Integrated Process for Urea and Melamine Production

Abstract

In an integrated process for urea and melamine production, urea is produced in a urea plant (10) comprising a high pressure urea synthesis section (11) from which an aqueous solution comprising urea, ammonium carbamate and ammonia is obtained and a urea recovery section (21) operating at low pressure, and melamine is produced in a melamine plant (40) wherein off-gases resulting as by-products of the melamine synthesis are discharged from said plant at a medium pressure and recycled to the high-pressure urea synthesis section (11).

Description

TECHNICAL FIELD[0001]The present invention relates to a process for the integrated production of urea and melamine.[0002]In particular, the present invention concerns a process of the above-identified type, wherein urea is produced in a urea plant comprising a high pressure urea synthesis section and a urea recovery section and wherein the off-gases resulting as by-products of the melamine synthesis are recycled to said high pressure urea synthesis section.[0003]In the following description and subsequent claims, with the expression “high pressure urea synthesis section” it is intended to mean a section operated at a pressure of at least about 120 bar, generally between 130-260 bar.[0004]More in particular, the process according to the present invention is of the type wherein the off-gases to be recycled have a pressure of at least 2 bar, generally between 2 and 30 bar.[0005]The present invention is also concerned with an integrated plant for carrying out such a process.[0006]As is ...

AI Technical Summary

Benefits of technology

[0015]The technical problem underlying the present invention is to provide an integrated process for urea and melamine production having functional features such as to fully overcome the drawbacks set forth with respect to the prior art and in which a higher production capacity for the urea plant can be achieved ensuring at the same time a high conversion yield of carbon dioxide to urea in a efficient way and with low energy consumption.

[0048]It follows that there is a significant increase in the conversion yield of the urea synthesis section, as well as of the overall yield of the H.P. Loop, to the great advantage of the efficiency and the energy consumption of the plant intended to carry out the process according to the present invention.

[0049]In addition, advantageously, the condensation of the off-gases is performed by exploiting the low amount of water already contained in the recycle carbamate aqueous solution obtained in the urea recovery section of the urea plant and which is anyway recycled to the high pressure urea synthesis section. Therefore, contrary to the processes of the prior art—no additional amount of water is added to the off-gasses when recycling them from the melamine plant to the urea plant. It follows that, thanks to the invention, a more concentrated carbamate solution is recycled to the high pressure urea synthesis section of the urea plant with the consequence that the urea conversion yield is advantageously increased and the energy consumption required for recovering urea and recycling the unconverted reagents to the synthesis section are advantageously substantially decreased.

Problems solved by technology

In fact, in the first case, high energy consumption are required to separate NH3 and CO2 from the off-gas liquid solution in the waste water treatment section.

Moreover, since the pressure in such a section is often much lower than the pressure of the off-gases discharged from the melamine plant, the off-gases have to be expanded before their condensation and recycle to the waste water treatment section of the urea plant, and thus there is also an energy waste in term of pressure loss.

Since water is a by-product of the urea synthesis, its presence in the reactant feed is detrimental for the CO2 conversion into urea.

The urea conversion yield is thus negatively affected by the water contained in the recycled off-gas liquid solution with an ensuing increase in the energy consumption required for recovering urea from the urea solution leaving the synthesis section and for recycling the unconverted reagents back to the synthesis section.

It follows that the implementation of urea / melamine integrated processes of the prior art, in addition to the above mentioned drawbacks, is particularly burdensome from the economic, functional and energy consumption points of view.

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