IL27801A - Process for the manufacture of urea - Google Patents

Description

«»««·S5«i»t?«·-»«-»—S3—SS-»S3«»-_=«■·SB·· Process for tke manufacture of urea Ci-26385 This invention relates to the manufacture of urea from NK^ and CO^, and particularly relates to a method of control and analysis for use therewith. A known process for the manufacture of urea involves the reaction of NHg and_C02 under suitable pressure and temperature conditions to produce a urea solution containing ammonium carbamate, and whereafter the ammonium carbamate present in the solution, including the free NH^ and CO,,, is substantially removed from the said solution as an Nl^/CO^ gas mixture, the removal being effected by stripping the solution with for example CO^ with simultaneous heating and at an elevated pressure of about 50 atmps-pheres or more, whereupon the evolving gas mixture containing NH^ and CO^ is subjected again to the ammonium carbamate forming reaction, and the carbamate thus formed is converted to urea. This known process may be carried out for example in the installation schematically shown in Figure 1 of the accompanying drawings.

Referring to Figure 1 an ammonium carbamate reactor A consisting of a tube system 1 installed in a pressure vessel 12, and a urea formation autoclave B, are connected by a conduit 13. The carbamate reactor may also be installed inside the urea formation autoclave. The installation further comprises a stripping column 6 and a washing column 14.

Liquid NH_ is fed via conduit 2 and C0_ via conduit 3 and conduit 8 through stripping column 6 and conduit 9 under synthesis pressure to the tube system 1 of carbamate reactor A. In this tube system the ΝΗ^ and CO^ feed condenses to form an ammonium carbamate melt, the heat produced being removed by formation of steam of water present in pressure vessel 12. The conversion of NHQ and CO to ammonium carbamate is complete and proceeds at a fast rate. The carbamate melt formed, which also contains some urea produced by transformation of ammonium carbamate into urea and water, flows through conduit 13 to the urea formation autoclave, where depending on the temperature and pressure conditions and on the molar NHL/CO ratio present in the reaction mixture, for example from 45 to 60 % of the ammonium carbamate is converted into urea and water. In consequence, the urea solution discharged from the urea formation autoclave B via conduit 5 still contains much dissolved ΝΗ^ and CC>2, possibly partly in the form of ammonium carbamate. To remove this NHg and CC>2 the urea product solution is supplied under urea formation pressure to the top of the stripping column 6 in which it descends as an even film along the inner walls of the tubes in countercurrent relation to an ascending flow of CO^ which is continuously supplied via conduits 3 and 8.

The tubes in the stripping column 6 are heated by steam, and owing to the supply of heat and to the rectifying action of the COg gas introduced therein, a large proportion, for example 90 %, of the ammonia and carbon dioxide present in the solution is carried off in the gaseous phase. The outgoing gases are returned to the ammonium carbamate reactor via conduit 9, urea solution being withdrawn from the reactor base 6 via conduit 7. By expanding the said urea solution to a much lower pressure, the remainder of dissolved NH^/CO^ gas mixture is expelled therefrom. These gases are preferably absorbed in water and the ammonium carbamate solution so formed can be recycled as washing liquid to the top of washing column 14. The CO used as the starting material invariably contains inert gases and these are vented through conduit 4 into the base of washing column 14, where NH^ and CO^ entrained by the vent gases are absorbed in washing liquid. The solution thus formed is fed into the base of urea formation autoclave B via conduit 15.

Although the condensation of NHg and CC>2 to ammonium carbamate, and the conversion of ammonium carbamate into urea and water, can readily be effected at a pressure higher than that present in the stripping column 6 it is desirable to operate the various parts of the plant at the same in the stripping column must not be too high so that the urea solution from conduit 7 will contain an unacceptably high biuret content, since urea decomposes into biuret and ammonia at elevated temperatures, but the pressure and temperature conditions in the urea formation autoclave must be sufficiently high for the formation of urea from the ammonium carbamate to proceed at a sufficiently fast rate, since otherwise an excessively large reactor space will be needed. Thus while it is important that for any given pressure, the temperature of the carbamate melt must be close to a particular optimum value, so that the rate at which the ammonium carbamate is converted to urea and water will then also be very close to the optimum rate, it is also important that, at a given pressure, this formation be effected at the highest possible temperature level, since the reaction heat removed can then be used for producing steam of a higher temperature level, It has now been found that the desired temperature conditions can be established if the molar ratio of NH^ and CO^, either as such or combined in a form other than urea or biuret in the carbamate formation or in the urea formation zone, is controlled.

The invention consists of a continuous process for producing urea by reaction of ΝΗ^ and CO^ at elevated pressure and temperature conditions to form an ammonium carbamate melt which is subsequently transformed into a urea solution which contains ammonium' carbamate and/or dissolved JGL. and C02, and stripping the said urea solution with H^ and/or CO^ and recycling the stripped ΝΗ^ and CO^ to the ammonium carbamate-formation zone or to the urea formation zone, 'after which, if so desired, the resulting urea solution is expanded to a low pressure to effect further removal of still dissolved NIL- and CO,,, which are thereafter absorbed in water and the solution so obtained is recycled to the carbamate or urea formation zone, and the urea solution freed of dissolved Η^ and C0o is removed for further processing, wherein in the said process the feed of and C02 present in the liquid phase as such or as NH^ and CO^ bonded otherwise than in the form of urea or biuret, is maintained at a value of from 2:1 to 3:1, and the said ratio in the urea formation zone is maintained at a value of from 3:1 to 6; 1.

The higher the temperature level and the larger the amount of solvent that is present in the liquid phase, the higher the molar MHg/CO ratio may be.

As hereinbefore mentioned, the molar ratio relates not only to all free NH^ and CO^ dissolved in solution, but to NH^ ana C02 bonded otherwise than to urea or biuret. For example, to maintain an optimum temperature of 185 °C in the urea formation reactor' at an operational pressure of from 110 to 150 atmospheres with an available amount of solvent (urea + water) equalling 59 % of the total liquid phase, the molar ratio of dissolved ΝΗ^ to CO^ is controlled to about 4.8:1, while in the formation zone of ammonium carbamate from ΝΗ^ and 2 at a pressure. of about 125 atmospheres in the absence of water and/or urea, the said molar ratio is o controlled to about 2.35:1 if the optimum temperature of 162 C is to be maintained.

In order that the urea-production process can be kept under control, it is desirable therefore to check during the process the NH^/CO^ molar ratios of the liquid phases formed in the process, and to keep them at the desired values according to the invention by controlling the addition of NH„ or CO into the system. Even minor deviations from the desired molar ratio will result in large temperature deviations and as analysis of the liquid phases is a laborious operation, controlling the process on the basis of an analysis of the liquid phases is not easy to realize and it has further been found as a preferred feature of the invention that process control can be very readily performed by adjustment in response to analysis of the gas phase that is in contact with the liquid, for in-stance by gas chromatography, which can be performed very rapidly. phase analysis, as a slight deviation from the desired ratio in the liquid phase correlates with a much larger deviation in the gas phase composition. It is surprising that notwithstanding the difference to be maintained between the molar NH /CO ratios in the liquid phases of the process, no such difference occurs in the gas phases in contact with said liquid phases, and that, provided the molar NHg/COg ratio in the gas phase remains between 1:1 and 6:1, the desired molar ratios in the liquid phases will establish themselves automatically. Thus a molar NHg/COg ratio of 1: 1 to 6: 1 present in the gas phase issuing from the top of the urea formation reactor, means that the molar NH^/CO,, ratio in the urea solution will be approximately between 3:1 and 6:1. Similarly if the molar ratio in the vent gases from the ammonium carbamate reactor is between 1:1 and 6:1, the molar NH^/COg ratio in the carbamate melt will be between 2:1 and 3:1.

The process can therefore be controlled by keeping the molar NH /CO ratios in the gas phases within the range 1:1 to 6:1. To achieve optimum temperature conditions in the process, the molar ^H^/CO^ ratio in the gas phases should be kept within narrower limits, particularly between 1.5:1 and 3.5:1.

One way of performing the invention is hereinafter described and illustrated schematically in Figure 2 of the accompanying drawing, which is similar to Figure 1, except that the inert gases are vented via a washing column connected to the carbamate reactor, and the low-pressure stage where the urea solution discharged from the base of the stripping column is further freed of dissolved ammonia and carbon dioxide with formation of a carbamate solution which is then recycled, is also indicated. Referring to Figure 2 the urea solution formed in urea formation autoclave B flows via conduit 5 into stripping column 6 where it is stripped under high pressure with CO fed to the base of stripper column 6 via conduit 3. autoclave via conduit 2. In the urea formation autoclave the feed gases bubble upwards through the liquid phase, being partly dissolved on their way up, the nondissolved portion being fed into tube system 1 of carbamate reactor A via conduit 4a. The said carbamate reactor comprises a pressure vessel 12 containing a tube system 1 which is surrounded by water. Owing to the strongly exothermic character of the ammonium carbamate forming reaction, in carbamate reactor A, this water is transformed into steam. The ammonium carbamate melt thus formed is transferred via conduit 13 to urea formation autoclave B, wherein gradually urea is formed with simultaneous production of water. To maintain the desired slug flow in the urea formation autoclave as much as possible and to prevent it from being disturbed by the gases introduced therein the autoclave is divided into a number of superposed compartments by means of sieve plates 23.' The gases that have not been condensed to ammonium carbamate in the ammonium carbamate reactor, i.e. inert gases, ΝΗ^ and CO^, are fed to a washing column 14 via conduit 4b, and the solution thus formed in the said column is returned to the base of the urea formation column via conduits 15 and 13. The gases are vented through the top of washing column 14. The urea solution withdrawn from the base of stripping column 7 is expanded to, for example 4 atmospheres absolute, and indirectly heated with steam in heater 17 so that dissolved NHg and CO^ are expelled, after which a liquid-gas separation is effected in separator 18. The urea solution formed is discharged via conduit 24 to be further processed to urea crystals or prills, while the gases are fed to a condensor 19 to be condensed therein to a carbamate solution by means of water and/or ammonia supplied via conduit 20. The heat of condensation is carried away by a cooling coil in the condensor. The liquid produced in the condensor is transferred via conduit 21, pump 22 and conduit 15 to the top of washing column 14 to be used as washing liquid.

For the purpose of analysis and control accordin to the in column B, and a sampling pipe Q in conduit 4b for analyzing the gas phase from the carbamate reactor. The sampling pipes P and Q are connected with gas chromatographic apparatuses (not shown) . Pipe R is provided with a volume meter to control the vent gas volume, vent gases consisting mainly . of inert gases which are continuously supplied mainly by the starting product CO . The supply of either ammonia or carbon dioxide is now controlled on the basis of the gas phase analysis. If the composition of the gas phase is such that the molar NH /CO in said phase is outside the range 1:1 to 6:1, the feed of ΝΗ^ or C02 has to be increased or decreased to achieve that the desired molar ratio will establish again in the liquid phases. Practice has shown that a satisfactory control can be realized by sampling only the gas issuing from the ammonium carbamate reactor (sampling point Q). If in another embodiment the ammonium carbamate reactor is arranged before the urea formation autoclave, a satisfactory control can be realized by sampling only the gas issuing from the urea formation autoclave. In the process illustrated the urea solution discharged from the urea formation autoclave is subjected to a stripping treatment with CO under a high pressure. The stripping treatment can also be carried out with NHg or with a mixture of C02 and W^; although this is less attractive economically, but the measures according to the invention remain applicable.

Claims (1)

1. I HAVIS¾ SOW particularly deecribed and ascertained the nature of our said invention and in a aanner the s me is to be performed, we declare that what ve claim Is: A continuous process for producing urea by reaction of NH^ anti CO at elevated pressure and temperature conditions to form an ammonium carbamate melt which is subsequently transformed into a urea solution which contains ammonium carbamate and/or dissolved NH^ and C02> and stripping the said urea solution with NH„ and/or CO and recycling' the stripped NHL and CO to the ammonium carbamate-formation zone or to the urea formation zone, after which, if so desired, the resulting urea solution is expanded to a low pressure to effect further removal of still dissolved NH^ and CO^, which are thereafter absorbed in water and the solution so obtained is recycled to the carbamate or urea formation zone, and the urea solution freed of dissolved NH and CO , is removed for further processing, wherein in the said process the feed of one or both pf the reaction components NH¾ and CO to the ammonium carbamate-formation zone is controlled so that the MI /CO molar ratio of the NHg and C02 present in the liquid phase as such or as NH^ and C02 bonded otherwise than in the form of urea of biuret, is maintained at a value of from 2;1 to 3:1, and the said ratio in the urea formation zone is maintained at a value of from 3:1 to 6:1. A process according to Claim 1, in which the said molar NH.-/C02 ratios in the liquid phases are adjusted by controlling the molar NH^/CO,., ratio in one or both of the gas phase(s) that are in contact with the liquid phase(s), the molar TSS^/QO^ ratio in said gas phases being kept within the range 1; 1 to 6.

1. A process accordin to Claim 2, in which the molar NHg/COg ratio in the gas phases is kept within the range from 1.5:1 to 3.5:1. A process TTIHI HIUIH vw ' wa--Fnv the manufacture of urea substantially as