DE1568845A1 - Process for the production of urea from ammonia and carbon dioxide - Google Patents

Description

'' Process for the production of urea from ammonia

and carbon dioxide

When ammonia and carbon dioxide are combined in a synthesis reactor forms, via the intermediate reaction product ammonium carbamate, at sufficiently high temperatures and a correspondingly increased temperature Pressure, urea with simultaneous elimination of water. However, the reaction is only incomplete, i.e. only about 50% of the reaction occurs The starting materials are converted to urea, while the remainder is in the form of caruamate and, as usual, ammonia is used in excess has been, free ammonia still remains dissolved in the synthesis melt. By separating from the urea formed and. Repatriation of products not converted to urea can, however, reduce the yield, based on the starting products. Various methods are known for performing this separation and recycling can be. For example, after it has been withdrawn from the reactor, the melt is expanded down to atmospheric pressure and the dissolved melt is added with the addition of heat Oarbamate decomposed "

The expelled gases, namely ammonia and carbon dioxide, are then absorbed in water and returned to the reactor as a solution. In order to reduce the recirculation of water, its presence The conversion of carbamate to urea in the reactor is known to be significant reduced, the relaxation, decomposition and absorption is carried out in several pressure stages.

This mode of operation requires increased expenditure on equipment. It has therefore been further proposed, the separation of ammonia and carbon dioxide from the melt at medium pressures of 10 to To carry out atmospheres by means of a stripping process with a circulating inert gas (British patent specification 853 220). The pressure level is counted so high that the separated reaction gases Do not let condense with relatively little water. The disadvantage of this method, however, is that it is significant Lengen have to circulate inert gas and it is also difficult to do that when Abortion process absorbed ammonia at 'the given pressure and Condense out again temperature conditions.

From a work by Prejaeques (Chemie ^e "t Industrie" _60 (1948), BAD ORIGINAL

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Pages 22 to 35, in particular from the one formulated on page 23 It can be seen from the equation that a shift in the concentration in the gas phase against> 4inen of the two borderline cases (pure ■ ammonia or pure carbon dioxide) the decomposition pressure of Garbamat quite significant compared to that with stoichiometric gas composition is increased. The use of excess carbon dioxide is much more effective. This is from a new release confirmed (Hydrocarbon Processing 44, No. 7, July 1965, pages 131-133), in which the newer urea synthesis methods directly can be reduced to the equation of PrSjacques. Under application This principle allows the synthesis melt to be largely freed from carbamate by using a countercurrent stripping process subject to one of the two reaction gases, even if the expulsion process at high pressures, e.g. at the pressure of urea synthesis, is carried out.

Various urea synthesis processes are known in which the decomposition of the carbamate not converted to urea is achieved according to the abortion principle described above (cf. German patent specification 511 465, US patent specification 2,056,283 and 3,046,307, Austrian patents 226 237 and 244 984).

It has now surprisingly been found that urea synthesis It can be carried out much more economically and simply by using the abortion process at much higher pressure than before and in contrast to all known processes, with a gas mixture carries out, which to a greater extent from carrier gases which are inert with respect to urea synthesis, and to a lesser extent from carrier gases one of the starting products, i.e. ammonia or carbon dioxide.

The inventive method for working up the in the synthesis synthesis mixture obtained from urea from ammonia and carbon dioxide in a synthesis reactor at elevated pressure and temperature

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by: Abortion of the contained not 'converted to urea Products is therefore characterized in that this abortion ■ jänr; with a bridge of 100 to 300 atmospheres and with a gas mixture carried out v / ird, which before the beginning of the assignment to a larger part of carrier gases which are inert with regard to urea synthesis and to a lesser extent consists of one of the two starting products.

The driving process according to the invention can either be carried out in a separate; Apparatus to which the synthesis mixture from the reactor is fed, or in the reactor itself.

The inert carrier gas can be circulated, with the The oil that is driven off the synthesis mixture and not converted to urea 'products are stripped off again in a downstream absorber-condenser v / earth, with an approximate total absorption of the one action component, while some of the other components are returned to the stripping apparatus with the carrier gas. It can Of course, a separate cycle can be selected, or the gas mixture can be used before or after the partial synthesis of ammonia Separation of the ammonia can be removed and after passing through the Abxreibe- and Absti "eif device again the Amnioniaksynthese as Feed gas are supplied.

On the other hand it is partly also possible to pass the inert gas only once to lead the abortion apparatus, i.e. through the separate column or through the i?, \ - "ithoeoruaktor and after passing through the downstream To route absorber-condenser to another process, which works in a similar pressure range.

So it is z.V. advantageous to use hydrogen or a mixture as the carrier gas from hydrogen and nitrogen to be used with a suitable Geiialt in carbon dioxide, whereby hydrogen or the mixture '.Tasserstoff-nitrogen after the stripping of the carbon dioxide and the Ammonia in an absorber condenser as feed gas for the aminonia * 'yr_th <? se is consumed. This process variant has the advantage data the processing of the urea syr.thesegeinisches in economic particularly advantageous for the production of the

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The ammonia required for the synthesis of nutrients can be linked.

In the production of ammonia, that is necessary for the synthesis Today, hydrogen usually comes from the gasification of solid fuels, such as coal or coke, or liquid hydrocarbons or of gaseous hydrocarbons by partial oxidation with oxygen or air or by means of a steam forming process from liquid or gaseous hydrocarbons or refinery gases e obtained, whereby in all cases a raw gas is obtained, which is the main cause is made up of hydrogen and carbon monoxide, depending on which one ends Process can also already contain a corresponding proportion of nitrogen. In a subsequent catalytic, In the conversion process, most of the carbon monoxide is converted into hydrogen and carbon dioxide with the addition of water vapor. That The resulting Konver ± gas must be introduced into the ammonia synthesis from the carbon dioxide it contains, which is usually carried out in an extensive washing system, e.g. in a ottasche wash, in a Konoethanolamine wash or in a pressurized water wash with subsequent cleaning. Lay out the washout jnnen partially omitted, i.e. much smaller or more simplified be laid out if at least part of the amount is required for ammonia synthesis first required converting gases for the abortion process according to the invention ι the urea synthesis is used. Detergent from the / abortion apparatus Grass mixture, consisting of the originally used Kon-3rtgas and the driven off (lawn, can in a subsequent Ab- ' Trapping device, for example in an absorber-condenser, the carbon dioxide and the main amount of ammonia is removed, so that a gas mixture accrues which the new Ineri ise, i.e. hydrogen, some carbon monoxide, possibly nitrogen and a small amount of ammonia, after passing through the _n the required carbon monoxide removal system, this gas mixture can can be used indirectly for ammonia synthesis, i.e. without

otherwise
; The carbon dioxide system required in the ammonia system thesis is run thinly, so that this latter system can in some cases be omitted, which results in significant operational costs : eu can be saved. In addition, the use has, ..: one uVh. · - ^1. · Yes aer -

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Ammonia synthesis, i.e. the converting gas, has the advantage that the The carbon dioxide content contained therein is already at the pressure level de « Conversion process, i.e. when 25 at or more is available, Normally the carbon dioxide is used in the processing of the converting gas for the ammonia synthesis in the carbon dioxide scrubbing plant Normal pressure relaxes and must therefore for use in. The urea synthesis from, this lower level again compressed to the synthesis pressure will. When using the method according to the invention, however, that portion of energy, which is necessary for the compression of the carbon dioxide, required for at least the first 25 atmospheres of urea synthesis pressure will be saved.

The method according to the invention is shown below with reference to the enclosed] Pig. I to 5 explained.

Fig. 1 shows an embodiment of the method in which the from inert carrier gases and an ammonia component is circulated. From the circulation fan 30, the gas is in the Abortion apparatus 3 initiated and in countercurrent to the urea synthesis mixture supplied, which flows to the apparatus 3 through the line 29 and is withdrawn again via the line 33 after it has been supplied with heat and largely from the abortion effect of the gas mixture Carbamate was exempt. After relaxing on a deeper pressure and Removal of the remaining ammonia, carbon dioxide and water content in a known suitable apparatus can then be pure urine material can be obtained ..

The carrier gas loaded with the expelled gases passes through the Line 17 in the absorber-condenser, which consists of a partial condenser 4 and a washer 5 consists. In the condenser 4, a part, e.g. 25 to 75 $, of the total heat of condensation to be dissipated is added to the gas mixture withdrawn at a temperature level that allows this heat either through direct heat exchange or via an intermediate generation; of steam for the company to use the heat consumer in the low pressure part of the process. In this partial condenser 4, most of the carbon dioxide, the entrained water vapor and a corresponding proportion of ammonia to a highly concentrated ammonium carbamate or carbonate solution.

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: ondensiert .. The carrier gas with the not yet condensed portion of ammonia, carbon dioxide and water is eführung in the scrubber 5 in countercurrent to iner at the top through the lines 21 and 24 applied liquid, which consists of ammonia or an ammonia-water mixture and the carbon dioxide still contained in the gas mixture washes out, the carbon dioxide-free gas passes through line 18 into the condenser 6, η which the ammonia is condensed out to about 5 to 20 $> residual content, i.e. as far as the available cooling water temperature allows. The resulting liquid ammonia is withdrawn via the line 22 and mixed with a part of the ammonia supplied through line 23 and fed back to the scrubber 5 via line 24. In the post-washer 7, a further part of ammonia can be washed out of sm through line 19 from the condenser 6 flowing gas. Via the line 20, the inert fraction to be removed from the system is discharged practically free of ammonia after washing with the aid of water that is supplied via line 36 for 3 seconds.

.e ammonia-water solution obtained in the sump is via line 21 fed in the scrubber 5 for the purpose of washing out the carbon dioxide. About the Line 14 and, if necessary, 15, the return to be returned with 2 to 15 promt Carrier gas laden with ammonia is fed back to the blower 30,

1 a known ^ experience, which works with an inert gas circuit at separate pressures (British patent specification 853 220), according to the invention, the gas flowing from the condenser to the secondary scrubber contains 50 to 80 ammonia, depending on the pressure and cooling water conditions. Since it has become established that an ammonia content in excess of 15 f> in the circulating gas is unfavorable for the economic efficiency of the process, a significant amount of washing water must be added in this process to reduce the high inlet content, which then reaches the actuator and increases the turnover reduced. In contrast, the entry content of ammonia is the rfahren the invention in gas for post-3cher only 5 to 20 / <>, so with little wash water a -aus wash F 2 to 15 ° i ammonia possible ""

2 highly concentrated carbamate solution accumulating in the condenser 4 is over Ltung 25 the urea reactor 8 which is under the same pressure directly fed, so that a problem under these operating conditions

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matisehe carliaruat pump can be avoided. From the whole of the process' Via line 28 to be supplied carbon dioxide can a part via the pipe 28a directly into the reactor 8 and the other part via the Hohrleitung 28 ″ b are introduced into the condenser 4.

The same applies to that to be fed to the process via pipeline 27 Quantity NEW, some of which is fed via line 26 to reactor 8 or via line 23 to scrubber 5. The distribution of the currents depends on the selected temperatures and concentrations in reactor 8 and condenser 4 and is expediently carried out in such a way that that, taking into account the heat of reaction, the reactor can be operated without a device for heating or cooling. The one with the Carbon dioxide in the reactor geuraut portion of inert gas can over the Line 37 are passed into the V / Äscher 5. Arrived via line 29 the urea melt formed in the reactor to the stripping apparatus 3 · at the above-mentioned methods of the prior art, which with If a stripping process with carbon dioxide or ammonia work under reactor pressure, this must be chosen so low (e.g. 120 - 150 at) that the abortion process can work without extremely high temperatures, which could cause the urea to decompose. With the choice this pressure also gives the maximum temperature in the reactor, e.g. 170 - 190 G. Iai, in contrast, works in the case of the invention Process the reactor at 150-300 atmospheres, which means higher reactor temperatures, e.g. 190-210 0, with the same usage conditions and thus better sales values and shorter residence times can be achieved are.

With the same pressure level in the reactor and abortion apparatus, however, depending on the content of inert gases, the partial pressure of the condensing ■ b: - ": ren Gaoe are the same or lower than in the Abtr'eibeapparat of the known processes despite the higher total pressure. This is special Significant when the carrier gas enters the abortion apparatus, in that the residual concentration of the melt increases at the same temperature

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Garbamate and ammonia can be kept lower or a lower Temperature can be applied without a higher drainage concentration on ammonia and cartaat must be accepted. this leads to the further advantage that, thanks to the lower temperature, biuret formation is reduced.

FIG. 2 shows a similar embodiment of the method according to the invention, with the difference that instead of γοη ammonia, carbon dioxide is added as the starting gas component to the stripping gas mixture. Compared to the procedure according to FIG. 1, the following deviations should be mentioned. The ammonia condenser 5 is omitted the scrubber 5 is equipped, for example, with a cooling circuit 44 and a cooler 43, in order to dissipate the condensation heat of the vapors not retained in the condenser 4. A bicarbonate solution can additionally be fed to the scrubber 7 via line 42, which in the absorption of the in L ^r iederdruck part is formed expelled residual amounts of Cartjfnat. About line 21 is thus pulled off an I> icarbonatlösung which is the supplied via line 42 and 18 amounts of ammonia to the scrubber 5 returns _o From the bottom of the Partialkondensers 4 a highly concentrated carbamate and Mcarbonatlösung or melt passed through line 25 to reactor b.- Da The ammonia to be fed to the process via line 27 is fed to the length of the reactor 8, while the carbon dioxide feed is again divided into the substreams 28a and 28b in order to maintain an optimal temperature in the reactor 8.

3 explains a variant embodiment of the method according to the invention with a single passage of the carrier gas. That through the line 11 directly from or after the carbon monoxide conversion plant Outflowing raw gas for the ammonia synthesis is via line 12 a suitable portion of raw gas previously freed from carbon dioxide is added. The resulting gas mixture, which under the pressure of the Carbon monoxide conversion (e.g. 25 - 30 at or more) and that e.g. 3-30 percent by volume carbon dioxide, 0-15 percent by volume Carbon monoxide and 0-24 percent by volume May contain nitrogen in addition to hydrogen and Inert gases are fed to the compressor 2 through the line 13 this compressed to a pressure of 100-300 atm and introduced into the abortion apparatus 3. Otherwise the r'rozeus runs f: en: ii5 _

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Pig. 3 exactly analogous to the process described in Pig * 1 with the Exception that the carrier gas is no longer via lines 14 or 15 is returned to the abortion apparatus 3, but after extensive Au washing of the ammonia (depending on the required conditions) to one known carbon monoxide removal system not shown here, un from there to ammonia synthesis. Over the line 28 is only the proportion of what is used in the process that is not present in the carrier gas Carbon dioxide supplied and introduced directly into the reactor 8. The ammonia substream fed to the reactor can be used in the heating device 8 26 can be lifted.

According to another known process (British patent specification 981 265), a gas consisting of nitrogen, hydrogen and carbon dioxide is reacted with ammonia under pressures and temperature conditions that are suitable for the synthesis of urea, by placing this gas in the urea reactor and is fed in countercurrent to an absorbing means, the carbon dioxide being scrubbed out of the carrier gas and the resulting absorption solution increasing to the temperature of the urea synthesis. In contrast to this known method, in the method according to the invention the carbon dioxide-laden carrier gas is used for the expulsion process and then freed from the absorbed vapors by condensation, this process in no way taking place under temperature conditions that are used in an economical urea synthesis. For example . finds the carbamate condensation takes at about 150 C while the Har stofi'synthese et at v / a 200 ⁰ C is performed. In addition, the carrier gas used contains only part of the carbon dioxide required for the subsequent urea formation, while the remainder is not fed to the carbamate condenser, but to the urea reactor, whereby the reaction temperature can be kept so high without external Y / heat supply that an economical production of urea is possible.

Pig. 4 shows a further variant of the method with one-time run / ¹ ; des Trä ^ ergt.ses, at which the pressure in the stripper 3 is kept 50 to 100 atmospheres lower than in the reactor 8 and in the absorber

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Over condenser part 4 to 7. This can iia Abortion apparatus the residual concentration of Carhamat in the over line 33 draining melt can be further reduced or, with the same core concentration, the temperature in the stripping apparatus can be lowered. For this purpose, a relief valve 38 is provided in the connecting line 29, while the abortion apparatus 3 by means of the lines 39 and 40 between two stages or stage groups of the compressor

2 is switched. For the compression of the pressure of the abortion apparatus

3 on the pressure of the absorber 4 no gsondeptelrkoinpressor is necessary. Otherwise, this variant corresponds exactly to the variant according to FIG. 3.

Another embodiment with a single passage of the carrier gas is shown in FIG. Here, the gas mixture flowing in from line 13 first passes through a stripping apparatus 1, which is below the pressure of the carbon monoxide conversion is and frees the melt fed from the line 33 by the stripper 3, which is still Contains small proportions of carbamate dissolved, practically completely of Carbon dioxide and ammonia, so that the flowing off via line 35 Urea-water solution only needs to be freed from the water. Additional equipment for the displacement and absorption of the expelled Gases can therefore be omitted. The gas mixture leaving the apparatus 1 through the line 41, which has a relatively small Amount of ammonia and carbon dioxide is loaded, is led to the compressor 2 and e.g. in a pre-cooler or as shown in a Intermediate cooler 10, possibly with the addition of a small amount of water, the amount of ammonia and carbon dioxide vapors absorbed in the abortion apparatus 1 condenses out and the condensate obtained is conveyed by means of the pump 31 via the line 32 to the head of the stripping apparatus 3, while the carrier gas together with the amount of carbon dioxide brought along via the line 13 as a dry gas at the pressure of the stripping apparatus 3 is compressed. Otherwise corresponds to the process according to Pig. 5 the process already described according to Pig. 3.

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Embodiment of the execution s form according to FIG. 3

A gas stream consisting of 110 moles of H ₂ , 7 moles of GO and 60 moles of CQ ₂ , which comes directly from the CO conversion via line 11 under a pressure of 30 atm, is mixed with a gas in a COp discharge from COp "freed converting gas, consisting of 125 Hol H ₂ and Ö KoI CO. The resulting gas flow in line 13 is brought to a pressure of 250 atm by the gas compressor 2, introduced at the bottom into the outlet 3 and in the reverse flow to the outflowing pulp The melt flowing out of the lieactor 8 consists of 100 moles of urea. 149 Hol V / water, 43 moles of carbamate and 142 1.7% of free NH. After leaving the stripping apparatus 3 ", the melt still consists of 100 moles of urea 129 mol V / water, 10.5 mol KH, and 8.5 mol COp, while the! -Test ΚΉ,, CO ₂ and water with the. From drive gas to the condenser 4 is performed. 50 moles of COp are fed to the reactor 8 through line 28 and 100 moles of ITH through line 26, while 114.5 moles of NH3 flow through line 23 to the wash colloiijie 5. In Heaktor a temperature is maintained from 200 ⁰ C by means of heating. 9 From the top of the scrubber 7, 235 Hol H ₂ , 15 moles of CO and 4 moles of NH _{go through line 20 to the NH 5} synthesis solution, while 29 moles of wash water are added via line 36.

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Claims (13)

Dr.MEDlGER 1 ir ^ oo / r 15S8845 Patent claims: 1. Process for working up the synthesis mixture obtained in the synthesis of urea from ammonia and carbon dioxide in a synthesis reactor at elevated pressure and elevated temperature by driving off the contained therein, not to. Urea-converted products by means of an inert carrier gas, characterized in that this expulsion process is carried out at a pressure of 100 to 300 atmospheres and with a gas mixture which, at the start of the abortion, consists largely of carrier gases which are inert with respect to urea synthesis and a smaller portion of a of the two starting products. 2. The method according to claim 1, characterized in that the synthesis mixture from the synthesis reactor in a separate column is directed and there is subjected to the abortion process. 3. The method according to claim 1, characterized in that 'dasε the Synthesis mixture in the synthesis reactor itself is subjected to the stripping process. 4. The method according to claim 1 to 3, characterized in that the laden with the gases driven off from the synthesis mixture, gas mixture used for abortion in a downstream Stripping device freed from the expelled gases and in its original composition is returned to the abortion apparatus in the cycle. 5. The method according to claim 1 to 4, characterized in that to a lesser extent in that used for abortion Starting product ammonia contained in the gas mixture. 6. The method according to claim 1 to 4, characterized in that the » that to a lesser extent in that used for abortion Gas mixture contained from ^ ati / vproduct is carbon dioxide. 209816/1617 BAD ORIGINAL "~ -73 7. The method according to claim; 1 to 6 _T characterized in that the gas mixture used for the abortion • is largely freed of the discharged gases and the original proportion of an output product in a stripping device connected to the abortion apparatus and the remaining proportion of inert carrier gases Others working in the soapy pressure area end a process as sins at ζ gas is fed. 8, The method of claim 7> characterized in that the _t gelcenazeicnnet for Atotreitang used asgemisQü ©, 'suitable from a "raw gas ur & lersteiiuiig van. Asiffioniate ordered:, - wool ·! the larger fe-il contained therein. on! Srägergas primarily of hydrogen "be is t weleher naoh elem Burehlaufem the Afttreite- and Äftatr ^ ifelnriehtungen a AjBmaniatesynithe" © is as linsatzigas fed * 9. Terfabiren.aeh Ajis saying 1 to 8> this means that the öasgeisisoh before or after the partial ahsounion of the Ammoaiaksynthese- is taken and after going through; The abortion and stripping devices wled.er ^ er the ammonia synthesis as insert gas is supplied, 10. The method according to claims 1 "to 9" thereby gelcennzeiohnet that that used for abortion (gas mixture 5 to 30 "percent by volume Contains carbon dioxide. 11. Yerfahren naoh claims 1 to 10, characterized in that in the gas mixture used for driving only a part aes contain carbon dioxide required for urea synthesis, is. 12. The method according to claim 1 u ± a. 11, characterized in that the stripping process is carried out at a higher pressure than the stripping process «, 13. The method according to claim 1 to 12, characterized in that the drive-off process at high pressure with a drive-off process medium pressure is connected, whereby the same gas mixture is used for the abortion » 309816/1617 • Hf Blank page