PRESENTED BY P.BABOO

BRIEF PRINCIPLE OF OPERATION UREA STRIPPER used in this presses is a

falling film shell and tube type heat exchanger. The urea solution from reactor outlet carrying unconverted carbamate,urea,water and excess ammonia enters the vertical tube through ferrules(inserts) mounted on the top of tubes. Ferrules are provided with three(or four) tangential holes at 1200(or 900 ) angle which allow inside the tubes.

CONTD.- The film adheres to the tube surface while

flowing down to the bottom of each tube. The film is heated by means of saturated steam at 25 ata.The excess ammonia present in the urea solution evaporates thus reducing the partial pressure of the CO2in the vapour phase hence causing stripping of the CO2 from the carbamate present in the solution phase.

ADVANTAGE OF FALLING FILM HEAT EXCHANGER. 1-Higher rate of heat transferdue to high

velocity offilm and less convective resistance.

2-Reduced biuret formation due to less residence time.

3 Minimum internal pressure drop. 4-Less tube skin Temperature for same

solution outlet temperature.

HIGH PRESSURE CORROSIONAND PASSIVATION METHOD FOR THE TITANIUM STRIPPER ,THE DISSOLVE O2

IN THE BOTTOM STRIPPER SOLUTION SHOULD BE 2PPM EVEN LESS, WHEREAS FOR BIMETALLIC STRIPPER, THE DISSOLVE O2 IN THE BOTTOM STRIPPER SOLUTION SHOULD BE 5-6PPM

By increasing the temperature in the stripper bottom (if air is not injected directly), the dissolve O2 in the bottom solution should decrease as per following data:-

DESIGN FEATURE OF THE STRIPPER During initial plant construction,SNAM,

used special grade of stainless steel i.e.2RE69(25-22-2)as material of construction for tubes ,ferrules and tube sheet / channel side lining in plants at Italy & Spain. It was found that these strippers generally suffered from severe corrosion attack particularly during lower turn down plant operation due to less availability of O2 for passivation layer.

TEMPERATURE VS O2 STRIPPER BOTTOM

TEMPERATURE, 0C 204 205 206 207 208 209 210

O2 IN THE BOTTOM SOLUTION , PPM

6 5 4 3 2.5 2 2

ZIRCONIUM. Zirconium in ammonium carbamate solution is

HIGHLY CORROSIVE RESISTANCE, furthermore it does not show the ERROSION PROBLEMS typical of Titanium. When it is in contact with air, the Zirconium is covered by film formed by Zro2 that are able to protect the metal up to 3000C. Only few agent are attack the Zirconium: Floridic acid, copper and iron chloride.

The main limitation of the Zirconium use is the high cost of it.

O2 FOR TITANIUM To circumvent the above problem, stainless steel

as above was replaced by Titanium which was extremely good corrosion resistance and not require O2for passivation. For all plants in India and also around the world, similar construction was followed till recently by SNAM. However,the above change in material of construction unfortunately could not solve all the operation and maintenance problem and the stripper continued to attract greater attention by the plant operators and also of the designer.

STRIPPERS CATEGORIES.

STRIPPERS

IRREVERSIBLE REVERSIBLE MODIFIED DISTRIBUTION

IRREVERSIBLE STRIPPERThese are apparently 1st generation of plants

in 70s and commissioned in late 70s/early80s.these stripper were typically designed with following common features.

1.—About 6 meter effective length with higher load per tube

2.—Tube thickness 2.7mm(minimum wall)

3.INTERNAL FERRULE4.BELL DISTRIBUTION

5. STEAM ENTRY IN THE SHELL SIDE AT TOP AND CONDENSATE OUTLET AT BOTTOM.

Similar stripper were used in RCF,TROMBAY,GNFC,IFFCO,PHULPUR PLANTS

INITIALLY.

REVERSIBLE STRIPPER In view of the problem encountered as above, next

generation of the strippers of were designed in early 80s and commissioned during80s.These strippers are having nozzles and support saddles in such a way that they may be reversed without any modification.

These have following common features--- 1—Effective length of the tubes reduced to 4.5 meters. 2—More numbers of tubes ,hence less load per tube. 3—Thickness of the tubes increased to

3.5mm(minimum,wall thickness) 4—External ferrules mounted on the top of the tubes. (a) With metal to metal contact. (b) With Teflon sealing ring in-between ferrules and tube.

Contd--

5—Holding down plates to keep the ferrules in position.

6—Bell distributor. 7—Both steam entry and condensate outlet at the

centre. These kinds of strippers were commissioned in

the country in 84-85(RCF,THAL,KRIBHCO PLANTS) followed by other plants in the country(NFL).These strippers are still very much in operation with their fair share of problems and are generally due for reversal for the first set of the plants. The nature of the problems have been separately identified.

3.REVERSIBLE STRIPPER WITH MODIFIED DISTRIBUTION SYSTEM BOTH 1ST AND 2ND generation of stripper as

discribed had employed bell distributor for distribution of liquid to the tubes across the tube sheet. In fact during inspection it was clear that liquid load on the peripheral area that to near the end is higher than central portion leading non-uniform loss of thickness inside the tube as high around 0.25mm/yr. to 0.1mm/yr.

CONTD. New distribution system consists of sieve

perforated) tray and short packed bed over the ferrules(external).Ferrules are held in place hold down grid have wider area for gas outlet.

A study of new distribution system incorporated in the stripper used since 90 either in new plants displays remarkable difference in features. These difference may be attributed to consultant's effort to improve or even may be due to clients interaction based on their operating experience.

FEATURES MAY BE SUMMMMARISED AS UNDER:- 1: Tray distributor without packing

(GNFC/TROMBAY) 2: Tray distributor with

packing(CFCL/NFCL/TATA/PHUPUR) 3: Tray distributor with gas riser:-

(TATA/KRIBHCO) 4: Solution inlet inside the stripper to tray (a). Segmental –(CFCL/NFCL/TATA) (b). Cross over header-(GNFC)

MECHANICAL DESIGN REQUIREMENTS Titanium inherently offers excellent corrosion resistance but being

soft as metal has little resistance to corrosion. Due to process/operational reasons the exchanger is subjected to both corrosion and erosion. Therefore ,major causes of failures in stripper is attributable to erosion of tubes leading to thickness reduction.

Process requirement calls for effective liquid film inside the tube and also stripping of CO2by heating along the tube length. To fulfill the above requirements it is desire to have:

1. Good coupling between tube and ferrules to prevent bypassing of liquid through the mating surface.

2 Effective positive pressure on the on the top of the ferrules to secure a leakage free joint (external ferrules).

3. Provision of anti erosion (Teflon bush) in between ferrule and tube end.

4. To have least erosion damage to tube due to high velocity of fluid leaving at the end (tip) of internal ferrule.

INTERNAL Vs EXTERNAL FERRULES INTERNAL 1. Generally good coupling at

the tube mounting joint but form but deep erosion ring at ferrules tip.

2.Frequent inspection is necessary to monitor of depth of erosion.

3. Ferrules neck may be machined to lower the ferrules tip to cover erosion ring.

4. Usually erosion is more localized than found in external ferrules.

EXTERNAL1. Coupling at tube mounting is

not satisfactory.2. There had been attempts to

apply more pressure on the seating face to improve sealing of liquid but have not been very successful.

3. Introduction of Teflon ring between ferrule and tube OD also generally did not work as Teflon becomes hard and brittle at over 2000C

4. Bypassing of liquid between ferrules to tube OD were found resulting in damages to tube OD

COMPARATIVE SPECIFICATION OF UREA STRIPPER INSTALLED IN VARIOUS PLANTSDETAILS Kribhco IFFCO A

NFL indo IFFCOphulpur

GNFCBharuch

TYPE BEM falling film

BEM falling film

BEM falling film

BEM falling film

VENDER KOBEJAPAN

FBMITALY

FBMITALY

KOBE JAPAN

CapacityIn MTPD

1100 1100 1550 1800

DistributionSystem

Ring/Bell Ext.Ferr without tf ring

Ring/bell ext ferr with tf ring

Ext.Ferr+Tf ringTrays dist pall ring pkg

Ext Ferr+Tf ringTrays DiWithoutpkg

Operating press

146 150 150 150

Surface area 534.95 m2 632.21m2 877m2 980m2

Tube length 4500mm 4500mm 4500mm 4500mm

No. of tube 1892 2236 2784 3120

TRANSITION ASSEMBLY In view of the problem experienced with external

ferrules ,SNAM advised most of the plant having external ferrule presently to changed over to internal ferrule system through transition piece of 25-22-2 stainless steel.

It involve trimming the damaged tube of top, machining of OD and ID of tubes with some standard dimension.

KRIBHCO had carried out partial convection in two of the strippers but with unsatisfactory result.

From the maintenance point of view---- Concentric to the tube bore. All tube top are built up/machined at same level. OD &ID of the tube ends are of the same dimension after

repair. If not assembly problem with different size of transition pieces leading to further damaged is envisaged.

It is suggested that if any plant desire to carry out the modification from external to internal ferrule, may do so during reversal without transition piece.

ARRANGEMENT FERRULE HOLDING DOWN

Initially internal ferrules did not have any holding down arrangement at the top as they were self supported inside the tube.

The very design of external ferrules requires a holding down arrangement to keep them in position tightly.

The standard design found having too much clearance and was inadequate to effect the desired result.

SNAM proposed a second design which was later changed. Simpler design are being perused by client which are apparently

more effective. There is also an attempt to hold the ferrules in position by

screwed down nut for each tube fixed to holding plates. During inspection of newly commissioned urea plant where

stripper developed leakage at tube to tube sheet joint, it was found that spring were distorted during dissembly.Moreover,it was also observed that notch at the top of the ferrule to fit the spring are getting bent and would pose problem in future assembly.

FOURTH GENERATION STRIPPER SNAMPROGETTI has presently offered a fourth

generation stripper with bimetallic stripper having external tube of 2RE-69 (25-22-2) and tube of zirconium.Prior to final design, few tubes with combination have been experimented in an operating plant at Temi ITALY for over 10,000 Hrs.with reported good result.

Cost of bimetallic stripper is less by 40% than Titanium.

With the introduction of bimetallic stripper, it appears that the problems so far experience d in existing Titanium strippers are likely to be over.

CONSTRUCTION It consists of a vertical tube bundle exchanger,

with hemispherical head cylindrical channels at top and bottom

The heat needed for decomposition of carbamate ,for the separation of vapour and for increasing the temperature is supplied by steam condesing in shell side of stripper.

The solution inside the tube falls down along the internal walls of tube to ensure high residence time (to prevent burette formation). and high heat transfer.

Passivation air is injected at bottom channel by K-3 compressor.

MATERIAL OF CONSTRUCTION

PART Hemispherical

head(Top&bottom) CHANNEL(TOP,BOT) TUBE SHEET(TOP

&BOTTOM) TUBES FERRULES &OTHER

INTERNALS LENSE GASKET

MATERIAL 516-GR-60+2-RE-

69(LINER) A266-C12+2RE69A266-

C12+2RE 69OVERLAY 2-RE-69+Zr-702 2-RE-69

TITANIUM

BIMETALLIC TUBE STRIPPER DESIGN DATA

EQUIPMENT TAG NO. MANUFACTURER CODE

DESIGN PR & TEMP WORKING PR & TEMP TUBES TUBE SIZE CORROSION ALLOWANCE

31/41-E-1 FBM HUDSONITAL VEDI AND SNAM”S

SPECIFICATION 165Kg/Cm2,2250C 147Kg/CM2190/2070C 1677 25.4X20X6520 ZERO

BIMETALLIC TUBES

Tubes consist of two tubes an external tube of 2 RE-69 and an internal tube of Zirconium.

These tubes are fabricated separately according to specification of SNAMPROGETTI.

Then they are assembled and drawn together.

During drawing operation a proper mechanical bonding is obtained.

ABOUT 2RE 69 MATERIAL

It is a tailor made grade of Austenitic Stainless Steel ordered according to Licensor's . Specification and also known as 25-22-2.

According to snam's specification CR.UR.G510 1995 material shall meet to following requirement.

CHEMICAL COMPOSITION:- Cr-24-26%,Ni-21-23.5%,Mo-2-2.6%, C-.02,Mn-1.5-2.0,Si-0.4,S-

0.015,P-.02,N-.1-.15%

METALLOGRAPHIC PROPERTIES

Material in contact with process fluid shall have Austenitic structure.

Ferrite content shall not exceed 0.6%except for manual welds for which 1.0% is allowed.

Sigma phase shall be absent. Chromium carbide may be present in

amount only . Material shall pass Huey Test &shall be

performed according to ASTM A262 practic”C”.

2-RE-69 VS TITANIUM

ADVANTAGES:- Excellent erosion and corrosion resistance

specially in tube due to Zr. Lining. Cost effective. Easy maintenance. LIMITATIONS.:-Stripper bottom temperature is limited to 2070C

means less decomposition.High pressure air compressor is required for

passivation of bottom part.Plant can not be run at lower capacity than normal.

Operating experience of Bimetallic Stripper

As per M/S. SNAM,the maximum operating temperature of the Bimetallic Stripper with bottom passivation air is 2050C

It was reported that in Mexico,against the recommendation of M/S SNAM, Bimetallic Stripper was run at 2100C.After 3years of operation, on inspection, the bottom liner was found totally WORN-OUT.

On this experience, M/S SNAM HAS BEEN RECOMMENDING THE MAXIMUM BOTTOM TEMPERATURE OF BI-METALLIC STRIPPER AS 2050C.

CONT D.--

It was reported that minor problem has been faced at OSWAL CHEMICALS with Bi-Metallic Stripper.

It has been in operation without passivation air at the bottom(on account of non-availability of the air compressor) against the recommendation of M/S SNAM.Now the bottom temperature remains at 206-2070C at 115% plant load.

NEW UREA HIGH PRESSURE Zr.STRIPPER DEVELOPMENT

Profertil,origional urea HP pressure stripper suffered a set of several corrosion processes in first two years since beginning of the operation, with losses of 216000 tones urea shortening drastically its expected 30 years life and threatening Profertil business.

For this reason Profertil decided to build a new stripper with more reliable material.

Profertil found the Zirconium as a material that based in the field experience has an excellent performance with a corrosive media.

As the SNAM never built the stripper in Zr this one experience of Profertil shows that life cycle cost that include not only the equipment cost but also the business losses.

COTD.-

Any way we found that Ti experience and specific Zr applications in similar design are fully applicable.

The technology is mature to take a reasonable risk with the incentive of long term solution for the problems in this critical equipment.

A risk management assessment for the prototype is also included in the scope of work .

The new equipment under manufacturing will be finished in the fourth quarter of 2004.

.- New Urea HP Zr Stripper Development New Urea High Pressure Zr Stripper Development Jorge Versace( Profertil S.A., Argentina) Profertil original Urea HP pressure stripper suffered a set of several severe

corrosion processes in the first two years since the beginning of the operations, with losses

of 216.000 Urea tns, shortening drastically its expected 30 years life and

threatening Profertil business. For this reason Profertil decided to build a new stripper with a

more reliable material. Profertil found the Zirconium as a material that based in the field experience has

an excellent performance with the corrosive media. The experience of Profertil

shows that we should look for the minimum total life cycle cost that includes not only the equipment cost but also the business losses. For this reason the stripper with Zirconium, as a corrosion resistant barrier, has the lowest net present cost

between the alternatives evaluated.

OPERATION WITH PRE-

CONCENTRATORPRESENTED

BYPBABOO

OPERATING PROBLEMS IN VACUUM EVAPORATION SECTION

Performance of vacuum evaporation section is crucial as it affects the urea product quality.

Less vacuum causes high moisture, high free ammonia and less crushing strength of urea prills.the poor performance of vacuum evaporation section at NFL VIJAIPUR (line 1&2) can be attributed to the following design. operating problems.

POOR PERFIRMANCE OF E-14:- 1st stage vacuum evaporator(E-14) solution inlet temperature remains as high at 1250C against value of 900C due to inadequate flushing of urea solution across the L.P. holder(ME-13) level control valve (LV131).

CONTD.-

This is because of inherent design problem as solution has to pass through E-14 tubes instead of going through a flash separator for phase separation.

In the existing plant system, differential across E-14 is also high due to scaling of tubes with oil and rust and bulging of its inlet tube sheet due to two phase flow resulting into the reduction of the ID at the entrance point.

High solution temperature at inlet to E-14 (1250C against 900C )result into lower LMTD across E-14.

CONTD.-

LMTD would reduced from 340C to 210C assuming steam condensing at 1500Cand solution outlet temperature of 1330C in surface area (where vacuum pre-concentrator is not there).

The heat transfer coefficient on E-14 tube side also becomes low due to presence of flashed vapour in the in the solution as well as frequent scaling of tubes.

Thus low LMTD and low heat transfer coefficient on the tube side result into poor performance of heat exchanger E-14.

OVERLOADING OF 2ND STAGE VACUUM

MV-6 solution inlet and outlet temperature should be 1330C and 1280C respectively as per design(i.e.a temperature drop of 50Cdue to flashing at 0.3ata pressure).

In actual practice ,this temperature drop remains as highas200C.

At 0.4 ata pressure as the flushing that should have occurred on upstream of E-14

HIGH LOAD ON VACUUM EVAPORATION SECTION

30% urea solution obtain from dust scrubbing in bagging plant.

Spillage and lumps dissolving in new V-4 has to be continuously recovered in vacuum section.

High water recycle to reactor due to various reasons mentioned below not only aggravate the per pass conversion in urea rector but also increases load on vacuum.

LOAD ON VACUUM BY FOLLOWING MODIFICATIONS. High waste water section load due to

various modifications carried in view of pollution abatement.

Floor washing in dissolving area recovered in old V-4.

MV16/17 water recovered in old V-4. H.P.AMMONIA pumps packing sealing

modification from oil to cold condensate.

CONTD.-

1ST stage evaporation pressure is kept around 0.4 ata against design value 0.3 ata so as to avoid crystallization of urea solution into MV-6 outlet to E-15 lute choking

This cause the shifting of load from 1st stage to 2nd stage of vacuum evaporation. resulting into poor vacuum in 2nd stage .MV-7 pressure remains high as 0.08 to 0.1ata against design value of 0.03 ata resulting into HIGH MOISTURE AND FREE NH3 IN THE UREA PRODUCT

PRE-CONCENTRATORS USED IN DIFFERENT PROCESS. IN LATEST SNAMPROGETTI PLANT. In latest SNAM plant above problem has

been solved by providing vacuum pre-concentrator with an in built flash separator

This flash separator operates at 0.42 Kg/cm2.(abs)

MV-2 off gases is used for heating.

LATEST PRE-CONCENTRATORSNAMPROGETTI PLANT.

TO E-14 INLET

MV-2 OFF GASSES

TO E-7 INLET

ME-3 SOLUTION OUTLET

TO VACUUM SYSTEM

STAMICARBON FLASH TANK TYPE PRE-CONCENTRATOR

It may be noted that in stamicarbon plants the above problem has been taken care by sending L.P. decomposer outlet urea solution through a flash tank operating 0.42 kg/cm2 abs.

This help in reducing the load on 1st stage vacuum evaporator .

The urea solution (68.8% urea) after flashing through L.P.decomposer level control valve gets cooled from 1350C to840C at inlet of flash tank.

STAMICARBON PRE-CONCENTRATOR

TO VACUUM SECTION

L.P. DECOMPOSER OUTLET UREA SOLUTION

FLASH TANK

BAROMETRIC

LEG

UREA SOLUTION TANK

TO 1ST VACUUM

0.42 Kg/cm2 abs

ACES PRE-CONCENTRATOR. In ACES (Advanced Cost Energy Saving)

process of M/S. TEC also similar to similar arrangement of flash separator cum vacuum pre-concentrator has been provided in-between L.P. decomposer solution outlet and two stage vacuum evaporation section.

FIGURE- ACES PROCESSPRE-CONCENTRATOR

TO VACUUM SYSTEM

FROM L.P.DECOMPOSER

H.P. DECOMPOSER

OFF GAS

TO 1ST VACUUM SYSTEM

TO 1ST VACUUM EVAPORATION 140

mm Hg Ab.

CASALE VACUUM PRE-CONCENTRATOR. Casale's innovative variant of the conventional

vacuum pre-concentrator envisages replacement of the of fouling prone 1st vacuum evaporator(E-14) with a unitized exchanger consisting of twin vertical shell and tube rising film exchangers(E-14A/B) bolted together in series, having urea solution in tube side of the exchanger.

The equipment(E-14A/B) is a combination of Snam”s conventional vacuum pre-concentrator (E-104) and 1st vacuum evaporator (E-14) bolted to 1st vacuum separator(MV-6).

CONTD.- Heating fluid in the shell side of E-14 A being MP

decomposer(MV-2) off gases and LP steam in the shell side of E-14B.the exchanger E-14 A/B were to be designed with a much larger shell diameter and provided with 1”ODX14BWG tube s instead of the 3/8”ODX18BWG tubes the E-14 ,which were to sensitive to fouling.

The surface area of the exchange E-14 A/B were to be obtained on the basis of the heat recovery from the off gases of MP decomposer under varying plant conditions in lower part (E-14A) and the balance heat duty for evaporation of the urea solution to be realized from LP steam in the upper part (E-14B) plus 10%additional surface to provided for operational flexibility.

ADVANTAGES. Pre-concentrator is improvement over

conventional SNAM ammonia stripping plants, for conserving energy. In latest generation SNAM plants these are incorporated at inception stage.

Old plants are incorporating these scheme through revamping. BENEFIT ACHIEVE IS ABOUT 0.123 G.Cal/MT of urea.

A payback of less than two years was forecast for the CASALE option, which was quite attractive, as compared to a pay back of about three years for snam option.

COMPARISION OF PLANT OPERATING DATA BEFORE & AFTER RETROFIT(IFFCO)

UNIT PRE-RETRFIT

POST-RTROFIT

HP STEAM MT/HR 81.25 77.5

STEAM E-14/15

MT/HR 34.4 16.9

ME-3 UREACONC

% 74 70.4

BIURETE % .94 .95

MOISTUREFREE AMM

% PPM

.75 300

.55 230

CASSALE PRE-CONCENTRATORE-14 A/B

TO MV-6

LP STEAM

E-14 B

E-14 A

COND

FROM MV-2

OFF GASES TO E-7

FROM ME-3

CASALE PRE-CONCENTRATOR

E-14 A/B VS E-14.

ITEM E-14 E-14 A/B

HEAT DUTY 11.55 MKCAL

8.8+6.1 Mpv+steam

Heat Tras.Area M2

544 780+410=1190

No. of tube 3636 2444+2444

Tube OD 3/8” 1”+1”

Tube length 5.0 M 4.0+2.1 M

CONTD.-SHELL DIA 910 MM 1700 MM

IN/OUT TEMPDESIGNED

90/1300C 101/1090 C

MATERIAL OFConstruction

SS 304L SS316L+SS304L

BARE WEIGHT/Weight with water

8 MT/11.5 MT 23.2+13.2 MT35+22MT

EFFECT ON E-7 AND SPECIFICATION

ITEM PHULPUR-1 VIJAIPUR-1No of tube 833 990ID/OD ¾” ./ .05” 20/17Tube Length 6000 mm 6000 mmSurface Area 299 m2 334m2

Heat Duty 8.3 MK Cal 9.396 MKCalRated Capacity 1650 MT/Day 1310 MT/DayS.A/MT area 0.181 M2/MT 0.225M2/MTHeat/MTurea 0.120MKCal/MT 0.172MT/MT

BENEFITS ACHIEVED.IFFCCO REPORT

DESCRIPTION BEFORE AFTERLS TO E-14 34.5 MT 18 MTE-14 inlet/out 126/1280C 113/1300C2nd vacuum .09 abs .06 absFree ammonia 300-350 ppm 250 ppmE-14 inlet press. 2.5 Kg/cm2 1.0 Kg/cm2

LP Seam Press. 4.3 Kg/cm2 3.9Kg/cm2

Temp drop in MV-6

120C 2-30C.

By

P.Baboo