Process and plant for the production of urea

Details Process and plant for the production of urea Process and plant for the production of urea

2002-05-16

2003-04-22

O'Sullivan, Peter (Department: 1621)

Organic compounds -- part of the class 532-570 series

Organic compounds

Amino nitrogen containing

C066S067000, C066S071000, C066S072000

Reexamination Certificate

active

06552224

ABSTRACT:

DESCRIPTION
1. Technical Field
In its general aspect the present invention relates to an urea production high efficiency, low investment synthesis loop based on a CO
2
stripping scheme with horizontal lay-out.
2. Prior Art
Most of the today new urea plants are based on the so called “stripping” technologies, namely CO
2
stripping by Stamicarbon and Toyo and ammonia “self stripping” by Snamprogetti.
All above technologies are characterised by a practically isobaric loop where the main part of unreacted residual CO
2
and ammonia contained in the urea reactor effluent are removed in a first carbamate decomposer (stripper), which vapours effluents are at least partially condensed, generating steam in a carbamate condenser. Reactor, stripper and carbamate condenser being the main components of the isobaric loop.
In all above schemes the urea solution effluent from the “stripper” is processed in a downstream urea recovery unit to totally remove the residual content of ammonia and CO
2
from urea water solution, forming a water carbamate solution which is recycled to the loop. According to the Snamprogetti and Toyo schemes, a separate stream of liquid ammonia, besides the carbamate solution, is also recycled to the loop.
All above schemes operate with high content of water in the reactor, since most of the water contained in the carbamate water solution recycled from the downstream the loop urea recovery unit is fed to the reaction space (reactor). Such high water content (H
2
O/CO
2
mole ratio at reactor inlet: 0.5-0.8) prevent to obtain high intrinsic CO
2
conversion yields which are essential to minimise the size of equipment and therefore investments and energy consumption for low operating costs. High intrinsic CO
2
conversion yields can be obtained only with low water content such as 0.1-0.2 H
2
O/CO2 mole ratio at reactor inlet.
In addition, in all above schemes (except the Snamprogetti one and improved Toyo CO
2
stripping processes) most of the inert gas introduced in the loop—such as for instance air, hydrogen, etc. the main part of which is contained in the CO
2
feed (air is generally added for its oxygen content which is used to passivate and protect against corrosion the loop equipment)—reaches the reactor with negative effect on CO
2
conversion efficiency.
Key features of the just mentioned technologies are
A) The CO
2
stripping processes (namely Stamicarbon and Toyo): (a
1
) gravity circulation of vapours and liquid streams in the loop, leading to a “vertical lay-out” to secure the above by gravity circulation; (a
2
) all CO
2
feed is sent to the stripper and then to the reactor through the carbamate condenser where vapours are only partially condensed in order to secure the reactor thermal balance; (a
3
) high water content (high H
2
O/CO
2
ratio) in the reactor since most of the water contained in the carbamate solution recycle coming from loop downstream urea recovery unit, is sent to the reactor through the loop carbamate condenser.
Modifications in the CO
2
stripping processes have been recently introduced by Stamicarbon (AIChE Ammonia Safety Symposium, Boston September 1996) and by Toyo (U.S. Pat. No. 5,936,122), both aiming to achieve horizontal lay-outs to reduce investments and for easier maintenance and operation.
Above modified schemes can be summarized as follow:
Modified Stamicarbon CO
2
stripping process (
FIG. 1
according to AIChE Boston 1996 publication):—use of an horizontal “pool reactor” where the functions of carbamate condensation with steam generation and reaction to form urea are combined in one single horizontal equipment, being the circulation of liquid and vapour streams from “pool reactor” to stripper still by gravity. The horizontal “pool reactor” is located at a higher level above the top of the stripper to secure the gravity circulation of the loop streams.
Modified Toyo CO
2
stripping process (
FIG. 2
according to U.S. Pat. No. 5,936,122): use of a vertical “pool condenser” where the vapours coming from CO
2
stripper are totally condensed with steam generation and beginning of urea formation. The carbamate solution from carbamate condenser is recycled to the reactor by the use of an ejector with liquid ammonia as driving fluid. Only part of the CO
2
feed is sent to the stripper since part of it has to be sent to the reactor to secure the reactor thermal balance.
B) The Snamprogetti process (
FIG. 3
according to UK patent 1188051): (b
1
) no external stripping agent is used in the stripper, being reactants, including CO
2
, sent directly to the reactor; (b
2
) vapours from the stripper are totally condensed in the loop carbamate condenser; (b
3
) carbamate solution from carbamate condenser recycled to the reactor by the use of an ejector with liquid ammonia used as driving fluid; (b
4
) high free ammonia residual content in the urea solution effluent from stripper; (b
5
) high water content (high H
2
O/CO
2
mole ratio) in the reactor since most of the water contained in the carbamate solution recycle coming from loop downstream urea recovery unit, is sent to the reactor through the loop carbamate condenser; (b
6
) “horizontal lay-out” where all loop main equipment (reactor stripper and carbamate condenser) are located at ground level.
The following draw-backs are present in the above described loop schemes:
The improved Stamicarbon CO
2
stripping process still has a high water content (high H
2
O/CO
2
mole ratio) in the reactor since most of the water contained in the carbamate solution recycle coming from the loop downstream urea recovery unit is sent to the reacting zone through the carbamate condensation zone of the “pool reactor”. That means that the reactor cannot have an intrinsic high efficiency. In addition, the horizontal pool reactor has to be installed at high elevation with high investment costs. High investment costs are also required due to the complexity of the “pool condenser”.
In the above schemes high content of inerts are present in the reactor, causing lower CO
2
conversion efficiency.
The modified Toyo CO
2
stripping process realises the horizontal lay-out but, similarly to the Snamprogetti self stripping process but does not solve the problem of achieving a reaction intrinsic high efficiency with a low H
2
O/CO
2
mole ratio. In addition, the partial use of the CO
2
feed to the stripper (a substantial part of the CO
2
feed has to be sent to the reactor) significantly reduces the stripping efficiency with higher residual content of unreacted ammonia and CO
2
being left in the stripper urea solution effluent to be processed with high investment and operating costs in the loop downstream urea recovery unit. In addition, high investment costs are required due to the complexity of the “pool condenser”.
The Snamprogetti process: although it realises an horizontal lay-out, it does not solve the problem of obtaining intrinsic high efficiency with low ammonia H
2
O/CO
2
mole ratio like in the other above mentioned processes. In addition, the Snamprogetti self stripping process, with no use of an external stripping agent, causes a higher residual content of free ammonia in the stripper urea solution effluent with higher investment and operating costs in the downstream urea recovery unit.
SUMMARY OF THE INVENTION
The aim of the present invention is to conceive and make available a process and plant related to an urea production high efficiency, low investment synthesis loop based on a CO
2
stripping scheme with horizontal lay-outwhich comprises the steps of:
performing a reaction between ammonia (NH
3
) and carbon dioxide (CO
2
) in a vertical reaction space (reactor R) to obtain a mixture comprising urea with minimum residual content of unreacted CO
2
(maximum CO
2
intrinsic conversion yield) in presence of minimum content of water (H
2
O) and possibly of High Efficiency Trays (HET);
subjecting said reaction mixture to a stripping treatment with the use of CO
2
feed as stripping agent in a counter current apparatus (Stripper) to remove most of the unreacted CO
2
and NH
3
and to obtain a first

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