Biuret formation

Urea processes provide an aqueous solution containing 70—87% urea. This solution can be used directiy for nitrogen-fertilizer suspensions or solutions such as urea—ammonium nitrate solution, which has grown ia popularity recentiy (18). Urea solution can be concentrated by evaporation or crystallization for the preparation of granular compound fertilizers and other products. Concentrated urea is sohdified ia essentially pure form as prills, granules, flakes, or crystals. SoHd urea can be shipped, stored, distributed, and used mote economically than ia solution. Furthermore, ia the soHd form, urea is more stable and biuret formation less likely. 

A two-stage evaporative concentration under vacuum, with a limited residence-time in the evaporator to limit biuret formation, produces a urea stream containing about 0.5 per cent water which can be sprayed into a prilling tower. 

Purge stream assuming no biuret formation in the crystallizer and a final concentration in the purge of 4.5% biuret and a loss of miscellaneous CO2 and NH3 by evaporation. 

Texas) in 1964 and 1965. Toyo also will provide the acoustic vibration unit. In prilling, the urea melt is concentrated via vacuum evaporation to 99.8% and fed as quickly as possible into the bucket to minimize biuret formation. The liquid forms drops that then fall down a cylindrical concrete tower that has either induced, forced, or natural draft air flow. The prills solidify and are removed at the bottom by belt conveying to storage. Some plants have a fluidized-bed cooler in the prill tower bottom and others use an in-line cooler before storage. If low biuret product is to be produced, the urea melt from the last decomposition stage of the synthesis plant is first crystallized and the crystals are then melted just before prilling. 

To compensate for hydroxyl groups that are untreated due to imperfect mixing, or for excess isocyanate groups consumed in allophanate or biuret formation, it is standard practice to combine the isocyanate and polyol... 

Acids influence the NCO/OH reaction by accelerating chain extension a little, and retarding crosslinking. If p-nitrobenzoylchloride is added to a urethane system in which active hydrogen compounds must be present, this additive has a mild catalytic effect on chain extension, no effect on allophanate formation, and a strong retarding effect on biuret formation. If water is present the reaction is strongly catalyzed. 

The initial step to prepare polyurethane polymers for solution wet or dry spinning includes reaction of 1000-3500 molecular weight macroglycol with a diisocyanate at molar ratios of between about 1 1.4 and 1 2.0. Reaction conditions must be carefully selected and controlled to minimize side reactions, eg, allophanate and biuret formation, which can result in trifunctional branched chains and ultimately to insoluble cross-linked poljuners. For the prepolymer reaction, poly(tetramethylene ether) glycol [25190-06-1] and bis(4-isocyanatophenyl) methane [101-68-8] are currently the most commonly used macroglycol and diisocyanate. Several types of polyester-based macroglycols are included in spandex producers product lines, but with the exception of Dorlastan, made by Bayer AG in Germany, the polyester-based products represent only a minor part of their spandex fiber production. 

The derivatization of IPDI is easier to accomplish because of the different reactivities of the two isocyanate groups in the molecule. For example, preferential dimerization of the primary isocyanate group is observed in the trialkylphosphine-catalyzed reaction (46). Trimerization of IPDI in the presence of quaternary ammonium salts affords mainly the isocyanurate trimer (47). Attempted biuret formation from IPDI and Na2S04 IOH2O gives a mixture of ureas and biurets (48). A biuret derivative is also obtained from IPDI and HDA (49). 

Isocyanates react with alcohols and phenols to form urethanes. In general, rates of urethane formation decrease in the following order primary alcohols > secondary alcohols > 2-alkoxyethanols > l-alkoxy-2-propanols. Isocyanates can react with urethanes to form allophanates. This reaction is much slower than the reaction of isocyanate with alcohol. Isocyanates react rapidly with primary and secondary amines to form ureas. The reaction is much faster than the reaction of isocyanates with alcohols. Isocyanates can react with ureas to form biurets. Biuret formation is slower than urethane formation, but faster than allophanate formation. Isocyanates react with water to form imstable carbamic acids, which dissociate into carbon dioxide and an amine. The amine is so much more reactive that it reacts with another isocyanate (in preference to water) to form mea. The reactivity of water with isocyanates is somewhat slower than that of secondary alcohols, but much more rapid than that of imcatalyzed reaction with methanes or ureas. 

This reaction proceeds to a significant degree at about 120 140 °C but it could occur also at lower temperatures at high excess of isocyanates. Similar is the reaction of biuret formation from isocyanate and urea groups ... 

Biuret formation reaction proceeds to a considerable measure above 100 °C. Both reactions (8) and (9) are utilized to introduce crosslinks with the excess of isocyanate. The previously given reactions are the most frequent ones in the polyurethanes chemistry. There are other important reactions such as the reaction of isocyanate with itself, which may occur during storage or are intentionally carried out to obtain new products. Isocyanates (particularly the reactive aromatic ones) easily form dimers (uretdiones) ... 

In reality, allophanate and biuret formation is reversible at temperatures above 130°C [252]. Formation of isocyanurates causes a reorganization of the CLD akin to what happens in reversible polycondensations because of exchange reactions. 

Biuret formation is also favored by low ammonia content (law of mass action) [13]. Since the partial pressure of ammonia also depends on the overall pressure, biuret formation is negligible in the high-pressure synthesis but has to be taken into consideration in the medium and low-pressure sections of the plant. In these... 

Thus, the residence time at high temperatures should be reduced to minimize urea product losses through both reverse reactions of urea synthesis (Eqs. (3.15) and (3.16)) and biuret formation [13]. 

To obtain soM urea, in the first step, the water content of the urea solution is reduced by evaporation. The evaporator is operated with vacuum as fairly low temperatures reduce biuret formation (see Section 3.3.1). In addition, attention should be given to short residence times. If very low water concentration is required (e.g., for prilling), it can be necessary to realize two successive evaporation steps with decreasing operating pressure [13]. This is related to the complex crystallization behavior of the water /urea melt, which can lead to undesirable crystallization within a single-step evaporation at low water concentration. 

The Snamprogetti process is based on using ammonia to strip CO2 and NH3 out of the urea reactor effluent. One of the impurities that affects the performance of urea as a fertilizer is biuret. To avoid a yield loss and minimize product contamination, it is important to minimize biuret formation. The con-... 

Model urethane and urea linkages were created using a prepolymer process. The small molecules were characterized using solution and solid state NMR spectroscopy. The results of the project were identified peaks on the NMR spectra, reactions times, and a study of allophanate and biuret formations. 

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