1

Extraction Of Furnace Oil from

Low Sulphur Heavy Stock (LSHS) at

Tatipaka Refinery

BY

DEEPANKRAVALI

KARTHIKPRADEEP

2

ACKNOWLEDGEMENT

We are grateful to our external guide Shri K.Giridhar Rao DGM (P) for his technical guidance and

support in carrying out our project at mini refinery at Tatipaka, ONGC Ltd, Rajahmundry.

We would also like to thank Shri D.K.Gour CE (P), Shri K.V.Nagesh CE (P), Shri S.Perumal CC and

Shri G.Ravi Kumar AEE(E&T) for explaining the functions and activities of GCS,GCP,ETP, Refinery

and Quality control lab of Tatipaka complex.

Also, we would like to pay humble gratitude to Shri D.K.Datta CE (P), I/M – TPK REFINERY as

without his support and encouragement my project work would not have been possible.

Most humbly and respectfully, we would like to acknowledge the efforts of all those people who are

working at ONGC Tatipaka Refinery and who guided me through my project work in the refinery.

3

CONTENTS PAGE NO

1. ABOUT ONGC AND MINI REFINERY………………………………. 01

2. ABOUT TATIPAKA COMPLEX……………………………………….01

2.1 GAS COLLECTION STATION (GCS)…………………………...01

2.2 GAS COMPRESSION PLANT (GCP)……………………………02

2.3 EFFLUENT TREATMENT PLANT (ETP)………………………..04

2.4 QUALITY CONTROL LAB………………………………………...06

2.5 MINI REFINERY…………………………………………………….07

3. EXTRACTION OF FURNACE OIL FROM LSHS……………………13

3.1 VACUUM DISTILLATION UNIT (VDU)……………………………15

3.2 DESIGN ASPECTS…………………………………………………..17

4. CONCLUSION & RECOMMENDATIONS……………………………19

1

1. ABOUT ONGC & MINI REFINERY:

“MAHARATNA” ONGC Ltd is a national oil company of India focussed on exploration and production of oil and gas. It has interests in Refining, LNG, Power, Petrochemical and new sources of energy. ONGC is recognized as the world no:3 Exploration and Production Company and it is credited with the discovery of all the producing petroliferous Indian basins since its formation in 1956.

ONGC has a current daily production of over 1.2 million barrels of oil equivalent. it contributes to 65% of India’s domestic oil equivalent production.

ONGC produced 47.03 MTOE (23.71 MT Oil and 23.32 BCM of Gas) in the year 2012-2013 and achieved highest ever profit-after-tax of Rs 251.23 billion.

ONGC has established its first mini refinery at Tatipaka, East Godavari district of Andhra Pradesh with a capacity of 250 tonnes of crude oil per day on 3rd September, 2001. Refinery houses the sophisticated distributed control system (DCS) for the process dynamics control. The refinery established here is re-locatable skid mounted unit which is first of its kind in Asia. The speciality of this type of unit is that ease of transportation of the equipment of plant from the current place to the other if the resources have been depleted and should be moved to the other place where there is source of oil wells.

2. ABOUT TATIPAKA COMPLEX

Following units are present in the Complex

1. Gas Collecting Station (GCS)2. Gas Compression Plant (GCP)3. Effluent Treatment Plant (ETP)4. Quality Control Lab5. Mini Refinery

2.1 Gas Collecting Station (GCS)

Tatipaka GCS is a major installation in KG project with good infrastructural facilities. The GCS collects and processes the gas that is produced from different wells under tatipaka complex. This is the largest on-shore installation of KG project where all associated also exist.

Process description:

Gas from about (at present 19) wells are directed to the production header. This gas not only comprises of produced gas but also crude/condensate and water. So for the separation the gas is sent to a production separator with a liquid boot

Production Separator:

The well fluid is received in a 3-phase low pressure production separator, operates at a pressure 6.0 kg/cm2. Three phase separation is achieved inside the separator due to change in momentum of the well fluid and providing the required time for the liquid phase.

The gas stream from the top of separator is routed to the filter separators for the filtration and knocks out of entrained liquid particles. The produced gas from the LP production separator is routed to the LP gas compressor at tatipaka site.

2

The hydrocarbon liquid is separated in the production separator is routed to the existing condensate storage via two phase liquid separator. The separated water after passing through the basket filter is collected in the produced water collection tank.

Test separator has been provided at the site for testing performance of the individual wells at low pressures i.e. 6.0 kg/cm2. The gas separated from the test separator is metered and is connected to the separated gas from the production separator prior to filter separators. The hydrocarbon liquid stream from test separator is routed to two phase liquid separator and produced water stream to produced water collection vessel.

2.2 Gas Compression Plant (GCP)

The gas from the wells used to be obtained at high pressure. Slowly the pressure from the wells started to drop. Now, almost all wells under tatipaka complex were producing low pressure gas.

Hence, the GCP is established in 2010 to process and compress the LP gas.

3

Process description:

Liquid free low pressure gas after the filter separator is routed to the gas compressor suction KOD operating at a pressure 5 kg/cm2. Gas flow from the overhead of the suction KOD is metered before entering the compressor skid. Liquid collected in the suction KOD flow to the intermediate blow down (IBD) vessel.

Intermediate blow down vessel is designed to receive various hydrocarbon liquid streams from the suction KOD, discharge KOD, compressor package, trunk line KOD and the fuel gas conditioning system. The gas stream from the blow down vessel is routed to the flare.

There are three compressor units, each of 4 LSCMD capacities.

Gas form suction KOD is routed to the 1st stage suction separator where any small amount of liquid left over in the gas stream is knocked off. Gas from the suction separator flows into the 1st stage at 5 kg/cm2 and discharge pressure is 22-26 kg/cm2.

As the discharge temperature of 1st stage gas is 144 0C, it is cooled in air cooled heat exchanger up to 58 0C before entering the 2nd stage suction separator. Gas from overhead of the 2nd stage suction separator is routed to the second stage of the compressor where it is compressed to 60-62 kg/cm2

and with a discharge temperature of 138 0C. An air cooled after cooler, cools the gas at the outlet of the second stage to a temperature of 58 0C.

Compressed gas leaves the compressor skid and is routed to the discharge KOD. Liquid from discharge KOD flows to the intermediate blow down vessel. The gas from the discharge KOD goes to the GAIL terminal.

4

2.3 Effluent Treatment Plant (ETP)

Effluent is primarily water with hydrocarbon. Hydrocarbon mixed with water is characterized with two types of oil:

1. Free oil2. Emulsified oil

The treatment philosophy is largely based on oil water separation through specific gravity differences of two phases. This separation is carried out in stages by either coalesce of small oil droplets into larger droplets or through reducing the specific gravity by nucleating the micro bubbles to emulsified oil thereby allowing the oil droplets to rise to the surface, facilitating separation. The entire treatment scheme for Tatipaka GCS is designed to treat 500 M3 per day with average load of 21 M3 per day.

Oily water sump (OWS):

The raw effluent feed from the underground sewer is collected in the OWS sump.

Wash tank:

The effluent transferred from the OWS sump through the pump is collected in the wash tank capacity of 200 M3. The inlet of wash tank line will be connected to an on-line system.

The dosing of de-oiler chemical is done on-line to achieve de-emulsifying of oil from the effluent. The separated sludge is transferred to the existing chemical sludge sump through sludge transfer pump.

Balancing tank:

The effluent is transferred from wash tank to balancing tank capacity of 200 M3. In a balancing tank a separate slop oil overflow line is provided to collect the slop oil in the slop oil sump. The sludge is transferred to chemical sludge sump through sludge transfer pump.

Titled plate interceptor:

The outlet of the balancing tank that is oily water is connected to two TPI feed pumps to transfer the effluent to titled plate interceptor. These pumps transfer the equalized raw effluent for further treatment.

Flash mixer:

The outlet of the TPI treated effluent then passes through flash mixer. The flow from the TPI to the flash mixer is under gravity over flow. The TPI created effluent is then subjected to coagulation of solids with the help of alkali and alum dosing.

Flocculation tank:

The over flow of the flash mixer tank, flows into the flocculation chamber by gravity. Here polyelectrolyte is dosed for flocculating the alum-oil suspended solids flocks and creating large agglomerates.

Dissolved air flotation (DAF) unit:

Dissolved air flotation system removes flocculated solids by means of air flotation and sedimentation.

5

Filter feed sump:

The treated effluent from DAF unit is collected in the filter feed sump through gravity flow pipelines. The effluent is then transferred to a dual media filter in the downstream process through transfer pumps.

Polishing system:

The treated effluent after the process might carry traces of organic and suspended solids. These impurities removed through pressurised dual media filtration (DMF) process.

Backwash sump:

The dual media filtration unit is designed for continuous filtration process. When pressure increases more than 1kg/cm2 filter will be taken for backwashing.

A guard pond is provided to store the treated effluent after the filtration. The effluent from the pond is pumped by high pressure pump for underground disposal in the old reservoir.

Sludge and slop oil handling system:

The sludge generated through the treatment process is collected in sludge collection sump. The sludge is then dewatered through centrifuge process and the remaining sludge cake is transferred to sludge drying bed.

Quality Parameters:

Parameter Raw Effluent Treated EffluentpH 7.5 to 8.0 8.0 to 9.0Total Suspended Salts 200 mg/L <100 mg/LOil and Grease 300 mg/L <10 mg/L

6

2.4 Quality Control Lab

In this laboratory the quality and characteristics of crude and distillates are tested.

Online sample (test sample collected before entering the storage tanks) is tested for its purity based on its composition and properties for any change.

The various equipments in the laboratory and their functioning are:

Reid vapour pressure apparatus- to determine vapour pressure. Hydrometer- for measuring density. ASTM distillation unit - the quality of the product is tested. Copper corrosion apparatus. Abel flash point apparatus- to determine flash point. Pensky marten apparatus- to determine flash point. Smoke point apparatus- to determine smoke point of diesel and naphtha. Blumont lamp - to determine smoke point of kerosene. Auto kinematic viscometer- to determine viscosity. Otal sulphur UV - to determine sulphur content. Dean and stark apparatus - to determine associated water content. Pour point apparatus - to determine pour point.

7

2.5 Mini Refinery

On average crude oils are made of the following elements or compounds.

Carbon : 84%Hydrogen : 14%Sulphur : 1-3%Nitrogen : <1%Oxygen : <1%Metals : <1%Salts : <1%

These hydrocarbons are named as below depending on their structure.

Methane (C1), Ethane (C2), Propane (C3), Butane (C4) Naphtha(C5-C12), Kerosene (C12-C16), High Speed Diesel (C16-C32), Low Sulphur Heavy Stock (C-32 & above).

The raw material crude oil is refined into various types of products such as Naphtha, SKO, HSD and LSHS. Crude oil characteristics vary from light (which is straw coloured liquid) to heavy (tar black solid). Crude oil is also called sweet (<0.1ppm sulphur) and sour (>0.1ppm sulphur) crude oil depending upon the amount of sulphur it contains.

At tatipaka crude oil is received in tankers from the wells at Kesanapalli, Lingala, Nagayalanka, Raghavapuram, Nandigama, Vadaparru, Kanukollu, Matsyapuri and many other wells in KG basin.

Refinery process:

1. Storage of crude oil :

Two tanks each of 500 m3 capacity are used for storage of crude which is received every day from nearby fields constituting feed for refinery. One of these tanks receives crude oil from tanker intermittently and condensate from GCS on continuous basis. The other tank supplies feed to the refinery through centrifugal pumps.

2. Crude charging & pre-heating:

A mixture of crude and condensate with a composition range of 30% to 70% is fed in a distillation column by adding de-emulsifier, at a temperature of 3300C, after passing through two stages of preheating exchangers, a de-salter vessel and furnace.

In first stage of preheating the crude feed is made to exchange heat with Kerosene, Diesel & LSHS (cold LSHS) product streams in respective heat exchangers

After the first stage of preheating the crude enters the de-salter vessel which is positioned between the two sets of exchangers.

The salt content of the feed is separated in the de-salter vessel with the help of mixing water with the feed and then breaking the emulsion with temperature, high voltage and de-emulsifier.

In second stage of preheating the crude feed is allowed to exchange heat with kerosene circulating reflux, LSHS (hot LSHS) and diesel circulating reflux streams and then enters into furnace.

3. Crude furnace:

8

At a temperature of 170-190 0 C, feed enters the gas-fired furnace where crude gets partially vaporised gaining a temperature of 330 0 C

4. Atmospheric distillation column:

The crude column operates at atmospheric pressure and fractionates crude feed into Naphtha, Kerosene, Diesel & LSHS products.

The column contains 49 trays (48 valve trays and 01 sieve tray). Different fractions get liquefied at different trays at the temperatures just below their boiling

point. Heavy naphtha is drawn from tray-11, kerosene is drawn from tray-22, diesel is drawn from

tray-35 and LSHS from bottom of the distillation column.

5. Process at other various Individual sections:

a. Overhead sectionb. Heavy naphtha sectionc. Kerosene sectiond. Diesel sectione. Low sulphur heavy stock sectionf. Strippersg. Circulating refluxesh. Naphtha stabilizersi. De-salterj. Valve trays and sieve traysk. Heat exchanger

a. Overhead section:

The overhead vapour from the crude column at 114 0 C is withdrawn under pressure control and is condensed and cooled in overhead condenser to 55 0 C.

The condenser utilizes cooling water as the cooling medium and this condensed overhead product separates out as hydrocarbons and water in the overhead accumulator.

Water is withdrawn from the boot under level control and sent to the effluent treatment plant for treatment.

Crude column overhead pumps to the naphtha stabilizer pump part of the un-stabilized naphtha from the accumulator under level cascade flow control and the rest is sent back to the crude column as reflux.

Crude column top temperature & reflux controller sets reflux demand. A split range pressure control maintains the column overhead pressure by controlling fuel

gas make-up and vapour to flare.

A. Heavy naphtha section:

Heavy naphtha is withdrawn as side product from tray-11 of the crude column at a temperature of 144 0 C under level control sent to side-stripper bottom.

Light ends in the heavy naphtha steam are stripped in heavy naphtha side-stripper using superheated steam.

Stripped vapours sent back to tray-9 of the crude column and the bottom product, heavy naphtha is pumped by gear pumps and cooled in the heavy naphtha cooler using cooling water.

9

Part of this stream is blended with final diesel product (to maintain Cetane number of diesel oil) and the rest is mixed stabilized naphtha in the naphtha storage tanks.

B. Kerosene section:

Kerosene as Superior Kerosene Oil (SKO) is withdrawn as a side product form chimney tray, which receives liquid from tray-22 of the crude column at a temperature of 186 0 C under level control sent to side-stripper bottom.

Light ends in the kerosene stream are sent back to tray-19 of the crude column and the hot bottom product of kerosene stripper is pumped by gear pumps, under flow control to the crude pre-heating section to exchange heat with crude in crude/kerosene exchanger.

The kerosene product gets cooled to 60 0 C in the preheat train and further cooled in kerosene product cooler to 40 0 C using cooling water.

The cooled product is stored is stored in kerosene storage tanks.

C. Diesel section:

Diesel as High Speed Diesel (HSD) is withdrawn as a side product from tray-35 of the crude column at a temperature of 290 0 C under level control of side-stripper bottom.

Light ends in the diesel stream are stripped in the diesel side-stripper using superheated steam.

Stripped vapours are sent back to the tray-32 of the crude column and the hot bottom of the diesel stripper is pumped by diesel product gear pumps under flow control to the crude pre-heating train to exchange heat with the crude in the crude/diesel exchanger.

The diesel product gets cooled to 53 0 C in the preheating section and further cooled in diesel product cooler to 40 0 C using cooling water and the cooled product is stored in diesel storage tanks.

D. Low Sulphur Heavy Stock section(LSHS):

Stripped LSHS at a temperature of 330 0 C is withdrawn from the bottom of the crude column under level control cascaded with flow control.

It is pumped by LSHS centrifugal pumps to the pre-heating of crude. This hot LSHS exchanges heat with crude in crude/LSHS (hot) exchanger in first stage of

heating and crude/LSHS (cold) exchanger and in turn gets cooled to a temperature of 110 0 C in this pre-heating section in second stage of heating.

There after LSHS is sent to the storage tanks provided with steam heating coils to avoid LSHS congealing.

E. Strippers:

Stripper is also a process vessel (packed column) where heat transfer operations and mass transfer (stripping) operations occur.

Respective liquid streams drawn out from the nozzle are made to fall through liquid inlet in the region above the stripper column and from the bottom, steam is sent.

In the stripping column the liquid and steam contact occurs and then heat transfer occurs between them.

The steam being at lower temperature than that of the drawn liquid, by direct contact in counter-current flow, decrease of liquid temperature occurs.

Mass transfer (stripping) occurs between the steam and drawn liquid (from column), heavy liquid is drawn out through the stripper bottom and the lighter vapour is sent back to the column as reflux.

10

The importance of refluxing of vapours is to maintain column temperature, pressure balancing and to enhance product quality and quantity.

There are three (3) strippers, namely:1. Heavy naphtha stripper, liquid drawn from tray -11 and its vapour refluxing sent back to

tray-9.2. Kerosene stripper, liquid drawn from tray-22 and its vapour refluxing sent back to tray-19.3. Diesel stripper, liquid drawn from tray-35 and its vapour refluxing sent back to tray-32.

The design stripping steam flow rates are as follows:

Sections Steam flow rates (kg/hr)Main tower 64 – 68Heavy naphtha stripper 45Kerosene stripper 68

Diesel stripper 34 - 54

F. Circulating refluxes:

In order to maximize the heat recovery and balance the tower loading, heat is removed by way of circulating refluxes from kerosene and diesel sections.

Kerosene CR is withdrawn from kerosene product withdraw tray - 22 of the crude column at a temperature of 175-185 0C.

It is pumped by kerosene CR pumps, 20-P-102 A/B to the crude preheat section where it exchanges heat with the crude in the crude/ kerosene CR exchanger 20-E-104 A/B.

Diesel CR is withdrawn from diesel product withdraw tray - 35 of the crude column at temperature of 280- 290 0C.

Then it is pumped by diesel CR pumps, 20-P-103 A/B to the crude preheating section where it exchanges heat with the crude in the crude/diesel CR exchanger 20-E-105 A/B.

The diesel CR is routed back to tray - 32 of the crude column after exchanging heat in naphtha stabilizer re-boiler 20-E-112.

The diesel and kerosene CR draw rate is controlled by a flow controller.

G. Naphtha stabilizers:

The un-stabilized naphtha from the overhead naphtha accumulator of the crude column is stabilized by removal of light ends in the naphtha stabilizer column 20-C-105.

The light ends are recovered as fuel gas and stabilized naphtha at 140 0C is obtained from the bottom of the column under level control.

The necessary heat required for light naphtha in re-boiler is provided by diesel circulating reflux under temperature control of the naphtha stabilizer bottom, through thermo-syphon re-boiler 20-E-112.

Naphtha is further cooled in 20-E-113 using cooling water and sent to storage tanks 20-T-104 A/B.

The stabilizer is provided with a stub in exchanger 20-E-111 to condense the overhead vapour.

Water is used as the cooling medium and water flow is controlled by stabilizer top temperature control.

The fuel gas from the top of the stabilizer is sent to fuel gas system. Stabilizer pressure is controlled by a pressure control provided in the fuel gas line.

11

H. De-salter :

The crude feed enters the De-salter vessel after the first stage of heating. In this process the salts are removed from the crude feed. The chemicals are added to the injection water and then added to crude feed. The undesired salts are dissolved in the water and form an emulsion. The electric field is used to break the oil-water emulsion. By means of voltage (10KV-25KV) and temperature the fluid is separated into two phases. Then the desalted crude is further sent to the second stage of heating.

I. Valve trays and Sieve trays:

1. Valve trays: They have perforations which are covered by caps. These caps make the vapour to move laterally through them.

Features of valve trays: Provides best turndown and efficiency. Valves positioned parallel to the liquid flow allow the liquid to flow unopposed across

the tray.

12

Lateral vapour release assures uniform contact in all active areas.2. Sieve trays: They are simple metal plates with holes in them. There is no lateral

movement of vapour.

Features:

Most economical for low turndown Low pressure drop

Each tray has two extensions namely the down-comer (lower extension) and the weir (upper extension).

Continuous condensation and vapourisation occurs at the trays which help in separating the required fractions more effectively.

3. Heat exchanger:

Heat exchanger is a device built for efficient heat transfer from one medium to the other. In Tatipaka refinery, shell and tube heat exchangers are used.

A shell and tube heat exchanger has shell with a bundle of tubes inside it. The crude feed flows in the tubes and the other fluid flows in the shell. The shell is

provided with baffles which maximise their heat efficiency.

PROCESS DIAGRAM OF MINI REFINERY :

13

4. EXTRACTION OF FURNACE OIL FROM LSHS

Definition of Furnace Oil:

14

Furnace Oil is a dark viscous residual fuel obtained by blending mainly heavier components from crude distillation unit. Furnace oil can include distillates and blends of distillates and residue such as light diesel oil.Furnace oil in the current marketing range meets bureau of Indian standards specification IS: 1593 - 1982 for fuel oils, grade mv2.

Characteristics of Furnace oil:

1.

Viscosity:Viscosity is the most important characteristic in the furnace oil specification. It influences the degree of pre-heat required for handling, storage and satisfactory atomization. If the oil is too viscous it may become difficult to pump, burner may be hard to light and operation may be erratic. 2. Flash point:As per the controller of explosives classification, furnace oil falls in the class "c" category with minimum flash point standard of 66 0C. Since Pensky martens closed cup method is used, it is apparent that a small quantity of low boiling point hydrocarbons is sufficient to lower the flash point drastically.3. Pour point:The pour point is the minimum temperature below which the liquid becomes semi solid and loses its flow characteristics. Generally a liquid with high pour point is associated with high paraffin content. The maximum pour point of furnace oil is 180C4. Water:Water may be present in free or emulsified form and on combustion can cause damage to the inside furnace surfaces especially if it contains dissolved salts. Water content of furnace oil, maximum limit of 1% is specified in the standard.5. Sediment:Furnace oil being a blend of residues contains some quantity of sediments. These have adverse effect on the burners and cause blockage of filters etc. However, the typical values are normally much lower than the stipulated value of maximum 0.25%, by mass.6. Ash:Ash is incombustible component of the furnace oil and is expressed as a percentage mass of the furnace oil sample. Ash has erosive effect on the burner tips, causes damage to the refractories at high temperatures and gives rise to high temperature corrosion and fouling of equipments.7. Sulphur:The maximum amount of sulphur in furnace oil can be 4% by weight. The sulphur dioxide formed during combustion may come in direct contact with the product and may create adverse quality effects in the product.8. Calorific value:Calorific value of a fuel is the quantity of heat generated in kilocalories by complete burning of one kilogram weight of fuel. Typical calorific value of furnace oil is 10 Kcal/g.

Low Sulphur Heavy Stock (LSHS):

SL.NO CHARACTERISTICS FURNACE OILSPECIFICATION METHOD

1 DENSITY @ 150C TO BE REPORTED

IS-1448,P-32

2 FLASH POINT (PM), 0C 66 (MIN) IS-1448,P-21

3 POUR POINT, 0C 18 (MAX) IS-1448,P-10

4 WATER CONTENT,% V/V

1.0 (MAX) IS-1448,P-40

5 GROSS CALORIFIC VALUE, KCAL/KG

TO BE REPORTED

IS-1448,P-7

6 SULPHUR, TOTAL, % BY WT

4.0 (MAX) IS-1448,P-35

15

LSHS is a residual fuel processed from indigenous crude. The main difference with LSHS and Furnace Oil is in the form of higher pour point, higher calorific value & low sulphur content in LSHS. As this fuel as pour point higher than that of furnace oil it requires special handling arrangements. LSHS is handled hot at all stages & is maintained at 75 0C. Special care is also is taken so that no boil over of the product takes place in the storage tank.

VACUUM DISTILLATION UNIT (VDU):

Both atmospheric and vacuum distillation units have similar architecture of the main and secondary columns i.e., both have complex stream circuitries with pump around, heat exchanger networks and utilization of steam.

Only basic difference is that while we operate the VDU at lower pressure (30 – 40 mm hg), the operating temperatures will be lower than those in the atmospheric distillation unit (ADU).

Otherwise, the basic principles remain the same.

SL.NO CHARACTERISTICS LSHSSPEICIFICATION METHOD

1 DENSITY @ 150C TO BE REPORTED IS-1448, P-162 FLASH POINT (PM), 0C 66 (MIN) IS-1448, P-213 POUR POINT, 0C 60 (MAX) IS-1448, P-104 WATER CONTENT, % V/V 1.0 (MAX) IS-1448, P-405 GROSS CALORIFIC VALUE,

KCAL/KGTO BE REPORTED

IS-1448, P-7

6 SULPHUR, TOTAL, % BY WT 1.0 (MAX) IS-1448, P-34

16

Ejectors are used to create low pressures in the Vacuum Distillation Unit by converting pressure energy of motive steam into velocity.

Functioning of Ejector:

Major components of an ejector consist of the motive nozzle, motive chest, suction chamber, and diffuser.

High velocity is achieved through adiabatic expansion of motive steam across a convergent / divergent steam nozzle.

This high-velocity jet enters the mixing chamber and entrains the suction fluid being pumped.

The motive steam actually expands to a pressure below the suction fluid pressure. This expansion creates a low-pressure region, which draws suction fluid into an ejector.

The mixed motive and suction fluid then enter the converging inlet diffuser where the portion of velocity energy is converted into pressure energy.

The mixture is then compressed in the diverging outlet section to attain the final discharge pressure.

This pressure is normally 5 to 15 times the suction pressure.

Benefits of using ejectors:

Ejectors can operate using many other motive fluids like steam, air, organic vapour and other gases.

Can handle corrosive and sludge generating liquids, solid and abrasive suction fluids without damage.

Explosion-proof construction. Low initial and maintenance cost, long life.

Design aspects:

LSHS from the bottom of ADU is further distilled under reduced pressure to get the remaining fractions mainly, HSD and Furnace Oil.

Design of this column is more empirical than ADU.

Step-1. Determine the Minimum Reflux Ratio

17

The Underwood equation approximates the minimum reflux ratio (Douglas, 1988). The following equation can be used for multi-component systems with constant relative volatility. Relating mass balances and Vapour Liquid Equilibrium equations, the following equation was derived for which φ can be calculated.

F*(1-q)= ∑ ((αi * zi)/ (αi -φ))

Where,

F Feedq Feed qualityαi Relative Volatilityzi Composition of component (i) in Feed

The φ can be used to obtain the minimum amount of vapour (Vmin):

Vmin = ∑ (αi * Dxi)/ (αi - φ))

From a mass balance and the definition of reflux:

Lmin = Vmin – DRmin = Vmin / D

Step 2. Determine Theoretical Number of Trays

Fenske Equation:

A simplified approximate equation can be used to determine the number of trays (Douglas, 1988). This is an expression for the minimum number of trays, assuming total reflux and constant relative volatility. This equation takes into account the reflux ratio.

N = ln SF/ ln[α/√(1+1/R*ZF)]

WhereSF = β*δ/[(1- δ)(1- β)]

β Fractional recovery of light in the distillate

δ Fractional recovery of light in the bottom

α Relative volatility

R Reflux ratio

ZF Mole ratio of light component in feed

Step 3. Determine the Height of the column

The tower height can be related to the number of trays in the column. The following formula assumes that a spacing of two feet between trays will be sufficient including additional five to ten feet at both ends of the tower. This includes a fifteen percent excess allowance of space (Douglas, 1988). Htower = 2.3 Nactual

Step 4. Determine the Diameter of the column

Vapour Velocity:

Before we can determine the tower diameter, we need to determine the vapour velocity. The

18

vapour velocity can be derived from the flooding velocity. To limit our column from flooding, we chose a velocity 60 percent of flooding velocity (Douglas, 1988). This is assumed in the following equation:

V= 1.2 / √ ρG

Where V vapour velocity ρ density of the gas Diameter of Tower:

The diameter of a tower is relatively insensitive to changes in operating temperature or pressure. The main determinant of the diameter is the vapour velocity. The desired vapour velocity is dependent on the limitations of undesired column flooding. This equation allows for a twelve percent surplus in area (Douglas, 1998).

DT = .0164√(V) * (MG/ ρm )^.25

Where V Vapour VelocityMG Molecular weight

ρm Molar density

LSHS distillation characteristics:

SL.NO CHARACTERISTICS LSHSSPEICIFICATION METHOD RESULTS

(25.10.13 to 24.11.13 )1 DENSITY @ 15 0C To be reported IS-1448, P-16 0.8812-0.88532 FLASH POINT (PM), 0C 66 (MIN) IS-1448, P-21 66-793 POUR POINT, 0C 60 (MAX) IS-1448, P-10 (+) 454 WATER CONTENT, % V/V 1.0 (MAX) IS-1448, P-40 <0.05

5GROSS CALORIFIC VALUE, KCAL/KG

To be reported IS-1448, P-7 10754-10769

6 SULPHUR TOTAL, % BY WT 1.0 (MAX) IS-1448, P-34 --

LSHS distillation analysis:

Recovery Temperature 0C Recovery Temperature 0CIBP 191 45% 4295% 265 50% 437

10% 289 55% 44615% 327 60% 45420% 353 65% 46425% 376 70% 47530% 394 75% 49235% 406 80% 52240% 418 84% 552

Vacuum tower operating data:

19

Temperature of crude at flash zone 280oCPressure in flash zone 30 to 40 mm HgPressure for long residuum 40 to 60 mm HgTop Pressure 12 to 15 mm HgTemperature at top 225-250 o CSteam (at 370 o C) 0.3 to 5 kg per barrel

Conclusion:

The mini refinery at tatipaka is designed to process about 250 TPD of crude and condensate mixture to produce different products such as naphtha, kerosene, diesel and LSHS. From the above distillation analysis data of LSHS, it is found that 20% of diesel and adequate amount of furnace oil fractions are present in the LSHS product.

For the extraction of the above fractions vacuum distillation is a proven and widely used process in refineries. Vacuum distillation results in lowering of boiling points of the components and prevents thermal cracking of the feed.

Recommendations:

In view of the above, it is possible to extract furnace oil fractions from the existing LSHS by means of a vacuum distillation scheme along with other fractions like HSD, Lubricant oils etc.