Organic compounds -- part of the class 532-570 series – Organic compounds – Amino nitrogen containing
Reexamination Certificate
1998-01-28
2001-09-04
Kumar, Shailendra (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Amino nitrogen containing
C564S060000, C504S327000
Reexamination Certificate
active
06284921
ABSTRACT:
The invention relates to a method for preparing an oligomethylene urea product usable as a fertilizer by reacting urea and formaldehyde in several steps.
Oligomethylene ureas are reaction products of urea and formaldehyde, also called urea formaldehydes. Oligomethylene ureas are used for instance as slow nitrogen-release fertilizers, either as such or admixed with other fertilizers. Oligomethylene ureas having different molecular chain lengths decompose at different rates in the soil. The properties of oligomethylene ureas can also be assessed chemically by using the method called activity index analysis (official AOAC analysing process), whose activity index is determined as follows:
AI
=(
CWIN−HWIN)·
100
/CWIN
in which CWIN represents Cold Water Insoluble Nitrogen and HWIN Hot Water Insoluble Nitrogen.
This alone is not enough to characterize an oligomethylene urea composition, because the activity index AI is a relative variable. The absolute quantities of the various components are vital in terms of the fertilizer activity. Yet it is known that the HWIN fraction mineralizes at an extremely slow rate and is thus appropriate for fertilizing plants to be fertilized at very long intervals. The CWSN fraction (=N
tot
−CWIN) mineralizes relatively fast. The central fraction (CWIN−HWIN) is most suitable for many purposes of use and processes frequently aim at maximizing this fraction.
The processes for preparing oligomethylene urea compositions can be divided into two main categories.
In the concentrated solution process, a concentrated solution made of urea and formaldehyde is acidified, so that urea and formaldehyde are condensed into a solid product. This process has the advantage that liquid needs not to be processed in large amounts relative to the product amount, however, the process has the drawback of difficult neutralization of the solid product cake obtained, resulting but rarely in a commercial product with a homogenous pH value. In addition, the process is hard to perform under control so as to achieve the proper solubility characteristics for the product.
In the dilute solution process, aqueous urea and formaldehyde solutions are admixed in large amounts relative to the product amount. The particles generated after the condensation reaction at an acid pH are neutralized, whereupon the product is separated from the aqueous phase by filtering and/or centrifugating. The filtrate is recycled in the process and the product cake is dried so as to form a fertilizer. Although the dilute solution process results in better neutralization, it still has the drawback of requiring very large amounts of water to be processed.
The processes for preparing oligomethylene products can also be divided into single-step and two-step processes.
In the single-step process, an aqueous solution containing urea and formaldehyde is immediately acidified, yielding a condensate in solid state. This process is exemplified in DE patent specification 2 422 238, in which the formaldehyde solution is slowly added to an aqueous urea solution under strictly determined conditions. A second single-step process is described in U.S. Pat. No. 4,173,582, in which urea and formaline (aqueous solution of formaldehyde) are contacted, the solution formed is acidified in order to yield a solid condensate, and the product cake formed is filtered, whereupon the filtrate is returned to the urea dissolving vessel.
In the two-step process, the pH of an aqueous solution of urea and formaldehyde is first raised to an alkaline state in order to yield methylol urea products. After this, the reaction mixture is acidified in order to condensate the methylol urea products into solid oligomethylene urea products. Such a method has been described in DE patent specification 3 639 581, in which urea is reacted with formaldehyde in an aqueous solution, which has been raised to an alkaline state by means of salt of polyphosphoric acid. The methylol urea products thus produced are condensed by adding acid to the reaction mixture. The reaction mixture is totally dried.
The references cited above have not reached a satisfactory solution to the problem caused by difficult filtering and handling large amounts of water. Regarding these references, it is still true that effective filtering is achieved only by using very large amounts of water relative to the product quantity.
Thus the object of the invention is to provide a method for preparing an oligomethylene urea product usable as a fertilizer, which uses minimum amounts of liquid relative to the product yield. In addition, the method aims to achieve a product of optimally high quality and effective filtering in particular. These goals have now been achieved with a new method, which is mainly characterized by the following steps:
a) introducing at least urea and an aqueous solution of formaldehyde in the overall molar ratio urea to formaldehyde of the step 1:1-2:1 into a first reactor, having a retention time of at least 0.5 h, a temperature maintained in the range of 40 to 95° C. and a pH maintained in the range of 6.5 to 11, in order to yield a methylol urea composition,
b) conducting the methylol urea composition to a second reactor, having a temperature maintained in the range of 5 to 35° C. and a pH maintained in the range of 1.8 to 3.5 in order to yield a first oligomethylene urea slurry, or optionally,
c) conducting the methylol urea composition or the first oligomethylene urea slurry to a third reactor, having a temperature maintained in the range of 5 to 35° C. and a pH maintained in the range of 1.8 to 3.5 in order to yield a second oligomethylene urea slurry,
d) conducting an oligomethylene urea slurry, selected from the first or the second oligomethylene urea slurry, to a filter and filtering it to form an oligomethylene urea cake and a filtrate,
e) recycling the filtrate to the reactor, selected from the second or the third reactor, and
f) neutralizing and drying the oligomethylene urea precipitate and recovering it as said oligomethylene urea product.
REFERENCES:
patent: 3227543 (1966-01-01), O'Donnell
patent: 3759687 (1973-09-01), Nobell
patent: 2536545 (1976-11-01), None
patent: 3639581 (1987-05-01), None
Kemira Agro Oy
Kumar Shailendra
Locke Liddell & Sapp LLP
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