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Modeling Urea Processes: A New Thermodynamic Model and Software Integration Paradigm
(Special Shared Content with Virtual Materials Group)

Introduction

Nitrogen based fertilizers are the most widely produced types of fertilizers, accounting for 82.79 million tones produced worldwide between 1998 and 1999 (1). Amongst all nitrogen based fertilizers urea is the most widely produced, with 37.57 million tones produced between 1997 and 1998 (1). It is significant to notice that urea consumption is increasing significantly, jumping from 8.3 million tones in 1973-1974 to 37.57 million tones in 1997-1998 corresponding to about 46% of the total world consumption of nitrogen. The importance of urea production and the availability of modern flowsheeting tools motivated us to apply basic thermodynamic principles and software engineering for the creation of a tool that can be used for modeling the most significant aspects of the urea production processes currently used. Albeit several technologies are available for the production of urea (2, 3, 4, 5, 6), the Stamicarbon and Snamprogetti processes correspond to approximately 76% of the world market (7) and therefore our modeling efforts concentrated on these two production technologies. Basic Principles The commercial production of urea is based on the reaction of ammonia and carbon dioxide at high pressure and temperature to form ammonium carbamate, which in turn is dehydrated into urea and water:

	(1)
	(2)

Reaction 1 is fast, highly exothermic, and goes essentially to completion under normal industrial processing conditions, while reaction 2 is slow, endothermic and usually does not reach thermodynamic equilibrium under processing conditions. It is common practice to report conversions in a CO₂ basis. According to Le Chatellier's principles, the conversion increases with an increasing NH₃/CO₂ ratio and temperature, and decreases with an increasing H₂O/CO₂ ratio.

Different urea production technologies basically differ on how urea is separated from the reactants and how ammonia and carbon dioxide are recycled. Refinements in the production technology usually are concentrated in increasing carbon dioxide conversion, optimization of heat recovery and utility consumption reduction.

Stamicarbon Process