IL44844A - Process for reducing the biuret content in urea - Google Patents

Description

riR'HRa 011»an riVian nnnanV πο»σ PROCESS FOR REDUCING THE BIURET CONTENT IN UREA The present invention relates to a process for treating a urea melt or solution containing biuret -or a— ^^^5 metal complex; thereof to reduce the amount of biuret -or L_ oomplex thereof..

Heretofore a serious problem encountered in the manufacture of urea has been the biuret content in the end product . Biuret is an impurity of urea and it is a condensation product thereof, produced according to the following reaction: 2C0(NH ) ► NH CONH-CONH +NH The formation of biuret is a direct function of the temperature and the retention time; therefore, thes circumstances are avoided in all processes used. However,, in order to obtain prilled urea, the most commonly form found in the market, it is necessary to use a sufficiently concentrated and fluid urea solution that allows the formation of a prill with such a moisture content that it prevents the cracking of the stored and/ or packed product. In order to obtain a solution capabl of being prilled, it is necessary to operate at high temperatures. Besides, the reprocessed end product is subjected once more to the necessary heating for prilling same, thus the biuret content increases again.

Urea is mainly used as an agricultural fertilizer, and in one of its uses is dispersed on the foliage of growing plants; and in such an event the biuret is extremely noxious since it possesses a very active phyto toxic action. Therefore, in order to produce a foliar grade urea, that is, so that its solutions be dispers-able in the leaves of the plants, it is necessary that same has a maximum biuret content of 0.2%.

In addition to the above mentioned use, urea has a great variety of different usages in which the requisite is a low biuret content. One of these usages is, for example, the employment of urea in the production of synthetic resins and plastics . Likewise, a small amount of urea is used in pharmaceutical products and, logically, for this usage there is a serious restriction in the biuret content as well as other impurities . On the other hand urea is used in solutions for textile treating and finishing., In this event, the biuret contained in the urea, together with the formaldehyde necessary for the textile treating, causes turbidity in the solutions and consequently it destroys the brilliancy of the textile finish, this being totally undesirable .

The usual process of manufacturing urea consists of contacting ammonia and carbon dioxide at high pressures and temperatures in a closed system. In this manner initially ammonia and carbon dioxide are exo-thermically combined forming ammonium carbamate which, under the reaction conditions, is partially converted into urea and water. Subsequently, the resulting urea, ammonium carbamate, ammonia and water are treated by different processes in order to recover the ammonia and the carbon dioxide. Finally, the water is evaporated to obtain a relatively pure concentrated urea solution, which is subjected to a suitable process in order to obtain the desired final form of urea, for example, prilled urea . In. this last step and according to the aforementioned comments made in respect to the heating, it is not possible to avoid an undesirable percentage of biuret content in the end product .

Various solutions have been proposed to avoid the formation of biuret. For example, a proposal was to mount the evaporator unit, which is generally used, on the top of the prilling tower so as to immediately transfer the urea melt to the prilling operation „ This process is disadvantageous since it requires special supports fo the evaporator, as well as extra lines for the steam and condensate „ Another process as a solution to the problem of the biuret formation, consisted of ammoniation of the solutions containing biuret to' split the biuret mole-cula thus forming again urea „ This process is riot convenient because it is an expensive process, sirice' it requires high pressures during a substantial period and, therefore, the appropriate equipment results uneconomical .

Another process for solving the problem of biuret formation in'iirea: consisted of a partial crystallization of urea saturated solutions in order to provide relatively pure urea crystals, leavin most of the biuret in the mother liquors, arid of a further reprocessing of"the mother "liquors through the reactor. However, this process has riot yielded satisfactory results due to the fact" that the' recirculation of the mother liquors reduces the capacity of the urea plant and besides, the occlusion- of biuret in the final urea crystals is not prevented » Another process for the production of urea having a low biuret content consisted of evaporating ammonia, carbon dioxide and a certain amount of water and passing them rapidly . through an externally heated tube.

Then, the liquid and gas mixture evolving from the' tube was processed to separate the gas from the liquid; and subsequently the liquid was passed through a packed tower in countercurrent flow to a stream of hot air, thereby providing a final drying of the urea . This final drying could be changed by the steps of crystallisation and separation of crystals in a centrifuge. This process did not provide adequate results due to the several disadvantages it involves. That is, there was a premature crystallisation resulting in plugging the apparatus; the decomposition products including biuret in the urea substantially increased due to the long time required by the high temperature evaporation; a high water content was found in the resulting product, therefore a subsequent drying step was required involving a consequent decomposition; there were losses of the desired product due to decomposition and the resulting granules of end product were either larger or smaller than the required size.

Finally, some other processes for solving the problem of the biuret contents have been proposed, for example, one of them involves treating solid urea with a liquid solvent containing acetone to extract biuret from the solid urea and others are only variations in operating pressures and temperatures or applying different crystallization conditions . Nevertheless these methods have not provided a suitable and efficient solution to the problem of biuret content in urea .

It is an object of the invention to provide an improved process for reducing the biuret content in urea .

According to the present invention we provide a process for treating a urea melt or urea solution containing 1.0 to 99*9 of urea and an amount of biuret itself on a metal comple-y thereof , to reduce the amount /which procesg / of biuret nr rnmplnix thereoff comprxses contacting the urea melt or solution with an ion-exchanging resin.

For instance, an aqueous urea solution may be treated with the ion-exchanging resin. The process may be conducted at a temperature of 0°C to 200 °C.

With reference to the accompanying drawings, Figure 1 is a schematic sectional view illustrating an embodiment of an ion-exchange system employable with the present invention, and Figure 2 is a schematic view illustrating another embodiment of an ion-exchange system employable with the present invention.

The type of the ion-exchanger is not critical since any type work. However, ion-exchangers capable of regeneration and operating at high temperatures (200°C) are preferred; and of these ion-exchanging resins of a strong base anionic type are especially preferred.

Once the ion-exchange operation has been carried out and the biuret has been separated from the urea, the biuret may be displaced from the resin by any stronger anion, for example, bicarbonates , carbonates, chlorides, nitrates, sulphates and hydroxides. Nevertheless the preferred ones are the hydroxides because they are the only anions that totally displace the biuret, leaving the resin in a condition capable of again retaining the biuret .

The aforementioned ion-exchange can be carried out through a column containing a suitably supported ion-exchanging resin.

In one of the embodiments of the present invention (see Figure 1 of the accompanying drawings) a. column contains a strongly basic resin 12, for example, of the styrene type. The resin 12 is supported by means of graded sand 13 providing an adequate support, for avoiding loss of the resin 12 during the ion-exchange operation. The column 1 1 is provided with a surrounding jacket 14 through which low pressure steam passes for maintaining the required temperature, in order to avoid solidification of urea .

The volume of column 1 1 must be such as to include the graded sand 13 and the ion-exchanging resin 12 and to allow a clearance corresponding to 5% of the volume of resin 12 to minimise loss of same during the processing operations .

In this type of ion-exchanger, the feeding of the urea melt or urea solution to be treated is carried out through an upper line 1ζ> including its corresponding distributor (not shown), in order to immediately contact the urea to be treated with the ion-exchanging resin 12 . Once the urea is processed it discharges from column 1 1 through a lower line 16 .

The regeneration and rinse of the ion-exchanging resin 12 are carried out likewise through lines 1 5 and 16 . However, since a backwash is necessary, lines 17 are included for introducing the backwashxng liquid at the lower end of the column 1 1 and removing same through the upper end .

The process for reducing the biuret content in a urea solution in this type of apparatus can be carried out in the following manner: Water necessary for keeping the resin 12 fully covered, is partly drained until it is just above the level of the resin 12 , so as to prevent dilution of the urea solution and to avoid modification in the ion-exchange operation by air bubbles due to excess drainage. The urea solution is fed through line 1 5 to column 1 1 , thereby displacing the water retained in the ion-exchanging resin 12 which water lows through line 16 . Consequently, the liquid in the effluent at this time is essentially water, therefore it is eliminated through line 16 until the said liquid is a urea solution with concentration of 4 to ζ% urea, then the outflowing urea solution, which is a biuret free urea solution, is recovered as an end product. This operation is maintained until, by means of a qualitative test, biuret is detected in the effluent; the test is considered positive when the biuret content exceeds 0:1%.

The presence of biuret in the effluent indicates that the ion-exchanging resin 12 is exhausted. Therefore, a urea-recovering operation, as well as the operations of backwash, regeneration and rinse of the ion-exchanging resin 12 are necessary.

The, urea-recovering operation comprises feeding, water through line 15 to column 1 1 to displace the urea solution retained in the ion-exchanging resin 12 , The liquid, flowing through line 1 6 is a solution having a high urea content; therefore it can be reprocessed. When the concentration of the urea solution in the effluent flowing through line 16 drops to 3 or 4-% the said solution is considered as not being useful and this operation is concluded.

After the above operation, the backwash is performed. That is, the water flow is inverted by passing it through lower line 16 and flowing out through upper line 17 · When the backwash has been established, the rate of flow is increased for eliminating suspended solids and for raising the ion-exchanging resin 12 ; this situation is kept for a certain interval of time in order to appropriately clean the resin 12 .

After the backwash operation, the regeneration is initiated which, in this case will be carried out using caustic soda having a concentration of 15 to 45% . For this operation, the remaining water from the backwash operation, that necessarily entirely covers the resin 12 , is drained until reaching just above the level of the said resin 12 ; then the, feeding of caustic soda is started through line 15 . In this manner, the water contained in the ion-exchanging resin 12 is displaced, flowing out through line 16 . The first amount of outflowing liquid is eliminated since this represents the last backwash water, until the outflowing liquid reaches a concentration of about 3% of caustic soda . Once this concentration is reached, the effluent liquid is recovered, maintaining the regeneration operation according to the type of ion-exchanging resin 12 embedded in column 11 .

According to the above, it can be considered that the ion-exchanging resin 12 has been regenerated. But, it is necessary to recover the caustic soda still contained in the said ion-exchanging resin 12 and this is carried out by a water displacement that is continued until the effluent through line 16 has a caustic soda concentration of 3%· At this time, the effluent liquid is eliminated and the introduction of water is continued with an increased flowing rate. This operation is considered as a rinse operation and the same is ended when the effluent contains 500 p.p.m. of caustic soda . In this manner, the ion-exchanging resin 12 is ready for a new operating cycle .

As can be seen from the above, with this type of ion-exchanger, the process represents a batch process since the recovery, backwash, regeneration and rinse operations are required. If a continuous operation is desired, it will be necessary to have duplicate ion-exchanging columns so that, while in one of them the urea solution is processed in the other the several different operations can be performed.

Another possible embodiment of the process of the present invention can be carried out (see Figure 2 ) by means of a continuous ion-exchange system. In this system three columns 2 1 , 22 and 23 are located in such a manner that column 21 is connected at its lower end to column 22 by means of line 2 * including, its valve 25 > besides the said column 22.is connected at its lower end to column 23 by means of line 26, also including its valve 27 ; and that column 23 is connected at its lower end to column 2 1 through line 28 including valve 29) with which a continuous recycling system is obtained .

Moreover, each column 21 , 22 arid 23 comprises at its lower end, feeding inlets 30* 3 1 and.32 through which, by means of pumps 33 * 34 and 35* "the corresponding liquids to be fed to each column 21 , 22 and 23 are introduced. Likewise, at its upper ends , , columns 21 , 22 and 23 include outlet lines 36, 37 a d 38 respective ly, through which the liquids fed to each column 21 , 22 and 23 respectively ar flown out.

In this type of apparatus we find, for example, that column 2 1 contains a certain amount of an ion-exchanging resin 3 9* which can be of the same type as in the case of the batch operation previously described that is, a strongly basic styrene resin. Then, through inlet 30 a urea solution containing, biuret is pumped for contacting the ion-exchanging resin 3 9 whereby, through line 36, a biuret free urea is obtained. The pumping operation is thus continued until it is considered that a sufficient amount of resin 39> although not all, has been exhausted. This interval of time will be taken as a standard for further operating cycles .

After the selected period has elapsed, a certain amount of exhausted resin 39 is transferred from column 21 to column 22 ; allowin at the same time for. a corresponding amount of a regenerated resin 40, embedded in column 23 * "to be transferred to column 21 for keeping the total volume of resin in the column 21 constant.

Regeneration of exhausted resin 39 in column 22 is carried out using caustic soda . The caustic soda is pumped through inlet 3 1 and flown out through outlet line 37 keeping a flow during a suitable period for regenerating exhausted resin 39 .

After the resin 39 has been regenerated, the same is passed through line 26 to column 23 wherein it is subjected to a rinse operation. This rinse operation is carried out introducing water to the column 23 > through inlet 32 and by using pump 35 and flowing out of same the said water through line 38 . The water flow is continued during the time necessary for displacing almost all the caustic soda retained in the resin 39. In this manner, a washed and regenerated resin 40 is provided, that is ready for re-use in column 21 , as previously indicated.

The above-mentioned systems may be adapted or included in urea-synthesis plants for obtaining an end product with a very low biuret content . The location of the ion-exchange systems in the urea-synthesis plants is not critical, even in intermediate steps, provided the required final biuret content in the urea is taken into account., ' The efficiency of the process of the present invention is illustrated by the following examples in which various ion-exchanging resins were used and urea solutions were used, these being prepared from non-coated prilled urea.

EXAMPLE 1 \° A 50% urea solution containing 1,17% of, biuret on dry urea basis, was treated with 150 mlllilitres of a cationic resin "Amberlite" IR-120. The obtained results are shown in Table I. "Amberlite" is a Trade Mark.

EXAMPLE 2 Tests were carried out treating a 50% urea solution containing 1.17% of biuret on dry urea basis, with 150 mlllilitres of a weak anionic resin IRA-93. The obtained results are shown in Table II.

EXAMPLE 3 Several tests were carried out treating a 50% urea solution containing 1.12% of biuret on dry urea basis, with 200. mlllilitres of a strong anionic resin SBR-P.

The obtained results are shown in Table III.

EXAMPLE 4 A 50% urea solution containing 1.15% of biuret on dry urea basis was treated with 200 mlllilitres of an extra strong anionic resin "IONAC-935" . The results obtained in this example did not indicate the presence of biuret until 1600 millilitres of the urea solution had passed through the bed. That is, during the passage of the above-mentioned millilitres up to this value, the percentage of biuret on a dry urea basis in the effluent is considered to be 0%.

TABLE l Ml. of urea solution passed through the bed 100 200 300 400 500 % of biuret on dry urea basis, in the effluent 0.69 0.95 1.07 1.11 1.16 Regeneration With a 4% hydrochloride acid solution and a regeneration level of 9 lbs. HCl/ft3 of resin .

TABLE Ilj RUN I Ml . of urea solution passed through the bed 100 % of biuret on dry urea basis, in the effluent 0.62 Regeneration With a \% caustic soda solution and a regenerating level of 5 lbs Na0H/f 3 of resin.

RUN II Ml. of urea solution pas ed through the bed 100 200 % of biuret on dry urea basis, in the effluent 0.11 0.77 Regeneration With a 4% caustic soda solution and a regeneration level of 5 lbs of Na0H/ft3 of resin.

NOTE: Ammonia and sodium hydroxide were added until the pH was 13.4 at which time the biuret is subjected to comple .

TABLE IM. Ill RUN I Ml. of urea solution passed through the bed 100 200 300 400 % of biuret on dry urea basis, in the effluent 0.05 0.08 0.08 0.08 Regeneration With a % caustic soda so 5 lbs. Na0H/ft3 of resin.

RUN II Ml. of urea solution passed through the bed 100 200 3OO : OO -5 % of biuret on dry urea basis, in the effluent 0.25 0.36 0.36 0.36 0 Regeneration With a 4% ammonia solutio NH /ft3 of resin.

Ill TABLE (continued) RUN III Ml . of urea solution passed through the bed 100 200 300 400 50 % of biuret on dry urea basis, in the effluent 0.18 0.25 0.26 0.32 0.

Regeneration With a 4 ammonia solution NH^ft of resin.

RUN IV Ml . of urea solution passed through the bed 100 200 300 400 % of biuret on dry urea basis, in the effluent 0.11 0.11 0.09 0.08 Regeneration With a 4% caustic soda sol and with regeneration leve of resin and 5 lbs. NH /ft 3 PILOT PLANT EXAMPLE From laboratory examples it was determined that the resin providing the best results is definitely the extra strong anionic type, such as " IONAC-935" · Therefore, in a pilot plant, the runs were only carried out with extra-strong anionic resin " IONAC"-935 · The results obtained are shown in Table A.

TABLE A

Claims (8)

44844/2 WHAT IS CLAIMED IS:

1. A process for treating a urea melt or urea solution containing 1.0 to 99.9% of urea and an amount of buiret, to reduce the amount of buiret, which process comprises contacting the urea melt or solution with an ion-exchanging resin.

2. A process as claimed in claim 1, wherein an aqueous urea solution is treated with the ion-exchanging resin.

3. A process as claimed in claim 1 or claim 2, wherein the ion-exchanging resin is a strongly basic anionic resin.

4. A process as claimed in a preceding claim, which is conducted at a temperature of 0°C to 200°C.

5. A process as claimed in any preceding claim, which is conducted by batch ion-exchange.

6. A process as claimed in any one of claims 1 to 4, which is conducted by continuous ion-exchange.

7. A process as claimed in claim 1 substantially as herein described with reference to the accompanying drawings.

8. A urea melt or urea solution which has been produced by the process claimed in any preceding claim.