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