1. GENERAL

The following operating instructions have been prepared for the initial start-up and operation of the urea plant with a capacity of 3250 MTPD. Plant with pool condenser and evaporation section for integration with a Hydro Fertilizer Technology granulation section.

This manual has been compiled to assist those charged with the responsibility and supervision of the initial start-up and subsequent operation of the 3250 metric tons per day urea plant for Petrochemical Industries Development Management Company located at Bandar Assaluyeh, Iran.

Its primary objective is to provide flow descriptions and discussions of the processes involved and relating operating principles, together with suggested guideline procedures for the initial commissioning, start-up, normal shutdown and emergency shutdown of the plant.

It may also serve as a basis for the preparation of detailed operating instructions.

Such detailed instructions or plant operating manual depend on local conditions and should include instructions issued by equipment manufacturers (these are beyond Stamicarbon's scope). Under no circumstances should operations deviate from safety regulations and practices followed throughout the industry.

Operating conditions and techniques will evolve from actual operating experience and it is not possible to anticipate and present herein all potential circumstances, which may confront the operator during the commissioning, start-up, normal operation and shutdown of the unit. Consequently, this operating manual must be recognised as a guide and that conditions stipulated are not rigid standards, unless specifically noted as such.

Numerical values given in this manual are design figures indicating the ranges within which actual values may vary during normal operation and may need to be changed as a result of experience gained in the plant. Under no circumstances should these figures be regarded as guaranteed performance figures.

Special attention should be paid to housekeeping practices on the Urea Unit and the minimising of spillage/leakage of Urea from sample points etc. Regular clear-ups should be carried-out by picking-up any spillage/leakage and not by washing down.

Any and all information pertinent to the Stamicarbon proprietary process included in this Process Manual is disclosed in confidence for use in accordance with the agreement made between Chiyoda Corporation and STAMICARBON BV and should be treated accordingly.

Given the above, no claims for damages or losses in connection with this manual are accepted.

2. PRODUCT AND RAW MATERIALS

The raw materials for the preparation of urea are ammonia and carbon dioxide.

Carbon dioxide is obtained as a by-product from the ammonia plant.

Some applications of urea are: soil and leaf fertilisation, melamine production, urea-formaldehyde resins, nutrient for ruminant animals and miscellaneous other applications.

Urea product.

Urea solution at the exit of the Evaporation Section in the Urea Plant shall be about 96 wt. % urea (including biuret) solution which will be sent to the Granulation Plant for the production of Urea Granules.

The quality of the product urea solution from the plant shall be as follows:

Urea melt containing:

Urea plus Biuret                                                                    minimum    95.8 wt. %

Biuret                                                                                     maximum   0.70 wt. %

Free ammonia (indicator = methyl red)                                 maximum   650 ppm

Appearance and properties of urea (NH₂-CO-NH₂)

Urea is a white crystal, which is not inflammable, not conductive and has the following physical properties:

density (solid at 20 °C)                                                         : 1,335 kg/m³

melting point                                                                         : 132.6 °C

specific heat (melt)                                                                : 126 J/mol/°C

melting heat (melt point)                                                       : 13.6 kJ/mol

mol weight                                                                             :  60.056

                                                                                                        NH₂

structural formula                                                                  : C =  O

                                                                                                        NH₂

Appearance and properties of ammonia (NH₃)

Ammonia is under pressure, a liquified gas, which is recognizable at the smell.

Ammonia gas is lighter than air, and can be explosive and inflammable under certain circumstances.

Ammonia is soluble in water in an exothermic reaction.

Ammonia has the following physical properties:

density (liquid, 20 kg/cm², 25 °C)                                         : 603 kg/m³

melting point                                                                         : -78 °C

boiling point                                                                          : -33 °C

ignition temperature                                                              : 630 °C

lower explosion limit (in air)                                                  : 15 vol. % NH₃

upper explosion limit (in air)                                                  : 29 vol. % NH₃

mol. weight                                                                            : 17.03

                                                                                                               H

structural formula                                                                  : N            H

                                                                                                               H

Appearance and properties of carbon dioxide (CO₂).

Carbon dioxide is a colourless, odourless gas, which is not explosive and not inflamma­ble.

Carbon dioxide is heavier than air and has the following physical properties:

density (gas, 1 kg/cm², 25 °C)                                               : 1.8 kg/m³

triple point                                                                             : -57 °C and 5.1 atm.

critical point                                                                           : 31 °C and 72.8 atm.

mol. weight                                                                            : 44.01

                                                                                                               O

Structural formula                                                                   : C

                                                                                                               O

3. CONCISE PROCESS DESCRIPTION

Urea is produced by reacting liquid ammonia and gaseous carbon dioxide at about 170 ‑ 185 °C and 135 ‑ 145 barA according to the following reactions:

                        2 NH₃ + CO₂              <=====>        NH₂COONH₄                 (1)

                        NH₂COONH₄            <=====>        NH₂CONH₂ + H₂O        (2)

In the first reaction, carbon dioxide and ammonia are converted into ammonium carbamate. This reaction is fast and exothermic. In the second reaction, which is slow and endothermic, the ammonium carbamate dehydrates to produce urea and water.

3.1 Ammonia and Carbon dioxide compression

Liquid ammonia is supplied from the ammonia plant to the high pressure ammonia pump and compressed to about 171 barA. The pump discharges ammonia into the high pressure pool condenser via the high pressure ammonia ejector, where ammonia is the driving force for carbamate flowing over from the high pressure scrubber.

Carbon dioxide from the ammonia plant is supplied, together with a small amount of air, to the carbon dioxide compressor before it is compressed to about 150 barA.

A hydrogen converter is integrated in the carbon dioxide compressor. In this converter the hydrogen, present in the carbon dioxide, is removed by catalytic combustion. A portion of the supplied air is used for this catalytic combustion while the remainder is being used to passivate the equipment of the synthesis section and so prevent corrosion. The dehydrogenated carbon dioxide is introduced into the bottom part of the high pressure stripper.

The two feed stocks, ammonia and carbon dioxide, are fed to the synthesis section at a molar ratio of 2:1.

3.2 Synthesis

Ammonia with carbamate from the high pressure scrubber and carbon dioxide with off gas from the high pressure stripper is introduced into the high pressure pool condenser, which is a liquid submerged tube heat exchanger. The greater part of the gas will condense and convert with ammonia into carbamate.

The dehydration of ammonium carbamate into urea and water takes place in the pool condenser and subsequently in the urea reactor. The urea reactor effluent is distributed over the tubes of the high pressure stripper, which is a falling film type shell and tube heat exchanger. Here, the reactor effluent is contacted counter currently with carbon dioxide, causing the partial ammonia pressure to decrease and the carbamate to decompose. The heat, required for this purpose, is supplied by passing saturated middle pressure steam around the tubes of the high pressure stripper. This steam pressure is controlled by a pressure control valve so that the liquid, leaving the high pressure stripper, contains about 8.6 % by weight of ammonia.

The urea solution from the high pressure stripper, flows to the low pressure recirculation section whilst the high pressure stripper off gases are sent to the pool condenser which is a special design U-tube type heat exchanger. In the pool condenser condensation of strip gases takes place through the principle of pool condensation, i.e. the gases are dispersed into a pool of liquid, where the heat of condensation is being dissipated by submerged heat exchanger tubes. This heat of condensation is used to generate low pressure steam of 4.5 barA. This steam is used for heating and desorption as well as for the vacuum ejector.

The steam pressure at the tube side of the high pressure pool condenser is controlled by a pressure control valve in the steam discharge line of the LP steam drums. A change in this pressure will change the steam condensate temperature and hence the temperature difference between the shell side and the tube side. The steam drum pressure is set to such a value that the synthesis pressure is about 145 barA.

The pool of liquid in the pool condenser allows for a considerable amount of urea formation to take place here. The formed urea, non converted carbamate, excess ammonia and some non condensed ammonia and carbon dioxide are subsequently introduced into the bottom of the urea reactor where further conversion of carbamate into urea takes place. The urea reactor volume allows sufficient residence time for the reaction to approach equilibrium. The heat, required for the conversion and for heating the solution in the urea reactor, is supplied by additional condensation of ammonia and carbon dioxide.

The urea reactor contains five (5) high efficiency trays to ensure that the flow of liquid through the reactor approaches piston flow. Moreover the trays are designed such that negative effects (such as back-mixing, by-passing and stagnant zones in the reactor) are avoided.

The reactor effluent goes through the down comer to the high pressure stripper. The inert, introduced with the carbon dioxide and part of the unreacted ammonia and carbon dioxide, goes overhead to the high pressure scrubber which contains a shell and tube heat exchanger in the lower part and a packed bed in the upper part.

In the lower part of the high pressure scrubber the bulk of the ammonia and carbon dioxide are condensed, the heat of condensation being dissipated into tempered cooling water. In the upper part the gases, leaving the bottom section, are contacted counter currently with the carbamate solution which is formed in the low pressure recirculation section. The gases, substantially consisting of nitrogen and oxygen and containing only small amounts of ammonia and carbon dioxide, are vented to the granulation stack via an MP absorber operating at 7.8 barA.

The carbamate solution from the high pressure scrubber flows to the high pressure ammonia ejector. The ammonia feed pressure is such as to induce sufficient head in the high pressure ammonia ejector to convey the carbamate solution from the high pressure scrubber to the high pressure pool condenser.

3.3 Low pressure recirculation section

In this section essentially all of the small amounts of non converted ammonia and carbon dioxide are recovered from the urea / carbamate solution, leaving the bottom of the high pressure stripper. This solution is expanded to about 4 barA. As a result a portion of the carbamate, left in the solution, decomposes and evaporates. The remaining liquid is divided onto a bed of Pall rings in the rectifying column. The urea/ carbamate solution is sent from the bottom of the rectifying column to the recirculation heater where its temperature is raised to about 135 °C in order to decompose the remaining carbamate. The heat required is supplied by low pressure steam. In the separator (i.e. the bottom part of the rectifying column) the gas phase is separated from the liquid phase. The gases are sent to the rectifying column where they are cooled by the colder urea/ carbamate solution. This causes a portion of the water vapour contained in the gases to condense.

The gases leaving the rectifying column are introduced into the bottom part of the low pressure carbamate condenser where they are condensed almost completely. The heat of condensation is dissipated into tempered cooling water. Process condensate is also supplied to the low pressure carbamate condenser together with the condensed overhead vapours from the first desorber in order to control the water concentration in the carbamate solution.

The optimum ammonia / carbon dioxide ratio allows the water concentration to be as low as 31 % by weight. The pressure in the low pressure carbamate condenser is controlled at about 3.3 barA.

From the level tank of the low pressure carbamate condenser, the carbamate solution flows to the high pressure carbamate pump where its pressure is raised to about 154 barA and from where the carbamate solution is carried to the high pressure scrubber.

The urea solution, leaving the bottom of the rectifying column, flows to the flash separator via a level control valve. Due to the adiabatic flash to about atmospheric pressure, a portion of the water evaporates and some ammonia, carbon dioxide and inert are liberated. These vapours are partly condensed in the flash separator condenser and the remaining ammonia and carbon dioxide are scrubbed from the inert in the LP absorber by means of circulating process condensate and steam condensate. Condensate from the flash separator condenser is recycled to the reflux condenser.

3.4 Pre‑evaporation and evaporation

The solution from the flash separator is sent to the preevaporator.

A portion of the water in the solution is evaporated so as to increase the urea concentration from about 70 to 78 % by weight. The heat of evaporation is taken from the low pressure steam system. The recycled urea solution from the granulation section is introduced into the pre-evaporation feed line. Finally, the urea solution is sent to the urea solution tank.

The urea solution is pumped from the urea solution tank to the evaporator, where it is concentrated to about 96 % by weight. In the separator for the evaporator, the outlet from the evaporator is separated into a gas phase and a liquid phase. The vapour, leaving this separator, is condensed in the evaporator condenser together with the vapours from the vacuum flash separator. The urea solution from the separator for the evaporator flows to the suction side of the urea melt pump and is sent to the granulation section after mixing with urea formaldehyde solution. The condensate, leaving the evaporator condenser is sent to the ammonia water tank via a barometric leg.

3.5 Process condensate treatment

Process condensate from the evaporator condenser, containing ammonia, carbon dioxide and urea, is collected in the ammonia water tank and used as absorbent in the MP absorber and the LP absorber. Next the process condensate is pumped from the ammonia water tank to the first desorber via a desorber heat exchanger.

In the first desorber the bulk of the ammonia and carbon dioxide is stripped off by means of the overhead vapours of the second desorber and hydrolyser. The bottom effluent of this first desorber is pumped via a hydrolyser heat exchanger, where this condensate is heated from approximately 140 °C to 197 °C, to the top of the hydrolyser column. In the hydrolyser, the urea is decomposed into ammonia and carbon dioxide while being heated by means of live medium pressure steam, to about 195 °C. To obtain very small urea concentrations in the hydrolyser effluent (< 1 ppm wt), the process condensate is counter currently contacted with the live steam.

On leaving the hydrolyser the process condensate, containing traces of urea, goes via the hydrolyser heat exchanger to the second desorber. The overhead vapours of the hydrolyser being sent to the first desorber. After cooling the hydrolyser effluent in the hydrolyser heat exchanger to about 148 °C, this condensate is fed to the top of the second desorber. Here, the remaining ammonia and carbon dioxide is stripped off by means of live low pressure steam. The process condensate, leaving the second desorber, is cooled in the desorber heat exchanger and subsequently in the waste water cooler. It contains very small amounts of urea and ammonia (< 1 ppm wt ammonia and 1 ppm wt urea) and to be sent to the polisher unit in the ammonia plant for re-use. The overhead gases from the first desorber are condensed in the reflux condenser and are transferred as a carbamate solution to the low pressure carbamate condenser. The non condensed vapours are sent to the LP absorber.

4. BLOCK DIAGRAM UREA PROCESS