Improved urea synthesis process and apparatus therefor

Abstract: A vertical condenser is installed on or above a urea synthesis column to condense the mixed gas from the stripper by bringing it into contact with an absorption medium under cooling. A first down pipe for making the top part of the condenser communicate with the bottom part of the synthesis column is provided to allow the resultant condensate to flow down to the bottom part of the synthesis column by gravity. The condensate is subjected to urea synthesis together with feed ammonia or a part of feed carbon dioxide supplied thereto. The urea synthesis solution thus formed is introduced into the stripper by gravity through a second down pipe having an opening in the top part of the synthesis column. Unreacted ammonia and carbon dioxide are separated as the aforesaid mixed gas by the rest of the feed carbon dioxide and introduced into the bottom part of the aforesaid condenser so as to be condensed. Alternatively, the condensate from the vertical condenser is sucked by means of an ejector using preheated feed liquid ammonia as the driving fluid, so that it is introduced into the bottom part of the urea synthesis column and is subjected to urea synthesis.

Description: The invention relates to an improved urea synthesis process comprising separating unreacted ammonia and carbon dioxide under a pressure nearly equal to the urea synthesis pressure by means of carbon dioxide stripping, wherein the synthesis column and the like can be placed on the ground. It is well-known to produce urea by processes which comprise reacting ammonia with carbon dioxide in a urea synthesis zone under a pressure and temperature of urea synthesis, separating unreacted ammonium carbamate as a mixed gas of ammonia and carbon dioxide from the resultant urea synthesis solution, absorbing the mixed gas by an absorption medium to recycle the resulting absorbate to the urea synthesis zone, and providing a urea solution from the urea synthesis solution from which the ammonium carbamate has been separated. A variety of proposals have been made regarding the urea synthesis process of this type. Though depending on the process it employs and its production scale, the urea plant comprises a synthesis column, a carbamate condenser and a stripper, of which the synthesis column and the carbamate condenser have sometimes been placed at a height of 20 - 30 m from the ground level on their bottom basis. Where these equipments are placed at a height of 20 - 30 m from the ground level, structures, supporting these equipments only by structural members such as steel frames have never been employed and concrete and the like functioning as the foundation have been constructed up to this height or a combination thereof with steel-framed structural members has been employed. In view of this background, there have been developed urea synthesis processes in which the urea synthesis column and the like are placed on the ground, by reason that it takes much time and labor to install and maintain these equipments at the height, or by the like reasons. As an example of the processes suggesting the ground placement, an improved urea synthesis process disclosed in Japanese Patent Laid-Open Publication No. 149930/1978 (corresponds to US-A-4 208 347) is mentioned. According to this process, in an equal pressure double-flow recycle process for synthesizing urea with intermediate ammonium carbamate, which comprises reacting ammonia with carbon dioxide at a high ammonia to carbon dioxide molar ratio (hereinafter simply referred to as N/C), heat-treating the synthesis product in the presence of a stripping agent under substantially the same pressure as that of the synthesis, and recycling the remaining materials and the excess material separated from the synthesis product in two separate streams under the same pressure, (1) the urea synthesis product is treated in two continuous steps under the same pressure as that of the urea synthesis, and in the first step, the above-mentioned product is heated to decompose substantially all of the remaining ammonium carbamate and the decomposition product is discharged together with part of the excess ammonia, while in the second step, the remaining part of the excess ammonia is discharged by the addition of supplementary heat and the introduction of a carbon dioxide stream, and (2) the gaseous phase stripped off in the first step is recycled directly to the synthesis, and the gaseous phase stripped off in the second step is subjected to condensation and a residual gas purge treatment and then recycled to the synthesis in a liquid state. The synthesis pressure used therein is 100 - 250 kg/cm 2 , preferably 180 - 225 kg/cm 2 , and the synthesis temperature is 170 - 205°C, preferably 180 - 200°C. The synthesis part comprises two separate synthesis autoclaves continuously arranged in two steps or it is comparted into two steps by completely dividing a synthesis autoclave into the upper and lower sections by means of a partition plate provided in the lower part of the autoclave. As synthesis conditions in the synthesis autoclave located above (hereinafter referred to as the first zone) and the synthesis autoclave located below (hereinafter referred to as the second zone), the N/C in the second zone should always be larger than the N/C in the first zone. The N/C is 4 - 7 in the first zone, while it is 6 - 8 in the second zone. Separately, strippers are provided in a number of two, and the excess amount of ammonia dissolved in the liquid phase is recovered in the first step of the two stripping steps and introduced into the first zone, while the gaseous phase stripped off in the second step of the stripping steps is subjected to condensation and a residual gas purge treatment, and thereafter the condensate is recycled to the first zone. As an attendant effect of this process, it is described in the upper left column on page 6 in the aforementioned publication that by using a reactor having both the upper and lower sections, it is possible to avoid the installation of a gigantic and unmanageable scaffold that is usually necessary for installing a reactor at a height. This suggests that the autoclave is of a self-supporting type. Among other examples of the process suggesting the ground placement is EP-0329215A1. This patent publication discloses a process in which a carbamate condenser is installed at a high ground and a part of the stripper gas is introduced into the synthesis section by means of an ejector, suggesting that the synthesis column, stripper and the like can be installed directly on the ground. Although the above-mentioned two conventional techniques suggest the installation of the synthesis column, stripper and the like directly on the ground, they need to place the carbamate condenser at a height and sometimes to use an ejector. Then, an illustration is made on a.urea production process disclosed in Japanese Patent Laid-Open Publication No. 209555/1985 (corresponds to EP-A-0 155 735). Unlike the above-mentioned two conventional techniques, this process permits the achievement of a high urea synthesis rate and the recovery of the heat of condensation of a gaseous mixture from the stripping treatment by a small heat-exchanging area. In this process, in the production of urea comprising forming a urea synthesis solution comprising ammonium carbamate and free ammonia (excess ammonia) from carbon dioxide and ammonia in excess in a synthesis zone under a pressure of 125 - 350 bar, decomposing at least a part of the ammonium carbamate present in the urea synthesis solution by the addition of heat and a counter-current contact with a stripping gas in a stripping zone under a pressure not higher than the pressure in the synthesis zone, removing the decomposition product of ammonium carbamate from the stripping zone as a gas mixture together with a part of the excess ammonia and the stripping gas, condensing at least a part of the resultant gas mixture in a condensation zone, and treating the urea synthesis solution having undergone the stripping to obtain a urea solution or solid urea, at least 30% of the equilibrium amount of urea attainable under the reaction conditions is formed even in the condensation zone, and the mixture containing ammonium carbamate and urea is supplied to the synthesis zone. The condensation in the condensation zone is carried out under the synthesis pressure or the pressure of the stripping treatment lower than the synthesis pressure. Further, the condensation is effected in a submerged condenser, particularly in the shellside of a shell and tube heat exchanger arranged horizontally. The condensation can also be practiced in a condensation zone integrated with the reaction zone. In the upper left column on page 4 of the publication, however, it is mentioned that the dip-type condenser may be arranged horizontally or vertically and it is particularly preferable to carry out the condensation in a vertical submerged condenser. It is also described that where a horizontal condensation zone is applied, a synthesis zone and the condensation zone can be accommodated within an equipment so that a compact structure is obtained. However, it only suggests an example of the application of a vertical condensation zone, that is, the accommodation of the synthesis zone and the condensation zone within an equipment. Further, there is also a description that where a horizontally arranged submerged condenser is used, it can be disposed directly on the working floor, thus lowering the height of the equipment, reducing the cost of the equipment, and facilitating its assembly and dismantlement. This description however does not teach the ground placement of the equipment(s). It is an object of the present invention to provide an improved process capable of placing the equipment(s) on the ground in a urea synthesis process comprising a step of stripping unreacted ammonia and carbon dioxide by feed carbon dioxide under a pressure substantially equivalent to the urea synthesis pressure and a step of condensing the mixed gas from the stripping step. The present invention provides a urea synthesis process as described below and an apparatus therefor. One aspect of the present invention is a urea synthesis process comprising reacting ammonia with carbon dioxide at a urea synthesis temperature and pressure in a vertical urea synthesis zone, bringing the resultant urea synthesis solution containing urea, unreacted ammonia, unreacted carbon dioxide and water into contact with at least a part of feed carbon dioxide under heating under a pressure nearly equal to said urea synthesis pressure in a stripping zone to separate said unreacted ammonia and said unreacted carbon dioxide as a mixed gas of ammonia,carbon dioxide and water, treating further the urea synthesis solution containing unseparated unreacted ammonia and unreacted carbon dioxide to obtain urea, introducing the mixed gas separated in said stripping zone into the bottom part of a vertical condensation zone to bring it into contact with an absorption medium under cooling thereby to condense said mixed gas, and recycling the thus-obtained condensate to the urea synthesis zone, wherein said process comprises installing the vertical urea synthesis zone in a lower part and the vertical condensation zone thereon or thereabove, providing a first down pipe that has an opening in each of the top part of said vertical condensation zone and the bottom part of said vertical urea synthesis zone and is for making said vertical condensation zone communicate with said vertical urea synthesis zone, letting said condensate flow down to the bottom part of said vertical urea synthesis zone by gravity through said down pipe from the top part of said vertical condensation zone, subjecting the condensate having flowed down to the urea synthesis reaction while it goes up through said vertical urea synthesis zone, and letting the thus-formed urea synthesis solution flow down to the top part of said stripping zone by gravity through a second down pipe having an opening in the top part of said vertical urea synthesis zone. Another aspect of the present invention is a urea synthesis apparatus which comprises (a) a vertical urea synthesis column, (b) a vertical condenser with a cooler, installed on or above said vertical urea synthesis column, (c) a stripper for stripping unreacted ammonia and unreacted carbon dioxide, contained in a urea synthesis solution from said vertical urea synthesis column, by means of feed carbon dioxide to separate them from the solution as a mixed gas of ammonia, carbon dioxide and water, (d) a first down pipe that has an opening in each of the top part of said vertical condenser and the bottom part of said vertical urea synthesis column and is for making said vertical condenser communicate with said vertical urea synthesis column to allow the condensate to flow down from the top part of said vertical condenser to the bottom part of said vertical urea synthesis column by gravity, (e) a piping for introducing feed ammonia or feed carbon dioxide into the bottom part of said vertical urea synthesis column, (f) a second down pipe that has an opening in the top part of said vertical urea synthesis column and is for introducing the urea synthesis solution into the top part of said stripper by gravity, (g) a piping for introducing said mixed gas from said stripper into the bottom part of said vertical condenser, (h) a piping for introducing an absorption medium or an absorption medium and feed ammonia into the bottom part of said vertical condenser, (i) a piping for discharging inert gas, joined to the top part of said vertical condenser, (j) a piping for introducing at least a part of feed carbon dioxide, joined to the bottom part of said stripper, and (k) a piping for discharging the aqueous urea solution containing unseparated unreacted ammonia and unreacted carbon dioxide from the bottom part of said stripper for further treatments. According to a preferred embodiment, the urea synthesis process is a process, wherein the vertical urea synthesis zone and the vertical condensation zone are so integrated that the vertical urea synthesis zone is located in the lower part and the vertical condensation zone in the upper part, and these two zones have a partition wall inbetween to divide them and have a first down pipe that has an opening in each of the top part of said vertical condensation zone and the bottom part of said vertical urea synthesis zone and is for making said vertical condensation zone communicate with said vertical urea synthesis zone. According to another preferred embodiment, the urea synthesis process is a process, wherein the vertical urea synthesis zone and the vertical condensation zone comprise a vertical urea synthesis zone, a vertical condensation zone installed separately thereon or thereabove but integrated with said vertical urea synthesis zone, and a first down pipe that makes these two zones communicate with each other and has an opening in each of the top part of said vertical condensation zone and the bottom part of said vertical urea synthesis zone. According to still another preferred embodiment, the urea synthesis process is a process, wherein the temperature of the condensate in the vertical condensation zone is controlled at 165-190°C, the N/C (the molar ratio of ammonia to carbon dioxide) is controlled at 2.5-4.5, the H/C (the molar ratio of water to carbon dioxide) is controlled at 1.0 or less, and the residence time is set at 10-30 minutes. According to still another preferred embodiment, the urea synthesis process is a process, wherein the urea synthesis reaction is carried out at a synthesis rate of 20-60 % based on the carbon dioxide in the condensate in said vertical condensation zone. Another preferred embodiment is a urea synthesis process, wherein in said vertical urea synthesis zone a part of the feed carbon dioxide, or the feed ammonia is added to the condensate having flowed down through the first down pipe from said vertical condensation zone, so that the temperature of the condensate is controlled at 170-200°C, the N/C is controlled at 3.5-4.5, the H/C is controlled at 1.0 or less, and the residence time is set at 10-40 minutes. Still another preferred embodiment is a urea synthesis process, wherein a total urea synthesis rate of 60-75% is achieved based on the carbon dioxide in the vertical synthesis zone. Another preferred embodiment of the apparatus according to the present invention is a urea synthesis apparatus, wherein the vertical urea synthesis column and the vertical condenser are integrated into a vertical condensation synthetic reaction apparatus in which a partition wall is provided between the lower urea synthesis zone functioning as a vertical urea synthesis column and the upper condensation zone functioning as a vertical condenser to divide them from each other. Still another preferred embodiment of the apparatus according to the present invention is a urea synthesis apparatus, wherein the vertical condenser is installed separately on or above the vertical urea synthesis column, and the urea synthesis column and the condenser are integrated. According to still other preferred embodiments, the apparatus according to the present invention is a urea synthesis apparatus, wherein the vertical condenser and/or the vertical urea synthesis column have baffle plate in the inside thereof to make gas-liquid contact better. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a flowsheet illustrating an embodiment of the present invention, indicating the height of each equipment from the ground level; Fig. 2 is a flowsheet illustrating another embodiment of the present invention; In the present invention, the urea synthesis rate based on carbon dioxide is the percentage of the molar amount of the carbon dioxide converted to urea relative to the molar amount of the total carbon dioxide, which is simply referred to as a urea synthesis rate hereunder. Further, the inert gas represents a general name of the air introduced into the urea synthesis column, stripper, condenser and the like to prevent corrosion and impurities originally involved in the feed carbon dioxide. The vertical condenser is a heat-exchanger in which an ammonium carbamate condensate is caused to flow through the shellside, for example, a shell and tube heat-exchanger, which is vertically installed. Figs. 1 and 2 are flowsheets illustrating embodiments of the present invention, indicating the positions (heights from the ground level) at which the condenser, urea synthesis column, stripper and the like are installed. Although each of the equipments is described in detail later, it is explained in the following why each equipment can be placed on the ground in these embodiments. Where a urea synthesis solution is allowed to fall down from a urea synthesis column A to a stripper C only by gravity, the solution does not flow in the absence of a head difference. Formerly, when the solution was allowed to flow from the bottom part of a synthesis column to the top part of a stripper, the positional difference between the synthesis column, particularly the bottom part of the synthesis column and the top part of the stripper, particularly the feeding part to the stripper, constituted the aforementioned head difference. Conventionally, in a process for producing 1,000 - 1,700 t/d of urea, 10 - 15 m have been employed as the aforesaid head difference. However, the head difference does not significantly vary even with the variation of production scale. To attain the aforesaid head difference, a condenser B and a synthesis column A are integrated to provide a vertical condensation synthetic reaction apparatus of the integrated type, which has a first down pipe 3 for joining both the apparatuses directly to each other. The synthesis column employs such a structure that a second down pipe 40 provided below the vicinity of an internal partition plate 30 (made of the same material as the lining material of the synthesis column, e.g. improved 316LSS) is submerged in the synthesis column down to a position below an inlet provided in the top part of a stripper C, and connected from this position to the inlet provided in the top part of the stripper. By adopting this structure, the second down pipe can serve as a seal and secure a head difference of 10 - 15 m, so that it has become possible to place the equipment on the ground. In the vertical condensation synthetic reaction apparatus, the position of the internal partition plate is selected to be 10 - 30 m from the ground level, though it depends more or less on the production scale of urea. For example, the plate is installed at about 27 m from the ground level in a plant producing 1,700 t/d of urea. In the present invention, a vertical condenser is used. The reason is as follows. In a urea synthesis process to which the carbon dioxide stripping is applied, the feed to the condenser is an ammonium carbamate solution as the absorption medium and a mixed gas comprising carbon dioxide, that is the stripping gas sent from the stripper, excess ammonia, carbon dioxide and ammonia formed by decomposing ammonium carbamate, water, and inert gas. In the condenser, urea is not formed at a high urea synthesis rate unless the N/C is increased by causing the above-mentioned ammonium carbamate solution and the condensate formed in the condenser to absorb the aforesaid mixed gas fed to the condenser, though depending on the conditions including operating temperature and pressure. Further, it is necessary for the synthesis of urea to take a long residense time. From the above viewpoint, the vertical condenser, in which the foregoing solution are caused to flow through the shellside, can secure a liquid depth necessary for the gas absorption, and can prolong the gas-liquid contact time by providing baffle plates, etc. because the aforementioned mixed gas is supplied to the vertical condenser at its bottom to form an upward-flowing gas-liquid concurrent flow in the shellside thereof, in addition to that it needs only a small area for its installation. Therefore, the residence time can be prolonged so that a larger amount of urea may be produced. Further, by separating inert gas in the top part of the condenser, the proportion of the gas is greatly reduced in the synthesis column A described below, and therefore the urea synthesis reaction taking place essentially in the liquid phase becomes easy to proceed. Since corrosion preventive air, the aforesaid ammonium carbamate solution and the liquid condensed in the condenser are fully mixed in the condenser, dissolved oxygen necessary for preventing corrosion in the synthesis column is present in said mixed liquid, so that the synthesis column can be prevented from corrosion, even if the inert gas has been separated. Down pipes are illustrated. A synthesis solution containing the urea formed in the condenser B at a urea synthesis rate of 20 - 60%, unreacted ammonia, unreacted carbon dioxide, and the like overflows into a first down pipe 3 and flows down therethrough by gravity into the synthesis column A. Further, since the first down pipe is caused to reach the fully deep part in the lower section of the synthesis column, a liquid seal is formed so as to prevent the back flow of liquid and gas. As the material of the first down pipe, improved 316LSS is generally selected, which is the same material as that of the lining of the synthesis column. The down pipe may also be a pipe running outside the equipment. A high pressure stainless steel pipe will suffice in this case. A second down pipe 40, which has an opening below the vicinity of the partition plate 30, is arranged in the synthesis column A to provide a liquid seal effect. The urea synthesis solution flows through the second down pipe by gravity into the stripper. Feed ammonia or feed carbon dioxide can be supplied to the synthesis column so as to attain a temperature of 180 - 200°C in the column. When the feed material is supplied to the synthesis column, a density difference (hereinafter called a drafting force) occurs between the liquid having flowed down from the condenser to the synthesis column and the gas-liquid mixed fluid going up through the synthesis column. The drafting force is explained taking, as an example, the case of supplying feed ammonia through a line 1 to the synthesis column in Fig. 1. The feed liquid ammonia is prefeated in a heat-exchanger D and supplied to the synthesis column. At this moment, the N/C (3.5 - 4.5) in the synthesis column is maintained higher than the N/C (2.5 - 4.5) in the condenser, and the liquid density in the synthesis column is approximately 870 kg/m 3 . The liquid density in the condenser is about 1,000 kg/m 3 , and this difference of the densities constitutes the drafting force. The drafting force can further accelerate the flow by gravity and minimize the difference in height between the -condenser and the stripper. In the embodiments of Figs. 1 and 2, the volume ratio of the condenser to the synthesis column is selected to be approximately 3 : 7 - 5 : 5, which is described specifically below. Though depending on the production rate of urea, if the volume of the condenser is less than 30% of the sum of the volumes of the synthesis column and the condenser, the residence time in the condenser is sometimes insufficient, while if it exceeds 50%, the residence time in the synthesis column is sometimes insufficient. The relative positional relations of the condenser, urea synthesis column, and stripper in the embodiments of Figs. 1 and 2 are generally illustrated such that when the ground-base height of the top of the condenser is assumed to be 100, the ground-base height of the bottom of the condenser is 50 - 80, the ground-base height of the top of the stripper being 20 - 50, the ground-base height of the bottom of the urea synthesis column being 5 - 30, the ground-base height of the bottom of the stripper being 3 - 10, and the difference in height between the bottom of the condenser and the top of the stripper is 10 - 60. With such relative positional relations, introduction of the condensate formed in the condenser B into the urea synthesis column A, introduction of the urea synthesis solution from the urea synthesis column A into the stripper C, and introduction of the mixed gas from the stripper C into the condenser B are carried out smoothly by gravity. Therefore, it is not necessary to use an ejector or to install the urea synthesis column at a high ground. As shown in Example 1 and Reference Example 1 described below, if we assume the volume of the synthesis column of the conventional process, in which the reaction is carried out exclusively in the column, as 1, the sum of the volumes of the condenser and the synthesis column according to the present invention is less than 1, under the same urea synthesis conditions (at the same urea production rate), thus indicating reduction in the total volume. As mentioned above, since the synthesis column and the condenser are integrated and the condenser also has a urea synthesis function in the embodiments of Figs. 1 and 2, the process of the present invention is clearly simplified arid at the same time the integrated apparatus is made smaller in size, as compared with the conventional processes in which urea is synthesized exclusively in the synthesis column. Referring to Figs. 1 and 2, the present invention is illustrated in detail in the case of recycling the liquids and gas to the condenser B, synthesis column A and stripper C only by head differences. In these drawings and Figs. 3 and 4 shown later, common codes and numerals are used. In Figs. 1 and 2, a condenser.B is installed on a urea synthesis column A and they are divided by a partition plate 30, a first down pipe 3 being so arranged as to make the top part of the condenser B communicate with the bottom part of the synthesis column A. Further, there is provided, in the synthesis column A, an independent second down pipe 40 having an opening below the vicinity of the partition plate 30 in order to provide a liquid seal effect. Figs. 1 and 2 shows an apparatus in which a condenser B and a urea synthesis column A are integrated through a partition plate dividing both the apparatuses. However, the condenser may be installed separately above the urea synthesis column, or the condenser and the urea synthesis column provided independently may be integrated by joining them together with the aid of, for example, a cylindrical body (which does not need pressure resistance). The process of Fig. 1 is illustrated. A part of a recovery solution, which has been formed by recovering unreacted ammonia and carbon dioxide as an aqueous ammonium carbamate solution in a step not shown in the drawing, is introduced into a condenser B through lines 7, 8 and 11, while the rest is supplied to a scrubber F through a line 9. The recovery solution introduced into the vertical condenser B is brought into contact and mixed with a mixed gas comprising carbon dioxide, ammonia, inert gas, etc. fed to the condenser B through a line 5 from a stripper C described below, thereby absorbing the carbon dioxide and ammonia. In the condenser B, the operating pressure is the same as the operating pressure of the synthesis column mentioned below, the N/C being 2.5 - 4.5, the H/C being 0.0 - 1.0, and the residence time is 10 - 30 minutes. The operating temperature is controlled at 170 - 190°C, and, under these conditions, a urea synthesis rate of 20 - 60% can be achieved. When the N/C is less than 2.5, the condensation rate is decreased due to the increase of the partial pressure of carbon dioxide over the ammonium carbamate solution and the solidification temperature of ammonium carbamate is increased, while if the N/C exceeds 4.5, the condensation rate is also reduced due to the increase of the vapor pressure of ammonia. Therefore, it is preferable to avoid any of these N/Cs. Any H/C exceeding 1.0 reduces the equilibrium synthesis rate of urea and hence is preferably avoided. If the residence time is less than 10 minutes, the vapor pressures are increased and the condensation rate is decreased due to reduction in the urea synthesis rate. Further, even if it exceeds 30 minutes, the urea synthesis rate will not be improved so significantly in proportion to the length of time, and the volume of the condenser is uselessly and excessively increased. Therefore, any residence time outside the above range is preferably avoided. The operating temperature lower than 170°C leads to reduction in the urea synthesis rate, while that in excess of 190°C results in the reduction of the condensation rate due to the increase of the vapor pressures and in the corrosion of the material of the equipment. Accordingly, any of these operating temperatures is preferably avoided. The foregoing recovery solution supplied to the scrubber F absorbs ammonia and carbon dioxide in the inert gas containing small amounts of ammonia and carbon dioxide introduced into the scrubber F through a line 6. The recovery solution corresponding to the increment produced by absorbing the ammonia and carbon dioxide is fed through a line 10 to an ejector E where it is pressurized and introduced into the condenser B through a line 11. Separately, the inert gas is discharged into the air from the scrubber F at the top. In the drawing, an example of using an ejector E is illustrated. Needless to say, the present invention is not limited to this example. Where the ejector E is used, said recovery solution is divided in two between the scrubber F and the ejector E, one being used for scrubbing the inert gas in the packed bed of the scrubber F, while the other is used to increase pressure through the ejector E at an outlet of the scrubber F. However, if the scrubber F is installed at a position above the condenser B, the ejector E is not necessary as a matter of course. A part of feed carbon dioxide is supplied to a stripper C through a line 2. Although not described in.the drawing, corrosion preventive air is also supplied to the stripper C. A synthesis solution comprising urea, ammonium carbamate, ammonia, etc. is fed through a second down pipe 40 in the synthesis column A to the stripper C, where it undergoes stripping under heating by means of the carbon dioxide. Its operating conditions are usually a pressure of 140 - 200 bar and a temperature of 160 - 200°C. The rest of the feed carbon dioxide is supplied, for example, to a low pressure decomposition column, which is not shown in the drawing. A urea synthesis solution comprising urea and unreacted ammonia and carbon dioxide is taken out of the stripper at its bottom and sent to the low pressure decomposition step through a line 12. Feed liquid ammonia is preheated, for example, to 70 - 90°C, introduced through a line 1, further preheated through the scrubber F and a heat exchanger D, and supplied to the bottom part of the synthesis column A. In the synthesis column A, the liquid ammonia is joined with a synthesis solution having flowed down through a first down pipe 3 from the condenser B. The resultant solution moves up to the top part of the synthesis column and is supplied to the stripper C through a second down pipe 40 provided in the synthesis column and a line 4. The operating pressure of the synthesis column is selected to be 130 - 250 bar as in the conventional processes. The N/C, the H/C, and the residence time are selected to be 3.5 - 5.0, 1.0 or less, and 10 - 40 minutes, respectively. The operating temperature is controlled at 180 - 200°C, and a urea synthesis rate of 60 - 75% is attained under the above conditions. If the N/C is less than 3.5, the equilibrium synthesis rate is low, whereas if it exceeds 5.0, the vapor pressure of ammonia is increased due to its gasification. Therefore, any N/C outside the above range is preferably avoided. Any H/C exceeding 1.0 leads to reduction in the synthesis rate and therefore is preferably avoided. When the residence time is less than 10 minutes, the time is insufficient for the urea synthesis rate to achieve nearly its equilibrium value, while even when the time exceeds 40 minutes, the urea synthesis rate has already reached nearly its equilibrium value. Therefore, any residence time outside the above range is preferably avoided. The process of Fig. 2 is now described. It is a substitution for the process shown in Fig. 1 in the following way. In Fig. 1, feed liquid ammonia is fed to the synthesis column, while feed carbon dioxide is fed to the condenser via the stripper. The process of Fig. 2 however differs in that feed liquid ammonia is fed to the condenser, and a part of feed carbon dioxide is supplied to the condenser through the stripper, the rest being fed to the synthesis section. In these drawings, basic large differences are in the operating conditions, particularly the N/Cs in the condenser and the synthesis column. In the process of Fig. 2, the operating conditions of the condenser are an N/C of 3.0 - 4.5, an H/C of 1.0 or less, a temperature of 160 - 180°C, and a pressure equivalent to that of the synthesis column. The operating conditions of the synthesis column are such that the N/C is 3.0 - 4.0, the H/C being 1.0 or less, the temperature being 180 - 200°C, and the pressure is 130 - 250 bar. The present invention is further illustrated in detail with reference to examples. However, it is needless to say that the present invention is not limited only to the examples described below. Example 1: Urea synthesis was carried out by an improved urea synthesis process, in which an apparatus with a daily production of 1,400 tons of urea placed on the ground was used as shown in Fig. 1. 791 t/d of feed ammonia and 1,025 t/d of feed carbon dioxide were supplied through a line 1 and a line 2, respectively. The feed ammonia heated to 175°C by a steam condensate in a heat-exchanger D was fed to the bottom part of a synthesis column A through the line 1, while 969 t/d of the feed carbon dioxide and about 17 t/d of corrosion preventive air were fed to the bottom part of a stripper C through the line 2, the rest of the feed carbon dioxide being supplied to a low pressure decomposition column not illustrated in the drawing. Separately, 1314 t/d of a recovery solution as an absorption medium through a line 7, having a composition described below and a temperature of 108°C, were pressurized to 190 bar and divided in two between a scrubber F and an ejector E. urea 0.4% ammonia 34.8% carbon dioxide 40.0% water 24.4% The recovery solution supplied to the scrubber F was used for scrubbing inert gas in the packed bed, while that supplied to the ejector E was used for pressurizing a scrubber delivery solution (line 10) by means of the ejector E. The scrubber F was operated at 175 bar and 175°C. The scrubber delivery solution (line 10) and the ejector-driving recovery solution (line 8) were mixed together by the ejector E and sent to the bottom part of a condenser B through a line 11. A mixed gas comprising ammonia, carbon dioxide and water separated in the stripper C was sent to the bottom part of the condenser B through a line 5. The shellside of the condenser B was filled with the above-mentioned recovery solution and ammonium carbamate formed by condensation, by which the foregoing mixed gas was absorbed. The heat of absorption was removed by a cooler shown by lines 13 and 14. The condensate having absorbed the mixed gas stayed in the condenser for 15 minutes and flowed down through a first down pipe 3 by gravity to be fed to the synthesis column A. In the top part of the condenser, inert gas was withdrawn through a line 6. The condensate from the condenser B which had been supplied to the bottom part of the synthesis column A was mixed with the above-mentioned liquid ammonia heated to 175°C from the line 1, and while the mixture went up slowly through the synthesis column, the urea synthesis reaction further proceeded. The residence time for the reaction was 25 minutes. In the interior of the urea synthesis column A, baffle plates were arranged to accelerate the dispersion of gas and prevent the back mixing of liquid. The synthesis solution sent to the stripper C through a second down pipe 40 and a line 4 from the synthesis column A was heated there so that the ammonium carbamate was decomposed and separated as a mixed gas of ammonia and carbon dioxide. The stripper C was operated at a top temperature of 191°C, a bottom temperature of 179°C, and a pressure of 175 bar. As described above, the apparatus was operated by circulating the gas and liquids through the condenser B, urea synthesis column A and stripper C. The operating conditions and the compositions in the condenser B, synthesis column A, and stripper C are mentioned in Table 1. Reference Example 1 (corresponding to the process in Fig. 1): In a conventional urea synthesis process (the condenser is not placed on the synthesis column, but the condenser and the synthesis column are installed at positions of 30 m and 28 m from the ground level, respectively), operation was practiced under the same operating conditions as in Example 1. The volume of the synthesis column in this case being taken as 1, it was compared with the total volume of the condenser and the synthesis column in Example 1. The results are also shown in Table 1. Example 2: A urea synthesis process illustrated in Fig. 2 was put in operation under the operating conditions of the condenser B, synthesis column A and stripper C as described in Table 1. Example 1 Example 2 Ref. Ex. 1 Feed ammonia (t/d) 792 792 - Feed carbon dioxide (t/d) 1025 1025 - Recovery solution (t/d) 1314 1462 - Urea (%) 0.4 0.4 - Ammonia (%) 34.8 38.7 - Carbon dioxide (%) 40.4 36.4 - Water (%) 24.4 24.5 - Condenser B Pressure (bar) 175 170 - Temperature (°C) 185 170 - Urea (%) 21.1 15.9 - Ammonia (%) 34.3 47.5 - Carbon dioxide (%) 26.7 21.6 - Water (%) 17.9 15.0 - Urea synthesis rate (%) 42.6 35.1 - N/C 2.85 4.40 - H/C 0.67 0.76 - Residence time (minutes) 15 20 - Volume ratio 0.34 - - Synthesis column A Pressure (bar) 175 170 175 Temperature (°C) 185 185 185 Urea (%) 31.8 32.4 31.8 Ammonia (%) 36.8 34.8 36.8 Carbon dioxide (%) 12.2 13.4 12.2 Water (%) 19.2 19.4 19.2 Urea synthesis rate (%) 65.7 63.9 65.7 N/C 4.0 3.70 4.0 H/C 0.67 0.64 0.67 Residence time (minutes) 25 30 - Volume ratio 0.54 - 1.0 Stripper C (line 12) Urea (%) 48.1 46.3 - Ammonia (%) 13.2 15.5 - Carbon dioxide (%) 13.7 13.5 - Water (%) 25.0 24.7 - Stripper C (line 2) Carbon dioxide (t/d) 969 700 - Effects of the Invention: According to the improved urea synthesis process of the present invention, the following effects can be achieved. (1) Since a process, in which a synthesis column, and stripper are placed on the ground, could have been established, concrete and steel-framed structures, which have hitherto been needed, are unnecessary. (2) Where a vertical condenser is integrated with the synthesis column, simplification of the process is promoted. (3) Since the condenser is a vertical condenser, in which the feed gases and the like are fully absorbed to easily form ammonium carbamate, and employs such a structure that liquid is caused to flow through the shellside, urea is formed in the condenser by regulating the volume of the condenser. (4) Where the urea synthesis column and the condenser are integrated, urea can be synthesized in both the condenser and the synthesis column, and therefore the total volume of these apparatuses may be made smaller than the total volume of the conventional synthesis column and condenser.

Claims: A urea synthesis process comprising reacting ammonia with carbon dioxide at a urea synthesis temperature and pressure in a vertical urea synthesis zone, bringing the resultant urea synthesis solution containing urea, unreacted ammonia, unreacted carbon dioxide and water into contact with at least a part of feed carbon dioxide in a stripping zone under heating under a pressure nearly equal to said urea synthesis pressure to separate said unreacted ammonia and said unreacted carbon dioxide as a mixed gas of ammonia, carbon dioxide and water, treating further the urea synthesis solution containing unseparated unreacted ammonia and unreacted carbon dioxide to obtain urea, introducing the mixed gas separated in said stripping zone into the bottom part of a vertical condensation zone to bring it into contact with an absorption medium under cooling thereby to condense said mixed gas, and recycling the thus-obtained condensate to the urea synthesis zone, wherein said process comprises installing the vertical urea synthesis zone on the ground level and the vertical condensation zone thereon or thereabove, providing a first down pipe that has an opening in each of the top part of said vertical condensation zone and the bottom part of said vertical urea synthesis zone and is for making said vertical condensation zone communicate with said vertical urea synthesis zone, letting said condensate flow down to the bottom part of said vertical urea synthesis zone by gravity through said down pipe from the top part of said vertical condensation zone, subjecting the condensate having flowed down to the urea synthesis reaction while it goes up through said vertical urea synthesis zone, and letting the thus-formed urea synthesis solution flow down to the top part of said stripping zone by gravity through a second down pipe having an opening in the top part of said vertical urea synthesis zone. The process according to Claim 1, wherein said vertical urea synthesis zone and said vertical condensation zone are so integrated that the vertical urea synthesis zone is located in the lower part and the vertical condensation zone in the upper part, and these two zones have a partition wall inbetween to divide them and have a first down pipe that has an opening in each of the top part of said vertical condensation zone and the bottom part of said vertical urea synthesis zone and is for making said vertical condensation zone communicate with said vertical urea synthesis zone. The process according to Claim 1, wherein said vertical urea synthesis zone and said vertical condensation zone comprise a vertical urea synthesis zone, a vertical condensation zone installed separately thereon or thereabove but integrated with said vertical urea synthesis zone, and a first down pipe that makes these two zones communicate with each other and has an opening in each of the top part of said vertical condensation zone and the bottom part of said vertical urea synthesis zone. The process according to Claim 1, 2 or 3, wherein the temperature of the condensate in said vertical condensation zone is controlled at 165 - 190°C, the N/C (the molar ratio of ammonia to carbon dioxide) at 2.5 - 4.5, the H/C (the molar ratio of water to carbon dioxide) at 1.0 or less, and the residence time is set at 10 - 30 minutes. The process according to Claim 4 wherein the urea synthesis reaction is carried out at a synthesis rate of 20 - 60% based on the carbon dioxide in the condensate in said vertical condensation zone. The process according to any one of Claims 1 - 4, wherein in said vertical urea synthesis zone, a part of the feed carbon dioxide, or the feed ammonia is added to the condensate having flowed down through the first down pipe from said vertical condensation zone, so that the temperature of the condensate is controlled at 170 - 200°C, the N/C at 3.5 - 4.5, the H/C at 1.0 or less, and the residence time is set at 10 - 40 minutes. The process according to any one of Claims 1 - 6, wherein a total urea synthesis rate of 60 - 75% is achieved based on the carbon dioxide in the vertical urea synthesis zone. The process according to Claims 2 or 3, wherein the relative positional relations of said vertical urea synthesis zone, said vertical condensation zone and said stripping zone are such that when the height up to the top of said condensation zone from the ground level is assumed to be 100%, the height up to the bottom of said vertical condensation zone is 50 - 80%, the height up to the top of said stripping zone being 20 - 50%, the height up to the bottom of said vertical synthesis zone being 5 - 30%, the height up to the bottom of said stripping zone being 3 - 10%, and the difference in height between the bottom of said vertical condensation zone and the top of said stripping zone is 10 - 60%. A urea synthesis apparatus comprising (a) a vertical urea synthesis column placed on the ground level, (b) a vertical condenser with a cooler, installed on or above said vertical urea synthesis column, (c) a stripper for stripping unreacted ammonia and unreacted carbon dioxide, contained in a urea synthesis solution from said vertical urea synthesis column, by means of feed carbon dioxide to separate them from the solution as a mixed gas of ammonia, carbon dioxide and water, (d) a first down pipe that has an opening in each of the top part of said vertical condenser and the bottom part of said vertical urea synthesis column and is for making said vertical condenser communicate with said vertical urea synthesis column to allow the condensate to flow down from the top part of said vertical condenser to the bottom part of said vertical urea synthesis column by gravity, (e) a piping for introducing feed ammonia or feed carbon dioxide into the bottom part of said vertical urea synthesis column, (f) a second down pipe that has an opening in the top part of said vertical urea synthesis column and is for introducing the urea synthesis solution into the top part of said stripper by gravity, (g) a piping for introducing said mixed gas from said stripper into the bottom part of said vertical condenser, (h) a piping for introducing an absorption medium or an absorption medium and feed ammonia into the bottom part of said vertical condenser, (i) a piping for discharging inert gas, joined to the top part of said vertical condenser, (j) a piping for introducing at least a part of feed carbon dioxide, joined to the bottom part of said stripper, and (k) a piping for discharging an aqueous urea solution containing unseparated unreacted ammonia and unreacted carbon dioxide from the bottom part of said stripper for further treatments. The urea synthesis apparatus according to Claim 9, wherein said vertical urea synthesis column and said vertical condenser are integrated into a vertical condensation synthetic reaction apparatus in which a partition wall is provided between the lower urea synthesis zone functioning as a vertical urea synthesis column and the upper condensation zone functioning as a vertical condenser to divide them from each other. The urea synthesis apparatus according to Claim 9, wherein said vertical condenser is installed separately on or above said vertical urea synthesis column, and the urea synthesis column and the condenser are integrated. The urea synthesis apparatus according to any one of Claims 9 - 11, wherein said vertical condenser has baffle plates in the inside thereof to make gas-liquid contact better. The urea synthesis apparatus according to any one of Claims 9 - 12, wherein said vertical urea synthesis column has baffle plates in the inside thereof to make gas-liquid contact better. The urea synthesis apparatus according to claim 9, wherein the relative positional relations of said vertical urea synthetic column, said vertical condenser, and said stripper are such that when the height up to the top of said vertical condenser from the ground level is assumed to be 100%, the height up to the bottom of said vertical condenser is 50 - 80%, the height up to the top of said stripper being 20 - 50%, the height up to the bottom of said vertical urea synthesis column being 5 - 30%, the height up to the bottom of said stripper being 3 - 10%, and the difference in height between the bottom of said vertical condenser and the top of said stripper is 10 - 60%.

Inventors: FUJII HIDETSUGU ING, KOJIMA YASUHIKO ING, FUJII, HIDETSUGU, ING., KOJIMA, YASUHIKO, ING.
Applicants: TOYO ENGINEERING CORP, TOYO ENGINEERING CORPORATION
Application year: 1997
Main classification: C07C273
IPC codes: B01D5, B01J19, B01J10, C07C273, C07C275