Once-Through Urea Process

Other Processes. Flow sheets for typical partial-recycle process and typical once-through urea process are given in Figures 9 and 10, respectively. 

Figure 9.1. Typical Once-Through Urea Process.

The limited carbon dioxide conversion and the huge amounts of the by-product ammonia salt represent major disadvantages with respect to production of pure urea. Thus, once-through urea synthesis processes do not have any significant industrial relevance nowadays. Modified processes have been developed, which allow recycling of the nonconverted reactants, thus increasing overall conversion and yield. 

Because an excess of ammonia is fed to the reactor, and because the reactions ate reversible, ammonia and carbon dioxide exit the reactor along with the carbamate and urea. Several process variations have been developed to deal with the efficiency of the conversion and with serious corrosion problems. The three main types of ammonia handling ate once through, partial recycle, and total recycle. Urea plants having capacity up to 1800 t/d ate available. Most advances have dealt with reduction of energy requirements in the total recycle process. The economics of urea production ate most strongly influenced by the cost of the taw material ammonia. When the ammonia cost is representative of production cost in a new plant it can amount to more than 50% of urea cost. 

Preparation. The current basic comml procedure, the once-through process (Fig 1), uses liq ammonia and liq carbon dioxide at 1750 to 3000psiand 160—200°. These constituents react to form amm carbonate which is decompd at about 80psi to urea and w. Several variations of the process include a partial recycle process and, the most commonly used technique, the total recycle process. Fig 2 is a diagram of the latter process as adapted for use by the Chemical... 

Derivation Liquid ammonia and liquid carbon dioxide at 1750-3000 psi and 160-200C react to form ammonium carbamate, NH4C02NH2, which decomposes at lower pressure (about 80 psi) to urea and water. Several variations of the process include once-through, partial recycle, and total recycle. 

A once-through modification of a urea process is shown in Fig. 11.11. Recycle of unconverted ammonium carbamate and of ammonia and carbon dioxide formed in part from ammonium carbamate decomposition (Eq. 11.60) is practiced in other variations on this theme [64]. 

This process is based on the high pressure, high temperature reaction. Even though the once-through conversion of urea to melamine is merely 50%, this route is still the most economical of all the available processes. 

Urea Technologies Inc. fMarrtrric), which offers the Heat Recycle Urea Process, (HRUP1, whose main feature is the hot recirculation of the aqueous carbamate solution, which enhances the energy balance. This is a conventional technique, operating at 20 to 22. 10 Pa absolute, 190 to 2Q0 C, N/C ratio 4, and once-through CO, conversion S 71 to 72 per cent. 

In the partial-recycle process, part of the off-gas ammonia and carbon dioxide from the carbamate strippers is recycled to the urea reactor. Recycling is accomplished by absorbing the stripper gases in a recycle stream of partially stripped urea effluent, in process-steam condensate, or in mother liquor from a crystallization finishing process. In this manner, the amount of NH3 in off-gas is reduced. Any proportion of the unreacted ammonia can be recycled typically, the amount of ammonia that must be used in some other process is reduced to about 15% of that from a comparable once-through unit. 

Since the amount of ammonium carbamate that can be absorbed in the absorber solution described above is limited by its solubility in the system H20-urea-NH3, part of the ammonia and carbon dioxide cannot be recycled and must be used in the production of a coproduct nitrogen material, As in the once-through process, the operation of the urea plant still must coincide with that of the coproduct plant. 

Figure 3.7 Blockflow diagram of once-through process for urea synthesis with optional production of ammonia nitrate (AN).

There now are three prevailing process designs for urea manufacture, licensed by Stamicarbon, Mitsui Toatsu, and Snampro-getti. The main differences between these processes are in the methods used to handle the converter effluent, to decompose the carbamate and carbonate, to recover the urea, and to recover the unreacted ammonia and carbon dioxide for recycle, with the objective being minimum expenditure of energy and a maximum recovery of heat. In some processes, a liquid is used to recycle a solution of carbamate, carbon dioxide, ammonia, and water. In others, the amount of water recycled is minimized, and only carbon dioxide and ammonia are recycled. In the older plants, or once-through plants, the off-gases are used as feed to ammonium nitrate or ammonium sulfate plants. 

The first reaction is exothermic and goes almost to completion, while the second, endothermic, reaction converts less than 70% of feedstocks to urea. Consequently, the once-through process of urea synthesis had to be combined with production of other nitrogen compounds. Modern syntheses use a more expensive total recycle process in which all the unreacted CO2 and NH3 is separated from the effluent solution and returned to the reactor. Conversion rates range between 65 and 67% for each pass, and the overall reaction yield is on the order of 99%. Three commercial processes—Dutch Stamicarbon, Italian Snamprogetti, and Japanese Toyo—dominate today s world market for large (up to 2,000 t/day) urea plants using the total recycle process. ... 

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