Urea is manufactured by reacting ammonia and carbon dioxide in an autoclave to form ammonium carbamate. The operating temperature is 135oC and 35 atm pressure, the chemical reaction is an endothermic reaction and so ammonia is maintained in excess to shift the equilibrium towards urea formation. Urea production is based on two main reactions.
1. Formation of ammonium carbamate
2. Dehydration of ammonium carbamate to produce molten urea
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| Urea Production from NH3 and CO2 |
Description of flow sheet:
1. Ammonia pumping: Liquid ammonia is pumped from the multistage pump which maintains the reaction pressure in the vertical stainless steel vessel
2. Carbon dioxide compression: The ammonia plant directly boosts the carbon dioxide from the compression section as it readily forms at the CO2 section of the ammonia production plant.
3. Urea synthesis tower: It is lined with a film of oxides to protect it from corrosion. A catalyst bed is placed on the inner side of the autoclave structure and 180- 200 atm pressure at a temperature of about 180-200 degrees centigrade is maintained. Plug flow operation takes place and molten urea is removed from the top of the tower.
4. Distillation tower and Flash drum: This high-pressure slurry is flashed to 1 atm pressure and distilled to remove excess ammonia and decomposed ammonia carbamate salts are removed and recycled.
5. Vacuum Evaporator: The solution is fed to the vacuum evaporator for concentrating the slurry.
6. Prilling Tower: It is a dryer where the molten slurry is passed from the top of the tower into a bucket which rotates and sprinkles the slurry and air is passed from the bottom. All the moisture is removed as the urea forms into granules during its journey to the bottom of the tower. These granules are sent by conveyor to the bagging section.
Biuret Formation:
Two moles of urea are converted into one mole of biuret and one mole of NH3 by heating.
2 NH2CONH2-------------> NH2CONHCONH2 + NH3
Because the biuret is injurious to germinating seeds, pineapple and citrus trees wither when the fertilizer is sprayed on the leaf. The biuret content in fertilizer-grade urea on the world market is required to be below 1.0%. biuret forms almost everywhere in urea production steps.
The following conditions are favourable for biuret formation.
• High residence times.
• High temperature.
• Low amount of water.
Process Water Treatment:
As already pointed out in the process description, the liquid effluent treatment section consists mainly of a distillation column to purify the wastewater, a hydrolyser to decompose the small percentage of urea into ratio NH3and CO2 which are eventually stripped in the lower section of the same column.
The condensed vapours from the first and second vacuum systems, containing urea, ammonia and CO2 are collected in the process condensate tank. In this tank, the carbonate close drain is also fed by the centrifugal pump and is recycled.
A simple description which gives an idea of the urea manufacturing process with plant layout:
Reactor effluent:
The reactor effluent which consists of a liquid phase along with a certain percentage of inerts and reactants in a vapour phase, is fed to the H.P. stripper where the first carbamate decomposition occurs. The vapour phase containing most of the inert gases then flows to the carbamate condenser together with the carbamate recycle from the medium pressure section. Only before re-injecting the carbamate into the reactor, the inert gases are separated from the liquid phase in the carbamate separator and fed to the MP decomposer.
H.P. Stripper:
It is the falling film type heat exchanger. It contains 2429 tubes with some space above the tubes and below the tubes. In the above space, a 0.315m height pall ring bed is arranged. A sieve tray is fitted above the packed bed. The tubes are fitted with ferrules and have three tangentially drilled distribution holes. Tubes are made of titanium and shell side fluid is the medium pressure saturated steam.
The reaction product leaving the reactor flows to the steam-heated falling film stripper which operates at about 144-146Kg/cm2 pressure. The liquid from the feed distributor pipe is evenly distributed onto the packed bed through a preheated sieve having 1400 holes. The mixture is heated up as it flows into the vertical tubes of the falling film exchanger. The CO2 content of the solution is reduced by the stripping action of the ammonia as it boils out of the solution. The carbamate decomposition heat is supplied by medium-pressure saturated steam, where the latent heat of condensation of saturated steam is taken by carbamate solution. In the falling film exchanger, the principal advantages are a high rate of heat transfer, no internal pressure drop, short time of contact.
Decomposition is promoted by heating and stripping CO2 by vaporising excess NH3, under the same pressure level as the urea reactor. The stripper used is a falling film type, decomposed and vaporized gases and liquid effluent are therefore in counter-current contact and CO2 concentration in the liquid is gradually reduced from the top to bottom of the stripper tube. As NH3-rich gas (CO2lean gas) rises from the lower parts of the tube, then the gas at the upper parts of the tube becomes an NH3 rich gas as compared with the equilibrium composition and the decomposition reaction in the liquid phase corrects the deviation from the equilibrium (the stripping effect). Decomposition at high pressure requires high temperature which means that much biuret has formed and the liquid becomes corrosive, but excess ammonia and the use of titanium in the stripper permit minimizing the problems.
The urea solution with part of the inserts coming from the bottom of the stripper enters into the medium-pressure decomposition in the urea purification section. The overhead gases from the top of the stripper are mixed with the recovered solution from medium pressure absorber and then pressurized to 180kg/cm2 in H.P. carbamate pumps and preheated in carbamate pre-heater by using steam condensate flowing to battery limits this mixture enters tube side of carbamate condenser where heat of reaction of reaction-1 and condensation of carbamate gases is removed by production of steam at 3.5 to 4.5 kgf/cm2 on the shell side by vaporization of water. The condensate from the condenser with few inert gases is entered into the carbamate separator. The carbamate separator is the cylindrical empty vessel in which the separation of carbamate solution from inert gases will take place, carbamate solution from the bottom of the separator is recycled to the reactor utilizing an ejector.
The non-condensate gases from the top of the separator consist mainly of inert gases, with a small amount of NH3 and CO2 passed through the split range controller to the medium pressure decomposer holder to utilize the heat of these for that decomposition.
UREA PURIFICATION:
Urea purification takes place in three stages at decreasing pressures as follows: First stage at 18kgf/cm2 Second stage at 4.5kgf/cm2 Third stage at 0.35kgf/cm2 It is pointed out that the exchangers where the urea purification occurs are called decomposer. the upper part of the medium pressure inert washing tower consists of three valve trays. Where the inert gases are subjected to a final scrubbing or washing through some absorption water. In this way, the inerts are sent to the vent stack practically free from ammonia.Prevention of explosion hazard by gases vented to the atmosphere:
CO2 fed to the reactor normally contains a small percentage of H2, CH4 and CO in addition to inerts like N2 and Ar. These gases plus a few gases introduced into the plant with NH3 coming from B.L together with CO2 contained in passivation air could give rise to explosive problems when vented into the atmosphere from MP inerts washing tower. As a matter of fact, this problem is minimized in Snamprogetti urea plants. Since the quantity of passivation air used is far lower than the one used in other processes. Thus the O2 to the flammable gases ratio in the vented gases does not justify the use of an H2 removal system on the CO2 stream from B.L
Purification and recovery stage at 4.5 kg/cm2:
L.P.Decomposer (LPD):
This is also the falling film type heat exchanger. It is also constructed the same as to MP decomposer, the packed bed height, equipment divisions and construction are the same.
The lower the pressure, the better the prevention of NH3 and CO2 loss from the system, but the recovered solution becomes weaker. This means that excess water is recycled to the synthesis loop, and the operating conditions of the L.P decomposer are selected at 3.5kgf/cm2 pressure decomposer (falling film type). The gases leaving the top separator are mixed with the dilute carbon solution coining from wastewater treatment and sent to the ammonia preheater, where they are practically absorbed and condensed.
The ammonia preheater is the shell and tube(1-4 pass) heat exchanger, in which LPD vapours are condensed and feed NH3 to the reactor is heated. While depressurizing(drawing tube side NH3 loop, case must be taken to avoid freezing of water) solution on the shell side of this preheater.
From the above condensate with uncondensed gases and then enter the LP condenser, where the residue absorption and condensation heat are removed by cooling water. The liquid phase,, with remaining inert gases, is sent to the carbonate solution vessel.
The carbonate solution tank is a horizontal cylindrical vessel. It is constructed with an inert washing tower above the tank and is located slightly taper to the ground to maintain the solution head for pumps at a low level. In shutdown followed by emptying of high-pressure equipment, the recovered NH3, and CO2 in the low-pressure stage are also stored in their tank. The level of this tank should be maintained low to recover all carbonate in case of a shutdown.
The inert gases leaving from the carbonate solution tank enter into a low-pressure inert washing tower which is located on the tank with the packed bed. The inerts are washed in this tower by using water in the countercurrent flow. The inerts which are leaving the washing tower are vented to stack, which is practically free from NH3.
Purification and recovery stage at 0.35kg/cm2:
Vacuum pre-concentrator:
This is also the falling film type heat exchanger. It is also constructed the same as to above decomposers with a bell distributor.
The solution leaving(the bottom of the low-pressure decomposer is expanded at 0.35 kgf/cm2 pressure and enters the vacuum pre-concentrator) falling film types with the help of a tangential inlet duct. Top separator where the released flash gases are removed before the solution enters the tube bundle. Decomposition section where the last residual carbamate is decomposed and the required heat is supplied by the condensation of the gases coming from the medium-pressure decomposer separator.
The gases leaving the pre-concentrator top are routed to the vacuum duct where condensation takes place. The urea solution, collected at the bottom of the pre-concentrator holder is sent to the vacuum section by using the centrifugal pump. The pre-concentrator can save a lot of pressure stream in the evaporator permitting it to concentrate the urea solution from 70-75% to about 85-88% wt. Low-pressure section for urea production
UREA CONCENTRATION :
As it is necessary, to prill urea, to concentrate the urea solution up to 99.8% wt. The simplest and most widely used method is direct concentration, which consists of heating the solution under vacuum to remove water. Direct concentration is operated based on the equilibrium vapour pressure of the urea solution.
Theoretically, to concentrate the solution without the deposit of crystals, the operating pressure should be kept over 0.3kh/cm2 abs.., 1360C at the second vacuum system. The urea solution coming from the vacuum pre-concentrator holder is sent to the first vacuum concentrator where it is heated up to above the boiling point of that liquid at the pressure of the separator. The mixed phase coming out of the concentrator enters the gas-liquid separator from where vapours are extracted by the second vacuum system, while the solution is fed to the prilling section by using the centrifugal pump.
Both the 70-72% wt. urea solution from the L.P decomposer and the urea melt from the vacuum separator can be directed to the urea solution tank, to face any emergency situation in both the vacuum and prilling sections.
UREA PURIFICATION at M.P. Decomposer:
This falling film-type heat exchanger is divided into three parts. The top separator is where the released flash gases are separated, the middle decomposer is where the carbamate decomposition will take place and the bottom holder is where the concentrated urea solution will be held. The decomposer tubes are fitted with ferrules having four tangentially distribution holes with equispaced. A packing bed of pall ring with 1.3m height and sieve plate for distribution is provided above the decomposer in the separator. To promote more decomposition it is necessary to that higher temperatures or to reduce to lower levels. M.P. Decomposed is operated at 17kgf/cm2 (g) and 156-158OC decomposed heat is being supplied from outside of the tube by M.P. steam and M.P. condensate.
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| Urea medium pressure section flow sheet |
The NH3 and CO2-rich gas leaving the top of the separator are sent to the vacuum pre-concentrator, where they are partially absorbed in the aqueous carbonate solution coming from the urea purification section at 4.5kgf/cm2. The absorption and condensation of gases are removed by evaporating water from the urea solution, thus allowing a considerable saving of L/P/ steam in the evaporation stages. Then the gases enter the M.P. condenser where the residue absorption and condensation of the heat of gases are removed by cooling water. In the condenser, CO2 is almost totally absorbed. The mixture of the M.P. condenser flows to the medium-pressure absorber.
M.P. Absorber:
It is the bubble cap tray type column that contains 4 numbers of trays having bubble cap risers fitted with bell caps. It contains a sparger pipe distributor at the bottom. The absorber performs CO2 absorption and NH3 rectification.
Reflux NH3 is drawn as part from the NH3 booster pump and fed to the absorber on the top tray and the aqueous ammonia solution which is coming from M.P.inerts washing tower is fed on the third tray by means of a centrifugal pump and tray washing provision is also there.
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| Image of a medium-pressure section of a urea production |
Ammonia Receiver:
It is a horizontal cylindrical vessel fitted with an ammonia recovery tower. The tower is installed on the receiver with a 3m packing bed height of pall rings and contains a distribution sieve tray above the packed bed. The receiver is located slightly tapper to the ground.
The ammonia which is received from battery limits contains 5PPM oil. It causes the foaming in the synthesis section, to avoid this foaming the oil should be separated from ammonia. In the above receiving tank, the oil will separate by density separation and come towards the down end of the tank. This oil will drain periodically.
- The function of this receiver tank is to receive and act as buffer storage for ammonia received from the battery limit.
- To receive ammonia recovered during plant shutdown.
- To receive ammonia condensed in the recovery system.
The inert gases containing residual ammonia leaving the receiver enter the ammonia recovery tower, where the pure ammonia coming from B.L. is fed at the top of the tower. In the tower, the inert gases containing NH3 and pure liquid NH3 are brought in contact with each other in countercurrent flow to recover some ammonia from inerts.
The inert gases containing residual ammonia are sent to the medium pressure falling film absorber(inert washing tower) where they meet in a counter-current water flow which absorbs gaseous ammonia. The heat of absorption is removed by cooling water. From the bottom of the absorber, the water-NH3 solution is recycled back to the medium-pressure absorber through a centrifugal pump. The tower operates at a pressure of 2.5 kgf/cm2 before entering the distillation tower the process condensate is preheated in the exchangers where the heating medium is the purified condensate flowing out of the tower.
Since the solution is contaminated by urea, after a first stripping in the upper part of the tower, it is pumped into the hydrolyser where the urea is decomposed by means of the stream at 37 kgf/cm2 , 370oC. Before entering the hydrolyser, the solution is preheated in the exchanger with the solution coming out from the hydrolyser.
The hydrolysis reaction of urea is the opposite of that occurring in the reactor.
NH2CONH2 + H2O ------->2NH3 + CO2 + Heat
Therefore urea decomposition is favoured by high temperature, low pressure and NH3 & CO2 deficiency. Also, a sufficiently long residence time has proved to be an important parameter. To eliminate NH3 and CO2 as far as possible before feeding the hydrolyser the wastewater coming out from the vacuum condensers is first stripped in the column. Moreover, a series of baffles in the hydrolyser provided a plug flow effect, thus avoiding back mixing. Also, the continuous removal of the hydrolysis reaction encourages the decomposition of urea.
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| Urea High Pressure section |
UREA PRILLING: PRILL TOWER:
It is a cylindrical vertical tower with a height of 100m, in which urea prilling takes place. It consists of a prill section at the top and a scrapper at the bottom. The Prill tower contains bottom lowers(windows) and top lowers(windows). In the prill section bucket (Tuttle type) is there. The tower is coated inside with an anti-corrosive paint.This is a natural draft Prill tower.
The molten urea leaving the second vacuum holder is sent to the prilling bucket utilizing a centrifugal pump. The bucket contains no. of holes in the wall. The urea coming out of the rotating bucket in the form of drops falls along with the prilling tower and arid encounters cold air flow which causes its solidification.
The molten urea drops coming from the bucket contains at a temperature of133oC
There will be heat transfer from drops to air, thus reducing the temperature of drops and increasing the temperature of the air. The heated air tries to go up, and due to that flow of air, some vacuum is created in the glass. The bottom air will try to cover the above vacuum thus creating the natural draft. The air will enter the prill tower through the bottom lower and be vented to the atmosphere through the top lower.
The heated air with a few parts of urea dust enters the scrubbing section where the urea dust will recover from the air by scrubbing air with DM water and the free urea dust is vented to the atmosphere.
The molten urea drops from the bucket fall down along the prilling tower. Due to the countercurrent flow of air, the temperature of molten urea will decrease and form a prill. The solid prills falling to the bottom of the prilling tower are fed to a belt conveyor by a rotary scraper. From here they are sent to the automatic weighing machine and to the urea storage, and bagging section.
DEDUSTING SYSTEM:
The urea melt coming out of the bucket in the form of droplets and while falling inside the prill tower encounters a countercurrent flow of cold air causing solidification. Hot air leaving the prill tower top consists of fine urea dust and free ammonia. To prevent pollution caused during the process of prilling. During system has been incorporated at the prill tower top. The system also recovers urea, which is recycled back into the system.
Operation:
In the dedusting tank, air travels in two chambers and a stainless steel partition wall hanging from the top separates these two chambers. The three recirculation pumps take suction deduction chamber with the help of scrubber nozzles with an angle of 10 deg and due to this spraying action, sir is sucked into the first chamber (annual scrubbing chamber). Urea gets dissolved while exhaust air travels from top to bottom in the annular scrubber chamber and then it enters the second chamber of the dedusting sump, where demister pads are provided at the top. A process condensate pump is sprayed on demister pads. By nozzles with 90deg.angle, and this system is operated by PLC (programmable logic control). Before taking DDS in line top louvres are be kept closed. Make-up liquid for the dedusting sump is done by a control valve and after attaining the required concentration the solution is drained into the urea lumps dissolving tank. The maximum allowable urea dust to atmospheric air is 3Omg/Nm2 of air. An energy-efficient process for urea synthesis must fulfil the following parameters.
• The high conversion efficiency of CO2 in a urea synthesis reactor, to minimize the heat required for decomposition of unconverted carbamate.(Achieved by optimization of parameters in the urea reactor).
• Efficient decomposition of carbamate and efficient separation of carbamate decomposition products(CO2 and NH3 ), as well as of excess ammonia.(Optimization of process parameters in the stripper and decomposer)
• Maximum recovery and efficient utilization of heat formed by absorption and reaction of NH3 and CO2 released from the stripper and decomposition. (Optimization of process parameters in the carbamate condenser, the MP decomposes and the MP absorber).
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| Urea formation and high-pressure section photo |
