Submitted By-: UTKARSH SHARMA 09/CHE/15

DCM Shriram Consolidated Ltd. (DSCL) is a company with a core sector business portfolio comprising: Agri-Business (Urea Fertilisers, Sugar, Farm inputs marketing such as DAP) Pesticides, Seeds, Agri retailing Hariyali Kisan Bazaar Plastics (PVC and PVC compounds) Chemicals (Chlor-Alkali) DSCL Building Products (Fenesta door and window profiles) Business portfolio is of primarily two types i.e. Energy intensive products, Agri products (inputs as well as outputs), and services. It has manufacturing facilities at Kota (Rajasthan), Bharuch (Gujarat), and Ajbapur & Rupapur (Uttar Pradesh).

Commissioned in February 1969 with an installed capacity of 700 MTPD Based on Stamicarbons Total Recycle Process technology. Construction of the plant was done by Chiyoda Chemical Engineering and Construction Co. now known as Chiyoda Corporation. Installation of an additional 120 TPD ammonia synthesis loop in the ammonia plant and by making minor process modifications in the Urea plant has led to a production peak level of 1250-1300 MTPD Whole Urea Plant is totally dependent on the following plants: DM Plant Power Plant Ammonia Plant

Urea (identified 1773), the first organic compound prepared by inorganic synthesis (1828 Wohler) NH3 + HCNO CO(NH2 )2 Urea has been considered as slow release fertilizer since it undergoes two transportation Hydrolysis: CO(NH2 )2 + H2O 2NH3 + CO2 Nitrification: NH3 Nitrite or Nitrate (Microbes, moist and warm soil) India is the second largest producer and consumer of urea in the world. More than 90% of world production of urea is destined for use as a nitrogen-release fertilizer. Other uses include raw material for manufacturing of plastics specifically urea formaldehyde resin and urea-melamine-formaldehyde. The most common impurity of synthetic urea is biuret, which impairs plant growth.

CHEMISTRY OF UREA SYNTHESIS

When NH3 and CO2 are brought together under the synthesis conditions of 200 atm the following reaction occur:

2NH3 (liquid) + CO2 (gas) NH2COONH4 +38.06 Kcal/mol (carbamate) NH2COONH4 NH2CONH2 + H2O -522 Kcal/mol (urea) The first reaction is spontaneous and complete, provided the heat liberated thereby is discharged, while the second is a slowly involving equilibrium reaction, due to which only part of the carbamate is converted into urea. At higher N/C ratio in the original synthesis mixture, the equilibrium reaction 2 shift to the right by Le Chatelier's principle.

The Stamicarbon Process for the production of Urea using Ammonia and Carbon dioxide as raw material is realized as total recycle process in total recycling of NH3 (l) and CO2 (g) and carbamate solutions. The plant includes the following sections: NH3 and CO2 compression Synthesis 1st Stage Recirculation 2nd Stage Recirculation Crystallization, Drying and Remelting Prilling and Solid Handling

For synthesis reaction of Urea to happen, we need a finite amount of temperature (190 C) and pressure (200 Kg/cm2). For attaining this pressure, following procedures are followed:

CO2 is sent for compression at Ammonia Plant end only - using 3 reciprocating compressors to be compressed to about 200 Kg/cm2 after the mixing of CO2 with anti corrosion air. Liquid ammonia at 23 atm g pressure & a temperature of 40 C maximum goes for filtration in Ammonia Filter Tank and then to Ammonia Buffer Tank. Ammonia Reflux Pumps are then used to transfer the ammonia to the high-pressure Ammonia Feed Pumps . Pressure of the discharged ammonia from Ammonia Feed Pumps is about 210 Kg/cm2. Pressurized ammonia now heads up to Ammonia Pre-Heaters which are shell & tube type heat exchangers. After gain of sufficient heat to maintain desired reactor temperature (required ammonia temperature ~ 85 C), ammonia is transported to mixer.

SYNTHESIS SECTION

The feed components(pressurized NH3 and compressed CO2) are poured in the orifice mixer to mix thoroughly which also have concentrated Carbamate coming from the recirculation section. About 90% of the CO2 feed is spontaneously converted into Carbamate. The heat evolved during this reaction is being absorbed by the feed components themselves. 2 NH3 +CO2 H2N-CO2-NH4 + heat Total pressure inside the mixer is maintained by the use of let down valve PRC-502 at 200 Kg/cm2 which in turn results in the temperature of about 172-178 C. H2N-CO2-NH4 H2N-CO-NH2 heat

There is no retention time in the mixer and it is followed by the two reactors, A and B, placed in series. Under design conditions NH3/CO2 ratio is 4 and H2O/CO2 ratio is equivalent to around 0.64, the optimum temperature in the reactor remains at around 190 C. The overall heat of reaction inside the reactor is not sufficient enough to reach this required optimum temperature. Therefore, some extra heat as sensible heat has been put in the ammonia feed so as to control the optimum temperature. Inside the reactor the conversion of about 64% of total CO2 feed into urea has been observed; mentioned figure is being called the CO2 conversion per pass.

RECIRCULATION SECTION

RECIRCULATION

Effluent from the two reactors consisting about 32% urea, 13% CO2, 37% NH3, 18% H2O (by weight) and some of the unconverted Carbamate enters the first re-circulation section. This part of the plant practically separates all the carbamate, the excess of ammonia and part of water from the remaining urea solution. This separation takes place in two stages, the first stage operating at around 20 Kg/cm2 and the second at 3 Kg/cm2.

First stage The reactor effluent is throttled through the letdown valve to about 20 Kg/cm2 pressures. The temperature of solution reduces due to evaporation of large amount of excess ammonia and part of the available carbamate dissociating into ammonia and carbon dioxide. These vapour escape from the top of first stage rectifying to MPCC (Middle Pressure Carbamate Condenser).

The remaining solution is distributed over rings of first stage rectifying column.. The solution temperature increases meanwhile to about 134 C. The liquid from the first stage rectifying column is then fed to the first stage heaters and thus heated to about 160 C. The heat input causes an additional vaporization of ammonia, carbon dioxide and water from the solution, the gas phase is separated from the liquid phase in first stage separator and is returned to the bottom of first stage rectifying column. The outgoing solution is transported to the secondary recirculation system containing 58% urea, 4% CO2, 10% NH3, 28% H2O by weight.

The vapours leaving the first stage rectifying column are introduced in MPCC in which about 90% of the gaseous CO2 is converted into the carbamate. The non-absorbed gases will rise through the condenser, leave at the top and are together with carbamate solution, introduced into level tank. Carbamate solution produced has a temperature of about 100 C at the existing pressure and approximate composition by weight of 43% NH3, 40% CO2, 12% H2O and 5% urea. The solidification temperature of this is about 78 C. In the level tank the gas phase is separated from the liquid phase.

The concentrated carbamate solution is recycled to the orifice mixer by centrifugal carbamate pump(C.C.P.) or first stage carbamate pumps. The level in level tank is controlled by regulating the speed of carbamate pumps.

Heat evolved due to formation of carbamate and condensation of water vapour is removed by the conditioned cooling water flowing through the hairpin tubes of the condenser.The non-condensed NH3 and CO2 gases leave the top of level tank and are piped to the bottom of the washing column.

RECIRCULATION SECTION

Second Stage The solution leaving the bottom of the first stage recirculation separator passes through level control valve, LIC-502 which gives the pressure drop from 20 Kg/cm2 to 3Kg/cm2. Because of pressure drop, ammonia and CO2 will gasify during the expansion. The remaining solution is distributed over the stainless steel rings of the second stage-rectifying column.

In the second stage re-circulation heater almost whole of CO2 and NH3 still being present in the solution are removed by heating to about 150 C. The gas phase is separated from the liquid phase in second stage recirculation separator. The solution of about 70% urea, 0.3% CO2, 1% NH3, 28.5% H2O, 0.4% biuret (by weight) flows to flash drum.

The gas phase from the separator is returned to the bottom of second stage rectifying column in which, as described before . The major proportions of the ammonia and CO2 in the vapours leaving second stage rectifying column are condensed in second stage carbamate condensers. The solution and the non-condensed gases leave at the top of carbamate condenser are separated in separator/level tank. The liquid phase is transported by means of second stage carbamate pump to washing cooling column.

The pressure of the second stage re-circulation is controlled by FRC-505 in the off gas line from carbamate level tank, through which the inert are going to the vent stack.

RECIRCULATION SECTION

Washing Column

In the washing column the remaining water vapour from 1st stage get absorbed and about 95% of the gaseous CO2 is converted into the carbamate. The non-absorbed gases will rise through the column. In the top part of the column which is filled with raschig ring, a spray desk is fitted, onto which ammonical water is fed from the scrubber. Thus gaseous CO2 and water vapour are completely removed from the rising gases, so that a pure NH3 gas, with a temperature of about 50 C leaves the washing column and carried to Ammonia buffer tank. In the bottom of washing column a weak carbamate solution from 2nd stage carbamate condenser is introduced by means of 2nd stage carbamate pump.

Together with the gaseous CO2, condensed water vapour and NH3, a concentrated carbamate solution is produced. The solidification temperature of this solution is about 70 C.

Concentrated carbamte solution is recycled back to the first stage carbamate condenser(MPCC) so that it can be recycled to the orifice mixer by centrifugal carbamate pump or first stage carbamate pumps.The temperature is controlled by means of ammonia reflux. This ammonia reflux, being vaporized absorbs the heat of reaction.

Crystallization & Drying

The solution from second stage re-circulation separator enters flash drum, operating under the vacuum. Due to low pressure drop, a considerable amount of water vapour and some ammonia will escape from the solution, decreasing the solution temperature. The resulting concentration of urea in the solution being fed into urea buffer tank is about 70-75%. Urea solution pumps are installed to transport the urea solution via urea solution filter to the crystallization section

The incoming urea solution, part of which is going to the centrifuges for rinsing, is mixed in MLT with the liquor effluent from the centrifuges.

From MLT the urea solution is pumped to the evaporator. This flow is controlled, as per process requirement in the evaporator; the solution is concentrated under a pressure of 300-mm Hg-atm abs. Pressure is controlled through PRC-507 .

The concentrated solution of about 115 C is mixed in the MLT with sieve bends solution from partial separation and the crystal strain from the centrifuges. The liquor from the MLT is pumped to crystallisers in which a crystal suspension is in circulation.In crystallisers, evaporation of the solvent take place under a pressure of 95mm Hg abs and super saturation occurs in the surface region. The crystal suspension is extracted from the bottom and pumped by slurry pumps to sieve bends with a fixed capacity of about 90m3/hr.

Crystallization & Drying

Each by means of the sieve bends crystal slurry concentration is increased to about 45% by weight. From the sieve bend the slurry is introduced into No.1 divider box which transfer it to the centrifuges.

The mother liquor separated from the slurry by means of sieve bends flows into MLT.Biuret concentration in the mother liquor relates to a maximum concentration depending on the crystallisation temperature.

The centrifuges are designed such that the crystals may be washed with a part of the fresh urea feed, in order to lower the final biuret content.

Crystallization & Drying

The washed urea crystals separated off in the centrifuges fall into screw feeding system for the pneumatic drying system. The crystals still contain maximum 1.5% of water and are dried in the pneumatic drying system to final moisture content of 0.3%.

For pneumatic conveying of the crystals, air is taken in via a filter and a blower for drying purposes. This air is heated in a steam heater to a temperature of about 120 C.To add additional heat to the system, the pneumatic tube is provided with steam jackets, on which a constant steam pressure is maintained.

Remelting & Prilling

Remelting

The separation of crystals from the air is carried out in cyclone. The temperature is about 80 C. The product separated in the cyclones goes to the melting equipment i.e. remelter.The product is fed in to remelter where it is suspended in to a circulating stream of molten urea. The slurry is pumped from remelter to remelter heater where the temperature is kept around 138 C. The melt is returned to the remelter, out of which it over flows to a second heater. This heater is installed to melt the crystals still present in the remelter over flow. The retention time in remelting equipment is as short as possible in order to diminish biuret formation during remelting as much as possible.

Prilling and Solid Handling:

The concentrated urea melt from the remelter system is fed to the bucket, which distributes the urea melt in fine droplets over the prilling tower diameter. During their fall in tower which has a height of around 80 mt, the urea solidifies. The heat of solidification is carried off by air sucked in through openings at bottom of the tower with the help of the induced draft fans mounted on the top of the tower.The urea prill accumulating on the bottom of the tower are scrapped into a slit by scrapper. Conveyor loading chute transports the product into prill cooler where prills temperature is brought down and under size material is removed. The product is transported via the product conveyor to the storage.

Remelting & Prilling