Process for Urea Production

1. Field of the Invention

This invention relates to an improved process for the production of urea from ammonia and carbon dioxide via synthesis at adequate pressure and temperature. The urea formation takes place in a synthesis zone (or zones) where an excess of free ammonia is kept to favour high conversions.

This improved process covers, in particular, a new treatment step to recover and recycle the unreacted materials (free ammonia and carbamate) from the reaction zone (or zones) in an optimal way to minimize energy consumption and investment costs.

2. Description of the Prior Art.

It is known that high reaction yields are favoured by a high ammonia excess (compared with the stoichiometric ratio) which require however a high reactor operating pressure and, as a consequence, complex and energy consuming treatment sections downstream the reactor to remove and recycle said excess ammonia and the residual carbamate from the produced urea.

Some processes have been recently studied to minimize energy and investment requirement for the treatment sections downstream the reactor, but they are still complex and still require considerable amount of energy.

The U.S. Pat. No. 4,208,347 (Montedison), known as the IDR process (Isobaric Double Recycle), describes a two steps stripping treatment scheme where carbamate is removed with ammonia as stripping agent, in the first step, while free ammonia is removed with carbon dioxide as stripping agent, in the second step. A certain complexity of this scheme is evident. The U.S. Pat. No. 4,321,410 (Mitsui Toatsu Chemicals and Toyo Engineering) known as the ACES process (Advanced Process for Cost and Energy Saving) describes a two steps stripping treatment performed in a newly designed stripper where the urea reactor effluent is contacted with the gases (mainly NH.sub.3 and CO.sub.2) coming from a falling film exchanger in an adiabatic first treatment step where free ammonia is removed and successively treated in the falling film exchanger (second treatment step) counter-currently with carbon dioxide introduced as stripping agent to remove the residual carbamate.

With this process the amount of free ammonia that can be removed from the reactor effluent is limited due to the presence of NH.sub.3 in the gases contacting the urea solution in the adiabatic step, while a minimum content of free ammonia in the urea solution is desirable to obtain optimal carbamate removal in the subsequent CO.sub.2 stripping step.

The Italian patent application No. 24357A/80 (Snamprogetti) describes a process very similar to the Montedison one but with the two treatment steps at different pressure (non isobaric).

None of the above mentioned new processes achieve the direct recycle to the reactor of the free ammonia separated form the reactor effluent, which is optimal to minimize investment and energy consumption. The indirect recycle of ammonia in the downstream sections is made via acqueous solutions with the recycle of water in the reactor, which is detrimental for reaction yields.

The last generation processes, followed by the cited new generation ones, were dominated by the Stamicarbon CO.sub.2 stripping and Snamprogetti NH.sub.3 stripping processes both using only one high pressure treatment step. In the Stamicarbon CO.sub.2 stripping process, the reactor effluent with a low free ammonia content is directly treated in the CO.sub.2 stripper to remove the residual carbamate. The content of ammonia in the reactor is kept low to have optimal carbamate separation in the CO.sub.2 stripper, but reaction yields are low with consequent high investment and energy consumption.

In the Snamprogetti NH.sub.3 stripping process the reactor effluent with a higher free ammonia content is also directly treated in a "self stripping" treatment step to remove carbamate.

An important amount of free ammonia is still present in the urea solution leaving the stripper and is separately recycled to the reactor using pumps.

This scheme implies the use of a rectifying column to separate pure ammonia with high costs and energy consumption.

None of the last generation processes also achieves the direct recycle to the reactor of the free ammonia separated from the reactor effluents with minimum investment and energy consumption.

The direct recycle to the reaction zone of important amounts of ammonia is an optimal way of minimizing energy and investment costs which is the main objective of the present invention.

It has been surprisingly discovered that important amounts of ammonia can be economically separated from the effluents of reactors operating with high excess ammonia and therefore with high conversion yields, obtaining minimal excess of ammonia content in the urea solution which can be subsequently treated with maximum efficiency in a falling film exchanger with a counter-current of CO.sub.2 stripping stream to remove the residual carbamate.

Knowingly the presence of ammonia is detrimental to CO.sub.2 stripping efficiency.

The new process is described with reference to FIG. 1 which represents one of the possible embodiments of the invention.

The urea solution obtained in the high conversion yield reactor (R) with the presence therefore of a consistant amount of ammonia excess over the stoichiometric amount, is treated in the adiabatic step (S) where the major part of the excess ammonia is removed thanks to the intimate contact of the solution with a small amount of fresh CO.sub.2 fed to step (S) by line 1.

Line 2 feeds the urea solution from the reactor (R), which can be of conventional design, to the step (S), while the direct recycle of the separated excess ammonia from step (S) is made through line 3. The urea solution with minimum excess ammonia from step (S) is then fed (line 4) to a CO.sub.2 stripper (ST), also of conventional design, where carbamate is removed with maximum efficiency in a falling film exchanger with counter-current fresh CO.sub.2 used as stripping agent introduced through line 5.

The vapors (mainly NH.sub.3 and CO.sub.2 coming from carbamate decomposition) from stripper (ST) are fed through line 6 to the carbamate condenser (CC) where evolved heat is removed producing steam (line 7) utilized for the urea solution conventional treatment steps (not represented in the figure) downstream stripper (ST).

The carbamate condenser (CC) receives also the carbamate solution (line 8) from the above-mentioned, not represented, treatment steps and the inerts introduced with the CO.sub.2 which are vented from reactor (R) (line 9). Said inerts, after removal in the carbamate condenser (CC) of the residual NH.sub.3 and CO.sub.2, are vented from the system (line 20). The feed ammonia (line 10) is partially sent (line 11) to the reactor after preheating, for reactor heat balance purposes, in preheater (P) and partially sent to the carbamate condenser (CC) (line 12). Line 13 feeds the fresh CO.sub.2, the major part of which is sent to the stripper (ST) (line 5) and a minor part is sent to step (S) (line 1).

The urea solution (line 14) after treatment in stripper (ST), with optimal residual content of carbamate, is finally sent to the conventional treatment steps to obtain the desired final urea product. The carbamate solution from carbamate condenser (CC) is recycled to reactor by gravity (line 15).

It is critical that an intimate contact of short duration (a few seconds) between the urea solution with excess ammonia and the introduced fresh CO.sub.2 be obtained in the adiabatic ammonia separation step (S). FIGS. 1, 2 and 3, where an appropriate layer of mass transfer promoter (L) (for ex. rings or trays) is foreseen, represent different embodiments of the invention, where:

in FIG. 1 the layer of appropriate mass transfer promoter (L) is installed in a separate equipment (E) the top part of which (T) functions as separator for the evolved ammonia collection. An appropriate liquid distributor (D) is also foreseen;

in FIG. 2 the ammonia removal step (S) is located in the bottom part of the reactor (R) where the layer (L) of appropriate mass transfer promoter is installed in a reactor bottom empty space (ES);

in FIG. 3 the ammonia removal step (S) is also located in the bottom part of the reactor (R), where the layer (L) of appropriate mass transfer promoter is installed in a reactor bottom empty space (ES) of reduced diameter.

FIG. 4 represents the preferred embodiment of the invention. In the adiabatic ammonia separation step (S) the intimate contact between the urea solution with excess ammonia (stream 2) and the introduced fresh CO.sub.2 (stream 1) is obtained in a very short time, achieving a very high mass transfer, in a Venturi type mixer (VM). The evolved ammonia vapor is then separated from the urea solution in the separator (SEP).

The advantageous features of the invention can be evidenced by the following comparison of the energy consumption (steam consumption in the loop) of the above mentioned known processes (last and new generation processes) with those of the examples describing the present invention. The consumption figures of the known processes are taken from Dooyeweerd and Messen, Nitrogen issue n. 143 May 1983.

ACES Process (MT/TEC): 474 kg of 22 bar steam for 1000 kg urea

IDR Process (Montedison): 524 kg of 22 bar steam for 1000 kg urea

CO.sub.2 Stripping (Stamicarbon): 633 kg of 18 bar steam for 1000 kg urea

EXAMPLES 1 and 2: 190 kg of 22 bar steam for 1000 kg urea

EXAMPLE 3: 150 kg of 8 bar steam for 1000 kg urea

The features of the invention will be better illustrated by the following examples, where isobaric loops are described. The same improved results can be obtained with schemes where the stripper (S) operates at lower pressure than the ammonia separation step (S).

Reference is made to FIGS. 1, 2, 3 and 4 (isobaric loop)

In the downstream sections (not represented in the figure) for the removal and recycle of the residual NH.sub.3 and CO.sub.2 contained in the urea solution coming from the CO.sub.2 stripper, before final urea solution vacuum concentration to obtain finished product, the 6 to 7 bar steam produced in the carbamate condenser (CC) can be used. By the use of the technique of process to process direct heat recovery (multiple effect system) no extra steam will have to be imported from the plant battery limits.

Reference is made to FIGS. 1, 2, 3 and 4 (isobaric loop)

Reference is made to FIGS. 1-2-3 and 4 (isobaric loop). Compared to example 2, operating conditions have been modified to have the stripper (ST) operating in adiabatic conditions. In this case the stripper (ST) could be an apparatus different from a tube exchanger (ex. trays column) but to minimize residence time a falling film type tubes apparatus might still be the best choice as indicated in the figures.

In the downstream sections (not represented in the figure), for the removal and recycle of the higher residual NH.sub.3 and CO.sub.2 contained in the urea solution coming from the CO.sub.2 stripper, before final urea solution vacuum concentration to obtain finished product, the 7 to 8 bar steam produced in the carbamate condenser (CC) can be used.

By the use of the technique of process to process direct heat recovery (multiple effect system), a reduced amount of 150 kg for 1000 kg urea of 8 bar steam will have to be imported from the plant battery limits.

This example refers to the last generation Stamicarbon CO.sub.2 stripping process modified according to the invention (see FIGS. 1, 2, 3 and 4) in a case of a Stamicarbon CO.sub.2 stripping plant modernization to reduce energy consumption.

The 22 bar steam consumption in the loop (CO.sub.2 stripper) is reduced of 100 kg for 1000 kg urea with a modest investment for the installation of the ammonia separation step (S).

This example refers to the use of the invention for the revamping of the total or partial recycle conventional non stripping processes (Montedison, Mitsui Toatsu, etc), to reduce energy consumption. With the use of the ammonia separation and direct ammonia recycle step (S), to treat the urea solution from the reactor, before the first decomposition step, a smaller quantity of ammonia and, as a consequence, of water, will have to be recycled in the downstream sections, improving reactor conversion yields with the reduction of water vaporization. For both the above mentioned reasons (higher conversion yields and, consequently, less carbamate to be recycled and less vaporised water) a reduction of the 8 to 15 bar battery limits steam, of 300 kg per 1000 kg urea can be obtained.

This example refers to the use of the invention for the revamping of the Snamprogetti NH.sub.3 stripping plants, to reduce energy consumption and maintenance and operating costs.

With the use of the ammonia separation and recycle step (S), in this case located downstream the stripper, to remove the high excess ammonia content in the treated urea solution stream (the high excess ammonia of the urea solution from the reactor favours the NH.sub.3 self-stripping carbamate separation in the stripper), a smaller quantity of ammonia will have to be recycled in the downstream sections. The use of the rectifying column to separate and recycle pure ammonia with high costs and energy consumption, is so avoided.