Process for combined production of ammonia and urea

Details Process for combined production of ammonia and urea Process for combined production of ammonia and urea

2001-01-16

2004-02-24

Johnson, Jerry D. (Department: 1764)

Chemical apparatus and process disinfecting, deodorizing, preser

Chemical reactor

Ammonia synthesizer

C422S186220, C422S186220, C564S066000, C564S067000, C564S070000, C564S073000, C423S352000

Reexamination Certificate

active

06696026

ABSTRACT:

FIELD OF APPLICATION
The present invention relates to a process for the combined production of ammonia and urea in a plant comprising an ammonia synthesis reactor, a urea synthesis reactor and a urea recovery section.
In the following of the description and subsequent claims, with the term: “process for the combined production of ammonia and urea”, it is intended to mean a single process that integrates the ammonia production process with the urea production process.
In other words, according to this technology, urea is produced—at least in part—by causing ammonia obtained in a synthesis reactor to react with carbon dioxide contained in a synthesis raw gas flow comprising among other things hydrogen and nitrogen coming, for instance, from a reforming section. The synthesis raw gas flow, free from carbon dioxide, is thereafter sent to the ammonia synthesis reactor.
Processes of this kind allow to eliminate, or in any case, to reduce to a remarkable extent, the decarbonation section of the synthesis raw gas flow, the separation section of the ammonia produced in the corresponding synthesis reactor, and the carbon dioxide compression section. Besides, energy consumption and investment costs resulting from a single integrated system may be substantially lower than those resulting from two separate processes for ammonia and for urea.
The need to provide an integrated process is particularly felt in all cases where all or in any case most of the ammonia is converted into urea by causing it to react with carbon dioxide obtained as a by-product in the preparation of the synthesis gas.
In the following of the description and subsequent claims, with the term: “urea recovery section”, it is intended to mean the part of the plant downstream of the urea synthesis reactor, comprising generally one or two carbamate decomposers at medium pressure (about 18 bar a), respectively at medium and low pressure (about 4 bar a) and related carbamate condensers, whose function is to separate the produced urea from the reaction mixture coming from the corresponding synthesis reactor, allowing in this way to obtain a 60% to 75% concentrated urea solution.
The invention also relates to a plant for implementing the aforesaid process, as well as to a method for the simultaneous modernization of an ammonia production plant and a urea production plant.
In the following of the description and subsequent claims, with the term: “simultaneous modernization”, it is intended to mean a modernization that concerns—at the same time—both an existing plant for ammonia synthesis and an existing plant for urea synthesis, for the purposes of their integration.
The integration between the production processes of ammonia and urea, wherein the carbon dioxide contained in the synthesis raw gas and the synthesis ammonia are caused to react, producing a carbamate aqueous solution to be sent to the urea synthesis reactor, involves on the one hand a simplification of the plant—with special reference to the ammonia decarbonation and separation sections and to the CO
2
compression section—but on the other hand, a marked overloading of the sections correlated with the urea production, essentially due to the lack of formation heat and to the excessive molar ratio H
2
O/CO
2
in the urea synthesis reactor, with an ensuing low conversion yield and high energy consumption.
As a consequence, in the field of combined production of ammonia and urea there is increasingly felt the need of providing processes allowing to increase urea conversion yield, in a simple way, with low operating and investment costs.
PRIOR ART
In order to meet the above requirement, several processes for combined production of ammonia and urea have been proposed in the field.
For instance, in U.S. Pat. Nos. 3,303,215 and 3,310,376, it is disclosed a process for the combined production according to the prior art wherein suitably purified liquid ammonia is fed to a urea synthesis reactor where ammonia is caused to react with carbon dioxide comprised in a synthesis raw gas including also hydrogen and nitrogen.
In the urea synthesis reactor, ammonia and carbon dioxide react forming ammonia carbamate which, in its turn, is transformed into urea by dehydration.
A first drawback of this process lies in that the high development of heat produced during carbamate production and the presence of inert gases (hydrogen and nitrogen) which reduce the partial pressure of ammonia and carbon dioxide makes it necessary to operate—in the urea synthesis reactor—at high pressures to keep reactants in a liquid phase, with ensuing high energy consumption and operating costs.
Moreover, because of the introduction in the urea synthesis reactor of a high amount of water—for instance in the form of carbamate in aqueous solution—to facilitate the absorption of carbon dioxide in the ammonia solution and the subsequent reaction into carbamate, the H
2
O/CO
2
molar ratio in such synthesis reactor is relatively high and the conversion yield is unsatisfactory.
A further drawback lies in the structural and operating complexity of the urea synthesis reactor necessary for implementing the above described process, which must include a special unit for the separation of inert gases (hydrogen and nitrogen) from carbon dioxide and from ammonia in the vapour phase.
According to this process of the prior art, there is also provided a step of condensation and separation of the ammonia produced by unreacted gases, typical of ammonia production processes, which is rather demanding from the economic and energy consumption viewpoints.
In U.S. Pat. Nos. 3,349,126, 4,012,443, 4,013,718, and 4,320,103, it is disclosed another type of process according to the prior art, which comprises a separate section for carbon dioxide absorption and carbamate synthesis.
According to this process, ammonia coming from the corresponding synthesis reactor is separated from unreacted gases—generally by absorption with water in a special absorption section—and sent to the carbamate synthesis section, wherein its reacts with the carbon dioxide contained in the synthesis raw gas flow coming from a reforming section, forming ammonia carbamate, which is sent to the urea synthesis reactor.
Also in this case, carbon dioxide absorption and the subsequent reaction into carbamate takes place in a milieu rich in water, which is then sent together with the carbamate to the urea synthesis reactor.
In addition, carbamate formation heat that is released during carbon dioxide absorption with the ammonia solution causes a strong evaporation of the latter, which involves the need of an additional recovery of ammonia at the outlet of the carbamate synthesis section, with ensuing problems of excessive dilution of the carbamate. At the same time, as the urea conversion reactor lacks carbamate formation heat, the operating conditions in said reactor become more difficult.
Otherwise said, according to this process of the prior art, producing carbamate outside the urea synthesis reactor involves not only the loss of the related formation heat, but also requires an addition of water, which is in contrast with the subsequent dehydration to urea, and therefore does not allow to obtain satisfactory conversion yields.
In conclusion, the processes for combined production of ammonia and urea according to prior art, besides requiring very complex plants for their implementation, and involving high investment and operating costs, as well as high energy consumption, do not allow in any case to obtain a high urea conversion yield due to the excessive H
2
O/CO
2
molar ratio present in the corresponding synthesis reactor.
Because of these drawbacks, the above processes have not found till now a concrete application, in spite of the increasingly felt requirement in the field.
SUMMARY OF THE INVENTION
The problem underlying the present invention is to conceive a process for combined production of ammonia and urea, such as to allow on the one hand to obtain a high urea conversion yield, and to be, on the other hand, of simple implementation, with low operating and inves

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