Apparatus for the preparation of urea

Abstract: In 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 the 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 first down pipe that has an opening in the top part of said vertical condensation zone and is for supplying a condensate from the top part of a said vertical condensation zone to the suction side of an ejector, preheating the feed liquid annomina under a pressure of 150 - 300 bar to supply it to the ejector, expanding said liquid ammonia through the ejector so that the pressure difference between the delivery pressure and the suction pressure of the ejector may become 2 - 10 bar thereby to suck the condensate coming from the top part of said vertical condensation zone, introducing the mixture of said liquid ammonia and said condensate from said ejector into the bottom part of said urea synthesis zone, subjecting said introduced mixture to the urea synthesis while it goes up through said vertical urea synthesis zone, and supplying the thusformed urea synthesis solution from the top part of said vertical urea synthesis zone to the top part of said stripping zone.

Description: The invention relates to an improved urea synthesis apparatus for separating unreacted ammonia and carbon dioxide by means of carbon dioxide stripping. 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 absorbed solution 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 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. 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 equipments. It is an object of the present invention to provide an urea synthesis apparatus which comprises (a) a vertical urea synthesis column, (b) a vertical condenser with a cooler, installed on or above said urea synthesis column, (c) a stripper for stripping unreacted ammonia and unreacted carbon dioxide, contained in a urea synthesis solution from said 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 heat-exchanger for preheating feed liquid ammonia, (c) an ejector using the preheated feed liquid ammonia as the driving fluid, (f) a first down pipe that has an opening in the top part of said condenser and is for feeding the condensate from the top part of said condenser to the suction side of the ejector, (g) a piping for introducing the feed liquid ammonia and said condensate from the delivery side of the ejector into the bottom part of said urea synthesis column, (h) a second down pipe that has an opening in the top part of said urea synthesis column and is for introducing the urea synthesis solution into the top part of said stripper, (i) a piping for introducing said mixed gas from said stripper into the bottom part of said condenser, (j) a piping for introducing an absorption medium or an absorption medium and feed ammonia into the bottom part of said condenser, (k) a piping for discharging inert gas, joined to the top part of said condenser, (l) a piping for introducing at least a part of feed carbon dioxide, joined to the bottom part of said stripper, and (m) 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. In the present specification, 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. 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 for joining both the apparatuses directly to each other. The synthesis column employs such a structure that a second down pipe provided below the vicinity of an internal partition plate (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 residence 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. 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 and flows down therethrough by gravity into 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, which has an opening below the vicinity of a partition plate, is arranged in 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 first line to a synthesis column. The feed liquid ammonia is preheated 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. 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 according to a preferred embodiment of the invention 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 condenser B into urea synthesis column A, introduction of the urea synthesis solution from urea synthesis column A into stripper C, and introduction of the mixed gas from stripper C into 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, the process of synthesizing urea in the apparatus of the present invention is clearly simplified and 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. The apparatus of the present invention and the urea synthesis process that can be performed therewith, are described in detail in the case of recycling the liquids and gas to condenser B, synthesis column A and stripper C only by heat differences. Condenser B is installed on urea synthesis column A and they are divided by a partition plate the first down pipe being so arranged as to make the top part of condenser B communicate with the bottom part of the synthesis column A. Further, there is provided, in synthesis column A, the independent second down pipe having an opening below the vicinity of the partition plate in order to provide a liquid seal effect. According to an embodiment of the invention, in the urea synthesis apparatus 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 synthesizing urea 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, is introduced into condenser B through 3 lines, while the rest is supplied to a scrubber F through another line. The recovery solution introduced into vertical condenser B is brought into contact and mixed with a mixed gas comprising carbon dioxide, ammonia, inert gas, etc. fed to condenser B through a line from stripper C described below, thereby absorbing the carbon dioxide and ammonia. In 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 scrubber F absorbs ammonia and carbon dioxide in the inert gas containing small amounts of ammonia and carbon dioxide introduced into scrubber F. The recovery solution corresponding to the increment produced by absorbing the ammonia and carbon dioxide is fed to ejector E where it is pressurized and introduced into the condenser B. Separately, the inert gas is discharged into the air from scrubber F at the top. A part of feed carbon dioxide is supplied to stripper C, and corrosion preventive air is also supplied to stripper C. A synthesis solution comprising urea, ammonium carbamate, ammonia, etc. is fed through the second down pipe in synthesis column A to 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. 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 column. Feed liquid ammonia is preheated, for example, to 70 - 90°C, introduced through a line, further preheated through scrubber F and heat exchanger E, and supplied to the bottom part of synthesis column A. In synthesis column A, the liquid ammonia is joined with a synthesis solution having flowed down through the first down pipe from condenser B. The resultant solution moves up to the top part of the synthesis column and is supplied to stripper C through the second down pipe provided in the synthesis column. 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. In the alternative, feed carbon dioxide is fed to the condenser via the stripper. The process, however, may differ 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. Suitably, 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. When using the apparatus of the invention, feed liquid ammonia is introduced into heat-exchanger D under a pressure of 150 - 300 bar, where it is preheated to 100 - 200°C, and enters ejector G as the driving fluid. The feed liquid ammonia is expanded through the ejector so that the pressure difference between the delivery and suction pressures of the ejector may become 2 - 10 bar. In consequence, the condensate supplied to the suction side of the ejector through the down pipe in vertical condenser B is sucked, pressurized, and introduced into the bottom part of urea synthesis column A as a mixture with the driving fluid, the feed liquid ammonia. Of the feed carbon dioxide, carbon dioxide necessary for stripping unreacted ammonia and carbon dioxide is fed to the bottom part of stripper C, and the rest is supplied to the bottom part of urea synthesis column A or decomposed at low pressure. The mixture of the feed liquid ammonia and the condensate from ejector G and, if necessary, the carbon dioxide are reacted while going up through urea synthesis column A to attain a total urea synthesis rate of 60 - 75%. In urea synthesis column A, the pressure, the N/C, the H/C, and the residence time are preferably 130 - 250 bar, 3.5 - 5.0, 1.0 or less, and 10 - 40 minutes, respectively. If the N/C is less than 3.5, the equilibrium synthesis rate is low, whereas if it exceeds 5.0, ammonia is gasified to increase its vapor pressure. Therefore, these N/Cs are preferably avoided. When the residence time is less than 10 minutes, the time is insufficient for the urea synthesis rate to attain nearly its equilibrium value, whereas even when it exceeds 40 minutes, the urea synthesis rate has already achieved nearly its equilibrium value, so that no more increase is expected in the synthesis rate. Therefore, these residence times are preferably avoided. The urea synthesis solution comprising urea, unreacted ammonia, unreacted carbon dioxide and water formed in synthesis column A are drawn out of the synthesis column at its top, and fed to the top part of stripper C having a pressure substantially equal to or somewhat lower than the pressure in the synthesis column. The urea synthesis solution, while flowing down through stripper C, is brought into contact, under heating, with feed carbon dioxide supplied to the bottom part of stripper C to separate unreacted ammonia and carbon dioxide as a mixed gas of ammonia, carbon dioxide, inert gas and water. The mixed gas is introduced into the bottom part of condenser B from the top of stripper C. Air is also introduced into stripper C to prevent corrosion of the equipments. The operating conditions of the stripper are set at a pressure of 130 - 250 bar, particularly 140 - 200 bar, and a temperature of 160 - 200°C. The amount of feed carbon dioxide supplied to the stripper C and that of the carbon dioxide fed to urea synthesis column A or the low pressure decomposition step are properly determined taking into consideration such conditions as the amounts of unreacted ammonia and carbon dioxide to be separated in stripper C. From the bottom part of stripper C, an aqueous urea solution containing unseparated unreacted ammonia and unreacted carbon dioxide is taken out and sent to the low pressure treatment step where it is treated to produce urea. Scrubber F is integrated on the top part of condenser B. Needless to say, scrubber F and condenser B can be provided separately. A recovery solution from a low pressure recovery step is supplied to the top part of scrubber F as an absorption medium. The absorption medium is brought into contact with the inert gas from condenser B to absorb the accompanying ammonia and carbon dioxide for their removal. The inert gas is discharged. Separately, the absorbed solution (the absorption medium having absorbed ammonia and carbon dioxide) from scrubber F flows down to a receiver provided in the bottom part of scrubber F and then to the bottom part of condenser B through a line (down pipe) as an absorption medium for the mixed gas from the stripper. The absorption medium having flowed down is brought into contact with the mixed gas under cooling by a cooler to condense the gas. The condensate thus formed goes up through the condenser and is fed to the suction side of ejector G through the down pipe having an opening in the top part of the condenser. It is then sent to the bottom part of synthesis column A together with the feed liquid ammonia as described above. The operating pressure of condenser B is nearly equal to that of stripper C and is selected from 140 - 250 bar, and the temperature is controlled at 130 - 250°C, particularly 170 - 190°C. The N/C in the condenser B is 2.5 - 3.5, the H/C is 1.0 or less, and the residence time is 10 - 30 minutes. Under these conditions, a urea synthesis rate of 20 - 60% can be achieved. In condenser B, if the N/C is less than 2.5, the partial pressure of carbon dioxide over the condensate is increased, the condensation rate is reduced, and the solidification temperature of the condensate is increased, while if the N/C exceeds 3.5, the vapor pressure of ammonia is increased and hence the condensation rate is decreased. Therefore, any of these N/Cs is preferably avoided. Any H/Cs exceeding 1.0 reduce the equilibrium synthesis rate of urea and therefore are preferably avoided. When the residence time is less than 10 minutes, the vapor pressures are increased and the condensation rate is reduced due to the low rate of urea synthesis in the condenser, while even when it exceeds 30 minutes, a significant rise in the urea synthesis rate is not expected in the condenser in proportion to the prolonged length of time, which only increases the volume of the condenser. Any of these conditions is preferably avoided. Any operating temperature lower than 170°C reduces the rate of urea synthesis, while any temperature above 190°C leads to reduction in the condensation rate following increase in the vapor pressures and to the increase of the corrosion of the equipment. These operating temperatures are preferably avoided. Essentially, where the apparatus of the present invention is used, the liquids and gas can be circulated through urea synthesis column A, stripper C and condenser B by gravity. However, since the condensate from the condenser is sucked, pressurized, and sent to synthesis column A by means of ejector G driven by feed liquid ammonia, the pressure distribution among the synthesis column A, stripper C and condenser B can be determined more freely. A part of the recovery solution, which has been obtained by recovering unreacted ammonia and carbon dioxide as an aqueous ammonium carbamate solution is supplied as an absorption medium to vertical condenser B, while the rest is fed to scrubber F. The aforesaid recovery solution supplied to vertical condenser B is brought into contact with a mixed gas comprising carbon dioxide, ammonia, inert gas and water fed from stripper C to condenser B to absorb the carbon dioxide and ammonia. The operating pressure of the condenser is substantially the same as or somewhat lower than that of the synthesis column as described below and selected from 130 - 250 bar. The N/C, the H/C, and the residence time are selected from 2.5 - 3.5, 1.0 or below, and 10 - 30 minutes, respectively. The operating temperature is controlled at 130 - 250°C, particularly 170 - 190°C, and under the above conditions, an urea synthesis rate of 20 - 60% can be achieved in condenser B. In condenser B, if the N/C is less than 2.5, the condensation rate is reduced due to increase in the partial pressure of carbon dioxide over the ammonium carbamate solution and the solidification temperature of ammonium carbamate is increased. On the other hand, if the N/C is higher than 3.5, the condensation rate is reduced due to increase in the vapor pressure of ammonia. Therefore, any of these N/Cs is preferably avoided. Any H/C in excess of 1.0 reduces the equilibrium synthesis rate of urea and hence is preferably avoided. When the residence time is less than 10 minutes, the vapor pressures are increased and the condensation rate is reduced because the urea synthesis rate is still low. Further, even when it exceeds 30 minutes, the urea synthesis rate is not expected to improve markedly in proportion to the length of time, and the volume of the condenser becomes excessively large. Therefore, any of these conditions is preferably avoided. Any operating temperature below 170°C reduces the urea synthesis rate, while that in excess of 190°C leads to reduction in the condensation rate following increase in the vapor pressures and to the corrosion of the material of the equipment. Therefore, these conditions are preferably avoided. The recovery solution supplied to scrubber F absorbs ammonia and carbon dioxide in the inert gas containing small amounts of ammonia and carbon dioxide introduced into scrubber F and is recycled again to scrubber F. On the other hand, the inert gas is discharged into the air from the top of scrubber F. Due to the use of ejector E, the above-mentioned recovery solution is divided in two between scrubber F and ejector E, and one is used for scrubbing the inert gas in the packed bed of scrubber F, the rest being used for the pressure rise by ejector E at the outlet of scrubber F. A part of feed carbon dioxide is supplied to stripper C. Though not described in the drawing, corrosion preventive air is also supplied to stripper C. A synthesis solution comprising urea, ammonium carbamate, ammonia, etc. is supplied to stripper C through the second down pipe in synthesis column A and undergoes stripping by carbon dioxide under heating. Regarding the operating conditions of stripper C, commonly, the pressure is substantially the same as or somewhat lower than that of synthesis column A and selected from 140 - 200 bar, while the temperature is selected from 160 - 200°C. The rest of the feed carbon dioxide is supplied, for instance, to a low pressure decomposition column. A solution containing urea and unseparated unreacted ammonia and carbon dioxide is withdrawn from the bottom of the stripper, and sent to the low pressure decomposition step where it is treated to obtain urea. The feed liquid ammonia is preheated, for example, to 70 - 90°C, is imported to pass through a cooler of scrubber F, further passes through heat exchanger D at 150 - 300 bar to be preheated to 100 - 200'C, and is fed to ejector G as the driving fluid. On the other hand, the condensate having flowed down by gravity through the first down pipe from the top part of condenser B is supplied to the suction side of ejector G where it is pressurized and fed to the bottom part of synthesis column A together with the driving fluid, the feed liquid ammonia, to undergo urea synthesis. The urea synthesis solution is supplied to the top part of stripper C through the second down pipe from the top part of synthesis column A, as described above. The operating pressure of synthesis column A is selected to be 130 - 250 bar, as is the case with the conventional processes. Further, the N/C, the H/C, and the residence time are selected in the ranges of 3.5 - 5.0, 1.0 - or less, and 10 - 40 minutes, respectively. The operating temperature is controlled at 180 - 200°C, and under the foregoing conditions, a total urea synthesis rate of 60 - 75% can be achieved. In synthesis column A, if the N/C is less than 3.5, the equilibrium synthesis rate is low, while if it exceeds 5.0, the gasification of ammonia is caused to increase the vapor pressure. Therefore, these conditions are preferably avoided. Any H/C in excess of 1.0 is preferably avoided because the urea synthesis rate is caused to decrease. 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 it exceeds 40 minutes, the urea synthesis rate already has fully reached the vicinity of its equilibrium value. Therefore, these conditions are preferably avoided. Further, all or a part of the rest of the feed carbon dioxide supplied to the stripper can be supplied to synthesis column A so that the synthesis temperature may attain 180 - 200°C. Example 1 (Reference) 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. 791 t/d of feed ammonia and 1,025 t/d of feed carbon dioxide were supplied through two separate lines. The feed ammonia heated to 175°C by a steam condensate in heat-exchanger D was fed to the bottom part of synthesis column A, 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 stripper C, the rest of the feed carbon dioxide being supplied to a low pressure decomposition column. Separately, 1314 t/d of a recovery solution as an absorption medium having a composition described below and a temperature of 108°C, were pressurized to 190 bar and divided in two between scrubber F and ejector E. urea 0.4% ammonia 34.8% carbon dioxide 40.0% water 24.4% The recovery solution supplied to scrubber F was used for scrubbing inert gas in the packed bed, while that supplied to ejector E was used for pressurizing a scrubber delivery solution by means of ejector E. Scrubber F was operated at 175 bar and 175°C. The scrubber delivery solution and the ejector-driving recovery solution were mixed together by ejector E and sent to the bottom part of condenser B. A mixed gas comprising ammonia, carbon dioxide and water separated in stripper C was sent to the bottom part of condenser B. The shellside of 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. The condensate having absorbed the mixed gas stayed in the condenser for 15 minutes and flowed down through the first down pipe by gravity to be fed to synthesis column A. In the top part of the condenser, inert gas was withdrawn. The condensate from condenser B which had been supplied to the bottom part of synthesis column A was mixed with the above-mentioned liquid ammonia heated to 175°C 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 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 stripper C through the second down pipe from synthesis column A was heated there so that the ammonium carbamate was decomposed and separated as a mixed gas of ammonia and carbon dioxide. 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 condenser B, urea synthesis column A and stripper C. The operating conditions and the compositions in condenser B, synthesis column A, and stripper C are mentioned in Table 1. Reference Example 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 (Reference) 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 (Reference). The results are also shown in Table 1. Example 2: A urea synthesis process was put in operation under the operating conditions of condenser B, synthesis column A and stripper C as described in Table 1. Example 1 (Reference) Example 2 (Reference) 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 Urea (%) 48.1 46.3 - Ammonia (%) 13.2 15.5 - Carbon dioxide (%) 13.7 13.5 - Water (%) 25.0 24.7 - Stripper C Carbon dioxide (t/d) 969 700 - Example 3 (Reference): Urea synthesis was carried out by an improved urea synthesis process which employs an apparatus with a daily production of 1,400 tons placed on the ground. 792 t/d of feed ammonia and 1025 t/d of feed carbon dioxide were supplied. Ammonia with a pressure of 250 bar and a temperature of up to 175°C heated by steam condensate in heat exchanger D, was fed to the bottom part of urea synthesis column A through ejector G. Further, 969 t/d of the feed carbon dioxide and about 17 t/d of corrosion preventive air were respectively supplied to the bottom part of stripper C the rest of the feed carbon dioxide being supplied to a low pressure decomposition column. Separately, 1314 t/d of a recovery solution as an absorption medium had the following composition and a temperature of 108°C and was pressurized to 190 bar to be sent to scrubber F. Urea 0.4% Ammonia 34.8% Carbon dioxide 40.0% Water 24.4% Scrubber F was operated at 175 bar and 175°C. The delivery solution of the scrubber was supplied to the bottom part of condenser B through a receiver and a down pipe. Separately, inert gas was discharged from scrubber F at its top. A mixed gas comprising ammonia, carbon dioxide, water and inert gas separated in stripper C, was sent to the bottom part of condenser B. The shellside of condenser B was filled with the foregoing recovery solution and ammonium carbamate formed by condensation, and the foregoing mixed gas was absorbed by the solution. The heat of absorption was removed in a cooler. The condensate having absorbed the mixed gas stayed in the condenser for 15 minutes, flowed down through the down pipe by gravity, was pressurized by ejector G, and then was sent to the bottom part of synthesis column A. The feed liquid ammonia and the condensate fed to the bottom part of synthesis column A caused the urea synthesis reaction to proceed further, while going up through the synthesis column. The residence time for the reaction was 25 minutes. In the interior of synthesis column A, baffle plates were arranged to accelerate the dispersion of gas and prevent the back mixing of liquid. The synthesis solution fed to stripper C from the top of synthesis column A was heated in the stripper 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 top pressure of 177 bar. As described above, the apparatus was operated by circulating the gas and liquids through condenser B, synthesis column A and stripper C. The pressure difference, serving as the driving force for this purpose, between the suction and delivery sides of the ejector, which employed the feed liquid ammonia as the driving fluid, was about 3 bar. The operating conditions and the compositions in condenser B, synthesis column A, and stripper C are shown in Table 2. Example 4: A urea synthesis process was put in operation under the operating conditions of condenser B, synthesis column A, and stripper C, as described in Table 2. Example 5 (Reference): Using an apparatus with a daily production of 1,725 tons placed on the ground urea synthesis was put in operation under the operating conditions of condenser B, synthesis column A and stripper C, as described in Table 2. Example 3 (Reference) Example 4 Example 5 (Reference) Feed ammonia (t/d) 792 792 978 Feed carbon dioxide (t/d) 1025 1025 1264 Recovery solution (t/d) 1314 1314 1457 Urea (%) 0.4 0.4 0.5 Ammonia (%) 34.8 34.8 36.4 Carbon dioxide (%) 40.4 40.4 37.0 Water (%) 24.4 24.4 26.1 Condenser B Pressure (bar) top 175 175 156 Pressure (bar) bottom 176 176 157 Temperature (°C) 185 185 185 Urea (%) 21.1 21.1 29.2 Ammonia (%) 34.3 34.3 32.0 Carbon dioxide (%) 26.7 26.7 18.0 Water (%) 17.9 17.9 20.8 Urea synthesis rate (%) 42.6 42.6 54 N/C 2.85 2.85 3.18 H/C 0.67 0.67 0.74 Residence time (minute) 15 15 20 Volume ratio 0.34 0.34 - Ejector Pressure(bar) suction side 176 176 157 Pressure(bar) delivery side 179 179 160 Synthesis column A Pressure (bar) top 177 177 158 Temperature (°C) 185 185 188 Urea (%) 31.8 31.8 32.9 Ammonia (%) 36.8 36.8 35.3 Carbon dioxide (%) 12.2 12.2 12.6 Water (%) 19.2 19.2 19.2 Urea synthesis rate (%) 65.7 65.7 65.5 N/C 4.0 4.0 3.8 H/C 0.67 0.67 0.62 Residence time (minute) 25 25 20 Volume ratio 0.54 0.54 - Stripper C Pressure (bar) top 177 177 158 Urea (%) 48.1 48.1 49.0 Ammonia (%) 13.2 13.2 13.5 Carbon dioxide (%) 13.7 13.7 12.3 Water (%) 25.0 25.0 25.2 Stripper C Carbon dioxide (t/d) 969 969 863 According to the present invention, the following effects can be achieved. (1) Where a vertical condenser is integrated with the synthesis column, simplification of the process is promoted. (2) Since an ejector using feed liquid ammonia as the driving fluid is employed to supply the condensate to the urea synthesis column, the degree of freedom is increased in the installing location and pressure distribution of the stripper and condenser. (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 apparatus comprising (a) a vertical urea synthesis column, (b) a vertical condenser with a cooler, installed on or above said urea synthesis column (c) a stripper for stripping unreacted ammonia and unreacted carbon dioxide, contained in a urea synthesis solution from said 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 heat-exchanger for preheating feed liquid ammonia, (e) an ejector using the preheated feed liquid ammonia as the driving fluid, (f) a first down pipe that has an opening in the top part of said vertical condenser and is for supplying a condensate from the top part of said vertical condenser to the suction side of the ejector, (g) a piping for introducing the feed liquid ammonia and said condensate from the delivery side of the ejector into the bottom part of said vertical urea synthesis column, (h) 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, (i) a piping for introducing said mixed gas from said stripper into the bottom part of said vertical condenser, (j) a piping for introducing an absorption medium or an absorption medium and feed ammonia into the bottom part of said vertical condenser, (k) a piping for discharging inert gas, joined to the top part of said vertical condenser, (I) a piping for introducing at least a part of feed carbon dioxide, joined to the bottom part of said stripper, and (m) 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 1, 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 1, wherein said vertical condenser is installed separately on or above said vertical urea synthesis column, and the vertical urea synthesis column and the vertical condenser are integrated. The urea synthesis apparatus according to any one of claims 2 to 3, 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 2 - 4, wherein said vertical urea synthesis column has baffle plates in the inside thereof to make gas-luqid contact better. The urea synthesis apparatus according to claim 1, wherein the relative positional relations of said vertical urea synthetic column, said vertical condenser, and said stripper are such that when the height upto the top of said vertical condenser from the ground level is assumed to be 100 %, the height upto the bottom of said vertical condenser ist 50 - 80 %, the height upto the top of said stripper being 20 - 50 %, the height upto the bottom of said vertical synthesis column being 5 - 30 %, the height upto 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, KOJIMA YASUHIKO, FUJII, HIDETSUGU, KOJIMA, YASUHIKO
Applicants: TOYO ENGINEERING CORP, TOYO ENGINEERING CORPORATION
Application year: 1997
Main classification: C07C273
IPC codes: C07C273, B01J19, B01D5, B01J10