JPS63208563A - Production of urea - Google Patents

Abstract

PURPOSE:To enable size-reduction of a stripping-type synthesis pipe in the conversion of a urea synthesis process from a nonstripping-type solution circulation process to a stripping-type process, by using a synthesis pipe of the solution circulation process as it is. CONSTITUTION:Recovered liquid 42 of unreacted material obtained from the step after the stripping step is pressurized to a synthetic pressure with a pump 3. A part of the pressurized liquid is supplied to a gas scrubber 15 and the remainder (30-50%) is supplied to the former-stage synthesis pipe 11 which is a synthesis pipe of a solution circulation process and is reacted with freshly supplied CO234 and freshly supplied NH333 under synthetic condition. The obtained former-stage synthesis liquid is transferred to a carbamate condenser 14' and made to contact with mixed gas supplied from a stripper 13 to effect the condensation of at least a part of the mixed gas. The mixed flow of gas and solid discharged from the condenser 14' is sent to the latter-stage synthesis pipe 12 which is a stripping-type synthesis pipe and is made to react with freshly supplied NH3 under a synthesis condition of conventional stripping-type reaction.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical field to which the invention pertains] This invention relates to a synthetic step in a urea manufacturing process. For details, see a. (Non-stripping type) solution circulation method glass.
This invention relates to improvements in the synthesis process when converting to a stripping method.

[Problems with conventional technology] The urea production process can be roughly divided into the two types mentioned above.

Previously, there was a big difference in the synthesis conditions between the two.

That is, the NH3/co2 (not ratio) in synthesis is about 4 in the solution circulation method, while it is 3 in the stripping method.
It has been considered difficult to increase the not ratio in this way. However, due to advances in technology and the rise in energy prices, the stripping method has become technically and economically possible even with a large molar ratio. Therefore, there are cases where the solution circulation method is converted to a stripping method. One of the problems in this case is that in both cases, one of the synthetic tubes
There is a big difference in m and how it is arranged. That is, in case b, the tube is larger than in case a for the same production volume, and a baffle plate is installed inside. Furthermore, the method of installing the fluid inlet/outlet and the synthesis pipe is also very different from that in case a. Therefore, during conversion, various measures are required to repurpose the synthesis tube used in the solution circulation method. Since the synthetic tube belongs to an expensive unit in the entire plant, when converting the process from a to b, especially when increasing production, the synthetic tube from a is used. If a plant is currently operating in process a, losses due to stoppage during conversion must be minimized, and this consideration must also be taken. This invention solves these problems and provides an optimal method for converting the synthetic process.

[Invention and its structure] This invention first converts a urea synthesis solution containing 1q of urea, unreacted ammonia, and carbon dioxide, which has been produced in a synthesis process at a pressure and temperature suitable for urea synthesis from ammonia and carbon dioxide, under the same pressure. A part of the unreacted substances is separated in gaseous form by a stripping process using carbon dioxide gas, and then the remaining urea synthesis solution is depressurized and subjected to at least one unreacted substance separation process to remove the urea. Substantially all of the unreacted substances in the synthesis solution are separated into a gaseous state to obtain an aqueous urea solution, and on the other hand, the solution of unreacted substances recovered from the separation step after the stripping step is brought to the pressure of the synthesis step. A urea production process in which the pressure is increased and the mixture gas is subjected to a condensation step, unreacted substances from the stripping step and carbon dioxide used in the stripping are condensed, and the resulting condensate and uncondensed gas are circulated to the synthesis step. , before subjecting the unreacted product solution, which has been pressurized to the pressure of the synthesis step, to the pseudo-condensation step,
This urea production method is characterized in that at least a part of the gas is kept under urea synthesis conditions together with a part of newly supplied carbon dioxide gas and ammonia, and then subjected to the condensation step after urea is synthesized. The synthetic tubes for a and b can be combined in series, in parallel, or in series depending on their order. As a result of considering the technical, economical, and loss of business during remodeling,
This led to the method of this invention in which the synthetic tube for A is installed in front of the tubes in series. In this first stage synthesis (hereinafter referred to as first stage synthesis or first stage synthesis tube), the pressure is substantially the same as that in the second stage synthesis, but the temperature is preferably as low as possible. The reason for this is that it is better to minimize the amount of newly supplied carbon dioxide gas introduced here and use it for stripping. The reason why a portion of the unreacted material solution is sent here is for the same reason. For this reason, the newly supplied carbon dioxide gas fed here is 5 to 15% (by weight) of the total newly supplied carbon dioxide gas. Because of this small amount, H 20 /CO2 (mO1 ratio) in the first-stage synthesis becomes larger than usual, resulting in a decrease in the synthesis rate.

For this purpose, the newly supplied ammonia sent here will be
It is preferable to increase the amount to an extent that compensates for this decrease in the synthesis rate, so the amount is set at 30 to 50% (by weight) of the total newly supplied ammonia. As a result, NH3/Co in this first-stage synthesis
2 (mol ratio) = 3.5 to 5.0. The temperature is also regulated by preheating the freshly fed ammonia and is 175-1
It becomes 85℃. The first-stage urea synthesis solution obtained under such synthesis conditions contains 25 to 35% (by weight) of urea.

This synthetic liquid is then subjected to the kabamate condensation step described above to condense the mixed gas from the stripping step. After this, the process is exactly the same as the circulation to the synthesis step in a normal stripping process. However, in this invention, the synthetic tube for b is in one stage at the rear.

Next, this invention will be explained with reference to the drawings. 11 is the front synthesis tube, 12 is the rear synthesis tube, 13 is the stripper, 14.14
° is the carbamate condenser. Pressure 160-180k
g/cdG, temperature 185-195°C post-synthesis tube 1
2, the synthetic liquid containing urea synthesized to near the equilibrium value passes through the downcomer 39 and enters the top of the stripper 13 at the same pressure, and the newly supplied carbon dioxide gas (80 to 95% of it) is blown into the bottom from the line 35. (weight)) under heating in a countercurrent manner, and unreacted ammonia and a portion of carbon dioxide contained in the synthesis liquid are separated from the top of the column together with the blown carbon dioxide gas.

Temperature 17 at which the residual unreacted material became 20 to 30% (weight)
Synthetic liquid at 5 to 185℃ is heated to 14 to 20k by pressure reducing valve 21.
g/CdG, enters the shell side of the carbamate condenser 14, receives heat of condensation generated on the tube side, and further vaporizes at least a portion of the remaining unreacted substances.

After this, the mixture is subjected to gas-liquid separation or, if necessary, heated roughly with steam to reduce residual unreacted substances in the synthesis liquid. The unreacted gases separated in this step are absorbed in a corresponding (high pressure) absorption step. Treatment of the synthesis liquid and unreacted substances after this pressure stage follows well-known methods.

The recovered unreacted liquid (aqueous bamate solution) obtained from the process after the stripping process - line 42 - is pressurized to the previous synthesis pressure by the pump 3, and a part of it (
50 to 70%) is supplied to the gas scrubber 15 via the line 43, and the remaining (30 to 50%) is sent to the pre-stage synthesis tube 11 via the feed-in 44. At the same time, part of the newly supplied carbon dioxide gas (5-15% (weight)) and 120-160%
A portion of freshly fed ammonia preheated to 30-50 °C
% (weight)) is also fed.

Here, under the synthesis conditions described earlier, urea is at an equilibrium value of 90 to 9
Synthesized up to 6%. The pre-synthesis liquid thus obtained enters the tube side of the carbamate condenser 14° through the inlet 37 and comes into contact with the mixed gas from the stripper coming in through the line 48, so that at least a portion of this mixed gas is condensed. At this time, the heat of condensation generated is recovered and used for separating unreacted substances as described above. On the other hand, a part of the unreacted product recovery liquid inserted into the gas scrubber 15 absorbs ammonia and carbon dioxide gas accompanying the inert gas separated from the top of the second-stage synthesis tube 12, and passes through the line 46 to the carbamate condenser. 14, on which at least a portion of the mixed gas from the stripper condenses, as in 14'. The gas/liquid mixture stream from the carbamate condenser 14.14° enters the bottom of the post-synthesis tube 12 via line 38. At the same time, the remaining new supply ammonia line 32- is also inserted. The post-synthesis tube 12 is maintained under the synthesis conditions of a normal stripping process.

The following effects arise from the above.

[Effect of the invention] 1. When remodeling, the synthetic pipe for A can be used as is.

2. Simpler equipment is required than installing a synthesis process for b according to the vT standard.

A new synthetic pipe for 3.b can be made smaller than a completely new pipe. For example, around 20%.

4. Since urea and water are generated in the front synthesis tube, the condensation temperature in the carbamate condenser can be kept higher than before.

[Example] There is 1,000 tons of urea produced by Type 8 process.
This was remodeled into a 5-type process plant with an eight production capacity of 1,500 tons to increase production. Conventional 250kg/cifG2
The synthetic tube for a used at 00°C was used in its original position. However, the synthesis tube outlet pipe was connected to one of the carbamate condensers that would be newly installed as a result of the remodeling. Pressurized newly supplied ammonia can be fed not only to the synthesis tube for A but also to the synthesis tube for B, and the pressurized newly supplied carbon dioxide gas can also be fed to the stripper installed in fr. The piping was done as follows. On the other hand, the (carbamate) recovered liquid from the conventional high-pressure absorber was pressurized to the synthetic pressure and then piped so that a portion of it could be sent to the newly installed gas scrubber. Note that the mutual relationship among the newly installed synthesis pipe, carbamate condenser, and stripper is the same as usual. N11320.0. from a conventional (not shown) high pressure absorber. CO220,8
and II 2012. The recovered liquid at 103°C consisting of Ot/hr is pressurized to 190 kg/cnfG by the recovered liquid pump 3, of which 65.5% (weight) goes to the gas scrubber, and the remaining 34.5% (weight) goes to the front stage synthesis pipe. It was sent to the bottom of 11. On the other hand, the pressure was increased to 140°C by compressor 2.
A portion of the newly supplied carbon dioxide gas 45.8t/hr10
% (weight) goes to this front-stage synthetic tube, and the remaining 90% (weight)
was fed into the bottom of the stripper 13. Also pump 1
35.4 tons of newly supplied liquid ammonia pressurized by
/hr is 7 hr by steam condensate in the first preheater.
It was preheated to 0° C., and 65% (by weight) of it was sent to the second stage synthesis tube 12, and the remaining 35% (weight) was roughly preheated with steam and sent to the bottom of the first stage synthesis tube. As a result, in this synthesis tube, NH3/CO2 = 4.25mo
l ratio), the temperature is 177°C, and urea is 10.5 at the outlet.
NH313,5, CO24,1 and H207,5i/
A synthetic liquid consisting of hr was obtained and then entered the tube side of the carbamate condenser 14-. Here, some of the mixed gas from the stripper condenses into this synthetic liquid and the temperature rises to 1.
The temperature reached 86°C. The heat generated at this time was recovered and used as described below. On the other hand, the recovered liquid sent to the gas scrubber 15 absorbs ammonia and carbon dioxide gas accompanying the inner 1 gas separated at the top of the latter synthesis tube 12 and flows to the tube side of the carbamate condenser 14. Another portion of the gas mixture from the input stripper condensed thereto and reached a similar temperature, the heat generated being recovered as low pressure steam. Thus, the gas-liquid mixed stream generated in the carbamate condenser 14, 14- entered the bottom of the latter stage synthesis tube 12 and remained there together with the previously mentioned ammonia, during which urea was roughly synthesized. The temperature reached 190℃ and after separating the inert gas at the top, urea 67.4, NI + 374.3, CO2
A synthetic solution having a composition of 23,3 and tl 203B, 4j/hr was obtained. This synthetic liquid enters the top of the stripper 13 from the downcomer pipe 39 and is heated by high-pressure steam and the stripping action of the carbon dioxide gas sent to the bottom, resulting in Nh 59.3, CO27,8 and I+.
20 5.5 t/hr was mixed with the incoming carbon dioxide gas or separated as a gas. As a result, from the bottom of the stripper, urea of 176°C 64.2, N11316.8,
A synthetic liquid consisting of CO217.8 and 112031.9 t/hr was obtained, which was immediately reduced in pressure to 17 kg/cnfG and entered the shell side of the carbamate condenser 14-, where residual ammonia and carbon dioxide were removed by the heat generated on the tube side. Some of it vaporized. The gas-liquid mixture thus produced was then subjected to conventional processing to produce urea and the unreacted ammonia and carbon dioxide were recovered as a recovery liquid as described above.

By implementing this invention, the downstream synthesis tube can be made 20% smaller than if it were completely newly installed, and since the condensation temperature can be made higher than before, the heat transfer area of the carbamate condenser can also be made smaller by 24%.

[Brief explanation of the drawing]

1: Ammonia pump, 2: Compressor, 3: Recovery liquid pump, 11: First-stage synthesis tube, 12: Second-stage synthesis tube, 13 Ni stripper-1 14, 14-: Carbamate condenser, 15: Gas scrubber, 16.17 : Ammonia preheater

Claims (1)

[Claims] The urea synthesis solution containing urea, unreacted ammonia, and carbon dioxide obtained in a synthesis process at a pressure and temperature suitable for urea synthesis from ammonia and carbon dioxide is first subjected to a stripping process using carbon dioxide gas under the same pressure. Then, the remaining urea synthesis liquid is depressurized and subjected to at least one unreacted substance separation step to remove a portion of the unreacted substances in the urea synthesis liquid. The whole is separated into a gaseous state to obtain a urea aqueous solution, and on the other hand, the solution of unreacted substances recovered from the separation step after the stripping step is pressurized to the pressure of the synthesis step and subjected to a condensation step for stripping. In a urea production process in which unreacted substances from the process and a gas mixture with carbon dioxide used for stripping are condensed and the resulting condensate and uncondensed gas are circulated to the synthesis process, the pressure in the synthesis process is increased. Prior to subjecting the unreacted product solution to the condensation step, at least a part of it is maintained at urea synthesis conditions together with a part of each of the newly supplied carbon dioxide gas and ammonia, and then urea is synthesized.
A method for producing urea, which comprises subjecting it to the condensation step.