PROCESS FOR PRODUCING HIGH-PURITY AQUEOUS UREA SOLUTION IN UREA PRODUCTION PROCESS
20170283372 · 2017-10-05
Inventors
Cpc classification
International classification
Abstract
The present invention relates to a method for producing a high-purity aqueous urea solution, utilizing a urea production process at least including a urea synthesis step of synthesizing urea from a raw material feed to produce a urea synthesis liquid, and a urea purification step of purifying the urea synthesis liquid to produce an aqueous urea solution with high urea concentration. The present invention includes a urea crystallization step of separating a part of the urea synthesis liquid and/or a part of the aqueous urea solution and crystallizing urea contained in the separated urea synthesis liquid and/or aqueous urea solution to produce solid crystal urea, and a mixing step of mixing the crystal urea with water to produce a high-purity aqueous urea solution. A high-purity aqueous urea solution to be produced is an aqueous urea solution with high purity suitable as a reducing agent for SCR.
Claims
1. A method for producing a high-purity aqueous urea solution in a urea production process, the process comprising at least a urea synthesis step for synthesizing urea from a raw material feed to produce a urea synthesis liquid and a urea purification step for purifying the urea synthesis liquid to produce an aqueous urea solution having high urea concentration, wherein the method further comprises: a urea crystallization step of separating a part of the urea synthesis liquid and/or a part of the aqueous urea solution, and crystallizing urea contained in a separated urea synthesis liquid and/or aqueous urea solution to produce crystal urea being a solid; and a mixing step of mixing the crystal urea with water to produce a high-purity aqueous urea solution.
2. The method for producing a high-purity aqueous urea solution according to claim 1, wherein the urea crystallization step comprises the steps of: crystallizing urea contained in the separated urea synthesis liquid and/or aqueous urea solution to produce slurry containing crystal urea; subjecting the slurry to solid-liquid separation into crystal urea and a mother liquid; and further optionally sending back a part of the mother liquid after the solid-liquid separation to the urea synthesis step.
3. The method for producing a high-purity aqueous urea solution according to claim 1, wherein conditions for concentrating the aqueous urea solution to generate the crystal urea in the urea crystallization step are 60 to 80 mmHg (abs.) in pressure and 50 to 80° C. in temperature.
4. The method for producing a high-purity aqueous urea solution according to claim 1, wherein: the urea production process further comprises a urea concentrating step of concentrating the aqueous urea solution produced in the urea purification step to increase urea concentration in the aqueous urea solution; and the urea crystallization step separates and crystallizes a part of an aqueous urea solution between the urea purification step and the urea concentrating step and/or a part of an aqueous urea solution in the urea concentrating step.
5. The method for producing a high-purity aqueous urea solution according to claim 1, wherein the urea production process comprises a finishing step of producing solid granular urea from a concentrated aqueous urea solution.
6. The method for producing a high-purity aqueous urea solution according to claim 2, wherein conditions for concentrating the aqueous urea solution to generate the crystal urea in the urea crystallization step are 60 to 80 mmHg (abs.) in pressure and 50 to 80° C. in temperature.
7. The method for producing a high-purity aqueous urea solution according to claim 2, wherein: the urea production process further comprises a urea concentrating step of concentrating the aqueous urea solution produced in the urea purification step to increase urea concentration in the aqueous urea solution; and the urea crystallization step separates and crystallizes a part of an aqueous urea solution between the urea purification step and the urea concentrating step and/or a part of an aqueous urea solution in the urea concentrating step.
8. The method for producing a high-purity aqueous urea solution according to claim 3, wherein: the urea production process further comprises a urea concentrating step of concentrating the aqueous urea solution produced in the urea purification step to increase urea concentration in the aqueous urea solution; and the urea crystallization step separates and crystallizes a part of an aqueous urea solution between the urea purification step and the urea concentrating step and/or a part of an aqueous urea solution in the urea concentrating step.
9. The method for producing a high-purity aqueous urea solution according to claim 2, wherein the urea production process comprises a finishing step of producing solid granular urea from a concentrated aqueous urea solution.
10. The method for producing a high-purity aqueous urea solution according to claim 3, wherein the urea production process comprises a finishing step of producing solid granular urea from a concentrated aqueous urea solution.
11. The method for producing a high-purity aqueous urea solution according to claim 4, wherein the urea production process comprises a finishing step of producing solid granular urea from a concentrated aqueous urea solution.
Description
BRIEF DESCRIPTION OF DRAWINGS
[0037]
[0038]
[0039]
MODE FOR CARRYING OUT THE INVENTION
[0040] Embodiments of the present invention will be described based on Examples described below.
[0041] Further, the urea production process 100 according to the present embodiment is configured to be able to separate and draw a part of an aqueous urea solution from the urea purification step 20. In the present embodiment, a line (Ls) between the urea purification step 20 and the urea concentrating step 40 is branched. Further, there is also set a line (Lc) for enabling separation of a part of an aqueous urea solution in the urea concentrating step 40.
[0042] Furthermore, the urea production process 100 according to the present embodiment includes a urea crystallization step 60 and a mixing step 70 for mixing crystal urea produced in the urea crystallization step 60 with water to produce a high-purity aqueous urea solution.
[0043] The urea crystallization step 60 includes a step of concentrating an aqueous urea solution from the urea purification step 20 by evaporating water with heating and generating slurry by depositing crystal urea with temperature lowered in vacuum. Further, it includes a step of subjecting the slurry to solid-liquid separation. The solid crystal urea subjected to the solid-liquid separation in the urea crystallization step 60 is mixed with water in the mixing step 70 to produce a high-purity aqueous urea solution.
[0044] Further, to the urea crystallization step 60, there is set a line (Lr) for sending back, to the recovery step 30, at least a part of a mother liquid obtained by subjecting the slurry to the solid-liquid separation. The opening and closing of the line Lr is controlled with a control valve or the like, and can be performed according to an operation state, biuret concentration in an aqueous urea solution or the like.
[0045] The above-described urea production process 100 according to the present embodiment can be practiced by a urea production plant composed of a group of apparatuses for practicing respective steps. The urea production plant includes a urea synthesis apparatus for the urea synthesis step 10, a urea purification apparatus for the urea purification step 20, a collection apparatus for the recovery step 30, a urea concentrating apparatus for the urea concentrating step 40, and a finishing apparatus for the finishing step 50. Further, it includes a urea crystallization apparatus for the urea crystallization step 60 and a mixing apparatus for mixing crystal urea with water to produce a high-purity aqueous urea solution. The urea crystallization apparatus has, as main equipment, a crystallization tank for depositing crystal urea from an aqueous urea solution and a centrifugal separator for solid-liquid separation. Moreover, the mixing apparatus has a mixing tank as main equipment. These apparatuses are usually configured with a plurality of equipment such as plural towers and tanks, heat exchangers, pumps and auxiliaries. The configuration is the same as that of a general urea production plant, and no particularly new one is required. In addition, these apparatuses may be configured with single equipment alone.
[0046] In the present embodiment, material balances under two operation states were obtained for the above urea production process. Meanwhile, material balances shown in Examples and Comparative Examples below are those based on an apparatus for which a prilled urea production capacity of 1725 ton/day is set as a design production volume of a urea production plant.
EXAMPLE 1
[0047] In this example, there will be described a material balance in a case of producing a high-purity aqueous urea solution while operating a prilled urea production plant under ordinary operation conditions based on a design production volume. Tables 1 and 2 show compositions of feed in respective lines in and after the urea purification step 20.
[0048] The urea synthesis step 10 having received a raw material feed supply of ammonia and carbon dioxide generates a urea synthesis liquid. As to the urea synthesis liquid containing urea, ammonium carbamate, ammonia and water as main components, ammonium carbamate is decomposed, and ammonia and carbon dioxide are separated from the urea synthesis liquid to form an aqueous urea solution, in the urea purification step 20. The aqueous urea solution from the urea purification step 20 (line L1) is an aqueous urea solution containing a minute amount of ammonia and carbon dioxide, and, in the present example, it was an aqueous urea solution of 0.33% by mass in biuret concentration. The composition of the aqueous urea solution in the line L1 is as listed in Table 1.
[0049] A part of the aqueous urea solution from the urea purification step 20 is drawn for the urea crystallization step (line Ls). On the other hand, the remaining aqueous urea solution is sent to the urea concentrating step 40 (line L2).
[0050] The aqueous urea solution sent to the urea concentrating step 40 by the line L2 is heated and concentrated in vacuum. On this occasion, usually a molten urea liquid is obtained generally under two-stage operation conditions of 150 to 250 mmHg (abs.) and 25 to 30 mmHg (abs.) in pressure and 125 to 140° C. in heating temperature. Moreover, in the concentrating step, ammonia and carbon dioxide in the aqueous urea solution were drawn as gas, and were separated and collected with ammonia and carbon dioxide generated as a result of hydrolysis of a part of urea. In the present example, a urea solution was concentrated up to about 99.7%. Then, the concentrated urea liquid is sprayed in a shower-like state from an upper part, and is solidified and cooled by the contact with the air rising from a lower part to generate a prilled urea product, in the finishing step.
[0051] A part of the aqueous urea solution from the urea purification step 20 is introduced to a crystallizing tank in the urea crystallization step 60 by the line Ls branched from the line L1. The crystallizing tank was operated in vacuum of 75 mmHg (abs.) at 60° C. to generate crystal urea. Ammonia and carbon dioxide in the aqueous urea solution are separated with water and urea being a minute amount of mist in the crystallizing tank (accordingly, a urea amount in slurry after purification of crystal is reduced slightly). Subsequently, solid-liquid separation was performed with a centrifugal separator to give crystal urea (line L4). Meanwhile, a mother liquid obtained by separating crystal urea from slurry is sent back to a crystallization tank. In Example 1, a part of the mother liquid separated from slurry is not sent back to the recovery step. It is configured so that all urea introduced to the urea crystallization step 60 via the line Ls is converted to crystal urea.
[0052] The generated crystal urea was sent to the mixing step 70, in which water (8334 kg/h) was added and the crystal urea was dissolved with heating to produce a high-purity aqueous urea solution.
[0053] There are listed, in Table 1, material balances of the lines L1, L2, and L3 from the urea purification step 20 until prilled urea is produced in the finishing step, and there are listed, in Table 2, material balances of the lines Ls, L4, and L5 until the high-purity aqueous urea solution is produced.
TABLE-US-00001 TABLE 1 L1 L2 L3 Urea 72227 kg/h 68156 kg/h 67145 kg/h Biuret 352 kg/h 332 kg/h 543 kg/h Biuret 0.33 wt % 0.33 wt % 0.8 wt % concentra- tion Ammonia 582 kg/h 549 kg/h — Carbon 341 kg/h 322 kg/h — dioxide Water 33725 kg/h 31824 kg/h 136 kg/h Total 107227 kg/h 101183 kg/h 67823 kg/h
TABLE-US-00002 TABLE 2 Ls L4 L5 Urea 4072 kg/h 4063 kg/h 4063 kg/h Biuret 20 kg/h 20 kg/h 20 kg/h Biuret 0.33 wt % 0.48 wt % 0.16 wt % concentra- tion Ammonia 33 kg/h — — Carbon 19 kg/h — — dioxide Water 1901 kg/h 83 kg/h 8417 kg/h Total 6044 kg/h 4166 kg/h 12500 kg/h
[0054] From Table 2, it is known that a high-purity aqueous urea solution having a biuret concentration of 0.16% by mass can be produced in the present example. On the other hand, prilled urea produced from the urea concentrating step 40 via the finishing step 50 has biuret concentration of 0.8% by mass, and thus the biuret concentration increases relative to that in the aqueous urea solution from the urea purification step 20. This is because, in the treatment in or after the urea concentrating step, biuret is additionally generated by a partial dimerization of urea with the previous biuret left as it is. However, with regard to this example, the biuret concentration in the produced prilled urea is not so high, and therefore it is possible to produce a high-purity aqueous urea solution even by diluting the produced prilled urea.
EXAMPLE 2
[0055] This example assumes a case where a production volume of prilled urea lowers due to a shortage of a raw material in the same urea production plant as in Example 1. In this example, the aqueous urea solution (L1) from the urea purification step 20 was partially separated (Ls) so that the production volume of a high-purity aqueous urea solution was equal to that in Example 1. Then, prilled urea was produced from a remaining aqueous urea solution (L2).
[0056] In this example, too, slurry was generated in the urea crystallization step 60, and a mother liquid after solid-liquid separation is sent back to the crystallization tank. However, in this example, a part of the mother liquid is sent back to the recovery step 30 by the line Lr so that it is finally recycled in the urea synthesis step 10.
[0057] There are listed in Table 3 material balances in lines L1, L2, and L3 from the urea purification step 20 up to the production of prilled urea in the finishing step, and there are listed in Table 4 material balances in lines Ls, Lr, L4, and L5 up to the production of a high-purity aqueous urea solution, in Example 2. Meanwhile, operation conditions in the urea synthesis step 10, the urea purification step 20, the urea concentrating step 40 and the finishing step 50 in Example 2 are the same as in Example 1. Further, operation conditions of the crystallizing tank in the urea crystallization step 60 are also the same as in Example 1 (75 mmHg (abs.), 60° C.).
TABLE-US-00003 TABLE 3 L1 L2 L3 Urea 41871 kg/h 37618 kg/h 36989 kg/h Biuret 251 kg/h 226 kg/h 412 kg/h Biuret 0.4 wt % 0.4 wt % 1.1 wt % concentra- tion Ammonia 337 kg/h 303 kg/h — Carbon 198 kg/h 178 kg/h — dioxide Water 19551 kg/h 17565 kg/h 83 kg/h Total 62208 kg/h 55896 kg/h 37484 kg/h
TABLE-US-00004 TABLE 4 Ls Lr L4 L5 Urea 4253 kg/h 181 kg/h 4063 kg/h 4063 kg/h Biuret 26 kg/h 9 kg/h 17 kg/h 17 kg/h Biuret 0.43 wt % 3.55 wt % 0.4 wt % 0.13 wt % concentra- tion Ammonia 34 kg/h — — — Carbon 20 kg/h — — — dioxide Water 1901 kg/h 63 kg/h 82 kg/h 8421 kg/h Total 6044 kg/h 253 kg/h 4162 kg/h 12500 kg/h
[0058] In the case of Example 2, volumes of fluids flowing in respective steps lower due to the lowering of the production volume. However, the size of each equipment is designed so that prilled urea of a design production volume (1725 ton/day) can be produced. Consequently, retention times of a urea synthesis liquid and aqueous urea solution in respective steps increase, and thereby the generation amount of biuret will increase. With reference to Table 3, the biuret concentration of the aqueous urea solution (L1) from the urea purification step 20 is 0.4% by mass. The biuret concentration is slightly higher than that in Example 1.
[0059] The aqueous urea solution was partially drawn (Ls), and crystal urea was deposited and separated in the crystallizing tank in the urea crystallization step 60 to generate slurry. Solid-liquid separation was performed with a centrifugal separator to give crystal urea (L4). As to the mother liquid from the centrifugal separator obtained at that time, a part of the mother liquid was sent to the recovery step (Lr), and the rest was returned to the crystallization tank. Further, 8338 kg/h of water was added to the crystal urea from L4 in the mixing step 70, which was heated to produce a high-purity aqueous urea solution. The biuret concentration of the aqueous urea solution was 0.13% by mass, which falls within the standard of a high-purity aqueous urea solution for an SCR reducing agent.
[0060] On the other hand, the remaining part (L2) of the aqueous urea solution from the urea purification step 20 was concentrated (L3) in the urea concentrating step 40 to produce prilled urea. The biuret concentration of the prilled urea is 1.1% by mass. When the prilled urea is dissolved in water to produce a high-purity aqueous urea solution, about 112000 kg/h of water is required. Then, the biuret concentration of an aqueous urea solution to be produced is 0.36% by mass, which falls outside the standard. As described above, in Example 2, the biuret concentration increased in the aqueous urea solution due to the lowering of the operation load. As the result, the produced prilled urea was inadequate for a raw material of an aqueous urea solution for an SCR reducing agent. In this way, in Example 2, a high-purity aqueous urea solution within the standard was not able to be produced from prilled urea being a finished product. However, in this example, it was confirmed that a high-purity aqueous urea solution within the standard was able to be produced by utilization of crystal urea obtained by partially drawing and treating an aqueous urea solution.
REFERENCE EXAMPLE
[0061] A result of a case was examined where the mother liquid of slurry generated in the urea crystallization step 60 was not sent back to the recovery step 30 in Example 2 (a case where line Lr was closed).
[0062] The Reference Example assumes a case where a prilled urea production volume lowers due to a shortage of a raw material, as in Example 2. Further, the aqueous urea solution (L1) from the urea purification step 20 was partially separated (Ls) so that the production volume of a high-purity aqueous urea solution was equal to that in Example 1. Furthermore, in the urea crystallization step 60, all urea, which was introduced by returning, to the crystallization tank, a part of the mother liquid obtained from generated slurry by solid-liquid separation without sending it back to the recovery step, was formed into crystal urea. Further, prilled urea was produced from the aqueous urea solution (L2) being the residue of L1. There are listed in Table 5 material balances in lines L1, L2, and L3, and there are listed in Table 6 material balances in lines Ls, Lr, L4, and L5 in the Reference Example.
TABLE-US-00005 TABLE 5 L1 L2 L3 Urea 41871 kg/h 37799 kg/h 37170 kg/h Biuret 251 kg/h 225 kg/h 412 kg/h Biuret 0.4 wt % 0.4 wt % 1.1 wt % concentra- tion Ammonia 337 kg/h 304 kg/h — Carbon 198 kg/h 179 kg/h — dioxide Water 19551 kg/h 17650 kg/h 83 kg/h Total 62208 kg/h 56157 kg/h 37665 kg/h
TABLE-US-00006 TABLE 6 Ls Lr L4 L5 Urea 4072 kg/h Without 4063 kg/h 4063 kg/h Biuret 26 kg/h recycling 26 kg/h 26 kg/h Biuret 0.43 wt % 0.62 wt % 0.21 wt % concentra- tion Ammonia 33 kg/h — — Carbon 19 kg/h — — dioxide Water 1901 kg/h 83 kg/h 8411 kg/h Total 6051 kg/h — 4172 kg/h 12500 kg/h
[0063] From Table 6, the biuret concentration in the high-purity aqueous urea solution (L5) produced in the Reference Example becomes higher than those in Examples 1 and 2. This is because the biuret concentration in the aqueous urea solution L1 is high due to the lowering of an operation load. In the case of the Reference Example, the biuret concentration satisfies the reference value at least. However, it is considered that, when a case is assumed where the biuret concentration in an aqueous urea solution becomes higher, a high-purity aqueous urea solution to be produced in the case may not satisfy the standard. Further, in the Reference Example as in Example 2, a suitable high-purity aqueous urea solution cannot be produced from the prilled urea being the finished product. Accordingly, it is preferable to allow a part of mother liquid of slurry to be recycled selectively and optionally, assuming a change in operation conditions.
[0064] Next, there was examined a conventional urea production plant without setting of the urea crystallization step 60 with respect to the urea production plant according to the present embodiment described above.
COMPARATIVE EXAMPLE 1
[0065] The urea synthesis step 10 and the urea purification step 20 were operated under the same operation conditions as in Example 1. Further, as shown in
TABLE-US-00007 TABLE 7 L6 L7 Urea Content 4072 kg/h ← Concentration 67.37% 32.58% Biuret Content 20 kg/h ← Concentration 0.33 wt % 0.16% Ammonia Content 33 kg/h ← Concentration 0.54% 0.26% Carbon Content 19 kg/h ← dioxide Concentration 0.31% 0.15% Water Content 1901 kg/h 8366 kg/h Total quantity 6044 kg/h 12500 kg/h
[0066] From Table 7, the biuret concentration in the aqueous urea solution (L7) produced in this case falls within the standard. However, the solution does not satisfy the standard of reference value of 0.2% (max.) on alkalinity (in terms of ammonia). Accordingly, a step of removing ammonia is required when the aqueous urea solution is utilized. For example, it is possible to produce an aqueous urea solution, in which alkalinity also falls within the standard, by concentrating the solution before dilution with mixture of water.
COMPARATIVE EXAMPLE 2
[0067] Consequently, as shown in
TABLE-US-00008 TABLE 8 L6 L8 L9 Urea Content 4087 kg/h 4072 kg/h ← Concentration 67.45% 99.00% 32.58% Biuret Content 20 kg/h 33 kg/h ← Concentration 0.33 wt % 0.8 wt % 0.26% Ammonia Content 33 kg/h — — Concentration 0.54% — — Carbon Content 19 kg/h — — dioxide Concentration 0.31% — — Water Content 1901 kg/h 8 kg/h 8395 kg/h Total quantity 6059 kg/h 4113 kg/h 12500 kg/h
[0068] As known from Table 8, the aqueous urea solution (L9) produced in this comparative example has a biuret concentration and alkalinity both falling within the standard. However, the biuret concentration is relatively higher than that in Example 1. This is thought to be due to generation of biuret when the drawn aqueous urea solution was concentrated.
COMPARATIVE EXAMPLE 3
[0069] Next, the urea plant 300 in Comparative Example 2 was examined assuming a case where the production volume of prilled urea lowered due to a shortage of a raw material, in the same way as in Example 2. In this comparative example, the aqueous urea solution (L1) from the urea purification step 20 was partially separated (L6) so that the production volume of a high-purity aqueous urea solution was equal to that in Comparative Example 2. Then, the partially drawn aqueous urea solution (L6) was concentrated, and an aqueous urea solution after the concentration (L8) was mixed with water to produce a high-purity aqueous urea solution (L9). Compositions of respective solutions at this time are as follows.
TABLE-US-00009 TABLE 9 L6 L8 L9 Urea Content 4087 kg/h 4072 kg/h ← Concentration 67.31% 98.70% 32.58% Biuret Content 32 kg/h 45 kg/h ← Concentration 0.53% 1.1 wt % 0.36% Ammonia Content 33 kg/h — — Concentration 0.54% — — Carbon Content 19 kg/h — — dioxide Concentration 0.31% — — Water Content 1901 kg/h 9 kg/h 8383 kg/h Total quantity 6072 kg/h 4126 kg/h 12500 kg/h
[0070] As known from Table 9, the biuret amount increases due to the lowering of an operation load, and the influence extends into the final aqueous urea solution (L9). That is, the aqueous urea solution has a biuret concentration exceeding the standard value (0.3% by mass, max.), and therefore, cannot be used for an application of an SCR reducing agent. Comparative Examples 2 and 3 are based on a virtual plant having a concentrating step added to the conventional technology (PTL 1) in Comparative Example 1. It can be said that the virtual plant may at least suppress impurities (ammonia etc.) low in an aqueous urea solution. However, it is known that the virtual plant cannot follow the increase in the biuret amount that may be generated due to the change in an operation load.
INDUSTRIAL APPLICABILITY
[0071] The present invention is a method for producing a high-purity aqueous urea solution for an SCR reducing agent, using, as a raw material, a urea-containing solution from a urea synthesis step or a urea purification step in a urea production plant. According to the present invention, a high-purity aqueous urea solution within the standard (biuret concentration: 0.3% by mass, max.) can be produced, regardless of a biuret concentration in the urea-containing solution to be received.