ContentsAcknowledgement.............................................................................................................................................1

INTRODUCTION OF UREA...................................................................................................................................2

PHYSICAL AND CHEMICAL PROPERTIES..............................................................................................................3

USES OF UREA....................................................................................................................................................4

PROCESS TECHNOLOGY......................................................................................................................................5

ONCE THROUGH PROCESS..............................................................................................................................6

PARTIAL RECYCLE............................................................................................................................................6

TOTAL RECYCLE OR LIQUID CARBAMATE SOLUTION RECYCLE PROCESS:-................................................7

STRIPPING PROCESS.......................................................................................................................................8

THEORY OF STRIPPING...............................................................................................................................8

EFFECT OF PROCESS VARIABLES.........................................................................................................................9

EFFECT OF H2O/CO2 RATIO...............................................................................................................................10

EFFECT OF PRESSURE AND TEMPERATURE......................................................................................................10

EFFECT OF RESIDENCE TIME.............................................................................................................................10

BIURET IN UREA................................................................................................................................................11

PROCESS DESCRIPTION FOR UREA SYNTHESIS.................................................................................................11

HIGH PRESSURE SECTION.............................................................................................................................12

CO2 COMPRESSOR........................................................................................................................................12

COMPRESSION DETAILS................................................................................................................................13

AMMONIA RECOVERY......................................................................................................................................13

UREA SYNTHESIS...............................................................................................................................................13

CONSTRUCTION OF UREA REACTOR.................................................................................................................14

OVER FLOW FROM REACTOR.......................................................................................................................15

AMMONIA STRIPPING......................................................................................................................................15

STRIPPING PROCESS.........................................................................................................................................15

CONSTRUCTION OF STRIPPER.......................................................................................................................16

CONDENSATION AND SEPERATION..................................................................................................................16

MEDIUM PRESSURE SECTION...........................................................................................................................17

MEDIUM PRESSURE SEPARATOR......................................................................................................................17

MEDIUM PRESSURE DECOMPOSER..................................................................................................................18

MEDIUM PRESSURE UREA HOLDER..................................................................................................................19

MEDIUM PREESURE CONDENSATION..............................................................................................................19

MEDIUM PRESSURE ABSORBER........................................................................................................................20

ABSORPTION PROCESS.....................................................................................................................................20

DESCRIPSITION OF MEDIUM PRESSURE ABSORBER.........................................................................................22

MEDIUM PRESSURE AMMONIA CONDENSATION............................................................................................22

AMMONIA RECEIVING......................................................................................................................................23

MEDIUM PRESSURE ABSORBER AND INERT WASHING....................................................................................24

LOW PRESSURE SECTION..................................................................................................................................25

LOW PRESSURE SEPARATOR.............................................................................................................................25

LOW PRESSURE DECOMPOSER.........................................................................................................................26

LOW PRESSURE UREA SOLUTION HOLDER.......................................................................................................27

LOW PRESSURE CONDENSATION.....................................................................................................................27

CARBAMATE SOLUTION TANK..........................................................................................................................28

LOW PRESSURE ABSORPTION AND LOW PRESSURE INERT..............................................................................29

WASHING.........................................................................................................................................................29

VACUUM AND EVAPORATION SECTION...........................................................................................................30

FIRST VACUUM CONCENTRATION....................................................................................................................30

FIRST VACUUM SAPARATION...........................................................................................................................31

SECOND VACUUM CONCENTRATOR................................................................................................................31

SECOND VACUUM SEPARATOR........................................................................................................................32

FIRST VACUUM SYSTEM...................................................................................................................................33

SECOND VACUUM SYSTEM...............................................................................................................................33

FINAL CONDENSATION.....................................................................................................................................34

UREA MELT PUMPING......................................................................................................................................34

PRILLING SECTION............................................................................................................................................35

AcknowledgementUrea production is based on Snam Progetti's self stripping process. CO2 enters the centrifugal compressor and leaves it at a pressure of about 160 atm. Liquid NH3 from NH3 plant is collected in Ammonia Receiver Tank. From here it is compressed to 25 atm pressure. Part of this ammonia is sent to Medium Pressure Absorber the remain-ing part enters the high pressure synthesis loop after it is compressed to pressure of 240 atm.

Liquid ammonia and gaseous carbon dioxide react in Reactor at a pressure of 150 kg/cm2 and 188 deg C to form ammonium carbamate, a portion of which dehydrates to form urea and water. The reaction products leaving the reactor flow to the steam heated falling film Stripper which operates at the same pressure as the Reactor. The unreacted carbamate gets stripped off as NH3 and CO2. Urea solution leaving the bottom of the Stripper still contains some amount of carbamate.

Purification of urea takes place in the medium and low pressure sections operating at 18atm and 4.5atm pressure respectively. Decomposition of carbamate takes place in medium and low pressure decomposers. The concentration of solution leaving the low pressure section is 72% of urea. Vacuum concentrators are provided to concen-trate the solution to 99.8% in two stages operating at 0.3 and 0.03atm pressure re-spectively. Urea melt from the concentration section is pumped to the top of the prilling tower and sprayed by means of rotating prill bucket.

The fine droplets, while descending through the tower, come into contact with cold air and solidify to form prills. Product urea from the bottom of Prilling Tower is sent to storage or bagging. In the high pressure section gases leaving the Stripper thor-oughly mixed with the recovered carbamate from the medium pressure section and gets condensed to ammonium carbamate from the medium pressure section and gets condensed to ammonium carbamate in two carbamate condensers operating in series. The heat of reaction is utilised to generate 4.5atm steam. The carbamate thus formed is recycled back to Reactor.

Ammonia and CO2 present in the decomposed gases of medium and low pressure sections are recovered in a series of condensers and absorbers. Traces of gases present in the condensate from Vacuum Section are removed in a distillation tower. Waste water from the distillation tower is sent to effluent system. The recovered am-monia solution is recycled back to the LP Section and ammonium carbamate from LP Section is recycled back to the MP Recovery Section.

INTRODUCTION OF UREAUrea synthesis is of historical importance, it is being the first organic compound to be synthesized from inorganic compound in laboratory. WHOLER in 1828 obtained urea from ammonium carbonate. Previous to this urea had been separated from urine and experiment from WHOLER showed that organic chemical can be separated from an inorganic chemical. Commercial production of urea was started in 1920 by I.C. FOR-BEN in Germany based on Ammonium carbonate process. Since then Considerable in-genuity was us to overcome process difficulties such as: corrosion problems, recovery of off gases and economic process routes which resulted in present day develop-ment.

The various commercial urea manufacturing process of today use reaction between liquid ammonia and CO2 gas to form carbamate and subsequent dehydration of car-bamate to form urea and water. Ammonia and CO2 are usually available from same site, as CO2 is a by product of ammonia plant using hydro carbon as feed stock.

Four streams of urea plant having capacity to produce 1320X4mt/day of urea fertil-izer .The technology is based on SNAM PROGETTI, Italy.

Liquid ammonia and gaseous carbon dioxide s made available by ammonia plant and sent to reactor after compression and pumping .In reactor ammonia and carbon diox-ide react to form carbamate which is further dehydrated to form urea. The output of the reactor is sent to the stripper where urea is further concentrated by removing an dehydrated carbamate using ammonia for stripping .Urea solution leaving the strip-per will contain some carbamate which is further purified in medium pressure section and low pressure sections. Vapour containing ammonia and carbon dioxide obtained from these vessels are converted to carbamate and recycled to reactor.

Concentration of urea solution is important because increase in temperature encour-ages biuret formation which in poisonous to crop, Therefore concentrated under va-cuum concentrator. After concentration urea melt is pumped to urea prill tower where it is sprayed by rotating prill bucket and fine droplets of urea are solidified by means of natural air draft. At bottom of prill tower, urea prills are collected and sent to product handling plant by means of scraper and belt conveyor.

PHYSICAL AND CHEMICAL PROPERTIESMolecular weight 60.05

Boiling point Decomposes at atmosphericPressure before boiling

Density at 200C 1.335Heat of combustion 2531Heat of solution in water 578 cal/gmCritical humidity 70.1Viscosity at 1500C 2.16 cpCrystallisation heat 47 Kcal/kgFusion heat 59.95 Kcal/kgThermal conductivity 0.197 Kcal/kg

Specific heat: cal/ gm/ 0C

At 200C : 0.321 At 98.40C : 0.158 At 120.50C : 0.194 At 160.30C : 0.224 At 2200C : 0.288

Solubility in water by wt %:

Temperat-ure

0 20 40 60 80 100 120

Urea in gm per 1000 gm of water

62 105 163 246 396 725 2244

Urea is a white crystalline chemical product with m.p. of 132.60C .It is readily soluble in water. On heating beyond its m.p. it decomposes giving CO2, NH3 and other com-plex compound of carbon, nitrogen and oxygen. At 1600C it decomposes to yield am-monia, biuret, and higher condensation product. The longer urea is held above its m.p. further reaction proceeds. Urea in fact is the diamide of carbonic acid with a chemical formula represented by:

USES OF UREAMain applications of urea are as follows:

(1) As a fertilizer in agriculture.(2) As cattle feed.(3) As an industrial raw material for a number of industrial/household

product like urea formaldehyde, melamine and urea furfural. Due to high nitro-gen content of urea the demand for fertilizer grade urea is rising rapidly. Urea today account for a large percentage of nitrogenous fertilizer.

(4) Urea is used as cattle feed in western countries. Sheep and cattle are capable of digesting urea up to about 40%.

(5) Urea also finds extensive use in preparation of adhesive, textile, anti shrink compound in exchange resin and as intermediate in preparation of pigments.

PROCESS TECHNOLOGYUrea is produced by synthesis of liquid ammonia and gaseous carbon dioxide. Ammo-nia and carbon dioxide react to form carbamate, a portion of which dehydrates to form urea and water. The fraction of carbamate that dehydrates is determined by the ratio of various reactions and various reactants, the operating pressure, temperature and residence time in reactor .The reaction of ammonia and carbon dioxide to pro-duce urea takes place in two stages at elevated pressure and temperature.

2NH3 (l) + CO2 (g) NH2CONH4 + 38.1 Kcal/gm mol oC

Liquid Gaseous Carbamate Heat

Ammonia Carbon dioxide

NH2COONH4 NH2CONH2 + H2O -7.1Kcal/gm

Ammonium Carbamate urea water heat

The first reaction is highly exothermic & therefore heat is liberated as reaction occurs with excess NH3. The CO2 conversion to carbamate is almost 100%. Provided solution pressure is greater than the decomposition pressure. The decomposition pressure is the pressure at which carbamate will decomposes back in to CO2 and NH3.

NH2COONH4 2NH3 + CO2

Urea Ammonia Carbon dioxide

Decomposition pressure is the function of NH3 concentrate in feed and the solution temperature and increased if either temperature or NH3 recycle is increased. It is de-sirable if operating pressure is quite high enough to prevent carbamate from decom-position in to NH3 and CO2. This will maximize CO2 conversion to urea and towards re-action II.

The second reaction is endothermic therefore heat is required for reaction to start. The heat of this comes from formation of carbamate. This reaction is function of tem-

perature and concentration in feed, the solution effluent from the reaction being a mixture of urea solution. Ammonia, water and carbon dioxide is extremely corrosive in nature. The subsequently stages of process consist of decomposition of unconver-ted carbamate recovery of resulting ammonia and carbon dioxide for recycle concen-tration and prilling of urea solution.

ONCE THROUGH PROCESSThis reaction is simple in operation, in this process ammonia and CO2 are fed to auto-clave and effluent solution is stripped of unconverted ammonia and carbon dioxide. The solution is processed further for concentration and for production of urea plant. The off gases concentrating ammonia and carbon dioxide are neutralised with acid for production of over ammonia fertilizer, because this process requires an invest-ment for other plant to use more expensive ammonia. The capital cost of urea produ-cing unit will go up.

Although the initial cost of urea pant is low and conversion efficiency is high. The once through process is no longer an economic proposition.

Besides the exceedingly huge demand of high nitrogen fertilizer has pushed the once through process in back ground.

PARTIAL RECYCLEIn this process the CO2 from off gases is absorbed in mono ethanol amine. Excess am-monia is recovered by condensation and recycle whereas CO2 is regenerated and ren-ted. This process gives a better temperature control of reaction. But suffer from the defect that CO2 is lost MEA used for CO2 absorption get perpetually degraded.

TOTAL RECYCLE OR LIQUID CARBAMATE SOLUTION RECYCLE PROCESS:-Today all conventional total recycle process are much the same. All similar reactor conditions, reactor pressure of about 200kg/cm2 and a temperature of about 185-190oc. They maintain a NH3/CO2 mole ratio of about 4:1 in synthesis section. A con-version of about 64% per pass is obtained.

Reactor effluent solution is let down and decomposition of carbamate is carried out by heating with steam. The off gases are condensed and recycled as solution to re-actor. The excess ammonia used is condensed and fed back to reactor

Major variation however exists in recycle system in temperature and pressure levels. The various solution recycle processes endeavour-

To maintain heat recovery

A> To minimize the amount of carbamate recycled and amount of water recycled back

B> To minimize power requirementC> To maximize ammonia recoveryD> To minimize investment

Put together such amount to find a balance between consumption, maintenance and investment.

STRIPPING PROCESSEfforts to find addition driving force beyond the usual addition of heat. The stripping is introduced from bottom which travels upwards. Medium pressure steam condens-ing on shell side of stripper supplies heat for decomposition of carbamate. Currently per day consumption does not require injection of ammonia vapours at stripper bot-tom. Only higher molal ratio of NH3/CO2 is maintained in reactor. Liberated in stripper ammonia is adequate to carry out stripping efficiency since the reactor stripper con-denser system operates at nearly same temperature. It is possible to feed recycle carbamate either by gravity flow which however necessitate the installation of car-bamate condenses at which a much higher elevation so as to give the require heat or to use a recycle injector using a part of feed reactant as a motri fluid . Snam Progetti was first to develop the use of such recycle ejector and locate condenser at ground level. Since a portion of unconverted carbamate is decomposed and recycle to re-actor at synthetic load on down stream decomposition and recycle section is compar-atively much less resulting in lower utility consumption.Further since the synthetic pressure in stripping process in lower it is possible to employ, steam drives centrifu-gal CO2 compressor which has lower maintenance process. Low in pressure steam is generated in carbamate condenser which has resulted in lower consumption of steam and cooling water.

THEORY OF STRIPPINGThe theory of stripping in s based on Henry’s law. The concentration of compound ‘s of every component in solution which in equilibrium with vapour phase is directly proportional as l to partial pressure of component in vapour phase .By changing par-tial pressure of component , concentration of solution can be changed. This can be il-lustrated as follows:

CA : concentration of component A in solution

CB : concentration of component B in solution

PA & PB : Partial pressure of component A & B vapour phase

P : Total partial pressure

In stripper we introduce excess NH3 along with reactor effluent .This NH3 is released as vapour by heating in stripper and this increases partial pressure. Pressure of am-monia over the solution as total pressure remains same. Partial pressure of CO2 re-duces to a lower value. In accordance with Henry’s law carbamate decomposes to in-crease partial pressure of CO2 in vapour phase to approach equilibrium concentra-tion. Stripping action is thus carried out and heat of decomposition is supplied from an external source by condensation of medium pressure steam. Partial pressure of either of component can be changed and this will result in decomposition of car-bamate.

EFFECT OF PROCESS VARIABLESCabamate formation takes place with liberation of heat and urea formation takes place with absorption of heat. The former reaction is rapid and later is slow. The equilibrium conversion to urea will be favoured under following conditions:

(1) Higher NH3 concentration(2) Less H2O concentration(3) Higher temperature(4) Higher pressure(5) Increased residence time

EFFECT OF H2O/CO2 RATIOWater is by product of urea formation. One mole of water is formed when one mole of urea is produced. In presence of excess water, equilibrium reaction shift in reverse direction and yield of urea is poor. However has to add for recycling unconverted NH3

and CO2 back to reactor. Lower the amount of water in the reactor, higher the yield of urea to low water concentration in low pressure recovery system and result is higher carbamate concentration and this causes pumping problem and clogging in piping system. Excess water in reactor also reduces effective volume for urea forma-tion and additional energy is required to get rid of this. Study shows that presence of one mole of excess water per mole of carbamate equilibrium yield of urea to half.

EFFECT OF PRESSURE AND TEMPERATUREAs per Le Chatliers principle higher pressure favours carbamate formation. As the op-erating condition of carbamate formation is almost instantaneous and reaction tends to completion. Provided reaction heat is removed simultaneously. Lower temperat-ure favours carbamate formation being an exothermic reaction. In case of urea form-ation higher temperature is favourable because reaction is endothermic. The reaction is such that when temperature increases the conversion increases. Maximum equilib-rium conversion is achieved at around 190 to 2000 C. Reactant are highly corrosive at higher temperature. Operating pressure is totally dependent on temperature at which conversion takes place in liquid phase. So equilibrium pressure increases when temperature rises.

EFFECT OF RESIDENCE TIMEUrea conversion reaction is slow and takes place in 20 min to attain equilibrium con-sidering slowness of reaction. Urea reactor is so designed that residence time should be more than 20 min. The higher residence time favours equilibrium conversion and normally reactor are designed for residence time of 30 min to 1 hour depending on other operating parameters. 68% urea conversion takes place with residence time of 30 min at a mole ratio of 4:1 and temperature at 188C whereas to achieve 60% con-version with a mole ratio of 2.8:1 at 181C, almost a residence time of 55 min is re-quired.

Residence time in urea reactor plays an important part on equilibrium conversion where operating parameter including mole ratio are not favourable for a good yield. The higher residence time contribute to some extend to achieve a better yield. But this is done by providing higher reactor volume which increases capital investment.

BIURET IN UREAA problem facing by every urea manufactures is formation of biuret. In production process it is not a desirable substance at is toxic to plant. It should not exceed more than 1.5% in urea as fertilizer control order. When urea solution is heated in absence of free ammonia an objectionable ingredient called biuret is formed according to fol-lowing reaction:

2NH2CONH2 NH2CONHCONH2 + NH3

The formation of biuret is favoured by higher temperature, higher concentration of urea solution, lower ammonia content and higher residence time.

PROCESS DESCRIPTION FOR UREA SYNTHESISThe technology used is urea plant is “SNAM PROGETTI” from Italy. The whole process has been divided in four main sections:

(1) High pressure section(2) Medium pressure section(3) Low pressure section(4) Prilling section

In high pressure section, reactor, high pressure stripper and carbamate condenser are main equipments used.

In medium pressure section, medium pressure decomposer, medium pressure ab-sorber, ammonia absorption tower and inert washing tower are the main equip-ments. In low pressure section, low pressure decomposer, low pressure absorber and carbamate solution tank. In prilling section urea melt is converted in prills by natural draft in prilling tower.

In prilling section it is prilled from top and granules of urea are obtained which is sent to bagging plant, water treatment plant which is required for environment reasons.

HIGH PRESSURE SECTIONThe liquid ammonia is pumped at high pressure through a ejector which drive the carbamate from carbamate separator in to reactor. CO2 mixed with small quantity of air is compressed in a two stage CO2 compressor and is also fed to reactor. The liquid ammonia and CO2 react together here. The product formed in carbamate which de-hydrates in reactor itself, to form urea and water. The oxygen in air forms a passive oxide layer on inside of vessel surface to prevent corrosion by carbamate and urea.

The reaction products from reactor overflow to high pressure stripper where the un-converted carbamate is decomposed back into ammonia and carbon dioxide. Heat of decomposition is supplied by medium pressure steam admitted in stripper shell side. The condensate obtained is sent to high pressure decomposer shell side. Urea solu-tion thus obtained flows out to medium pressure solution through level control valve.

The vapour produced on decomposition in high pressure stripper enter high pressure carbamate condenser through a mixer along with carbamate solution from medium pressure section. Here they condense to form carbamate again and flow to high pres-sure carbamate separator.

CO2 COMPRESSORCO2 is compressed in a twin case centrifugal compressor entering reactor. CO2 is re-ceived at battery limit rate of 40601 kg/hr at 1.4 atm and 40oC. It’s moisture is re-covered by a knock out drum. Air is added to outlet at rate of 382 kg/hr. The gas then enter compressor. The compressor consists of two casing – low pressure casing and high pressure casing. Both casing consist of two stage with inter coolers. By success-ive compression and inter cooling. The moisture present in CO2 is removed. The gas is pressured from 1.4 atm to 160 atm and 1300C. It then flows to reactor at rate of 39722Kg/hr of dry CO2.

COMPRESSION DETAILSCompression used in ammonia plant is a turbo driven compression. This compression casing is made of carbon steel. Compressor has two casing:

1. Low pressure casing

2. High pressure casing

Both casing has two stages. Thus it is four stages turbo driven case CO2 compressor. After each stage the temperature of casing increases due to compression.

Thus after each stage inter stages cooler are used. These coolers are simply shell and tube type heat exchanger. The outlet CO2 rate is 39722Kg/hr dry CO2 + air, Pressure 160 atm. and temperature 1300C.

AMMONIA RECOVERYAmmonia for urea synthesis comes from ammonia plant at 23 atm. within plant bat-tery limit. Total 30694Kg/hr ammonia first enters in ammonia recovery tower where its temp raises up to 350C counter current contact with vapour from receiver. Liquid ammonia is then stored in ammonia receiver. Ammonia from ammonia receiver is sent to NH3 booster pump at rate of 52058Kg/hr. A small amount of NH3 4334kg/hr is sent to medium pressure pump, and remaining is sent to succession of NH3 feed pump, which is a high pressure pump. Thus booster pump work to boost up succes-sion pressure pump, Because discharge of high pressure ammonia pump is the only reciprocating pump in the ammonia pump. This pump is motion driven pump. This in-creases pressure for liquid ammonia from 25 atm. to 240 atm.

UREA SYNTHESISUrea synthesis section that is the “UREA SYNTHESIS REACTOR” is the heart of urea plant, as urea is formed only in reactor. Remaining all section of plant is for con-centration of urea. Liquid ammonia and gaseous CO2 at high pressure enter the re-actor bottom and react to form carbamate which further decomposes to form urea and water. Ammonia at high pressure serves as motion fluid in ejector and drives car-bamate from high pressure carbamate separator to reactor. Compositions of car-bamate on mixing with NH3 are:

NH3 70.39%CO2 17.88%H2O 11.73%

FLOW RATE 121393Kg/hr

CONSTRUCTION OF UREA REACTORUrea reactor in urea plant is a tubular plug flow type reactor .this is a vertical reactor, made up of carbon steel. It is internal is lined with 7mm thick stainless steel. The feed inlet is of disperse type so hole are provided for both NH3 and CO2 inlet.

Total number of holes for NH3 -> 350

Total number of holes for CO2 -> 200

Total 10 number of sieve plates are provided over length of 22.5 m from top .These plates gives mixing to liquid and gas and avoid the back mixing in the reactor thus maintaining plug though reactor. Data of the plug reactor are:

Design pressure 170kg/cm2

Operating pressure 160kg/cm2

Height 40mShell thickness 67mm

OVER FLOW FROM REACTORThe over flow from over flow line is taken out from reactor at a length of 2.5m from top. The over flow stream from reactor is sent to high pressure stripper from where carbamate is decomposed. The compositions of overflow stream are as follows:

NH3 33.88%CO2 13.34%UREA 33.8%WATER 18.98%FLOE RATE 161115kg/hrTEMPERATURE Vary throughout the reactorPRESSURE 156 atm.

AMMONIA STRIPPING

In stripping process carbamate is decomposed with medium pressure steam. In presence of excess ammonia in a falling film type stripper, unconverted carbamate present in reactor outlet solution of urea is decomposed in stripper.

Urea solution from reactor at 156 atm. and 1880C enter the tube side of stripper op-erating a 147 atm. The high pressure stripper is of falling film type heat exchanger. The stripper tubes are provided with liquid dividers are called FERRULES. These fer-rules have three equispaced on their periphery. The urea solution enters at top tube channel and from a static liquid head over ferrules. This have drive urea solution through holes and in to tubes where a thin film is created on tube inner surface. The ferrules not only ensure uniform.

STRIPPING PROCESSIn stripper introduce excess ammonia along with reactor effluent. The ammonia re-leased as vapour by heating in stripper. This ammonia vapour increases partial pres-sure of NH3 over the solution. as total pressure remain same, the partial pressure of CO2 reduces to the lower value in accordance with the HENRY’S LAW carbamate de-composes to increase the partial pressure of CO2 in vapour phase to approach equi-librium concentration. Stripping reaction is thus carried out heat of decomposition is

supplied from an external source by condensation of medium pressure steam. Con-densate from a stripper shall go out to a steam condensate separator and is send to a medium pressure decomposer shell, through level control valve. Stripped urea solu-tion is collected in bottom of stripper and goes to medium pressure decomposer through a level control valve. The valve in stripper is important because the higher level increases the residence time of urea solution in stripper thereby increasing bi-uret content. Compositions of urea solution coming out of the stripper are as follows:

NH3 24.3% by wtCO2 5.8% by wtH2O 24% by wtUREA 45.9% by wtFLOW RATE 118627 kg/hr

The top end of stripper tube is highly susceptible to carbamate corrosion thus to pro-tect it from exposure to high temperature, nitrogen is periodically injected to main-tain nitrogen blanket in upper shell.

CONSTRUCTION OF STRIPPERThe stripper is a falling film type heat exchanger. It has shell and tube type arrange-ments. Falling film is formed by using ferrules. Ferrules not only ensure uniform ar-rangement though all tubes but also aids in continuous film on tubes inner surface. Thus no tube on any of its portion is starved of liquid thereby preventing it from get-ting overhead it from getting overhead and subsequent carbamate corrosion.

CONDENSATION AND SEPERATIONThe vapours from stripper are condensed in high pressure condenser. The carbamate thus formed is recycled back to reactor. Vapours from stripper obtained by decom-position of unconverted carbamate. These first enter the carbamate mixer at 147 atm. and 1900C. These vapours contain mainly of ammonia and carbon dioxide mixed with carbamate solution from high pressure carbamate pump (high speed centrifugal pump). They take suction from medium pressure carbamate pump. It also maintains a minimum flow through high pressure carbamate pump by flow control valve in dis-charge line. If carbamate flow to high pressure section decreases which maintain pump though put by recycling more carbamate to high pressure condenser. A very

high condensate, flushing connection is provided on line going to kettle. The Compos-ition is as follows:

NH3 46.9%CO2 20.1%H2O 33.98%FLOW RATE 35713 kg/hr

MEDIUM PRESSURE SECTIONUrea solution from bottom of high pressure stripper now enters the medium pres-sure decomposer after pressure reduction through a level control valve. During ex-periment much of remaining cabamate flashes from NH3 and CO2 vapour, there by concentrating urea in solution. This urea solution is further led down in pressure by level control valve and enters low pressure section. It consists of three parts- the top most part is medium pressure separator, the middle is medium pressure decomposer and bottom is medium pressure urea solution holder.

MEDIUM PRESSURE SEPARATORThe vapours from high pressure decomposer are condense in an medium pressure condenser using ammonium carbamate solution from low pressure section with tempered cooling water on tube side. The cabamate solution overflows from me-dium pressure condenser in to medium pressure absorber where excess ammonia, inert, a little amount of carbon dioxide separates to form vapour. These vapours are purified in top section absorber with reflux ammonia. Ammonia with inert gases leav-ing top of medium pressure absorber are mostly condensed in ammonia condenser, with cooling water on tube side. From ammonia condenser both liquid and gases are sent to ammonia receiver, along with incoming liquid ammonia. The inert gases sat-urated with ammonia, leaving the receiver enter the ammonia recovery tower. Here ammonia is further condenses with direct contact with cold ammonia from battery limit and flow down to ammonia receiver. The inert with residual ammonia from tower are sent to medium ammonia absorber where later gets absorbed in cool con-densed and recycle to medium pressure absorber as ammonia water.

MEDIUM PRESSURE DECOMPOSERThe urea solution coming out of medium pressure stripper and containing 45.9% by wt urea is further concentrate to 63.28% urea by wt. In medium pressure decom-poser from stripper is led down through it. There is a flushing connection and a sampling point. The reactor drain lines also meet the stripper outlet line. It is motor operated valve which act as a quick closing isolation valve of medium pressure sec-tion. The urea solution from stripper at 147 atm. and 1200C is let down to 18 atm through level control valve and enter at top of medium pressure separator which has the following composition by wt.:

NH3 24.3%CO2 5.8%UREA 45.9%H2O 24%FLOE RATE 118627kg/hr

As a result of pressure let down some solution flashing producing vapour of NH3, CO2 and H2O. The heat of vaporization is taken from urea solution where the temperature falls from 2100C down to 1400CS. This solution is taken distributed over bed of ras-ching rings in medium pressure separator. The vapours rising from medium pressure decomposer come in to intimate contact with urea solution on raschig ring bed. Thus more carbamate decomposed by hot vapours. The vapour from medium pressure de-composer after imparting heat for carbamate decomposition in medium pressure separator, flows out at rate of 37113kg/hr at 18 atm. and 1450C to medium pressure condenser. The composition of vapour is as follows by wt:

NH3 73.36%CO2 16.09%H2Os 10.55%

The solution thus enriched in urea flow down and is collected on top tube sheet of medium pressure decomposer. The decomposer is falling film type of heat ex-changer. The tubes are tight fitted with ferrules which have four tangential holes on their periphery of size 4mm. The carbamate enter bottom of medium pressure de-composer shell and rises to top imparting sensible heat in solution. The carbamate thus decomposes liberating NH3 and CI2 vapours which rise up to packed bed in me-

dium pressure separator. The enriched urea solution falls down in medium pressure urea solution holder at bottom and collected there. 4532kg/hr of vapour and 473kg/hr inert from medium pressure carbamate separator enter medium pressure holder. These vapours have following composition by wt.:

NH3 97.97%CO2 2.03%

MEDIUM PRESSURE UREA HOLDERThese vapours enter the medium pressure urea holder above the liquid level. The in-ert in vapour contain oxygen which acts as passivating agent in medium pressure sec-tion. The medium pressure uses holder let out urea solution at 18 atm. and 1560C having following compositions by wt.:

NH3 7.02%CO2 1.16%UREA 63.28%H2O 28.54%

The pressure is then reduced from 18 atm. to 4.5 atm. And Solution then enters the low pressure decomposer, it then control the level in urea solution holder. Two level glasses are provided in medium pressure urea solution holder to monitor level phys-ically.

MEDIUM PREESURE CONDENSATIONThe vapours from medium pressure decomposer are condensed in medium pressure condenser with help of recycle carbonate solution from low pressure section. The va-pour from medium pressure decomposer enter bottom of medium pressure con-denser which operate at 18 atm. Before entering they are mixed with 14160 kg/hr of carbonate solution. This solution contains following composition by wt.:

NH3 61.38%CO2 13.18%H2O 25.44%

Carbonate from carbonate solution pumped by carbonate solution pump and enter medium pressure condenser at 25atm and 400C through sprayer. The cone sprayer helps proper mixing of carbonate solution with vapour. The sprayed liquid mixtures with vapours from medium pressure decomposer and enter condenser shell bottom at rate of 51273kg/hr at 18atm and 1300C. The composition of mixture by wt is:

The mixture enters the shell bottom of high pressure condenser. This is a shell and tube heat exchanger. The vapours condense in liquid carbonate medium and top of shell.

MEDIUM PRESSURE ABSORBERAbsorption in medium pressure section is carried out in medium pressure absorber. The vapours of ammonia, carbon dioxide and inert is in medium pressure condenser. Outlet solution are absorbed and rectified in medium pressure absorber so that va-pours leaving contain only ammonia and inert.

51213kg/hr of liquid and vapour pressure leaving medium pressure condenser at 17.5 atm. and 800C absorber. The medium pressure absorber operates at 17.5atm. It consists of two sections. The top section is called rectification section. It consists of four bubble cap Trays numbered from bottom to top. The bottom portion is called absorber.

ABSORPTION PROCESSAmmonia reflux is introduced on tray in rectification section. The liquid and vapour mixture from medium pressure condenser enter the absorber portion of medium pressure absorber. After entering it flows down to bottom medium pressure ab-sorber and comes out of a sprayer. A liquid level is also maintained above sprayer. The vapour pressure of ammonia, carbon dioxide and water comes out of sprayer and get absorbed in liquid above it. This liquid level is made up of carbamate from medium pressure condenser. Pure NH3 and NH3 solution from the rectification sec-tion of medium pressure ammonia absorber. Ammonia and water present in solution

NH3 43.79%CO2 8.52%H2O 47.69%

help to absorb the CO2 content of vapour and form carbamate. Thus CO2 in vapour which rise to rectification section of medium pressure absorber is reduced to min-imum. The carbamate solution in medium pressure absorber provides suction to high pressure carbamate pump at 17.5atm and 720C and has following composition by wt.:

NH3 46.84%CO2 20.5%H2O 32.66%FLOW RATE 35013 kg/hr

Level control in medium pressure absorber bottom is very important. Firstly because of it provide suction to high carbamate pump. Secondly if level is not sufficient, va-pours from sprayer will be absorbed in solution to little extent. This will increase the load on rectification section and enhance the change of CO2 escaping to medium pressure absorber over head off gas line. Medium pressure absorber level is con-trolled with high pressure condensate purge connection for both tapings. Flow from medium pressure condenser to medium pressure absorber is regulated and so in me-dium pressure absorber level. Five number of sight glasses are provided besides for physical verification. A high level in medium pressure absorber is dangerous because this may result in liquid may carry over in medium pressure absorber off gas. To safe guard against extraordinary high level an absorber drain is provided. This drain line is equipped with a control valve which let down to solution to carbonate solution tank in low pressure section. High and low pressure condensate flashing connections are provided on sides. Medium pressure absorber is also provided with a drain line to closed drain system with high pressure condensate connection. This is used for rain-ing the vessel before undertaking any maintenance jib. The vapours rising above the liquid level, before level medium pressure absorber much have only ammonia and in-ert. This is so because ammonia form vapours has to be condensed and returned to ammonia receiver. Presence of CO2 will choke takes and heat exchanger in these con-densers will reduce. Also ammonia carbamate in the ammonium condenser will not only corrode them but the ammonia receiver as well. It will also cause pumping prob-lem in ammonium booster pump by blocking the section line from ammonia receiver. To remove possibility of CO2 slip the vapours pass through the rectification zone. It consists of four bubble cap trays numbered from bottom to top. The top tray receives pure reflux ammonia at 33c at rate of 4334kg/hr. Tray number 3 receives 2247kg/hr

of ammonia solution from medium pressure ammonia absorber. It has following composition by wt:-

NH3 63.75%H2O 34.25%

The vapours come in intimate contact with ammonia solution from top. This solution absorbs the CO2 which react with ammonia to form carbonate. Temperature control is important in medium pressure absorber. Each tray is provided with a temperature tapping. Decrease in temperature profile from first tray indicates that all trays are functioning properly. It implies that CO2 is being progressively consumed to form car-bamate. The overhead gas temperature should be minimum and around 42.50C.

DESCRIPSITION OF MEDIUM PRESSURE ABSORBERThe medium pressure absorber consists of four bubble trays. Each tray has 154 num-ber of bubble caps. Vapour liquid mixture enter through an enter pipe which run down inside column and divided equally in to four perforated arms.

These perforated pipes are submerged in carbamate solution, From where unab-sorbed gas rises. The rising vapours pass through the distributor to the rectification section. A liquid seal of 50mm is maintined in each bubble tray fourth and third trays are feed with pure reflux and aqueous NH3. Respectively solution which eliminated residual CO2 and H2O from NH3 leaving medium pressure absorber. Flushing water connections are provided on 4th, 2nd and 1st tray for washing carbamate deposits over the trays.

MEDIUM PRESSURE AMMONIA CONDENSATIONThe ammonia in medium pressure absorber off gas is condensed in ammonia con-denser. The ammonia condenser consists of two shells and the heat exchanger in series. Vapours enter shell side and cooling water enters shell side. Both ammonia condensers operate at 17.5atm. At this pressure it is possible to condense ammonia with help of available cooling water at 360C. The vapour from medium pressure ab-sorber first enter ammonia condenser. Much of ammonia vapours condense here. The uncondensed vapours along with condensed ammonia flow to second ammonia condenser. Here further condensation of ammonia takes place. Thus out of a total of 22841 kg/hr of pure ammonia and 473 kg/hr of inert entering ammonia condenser,

18332 kg/hr is condensed. The remaining 4519kg/hr of ammonia stays in vapour phase along with 473 kg/hr of inert. Both liquid ammonia and vapours from ammonia condenser enter the ammonia receiver through separator line at 17.2atm and 1400C.

Ammonia receiver can be isolated from by two isolation valves in liquid line and one valve in vapour line. Cooling water first enter condenser shell, then goes to ammonia condenser. Cooling water outlet from is required by control valve. A line is taken from the cooling water and outlet line. It is connected to cooling water circulation pump, discharge line of medium pressure condenser to supply cooling water when necessary. High pressure condenser flushing connection is provided in ammonia con-denser to ammonia receiver.

AMMONIA RECEIVINGLiquid ammonia from plant battery limit and recovered ammonia from ammonia con-denser and medium pressure section is collected and stored in a ammonia receiver. Ammonia receiving system consists of a receiver portion and an ammonia recovery tower. The tower is installed on receiver as an integrated part. The receiver holds ammonia at 17.2 atm and 350C. Liquid ammonia at 23atm pressure and 120C temper-ature comes to plant through isolation valves. It enters through a filter. The filter can be bye passed also. This first filter is connected to vent connections. Then the incom-ing liquid passes through flow totaliser. This send impulse to a control panel, this can be isolated sand bye passed. 30694 kg/hr of liquid ammonia enter the ammonia re-ceiver through level control valve. A portion of this ammonia, enter the top of ammo-nia recovery tower.

The tower is a packed column which operates at 17atm vapour consisting of ammo-nia and inert rise from the ammonia receiver and enter bottom of ammonia recovery tower. The vapour on rising come in intimate contact in packed bed with fresh liquid ammonia falling from top.

The ammonia vapours thus condense giving away heat of condensation to liquid am-monia. The liquid ammonia then flows down to ammonia receiver. The vapours from ammonia recovery tower containing 147kg/hr of inert at 1.7atm and at 350C flow to medium pressure ammonia absorber. The recovered liquid ammonia from ammonia condenser flows down to ammonia receiver at a value of 18322 kg/hr at 17.2atm and 490C.

MEDIUM PRESSURE ABSORBER AND INERT WASHINGVapours from top of ammonia recovery tower are scrubbed and washed in medium pressure ammonia absorber and inert gas washing tower.

This equipment consists of an absorber portion. It is a single pass shell and tube fall-ing film heat exchanger.An inert washing tower is mounted on its top. This tower is equipped with three trays. Cooled condensate is introduced on top trays of inert washing tower. Ammonia vapour and inert enter the bottom tube channel of me-dium pressure absorber. Cooling water is circulated on its shell side. The vapours form ammonia recovery tower enter bottom tube channel of medium pressure ab-sorber consist of 1477 kg/hr of inert. Cooled condensate introduced in inert washing tower above comes on top tube. Shell tubes are fitted with ferrules. The cold con-densate comes down through tangential holes of ferrules along the walls of tubes. It forms weak ammonia solution on absorbing ammonia vapours. This weak ammonia solution flows down the tubes. In this process it absorbs the ammonia vapours rising from bottom tube channel to medium pressure ammonia absorber. The resultant heat of absorption is removed by cooling water on shell side. The ammonia solution thus form flow out of tubes and is collected in bottom of medium pressure ammonia absorber at rate of 2247kg/hr. This solution has following composition:

AMMONIA 65.75%WATER 34.25%

This is pumped by ammonia solution pump to medium pressure absorber provided in this line central level in medium pressure ammonia absorber bottom. The vapour from medium pressure absorber enter the bottom tray of inert washing tower con-densate is cooled by cooling water in steam condensate. Cooler enter top tray of in-ert washing tower. The residual ammonia

Vapour come in intimate contact with cool condensate in each of three trays and get absorbed. Thus ammonia is complete washed from the vapour leaving only trays.

LOW PRESSURE SECTIONThe urea solution from medium pressure decomposer bottom enters the low pres-sure decomposer, after expansion through a level control valve. As a result of expan-sion most of remaining carbamate undergoes decomposition. Thus urea solution fur-ther concentrated and sent to vacuum section through a level control valve. The va-pours enter the low pressure condenser shell and get absorbed in an aqueous carb-mate solution from waste water solution. Low pressure condenser has cooling water on tube side.

The liquid thus formed goes to carbamate solution tank from where it is recycled back to medium pressure condenser. The inert gases from tank containing ammonia is absorbed in cooled condensate in low pressure ammonia absorber, washed in inert washing tower and sent to vent stack. The liquid flows down to tank. The urea solu-tion flowing out of medium pressure urea solution holder and having 63.28% by wt. Urea is further purified by 71.12% by wt. Urea in low pressure decomposer. The low pressure decomposer consists of three parts:

The top part low pressure separator

The middle is low pressure decomposer and

Bottom is low pressure urea solution holder.

LOW PRESSURE SEPARATORThe urea solution from medium pressure solution holder at 18atm and 156c has fol-lowing compositions:

NH3 7.02%CO2 1.16%UREA 63.28%H2O 28.54%FLOW RATE 86040 kg/hr

It is let down to 4.5atm through level control valve and enters at low pressure separ-ator. The reduction in pressure causes some of unconverted carbamate solution to

flash and generates vapours of NH3, CO2 and H2O. The heat of vaporization is supplied by solution itself, whose temperature consequently falls from 1560C to 1280C. The solution is then distributed over a bed of rasching ring in low pressure separator. The hot vapours from low pressure decomposer come in to intimate contact with urea solution on rasching ring bed. The heat exchange between rising vapourS and down going solution results in decomposition of carbamate still present in solution.

LOW PRESSURE DECOMPOSERThe vapours from low pressure separator flow out to low pressure condenser. The composition of this vapour steam is as following:

NH3 49.15%CO2 4.07%H2O 46.77%FLOW RATE 9480 kg/hrINERT 52 kg/hr

The solution thus enriched in urea flow down and is collected on top tube shell of low pressure decomposer is a falling film type of heat exchanger. The tubes are filled with ferrules or liquid dividers. These ferrules have tangential holes on their periphery. The urea solution flow down in test tube of low pressure decomposer and form a level over ferrules. The liquid head thus formed drives urea solution through holes of ferrules along inner wall of tubes. In this manner thin film is created on inner surface of tube. 54kg/hr at 4.5atm saturated steam is supplied to shell side of low pressure decomposer. The urea solution film on tube inner surface take heat from 4.5atm steam and remaining carbamate in urea solution decomposes into NH3 and CO2. These hot vapours then rise to packed bed in low pressure separator and carry on composition.

LOW PRESSURE UREA SOLUTION HOLDERThe urea solution finally flows out of low pressure decomposer and is collected in low pressure urea solution holder. The urea solution contain following composition:-

NH3 1.8% by wtCO2 0.8% by wtUREA 71.2% by wtH2O 26.28% by wtFLOW RATE 70560 kg/hr

Level in low pressure urea solution holder is controlled and let down the urea solu-tion at 4.5atm & 1380C to evaporation section which operate under vacuum.

LOW PRESSURE CONDENSATIONVapours from low pressure separator are condensed in low pressure condenser with help of recycle carbonate from distillation tower reflux pump. Low pressure con-denser is a shell and tube type heat exchanger. Vapours and recycle carbonate enter shell side vapours at rate of 9486 kg/hr are mixed at first with recycle carbonate solu-tion from distillation tower reflux pump. The composition of vapours by wt is as fol-lows:

NH3 49.15% by wtCO2 4.07% by wtH2O 46.77% by wtINERT 52 kg/hr

This recycle carbonate line is provided with low pressure condensate flushing con-nection. The recycle carbonate has following composition by wt:

NH3 34.87% by wtCO2 18.55% by wtH2O 46.55% by wt

The recycled carbamate is distributed through a sprayer in low pressure separator to low pressure condenser. This ensures the proper mixing of vapour and liquid. Mix-ture then enters the low pressure condenser shell at bottom and rise to top. The va-pours condense in liquid medium while rising. The heat of condensation is carried away by cooling water on tube side. Most of vapours get condensed in low pressure condenser. The uncondensed vapours, inert and carbamate together overflow out low pressure condenser. Air vapours and inert disengage from carbamate solution and flow out to arbamate solution tank from separator top. The carbamate solution flows down to carbamate solution tank from separator bottom. The carbamate solu-tion at outlet of low pressure condenser at 4atm and 400C has following composition by wt.:

NH3 44.61% by wtCO2 8.68% By wtH2O 46.7 by wtFLOW RATE 13900 kg/hrINERT 52 kg/hr

CARBAMATE SOLUTION TANKThe carbamate solution from low pressure condenser is finally collecting carbonate solution tank at 4atm and 400C. Carbamate solution from low pressure ammonia ab-sorber and inert washing tower is also connected here. The carbonate solution tank stores carbonate solution having following composition:

NH3 43.29% by wtCO2 8.52%by wtH2O 37.69%by wt

A steam sprayer is provided at bottom of carbonate solution tank. Low pressure steam is introduced in sprayer whenever the carbonate solution temperature falls down especially during long shutdown. In this way carbonate is prevented from crys-tallizing. The carbonate solution tank provides suction to medium pressure carbonate solution pump. The suction line is provided with a low pressure condensate flushing connection. A cooled condensate connection from steam condensate cooler is also

provided to suction line to build up level in carbonate solution tank whenever re-quired.

LOW PRESSURE ABSORPTION AND LOW PRESSURE INERT

WASHINGThe vapours and inert from carbonate solution tank are absorbed in low pressure ammonia absorber and subsequently washed in low pressure inert washing tower before being vented out. Low pressure ammonia absorber is a shell and tube type Heat exchanger. Low pressure inert washing tower consisting of three trays are mounted directly mounted on carbonate solution tank. Cooled condensate from steam cooler flows on top tray of low pressure inert washing tower at following con-ditions:-

FLOW RATE 260 kg/hrPRESSURE 31 atm.TEMPERATURE 400C

The condensate flow rate is controlled which also reduces the pressure of that sys-tem. This flow rate includes cold condensate flow suction line. Cooling water flows in a shell side of low pressure ammonia absorber from bottom to top. The vapour from carbonate solution tank enter bottom of low pressure ammonia absorber. Cooled condensate entering low pressure inert washing tower flows down to three trays. The inert rising from carbonate solution tank enter low pressure inert washing tower. The residual NH3 and CO2 are washed off by the cooled condensate on trays forming a weak carbonate solution. The inert then go out at rate of 52 kg/hr. The weak car-bonate solution then flow down into tubes of low pressure ammonia absorber from top through ferrules. The solution flow down in form of a thin film along walls. It comes in contact with vapour rising from carbonate solution tank. The NH3 and CO2 vapours are absorbed in weak carbonate solution which then flows out and is collec-ted in a carbonate solution tank. The heat of absorption is disugrated to cooling wa-ter on shell side of low pressure ammonia absorber.

VACUUM AND EVAPORATION SECTIONConcentration urea solution becomes 70.7% after low pressure decomposition reac-tion. This solution is further concentrated in vacuum system with a pressure of 3 atm. and 0.3atm. Concentration is done at vacuum concentration, it may be done with low temperature of system.

FIRST VACUUM CONCENTRATIONLow pressure solution from low pressure urea holder having 71.10% by wt. Urea is concentrated at about 9.5% by wt. Urea in vacuum concentrator which operates at 0.3atm.Low pressure urea solution forming out from low pressure urea solution holder is led down from 1.4atm and 1390C to 0.3atm through level control valve dur-ing this pressure reduction much of water in urea solution flashes. The heat of vapor-isation of water is supplies low pressure urea solution itself, thereby reducing tem-perature 1380C to 1000C. The low pressure urea solution them goes to bottom of 1st vacuum concentrator through a three way valve. Low pressure condensate flushing and drain connection are provide in this line. A three way valve is provided in low pressure urea solution line from low pressure urea solution urea holder. This may be used to stop low pressure urea solution flow to first vacuum concentrator and divert it to urea solution tank in case of shutdown of vacuum evaporation section. A control valve is provided to regulate the flow of low pressure urea solution to urea solution tank. Line from 3 way valve is joined by a recovery line measure flow going in to I va-cuum concentrator. It is a climbing film shell and tube type heat exchanger operating at 0.3atm. Low pressure solution enters the bottom channel of 1st vacuum concen-trator and travel upward forming a film on inner surface of tubes. The low pressure saturated steam on shell side supplies heat to low pressure urea solution film in tubes. The water contained in solution start vaporising taking heat from steam. Fi-nally urea solution comes out from top of first vacuum. Concentrator and flashes in to first vacuum separator.

FIRST VACUUM SAPARATIONUrea solution from 1st vacuum concentrator is separated from vapours in 1st vacuum separator. The urea solution coming out of 1st vacuum concentrator also contains va-pours of water, ammonia and carbon dioxide. Urea solution enters tangentially into circular hood. This led to separation of vapours containing mostly water and little am-monia and carbon dioxide from urea solution. The urea solution after impingement falls down to bottom of 1st vacuum separator by gravity and vapour rise to top. The urea solution then flows out to 2nd vacuum concentrator at 0.3atm and 1280C. It has following composition:

NH3 7% by wt

CO2 3.11 by wtUREA 0.49 by wtH2O 89.41 by wtFLOW RATE 19515 kg/hr

Three sight glasses are provided in bottom of 1st vacuum separator for visual inspec-tion. The vapour contain following composition:

NH3 0.02%by wtCO2 0.01%by wtUREA 94.97%by wtH2O 5%by wtFLOW RATE 57255 kg/hr

SECOND VACUUM CONCENTRATORFirst vacuum separator from vacuum 1st separator in concentrated from 94.97% by wt urea to 99.7% by wt in second vacuum concentrator which operates at 0.03atm. first vacuum urea solution at 0.3atm and 1280C from first separator flows out to second vacuum concentrator which operate at 0.03atm, thereby creating pressure gradient. Second vacuum concentrator is a climbing film shell and tube type ex-changer. First vacuum urea solution flows in tube side and low pressure saturated steam in shell side. First vacuum urea solution enter bottom of second vacuum con-centrator tube channel and climbs up forming a film in tube inner surface. The re-

maining water, ammonia and carbon dioxide in first vacuum urea solution vaporize at this low pressure at 0.03atm. During its upward climb the heat of vaporization is sup-plied by low pressure saturated steam on shell side. Thus first vacuum urea solution get concentrated to 99.7 % by wt Urea and flows out to second vacuum separator at 1400C. The lines from first vacuum separator to second vacuum separator form a lute. This lute is provided with a drain connection. Low pressure condensate and low pressure steam flushing connection are also provided to lute to clear this line if it gets check with urea.

SECOND VACUUM SEPARATORThe urea melt from second vacuum concentrator flashes into second vacuum separ-ator where it is separated from water, ammonia and CO2 vapours. The solution va-pour mixtures the top of separator tangentially which facilitates the separation of two phases. There is circular hood all around. The urea melt after separation falls down by gravity into a holder at 0.03atm and 1400C. It has following composition by wt.:

UREA 99.7%WATER 0.3%FLOW RATE 54350 kg/hr

Level control in holder is very important as it provided suction on melt urea pump. As holder is under a low pressure of 0.03atm at a minimum liquid head must be main-tained above suction pump to provide necessary NPSH. This is ensured by maintain-ing level in holder. A high level will increase residence time of urea melt at 1400C and thus more biuret will be formed. The vapour separated from melt urea rise to top at 0.03atm and 1400C. It has following composition-

NH3 0.41% by wtCO2 0.2% by wtUREA 6.51% by wtH2O 92.85% by wt

A low pressure flushing condensate is provided to top dome of second vacuum separ-ator with a spray nozzle for better flushing. The vapour outlet nozzle from second va-cuum separator is provided with a washing tore with holes. Low pressure condensate is provided to washing tower.

FIRST VACUUM SYSTEMFirst vacuum separator controls the vacuum pressure in first vacuum concentrator and first vacuum separator. Water, ammonia and carbon dioxide vapour from first vacuum separator at 0.3atm and 1300C flow to shell side of first condenser. This is a horizontal shell and tube heat exchanger. Cooling water from outlet of first con-denser flows in to tube side of first condenser. Low pressure condensate flushing connections are provided at several points on shell side of first condenser. The above maintained vapours get condensed here to quite an extent. The condensate then flow down to waste water tank. The uncondensed vapours are ejected out to second condenser. These vapours enter shell side of second condenser where they condense with the help of cooling water in tube side. The operating pressure in second con-denser is higher than first condenser.

SECOND VACUUM SYSTEMSecond vacuum system controls the vacuum pressure in second concentrator. The vapour from second vacuum separator at 0.03atm and are first boosted by steam jet ejector to a higher pressure. It is required to boost the pressure here as it is difficult to condense the vapour at low pressure of 0.03atm with help of cooling water at available temperature. Low pressure steam is used as motive fluid in steam jet ejector. The vapours are discharged to shell side of condenser. Cooling water flows in tube side and help in condensing the vapour. The condensate flows out of the waste water tank. Low pressure condensate flushing connections are provided t several point on shell side along the length of first condenser to remove urea deposits. These deposits reduce heat transfer. The uncondensed vapours from first condenser are driven to second condenser by first stage ejector. Low pressure steam acts as motive fluid. The vapour and steam enter the shell side of second condenser. Cooling water flowing on tube side condense these vapour. The condensate flows down to waste water tank. The uncondensed vapours from second condenser are ejected out to fi-nal condenser by second stage ejector.

FINAL CONDENSATIONThe uncondensed vapours from first vacuum system and second vacuum system are finally ejected to final condenser which operates at slightly higher pressure than at-mospheric pressure. The vapour enters the shell side. The cooling water flowing on tube side condense the vapours. The condensate flows down the waste water tank. The non condensed vapour flows out to pot where entrained liquid separates and flows out. The non condensable are verged out to atmosphere from pot’s top.

UREA MELT PUMPINGUrea melt from holder having 99.7% by wt. Urea is pumped by melted urea pump. Melted urea pumps are centrifugal pump located sufficiently below the holder so as to provide the required NPSH. Holder collects urea melt from second vacuum separ-ator and provides suction to urea melt pump. The common suction line from holder to melted urea pump is steam jacketed so as to any possibility of crystallisation of urea melt. This line is then divided into branches with separate isolation valves to provide suction to both melted urea pump. Low pressure steam and connection are provided to each of suction lines.

UREA 99.7% by wt.H2O 0.3 % by wt.

The urea melt is then pumped to a pressure of 15 atm and discharged to the top of prilling tower. The discharged line from each pump is provided with a pressure con-densate flushing connection. The common discharge line is steam jacketed is provided in discharged line which controls the level in holder.

PRILLING SECTIONMelt urea from the discharged of melted urea pump is sent to top of prilling tower where urea prills are made.

Urea melt enter the prilling section from both urea units 11 and 21 through steam jacketed lines. Before entering the prilling bucket both these urea melt lines join to-gether. Two three way valves are provided individually on both the lines. These three way valves will normally allow urea melt to flow to prilling bucket. But in case when prilling has to be diverted, these ways valve will divert the urea melt to urea solution tank. Both these three ways valves are operated by manual switches.

As long as prilling is being done the urea melt return lines to urea solution tank. Which are continuously kept flushed with washing stream. At that time washing stream to line from prilling bucket stand close. It will open when urea melt is diverted to urea solution tank.

Prilling tower is natural draft type of tower. Melt urea at rate of 54350kg/hr from each unit and at about 15 atm and 1490C is pumped into a prilling bucket at the top of prilling tower. The prilling bucket is housed in the ceiling of prilling tower. Only one prilling bucket is kept in line and other is kept spare. Prilling bucket is driven by an electric motor. Speed of prilling bucket can be varied to control prill stze. It is in-creased to reduce the size. A high temperature switch is provided at top of prilling bucket which will sound an alarm at 1300C. This in turn will imply that level in prilling bucket is going high. Thus its speed must be increased to avoid overflow. Melt urea comes out of holes of rotating prilling bucket in form of fine droplets. These droplets are distributed uniformly throughout the cross section of prilling tower by centrifugal force imparted by rotation of prilling bucket. These droplets then start falling down the height of prilling tower. Ambient air enters the bottom of prilling tower through louvers and rise upwards. During its rise the air comes in contact with urea melt droplets. Thus the urea melt droplets at 1490C first cool down to 132.70C where they solidify into prills. On travelling down further they are sub cooled to about 600C. The heat generated during urea solidification and cooling is dissipated to incoming ambi-ent air. In this process the air get heated becomes lighter and thus rises upwards cre-ating natural draft. The solidified prills falls on rake floor at bottom of prilling tower. They are then separated by a rotating scrapper though a slit onto prilling tower belt conveyor. From conveyor the urea prills falls on the lump separator. The urea lump

are separated from prills and conveyed to a urea lump dissolving tank by urea recycle belt conveyor. The urea prills pass through the lump separation onto the product belt conveyor.

The heated air along with some urea dust rise upward and enter urea dedusting sys-tem where urea dust is reclaimed. The holes in prilling bucket normally get clogged by urea melt and other foreign particles after being in constant operation, this result in improper prill size distribution. Thus the prilling bucket in operation needs periodic washing and cleaning. For this purpose a prilling bucket cleaning water tank is provided n prilling tower top.