. Quench and add additives

5. Mill

6. Deaerate

7. Filter

8. Package

Complexing agents are added to grease to increase their high temperature

stability. These are very stable soaps generated by reacting an alkali with a high and

low molecular weight fatty acid.

The reaction of a fatty acid and a base is very similar to the reaction of an acid and a

base. The acid and base react to form salt and water. The fatty acid and base forms

soap.

Acid + Base = Salt + Water

“Fatty Acid” + Base = Soap

Grease is typically made up of 75% to 96% percent oil. The oil is designed to separate

from the thickener, so it is not uncommon to see some oil separation if the grease has

been sitting.

12-hydroxy stearic acid is the most common type of fatty acid used in grease

manufacture.

49

5.5 Grease Soaps and Thickeners:

In addition to conventional and complex soaps, other materials may be used to thicken

grease. Here is a partial list of thickeners.

1. Soap Base

a. Aluminum, Aluminum complex b. Barium, Barium complex

c. Calcium, Calcium complex d. Lithium, Lithium complex

e. Sodium, Sodium complex

Mixed soap such as sodium calcium

2. Non-soap Base

Polyuria, Modified betonies and other clays like Colloidal, silica, Organic

compounds

Fluorinated compounds

5.6 Grease Additives:

The additives used in greases are very similar to those used in lubricating oils, which

are listed in Section 2C. Some additives that you would expect to find in grease

include: Antioxidants or oxidation inhibitors.

1. Corrosion inhibitors :

A. 30 Color stabilizers Dyes.

B. Film strength agents metal deactivators rust inhibitors stringiness additives structure modifiers for soap-oil systems.

5.6 Grease Fillers:

In addition to soap, base oil and additives, solid fillers can be added to grease to

51

enhance its lubricity and load carrying ability. Some of the fillers used in lubricating

greases are listed below:

1. Graphite colloidal flake powdered.

2. Lead powder molybdenum disulfide. 3. Red lead powder.

4. Zinc oxide. 5. Copper flake. 6. Zinc dust.

5.6 Finished products:

Using the above intermediates, a variety of plastics, rubber, fi bre, solvent, paint, etc,.

are manufactured. Polymerisation reactions are carried out for these monomers or

intermediates to various polymers, resinous and liquid products. Plastics are available

in the form of extrudates, granules, powders, beads, etc., from the manufacturing units

as the fi nished products. These are converted into plastic commodities, such as bags ,

fi lms, furniture, and products of various shapes and sizes by casting, moulding, or

blowing machines, as the marketable products. Plastics are classifi ed into two types,

namely, thermoplastic (or thermoplast) and thermosetting plastics or (thermoset).

Thermoplasts, usually linear in molecular structure, can be melted (or softened) by

heating and solidifi ed (or hardened) by cooling. This heating and cooling cycle can

be repeated indefi nitely without loss of the original properties. But thermosets will be

permanently transformed to a chemically cross linked or non- linear structure and

cannot be returned to their original property during a heating and cooling cycle.

51

REFERENCES:

Gary J.H., Handwerk G.E., Petroleum Refining: Technology and Economics,

Taylor & Francis, 2005.

Jones D.S.J., Elements of Petroleum Processing, John Wiley & Sons, 1995 .

Petroleum Refinery Process Safety Management National Emphasis Program,

USA, 2005.

Dolomatov M. Y., Lomakin S. P., Krakhmaleva G. V., Kuzmina Z. F., (1988).

Interrelation between carbon residue and relative density of petroleum resids,

Chemistry and Technology of Fuels andOils, Springer, NewYork.

Jones D. S. D. and Pujado P. R., (2006). Handbook of Petroleum Processing,

Springer, Netherlands.

Maples R. E., (2000). Petroleum Refinery Process Economics, 2nd Edition,

Pennwell Corporation,USA.

Maxwell (1950). Data book on hydrocarbons, 9th Edition, Von Nostrand

Reinhold, Company,USA.

Technical Data book – Petroleum Refining (1997), 6th Edition, American,

Petroleum Institute (API).

Mall, I.D. “Petrochemical processes technology”, First Edi., New Delhi,

Macmillan India, 2007.

Raseev, S. “Thermal and catalytic Processes in Petroleum Refining”, Marcel,

Dekker, Inc, New York 2003.

B.K. Bhaskara Rao, “Modern Petroleum Refining Processes” Edn. 3, Oxford

& IBH Publishing Company Pvt. Ltd. New Delhi.