8/12/2019 Urea Synthesis Process

1/5

Urea synthesis and high pressure recovery

Urea is produced by synthesis from liquid ammonia and gaseous carbon dioxide.

In the reactor R-1 the ammonia and carbon dioxide react to form ammonium carbamate, a

portion of which dehydrates to form urea and water. The reactions are as follows

!"#$ % &'! "#! - &''"#(

"#! - &''"#( )**********+ "#! - &' - "#! % #!'

t synthesis conditions T * 1 /&, 0 * 1 2g3cm! abs4 the first reactionoccurs rapidly and goes to completion. The second reaction occurs slowly and determines

the reactor 5olume.

The fraction of ammonium carbamate that dehydrates to form urea is determined by

the ratios of the 5arious reactants, the operating temperature and the residence time in thereactor. The mole ratio of ammonia to carbon dioxide is around $.( to $.6.

The mole ratio of water to carbon dioxide is around /. to 1. The liquid ammonia

from battery limits is boosted by 0-6 37 and collected in the ammonia recei5er 8-(.The ammonia before entering 8-( passes to the column &- where it encounters the gas

coming from the "#$ recei5er. #ere it acts as cooling agent in order to reduce the

amount of "#$ gas entering the 9.0 ammonia absorber :-11. In this way we obtain a

reduction in the :-; cooling water duty, ta2ing ad5antage of the low temperature of theammonia feed.

-1 where the carbamate from 8-1 is compressed to the

synthesis pressure. The liquid mixture of "#$ and carbamate enters the reactor where itreacts with compressed carbon dioxide.

The carbon dioxide from the urea plant battery limits, normally at about 1./6 tapressure and about !3 /& temperature, enters the reciprocating, electric motor dri5en

compressor ?-1 and lea5es it at a pressure of about 16/ ta. small quantity of air is

added to the carbon dioxide at the compressor suction in order to passi5ate the stainlesssteel surfaces, thus protecting them from corrosion due either to the raw material or the

reaction products.

8/12/2019 Urea Synthesis Process

2/5

The reaction products, lea5ing the reactor, pass to the steam heated falling film stripper

:-1, which operates at essentially the same pressure as the reactor. The mixture is heated

as it flows down the tubes of the falling exchanger.

The carbon dioxide content of the solution is reduced by the stripping action of

the excess ammonia as it boils out of the solution.

#eat supplied by the condensation of !( ta steam on the shell side of the

stripper decomposes the carbamate. The o5erhead gases from the stripper and thereco5ered solution from the medium pressure absorber &-1 after pressure ele5ation in

#.0 carbonate pumps 0-!37 are routed to the high pressure carbamate condenser :-

where the total mixture, except for a few inert, is condensed and recycled to the reactor

by means of e=ector >-1.

&ondensing the gases at high pressure and temperature permits the production

of steam at (. ta in the high pressure carbamate condenser.

The incondensable gases from the top of the carbonate separator 8-1 consists

mainly of inert gas, with a small quantity of "#$ and &'!. These are passed underpressure control to the medium pressure decomposer :-!. The controller is split range,

with a facility to pass gas to 7.@ on high pressure.

Urea Purification

Urea purification ta2es place in two stages at decreasing pressure as follows

1st stage at 1ta pressure 9edium 0ressure Aection4

!nd stage at (. ta pressure >ow 0ressure Aection4

The exchangers where urea purification occurs are called decomposers, because

in these apparatus the residual carbamate decomposition ta2es place

Medium Pressure Section

The solution with a low residual &'! content, lea5ing the bottom of the stripper

is expanded to the pressure of 1 ta and enters the medium pressure decomposer :-!falling film type4.

This is di5ided into two parts-

Top portion 8-! where the released flash gases are remo5ed before the solution

enters the tube bundle.

8/12/2019 Urea Synthesis Process

3/5

@ecomposition portion where the residual carbamate is decomposed and the

required heat is supplied by means of !( ta steam condensate, from the shellside stripper and from a6 ta steam in the lower part of the decomposer.

The "#$ and &'! rich gases lea5ing the separator 8-! are sent to the medium

pressure condenser :-B, where the &'! is totally absorbed and the "#$ partiallyabsorbed in an aqueous carbonate solution coming from the low pressure reco5ery

section. The absorption heat is remo5ed by a cooling water circuit, designed to maintain

the water inlet temperature abo5e (/ /&. This is achie5ed by re-circulating a controlledportion of the heated cooling water by the pumps 0-(37.

The mixture from :-B flows to the medium pressure absorber &-1, where the gaseousphase coming from the solution enters the rectification section.

This column has bubble-cap type trays and performs &'! absorption and "#$

rectification. The tray is fed by pure reflux ammonia, which eliminates residual &'! and#!' contained in the inert gases.

Reflux "#$ is drawn from the ammonia recei5er and sent to the column by meansof the centrifugal pump 0- 37. The bottom solution is re-cycled by the pump 0-!37

to the synthesis reco5ery section. n inert gas stream saturated with "#$ plus a &'!residue !/-1// ppm4 passes from the top of the rectification section to the ammonia

condenser :-; where it is mainly condensed.

The inert gases, saturated with ammonia, lea5ing the recei5er, enter the ammoniareco5ery tower &-, where an additional amount of ammonia is condensed.

The inert gases, with residue ammonia content, are sent to the medium pressure falling

film absorber :-11, where they meet a countercurrent water flow which absorbs gaseousammoniaC the absorption heat is remo5ed by cooling water.

8/12/2019 Urea Synthesis Process

4/5

8/12/2019 Urea Synthesis Process

5/5

The solid prills are collected in the bottom of the 0rilling tower by means of acone >-1 and discharged onto an existing belt con5eyor.