MEROX OPERATING MANUAL

MEROX OperAting manualINDEX

S. No.DESCRIPTIONPage No.

1INTRODUCTION

2MEROX PROCESS DESCRIPTION

MEROX PROCESS EQUIPMENT

PRETREATMENT

EXTRACTION SECTION

SWEETENING

POST TREATMENT

MEROX CATALYSTS

3U-21 ATF /SK MEROX

UNIT CAPACITY

FEED SPECIFICATIONS

PRODUCT SPECIFICATIONS

TYPE OF CATALYST USED

PROCESS DESCRIPTION

DETAILED DESCRIPTION OF P&ID

PRE-COMMISSIONING OPERATION

STARTUP PROCEDURE

REACTOR BED WATER WASHING AND RE-IMPREGNATION

SHUTDOWN PROCEDURE

EMERGENCIES

OPERATING VARIABLES

CHEMICALS AND CATALYSTS

MODIFICATIONS

4U-22 STRAIGHT RUN LPG MEROX

UNIT CAPACITY

FEED SPECIFICATIONS

PRODUCT SPECIFICATIONS

PROCESS DESCRIPTION

DETAILED DESCRIPTION OF P&ID

PRE-COMMISSIONING OPERATION

STARTUP PROCEDURE

SHUTDOWN PROCEDURE

EMERGENCIES

OPERATING VARIABLES

CHEMICALS AND CATALYSTS

MODIFICATIONS

S. No.DESCRIPTIONPage No.

5U-23 VBN MEROX

UNIT CAPACITY

FEED SPECIFICATIONS

PRODUCT SPECIFICATIONS

TYPE OF CATALYST USED

PROCESS DESCRIPTION

DETAILED DESCRIPTION OF P&ID

PRE-COMMISSIONING OPERATION

STARTUP PROCEDURE

REACTOR BED WATER WASHING AND RE-IMPREGNATION

SHUTDOWN PROCEDURE

EMERGENCIES

OPERATING VARIABLES

CHEMICALS AND CATALYSTS

6U-24 CRACKED LPG MEROX

UNIT CAPACITY

FEED SPECIFICATIONS

PRODUCT SPECIFICATIONS

TYPE OF CATALYST USED

PROCESS DESCRIPTION

DETAILED DESCRIPTION OF P&ID

PRE-COMMISSIONING OPERATION

STARTUP PROCEDURE

SHUTDOWN PROCEDURE

EMERGENCIES

OPERATING VARIABLES

TROUBLE SHOOTING

7U-25 CRACKED FCCG MEROX

UNIT CAPACITY

FEED SPECIFICATIONS

PRODUCT SPECIFICATIONS

PROCESS DESCRIPTION

DETAILED DESCRIPTION OF P&ID

PRE-COMMISSIONING OPERATION

STARTUP PROCEDURE

SHUTDOWN PROCEDURE

EMERGENCIES

OPERATING VARIABLES

MODIFICATIONS

8SAFETY & FIRE FIGHTING FACILITIES

1. INTRODUCTION:

Straight-run LPG, gasoline and kerosene fractions obtained from atmospheric distillation may contain hydrogen sulphide and mercaptans. the extent of which mainly depends upon the type of crude processed. Similar products from secondary processes such as FCC also contain hydrogen sulphide and mercaptans to a greater degree compared to straight-run products. Hydrogen sulphide is corrosive and should be remove in order to meet specifications on corrosion rate. The specification for LPG, gasolene, Kerosene and ATF include copper strip corrosion test which is a measure of rate of corrosion on copper containing materials.Mercaptans are substances with obnoxious odour and, therefore, in order to handle and store them, mercaptan level will have to be brought down to a acceptable odour level. The specifications of above products include 'Doctor Test' which must be negative and is generally related to the extent of mercaptan present. Hydrogen-sulphide can be easily removed by washing with dilute caustic solution. However, for reducing the mercaptans level many processes are available like:

Strong alkali-wash

Copper sweetening

Doctor sweetening

Merox process

Hydrodesulphurisation

Alkali-wash is effective only if low molecular weight mercaptans are involved. Hydrodesulphurisation is normally employed only if reduction of total sulphur level is also required. Both investment and operating costs are higher in case of hydrodesulphurisation. Out of other proceses available, Merox process is considered a superior and proven process.

2. MEROX PROCESS DESCRIPTION

The Merox process licensed by M/S Universal Oil Products Co., (UOP), USA, is for the chemical treatment of LPG, gasolene and distillates to remove mercaptans into disulphides. The removal of mercaptans may be either partial or full. The chemical treatment is based on the ability of Merox catalysts to promote the oxidation of mercaptan to disulphide using air as the source of oxygen. The overall reaction is as follows:

2RSH + 1/2O2 -> RSSR + H2O

The oxidation is carried out in the presence of an aqueous alkaline solution such as either sodium hydroxide or potassium hydroxide. The reaction proceeds at an economical rate at normal rundown temperature of refinery streams.

Low molecular weight mercaptans are soluble in caustic solution and therefore when treating LPG and light gasoline fractions, the process can be used to extract mercaptan to the extent, they are soluble in caustic. Extraction of mercaptan reduces the sulphur content of the treated product. Alternatively mercaptans can be converted to disulphides without removing any sulphur from the treated stock in which case the operation is referred to as sweetening. In the treatment of heavier boiling fractions such as heavy naphtha and kerosene only sweetening is possible.

MEROX PROCESS EQUIPMENT:

In order to understand the function of various Merox process equipment. the equipment can be broadly divided into following sections :

Pretreatment for removal of hydrogen sulphide and naphthenic acids, if present. The method varies with properties and conditions of feedstock and in some cases may not be required.

Extraction section where required, for removal of caustic soluble mercaptans and thus reduce sulphur in the treated product.

Sweetening for conversion of mercaptans to disulphides. For a given capacity plant, the Merox reactor size can vary depending on the case of sweetening due to the type of mercaptans present and also on product requirement.

Post-treatment to remove caustic haze and to control properties not affected by Merox process. Hence post-treatment needed depends on products, utilisation and type of contaminants present in the feedstock.

Taking each section in turn, function of each equipment can be described.

PRETREATMENT

Petroleum fractions may contain hydrogen sulphide and stocks boiling higher than 180C may also contain naphthenic acids. Hydrogen sulphide is not a catalyst poison as such, but will dilute the caustic containing Merox catalyst by reacting with caustic. Further it blocks some of the catalyst activity sites slowing down the normal reaction and also consumes part of the oxygen available. Hence, it is recommended that hydrogen sulphide is removed by washing with dilute alkali solution before the distillate is sent to reactor for treatment.

Naphthenic acids also interfere with treating operations and must be removed prior to treatment. The reactor contains caustic and if naphthenic acids are not remove they form sodium naphthenates which coat the catalyst and block the pores. For removal of napthenic acids, the procedure used is to wash with dilute caustic. Dilute caustic is used so as to avoid formation of emulsions. There could, however, be some carry-over of haze depending on the acidity of stock treated. The haze can easily be removed by coalescing through a sand filter.

Feedstocks, where carry-over of water from distillation units can be expected must be passed through a coalescer for removal of suspended water prior to caustic wash, which would otherwise dilute the caustic used for pretreatment.

EXTRACTION SECTION

As previously stated, low molecular weight marcaptans are caustic soluble and can easily be removed by washing with caustic in a counter current tower. Improved extraction is favored by:

Low temperature.

High concentration of caustic.

Lower molecular wt. of mercaptans

Type of mercaptans, viz. normal mercaptans are easily extractable, tertiary mercaptans least extractable and secondary being in between.

The mercaptan enters the caustic solution and reacts as follows:

RSH + NaOH NaSR + H2O

This is being a reversible reaction the degree of completion of reaction is governed by normal equilibrium laws.

The sodium mercaptide is readily oxidised to disulphide in the presence of Merox catalyst as shown :

2NaSR + l/2O2 + H2O -> 2NaOH + RSSR

This is not a reversible reaction and the reaction rate is speeded up by:

Raising the temperature.

Use of excess air.

Increasing the intimacy of contact.

Increasing the catalyst concentration.

The oxidation of mercaptides is carried out in oxidiser in the presence of merox catalyst. The disulfides oil, which is formed, separates out from caustic as it is insoluble in caustic. Caustic can be reused for extraction. The presence of Merox catalyst in extraction caustic does not however, affect the amount of mercaptans extracted. and extraction is dependent only on parameters explained earlier .

SWEETENING

Sweetening can be defined as conversion of mercaptan sulphur present in a hydrocarbon stream to disulphide sulphur without actually reducing sulphur content of treated stock. The sweetening process is based on the ability of Merox catalyst to promote the oxidation of mercaptans to disulphides using air as the source of oxygen. The reaction is as follows:

RSH + NaOH NaSR + H2O

2NaSR + l/2O2 + H2O -> 2NaOH + RSSR

As can be seen from reactions, the oxidation is carried out only in the presence of alkali solution.

The Sweetening can be accomplished either in solid bed sweetening, where the hydrocarbons and caustic are simultaneously controlled over a solid support impregnated with Merox catalyst.

Liquid-liquid sweetening where hydrocarbon, air and caustic containing Merox catalyst, air simultaneously controlled in a mixer.

Solid bed sweetening consists of a reactor, which contains a bed of activated charcoal impregnated with Merox catalyst and kept wet with caustic solution. Impregnation of catalyst on bed is achieved by dissolving the catalyst with ammonia solution and pumping ammonia solution over charcoal. Air is injected ahead of reactor and in the presence of merox catalyst the mercaptans are oxidised to disulphides. The reactor is followed by a settler which serves as reservoir of caustic. Caustic is intermittently circulated from the settler over the catalyst bed to wet the charcoal.

For liquid-liquid sweetening, the most common type of mixer used is the orifice plate mixer, which is a vessel, fitted with a series of plates with orifices. The vessel provides adequate residence time and the orifice plates create enough turbulence to bring about the intimate contact between hydrocarbon, caustic, catalyst and air. The problem of accomplishing liquid-liquid sweetening is one of getting the difficulty soluble mercaptans into the caustic phase for sufficient time to permit their oxidation. The higher the molecular weight or the more highly branched the mercaptan is, the more difficult it is to accomplish necessary mixing. Hence heavy gasoline and Kerosene may have to be treated using fixed bed reactor.

POST TREATMENT

The product from the merox reactor will at times contain caustic haze. Post treatment is required if the product is to go to storage, clear and bright. In most cases provision of caustic settler and sand filter is adequate to remove caustic haze. However, for treatment of ATF, which has to meet stringent specification caustic must be removed by water wash after caustic settling. Water wash removes entrained caustic as well as water soluble surfactants, Water wash is followed by a salt filter to remove entrained water and part of the dissolved water. This may be followed by clay filter to remove copper and water insoluble surfactants, if present in feed.

MEROX CATALYSTS:

There are two types of Merox catalyst, each one being used for specific service. Catalyst FB is to be used on units equipped with solid bed sweetening reactors. Catalyst WS is used for liquid-liquid sweetening in mixers. This is a caustic dispersible catalyst. This is also used for oxidation of extraction caustic in oxidisers.

3. U-21 ATF /SK MEROX :

UNIT CAPACITY:

The Merox Unit for treating kerosene & ATF streams from the Atmospheric Distillation unit has been designed to feed 1.5 million metric tons per year of kerosene while processing a 50: 50 mixture of Light Arabian: North Rumalia crudes. The Unit shall operate for 345 stream days in a year.

FEED SPECIFICATIONS:

The kerosene to be treated in the Merox Unit shall have substantially the following properties:

(i)Boiling Range IBP/FBP oC

140-270 Max.

(ii)Specific gravity at 15.6C

0.795 to 0.82

(iii)Total acidity, Mg KoH/Gm

0.02

(iv)Mercaptan Sulphur wt ppm

150

(v)Total Sulphur wt.%.

0.22

(vi)H2S wt ppm

10 max

(vii)Colour, Saybolt

+ 30

PRODUCT SPECIFICATIONS:

The Product kerosene/ATF after Merox treatment shall meet the following specifications:

(i)Appearance

Visually, clear, bright and free from solid

matter andundissolved water at normal ambient temperature.

(ii)Corrosion copper strip

No 1 Maximum

(iii)Corrosion silver strip

No 0 Maximum

(iv)Mercaptan sulphur wt ppm

10-20 ppm (max.)

(v)Doctor test

Negative

(vi)Colour loss, saybolt

5 Maximum when feed stock is 25 min.,

provided that the feed stock comes directly from the fractionation facility.

(vii)Thermal stability:

(a)Filter pressure drop

25.0 maximum

in mm Hg.

(b)Tube Rating visual

Less than code 3

(viii)Water separameter index

90 minimum

(ix)Acidity, mg. KOH/Gm

0.012 maximum

(a)Water Separometer 90 minimum

index (modified)

(b)WATER REACTION

Interface Rating

1.0 maximum

Volume Change

1.0 maximum

(x)Conductivity (ATF)

50-450 psm

"H2S free fresh stock charge" shall mean fresh stock charge containing no H2S or if the fresh stock charge contains H2S, it shall be scrubbed out with caustic soda solution or sodium carbonate solution in the laboratory without exposure to oxygen.

TYPE OF CATALYST USED:

Catalyst used for sweetening kerosene/ATF Stream in this treating unit is merox catalyst FB which is supplied by the Universal Oil-Products Company. This catalyst is used in the two sweetening Reactors 021 R1 A and B. The catalyst is impregnated on activated charcoal beds in the reactors. Quantity of catalyst required for one impregnation in the reactors is 250 Kg. of active ingredient. The catalyst is supplied in liquid from each US gallon containing 1 kg. of active ingredient.

PROCESS DESCRIPTION

Pretreatment for kero/ATF consists of a coalescer and caustic prewash vessel, for removal of suspended water and hydrogen sulphide, Naphthenic acid etc.

Sweetening is achieved in supported catalyst bed reactor. Two reactors in parallel have been provided. Air requirement of all Merox units is supplied from a common compressor. Also, storage tanks for receipt, dilution and storage of caustic and storage for methanol have been provided.

Post treatment for kero/ATF consists of caustic settling. water wash to wash of any carry-over caustic salt drier for drying and clay filters for improving colour stability of treated product. Clay adsorbs coloured nitrogen compounds.

DETAILED DESCRIPTION OF P&I DIAGRAM

Kerosene/ATF as obtained from distillation unit stripper under flow control is charged to the unit charge pumps 021 P4A/B. Pretreatment for kero/ATF consists of coalescer for removal of water and caustic prewash for removal of hydrogen sulphide, etc. The feed goes to coalescer 021-V-9 where feed passes through pass blanket as coalescing media to coalesce tiny water droplets into sufficiently big droplets which settle down in the water boot provided in the coalescer. Float type trap is provided in the water boot to drain water. Water from boot will have to be periodically drained to oily water sewer. Provision also exists for bypassing this vessel and directly feeding to caustic prewash vessel.

Kero/ATF from coalescer is sent to caustic prewash vessel 021-V1 (4750 mm OD x 9150 mm) where the feed bubbles through caustic and rises to the top of vessel due to gravity differential between feed and caustic. The feed is charged into the vessel through a distributor pipe mounted inside the vessel and holes drilled to a specified pattern on the distributor pipes. There are three inlet nozzles provided on the vessel. Normally the feed should be maintained thro' the B" nozzle. Maintain caustic level and charge caustic when the caustic is spent as indicated by its strength. The vessel is also provided with a wiremesh in its top portion to remove any caustic entrainment.

The feed is then mixed with air as necessary for reaction, on flow control 021 FRC 16 in the air-mixer 021 Y2 where intimate mixing of air and feed is achieved. The feed then passes on to two reactors operating in parallel (021 RI A&B) (4000 mm OD x 7200 m). The reactor consists of a bed of charcoal impregnated with Merox catalyst. Impregnation being carried out as per laid out procedures using ammonia as an agent for impregnation. The reactor bed also may need water washing and reimpregnation to restore activity whenever catalyst activity drops. the period depending on the type of feed proceesed in the unit.

The feed ex reactors is passed on to the caustic settler 021 V2 (3600 mm OD x 12200 mm). In the caustic settler. required settling time is provided for separation of any carry-over caustic. Since the unit is also designed to treat ATF, which has to meet stringent specifications, water wash and salt drier also have been provided. Hydrocarbon from caustic settler passes into water wash vessel 021 V3 (4750 mm OD x 9150 mm). The water wash vessel is continuously charged with demineralised water taken from the refinery source through a water tank 021 V7 provided with a level controller 021 LC 27 which regulates water entry into water tank. From the water tank, water is charged into water wash vessel by pump 021 P3 A & B. Flow control is by 021 FIC 24. Water hydrocarbon interface level is maintained by 021 LC 24. The vessel is also provided with a wire mesh in its top section to reduce water carry-over by hydrocarbons.

After water wash, hydrocarbon stream enters the salt drier 021 V4 (5000 mm OD x 8000 mm) for removal of any last traces of water. The salt drier is filled with rock salt and the feed is distributed at the bottom of the vessel and leaves from the top.

The final step of treatment is filtering through clay towers to remove water insoluble contaminants. For this purpose two clay filters 021-V5 A&B (4750 mm OD x 7500) have been provided which are operated in parallel. The hydrocarbon is distributed on top of clay filter and collected from the bottom using collector pipe assembly wrapped with wiremesh screens. The outlet of each clay filter is combined into a common product run down line where flow is recorded by 021-FRQI-55. The necessary back pressure to the unit is maintained by 021-PIC-32 which ensures constant pressure in the reactor and other vessels. The treated product is run-down to storage.

Feed stocks which do not call for complete treatment but which meet product specifications after caustic pre-wash, can be diverted to storage after caustic prewash, vessel 021-V1 using the bypass line between 021-V1 outlet and upstream of 021-PIC-32.

For meeting the requirement of caustic for all Merox units, two caustic storage tanks 021- T1 & 021- T2 have been provided in this unit. 021- T1 is meant for preparing and storing 3 deg. Be caustic required for caustic , prewash vessel 021-V1. 021- T2 is meant for 10 deg. Be caustic required for reactors and other Merox units. This unit receives 48.8 deg Be caustic from which 3 deg. Be and 10 deg. Be caustic solution will be I prepared. DM water is used for diluting caustic.

The air reqirement of all Merox units is met by air compressors 021-C1-A & B. Compressed air from 021-C1-A&B is supplied to other Merox units.

PRE-COMMISSIONING OPERATIONS:

I. CHECKING OF COMPLETION OF CONSTRUCTION WORK, INSPECTION AND BOXING UP OF EQUIPMENT

All unit equipment and pipelines must be checked to see that they conform Strictly to the specification of design and as per drawings. Location of vents, drains, gauge glasses, pressure gauges, sample points etc. on equipment must be checked to ensure that they are installed in accessible locations. Also, the internal fittings in columns and vessels like trays, packing supports screws, distributors etc. must be checked in position and in order.

When the equipments have been satisfactorily cleared as to their internal installations and cleanliness of construction debris, the manholes can be boxed up.

II. PRESSURE TESTING OF EQUIPMENT AND PIPELINES :

After inspection and box-up of equipment as mentioned above the entire unit has to be pressure tested with water to maximum allowable pressure to ensure that the plant equipment and lines can safely with-stand operating pressures. According to convenience, equipment are tested individually or divided into sections depending on locations and test pressure of various equipment and pipelines. proper isolation by blinds of equipment or section under test must be done so as not to over pressurise connected lines and vessels of lower pressure rating. Before the hydrostatic test is undertaken, relief valves and orifice plates must be removed from the system and blinded off. After carrying out hydrostatic tests on unit equipment and pipelines water must be drained out completely. In order to prevent vacuum pulling in vessels, their top vents must be opened up before attempting to drain water.

III. WATER FLUSHING OF THE UNIT :

On completion of pressure test of the unit, water flushing of the different systems in the unit is carried out with all the pumps running on line to flush out all muck, scale and other construction debris contained in the lines. Suction screens must be installed on each pump to protect them from damage. In the initial stages of commissioning of the unit, it is better to install a finer (60") mesh on the normal filter of each pump. This finer filter can be removed only 2 to 3 months of actual run of the unit when frequency of cleaning comes down to normal. The pumps are to be run at least for 24 hours to ensure that they are running freely and smoothly. The discharge valves of the pumps have to be suitably throttled so as not to overload the motors during the water flushing operations. Periodically, the pumps have to be stopped and the debris accumulated in the suction screens to be cleared. This operation has to continue till the suction screens remain clear continuously at least for eight hours.

When the water flushing operation has been successfully completed, all water from the systems has to be drained out completely, especially from the low points. Air blow the system thoroughly to remove the traces of water from equipment and lines.

All orifice plates should be checked for proper specifications before their installation in place. All instruments must be checked and calibrated before the unit start-up.

IV. SCREENING AND LOADING OF ACTIVATED CHARCOAL SALT & CLAY

Activated charcoal has to be screened properly before it is loaded in the reactors. Screening is done with 10 to 30 mesh sieve screens. To reject fines etc. which are formed during transportation and handling coarser than 10 mesh and fines after 30 mesh are rejected from the charcoal supply. The reactors are isolated from other equipment and their top and side man ways are opened up. The inlet distributor pipe assembly at top has to be suitably covered to prevent any charcoal entry into the distributor while loading. Loading is done through a hopper and adjustable loading sock which extends to the bottom of the reactors. As the loading progresses, the length of the sock is adjusted so that no cone formation occurs on the charcoal surface. Improper leveling will cause maldistribution of hydrocarbon flow through the bed. Loading personnel must wear dust masks covering the nose and the mouth to prevent inhalation of charcoal dust. While loading, it must be ensured that the bottom Johnson screen collector pipe assembly is fully submerged in charcoal by physical checking from the bottom man way. The person going inside for checking must wear a fresh air masks. The bottom man way is then boxed up and loading continued with the sock till the inlet distributor pipe is 230 mm from the charcoal bed. The Top surface of the bed is leveled after the dust has settled. Here again, the person entering the reactor for leveling the bed must wear a fresh air mask. A record of loading in the reactors has to be kept for reference and accounting, When loading to the required height is over, the loading hopper and sock are removed the inlet distributor covering is taken off and the top man way boxed up properly.

V. ROCK SALT LOADING :

After inspection of salt drier internals for cleanliness and fittings, it is loaded with rock salt up to a height of 600 mm below the top tangential line. Then the top man way is boxed up. The surface must be leveled similar to the procedure followed for charcoal loading.

VI. LOADING OF CLAY IN CLAY FILTERS:

The clay filters are checked for internal fittings and are cleaned up dry, prior to loading clay. The inlet distributor pipe has to be covered suitably to prevent clay entering into it while loading. The recommended grade of clay of the correct mesh, 30-60, is loaded from the top of the vessels. periodically leveling the surface so that no cone formation takes place. The loading personnel going inside the vessels for leveling must wear dust mask. The height of loading in each reactor will be up to 920 mm from the top tangential line. After leveling of the top surface of clay in the filters, their distributors are uncovered and the top man ways can be boxed up.

VII. IMPREGNATION OF CHARCOAL BED IN KERO/ATF MEROX REACTORS WITH UOP REAGENT FB.

The two reactors will be impregnated as a rule one after the other, as noted below:

Start filling 21 R 1 A with DM Water. Block off Reactor outlet lines.

Start circulation pump 21P-2 after DM water level is 1 metre below top distributor pipe. Continue to fill DM water.

When water level rises to 1 metre above the distributor pipe, stop addition of DM water ( Be careful, not to float charcoal out of reactor )

Continue water circulation for 2 to 3 hours

Start Ammonia gas injection after the cylinder is connected in to the circulating water. Regulate the outlet pressure at 3.0 kg/cm2, inject approximately 150 kg of Ammonia in about 5 to 6 hours time. (Each cylinder lasts for 21/2 hrs). Measure the rate by keeping the cylinder on a weigh bridge.

Keep a steam hose playing at the gas injection pipe to prevent freezing on the line.

Circulate the Ammonia solution for about 2 to 3 hrs. keeping the pump capacity to maximum i. e. 58 M3/hr . Sample the solution and send to lab for analysis. Ammonia concentration should be 0.2 + or - 0.02 wt. %. Add or dilute the solution as necessary.

Meanwhile get the catalyst dissolving drum system ready with the hoses connected. Ordinary drum of 200 liters will solve the purpose.

After achieving the required Ammonia concentration in the circulating water. route a slip stream from 21 P2 discharge to the catalyst drum. Fill it up to 50 to 60%. Start gentle air blowing in the Air Sprayer.

Add 7 bottles of Merox FB after shaking. Stir the mixture to help dispersing the catalyst if required. Make up the volume to 180 litres.

Start educting the catalyst solution by routing the slip stream through the eductor 21 G-2 from 21 P-2 discharge to suction.

Regulate the catalyst eduction rate to 3 to 4 litres/minute.

After establishing the above rate, fill the drum from Pump discharge with a rate equal to eduction rate. Add 1 bottle of Merox FB every 10 minutes.

After about 11/2 hrs. start checking the color of the circulating Ammonia at the pump suction before educting point. This color should be clear or faint blue. If the color is dark blue, it means that there is channeling in the charcoal bed In such a case the impregnation is to be stopped and Reactor bed should be flushed with air to rectify the bed channeling.

Add 45 bottles of Merox FB.

After the last batch of solution is educted, flush the catalyst drum several times with Ammonia solution and educt to the Reactor .

Continue circulation for about 4 hrs.

Start transferring Ammonia solution to the second reactor R1 B by opening the inlet block valve to top distributor when the solution level in R1 A falls below top distributor, shut off the inlet and continue to transfer all the circulating Ammonia solution into R1B.

When the pump loses suction shut off the pump. Block in R1A. Open hydrocarbon outlet from R1 B and start circulation back to the inlet. Start taking DM water to fill R1 B until the level is 1 metre above the distributor pipe.

Stop DM water flow to the Reactor, but continue the circulation. Sample the solution and analyze for Ammonia concentration.

Inject Ammonia gas again into the circulating solution to get 0.2 + or - 0.02 wt %.

Impregnate R1 B charcoal bed similar to R1 A. (Add 45 bottles of Merox FB).

After completing the impregnation of both charcoal beds, the circulating solution can be discarded to sewer.

Drain out all the remnant Ammonia solution from both reactors to sewer. Flush and drain each reactor with DM water to remove residual Ammonia (keeping checking the effluent pH).

Close the top man ways of both reactors.

Start alkalising each reactor bed with 10% caustic soda solution and then circulate the same for 2 hrs. About 8M3 of NaOH for each reactor will be sufficient.

Now the reactors are ready for commissioning with Kero/ATF.

Requirement of Catalysts/Chemicals:

(a)Merox FB catalyst

-90 Bottles.

(b)Ammonia

-5 Cylinders of 60 Kg net wt.

(c)DM water

-250 M3

VIII. CHARGING OF CHEMICALS:

The caustic storage tanks 021- T1 and T2 are meant to prepare and store 3Be and 10Be caustic solutions respectively in them. These two tanks take care of the caustic requirements of all the five Merox units. 26.5Be' caustic is received in the unit tanks prior to unit start-up. The caustic is then diluted and the solution of required strength prepared in each tank. 3Be' caustic is used for pre-wash in vessel 021-V1 while 10Be. Caustic is used in the reactors R1A and B as well as in other Merox Units. Solutions of required strength are prepared in the unit by diluting with demineralised water.

START UP PROCEDURE

As soon as Kerosene/ATF becomes available from the Atmospheric Vacuum Distillation Unit, slowly fill up the unit vessels viz., the coalescer V9, caustic pre-wash vessel V1, Merox Reactors R1A and R1 B. caustic settler V2, water wash vessel V3, Salt Drier V4 and the clay filters V5A and V5B one by one. Ensure all air from the resepctive vessels is removed by opening the top vents. before filling them with the product kerosene/ATF. Proceed further, as follows:

Fill up 3Be' caustic solution in the pre-wash column V-1 to the specified operating level so that the kerosene/ATF stream bubbles through caustic solution which will absorb all the hydrogen sulphide and naphthenic acid contaminants from the feed stock.

In a freshly charged Merox reactor no circulation of caustic is needed in the beginning. Hence commissioning the feed charge pumps 021-P4 A/B Kerosene is charged through the coalescer. pre-wash column, reactors and caustic settler and the pressure are raised in the vessels to their normal operating levels.

Setting the back pressure controller 021-PC-32 on the product rundown line, for giving the desired pressures in the vessels, air injection to the mixer 021-Y-2 is started at the specifed rate through flow controller FRC-16. The rate of air is approximately 30 lbs/lb of mercaptan sulphur in the feed. The back pressure controller PC-32 is set such that all air used for the process is kept in the hydrocarbon solution.

Initially, the product after treatment is to be routed to the slop tank. When doctor test is consecutively negative for 2 hours the kerosene stream may be routed to its normal storage tanks.

Water washing the product to remove the entrained caustic has to be done. Hence, start water-washing the product, pumping DM water into the water wash vessel 021-V3 on flow control F1C-34 through the distributor in the vessel. A steady level has to be maintained by operating the controller LC-24 which controls the spent water effluent from V-3 bottom.

The water washed product stream is passed through the drier which removes traces of water carried over. Finally, route the product through the clay filters 021-V5A and B to remove water insoluble contaminants and give clarity to the product. After passing the quality control tests which are very stringent in the case of ATF, the product is routed to the normal storage tanks. Off-spec material when treating ATF stream is diverted to Kerosene tanks. If the feed Kerosene contains very little mercaptans i. e. less then 10 ppm, then the entire Merox treatment may not be required at all. It can be directly routed to run-down by-passing the treating section. In cases where feed contains traces of H2S. but very less mercaptans, the feed can be routed through caustic pre-wash and then direct to product run-down by passing reactors etc.

When the plant has been fully in operation, set the feed rate to the desired level. Check the air injection rate for mixing before reactors is adequate and ensure proper levels are maintained in all vessels. Pressure, as already mentioned has to be set by the pressure controller PC-32 on the product rundown line.

REACTOR BED WATER WASHING & REIMPREGNATION

I. WATER WASHING

Over a period of time, the catalyst impregnated on charcoal in the reactors loses its activity. Wetting the catalyst with fresh caustic circulation also may not help in bringing the product to specifications. At this stage, the charcoal bed has to be hot water washed to remove the organic and soap deposits from its pores and bring back the active surface of the catalyst, for reaction. This deposition of cold organic material on charcoal takes place gradually and hence the water washing step can be conveniently planned to suit the product requirements. The water used for this washing must be free of dirt, suspended matter, hardness salts and active chlorine. Hence, steam condensate or DM water is used for washing. Proceed as follows:

Stop air injection and hydrocarbon charge to the reactors.

Pump out all the kerosene/ATF from the bottom of the reactors under steam pressure.

Steam out the reactors for about an hour giving steam through the beds at a rate equal to 10 lbs per cu feet of bed, i. e. about 14 MT/hr for each reactor.

Commission the water heater 021-J1 and start giving hot water at a temp. of about 85-95C to the reactors through the distributor pipes at a rate of 8 gallons/minute for 100 cu. feet of charcoal i.e. about 55 M3/hr. fill up the reactors and then drain them to the sewer at the same rate as filling water into them.

It was experienced that while using water heater (21 J1) lot of hammering was taking place resulting severe vibration of the lines and therefore it was difficult to maintain the water temp, Later on BFW line connection was given d/s of DM Water B/V. This does not require water heater services. BFW temp. can be controlled by controlling the steam to deareator.

Initially, the draining effluent will be dark brown and foamy which will change colour and eventually becomes colourless. When the water from the drain is clear and clean, the water washing can be stopped. Stop the water flow and blowout as much water as possible from the reactor beds with steam given to reactor top. Keep the steam flow through the beds for another 30 minutes to remove as much residual heavy oil as possible from the charcoal bed.

Stop steam injection and block off water heater. Introduce air into the reactors and blow the reactors well for an hour or so that there is no water drip noticed from the bottom drain.

Next alkalize the beds with fresh 10 deg. Be caustic solution and then return the unit to normal working condition as discussed under "Start-up Procedure".

II. CATALYST REIMPREGNATION

This step is necessary only when hot water washing, higher feed temperatures, stronger caustic saturation, reduced feed rates etc., do not help in mercaptan oxidation and the product remains doctor positive. The reimpregnation interval may vary from three months to one year depending on proper prewashing feed mercaptan content, crude source etc.

III. REIMPREGNATION OF KEROSENE/ATF REACTORS

Hot water washing.

Acidising the reactor bed.

Impregnation

IV. HOT WATER WASHING

Isolate Reactor (21 R-1 A) from the rest of the unit. Start pumping out the kerosene, keep reactor under positive pressure by steam.

After kerosene is pumped out, start steam flow from the water heater (21J-1) to the Reactor (Keep the bed temp. 190 deg C. Drain out the condensate from Reactor bottom. This is to remove the residual kerosene and heavy hydrocarbon in the bed.

Start introducing DM Water through the water heater into Reactor. Keep the water temp. (21 T1-15) around 90-95 deg C by introducing steam into the water heater. When the Reactor is full, open reactor bottom to drain. Keep the drain rate close to the hot water fill rate i.e. about 50 M3/hr.

NOTE:Instead of using water heater, BFW can directly be taken.

Check the drain water color. When the colour is clear, check the reactor effluent pH.

Continue hot water washing till reactor effluent pH is down to 8,9.

Follow step 1.1 to 1.5 for another reactor (21R-1B) and 23R-1.

V. ACIDISING OF REACTOR BED:

Stop the water and steam flow. Flush out the caustic circulation system by opening up the suction lines from the reactor bottom to the circulation pump 21 P-2 and pump discharge to spent caustic tank.

Open the top manway of the Reactor.

Refill reactor (21 R-1A) with DM water after the reactor has been pumped out till water level is 1 M below the distribution pipe, Block in Reactor outlet line.

Start circulation from bottom to top of the reactor. Both the kerosene outlet and the bottom pump out of the reactor are open to the pump suction. Continue to fill DM Water till water level is 1 M above the distributor pipe.

Inject steam occasionally to keep the circulating water temp. at 50-65C.

Take about 0.1 M3 glacial acetic acid to the Acetic acid addition pot 21V-8.

Route a slip stream from the circulating pump discharge to the eductor 21J-2. Start educting acetic acid to the circulating system & it should be added to the system over a 5-10 minutes period.

After circulating acidified water for about one hour check the reactor effluent pH. If pH is higher than 6.5, add another batch of about 0.1 M3, This acetic acid addition is necessary to neutralise the trace alkalinity on the bed and to provide a slight acidic medium.

Transfer the acidified water to reactor 21 R-1B and continue the circulation in 21 R-1A, Stop circulation when the level is 1 M below the distributor pipe.

Follow step 2.6 to 2.8 for acidising the bed of 21R-1B.

Transfer the acidic solution to 23R-1 from 21 R-1B with circulation on. Stop circulation when level is 1 M below the distributor pipe.

Keep a moderate steam flow through each reactor for 30-60 minutes.

Air purge each reactor for about 60-90 minutes.

VI. IMPREGNATION

Start filling reactor 21 R-1A with DM water. Block in reactor outlet lines.

Start circulation pump 21 P-2 after reactor level is 1 M below the distributor pipe, Continue to fill DM water.

When water level rises to 1 M above the distributor. Stop water addition (Be careful not to flood the charcoal out of the reactor).

Continue circulation for about 2-3 hours.

Start Ammonia injection after the cylinder is connected. Regulate the outlet pressure at about 3 kg/cm2. Inject approx. 150 kg of ammonia i.e about 5-6 hours (one cylinder in 2-21/2 hrs.) Measure the rate by weighing scale.

Put steam hose to prevent freezing of pipe due to gas expansion.

Circulate ammonia for about 2-3 hours keeping the rate at pump maximum i. e. 58 M3/hr. Sample the solution and send to lab analysis. Ammonia concentration should be 0.2 + or - 0 02 wt.%. Add or dilute solution if necessary.

After confirmation of solution concentration, route a slip stream from 21 P-2 discharge to the catalyst drum (200 litre drum). Fill it up to 50-60%. Start gentle air blowing in the air sprayer.

Add 7 bottles of Merox FB after shaking. Stir the mixture to help dispersing the catalyst, if required. Make up the volume to 180 litres.

Start educting the catalyst by routing the slip stream through the eductor 21J-2 from 21 P-2 discharge to suction.

Regulate the catalyst solution rate at about 3-4 litre/min.

After establishing the eduction rate, fill the drum from pump discharge with a rate equal to the educting rate. Add 1 bottle of Merox FB every 10 minutes.

Approx. after 11/2 hour start checking the colour at pump suction before the educting point. The colour of the solution should be clear as a very light tint blue. If the blue colour is observed, impregnation must be stopped & reactor bed be 'fluffed' with air to correct bed channeling.

Add 45 bottles of Merox FB.

After the last batch of solution is educted, flush the catalyst drum several times with ammonia solution and educt it into the reactor.

Continue circulation for about 4 hours.

Start transfer ammonical solution to reactor 21 R-1 B by opening the inlet distributor block valve. When the level in reactor 21R-1A falls below the distributor pipe, shut off the inlet. Continue/transfer to ammonical solution into reactor 21 R-1 B.

When the pump loses suction, shut off the pump. Block in reactor 21 R-1A. Open HC outlet from reactor 21R-1B and start circulation to its inlet. Start taking DM water to fill the reactor until the level is 1 M above distributor pipe.

Stop DM water flow to the reactor but continue circulation. Sample the solution and analyse for ammonia.

Inject ammonia to get 0.2 + or - 0.02 wt. % of ammonia in the solution.

Impregnate R1 B charcoal bed in the same as way as that of R1 A (add 45 bottles of Merox FB).

After impregnation, ammonia solution is to be transferred to VB Naphtha reactor (To be impregnated).

Drain out all remaining ammonia solution in both reactors. Flush and drain each reactor with DM water to remove residual ammonia, 2 hours is adequate.

Sample the charcoal in plastic bag for each reactor at three/four points.

Close the top manways.

Start alkalizing each reactor with 15Be' caustic, use 8.5 M3 of caustic for each reactor. Circulate the solution for 2 hours.

The reactors are ready for kero/ATF introduction.

REQUIREMENT OF CATALYST/CHEMICALS

-Merox FB catalyst

-90 bottles

-Ammonia cylinder

-5 cylinders of 60 kg each

-Glacial acetic acid-200 litres (approx.)

-DM Water

-250 M3 SHUT DOWN PROCEDURE:

For planned shut down for inpsection of equipment and maintenance work in the unit, follow the steps given below:

Shutdown air injection to the mixer and shut off feed charge pumps P4A/4B to the coalescer 021-V9. Shut off water wash pumps P3A/3B.

Empty out the salt-drier and clay filters into the kerosene storage tanks.

If the caustic is spent in settler 021- V2, it can be pumped out to spent caustic system for disposal. Empty out the reactors also of the spent caustic.

Kerosene remaining in the coalescer has to be drained out.

When all the vessels have been emptied out of their contents, they are to be isolated by blinds. Blind list of the unit has to be kept upto date.

Steam and water wash individual equipment and systems to make them free of hydrocarbons, caustic etc. and safe for entry. Detailed instruction for this step will be given at the time of actual shutdown.

Entry to the vessels should be given only after making sure that they have been isolated properly, ventilated enough, gas tested free and safe to enter. Recommended safety equipments must be worn by workman entering the vessels which contained caustic etc. Dust masks and fresh air equipment must be used when charcoal loading/unloading and levelling are done inside the reactors.

EMERGENCIES:

Emergencies can result from equipment failure and from interruption of feed and utilities. Operators should be thoroughly familiar with the emergency procedures and understand the reason for each move. Nature and degree of emergency varies from time to time and hence good judgement and discretion should be exercised to tackle such situations.

I. ELECTRIC POWER FAILURE:

If it is a general power failure, feed to the unit will be interrupted with the failure of the pumps. Hence a shutdown of the unit is inevitable. Shut off air injection to the mixer immediately and block off feed to the unit. Shut off discharge valve of unit pumps and close pressure controller at unit limits and hold system pressure. Start up the unit as per procedure when power supply is resumed.

II. STEAM FAILURE:

Local steam failure will not affect the unit running immediately, but if AVU running is affected by the emergency, feed to the unit might be interrupted. In such a case, the unit will have to be shutdown and kept in a standby condition pending resumption of feed supply from AVU.

III. INSTRUMENT AIR FAILURE:

Instrument air failure will result in all unit control valves operating in the fail-safe position, i. e., they will all close. Close off feed to the caustic pre-wash vessel V-1. Shutdown caustic circulation pump P1, circulation pump P2 and water injection pump P3 and block level controllers with isolation valves. Monitor all levels. Hold system pressure by blocking off pressure controller at unit limits.

Revert back to normal operations, starting the unit following standard procedure, once air supply to the instruments has been resumed.

IV. COOLING WATER FAILURE :

A total failure will effect feed supply to the unit and pumps. Hence, unit will have to be shutdown immediately. Isolate all the pumps after stopping them. Close the pressure controller and hold system pressure. Maintain levels in the vessels. Start-up the unit in the normal way, when cooling water supply becomes available and feed from AVU is restored to the unit.

V. AIR COMPRESSOR FAILURE:

If the duration of the failure is only short, the product may not go off specification. But the mercaptan conversion efficiency drops off rapidly and the product will have to be slopped as soon as it goes off quality. Air injection block valve has to be immediately shut-off to the reactors R1A and B, on air compressor failure. On resumption of air supply, check the product quality and when it is on specification, route it to storage tank. If both the compressors are not available for a longer duration due to break-downs and maintenance, the Unit will have to be shut-down.

OPERATING VARIABLES:

The caustic solution used for Merox treatment viz. In pre-wash column and in reactors become spent eventually. Weak acids like CO2 picked up from air, naphthenic acids and other aliphatic acids present in the feed stock make the alkalinity of the caustic lesser and its ability to extract mercaptans will suffer. Hence, it becomes necessary to remove part of the spent caustic and replace from time, to time to restore its alkalinity.

The feed stock should be freed of all hydrogen sulphide by passing it through the pre-wash column. Otherwise the oxidation reaction will be suppressed in the reactors. It will give rise to increased caustic consumption.

Lowering hydrocarbon feed rate and caustic temperature will improve mercaptan extraction

Naphthenic acid in feed stock may give rise to emulsion problems and hence must be removed by prewash with 3Be caustic solution.

Oxidation rate is increased by increasing the temperature and catalyst concentration. About 30 cubic feet of free air is required for each pound of mercaptan sulphur to be oxidised.

The amount of Merox Catalyst needed will be approximately one pound for 1000 gallons of 25Be caustic solution.

Sufficient back pressure must be maintained at each extraction stage to prevent vaporisation and caustic entrainment.

Presence of acid oils upto a limit of 8% will increase solubility of mercaptans in the caustic.

CHEMICALS & CATALYST

SrVessel No.Initial fill,M3Approx. level of initial fillCaustic strength %wtMake up / change out when requiredQuantityExpected duration with design crude

Caustic21V19.330-50% of upper LG2.1When caustic strength to 1.0%About 70% of initial fill (drain up to bottom LG & make up)Every 9 days

Caustic21R1A/B51.5Refer 10oBe caustic tank calibration chart6.9Circulate reactor caustic once a days. After about 5 circ. Charge caustic. Caustic strength would be around 6.2%51.5 M3Once in 5 days

Acitic acid 21R1A/B---About 250 litres for each reimpregnation-About once in 4 months.

Activated charcoal 21R1A/B171--- --

Rock salt 21V4161 (200MT)--After every 3 months run of the unit. Check the salt level and make up accordingly--

Filter clay 21V5A/B256-----

UOP Catalyst FB 21R1A/B90 bottles of 7.5 litres each. (active ingredient per litre is 0.325 kg)--90 bottles required during each impregnation-About once in 4 months at design throughput

MODIFICATIONS :

I. ANTISTATIC DOSING FACILITY

This facility is given to maintain ATF conductivity between 50 to 450 psm (PICO simen per meter) to meet the specification.

Antistatic Agent is stored in a small vessel (0.088 M3). Two dosing pumps, 21P-6A/B have been provided to dose ASA to D/S of 21PIC-32 as per specification.

II. ATF REPROCESSING FACILITY

With AVU revamp, ATF production has gone high. During ATF crude processing part of untreated ATF is sent to separate tank 404 directly from AVU B/L. this untreated ATF stored in tank can be reprocessed in kero/ATF unit at the time when AVU is on BH crude processing. A line is layed from tank 404 to unit 21. A pump 21P7 is given which takes suction from tank 404. Pump discharge line joins to 21P4A/B suction line with a C/V and flow transmitter namely (21FRC40). This facility can also be used for blending BH ATF with imported ATF.

4. U-22 STRAIGHT RUN LPG MEROX

UNIT CAPACITY:

The Merox Extraction Unit for straight run LPG has been designed to process 70,000 MT/year of an essentially C3/C4 mixture obtained from the stabilizer column of the Atmospheric distillation unit. The unit shall operate for 345 stream days in a year.

FEED SPECIFICATIONS:

The straight-run LPG feed to the Merox unit shall have substantially the following properties:

Vapour pressure at 65C

: 17 kg/cm2 max.

C 5's mol.%

: 1.0 max.

Specific gravity at 15.6C

: 0.56

Mercaptan sulphur wt. ppm.

: 900 max.

Hydrogen sulphide wt. ppm

: 200 max.

PRODUCT SPECIFICATIONS :

The straight run LPG after Merox treatment shall meet the following specifications excepting its vapour pressure shall not exceed 17.0 kg/cm2 at 65C.

Total Sulphur wppm: 15 max

RSH

wppm: 5 max

H2S

wppm: Nil

Cu Corrosion

: No worse than H2S Free feed

PROCESS DESCRIPTION:

Amine absorber for removal of H2S and caustic prewash vessel are provided for pretreatment.

Treatment for SR LPG consists of only an extractor with caustic, as all mercaptans present in feed are in the extractable range. Extraction caustic is oxidised in a common oxidiser section.

Caustic settler has been provided for settling and separation of carry-over caustic from extractor.

DETAILED DESCRIPTION OF P&IDLPG obtained from crude unit stabiliser overhead is charged to unit by unit charge pumps 022P-1A/B. The feed is sent to the bottom of amine absorber 022C1 (1000 mm x 19500 mm). The column is provided with 4 sieve type trays. Regenerated DEA from amine regenerator is received in a surge drum pumps provided in cracked LPG merox unit. DEA from surge drum is pumped to the column top on flow control 022 FRC5. Rich DEA from column bottom is sent to amine regenerator located in Bitumen/Sulphur unit area via level control 022 LC 4, under column pressure. The H2S free LPG leaves from top of 022 C1 to caustic prewash vessel 022V1 ( 1100 OD x 6100 mm ) on flow control 022 FRCQI-1. Provision for bypassing 22C-1 in running condition is made by giving a jump over from inlet to outlet bypassing the Amine absorber. In order to keep up the extraction efficiency constant at lower throughputs, a recirculation stream back to crude unit stabiliser is provided. This recirculation flow can be maintained by flow controller 022 ERC 13.

In LPG caustic prewash vessel, LPG is introduced at the bottom of the vessel through a distributor pipe. For this purpose, three nozzles at different levels have been provided. The vessel is provided with a wire mesh at top to remove caustic mist. Caustic in the vessel has to be replaced when it becomes spent.

LPG is then sent to the extractor 022-C-2 (1000 OD mm x 16400 mm) where mercaptans are extracted by caustic from caustic regenerator section. The extractor is provided with 11 sieve type trays. Caustic is charged on flow control 022 FRC-9. Rich caustic from extractor is sent to caustic regenerator section on level control 022 LIC-8.

LPG after extractor goes to Caustic settler 022 V2 (1000 mm OD x 4000 mm). After caustic settling LPG is run down to storage through a back pressure controller 022PC-12 to ensure constant back pressure in the Unit. To run-down LPG, mercaptan is added from odorant pot 022V03 by odorant pumps 022P02 to maintain odour specification. Caustic from settler can be drained into extractor caustic line going for regeneration.

NOTE:Mercaptan dosing pumps have been removed for some other services and dosing lines have been blinded as dosing is not required.

PRE-COMMISSIONING OPERATIONS: I. CHECKING COMPLETION OF CONSTRUCTION WORKS, INSPECTION AND BOXING UP OF EQUIPMENTS

The following preliminary operations have to be carried out to ensure a successful start-up of the unit:

Check that all mechanical works of construction have been completed, equipments inspected, boxed up and signed off on unit check lists made for this purpose.

Scaffolding, debris, tools and other construction materials removed from the unit area.

Make a final list of blinds which should be in position or taken out before starting purging operations. Make sure all blinds have been installed at the proper sides of valves and signed off in the list.

Check and ensure Fire-fighting and safety equipment is in place and in good working condition.

Ensure all utility systems and flare release headers are in service and ready for use.

Isolate the unit from other plants and tankages at the unit limits with block valves.

Keep the fuel gas and flare header isolated from the unit pending purging operations.

Isolate or remove orifice blocks of all flow meters for flushing purposes.

II. WATER FLUSHING OF THE UNIT :For water flushing of a new unit, use ordinary water.

Water flushing of the entire unit has to be done with all pumps running on line so that all muck, scale and construction debris are washed out of all equipment and unit pipelines. The unit can be conveniently taken in sections or individual equipment wise and the flushing operation is carried out with temporary connections from the Fire water header. Suction screens have to be installed on the pumps during the flushing period to protect them from damage. They have to be run at least for 24 hours continuously to ensure their free and smooth service. During the flushing period, care should be taken to throttle the pump discharge valves suitably so as not to overload the motors. The screens on the pump suction lines have to be cleared off all debris collected periodically after stopping the pumps. till they remain clear continuously at least for eight hours. Disconnect all instruments lead lines and flush the leads thoroughly. Make sure that all process lines, control valve loops are thoroughly flushed, to atmosphere to remove muck, construction debris, etc. Better to drop each C/V and flush the assembly and bypass also thoroughly. Then the C/V can be refixed in position. Make sure all low points bleeders are clear and all columns and vessels vents and drains should be clear.

When water flushing operation is completed, we have to pressure test the entire unit with water as detailed below:

III. PRESSURE TEST OF EQUIPMENT & LINES :

In order to check for leaks on equipment and lines after water flushing, they are all subjected to water pressure of about 16-20 kg/cm2 .For this purpose LPG inlet to amine scrubber and its outlet after the pressure controller are isolated by block valves. Rich amine and rich caustic outlets from scrubber 22C1, pre-wash vessel 22V1, extractor 22C2 must also be blocked off. Initial filling of the entire unit can be done with Fire water for removal of air. For pressurising we have to use Boiler feed water.

Keep running the pump out of 19 P14 A/B/C group and through the permanent supply line take BFW to this unit and slowly pressurise the entire system to normal operating pressure for a period of 15 to 30 minutes. If necessary one or two drains may be kept open to have a small discharge from the pump.

During pressure test safety valve down-stream flanges may be kept wedge open or to check tight shut-off of safety valves before their set pressure.

Tighten all leaky flanges, valves glands etc. where required. Gaskets may have to be replaced after pressure test. Keep a record of such flange joints for subsequent testing before cutting in of feed LPG.

After the system pressure test is successfully completed depressurise the unit, but keep the system filled with water. Now the system is ready for taking in Fuel gas. This is the best way of keeping air out of the unit.

LPG can be taken in to the unit from Atmospheric Unit in small quantities. LPG can be lined up slowly to pressurise the unit to a slight positive pressure with gas. This has to be done slowly and carefully to avoid chilling. If. however, fuel gas is available, gas can be taken directly. LPG vapourises slowly giving positive pressure. Now drains off the water from the entire system under gas pressure.

IV. CHARGING OF CHEMICALS:

After pressurising the unit with fuel gas, inlet valve to the scrubber 22C1 is blocked off. Ensure that DEA inlet and outlet from 22C1 are also blocked off. Similarly, ensure that caustic charge line to 22 V1 and its exit as well as Merox coustic inlet to extractor 22 C2 are all shut off. The Merox regeneration system is also pressured up with fuel gas upto the disulphide separator in cracked LPG Merox unit and the extractor 22C2 bottom valve is blocked off. Make sure air inlet valve to the oxidiser 024-V3 remains blocked off. Alternatively regeneration section can be filled with caustic and vented at suitable points to remove air .

In preparation for charging chemicals into the unit, vent gas from the caustic regeneration system till the system pressure drops to 0.5 kg/cm2. prepare 20 Be caustic solution in the storage tank provided and transfer to disulphide separator 024-V4,with a good level in 24V4, start circulation pump 024-P1A/B and establish a level in LPG extractor 22C2. Then line up the pump 24P1 A/B for circulating caustic to extractor 22C2, through bottom LIC to the caustic heater 024-E1, oxidiser 024-V3 and back to disulphide separator 024-V4. Adjust the circulation on FRC control at the stipulated rate.

From the 10 Be caustic solution tank in Kerosene Merox unit, transfer enough material into prewash vessel 22V1 to hold sufficient level. Line up lean DEA from sulphur Recovery Unit and establish working level in amine scrubber 22C1. Establish DEA circulation putting into commission the level controller at bottom of the column to control the rich amine solution returning for regeneration in the sulphur recovery unit.

START-UP PROCEDURE:

After the precommissioning activities have been completed and all equipment and piping etc. are purged to remove air proceed as follows:

Establish Merox caustic circulation to top of extractor column 22C2 after establishing a working level in disulphide separator 24V4 in cracked LPG Merox Unit.

when normal level of caustic is obtained in 22C2, commission the bottom level controller to regulate rich caustic flow back to disulphide separator 24V4.

Charge lean amine on flow control to the amine scrubber 22C1. Maintain level at the bottom of 22C1 by commissioning the level controller regulating the rich amine flow to regenerator section in ARU.

Lining up the product flow upto the rundown valve at battery limits, charge straight run LPG to the amine scrubber by means of pumps P1A and B. Control the flow rate from the scrubber to the caustic prewash vessel 22V1 with the FRC on line. Route LPG to rundown passing through the extractor 22C2 and caustic settler 22V2 displacing all the fuel gas to a horton-sphere lined up at LPG receiving station.

When operating conditions become steady, with the level controllers of 22C1 and 22C2 maintaining adequate level, adjust the flow rate of LPG to the prewash vessel 22V1 to normal rate. Commission the recycling line of LPG back to the stabilizer column of AVU. Use the recycle line if 22C1 operates at low throughputs.

Operate the rundown line pressure controller to maintain the desired back pressure in the system.

Check the product after caustic settler for quality. When it is doctor negative. it can be routed to the regular storage spheres, kept ready for the purpose.

Commission odorant injection to the rundown LPG after caustic settler at the specified rate. Check the odorant injection facility is ready in all respects beforehand. Then start the injection pump 22P2 and set dosage. as instructed.

NOTE:Odorant injection is no more in use nowadays.

Check frequently LPG for H.S after the prewash vesel1 22V1 and replace caustic as necessary. 22V1 caustic is be replaced when caustic becomes 50% spent.

Check the regenerated caustic being charged to the extractor 22C2. For extractable sulphur in it. This will give an idea of the efficient operation of disulphide separator.

SHUTDOWN PROCEDURE:

Inform all concerned of intention to shutdown the unit and proceed as follows:

Shut off DEA charge pump to the scrubber 22Cl. Shut off LPG charge pump to 22C1 immediately after this. Stop odorant injection pump 22P-02.

Stop caustic circulation through the extractor 22C2 from the disulphide separator 24V4.

Stop caustic supply to pre-wash vessel 22V1.

Empty out caustic levels from 22V1, 22C2 and 22V2 to spent caustic disposal or to the disulphide separator 24V4 in the cracked LPG Merox unit, as instructed.

Empty out DEA level in 22C1 to its regeneration section in Bitumen/Sulphur area.

Depressure the vessels and columns slowly to the flare system, after isolation at unit limits.

Isolate the vessels and columns by blinds and make them gas and chemicals free by water washing repeatedly, as per special instruction that will be issued at the time of shutdown.

Keep an upto date record of the blind list.

Entry to the vessels has to be given only after ensuring that they are absolutely free from all gases and chemicals used in the plant. Gas test must be done before entry permit is given. Proper ventilation inside the vessels must be ensured. Personnel entering the vessels must wear proper protective equipment as mentioned in the clearance permits.

EMERGENCY PROCEDURE

I. POWER FAILURE:

In case of general power failure LPG supply to the unit will stop and the unit pumps running will be interrupted. DEA solution circulation also will stop.

Block off pressure controller at unit rundown line and hold pressure in the system.

Monitor all levels.

When power supply and LPG feed are restored, put the unit back into normal operation, following standard procedure.

II. STEAM FAILURE:

In case of general steam failure, LPG to the unit will be affected and hence unit will have to be shut down and kept under pressure till feed supply is resumed from AVU. If the steam failure is confined to the Merox unit only then LPG Merox section can run without any interruption.

III. COOLING WATER FAILURE:

Failure of cooling water will affect the feed supply to the unit as well as the unit pumps. Unit will have to be shutdown and kept standby till water supply is restored.

IV. INSTRUMENT AIR FAILURE:

On instrument air failure, all the control valves in the unit will close. If AVU is also affected by air failure, feed to the unit will be interrupted. Close off the pressure controller on LPG rundown line and maintain pressure in the system. Monitor all levels till air supply is resumed to normal. Revert to normal operation.

OPERATING VARIABLES :

I. LPG PREWASH:

LPG is passed through amine scrubber 22C1 and caustic prewash vessel 22V1 to remove all traces of hydrogen sulphide. The DEA solution should be kept at the specified value of 25% by wt. The amine circulation rate also should be at the design rate of about 800 kg/hr. Caustic concentration for prewash should be 10-20 Be, when it becomes spent, caustic must be replaced with fresh stock.

II. LPG EXTRACTOR:

Mercaptan extractor 22C2 removes these undesirable components from LPG stream by intimate mixing with a caustic solution containing Merox catalyst. The caustic concentration for the circulation should be 20 Be. The catalyst concentration in the caustic should be 100 ppm. Merox solution circulation rate should be 0.17 M3/hr.

Operating pressure which is maintained by the pressure controller on the rundown line should be about 18.0 kg/cm2 at the extractor 22C2. Reducing any of these variables excepting the pressure will affect mercaptan extraction efficiency. Too Iow an operating pressure favours amine and caustic entrainment in LPG stream.

III. TYPICAL OPERATING DATA AS OBSERVED IN THE NORMAL RUN OF THE UNIT

ACTUAL-1ACTUAL-2

LPG flowM3/hr11.7624

Amine flowM3/hr0.340.8

Caustic flowM3/hr0.17Nil

Back pressureKg/cm21212.5

CHEMICALS & CATALYSTS:

Vessel noInitial fill M3Approx. level of initial fillCaustic strength % wt.Make up/ charge out when reqd.Quantity Exp.to run when design crude

Caustic 022V1330-50% of upper LPG6.9Caustic strength down to 1.5%About 70% of initial fill (drain upto bottom LG and make up)Every month

Ethyl mercap-tan---As required to maintain run-down LPG odour at level 2(min)Not in use now

MODIFICATIONS:

I. Facility to route SR LPG to CR LPG unit and vice-versa.

II. Facility to route CR LPG to SR LPG Xerox (unit 22) partially.

III. After AVU revamp, SRLPG make has gone up and now SRLPG is being treated in 24 Unit which was earlier treating CRLPG.

Now SR LPG is treated In Unit 22. Following changes in operation have been done to achieve LPG specifications.

Caustic circulation in extractor has been stopped due to caustic, carryover 8 M3 caustic is taken in this column which is acting as another prewash. Due to high LPG production. caustic carry over continued resulting LPG falling in Cu corrosion test. Now BFW is being taken in this column to wash the caustic haze carried from 22V1.

Name of the SchemeScheme for conversion of 22C2 to amine absorber

Scheme NoMR/PS/300/2001/12

Scheme implemented onApr02

Description of SchemeConversion of water wash column 22C2 into Amine absorber in cracked LPG Merox

Reason for modificationDue to higher LPG generation of around 36-38 m3/hr, load on 22C1 had increased leading to very often amine carryover from 22C1.Water wash column if converted into amine absorber would take part of the LPG load and would lead result in better extraction of H2S from LPG

Operating Instructions1. 22C2 will now be operated as amine wash column. LPG from FCCU would get distributed in 22C1 and 22C2.10 M3/hr of LPG flow would go to 22C2 controlled through 22FRC13.

2. 22FRC13 would be given a set-point of 10 m3/hr.

3. Amine flow through 22C2 would be maintained through 22FRC9. This would be given a set point of 0.7 M3/hr.

4. Amine level in 22C2 would be maintained by 22LIC 8. This would always remain on auto.

5. Amine and LPG level interface is maintained in the lower half of the column.

5. U-23 VISBREAKER NAPHTHA MEROX

UNIT CAPACITY

The Merox treating unit for sweetening the naphtha stream obtained from the visbreaker unit has been designed to feed 65,000 MT/year of naphtha. The unit shall operate for 330 stream days in a year.

FEED SPECIFICATIONS

DESIGN

ACTUAL

H2S wppm

10 max

70-110

Mercaptan wppm

3000 max

3000-4000

Density at 15C

0.74

0.71-0.72

Distn.

IBP

50

52

FBP

116

150

PRODUCT SPOECIFICATIONS

Merox treated naphtha shall meet the following specifications

(i) Copper strip corrosion

1 max. ASTM

( 3 hours at 50C )

D-130-65

(ii) Mercaptan sulphur wt ppm

5 max. UOP

163-62

(iii) Oxidation stabilityminutes.(after injection 390 min ASTM

of UOP No.5 copper deactivator)

D-525-55

TYPE OF CATALYST USEDMerox catalyst ws which is dispersed in caustic solution is used in the extractor column 023-C2 of this treating unit. Merox Catalyst FB is used in the sweetening Reactor 023-R1 of the unit. This is impregnated on activated charcoal bed in the reactor both the catalysts are supplied by UOP. Initial charge of catalyst WS is 4.0 gal. and it is added whenever spent caustic is dumped and fresh caustic solution is made up in the extractor column for one impregnation on the reactor bed. 7.3 gal. of catalyst FB is used.

PROCESS DESCRIPTION

Caustic prewash vessel is provided for removal of hydrogen sulphide etc.

For visbreaker naphtha, both extraction and sweetening sections are provided. After caustic prewash naphtha goes to extraction tower where partial extraction of mercaptans, to the extent possible are extracted. Extraction is followed by sweetening in reactor with supported merox catalyst bed. Air requirement is met from the common compressor .

Post treatment consists of a caustic settler, and a sand filter for removing caustic haze. The treated product requires anti-oxidant injection, since visbreaker naphtha is a cracked product.

DETAILED DESCRIPTION OF P&ID

Visbreaker naphtha as obtained from Visbreaker stabiliser bottom is fed to the unit by feed pumps 023-P1 A&B.Flow control 023-FRC-3 on discharge of pump is cascaded with visbreaker stabiliser bottom level control. Visbreaker naphtha can be diverted to slops, when necessary,downstream of 023-FRC-3. Caustic prewash vessel 023-V-1 ( 1000 MM x 6100 MM ) is provided for removal of hydrogen sulphide etc. The vessel is also provided with a stainless steel wiremesh blanket at the top to remove any caustic entrainment. There are three inlet nozzles like in other prewash vessels. Caustic in the vessel is to be changed out as indicated by its strength.

The feed then goes to extractor column 023C-1 ( 1000 MM x 13300 MM ) and is introduced at the bottom of column where counter-current contact is made with caustic under circulation. Regenerated caustic from caustic regenerator section is introduced at column top on flow, control 023-FRC-8. Through the extractor trays caustic flows downwards due to its higher gravity. Caustic after extraction is sent to caustic regeneration section by maintaining the 023-CI bottom caustic level by 023-LIC-7.

In the extractor, mercaptans are partially extracted to the extent these are soluble in caustic. About 40-60% mercaptans are normally extracted. This will vary with the type of feed to visbreaker and operating conditions of visbreaker .

The extracted naphtha then goes to Merox Reactor 023-R1 ( 1400 MM x 4500 MM ) for sweetening of unextracted mercaptans. The reactor consists of a solid bed of activated charcoal impregnated with merox catalyst No.1. Oxidation air is added to visbreaker naphtha on flow control 023-FRC-15 and mixed thorougly with feed in the air-mixer 023 M x 1 before it goes to reactor. Uniform distribution of feed over reactor bed is ensured by the distributor pipe assembly placed inside the reactor. The collector assembly at the bottom is wrapped with Johnson screen to ensure filtering of reactor bed material from the hydrocarbon stream.

From the outlet of reactor, caustic removal from visbreaker naphtha is achieved in the caustic settler 023V2( 1100 x 4700 ). The feed to the settler is distributed by a slotted vertical pipe.

The treated product from settler is sent to storage via a sand filter 023-V3 ( 1400 x 3800 ) for removal of caustic haze. The inlet distributor consists of slotted pipes assembly. Sand is filled from half way up the vessel and supported on the packing support provided at that level. Below the sand packing segmented baffle is provided in front of outlet nozzle' to ensure clear hydrocarbon withdrawal from sand filter. The treated product is sent to storage after the pressure control 023-PC-19 which ensures constant back pressure to the unit. The sand filter is also provided with a bypass, for use when necessary.

Visbreaker naphtha being a cracked product requires to be dozed with oxidation inhibitors. UOP No.5 inhibitor from drums can be transferred to inhibitor melting pot 023-V6. Three inhibitor metering pumps 023P-3A, B&C are provided. 023P-3A is for injecting inhibitor into visbreaker naphtha, 023P-3C is for injecting to FCC gasoline and 023P-3B is a common spare. Inhibitor is added upstream of 023PC-19 control valve which ensures constant pressure.

For circulation of reactor caustic as well as for water wash and methanol circulation, pumps 023-P2 A&B are provided. For impregnation of catalyst, catalyst addition pot 023-V4 and flow indicators 023FI-10 & 023FI-11 are to be used when impregnation is carried out in line with standard impregnation procedure. For acidifying charcoal bed during reimpregnation acetic acid addition pot 023-V5 and eductor 023-J2 are provided.

The treated and inhibited visbreaker naphtha which is

doctor sweet shall normally be used as a motor gasoline-blending component.

PRECOMMISSIONING OPERATIONS

I. CHECKING OF CONSTRUCTION WORK COMPLETION, INSPECTION AND BOXING UP OF EQUIPMENT.

The following preliminary operations are to be carried out in order to ensure a trouble free and smooth startup of the unit.

Check that all mechanical works of construction have been

completed. All equipments and lines in the unit must be hydraulically tested at pressure depending upon design conditions so that we can be certain that they will stand operating pressures. After the hydraulic test, equipment are finally inspected for proper internal fittings and fixtures as well as cleanliness. Clearance are given to box up equipment after final inspection as above. Checklists are signed off as complete in all respects for individual equipment and pipelines.

Make a final blind list indicating which blinds are to remain in position and which are to be taken out before starting gas-purging operations. Ensure that all blinds have been installed at the proper sides of valves and entered in the list.

Check and ensure that fire fighting and safety equipment are readily available in the unit in good working condition.

Ensure that all utility systems and flare release header are in service and ready for use.

Ensure free movement in the unit area. Get scaffoldings, construction debris tools and tackles etc. removed from the unit area.

Isolate the unit from other plants and storage tanks at the unit limits with block valves/blinds for carrying out flushing and purging operations.

Isolate or remove instrument orifice blocks for flushing purposes.

II. WATER FLUSHING OF THE UNIT

In preparation for start-up of the unit, water flushing of the entire unit has to be carried out to remove and wash out all muck, scales construction debris etc. from equipment and lines for the purpose of water flushing, the unit can be taken up in convenient sections as well as individual equipment-wise also. Hooking up temporary fire water hose connections on equipment or lines, thorough flushing of all equipment and lines are carried out.

All the unit pumps are tried out during water flushing with suitable wire mesh screens ( preferably 100 each )fitted on to their suction spools to prevent them from damages. The pumps have to be run at least for 24 hours continuously to ensure their free and smooth functioning. Care has to be taken while running the pumps not to overload their motors, during the water flushing operations Their discharge valve have to be suitably throttled to take care of the loads on motor. During the pumps running-in period, the suction screen will get choked up with debris washed out into them and they will start losing suction. Immediately the pumps are to be stopped. screens opened up, cleaned and then pumps restarted. This cleaning operation will have to be continued till the suction screens remain clear of all debris and muck at least for 8 hours.

On satisfactory completion of water flushing of the unit. All the water from equipment and lines are to be drained out, opening drains and flanges at low points. Care should be taken not to pull any vacuum in the equipment while draining water. Then, thoroughly air blow the entire unit to remove all traces of water remaining after water washing.

III. PRESSURE TESTING OF EQUIPMENT & PIPELINES

Subsequent to water flushing of equipment and lines in the unit, a pressure test is carried out with water to about 10 kg/cm2 to detect leaks, if any, in the system and repair them in the time before the actual start-up of the unit and to tighten most of the flanges. During the test safety valve downstream flanges are opened up to check holding of safety valves. Any safety valve which requires resetting is removed and sent to workshop for repair.

Pressure test with water can be conveniently carried out by running the feed pump or caustic pump and pressure up the unit by restricting the flow at the plant outlet till the desired pressure is reached. Care has to be taken during pressure test to isolate equipment and piping which are designed for lower pressure, like e.g. methanol system. These can be tested separately at lower pressure.

After pressure test is satisfactorily completed, drain water from system at all low point and air blow it thoroughly to remove all moisture.

IV. SCREENING AND LOADING OF CHEMICALS

ACTIVATED CHARCOAL

Activated charcoal which merox catalyst impregnation will be

screened properly before it is loaded in the reactor 23R1. Standard ASTM Sieve Nos. 10 and 30 screens have to be used and charcoal coarser then No.10 and fines after No.30 screen are separated out. The charcoal thus screened is loaded in 23R1 after isolating it from other equipment. Its top and side manways are kept open. Its inlet distributor pipe assembly has to be kept 'Covered during loading to prevent charcoal entry into the pipe. Charcoal loading is done through a hopper and adjustable canvass chute, which extends to the bottom of the reactor. As loading progresses, the length of the chute is adjusted. No cone formation is to be allowed on the charcoal surface in the reactor. Improper leveling will cause non-uniform distribution of hydrocarbon flow through the charcoal bed.

Personnel must wear dust masks covering their noses and

mouths to prevent inhalation of charcoal dust, while loading for leveling inside the reactor, the person entering must wear a fresh air mask. While loading, it has to be ensured that the 'bottom Johnson screen collector Is fully submerged in the charcoal by physical checking, from the bottom manway. The charcoal loading is continued till the inlet distributor pipe is about 900 mm from the charcoal bed. Final leveling is done entering the reactor top with protective equipment. Record of quantity of charcoal loaded in the reactor is kept for reference. The loading hopper and canvas chute are removed after loading, the cover on distributor pipe is taken out and the top maway of the reactor is boxed up tight.

SAND

Check the internals of sand filter 23-V3 for proper

fittings and support for sand 'bed, Check also it is wiped clean from inside sand screened to the specification of 8-16 ASTM Nos. is then loaded to a height of 1.4 meters in the filter. The surface is levelled properly and then the vessel is boxed up finally.

V. PRESSURE TEST FOR TIGHTENESS

When all chemicals have been loaded as required and all manways, flanges, etc boxed up the unit is ready for impregnation of catalyst on charcoal bed. Before impregnation of catalyst, it is preferable to test the unit for tightness of flanges, manways. etc. under air pressure. Compressed air ex 21 CI A, B can be used for pressuring up the unit to about 10 kg/cm2' pressure.

After the unit is pressured up. check all flanges manways, etc for tightness with soap solution. Tighten as required. When pressure test is over release the pressure gradually in order not to disturb the bed of charcoal.

VI. IMPREGNATION OF CATALYST ON REACTOR 23-R1

After the reactor is filled with charcoal to the required level and boxed up proceed to impregnate charcoal with Merox catalyst FB as detailed in the following steps.

Fill the reactor with DM water being careful not to add so much water as to float charcoal out of reactor.To ensure this. reactor top manway can be kept open. Once level is ensured manway can be kept in closed position without bolting to enable inspection of bed after impregnation.

Stop D M water supply and line up reactor bottom to reach

circulation pump 23-P2 A. B and establish from bottom of bed to top of bed.

Estimate the amount of water in the system i. e. reactor, lines etc. If water is charged from a tank, this can be estimated conveniently from tank dips. Estimate should take into account piping, dead spaces etc. The total quantity will be about 6-7 M3.

Calculate the amount of ammonia required to raise the

concentration of water to a 0.2% wt solution of ammonia. The quantity of ammonia required will be about 12-14 kgs.

With water in circulation, inject the desired amount of ammonia into the water upstream of suction line to pump 23-P2 A. B. Ammonia is injeted from a cylinder mounted on a suitable weighscale so that the quantity of ammonia injection can be controlled. Adjust ammonia injection rate in such a way that the amount needed will be introduced during the time it takes to pump the entire water inventory through this system twice; with pump this would takes 3 to 4 hrs and hence adjust ammonia injection to last this period.

When ammonia concentration is uniform in the system. fill the catalyst mix drum with ammonia water. Add the content of 3 bottles of 2.5 gallons each of merox FB catalyst. This contains 7.5 kg of active ingredient. Mix ammonia water and catalyst solution thoroughly using an air sparger. In the catalyst mix drum do not exceed 80% level.

With water still circulating from the bottom of the reactor to top, educt the catalyst-ammonia water into the system at a rate of approximately 4 liters per minute.

After emptying the mix-drum, flush it several times with Ammonia water and educt the water into the reactor to get all of the residual catalyst into the reactor. Continue circulation of ammonia water for one more hour after all catalyst has been educted. Finally drain all water from the system.

The catalyst is now ready for alkalizing with a 10Be solution of caustic. Transfer caustic of this strength to the settler 23V2 to about 30% of its volume. Start caustic circulation with pump 23-P2 A/B through the reactor 23-R1. Initially keeping the drain valve open, remove all the free water till caustic appears at the drain. Close off the drain and continue circulation at the maximum discharge rate for about two hours. If necessary. take additional caustic from storage tank into settler 23-V2. Stop circulation after two hours and the reactor is now ready after impregnation to receive hydrocarbon feed for sweetening.

VII. CHARGING OF CHEMICALSCaustic requirements of the unit is met from the storage facility available in the kerosene merox section. Fresh caustic of 26.5 %Be strength is received in storage tanks 21 TI & T2 and diluted with demineralised water to the desired strength. 10oBe caustic is charged into the prewash vessel 23-V1 till a working level is established in 23-V1. 20Be caustic from the disulphide separator 24-V4 of the cracked LPG Merox Unit is charged to the extractor column 23-C1 top through a 11/2" line by the transfer pump at cracked LPG Merox Unit. This caustic drop after extraction of mercaptans is sent back to the regeneration section on level control from the bottom of extractor column. To begin with 20Be caustic is transferred to the disulphide separator from kero merox unit to establish sufficient level for subsequent transfer to other merox sections.

START-UP PROCEDURE

It is assumed that the merox system is fully lined up for startup. Proceed to start-up the unit as follows :-

When visbreaker naphtha becomes available commission the charge pumps 23-P1 A/B and slowly introduce naphtha into the prewash vessel 23-V1 and fill it up. Let it pass through the extractor and then the reactor and caustic settler. Finally, pass naphtha through the sand filter 23-V3 to the rundown line to storage. Vent out all air before filling the vessels.

Commission the pressure controller 23-PC-19 to have the required back pressure in the system.

Maintain proper levels of interface in 23-V1 and 23-C1.Commission the bottom 23-LC7 of 23-C1.

Start air injection thro' mixer 23 MY1. Take the flow on control 23-FRC15 and set the flow rate to the requirement (30 kgs/hr.) of mercaptan sulphur in the feed. 23 PC-19 should be set such that all air injected is in hydrocarbon solution.

In a freshly charged merox Reactor, no caustic circulation will be needed on the initial start-up period. Hence it is enough to pass the feed through the reactor, with air mixing as already mentioned above. When catalyst activity declines over a period of time, resaturation with caustic circulation is resorted to.

Caustic carried over alongwith naphtha is settled in 23-V2 and drained back to the circulating pump suction. Any traces of caustic haze still remaining is removed in the sand filter 23-V3 and the treated naphtha is routed to storage from the sand filter.

Commissioning the inhibitor injection facility, the metering pump 23-P3A/B is started to inject the measured quantity of inhibitor into the naphtha stream going out to storage. The present injection rate is 80 to 90 ppm so as to get a minimum induction period of 360 minimum.

Initially, the product stream is routed to the slop tank till satisfactory results are obtained consistently for about 2 hours. Then naphtha can be routed to the regular storage tanks. Set the feed rate to normal consistent with other control systems parameters.

REACTOR BED WATER WASHING & CATALYST REIMPREGNATIONOver a period of time, catalyst in the reactor bed loses its activity and even wetting with fresh caustic circulation does not help in restoring the activity and the product continued to go off specification