A Look at Urea Production

Date Published:   01/13/2020

The urea production uses CO2 gas and liquid ammonia as raw materials supplied from an Ammonia plant. This article will divide urea production into six sections: 1) Synthesis & HP Recovery; 2) MP Purification & Recovery; 3) LP Purification & Recovery, 4) Vacuum Concentration; 5) Condensate treatment; and 6) Prilling.

 

SYNTHESIS AND HIGH PRESSURE (HP) RECOVERY

In addition to the HP machinery required to feed ammonia and carbon dioxide and to recycle ammonium carbamate solution, this section includes: the reactor where urea is formed; the stripper necessary to strip out as vapors, from the urea solution leaving the reactor, a large amount of ammonia and carbon dioxide not converted to urea in the reactor; the carbamate condenser that condenses these vapors; the ejector that recycles the ammonium carbamate solution to the reactor. In this equipment the pressures are of a similar level, 150 bar, while the temperatures of the outlet solutions are 188, 205 and 155°C for the reactor, the stripper and the carbamate condenser, respectively.

Inside the reactor a matching number of trays of a very simple design are installed to improve the conversion. Under these conditions 62-64% (conversion) of the total CO2 entering the reactor is converted to urea. The total carbon dioxide conversion in the HP section (or loop) is 85-90%. All the equipment in this section, the heaviest of the urea plant, is installed at ground level, thus providing a horizontal layout with all the relevant benefits.

The stripper and the hydrolyzer in the waste water section are the only two items of equipment in the plant that consume medium pressure steam. The amount of steam consumed in the stripper is practically completely recovered in the carbamate condenser: its pressure is lower but still sufficient to be used in the urea plant itself. The quantity of oxygen introduced into the plant as air is 0.25% vol of the fresh feeding carbon dioxide. This minimum amount guarantees, at the same time, excellent equipment passivation and the absence of explosive mixtures where the “inerts” are released from the plant into the atmosphere after practically all the ammonia contained therein has been washed.

Thanks to the proper choice of the materials in contact with the process fluids and to the presence of excess ammonia, it is possible, during unscheduled shutdowns lasting only a few days, to bottle-in the high-pressure synthesis loop by operating a few valves, thus keeping all the process solutions inside the loop. In this way obvious pollution and start-up problems are completely averted. All kinds of machinery (reciprocating and centrifugal) is utilized according to local conditions.

 

MEDIUM PRESSURE (MP) PURIFICATION AND RECOVERY

The purpose of this section is to partially strip out the reactants, ammonia and carbon dioxide from the urea solution and, after their condensation in water, to recycle the obtained solution to the reactor, together with the ammonia and carbon dioxide aqueous solution resulting from the downstream sections of the plant.

The ammonia excess is separated in this section and recycled to the reactor separately. A distillation column is provided for this purpose. The operating pressure is 17 bar g. A particular feature is included in this section. Ammonia and carbon dioxide are partially condensed in the shell of a preheater within the vacuum section, thus recovering some energy in the form of 200 kg of steam per ton of urea, with an investment cost that, even in existing plants, has a pay-back time of less than two years. Another particular characteristic of the MP section is the washing of the so-called inerts (CO, H2 and CH4 contained mainly in the carbon dioxide and the passivation air).

For the complete abatement of the ammonia contained in the inerts, in completely safe conditions with regard to explosions, The inerts are washed with water after the addition of a quantity of flammable gas, as for example natural gas, in such an amount that after the ammonia has been eliminated, the composition of the inerts is out of the explosive field due to the excess of flammable gas. The washed inerts are sent to a burner together with the natural gas.

It should be emphasized that the presence of the MP section provides great plant flexibility, which can be operated over a wide range of NH3/CO2 ratios, with excess ammonia present in the urea stream from the stripper being recovered and condensed by the MP section.

 

LOW PRESSURE (LP) PURIFICATION AND RECOVERY

Further stripping of ammonia and carbon dioxide is made in the LP section, operating at 3.5 bar g. The vapors, containing ammonia and carbon dioxide, are condensed and recycled to the reactor via the MP section. An appropriately sized tank is provided in this section to collect all the solutions from the plant when it is shut down for long time. Therefore, in no circumstances are solutions discharged from the plant.

 

VACUUM CONCENTRATION

The urea solution leaving the LP section is about 70% b.w. and contains small quantities of ammonia and carbon dioxide. The final concentration of the urea solution (99.8% b.w.) is made under vacuum in two steps at 0.3 and 0.03 bar abs. for the prilled product, and in one or two steps for the granular product, according to the granulation technology chosen. An important feature of this section is the preconcentration of the urea solution to about 86% b.w. The necessary heat is provided by partial condensation of the vapors (ammonia and carbon dioxide essentially) from the MP section evaporator.

A simple solution has been found to the problem of lump formation in the second vacuum separator: lump formation is prevented by wetting the internal walls of the separator by means of a small recycle of molten urea.

 

PROCESS CONDENSATE TREATMENT

This stage discharges a large amount of process water with 1 ppm of urea and 1 ppm of ammonia lowers specific consumption of ammonia and reutilizes process water as boiler feedwater). All possible and convenient heat recoveries are generally introduced into this section in order to minimize energy consumption.

 

FINISHING: PRILLING

Prilling is the easiest technology to manufacture solid urea with commercially valid chemical and physical characteristics. Molten urea (99.8% b.w.) is sprayed at the top of the prilling tower, at a height of 55-80 m, according to climatic conditions; at the bottom, essentially spheroidal urea particles, namely prills, are collected and sufficiently cooled in order to be sent to storage or directly to the bagging section without screening, coating or any other treatment. A rising draught of air inside the prilling tower is the cooling medium that removes the solidification heat and cools the prills.

 

MAJOR TECHNOLOGY LICENSORS

There are multiple technology licensors with proprietary plant designs and features. The main urea process licensors are Snamprogetti, Urea Casale, Kellogg and Stamicarbon.

 

COST AND UTILITIES OF AN UREA PLANT

A new plant constructed in Azerbaijan in 2018 by Samsung Engineering, with licensing in place with Haldor Topsoe for the production of ammonia and Stambicarbon for the production of urea, produces 1,200 tons of ammonia and, subsequently, 2,000 tons of urea per day. The plant will consume 25-26 MW of electricity per hour. The urea plant will require 1.3 million cubic meters of natural gas per day for operation at full capacity. The cost of the project is 770-780 million euros.

References
Stock# Urea Plants
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