Brief description of the invention

The present invention refers to a process for the recovery of energy from urea plants, this recovery consisting in the generation of electric power inside the urea plants. This electric power is at disposal at the battery limits of the plants and can be conveyed to the electric network of the Factory. The process of the invention essentially consists in the introduction in a urea plant of a turbine-electric power generator group, wherein the motive fluid in the turbine consists in a plant process flow (urea solution) expanding from a first section of the urea plant, operating at high pressure, to a second section of the plant, operating at considerably lower pressure.

The electric power generator, coupled with the turbine, can be of any type at disposal on the market, provided that the generated power has the same characteristics of the Factory electric power.

The generated electric power represents a net recovery of energy compared to a urea plant not provided with the process of the invention. Background of the invention

It is known that all the industrial processes for the production of urea are based on the reaction:

2NH ₃ + CO ₂ = (NH ₂ ) ₂ CO + H ₂ O (I) wherein two moles of ammonia and one mole of carbon dioxide give rise to one mole of urea dissolved in one mole of water.

From said solution, by water evaporation, urea is obtained as solid product in prilled or granular form.

Reaction (I) appears very simple, but in fact is complicated by the fact that it takes place, in the urea synthesis reactor, in two consecutive reaction steps, the first one leading to the ammonium carbamate formation

2NH ₃ + CO ₂ = NH ₂ - CO - ONH ₄ and the second step being represented by the dehydration of ammonium carbamate to urea

NH ₂ - CO - ONH ₄ = (NH ₂ ) ₂ CO + H ₂ O this second reaction being an equilibrium depending, among other parameters, on temperature and pressure.

Therefore the output from the urea synthesis reactor is an aqueous urea solution essentially consisting in urea, water, free ammonia, free carbon dioxide and in the intermediate product ammonium carbamate .

In order to obtain in a plant the complete transformation of the reactants, ammonia and carbon dioxide, as per reaction (I), into urea and water, except for some quantities, which for the economics and environmental pollution must be, in any case, reduced to the minimum, free ammonia, free carbon dioxide and ammonium carbamate must be removed from the urea aqueous solution and returned to the reactor.

The way this purification, ammomium carbamate decomposition and recycle are carried out, characterizes the different urea processes utilized in industrial plants. Among the various processes realized in the last four decades, two main groups have to be pointed out: the so called conventional processes and the so called stripping processes. In both groups of processes the purification and the ammonium carbamate decomposition from the urea solution exiting the synthesis reactor, and the recycle to the reactor of the reactants, ammonia and carbon dioxide, not transformed into urea, are carried out, downstream the reactor, in at least two steps at decreasing pressures, the last one at low pressure (generally between 0.1 and 0.6MPa) followed by the final concentration of the urea solution from reaction (I), leading to solid urea prilled or granular.

Conventional processes, based on which some plants in the world are still in operation, foresee the first expulsion of free ammonia and carbon dioxide (purification) , as well as the first decomposition of ammonium carbamate, at a pressure (1.5- 2.5MPa) substantially lower than the pressure of the reactor (15-25MPa) . Stripping processes, based on which is the majority of the urea plants in operation and the totality of the new urea plants, foresee the first expulsion of free ammonia and carbon dioxide, as well the first decomposition of the ammonium carbamate, at a pressure essentially the same of the pressure of the reactor (12-20MPa) , the practically negligible difference being represented by the pressure drop of the urea solution exiting the reactor and entering the first purification and decomposition step. In the stripping processes the first step of purification and ammonium carbamate decomposition above mentioned are performed in a heat tube bundle exchanger called high pressure decomposer or, most frequently, stripper, as it will be called in the following. The stripping action is performed with the aid of a stripping agent such as ammonia (for the Snamprogetti process) or carbon dioxide (for Stamicarbon and T. E. C. process), or both of them (for the Montedison process) . The global reaction (I) for the formation of urea from ammonia and carbon dioxide is strongly exothermic: nevertheless any process requires a considerable amount of energy (as steam at 1.2-2.5MPa) for the various steps of purification and ammonium carbamate decomposition and for the final step of getting solid urea from an urea aqueous solution.

Process Owners have worked out several processes in order to reduce the steam consumption of a urea plant, and in many cases they have been successful. The best result is represented by the performances of the stripping processes, in comparison with the conventional ones, by which a strong steam reduction consumption was achieved.

Similar efforts from process Owners were not performed as to the reduction of electric power consumption, even though the introduction of the stripping process had the consequence of a lower electric power consumption both because of the reduced pressure of the synthesis reactor and the reduced quantity of recycled solution from the low (and medium) pressure section to the high pressure one. It is the purpose of the present invention to introduce in the urea plant an electric power generation which can be connected with the electric network of the Factory, that means, in fact, a reduction on the electric power consumption of the urea plant. This reduction is rather remarkable and can represent even more than 50% of the electric consumption of the urea plant .

The process of the invention may be applied on new plants as well as on existing plants, based both on conventional and on stripping processes.

Detailed description of the invention

As per the last above paragraph the process of the invention can be applied on stripping process (existing or new plants) as well as on conventional processes (existing plants only, because no new plant is based on conventional process) .

In the following the present invention is disclosed for both groups of processes, stripping or conventional ones, even though the large majority of urea plants, existing or new, are based on stripping process. As it is well known, in the stripping processes the urea solution exiting the reactor flows into the stripper, a vertical heat tube bundle exchanger, nearly at the same pressure of the reactor, wherein the first purification and the ammonium carbamate decomposition take place, in a strongly endothermic reaction wherein the necessary heat is supplied by steam condensing in the shell of the stripper.

In the bottom head of the stripper the concentrated urea solution is collected at the pressure of 12-20MPa (according to various processes) and then expanded to the pressure of the downstream section, that may be a low pressure of 0.1-0.6MPa or a medium pressure generally of 1.2-2.5MPa, but in some case even 7.5MPa . This expansion takes place in the regulating valve, connected to the solution level in the stripper bottom head, and represents a complete loss of energy.

It is the purpose of the present invention not to loose said energy, but to make advantage of the expansion to produce energy, more precisely by generation of electric power.

In a stripping process for urea plants said purpose is achieved by the process of the invention simply introducing, downstream the stripper, in lieu of the expansion valve, a group turbine-electric power generator. The motive fluid in the turbine is represented by the urea solution leaving the stripper and expanding from the pressure in the stripper to the pressure of the downstream section of the plant where said solution is flowing to. The amount of electric power that can be generated is different according to the stripping process considered. Considering the stripping process with carbon dioxide, wherein the urea solution from the stripper pressure is expanded to about 0.3MPa, and the stripping process with ammonia, wherein the urea solution is expanded to a pressure of about 1.8MPa, and considering the quantity of expanded solution that is higher for the ammonia stripping process, the total result is that for the ammonia stripping process the amount of generated electric power is about 10-15% higher that for the carbon dioxide stripping process.

For the last one, and for a urea plant having the capacity of l,000T/D, the generated electric power is about 3,000kWh/h. To have an idea of the considerable amount of this recovered energy, it has to be considered that said urea plant, having the carbon dioxide compressor moved by an electric motor and producing prilled urea, consumes about 5,200kWh/h: that means that the generated electric power is about 57% of the urea plant electric power consumption. Furthermore, considering the same l,000T/D prilled urea plant, but with the carbon dioxide compressor moved by steam turbine, the total electric power plant consumption is about l,040kWh/h: in this case the generated electric power, 3,000 kWh/h, is about three times the electric power consumption of the urea plant.

For the same capacity plant producing granular urea the generated electric power is the same, while the plant electric power consumption is about 6,700 and 2,500kWh/h for the plant with carbon dioxide compressor moved by electric motor and for the plant with carbon dioxide compressor moved by steam turbine, respectively.

As mentioned above, the process of the invention can be applied in the plants (new or existing) based on the stripping process as well as in the plants (existing) based on the conventional process.

In the second case the turbine-electric power generator group of the invention utilizes the urea solution expanding from the pressure of the synthesis reactor to the pressure of a downstream section of the plant at a considerably lower pressure .

As the working pressure of the synthesis reactor, in the conventional process, is at least 20MPa and the urea solution flow rate much higher than that one from the stripper in the stripping process, the amount of the generated electric power in the conventional process can be much higher, i.e., 1.6-2.0 times higher.

Brief description of the drawings For a better comprehension of the invention, reference is made to the attached drawings that, in any case, have not to be considered limiting the scope of the invention.

Figure 1 shows a schematic view of the stripper, in a stripping process plant, with the immediate downstream section;

Figure 2 shows the same view of Figure 1 together with a schematic view of the process of the invention; Figure 3 shows a schematic view of the urea synthesis reactor, in a conventional process plant, with the immediate downstream section and

Figure 4 shows the same view of Figure 3 together with a schematic view of the process of the invention. In the above Figures 1 and 2, relevant to a stripping process, are clearly indicated: the stripper (1), with the connected lines of the urea solution inlet (2) from the reactor, the stripping agent inlet (3), the urea solution outlet (4), the vapours outlet (5), the steam to the shell (6) and the relevant condensate discharge (7), the urea solution after expansion (8) and (9), respectively, after the expansion valve (10) and the turbine (11), the immediate downstream section (12), the connecting line (13) of the urea solution from section (12) to downstream section (s) of the plant (not indicated in the Figures) and finally the electric power generator (14) coupled with the turbine (11) .

In the Figures 3 and 4, relevant to a conventional process, are clearly indicated: the synthesis reactor (15) with the connected inlet lines of the reactants, ammonia (16) , carbon dioxide (17), ammonium carbamate recycle solution (18), the reactor urea solution outlet (19), the urea solution after the expansion (20) and (21) respectively after the expansion valve (22) and the turbine (23), the immediate downstream section

(24), the connecting line (25) of the urea solution from section (24) to the downstream section (s) of the plant (not indicated in the Figures) and, finally, the electric power generator (26) coupled with the turbine (23) . In the urea stripping process wherein two strippers in series are used, the process of the invention is applied downstream the second stripper.

The Manufacturer of the turbine (11), for the stripping process, or (23), for the conventional process, has to take into due consideration the fact that during the expansion of the urea solution in the turbine a formation of vapours takes place which consists in ammonia, carbon dioxide and water. These vapours expand in the turbine (11) or (23) and flow, together with the solution, to the immediate downstream section (12) , in the stripping process, and to the immediate downstream section (24), in the conventional process, respectively. At the start-up of a urea plant, based on stripping process provided with the process of the invention, the urea solution from the stripper (1) flows, through line (4) and (27), to the expansion valve (10) and, after expansion, through line (8), to the downstream section (12) , operating at low or medium pressure, according to the process considered.

Step by step, valve (10) is closed and the urea solution flows through line (28) to the turbine (11) and then the expanded solution, together with the expanded mentioned vapours, flows to section (12) . Similarly at the start up of a urea plant, based on conventional process provided with the process of the invention, the urea solution from the synthesis reactor (15) flows, through the lines (19) and (29) , to the expansion valve (22) and, after expansion, through line (20), to the downstream section (24) operating at medium pressure.

Step by step, valve (22) is closed and the urea solution flows through line (30) to the turbine (23) and then the expanded solution, together with the mentioned vapours, flows to section (24) . During normal plant operation both expansion valves (10) and (22) remain closed.

The process of the present invention can be utilized for urea new plants as well as for the existing ones. In case of existing plants _/ it has to be pointed out that, in general, it is not necessary for the installation of the equipment of the invention to make extra plant shut-downs further to the scheduled ones, because, once tie-ins are installed, the installation of the necessary equipment of the process of the invention can be performed also with the plant in operation.

In order to highlight the favourable economics of the introduction of the process of the present invention in a urea plant the following examples are given. Example 1

In an existing prilled urea plant, having the capacity of l,000T/D and based on the carbon dioxide stripping process, the process of the invention, was introduced, i.e., a turbine- electric power generator group, together with the necessary piping, instrumentation etc. downstream the stripper. The total investment cost, including installation, was equal to € 2,600 Millions. The generated electric power was equal to 3,000kWh/h that gave rise, considering 330D/Y and a cost of electric power equal to 0.09 €/kWh, to the amount of € 2,138 Millions per year not paid to the electric power supplier and therefore spared by the Factory. The pay back time of the investment was about 15 months. Example 2

In an existing prilled urea plant, having the capacity of l,750T/D and based on conventional process, the process of the invention was introduced, i.e., a turbine-electrical power generator group, together with the necessary piping, instrumentation etc., downstream the synthesis reactor. The total investment cost, including installation, was equal to € 3, 600 Millions.

The generated electric power was equal to 8,920kWh/h that gave rise, considering 330D/Y and a cost of electric power of € 0.09/kWh, to the amount of € 6,350 Millions per year not paid to the electric supplier and therefore spared by the Factory. The pay back time of the investment was about 7 months.