Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Including external recycle loop
Reexamination Certificate
1997-08-25
2001-09-11
Knode, Marian C. (Department: 1764)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Including external recycle loop
C422S186220, C564S066000, C564S067000, C564S069000, C564S070000, C564S073000, C159S047200
Reexamination Certificate
active
06287525
ABSTRACT:
TECHNICAL FIELD
In its general aspect the present invention relates to a method for modernizing a plant for urea production of the type comprising:
a urea synthesis reactor;
a stripping equipment for subjecting a first reaction mixture leaving said reactor to a treatment of partial decomposition of the carbamate and partial separation of the free ammonia in aqueous solution present in said first mixture;
means for condensing at least partially the vapors leaving said stripping equipment and recycling a first carbamate solution to said reactor;
a recovery section for separating the urea produced in said reactor from a second aqueous carbamate solution.
The present invention also relates to a plant for producing urea obtainable by the modernization method of the present invention.
As is known, in the field of urea production the need is ever more growing of plants having greater capacity and operating flexibility on the one hand and, on the other hand, requiring ever smaller investment and operating costs, in particular in energy terms.
BACKGROUND ART
To this end, there have been proposed and implemented in the art a series of urea production processes essentially based on conversion reactions with differentiated yields in reaction spaces placed in parallel as described e.g. in European patent application EP-A-0 479 103.
In these processes, the total urea production is distributed between a main reaction space designed to cover the greater part of the required production capacity (generally from 60% to 80% thereof) and operating under low-yield conditions, and an auxiliary reaction space—so-called “once through”—operating under high-yield conditions and designed to bring production capacity to the final amount required.
Within the framework of the above mentioned processes, the reaction mixture from the main reaction space is subjected to a preliminary purification treatment to obtain a concentrated solution of carbamate recycled to the reaction space and a urea solution which is further processed and purified—together with the reaction mixture leaving the auxiliary reaction space—in a separation and recovery section.
From this latter section, a dilute carbamate solution and a substantially pure urea solution are obtained.
In accordance with the constant teaching of the prior art and for the purpose of increasing as much as possible the conversion yield in the auxiliary reaction space, this space is fed exclusively with substantially pure carbon dioxide and ammonia while the dilute carbamate solution is recycled exclusively to the main reaction space, to which is accordingly delegated the duty of converting into urea all the carbamate obtained in the purification sections located downstream thereof.
Although essentially meeting the above mentioned need, these processes exhibit both a weighted average yield limited by the rather poor yield in the main reactor and a series of plant limitations linked to the need of sending high recycle flowrates to the main reactor and provide an auxiliary reactor of large size and high cost.
The large quantity of water in the recycle solution to the main reaction space, furthermore, poses—despite the high conversion yield of the auxiliary reaction space—an upper limit to the average conversion yield achievable by the plant, which in turn limits the energy savings achievable in terms of high-pressure steam consumption reduction.
As disclosed in European patent applications EP 0 544 056 and EP 0 624 571, also known in the art are processes for the industrial synthesis of urea carried out by brand new plants wherein highly pure ammonia and carbon dioxide are reacted in a first reaction space, while a solution of recycled carbamate coming from an urea recovery section is sent to a second reaction space in which they react with an unreacted portion of ammonia and carbon dioxide.
Neither EP 0 544 056 nor EP 0 624 571, however, afford the problem of modernizing pre-existing urea production plants including a stripping equipment for treating the ureacontaining reaction mixture with the aim of enhancing production capacity while having at the same time a high weighted average yield.
DISCLOSURE OF INVENTION
The technical problem underlying the present invention is accordingly to conceive and make available a method for modernizing a plant for urea production which allows to overcome the drawbacks of the above mentioned prior art.
In accordance with a first embodiment of the present invention, this problem is solved by a method of the above mentioned type which is characterized in that it comprises the additional steps of:
providing a second urea synthesis reactor upstream of said stripping equipment;
connecting said second reactor with said stripping equipment;
providing means for recycling to said second reactor the second carbamate solution obtained in the recovery section.
In accordance with the present invention, it was surprisingly found that it is possible to further reduce steam consumption and further simplify the plant delegated to carry out the aforementioned process, by producing urea under high-yield conditions in the main reaction space (the major part in production terms) and low yield in the auxiliary reactor (minor part in production terms).
In accordance with the present invention, the above mentioned high-yield conversion conditions in the main reaction space may be achieved by feeding thereto the pure reagents and only the concentrated carbamate solution coming from the partial purification operations (partial decomposition of the carbamate, partial separation of the free ammonia and condensation) of the reaction mixture leaving the main reaction space.
In sharp contrast to the constant teaching of the prior art, the dilute carbamate solution coming from the urea separation and recovery section located downstream of both the main and auxiliary reaction spaces is recycled only and exclusively to the auxiliary reaction space.
The latter will then operate both in low-yield conditions because of the high quantity of water in the recycle solution and in low production terms.
In an alternative embodiment, the above-identified problem is solved by a method of the above mentioned type which is characterized in that it comprises the additional steps of:
providing a second urea synthesis reactor upstream of said stripping equipment;
providing means for recycling to said second reactor the second carbamate solution obtained in the recovery section;
connecting said second reactor with distillation equipment for subjecting a second reaction mixture leaving said second reactor to a treatment of partial decomposition of the carbamate and partial separation of the free ammonia in aqueous solution present in said second mixture;
providing means for recycling the vapors leaving said distillation equipment to said second reactor;
connecting said distillation equipment with the urea recovery section.
MODES FOR CARRYING OUT THE INVENTION
In accordance with a first embodiment of the present invention, the synthesis reaction in the main reaction space is carried out in accordance with the following process parameters;
NH
3
/CO
2
mol
2.8-3.4, preferably 3.0
H
2
O/CO
2
mol
0.1-0.25, preferably 0.18
Reaction temperature
180-195° C.; preferably 190° C.
Pressure
140-155 bar; preferably 145 bar
CO
2
conversion yield
69-71%.
Advantageously, by operating with an ammonia/carbon dioxide molar ratio below 4 and preferably about 3, a reduction of the volume of the high-yield reaction space and of the heat requirement for preheating the reagents to the reaction temperature may be achieved.
The synthesis reaction in the auxiliary reaction space is carried out in accordance with the following process parameters:
NH
3
/CO
2
mol
4.2-4.6; preferably 4.5
H
2
O/CO
2
mol
1.2-1.6; preferably 1.5
Reaction temperature
180-192° C.; preferably 190° C.
Pressure
140-155 bar; preferably 145 bar
CO
2
conversion yield
56-60%.
In accordance with this embodiment of the present invention, the reaction mixture leaving the auxiliary reaction space is subjected to a partial purification tr
Doroshenk Alexa A.
Knode Marian C.
Sughrue Mion Zinn Macpeak & Seas, PLLC
Urea Casale
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