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Biuret is formed from urea according the following overall reaction: 2 Urea <=> Biuret + NH3. In fact this reaction proceeds in three steps: When heating a concentrated urea solution in water or a urea melt, urea is present in two forms, a keto and an enol form. The enol form of urea primarily decomposes into NH3 and HNCO (cyanic acid). Cyanic acid reacts with another urea to biuret. All these reactions are equilibrium reactions. The biuret formation according these reactions is favored by higher residence times, higher temperatures and lower ammonia partial pressures. Biuret is considered an impurity in urea when used as a fertilizer as it has been identified as injurious to citrus foliage and to certain germinating seeds, but which biuret level really causes damage to the plants is disputable. The earliest urea plant designs produced urea with up to 5 wt% biuret. From 1955 to present, many urea plants were designed to produce low-biuret product with not more than 0.3 wt% biuret by means of a crystallization section. The lower biuret concentration was found to be safe for these special applications which amount to about two percent of total urea fertilizer consumption. Modern plant designs are successful in producing urea prills and granules with 0.8 wt% biuret. These plants can handle 98 percent of the urea market with simpler and more cost-effective urea finishing designs that produce higher quality products with better storage and handling characteristics.
Mr. Mohammed Al-Jeshi of Safco in Saudi Arabia posts an interesting operational question: What is the relation between Plant load & Biuret content in the final product?
 
 

缩二脲由尿素按照下述总反应生成:2 尿素 <=> 缩二脲+氨。事实上,该反应分三步进行:当加热尿素水溶液或者熔融尿素时,尿素以两种形式存在:酮式与烯醇式。烯醇式尿素会分解成NH3和HNCO(氢氰酸)。氢氰酸和另一种形式的尿素反应生成缩二脲。所有这些反应都是平衡反应。当停留时间越长、温度越高和氨分压越低时,缩二脲越容易按照这些反应生成。当尿素作为肥料时缩二脲被当做杂质,因为其已被认定对柑橘属叶和特定的种子是有害的,但是何种缩二脲含量能对植物造成真正的破坏是有争议的。最早的尿素装置所设计的尿素产品中缩二脲的含量高达5wt%。自1955年至现在,很多尿素装置通过结晶单元生产不超过0.3wt%的低缩二脲含量的产品。人们发现低缩二脲含量对这些特定应用是无害的,这些特定应用的尿素使用量占整个尿素肥料消耗的2%。现代装置的设计可成功生产0.8 wt%缩二脲含量的高塔颗粒和大颗粒。这些装置可满足98%的尿素市场需求量,这些装置的最终造粒方式更为简单和更有成本优势且操作性能好,可生产更 高质量的、更易贮存的产品。

沙特阿拉伯Safco公司的Mohammed Al-Jeshi先生提出一个有趣的操作问题:最终产品中缩二脲含量和装置负荷有何关系?

 
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