LOW STEAM CONSUMPTION PROCESS FOR THE MANUFACTURE OF UREA

Abstract: An I.D.R. process for the manufacture of urea, comprising two subsequent loops, wherein: A) In the first (high pressure) loop the solution coming from the synthesis zone contains, after the first isobaric strip per (E1), a strong NH₃ excess and a low CO₂ percentage and contains, before the second isobaric stripper (E2), an amount of carbamate lower than the amount of carbamate contained in the solution leaving the same (E2) stripper, wherein the consequent overproduction of carbamate (in E2) releases an amount of heat sufficient to displace a major proportion of the residual NH₃ contained therein; B) In the second (low pressure) loop, the solution leav­ ing (E2) is flashed at 20-30 ata and enters a first (medium pressure) still (E3), heated by a low pressure recovery steam, coming from the isobaric condensation (in E8) of the vapors leaving the top of (E2) said recovery steam having such a thermal level to decompose at least said overproduction of carbamate; C) the solution leaving (E3) is flashed substantially at 5 ata and enters a second (low pressure) still (E4) and then a vacuum concentrator (E5) wherein the heat of the vapors leaving the head of (E3) is exploited for the working run of (E4) and (E5).

Description: LOW STEAM CONSUMPTION PROCESS FOR THE MANUFACTURE OF UREA The invention concerns a process having a low steam consumption for the manufacture of urea. Improvements to urea manufacturing plants have allowed, until now, to lower the steam consumption from 1500 to 1000 kg per ton of urea or, in the most efficient case, 550-600 kg/t; U.S patent 4,208,347 deals with a new, double loop process (I.D.R. process), but describes only the high pressure loop and does not take into account the final, low pressure, unit (the so called "finishing). The Applicant (see: "New process gives urea with less energy" (Hydrocarbon Processing, Nov. 1982).) has later described an I.D.R. complete manufacturing line, "finishing" included, comprising a high pressure loop (reactor, two subsequent isobaric strippers and a carbamate condenser) and a "finishing" comprising: I - a medium pressure still (20-30 ata), the vapors of which are condensed within a vacuum concentrator, substan tially at 0.35 ata (see item II), thus realizing a multiple effect evaporation, and a low pressure still, working substan tially at 5 ata (atmospheres, absolute); II - a two-step vacuum concentration section (respectively -and approximately- at 0.35 and 0.05 ata) and one unit for the recovery of NH3 from the reaction waters and from the scrub ber waters coming from the washing of the synthesis vent. The steam consumption of the complete I.D.R. line is slightly lower than 600 kg/t and the Applicant has foreseen a further decrease to 500 kg/t. The present invention concerns an improved process for the manufacture of urea, which process allows to lower considerably, in a new and original way, the steam consumption of said I.D.R. line; i.e. the high pressure loop and the finishing unit of the present invention were matched in an original way, so as to improve synergistlcally (and therefore surprisingly) the whole I.D.R. line, while lowering the (outside) steam requirement to unexpected levels. In its most general forum (see figure 1) the invention concerns an I.D.R. process for the manufacture of urea, comprising two subsequent loops, wherein: A - in the first (high pressure) loop the solution coming from the synthesis zone contains, after the first isobaric strip per of.said I.D.R. process (El), a strong NH3 excess (NH3:C02 6 by moles) and a low C02 percentage (C02: :urea s 25% b.w.) and contains, before the second isobaric stripper (E2) of said I.D.R. process, an amount of ammonium carbamate (CBE) lower than the amount of carbamate (CBU) contained in the solution leaving the same (E2) stripper, wherein the conse quent overproduction of carbamate (in E2) releases an amount of heat sufficient to displace a major proportion of the residual NH3 excess contained therein, which allows a reduc tion of the steam wasted in (E2); ; B - in the second (low pressure) loop, the solution leaving (E2) is flashed at 20-30 ata and enters a first (medium pressure) still (E3), heated Indirectly and completely by low pressure recovery steam, coming from the isobaric condensation (in E8) of the vapors leaving the top of (E2), said recovery steam having such a thermal level to decompose at least said overproduction of carbamate, originated in (E2); C - the solution leaving (E3) is flashed substantially at 5 ata and enters a second (low pressure) still (E4) and then a vacuum concentrator (E5), wherein the heat of the vapors leaving the head of (E3), rich in NH3 and CO2, is exploited for the working run of (E4) and (E5). DETAILS - The pressure of the "high pressure" loop is usually comprised from 120 to 250 kg/cm2. - The temperature of the solution leaving the first isobaric strip per (El) is normally from 150 to 2300C. - Said NH3:C02 molar ratio of the solution leaving the first iso baric stripper El (a6 by moles) concerns the total amounts of NH3 and CO2, not only chemically bound (e.g. as ammonium carbamate), but also for the amounts bound in the form of urea. - Said second isobaric stripper (E2) (see European patent appli cation 83/105592.6) is a falling-film tube-bundle heat exchanger, where the solution to be stripped meets a countercurrent stream of CO2, injected from outside, the pipes being heated by condensing steam only in their uppermost portion. - Said recovery steam (coming'from E8) is at a pressure 26.5 abso lute. atmospheres. - The usual working pressure of the process side of said vacuum concentrator (E5) is approximately 0.35 absolute atmospheres. Because of the high yields of the I.D.R. process, the ratio between the fresh CO2, fed to the second isobaric stripper (CO2 inj.), and the carbamate (CBE) entering the same stripper attains a very high level; in general: (CO2 inj.):(CBE) > 2 (by moles) According to a preferred embodiment of the invention, the solution leaving (E5) must be further concentrated in (E6) (as an optional and advantageous way of carrying out the invention) exploiting the heat of the recovery steam herein-above and the resulting concentrated solution (urea = 90-95% b.w.) is transferred to the final vacuum concentrator (E7); the vapors leaving the vacuum concentrators (ES, E6 and E7), containing a certain amount of NH3 and C02, are then condensed in a usual way and a recovery tower (C3) allows to obtain a recovery stream to be condensed (in Ell), together with the vapors coming from said still (E4). The resulting condensate is recycled to said (E4) still (shell side); the condensed vapors leaving (E5) are recompressed (by pump PC1) and recycled to the high pressure loop. The first isobaric stripper (El) of the I.D.R. process can be advantageously a vertical heat exchanger having a tube-bundle and being selected from the group comprising the so called "falling-film" and "upflow" exchangers. The supply of steam (at 18-30 ata) to said second isobaric stripper (E2) may be also lowered to zero, thereby performing a purely adiabatic stripping. The determination of the size of said second (E2) stripper is one of the most delicate problems of the whole process; the ratio: R = uppermost surface of the pipes, heated by condensing steam remnant portion of the surface of the pipes should be R < 1 and preferably from 1/2 to 1/4; i.e., the uppermost surface of the tubes should be from 1/2 to 1/5 (preferably from 1/5 to 1/3) of the total surface. The residence time of the urea solution, in fact, in said uppermost portion of the exchanger must be very short, i.e. equal to or lower than 3 s (preferably 2.5 and even better from 1.5 to 2.5 s), the ratio: residence time in the lowermost portion residence time in the uppermost portion thus being #1 (pref. a2, even better a4); furthermore, the ratio between the exchanging surface (ScO ) of said stripper (E2) and the exchanging surface ( (SNH ) of said stripper (El) must be preferably greater than one, namely: SCO2: SNH3#1 According to a narrower form of the invention, it is advanatageous to carry out a process in which: : a) the urea synthesis is performed at 150-250 C and 170-220 bar; b) the temperature of the solution entering said second stripper (E2) is from 190 to 220 C; c) the temperature of the solution leaving the bottom of the same stripper (E2) is comprised between 150 and 1900C, preferably between 160 and 1900C and advantageously between 160 and 175 C; d) the temperature of the CO2 (carbon dioxide), injected as a stripping agent into the same stripper (E2), is 80-140DC; e) the solution leaving said first stripper (El) and entering said second stripper (E2) contains such a low amount of CO2 that: CO2 : urea s 22% b.w.; f) the NH3:C02 molar ratio, in said solution entering said second stripper (E2), is equal to or higher than 10. The condensation pressure in (E5), and consequently also in (E4), of the heating vapors coming from (E3) is very low; therefore the pressure of the solution to be distilled in (E3) can be lowered to such a level that the heating of the same still (E3) can be carried out very advantageously by-means of recovery steam (at 6.5-9 ata) coming from condenser (E8). The lowest temperature of the whole low-pressure loop (about 900C) is registered at the inlet of the first vacuum concentrator (E5), whereafter the temperature gradually increases with the degree of concentration of the final urea solution; this is the reason why the condensation temperature on the shell side of said concentration (E5) is also very low (115-120"C) and this is why, consequently, the pressure of the whole low-pressure loop can be kept at a lower level. The basic idea of the new process resides in the transfer of a portion of the thermal load from said second stripper (E2) to the first medium-pressure still (E3), this latter apparatus, because of the lower working pressure, being very advantageously heated by recovery steam; all this is the counterbalance of the fact that the very satisfactory stripping of ammonia in said stripper (E6) is bound to the parallel formation of carbamate, the amount of which carbamate, at the outlet of stripper (E2), is greater than at the inlet. The improvement according to the invention is satisfactory, not only when performed within an I.D.R. process, defined in a narrow sense, but also when performed in those double-stripping processes in which the second stripper (E2) (in countercurrent with CO2) is working at a pressure slightly different (several tens of atmospheres) from the pressure of the first stripper (El); see e.g. Italian patent publication 24357 A/80. In "a narrow sense" it is meant that the two strippers (El) and (E2) are isobaric with each other and with the synthesis reactor, apart from the pressure drops bound to hydraulics (and similar). As it is known, according to the equation: EMI6.1 300 kg of water are by-produced per ton of urea and such water is removed, for the most part, within two groups of vacuum concentrators, working at a different degree of vacuum. Until now, the vapors coming from said concentrators, containing 5-76 b.w. NH3 and 2-3% b.w. CO2, were condensed and pumped to a recovery tower (C3 in figure 1), so as to recover the NH3 contained therein in order to lower the amount of residual NH3 -in the waste waters- at least to 100-200 ppm, while obtaining, from the head of the same tower C3, vapors containing 25-30% b.w. NH3 (depending on the working pressure) and these vapors were condensed and recycled; the steam required by such a recovery unit was approximately 120 kg/t of urea. The new low-pressure loop of the present invention (see figure 2) is an advantageous alternative, with respect to the old kinds of process, in that said steam consumption (120 kg/t) can be completely suppressed. According to said alternative (and to figure 2), the vapors coming from the vacuum concentrator (E6) are condensed within a water-cooled apparatus (E13), into which is also conveyed a portion of the vapors (15) coming from the final vacuum concentrator (E7) (previously compressed in EJ2), the remnant portion (25) being fed, as a liquid phase, to the recovery tower (C3); the basis of such an alternative resides in that said (C3) tower contains a vertical falling-film tube-bundle, heated by the vapors (11) coming from said low-pressure still (E4), and in that the vapors (26), containing NH3 and coming from said (C3) tower, are also condensed within said water-cooled apparatus (E13), whereafter the liquid condensate (27) leaving said apparatus (E13) is recycled to said tower (C3), the bottom of which tower, practically free from NH3 and usually at 70-720C, is partially wasted (line 30) and partially (line 29) utilized for the scrubbing of the synthesis vent (22). The vapors (rich in recovered NH3) leaving said apparatus (E13) (see line 31 in figure 3) are treated in a usual way; they can be precondensed in E14), compressed, for instance, by a vacuum pump PV1 (e.g. at 1-1.5 kg/cm2, absolute) and then condensed (in E15), further compressed (by pump PC2), cooled in (E16), together with the scrubbing liquid (35) from tower (scrubber) (C2). The steam consumption is thus suppressed, while there is a slightly higher power need, (approximately 2 Kwh per t of urea). The performance of said apparatus (E13) can be surprisingly improved, when using, as shown in figure 3, a vertical exchanger, containing a tube-bundle, wherein the cooling medium, e.g. H20 flows within the bundle pipes, said exchanger being characterized in that the vapors to be (at least partially) condensed enter the lowermost portion of the shell and leave the same shell in its uppermost portion, thus flowing in countercurrent to the liquid condensate, which falls downwards, at least partially along the outer surface of the pipes, and which leaves the lowermost portion of said shell, in a lower position, with respect to the inlet of the vapors. The following examples illustrate the invention without however limiting in any sense the scope thereof. EXAMPLE 1 Operative conditions and composition of the process lines are recorded on Table 1. According to figure 1, the effluent (3), coming from reactor R1, was transferred to the first (falling-film) stripper (El), isobaric with said reactor, in countercurrent to a NH3 stream (2). It is incidentally noted that this stripper (E1) can obviously also work in a purely thermal way, namely without ariy injection of NH3 from outside; furthermore, the falling-film apparatus can be replaced by an "up-flow" apparatus, like the one indicated in figure 4. The solution leaving (E1) flowed into the second (falling-film) isobaric stripper (E2), in countercurrent to a C02 stream (24). The solution (6) leaving (E2) was flashed from 195 to 25 kg/cm2 (absolute) and flowed to the medium pressure (falling-film) still (E3) and the solution (8) leaving the bottom of (E3) was in turn flashed to 5 kg/cm2 (absolute) and transferred to the low-pressure (falling-film) still (E4). The steam condensates coming (at about 2250C) from (El) and (E2) were used for heating part of the NH3 feed (at 2000C), while the remnant portion of NH3 was pre-heated (at 150-1600C) by means of recovery steam. The vapors (9) leaving the head of (E3) supplied, while condensing within the shell side, all the heat required by said still (E4) and by the first vacuum concentrator (E5), working at about 0.35 kgicm2 (absolute), where the solution was concentrated up to about 85% b.w. (of urea); ; a further vacuum concentrator (E6), also working at 0.35 kg/cm2, and the final vacuum concentrator (E7), working at 0.05 kg/cm (absolute), brought the concentration up to 99.7X b.w., whereafter the molten urea (13) was ready to flow to a final trilling section (P) or to other treatment or use, of quite current nature. The vapors (14) and (15) leaving the head of the vacuum concentrators (E6) and (E7) were condensed within a usual condensation system and were transferred, in the liquid state, to the recovery column (C3), from which a recovery vapor (20) was obtained, that was condensed in (Ell), together with the head vapors (11) of the still (E4); the resulting condensate (21) entered the shell side of the same (E4) still. The condensed vapors (23) leaving (E5) were recompressed from 25 to 195 kg/cm2 (absolute) and then conveyed (in part, namely about 90%) to the high pressure (isobaric) condenser (E8), by means of ejector (EJ1); the remnant (about 10%) was used as a scrubbing liquid for the vent coming from the reactor (tower C1 in figure 1). The steam balance is recorded on table A; as the urea output was 43,863 kg/h, the medium-pressure steam waste (at 25 kg/cm2) was: (13,158:43,863)x1000 = 300 kg/t of urea and the low-pressure steam waste (at 8 kg/cm2) was only: (40,120-36,845):43,863x1000 = 75 kg/t. In other words, the process was nearly self-supporting (as to the low-pressure steam requirement) and the medium-pressure steam consumption was limited to 300 kg/t, which was indeed new and surprising. These results can be moreover improved by further decreasing the low-pressure steam consumption; in particular, a new recovery system (i.e. recovery of NH3 from the reaction waters), allowed to suppress completely the low-pressure steam. The basic idea of such a new system was the evaporation of said waters at the same pressure of the first vacuum concentrator (E5), the heat being supplied by a partial condensation of the vapors coming from (E4); said new system is described in detail hereinafter in Example 2. EXAMPLE 2 Example 1 was repeated, while replacing the recovery operations of figure 1 (hatch area) by the operations shown in figure 2; data and results were recorded on Table 2 and the steam balance on Table A. According to said figure 2, to said Table 2 and to the preceding figure 1, said vapors (14) coming from (E6) were condensed within said apparatus (E13), and said vapors (15) coming from (E7) were condensed (for the most part) first in the water-cooled apparatus (E17) and then in (E13), after a pressure Increase, by means of ejector EJ2, driven by low-pressure steam. A condensated portion (25) of said vapors (15) left the (E17) cooler and entered the recovery tower (C3), consisting of a vertical falling-film tube-bundle, heated by the vapors (11) coming from the still (E4). Also the vapors (26) coming from the (C3) tower entered said apparatus (E13), whereafter the liquid condensate (27) leaving same apparatus (E13) was recycled to said tower (C3), the bottom of which tower, practically free from NH3 and about at 70"C, was partially wasted (line 30) and partially (line 29) utilized in (C2), namely in the scrubber-of the vent gases (22). Said waste (30) to disposal practically corresponds to the synthesis water plus the amount of steam used for the driving of ejector EJ2. The vapors (31), rich in recovered NH3 and leaving the head of (E13), were partially condensed within the (E14) cooler; the condensate (through pump PC7) and the residual vapors (through vacuum pump PV1) were transferred to a condensation system (E15) working at about 1.5 kg/cm2 (absolute). The condensate herefrom was transferred, through pump (PC2), to the final condensation system (E16), working at 5 kg/cm2 (absolute), to which also flowed the condensing mixture (28) coming from tower (C3), the cooling medium being the fresh NH3 of the feed, which was thus pre-heated to 70 C. The solution (34) thus obtained joined solution (35) coming from abatement scrubber (C2) and the mixture flowed to the shell side of the low-pressure still (E4). Such a recovery cycle allowed to remove all the synthesis water, while using recovery steam; the steam requirement (from outside) for the distillation (stills E3 and E4) and for the first vacuum concentrator (E5) was therefore zero, while the power requirement for the vacuum pump (PV1) was merely about 2 Kwh/t of urea. With respect to example 1, we registered the output of an excess of low-pressure steam (at 8 kg/cm2, absolute), that can be calculated as follows (see Table A): (36,845-34,406):43,863x1000 = 55 kg/t. EXAMPLE 3 Example 2 was repeated, while modifying the first isobaric stripper (El), which was an "up-flow" equicurrent apparatus, instead of the former falling-film (countercurrent) exchanger (see figure 4); no stripping gas was injected from outside, as a coadjuvant agent. Furthermore, the working of the second isobaric stripper (E2) was adiabatic; no indirect heat (namely no condensing steam) was supplied to (E2) and the displacement of the NH3 excess was merely bound to the parallel formation of carbamate and to the "self-cooling" of the solution. Moreover, a portion (36) of the vapors leaving the head of said second stripper (E2) was recycled directly to the synthesis reactor (R1), because of the thermal balance in a point higher than the inlet of the vapors coming from said first stripper (eel). Within said reactor the NH3:C02 molar ratio was 4.5, the H20/C02 molar ratio was 0.4 and the carbamate conversion per pass was 73%. Data and results are on Tables 3 and 4; the NH3 recovery cycle (from the synthesis waters) is still the cycle of example 2 and the steam balance is recorded on Table A. The reduction of the medium-pressure steam consumption to 150 kg/t (see Table A) allows to approximate considerably the exciting threshold of a zero steam requirement, namely the thermal independence. EXAMPLE 4 Example 3 was repeated, apart from the differences hereinafter: more heat was supplied to the (E3) (by means of an increase of the recovery steam coming from E8), namely more than the heat required by (E4) and (E5); the consequent excess of heat was recovered by cooling the vapors (9) leaving (E3), rich in NH3 and C02, within a proper exchanger (ElO/bis), which pre-heated the NH3 fresh feed (see figure 5). Such a new exchanger was connected in series with (E4) and (E5) and was fed not only by vapors (9) described herein-above but also by a liquid phase (21) in order to speed up the condensation of the same vapors (9) (see figure 5). The steam balance was recorded on Table A. Should the exchanger ElO/bis be connected in parallel, results would be nearly the same. TABLE A AMOUNTS OF STEAM EXAMPLE 1 EXAMPLE 2 EXAMPLE 3 EXAMPLE 4 Waste of medium pressure saturated (at 25 kg/cmê, absolute): - to the first isobaric stripper (E1) 10088 kg/h 10088 kg/h 6580 kg/h 6580 kg/h - to the second isobaric stripper (E2) 3070 kg/h 3070 kg/h - Total 13158 kg/h 13158 kg/h 6580 kg/h 6580 kg/h Waste of low-pressure saturated steam (at 8 kg/cmê, absolute): : - to still (E3) 17545 kg/h 17545 kg/h 15483 kg/h 17676 kg/h - to vacuum concentrator (E6) 5263 kg/h 5263 kg/h 5263 kg/h 5263 kg/h - to vacuum concentrator (E7) 3948 kg/h 3948 kg/h 3948 kg/h 3948 kg/h - to vacuum system (driving of a battery of ejectors) 1754 kg/h 420 kg/h 420 kg/h 420 kg/h - to recovery tower (C3) 4380 kg/h - - - to NH3 pre-heating 4380 kg/h 4380 kg/h 5910 kg/h 1530 kg/h - to steam-tracings 2850 kg/h 2850 kg/h 2850 kg/h 2850 kg/h Total 40120 kg/h 34406 kg/h 34406 kg/h 33874 kg/h 31687 kg/h Recovery of saturated steam (at 8 kg/cmê, absolute) from (E8) 36845 kg/h 36845 kg/h 31669 kg/h 31669 kg/h Summary (unitary amounts) Waste of 25 kg/cmê steam (kg of urea) 300 kg/t 300 kg/t 150 kg/t 150 kg/t Recovery (balance) of 8 kg/cmê steam -75 kg/t(*) +55 kg/t -50 kg/t(*) (*) Waste TABLE 1 Page I LINE 1 2 3 4 5 6 State Liquid Gas Liquid Vapor Liquid Liquid P (kg/cmê abs.) 195 195 195 195 195 T( C) 200 194 195 203 157 COMPOSITION kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. Urea - - - - 43,683 34.50 - - 43,863 37.22 43,863 45.77 NH3 24,856 100.00 7,500 100.00 45,634 35.89 11,396 67.81 41,738 35.42 14,135 14.76 CO2 - - - - 13,789 10.84 5,043 30.00 8,746 7.42 15,343 16.01 H2O - - - - 23,865 18.77 367 2.19 23,498 19.94 22,480 23.46 TOTAL 24,856 100.00 7,500 100.00 127,151 100.00 16,806 100.00 117,845 100.00 95,821 100.00 TABLE 1 Page II LINE 7 8 9 10 11 12 State Liquid Gas Liquid Vapor Liquid Liquid P (kg/cmê abs.) 195 25 25 5 5 0,35 T( C) 200 157 160 135 135 115 COMPOSITION kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. Urea - - 43,863 60.20 - - 43,863 72.00 - 43,863 92.26 NH3 27,603 50.94 5,422 7.44 8,713 37.95 914 1.50 4,508 37.74 119 0.25 CO2 25,569 47.18 2,889 3.96 12,454 54.25 305 0.50 2,584 21.64 47 0.10 H2O 1,018 1.88 20,690 28.40 1,790 7.80 15,839 26.00 4,851 40.62 3,516 7.39 TOTAL 54,190 100.00 72,864 100.00 22,957 100.00 60,921 100.00 11,943 100.00 47,545 100.00 TABLE 1 Page III LINE 13 14 15 16 17 18 State Liquid Vapor Vapor Liquid Liquid Liquid P (kg/cmê abs.) 0,05 0,35 0,05 - - T( C) 140 120 140 40 40 40 COMPOSITION kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. Urea 43,863 99.70 - - - - - - - - - NH3 - - 795 5.94 119 3.35 914 4.89 - - 758 4.89 CO2 - - 258 1.93 47 1.32 305 1.63 - - 253 1.63 H2O 132 0.30 12,323 92.13 3,384 95.33 17,457 93.48 14,777 100.00 14,489 93.48 TOTAL 43,995 100.00 13,376 100.00 3,550 100.00 18,676 100.00 14,777 100.00 15,500 100.00 TABLE 1 Page IV LINE 19 20 21 22 23 24 State Liquid Vapor Liquid Gas(*) Liquid Gas P (kg/cmê abs.) 5 5 25 195 25 195 T( C) 60 80 50 about 195 125 130 COMPOSITION kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. Urea - - - - - - - - - - - NH3 304 8.73 1,062 25.25 5,570 34.39 148 - 14,283 - CO2 210 6.03 463 11.01 3,047 18.87 158 - 15,501 39.64 32,166 100.00 H2O 2,968 85.24 2,680 63.74 7,531 46.64 - - 9,321 23.84 - TOTAL 3,482 100.00 4,205 100.00 16,148 100.00 306 100.00 39,105 100.00 32,166 100.00 *Inert gas + NH3 + CO2 TABLE 2 Page I LINE 25 26 27 28 29 30 State Liquid Vapor Liquid Liquid+Vapor Liquid Liquid P (kg/cmê abs.) 0.05 0.35 0.35 5 0.35 1 T( C) 40 60 40 85 70 70 COMPOSITION kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. Urea - - - - - - - - - - - NH3 71 2.08 176 4.83 105 0.75 4.508 37.74 - - - - CO2 47 1.38 75 2.05 28 0.20 2,584 21.64 - - - H2O 3,295 96.54 3,400 93.12 13,852 99.05 4,851 40.62 300 100.00 13,447 100.00 TOTAL 3,413 100.00 3,651 100.00 13,985 100.00 11,943 100.00 300 100.00 13,447 100.00 TABLE 2 Page II LINE 31 32 33 34 35 State Vapor Liquid Liquid Liquid Liquid P (kg/cmê abs.) 0.35 0.35 5 5 25 T( C) 50 50 40 50 50 COMPOSITION kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. Urea - - - - - - - - - NH3 914 25.40 390 12.93 914 25.40 5,422 34.88 148 24.42 CO2 305 8.47 305 10.12 305 8.47 2,889 18.59 158 26.07 H2O 2,380 66.13 2,320 76.95 2,580 66.13 7.231 46.53 300 49.51 TOTAL 3,599 100.00 3,015 100.00 3,599 100.00 15,542 100.00 606 100.00 TABLE 3 Page I LINE 1 2 3 4 5 6 State Liquid Gas Gas Vapor Liquid Liquid P (kg/cmê abs.) 190 190 190 190 190 T( C) 200 190 200 200 165 COMPOSITION kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. Urea 43,863 34.30 - - 43,863 37.27 43,863 49.25 NH3 24,856 100.00 3,500 100.00 51,752 40.47 9,697 95.16 42,055 35.73 12,055 13.54 CO2 11,897 9.30 343 3.37 11,554 9.82 13,915 15.63 H2O 20,367 15.93 150 1.47 20,217 17.18 19,217 21.58 TOTAL 24,856 100.00 3,500 100.00 127,879 100.00 10,190 100.00 117,689 100.00 89,050 100.00 TABLE 3 Page II LINE 7 8 9 10 11 12 State Vapor Liquid Vapor Liquid Vapor Liquid P (kg/cmê abs.) 190 25 25 5 5 0.35 T( C) 295 155 155 135 135 115 COMPOSITION kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. Urea - - 43,863 61.31 - - 43,863 72.00 - 43,863 92.26 NH3 28,088 49.34 4,792 6.70 7,263 41.48 914 1.50 3,878 36.51 119 0.25 CO2 27,906 49.02 4,515 6.31 9,400 53.69 305 0.50 4,210 39.64 47 0.10 H2O 936 1.64 18,372 25.68 845 4.83 15,839 26.00 2,533 23.85 3,516 7.39 TOTAL 56,930 100.00 71,542 100.00 17,508 100.00 60,921 100.00 10,621 100.00 47,545 100.00 TABLE 3 Page III LINE 13 14 15 State Liquid Vapor Vapor P (kg/cmê abs.) 0.05 0.35 0.05 T( C) 140 120 140 COMPOSITION kg/h % b.w. kg/h % b.w. kg/h % b.w. Urea 43,863 99.70 - - - NH3 - - 795 5.95 119 3.35 CO2 - - 258 1.93 47 1.32 H2O 132 0.30 12,323 92.13 3,384 95.33 TOTAL 43,995 100.00 13,376 100.00 3,550 100.00 TABLE 3 Page IV LINE 21 22 23 24 State Liquid Gas(*) Liquid Gas P (kg/cmê abs.) 25 190 25 190 T( C) 70 circa 190 125 130 COMPOSITION kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. Urea - - - - - - - NH3 4,940 33.42 148 - 12,203 37.74 - CO2 4,673 31.52 158 - 14,203 43.52 32,166 100.00 H2O 5,213 35.16 - - 6,058 18.74 - TOTAL 14,826 100.00 306 100.00 32,334 100.00 32,166 100.00 TABLE 4 Page I LINE 25 26 27 28 29 30 State Liquid Vapor Liquid Liquid+Vapor Liquid Liquid P (kg/cmê abs.) 0.05 0.35 0.35 5 0.35 1 T( C) 40 60 40 85 70 70 COMPOSITION kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. Urea - - - - - - - - - - - NH3 71 2.08 176 4.83 105 0.75 3,878 36.51 - - - CO2 47 1.38 75 2,05 28 0.20 4,210 30.64 - - - H2O 3,295 96.54 3,400 93.12 13,852 99.05 2,533 23.85 300 100.00 13,447 100.00 TOTAL 3,413 100.00 3,651 100.00 13,985 100.00 10,621 100.00 300 100.00 13,447 100.00 TABLE 4 Page II LINE 31 32 33 34 35 36 State Vapor Liquid Liquid Liquid Liquid Vapor P (kg/cmê abs.) 0.35 0.35 5 5 25 190 T( C) 50 40 40 50 50 200 COMPOSITION kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. kg/h % b.w. Urea - - - - - - - - - - - NH3 914 25.40 390 12.93 914 25.40 4,792 33.70 148 24.42 1,912 49.34 CO2 305 8.47 305 10.12 405 8.47 4,515 31.75 158 26.07 1,899 49.01 H2O 2,380 66.13 2,320 76.95 2,380 66.13 4,913 34.55 300 49.51 64 1.65 TOTAL 3,599 100.00 3,015 100.00 3,599 100.00 14,220 100.00 606 100.00 3,875 100.00

Claims: CLAIMS 1.- An I.D.R. process for the manufacture of urea, comprising two subsequent loops, wherein: A) in the first (high pressure) loop the solution coming from the synthesis zone contains, after the first isobaric stripper (El), a strong NH3 excess (NH3:CO2 > 6 by moles) and a low C02 percentage (CO2: :urea < 25% b.w.) and contains, before the second isobaric stripper (E2), an amount of ammonium carbamate lower than the amount of carbamate contained in the solution leaving the same (E2) stripper, wherein the consequent overproduction of carbamate (in E2) releases an amount of heat sufficient to displace a major proportion of the residual NH3 excess contained therein; B) in the second (low pressure) loop, the solution leaving (E2) is flashed at 20-30 ata and enters a first (medium pressure) still (E3), heated indirectly and completely by a low pressure recovery steam, coming from the isobaric condensation (in E8) of the vapors leaving the top of (E2), said recovery steam having such a thermal level to decompose at least said overproduction of carbamate, originated in (E2); ; C) the solution leaving (E3) is flashed substantially at 5 ata and enters a second (low pressure) still (E4) and then a vacuum concentrator (E5), wherein the heat of the vapors leaving the head of (E3), rich in NH3 and CO2, is exploited for the working run of (E4) and (E5). 2.- A process according to claim 1, wherein: - the pressure of the high pressure" loop is from 120 to 250 kg/cm2; - the temperature of the solution leaving the first isobaric stripper (E1) is from 150 to 2300C; - said NH3:C02 molar ratio ( > 6 by moles at the outlet of the first stripper) concerns the total amounts of NH3 and CO2, not only if chemically bound, but also for the amounts bound in the form of urea; - said second isobaric stripper (E2) is a falling-film tube-bundle heat exchanger, where the solution to be stripped meets a counter current stream of CO2, injected from outside, the pipes being heated by condensing steam only in their uppermost portion; - said recovery steam (coming from E8) is at a pressure > 6.5 absolute atmospheres; ; - the working pressure of the process side of said vacuum concentra tor (E5) is substantially 0.35 absolute atmospheres; - the ratio between the fresh CO2, fed to the second isobaric stripper (C02 inj.), and the carbamate (CBE) entering the same stripper attains a very high level, namely: (CO2 inj.):(CBE) > 2 (by moles). 3.- A process according to claim 1, wherein said first isobaric stripper (E1) of the I.D.R. process is a vertical heat exchanger having a tube-bundle and being selected from the group comprising the "fallingfilm" and the "upf0ow1, exchangers. 4.- A process according to claim 1, wherein the supply of steam (at 18-30 ata) to said second isobaric stripper (E2) is lowered to zero performing thereby a purely adiabatic stripping. 5.- A process according to claim 1, wherein the solution leaving (E5) is further concentrated in (E6) exploiting the heat of the recovery steam herein-above, and the resulting concentrated solution is transferred to a final vacuum concentrator (E7), wherein the vapors leaving the vacuum concentrators (E5, E6 and E7), containing some amounts of NH3 and CO2, are condensed in a usual way and wherein a recovery tower (C3) allows to obtain a recovery steam to be condensed (in Ell), together with the vapors coming from said still (E4), the resulting condensate being recycled to said (E4) still (shell side) and the condensed vapors leaving (E5) being recompressed and recycled to the high pressure loop. 6.- A process according to claim 5, wherein (within said second stripper E2) the ratio: uppermost surface of the pipes, heated by condensing steam remnant portion of te surface of the pipes is R < 1, and preferably from 1/2 to 1/4, the uppermost surface of the pipes thus being from 1/2 to 1/5 (preferably from 1/5 to 1/3) of the total surface. 7.- A process according to claim 5, wherein the residence time of the urea solution, in said uppermost portion of the exchanger (E2) is equal to or lower than 3 s (preferably 2.5 and advantageously from 1.5 to 2.5 s), the ratio: residence time in the lowermost portion residence time in the uppermost portion thus being > 1 (pref. > 2, advantageously > 4). 8,- A process according to claim 5, wherein: a) the urea synthesis is performed at 150-2500C and 170-220 atmos pheres; b) the temperature of the solution entering said second stripper (E2) is from 190 to 2200C; c) the temperature of the solution leaving the bottom of the same stripper (E2) is from 150 to 1900C, preferably from 160 to 1900C and advantageously from 160 to 1750C; d) the temperature of the C02, injected as a stripping agent into the same stripper (E2), is from 80 to 1400C; e) the solution leaving said first stripper (El) and entering said second stripper (E2) contains a low amount of CO2 so that: CO2 : urea < 22% b.w.; f) the NH3 : CO2 molar ratio, in said solution entering said second stripper (E2), is equal to or higher than 10. 9.- A process according to claim 5, wherein the ratio between the exchanging surface (SC0 ) of said stripper (E2) and the exchanging surface ( H ) of said 2 stripper (E1) is greater than one, wherein namely: 3 SCO2 : SNH3 > 1. 10.- A process according to claim 5, where a portion of the vapors leaving the head of said stripper (E2) was recycled directly to the synthesis reactor, without flowing through high pressure condenser (E8), in a point higher than the inlet of the vapors coming from said first stripper to1). 11.- A process according to claim 5, wherein the vapors (14) coming from vacuum concentrators (E5) and (E6) are condensed within a water-cooled apparatus (E13), into which is also conveyed a portion of the vapors (15) coming from the final vacuum concentrator (37) (previously compressed in E12), the remnant portion (25) being fed, as a liquid phase, to the recovery tower (C3), wherein said (C3) tower contains a vertical falling-film tube-bundle, heated by the vapors (11) coming from said low-pressure still (E4), and wherein the vapors (26), containing N!I3 and coming from said (C3) tower, are also condensed within said water-cooled apparatus (E13), whereafter the liquid condensate (27) leaving said apparatus (E13) is recycled to said tower (C3), the bottom of which tower, practically free from NH3, is partially wasted (line 30) and partially utilized (line 29) for the scrubbing of the synthesis vent (22). 12.- A condensing apparatus, particularly suitable for the working of the apparatus (E13) of claim 11, said condensing apparatus being a vertical exchanger, containing a tube-bundle, wherein the cooling medium flows within the bundle pipes, said exchanger being characterized in that the vapors to be (at least partially) condensed enter the lowermost portion of the shell and leave the same shell in its uppermost portion, thus flowing in countercurrent to the liquid condensate, which falls downwards and which leaves the lowermost portion of said shell, in a lower position, with respect to the inlet of the vapors.

Inventors: PAGANI, GIORGIO
Applicants: MONTEDISON S.P.A.
Application year: 1984
Main classification: C07C126
IPC codes: C07C273, C07C67, C07C239