DE2154946A1 - Method for controlling a urea synthesis process - Google Patents

Description

Method for controlling a urinary thesis

procedure

The Invention ·; If there is a new method for the continuous determination of the ^{concentration of the} arbunoat solution, which is fed back into the reactor supply of the starting products and added to it in a process is, and a new method for the automatic control of the internal "Jfeuisergleichgewichtes in a Flarnatoffsynlheseverfahren. with liquid carbamate-Fdbk ^r ihrungskreislauf.

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Urea is obtained commercially by _{reacting NH- and GO 2} in a reactor at elevated temperature and pressure. Under conditions of brain matter analysis, NH, and 00 «react immediately and completely atoichiometrically with one another to form ammonium carbamate. The latter is partly converted into urea and ^f .Va3ser within a residence time of 20 to 30 minutes in the reactor. An excess of ammonia over the stoichiometric ratio to CO 2 is used in the reactor to increase the conversion of the carbamate to urea. The urea synthesizer actuator usually contains urea, water, excess ammonia and unconverted ammonium carbamate. For the purpose of separating the urea solution formed from the not yet converted reagents, the stream flowing out of the urea reactor is usually expanded by means of a reduction valve and heated in a decomposer, which is usually operated under a constant pressure, and in this way the unconverted ammonium carbamate is converted decomposed into gaseous NH and COp, and the gas is withdrawn from the urea end product solution together with the excess ammonia which has evaporated from the urea end product solution with some water. The NH and CO «Ga3 obtained together with the water are usually absorbed in water and form an aqueous ammoniacal solution of aramonium carbamate, and the solution obtained in this way is returned to the urea synthesis reactor for complete recovery.

The Yasser contained in the returned Carbamate Solution is for the conversion of ammonium carbamate into urea disadvantageous in the urea synthesis reactor. To the harmful Influence of the cup on the conversion of the carbide into To reduce urea in the reactor, the water content in the recycled carbamate solution will usually increase held to the deepest possible 'Vert. This minimum value is from the ammonium carbamate content of the recycled carbarite solution and consequently #on the Ausf "ill tempera door for the card

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bamat dependent. The water content in the recycled carbamate solution is usually kept at such a level that the precipitation of the ammonium carbamate from the in the Reactor recirculated solution is prevented.

In practice, therefore, there is a continuous control of the Yaaser content in the recycled carbimate solution required in order not to let the urea synthesis reactor work uneconomically and on the other hand To prevent crystallization of the returned carbamate solution, which obviously raises mechanical problems in the apparatus for recycling the carbamate solution.

In practice, such is continuous control des Va s ί. get it in the returned carbamate solution difficult to achieve due to the methods normally used to measure the water content in the recirculated water Determine carbamate solution. Usually analytical and various other methods are used to determine the concentration in the recycled carbamate solution.

One of the most widely used methods is analytical, which mainly consists in taking a sample from the recycled Knrbimat solution and analyzing it in the laboratory; leading zx of a complete determination of the ammonium carbamates, expressed as equivalent Goo ^ of the equivalent ammonia, da3 is present as ammonium carbamate and free dissolved in water, ammonia, both printed as the total amount of ammonia · The water content is obtained as a difference by the determined values of subtracted from the returned carbine solution.

The disadvantages of such an analytical method are apparently. Because of the usual minimal time lag of 6-7 hours between the time of sampling from the returned carbine solution and the time to

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the results of the laboratory analyzes are available, on the basis of which the operator of the urea plant can decide, how much water is added to the returned carbamate solution must be, \ i: _i, due to the various possible sources of error, which adhere to the sampling and analysis of the returned carbamate solution, an effective control cannot be carried out will.

In the past, various other methods and techniques have been suggested for more or less Continuous determination of the concentration in the recirculated Carbimate solution. Serve other mead! desolate exist mainly in the control of one of the following four specific physical properties of the returned Carbimate solution: density, viscosity, electrical conductivity or specific speed of sound in the solution.

These four basic methods, which are based on the measurement of one of the specific physical properties of the recycled carbimate solution given above, had the property that they gave rise to mechanical problems that were due to the contact of the sensitive moving parts of the measuring apparatus with the very corrosive To reduce the Karbaraat solution, the problem in common that it is relatively imprecise and inconstant, which can be traced back to a common cause. This common cause is based on the fact UISS each of the three components of the ^r: ick: j; eführten Karbimatlösung, namely Ammoniunkarbamat, free ammonia and Waeser, a completely different specific weight, different viscosity, different electrical and various sound conductivity boaiuzt. The difference between these four physical properties is relatively large, especially for water and free ammonia.

E.g. the 3specific weight for '.Vaas er 1 and for liquid Ammonia, when mixed with ', Yasser i ^ t, about 0.71. the Yasser viscosity is equal to one centipoise and the of liquid NH, i3t approximately equal to 0.16 centipoise. ain

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there is a similar relative difference between the others two physical properties of water and ammonia.

It is obvious that with the same ratio of Ammonium carbamate to 7 / ater the four physical properties such a solution described above, namely specific gravity, viscosity, electrical and sound conductivity, by the slightest variation in the free NH content in which the carbamate solution carried back can be influenced.

Since the crystallization point of the recycled carbamate solution depends mainly on the ratio of the dissolved ammonium carbamntes to the water contained in the solution and, to a much lesser extent, on the ratio of the dissolved free ammonia to the water contained in the solution, it is obvious that a relative 'discrimination in the ^{crystallizer:} 5ationspun the rückgef'i'ar th Karbamatlösung ct can not be reliably licking ent by measuring one of said top anrefihrten four physical properties of rückrefUhrten Karbamatlösung.

In the above-rides case, a relative ^v mlerung can in one of the above-rides four physical properties of the back / 'eführten Karbam IT solution either the rückgeföhrten Karbamatlösung be interpreted as a "change de3 Kri3talli3ationspunkte3 that the a change in the ratio of the dissolved Ammoniumkirbamates to water contained in the solution, or as a change in the ratio of the dissolved free ammonia to the water contained in the solution without any consequential, measurable change in the crystallization point of the returned carbamate solution.

In a special case where the observed physical Property, namely either the specific weight, the Viakouitüt, the electrical or the sound conductivity, remained relatively constant, but at the same time the proportion of the ammonium karbaraatea to 'Vaauer and consequently the Kriatalli-

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sationspunkt the rlickgefihrten Karbamatl'i3ung, including anv-UCHs namely by dena same equivalent value in 3ezug on the under observation 3pezielle physical üigen3Chaf I _9-free by which the ratio of ammonia to de3 '/ asoer decreased, it was found that the Xri3talli3itionspunkt the r 'iclcgefEhrten Karbamatlösung and hence their concentration is more or less changed, but this relative Xnderu g could not by observing the speziflachen above it-zrihnten be discovered physikali3chen properties, v / eil aich serve specific observed physical characteristic fir the entire ^r: ickgef ^; her carbamatl'is mg did not change at all. In this case acts against the relative growth of the 'erte3 the observed physical property of the respective waste thereof so that in the end, no change of physical property of the entire Rucli ^ ef'ihrten Karbamatlösung can be registered / ert by exactly the same. 7

Various other methods have been used in the past Control of the concentration of the carried Karbaaat lounges suggested e.g. 3. in U.S. Patent No. 3,270,050. A disadvantage of the method presented in this patent is the fact that the content of the by-product biuret is in a Urea solution accommodates the temperature changes that occur in a Degasser evaporator occur, changed

Another disadvantage of the above-mentioned procedure is the fact that that lie relationship between the concentration of a concentrated aqueous ammonium carbamate solution and its Viscosity is inconsistent.

The present invention now provides a new method to control the internal water balance in one Urea synthesis process with liquid carbate recirculation.

Contrary to common practice, this method includes the Change in pressure in one or both of the pressure-decomposition-absorption-absorbency-oetemes for the purpose of controlling the

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Amount of water removed from the aqueous solution of urea Product is evaporated and is consequently returned to the reactor

Veiterhin has now been found that the concentration and the IIriatallisation3punkt the rückgefihrten Karbamatlösung Jenau and can be quickly determined by measurement of its refractive index by a suitable Refraktometereinrichtung.

The main reason for this is the fact that the values for the refractive indices of water and liquid ammonia are relatively close and that the value for the refractive index of ammonium carbamate is quite considerably different from those inherent in liquid ammonia and water.

This new method of pressure variation through which the desired internal water balance control is achieved, further improves the synthesis process, since the continuous display the relative refractive index of the recycled carbamate solution, which is determined by a suitable measuring device for the refractive index can be used to control the pressures, which in turn control the water balance.

As an exemplary embodiment, the figure represents a schematic 31ock diagram shows a urine synthesis process which present invention.

For explanation, the refractive indices of the are below individual components of a return system specified *

Refractive index

Liquid water 1.3330

Liquid NH ₃ 1.3250

Solid ammonium carbamate 1.54 (n ^: approximate)

Separate the vessel in which the recycled carbamate solution is formed

is, at almost constant pressure and constant temperature

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was operated, it has been found that the ratio of the free dissolved ammonia in the recycled carbamate solution to the water read in the recycled carbamate solution varies within a relatively narrow range when the ratio of the ammonium carbamate dissolved in the recycled carbamate solution to the water present in the recycled carbamate solution varies over a relatively wide range. This fact, together with the fact that the values for the refractive index of water and ammonia are relatively close to one another, and that a relatively small change in the ratio of free ammonia to water in the carbamate return solution has a negligible influence on the refractive index of the carbamate return solution possessed, which is due to the relatively large difference between the refractive index of ammonium carbamate and that of water and liquid ammonia, ultimately lead to the result that the concentration of ammonium carbamate in the recycled carbamate solution as the ratio of the dissolved ammonium carbamate to the solvent aqueous ammonia can be expressed. In simple terms, the ratio of CO ₂ * present as ammonium carbamate can be related to the water present in the recycled carbamate solution.

It has been found that this ratio of 00 to water is proportional to the refractive index of the recycled carbamate solution and thus ultimately the crystallization temperature of the recycled carbamate solution in a very wide range of the weight ratio of CO ₂ to water: e.g. in the approximate range of 0, 3 to 4 ; and this proportionality has a relatively high accuracy and continuity: for example, there is a maximum error in the reproducibility or deviation of approximately 0.6 ⁰ C (1 ⁰ P) from a mean Kir_bamatkriotallisierungkurve, which on a carbumate recirculation solution in the above-mentioned weight ratio range from COp to Water of approximately 0.3 to 4 has been determined.

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In this way, the continuous indication of the relative refractive index of a kirbamate recycle solution obtained from suitable refractometer equipment can be converted into a suitable continuous proportional signal, pneumatic, electrical or similar, capable of controlling a valve, by which a z.3. the exact amount of water can be introduced into the vessel by forming the carbamate recirculation solution for the purpose of maintaining constant the ratio of COp to water in the carbamate recirculation solution continuously and automatically at a predetermined and preferably optimal value.

example 1

Referring to the figure, a standard three stage decomposition and absorption urate off-theate procedure is used with a return circuit for the entire liquid carbamate shown in a scheraatic manner.

It is anticipated that a urinary disinfection procedure with a partial recirculation circuit for liquid carbamate in which one or more of the gaseous streams from the decomposer ,, the NH ,, COp and containing water vapor, to an adjacent one Ammonium nitrate or sulphate plant for ammonia recovery can also be operated in a manner similar to that described in this example is. The same applies to a system with a smaller or larger number of decomposition absorption stages than in which is described in the example.

Reactor 1 is operated in the pressure range from approximately 105 to 422 kg / cm (1500 psig to 6000 paig) and in the temperature range from approximately 174 ⁰ C to 215 ⁰ C (345 ⁰ P to 420 ⁰ P), with an approximate total molar ratio NH, to COp in the range of 2.2 to 1 to 6 to 1, an approximate total molar ratio of CÜ2 to water in the range of 0.1 to 1.5 and an approximate conversion range of 40 to 80 fi, expressed as the percentage of CO2 des in the reactor

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urea formed from the total amount of GOp in the form of the carbamate initially present in the reactor will.

The stream 2 flowing out of the reactor contains urea, water, unconverted ammonium carbamate and excess ammonia. The stream 2 flowing out of the reactor is placed under reduced pressure of approximately 21 kg / cm (300 psig) by means of the reducing valve 3 and heated for the purpose of separating the aqueous urea end product solution formed in the reactor from the remaining unconverted ammonium carbamate and the excess ammonia, which in are present in the reactor effluent. In practice, this is done by heating the solution flowing out of the reactor at reduced pressure for the purpose of decomposing unconverted ammonium carbamate into gaseous ammonia and GO ₂ and then extracting the gas formed from the aqueous solution of the urea producta together with the excess ammonia » which has been evaporated from the aqueous solution of the urea product

it is obvious that a certain amount of water vapor in the gaseous ΝΗ, -CO «phase is present by a Decomposition system originates and depends on the operating conditions in the decomposition system. For the sake of simplicity Three levels of decomposition and absorption are shown in the figure, with these levels 1 to 3 corresponding with 4 »9 and 14 are designated.

For example, the first decomposer 4 is said to be at about 21 Kg / cm (300 psig) and 154.4 ⁰ G (310 ⁰ P), the second decomposer 9 at about 2 kg / cm ² (30 psig) and 126.7 ⁰ G ( 260 ° P) and the third decomposer 14 at about 0.56 kg / cm ² (8 psig) and 121 ⁰ G (25O ⁰ P) to be operated.

As already mentioned above, the flow 2 flowing out of the reactor is increased to approximately 21 kg / cm through the reduction valve 3 (300 psig) reduced in pressure and into the first decomposer 4

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initiated. In the heating coil 5 to keep the steam temperature of the product in the first decomposer 4 to about 154.4 ⁰ C (310 ° F) is initiated. _{The gas NH and CO 2} * which is formed in this way in the first decomposer 4 and which contains some hydrogen vapor, is discharged from the decomposer 4 through the line 6.

An aqueous solution of urea product containing some residual ammonium carbamate and an excess of ammonia is discharged from the first decomposer 4 by way of stream 7, reduced in pressure to approximately 2.1 kg / cm (30 psig) by means of the pressure reduction valve 8 and into the second decomposer 9 initiated. The heating coil 10 is supplied with steam to maintain the temperature of the product solution in the second decomposer 9 at about 126.7 ° C (260 ⁰ P). The NH, - and COp gas which is formed in this way in the second decomposer 9 and which contains some water vapor is discharged from this decomposer through the line 11.

An aqueous solution of urea product containing a residue Contains ammonium carbamate and an excess of ammonia discharged as stream 12 from the second decomposer 9, under pressure by means of the pressure reduction valve 13 to approximately 0.56 kg / cm (8 psig) and introduced into the third decomposer 14.

In the heating coil 15, steam is introduced to maintain the temperature of the product solution in the third decomposer 14 at approximately 121.1 ° C (25O ⁰ P) · NH thus formed in the third decomposer 14, - and COP-gas containing some water vapor is discharged from the third decomposer 14 through line 16.

An aqueous solution of the urea end product, which essentially contains the urea and the water which have been formed from the ammonium carbamate in reactor 1, is discharged as stream 17 from the third decomposer 14 and further in a plant (not shown) into its solid state ο let other desired shape brought.

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The gaseous stream 16, which ,, NH COp and -Yasser contains, in the third capacitor 18 ^(c 130 F), with approximate values of 54.4 ⁰ C and 0.56 kg / cm ² (8 psig) to a solution condenses in that the heat of reaction and condensation is dissipated with a cooling water circuit through the cooling coil 19 », which is provided with a control valve 20. The pressure in the third condenser 18 is controlled in that the working temperature is changed, whereby the vapor pressure of the solution formed in the third condenser 18 by the condensation of the gaseous stream 16 is changed. This change in temperature of the solution formed in the condenser 18 is in turn achieved in that the cooling water flow through the cooling coil 19 is varied by means of the control valve 20.

In this way, by changing the pressure in the third condenser 18 the pressure in the third decomposer 14 also changed so that that of the aqueous urea product solution 17 evaporated amount of water is changed and consequently the water vapor content of the stream 16, the third Leaves decomposer 14, also varied.

The pressure in the third decomposer 1-1 can be controlled by means of a Valve 58, which is located in the line 16 emanating from the third decomposer 14, can be controlled directly. In this case the pressure in the third condenser 18 is kept constant at a lower level than in the third decomposer 14, which at the expense of a certain loss of effectiveness in the Capacitor 18 happens.

In the condenser 18 the water vapor becomes liquid water condenses, part of the total NH present in the gaseous 3 stream, reacts with GOp to form ammonium carbamate form, v / elohes is dissolved in the liquid water, and the remaining part of the total present in the gaseous stream 16 NH, dissolves in the liquid water. The watery one ammonia -ehe solution of the ammonium carbamide formed in this way

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is drained from the third condenser 18 through line 21, it is pressurized to approximately 2.1 kg / cm (30 psig) by passing it through pump 22, and it is introduced into the second condenser 25 through the control valve 23 and the line 24.

An automatic level controller 26 provided on the third condenser 18 maintains the level in the third condenser 18 formed liquid at a constant height by opening the level control valve 23 through which the Solution formed in the third condenser 18 passes into the second condenser 25, is regulated.

Under normal operating conditions, the amount of water vapor which is usually contained in the gaseous stream 16, and in the third condenser 18 to liquid water is condensed, sufficient to keep the carbamate formed in solution and to prevent this in the third Capacitor 18 crystallizes.

The second capacitor 25 works in a completely analogous manner to that just described third capacitor 18.

The gas stream 11 from the second decomposer 9, which contains NH *, CO ₂ and water vapor, is introduced into the second condenser 25 in line 24 together with the stream withdrawn from the third condenser 18 described above. The gas stream 11 is condensed and is dissolved responds to a solution in the second capacitor 25, the / cm ² is carried out at the approximate values of 54.4 ° 0 (13O ^C P) and 2.1 kg (30 psig), the heat being removed from the condenser 25 by means of a cooling water circuit through the cooling coil 27 provided with the control valve 28.

The pressure in the second condenser 25 is controlled by changing the operating temperature and consequently the vapor pressure in the second condenser 25 by condensation

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sation of the gas stream 11 and by mixing this stream with the solution coming from line 24 resulting solution is changed.

The change in temperature of the solution formed in the condenser 25 is in turn caused by a change in the K'ihl-A-water flow achieved by the cooling coil 27 by means of the control valve.

Analogous to the operation of the third decomposer 14 described above, by changing the pressure in the second condenser 25 also changed the pressure in the second Ir decomposer 9 so that that of the aqueous urea product solution The amount of water evaporated in line 12 also varies, and accordingly the water vapor content of the stream 11 leaving the second decomposer 9.

The pressure in the second decomposer 9 can, by means of the valve 59, be that in the line from the second decomposer 9 11 can be controlled directly. In this case, the pressure in the second condenser 25 can be at a lower Level must be considered constant in the second decomposer 9, which happens at the expense of some loss in efficiency in the second capacitor 25.

The water vapor contained in the gas stream 11 is condensed into liquid, a part of the total ir. Learning Gas stream 11 present NH- * reacts with CO ^ t to ^ ammonium- to form carbamate, which in the liquid water. he solved and the remainder of the remaining portion of the ^ velvet in that Gas stream 11 existing NIU dissolves in the liquid phase. The total amount of the aqueous ammoniacal solution of the in this way by condensation of the Sasstromea 11 and through Mixing of the resulting solution with the liquid introduced into the second condenser 25 through the line 24 has been formed ammonium carbamate is withdrawn by line from the second condenser 25, its pressure is increased

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about 21 kg / cm (300 psig) by means of pump 30 and it is introduced into the first condenser 33 through the level control valve 31 and through line 32.

The automatic level controller 34 attached to the second condenser 25 maintains the level in the second condenser 25 formed liquid at a constant height by opening the level control valve 31 through which the Solution formed in the second condenser 25 passes into the first condenser 33, is regulated. The gaseous stream 6 from the first decomposer 4, which contains NH *, CO «and water vapor, is withdrawn from the previously described condenser 25 together with the liquid stream in line 32 has been introduced into the first capacitor 33. The «gaseous stream 6 is partially condensed and reacted and dissolved in the first condenser 33 to a solution, with the condenser 33 at the approximate values of 93.3 ° C (200 ° F) and 21 kg / cm (300 psig) is operated by a A predetermined amount of heat is withdrawn from the condenser 33 by means of a cooling water circuit through the cooling coil 35.

The water vapor contained in the gaseous stream 6 is condensed into liquid and mixed with the solution introduced into the first condenser through the line 32, a portion of the total NH present in the gaseous stream 6 _{reacting with CO 2} to form ammonium carbamate, which is dissolved in the solution present in the first condenser 33, and the remaining remaining portion of the total NH, from stream 6, which remains unabsorbed, is in the packing part 36 and in the tray section 37 of the condenser 33 from the remaining CO «Cleaned and passed through the line 38 to the water-cooled NH, condenser 39.

The pressure in the ammonia condenser 39, in the first Capacitor 3 "* and the first decomposer 4 is set to approximately 21 kg / cm (300 psig) maintained by the inerts through the, line 40 and through the control valve 41 from the ammonia

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- 16 _
condenser 39 can be drained.

The liquid NH condensed in the condenser 39 is withdrawn from the condenser 39 through the line 42 and divided into two Streams split; the greater portion of the liquid NH from stream 42 is passed through line 43, with the fresh treated liquid NH, mixed in line 44, passed through line 45, by pump 46 to an elevated level Pressured to about 225 kg / cm (3200 psig) in the liquid NH, pre-heater 47 heated with steam and introduced into reactor 1 through the line.

W The smaller part of the stream 42 is passed through line 49 and introduced into the upper part of the floor area 37 for the purpose of cleaning the ascending gaseous ΝΗ, stream from the last traces of COp.

The ammoniacal aqueous solution of the ammonium carbamate formed in the first condenser 33 is withdrawn from the condenser 33 through line and returned to the circuit through line 51 via the packing part 36, which has the purpose of coming into contact with the ascending flow of the gaseous NH in a countercurrent process to come and clean it of most of the CO ₂ zn.

The level of fluidity in the first condenser 33 becomes kept constant by using a suitable amount of the ammoniacal aqueous solution of Ammo.iiumkarbamat withdrawn from line 50 through line 52 by piston pump 53 to a pressure of approximately 225 kg / cm (3,200 psig) and introduced into reactor 1 via line 54.

The automatic level controller 55 on the first condenser keeps the fluid level in the condenser 33 constant by controlling the speed of the carbamate piston pump 53 and

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- 17 regulates your liquid throughput.

Gaseous GO ₂ at approximately 225 kg / cm (3200 psig) is introduced into reactor 1 through line 56.

The concentration of the ammoniacal aqueous solution of ^ mraoniumkarbamat, which through line 54 in the Reactor is reintroduced, or more precisely the ratio of GCpf that is present as carbamate to the '-'asser that is in the Contains carbamate recycle solution from line 54, depends on the amount of steam in CJasstrcSraen 6, 11 and 16 from that of the urea product solutions in the first decomposer 4, in the second decomposer 9 and in the third decomposer " setters 14 have each been evaporated. The total amount of ammonium carbamate, contained in the carbamate recycle stream 54 depends on the total conversion of the karbama.tes into urea in the reactor 1 and thus on the amount of unconverted ammonium carbamate that is in the from the reactor outflowing 3trom 2 is included.

If the conversion in the reactor 1 increases, there will be correspondingly less unconverted aramonium carbamate in the stream 2 flowing out of the reactor and consequently in the recycled carbamate stream 54. In this case the ratio of to keep GOP '-7a3ser in stream 54 constant, it is to reduce a NOTV / endir that -Vasserdampfmenge that of the aqueous Harnstoffpnduktl "solution in the first decomposer 4, in the second decomposer 9 and in the third decomposer 14 In order to achieve this, it is sufficient in practice to control the amount of water evaporated in one or both of the decomposers 9 and 14. In this case, the water balance is controlled by increasing the pressure in the decomposers 9 and 14, which in turn is achieved by reducing the cooling water flow through the respective cooling water control valves 28 and 20 and cooling coils 27 and 19, as described above.

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In contrast, if the conversion in reactor 1 decreases, the flow of cooling water through one or both control valves 28 and 20 are increased so that the pressure in the second decomposer 9 and in the third decomposer, respectively increases and thus increases the amount of water evaporated in the decomposers 9 and 14, whereby the iaaser content in the corresponding currents 11 and 16 increases.

Any change in the rate of formation of the liquid phase in the capacitors 25 and 18 is immediately reflected by the corresponding one Fluid level controllers 34 and 26 are displayed, which in turn the corresponding control valve 31 and 23 compensates and thus the 'corresponding flow through the lines 32 and 24 varies.

The final result will be the ratio of GOp to Tasser in the recycled carbamate solution 54 is kept constant at a predetermined and desired Vert, so that crystallization The solution in lines 50, 51, 52 and 54 is avoided, as shown below in the explanatory examples will.

The line 50 emanating from the first capacitor 33 is connected to a refractometer device 57 for continuous fe determining the refractive index of the recycled carbamate solution provided, which corresponds to the principle described above, so that the smallest relative change in the GOp to water ratio in the recycled carbamate solution of stream 54 (and hence streams 50, 51 and 52) through refractometer 57 in Form of a relative proportional change in the refractive index of the recycled carbamate solution from otrom 50 will. This proportional change in the index of refraction is converted into a suitable proportional signal, as is will be described below so that proportional actuation of either or both control valves 28 and 20 to the corresponding cooling coils 27 and 19 and one or both valves 58 and 59 is possible.

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By the method described above, the refractometer will hold indirectly the ratio of GOp to water in the recirculated Carbamate solution 54 constant simply in that one or both control valves 28 and 20 and / or one or both Valves 58 and 59 are regulated.

A refractometer device 57 can also be attached in the lines 51 »52 or 54, which is even more cheaper than in line 50. However, the situation in Line 50 preferred as shown »

It can be a conventional refractometer device 57 or any other similar device for determining a Refractive index of a liquid can be used.

Preferably, the refractometer device 57 is interconnected with a conventional control system 70 which converts the refractive index measurements into pneumatic signals. Such a control system 70 is implemented, for example, by conventional transmitters and pneumatic control systems: It goes without saying that the control system is shown in the drawing as a single element only as a three-part valve 20, 28, 58 and 59 are preferably pneumatic valves when transducers and control systems described above are used. Conventional valves can be used. The actuation of such valves and the controls associated therewith are known to those skilled in the art. The level controllers shown in the drawing are also of conventional design. Conventional level controllers were used. The pump speed of pump 53 is typically controlled by a conventional control unit.

The valves 20, 28, 58 and 59 can be operated in various combinations or individually to control the urea synthesis-V *. 4hrs to control. The simplest and most effective control however, actuation of only one or both valves

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tiles 20 and 28 for pressure control according to the rotation index changes achieved.

Alternatively, or in conjunction with the operation of any one or all of the valves 20, 28, 58 and 59, the control valves 71 and 72, the pressure-vent valves are to be actuated to the ^Dr: icke in the capacitors 18 and to control. However, this is a less effective way of controlling pressure in the system. Pressure control in the condensers 18 and 25 by the respective air valves 71 and 72, respectively, also indirectly affects the pressure in the decomposers 9 and 14 because of the existing between them. Links.

Ea is another possible case, wherein the conversion in the reactor does not change, however, the 7 / aasermenge in the urea product solution 17 for some reason varies what 2.B. may be the case in one or all of the corresponding decomposers 4, 9 and 14 due to a sudden low temperature fluctuation.

The urea product solution 17 normally contains about 1 mole of water per mole of urea formed in the reactor 1. If z.3i, as mentioned above, the temperature in one or in all three decomposers 4, 9 and 14 suddenly increases, more water is evaporated from the aqueous urea solution and accordingly the aqueous urea product solution in stream 17 becomes less than 1 Moles of water per mole of urea formed in the reactor 1 contain. In this case, a corresponding small excess of water vapor which is present in one of the gas lines 6, 11 and 16 will condense in the respective condensers 33, 25 and 18 and possibly accumulate in the first condenser J> .

The consequence of this partial accumulation of water in the condenser 33 will be a gradual dilution of the recycled Carbamate stream 54 and in this way a gradual one

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Lowering the ratio of OOof that in the form of carbamate is present to v / ater that is present in the recirculated carbamate stream 54 is included, be-.viricen. This gradual decrease in the ratio from CO «to Tasser causes a partial decrease in the Refractive index of the returned carbamate solution 54 »what through the refractometer 57 connected to line 50 registers ". Slide regiatrated by the refractometer 57 decrease of the 3i-calculation index is converted into a proportional signal, e.g. into a pneumatic or electrical signal, which in turn is used to determine the pressure in one or both Increase time 9 and 14 by increasing the flow of cooling water through the corresponding cooling coils 27 and 19 is reduced by means of the corresponding control valves 28 and 20 or by the valves 59 and 58 in the respective gas lines 11 and 16 are closed appropriately. If the pressure is in an or at Jen decomposers 9 and 14 is increased, less, Yasser is evaporated from the aqueous brain matter product solution and consequently in the corresponding Ga ^ streams 11 and 16 be present, which leads to Consequence hac that the accumulation of water in the first condenser 33 and there: uit canceled in the returned carbamate solution 54 and the originally specified 00 «to Yasser ratio is restored in stream 54.

Exactly the opposite of what is described above happens when z.3. the temperature in one or both decomposers 9 and 14 suddenly decreases.

Noel, another case i3t possible in the water or water vapor is suddenly introduced into the process system shown in the figure from an external source, either into one of the corresponding streams 44 and 56 of the output products, the NBU and G0 «included, or in the water is fed into one or more of the capacitors 33, 25 and 18 directly.

Again, similar to that described above, in this case the sudden drop in the CO ₂ increases. Water ratio in

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the otrom 54 and thus in its refractive index to an automatic increase dea pressure either in one or both Zer3etzern 3 and 14 ferries, further -en to INEM = ζ itweiligen growth of Va ^ serge'.altes the x'lasrl ^ Harnatoffproduktl ^Sung 17 and finally e3 will lead to a multiple division of the CC> 2 'u' iasser ratio in Jtrxa 54 to its original uni given 7 / ert.

Another pail is possible with the urine . οff in a common form of a more or less concentrated aqueous solution is continuously introduced from an external source in the process ayat of the figure in an im w. via the third capacitor 18. In a first approximation w ^r lre Ά3.Ώ. inclined to uneiiaen that the presence of urea as a fourth component in the r ^; ickjef.ihrfcen Karo imatlösung 54 influences the relationship between the COp to water ratio in stream 54 and 3 a refraction index in such a way that there is no longer any consistency Urine tof fotroa, which is introduced into the urine tof fsyntheseverwirren of the figure, is usually kept constant. Such a urea stream returned to the urea synthesis system usually originates from a device for working up and final processing of the urea solution, which is not shown in the figure and which can be either a urea crystallization or a urea vaporization device or a urea spray solidification device (prilling section). In view of the fact that this flow of recycled urea into the urea synthesis process is usually kept fairly constant, it has been found that the ratio of the recycled urea to the water contained in the recycled carbamate flow 54 is also constant. And it was again found that a relative change in the refractive index of the total recirculated solution stream 54 over carbamate is essentially proportional to the COp-water ratio in the recirculated carbamate solution in stream 54 Isfc »

209821/1058

BATH ORIGINAL

In conclusion it can be said that it was found that the refractive index of the recycled carbamate solution 54 pro is proportional to the COp- ^ water ratio of the stream 54, regardless of whether urea is present in the Jtroia 54 is or not.

It is understood that the material in line 51 as part the liquid phase which is returned to the reactor 1 can be viewed.

Example 2

With reference to the figure, the stream flowing out of the reactor in line 2 contains 11340 ki / h (25000 L33 / HR) urea, 8280 kg / h (18260 LB3 / HR) unconverted ammonium carbamate, 8420 kg / h (18540 LBS / HR) excess ammonia and 7160 kg / h (15799 LBS / HR) ',' / water. Reactor 1 is at the approximate ferten of 190.6 ⁰ G (375 ° F) and 225 kg / cm (3200 psig).

The current flowing out of the reactor in line ? Its pressure is reduced to about 21 kg / cm (300 psig) through the reduction valve 3 and heated in the first replacer 4 by steam, after which it has the approximate values of 154.4 ° G (310 ° F) and 21 kg / cm ² (300 psig).

The gaseous phase formed in the decomposition tar 4 is passed through line 6. It contains 4264 kg / h (9400 L3S / HR) 0O ₂ , 10886 kg / h (24000 L »S / HR) NH ₃ and 2205 kg / h (4360 L3S / HR) water vapor.

The remaining liquid Iase separated in decomposer 4 is withdrawn through line 7. It contains 11 340 kg / h (25000 LBS / ΗΙί) urea, 724 kg / h (1595 LBS / HH) the rest Karbaaat, 819 kg / h (18G5 LBS / HR) residual free ammonia and 4970 kg / h (10939 LBS / HR) water.

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Stream 7 is depressurized to approximately 2.1 kg / cm (30 psig) through reduction valve 8 and is in the second decomposer 9 to about 126.7 ° G (260 ° F) Steam heated.

The gaseous phase formed in the decomposer 9 is drawn off through the line 11. It contains 204 kg / h (45Q LBS / HR) GO ₂ , 590 kg / h (1300 LB3 / HR) NH ₅ and 771 kg / h (1700 LBS / HR) water vapor.

The remaining liquid separated in decomposer 9 Phase is withdrawn through line 12. 3ie contains 11 340 kg / h (25000 LBS / HR) urea, I61 kg / h (354 L3S / HR) residual ammonium carbamate, 112 kg / h (246 LB.l / HR) residual free ammonia and 3740 kg / h (8239 LBS / HR) water,

In stream 12, the pressure is reduced to approximately 0.56 kg / cm (8 psig) by reduction valve 13 and the stream is in the third decomposer 14 to approximately 121.1 ° C (250 ° F) heated.

The gaseous phase formed in the decomposer 14 is passed through line 1.6. It contains 68 kg / h (150 LB3 / HR) CO ₂ , 150 kg / h (330 LB3 / HR) NH ₅ and 335 kg / h (740 LB3 / HR) water vapor «

The liquid phase separated in the decomposer 14 is drawn off through line 17. It contains 11,340 kg / h (25000 LBS / HR) urea, 40 kg / h (89 LBS / HR) residual carbamate, 14 kg / h (31 LBS / HR) residual free ammonia and 3402 kg / h (7499 LBS / HR) Waseer.

The liquid stream 17 is the end product of the aqueous Urea flow due to the equilibrium in this urea synthesis process with regard to the water content Moles of water per col in the reactor 1 formed urea contains.

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When water from an external source into the urea synthesis system is initiated, is maintained when the water balance is maintained same amount of Vasaer in addition to the stoehiometric amount of 3402 kg / h (7499 LBS / HR) des Water formed in the reactor 1 is withdrawn from the process system via line 17.

The gas stream from line 16 is in the third condenser

ft ft ρ

at the approximate values of 54> 4 C (130 F) and 0.56 kg / cm (8 paig) condensed. The heat of reaction is dissipated with a cooling water circuit through the cooling coil 19 The pressure in the third condenser 18 and accordingly in the decomposer 14 is regulated by the operating temperature the solution in the condenser 18 is controlled by means of the valve 20 in the cooling water supply line to the cooling coil 19 will.

The pressure in the condenser 18 can also be controlled by adding a certain amount of (e.g. through valve 72 as discussed above, but this method is somewhat inefficient, once because of the Loss of valuable HH and COg gas to the atmosphere and on the other hand because of the additional costs incurred for a suitable NH ~ and COo recovery method for the discharged gas.

The liquid formed in the third condenser 18 contains 121 kg / h (266 LBS / HR) ammonium carbamate, 102 kg / h (214 LBS / HR) free ammonia and 336 kg / h (740 LBS / HR) water and is withdrawn from condenser 18 through line 21, under pressure increased to approximately 2.1 kg / cm (30 psig) by pump 22 and introduced into the second capacitor 25 through the conduit.

The level controller 26 in the condenser 18 keeps a constant one Liquid level by actuating a control valve 23 upright, so that the entire amount of the in the capacitor 18

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formed liquid is continuously transferred to the second condenser 25 at the rate at which it is formed can be·

The gaseous stream starting from the second decomposer 9 is condensed on line 11 in the second condenser 25 at the approximate values of 54 _f 4 ° C. (130 ° I) and 2 _f 1 kg / oa ² (30 paig).

The heat of reaction in condenser 25 is completely analogous to that in the third condenser 18 by cooling water in line 27 derived »and the pressure in the condenser 25 is increased to **" by means of a valve 28 in the cooling water supply line. Cooling coil 27 controlled In a less economical way the pressure is regulated by a relief valve 71

The liquid formed in the second condenser 25 contains 683 kg / h (1506 LBS / HR) ammonium carbamate, 805 kg / h (1774 IBS / HR) free ammonia and 1560 kg / h (3440 LBS / HR) water and is aua the condenser 25 through the line 29 peeled through, pressurized to about 21 kg / cm (300 psig) the pump 30 is stepped up and introduced into the first condenser 33 through the line 32

The level control 34 maintains a constant liquid level in the condenser 25 by actuating a control valve 31, so that the entire amount of the in the condenser 25 is continuously transferred into the first condenser 33 at the rate at which it is formed will.

The gaseous stream 6 emanating from the first decomposer 4 is partially condensed in the first condenser 33 at the approximate values of ^{93-3 ° 0 (200 ° F) and 21 kg / cm 2} (300 psig). The heat of reaction is dissipated by cooling water in a coil.

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A part of the excess ammonia in stream 6 cannot be dissolved in the solution formed in condenser 33, so that it is necessary _{to clean this portion of the unabsorbed excess free ammonia in packed column 36 from the remaining CO 2} , over which part the solution formed in condenser 33 is flushed through lines 50 and for better contact and separation of the gaseous excess ammonia in the blessing flow process; The last traces of CO ₂ are removed from the excess gaseous NH in the bottom region 37. Approximately 4536 kg / h (10,000 LBS / HR) of recycled liquid NH are fed through line 49 into the upper part of the tray area 37 _{to remove CO 2} from the excess gaseous ammonia and to remove the heat of reaction from the tray area and τοη of the column 36 to be driven off by evaporation

A mixture of 6240 kg / h (13730 LBS / HR) of excess gaseous ammonia from the process and approximately 4536 kg / h (10000 LBS / HR) of refluxing gaseous ammonia is discharged from the upper part of the tray area 37 through the duct. 38 withdrawn at about 54.4 ° 0 (130 ° F) and condensed to liquid ammonia in the water-cooled condenser 39 at about 35 ° C (95 ° F). The pressure in condenser 39 is held constant at approximately 21 kr / cm (300 peig) by venting a controlled amount of inert gases through line 40 and control valve 41. The inerts comprise the corresponding amounts of H ₂ , R ₂ ⁰ ^ ® & λ * & * ■ * usually present as impurities in the liquid Aoraonia inlet stream 44 and in the gaseous COg outlet stream 46. Approximately 10730 kg / h (23730 LBS / HR) of liquid RH ₃ is discharged from condenser 39 through line 42. Approximately 4536 kg / h (10,000 LBS / HR) of this amount is fed to the upper part of the floor area 37 through line 49 and 6240 kg / h (13,730 LBS / HR) liquid RH ₃ is sent through line 43 and old 6460 kg / h (14,236 LBS / HR) of starting liquid ammonia from line 44 mixed. The mixture of these two streams in line 45 »which contains 12,670 kg / h (27,966 LBS / HR) liquid RH _? the high pressure piston

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pump 46 at an elevated pressure of approximately 225 kg / cm (3,200 psig) set in the heater 47 to approximately 62.8 ° C (145 ° F) and preheated through line 48 into reactor 1 initiated.

8340 kg / h (18,383 LBS / HR) gaseous GOg at the approximate values of 225 kg / cm ² (3,200 psig) and 121.1 ° C (250 ° F) is introduced into reactor 1 through line 56.

Reference is made again to the first capacitor 33. The solution formed in the condenser 33 is returned to the circuit through the lines 50 and 51 via the filler body part 36. The amount of solution formed in the condenser 33, which contains 8250 kg / h (18,170 LBS / HR) ammonium carbamate, 2170 kg / h (4,780 LBS / HR) free ammonia and 3765 kg / h (8,300 LBS / HR) water »is passed through the line 52, pressurized to approximately 225 kg / cm ² (3,200 psig) by high pressure piston pump 53, passed through line 54, and introduced into reactor 1. The automatic liquid level controller 55 maintains a constant level of the solution in the condenser 33 by controlling the speed of the high pressure piston pump 53 so that the pump 53 is continuously releasing the recirculated carbamate solution in line 54 into the reactor 1, which corresponds to the same rate which the recycled carbamate solution is formed in the condenser 33.

The refractometer 57 in line 50 registers the refractive index of the recycled carbamate solution in the condenser 33 is formed.

In this example, the ratio of COp to water in the recycled carbamate solution, at which operation in the first condenser 33 is desirable, has the following value: 8,250 kg / h (18,170 LBS / HR) ammonium carbamate is equivalent to 4650 kg / h (10,250 LBS / HR) CO ₃ : it follows

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- 29 ^C0 2 10 250 _{Λ 9Α}

If this ratio increases above 1.24 and accordingly the crystallization point of the recycled carbamate solution also increases in stream 54, the refractometer will increase the refractive index of the solution in proportion displayed · This increase in the refractive index is in turn converted into a proportional signal »which one or both control valves 28 and 20 to a greater flow of cooling water through the corresponding cooling coils 27 and adjusts · This results in a decrease in temperature and consequently in the vapor pressure of the solution in condensers 25 or 18 and a relative pressure decrease, which is due to the corresponding Decomposers 9 and 14 is passed on. This temporary pressure decrease in the decomposers 9 and 14 is in turn in turn cause more water from the aqueous urea solution is evaporated and more water vapor is present in the respective upper lines 11 and 16. By by this mechanism there will be more water in streams 24,32 and finally in stream 54.

Once the original CO ₂ to water ratio of approximately 1.24 is restored, the refractometer will automatically reset control valves 28 and 20 to maintain the desired and predetermined CJO 2 to water ratio of approximately 1.24.

Exactly the opposite will happen when the COp to HpO ratio below the value 1.24 '! AInICt. Then the pressure in the Decomposers 9 and 14 temporarily increased, so for a certain time the amount of water that was removed from the aqueous urea solution is evaporated in the corresponding decomposers 9 and 14, is reduced and in this way the water vapor content is decreased in streams 11 and 16, respectively.

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It is obvious that the same bottom line is achieved by the Change in pressure in decomposers 9 and 14 caused can be »« as either directly through the control of the valves 59 and 58 in the gas streams 11 and 16, respectively can or indirectly, by a larger or smaller amount of gas from the condenser 25 or 18 through the valves 71 and 72 is drained. But in these cases the urea synthesis process will be less effective compared to that Case in which the bridge in decomposers 9 and 14 is indirect by changing the cooling water flow through the cooling coil 27 or 19 is varied

The level control and all others shown in the figure Components are standard parts that are known to those skilled in the art.

A stream containing ammonia, carbon dioxide and water, or a stream of carbon dioxide, may come from an external source of either one or all of said capacitors 18, 25 or 33 can be added, such as an exhaust gas jet from a melamine plant or from other sources of ammonia and / or carbon dioxide.

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Claims (14)

Patent claims Process for urea synthesis with a recirculation circuit for the ammonium carbamate solution, in which an ammonia flow, a carbon dioxide flow and a solution comprising ammonium carbamate are reacted at elevated pressure and temperature to react in a urea reactor (1) to synthesize, whereby the stream (2) flowing out of the urea reactor, which contains urea * water, unconverted ammonium carbamate and ammonia, is separated into an aqueous urea product solution and the unreacted reagents ammonium carbamate and ammonia, this separation by heating the above The stream (2) flowing out of the reactor is carried out in a decomposition system (4.9tH) under reduced pressure in order to decompose the ammonium carbamate which has not been converted into gaseous NH and CO2, to evaporate the ammonia and the entire mixture τοη gaseous HH. and expelling COn together with a certain proportion of water vapor from the aqueous urea product solution, this mixture of gaseous HH, and COp containing some water vapor, partly or entirely in a condensation system (33 * 25, 18) after separation from the remaining aqueous urea product solution is condensed, which contains ammonium carbamate, ammonia and water and is returned to the urea reactor (1), characterized in that the pressure in the decomposition system (4 * 9 * 14) depends on the ratio of ammonium carbamate to water in the reactor returned liquid phase is regulated " 2. The method according to claim 1 *, characterized in that the Pressure in the decomposer system (4 * 9 * 14) is regulated in this way * that the ratio between ammonium carbamate and water remains in a predetermined range 209821/1059 3. The method according to claim 1 or claim 2, characterized in that that in the liquid phase, which is returned to the reactor, urea in an amount of 0.005 to 4 »0 kg of urea per kg of the ammonium carbamate contained in the liquid phase (54) is present. 4. The method according to any one of claims 1 to 3t, characterized in that that ammonia and carbon dioxide or Ammoniumkar bamat added to the liquid phase from an external source and the resulting mixture of aqueous ammoniacal ammonium carbamate solution in the Urea reactor (1) is returned. 5. The method according to any one of claims 1 to A _9, characterized in that the refractive index of the liquid phase returned to the reactor is determined and the pressure in the decomposition system (4,9,14) is set as a function of the measured refractive index, the one Puncturing the ratio of ammonium carbamate to water in the liquid phase returned to the reactor. 6. The method according to any one of the preceding claims, characterized It is known that the decomposition system (4,9,14) is in pressure coaaunilcation with the condensation system (33 * 25,13) and the bridge in the condensation system as a function of the ratio of ammonium carbamate to water in the liquid phase returned to the reactor (1) is regulated, the pressure in the decomposition system being equal to the pressure im Condensation system varies. 7. The method according to claim 6, characterized in that the Pressures in the condensation (33f25 # 18) and in the decomposition system (4,9,14) are regulated so that the ratio is within a predetermined area is held. 209821 / 10B9 8. The method according to claim 6 or 7 »characterized in that the pressures in the condensation (33.25» 18) and in the decomposition system (4 »9» 14) are regulated by the fact that the coolant supply (35 »27» 19) for the condensation system is dependent on on the ratio of ammonium carbamate to 7 / water in the liquid returned to the reactor (1) Phase is regulated. 9. The method according to any one of claims 6 to 8, characterized in that that the refractive index of the liquid phase returned to the reactor (1) is determined and the pressure in the condensation system (33 »25,18) depending on the measured refractive index, which is a puncture of the ratio from ammonium carbamate to water in the in the reactor (1) recycled liquid phase is regulated. 10. The method according to claim 9 »characterized marked '. That the pressure in the condensation system (33» 25 »18) by regulation the coolant supply (35 »27, 19) for the condensation system depending on the ratio of ammonium carbamate is regulated to water in the liquid phase returned to the reactor (1). 11. Process for urea synthesis with a recirculation circuit for the ammonium carbamate solution, in which an ammonia flow, a carbon dioxide flow and a solution comprising ammonium carbamate at elevated pressure and temperature reacted to in a urea reactor (1) To synthesize urea, whereby the stream (2) flowing out of the urea reactor, the urea, water, not contains converted ammonium carbamate and ammonia into one . aqueous urea product solution and the unreacted Reagents ammonium carbamate and ammonia is separated, this separation by heating the above The stream (2) flowing out of the reactor is carried out in a decomposition system (4,9,14) under reduced pressure 209821/1059 - 34 - is used in order to decompose the unconverted ammonium carbanate into gaseous NHL and GO «, to evaporate the ammonia and to drive off the entire mixture of gaseous HH and GOp together with a certain amount of water from the aqueous urea product solution, v / above this somewhat 'Steam-containing mixture of gaseous NH and CO' after separation from the remaining aqueous urea product solution is partly or wholly condensed in a condensation system (33,25,18) to form a liquid phase which contains ammonium carbamate, ammonia and water and is returned to the urea reactor (1), characterized in that the refractive index of the liquid phase (54) returned to the reactor is determined and the ratio of ammonium carbamate to water / water in this liquid phase is regulated as a function of the measured refractive index. 12. The method according to claim 11, characterized in that the pressure in the decomposition system (4,9,14) is regulated, about the said relationship in a predetermined one 3 to keep rich. 13. The method according to any one of claims 11 or 12, characterized in that in the in the reactor (1) zur'ickge— led liquid phase (54) urea in an amount of 0.005 to 4.0 kg of urea per kg of the Annaoniuiakarb- ^ niates available contained in the liquid phase returned to the reactor. 14. The method according to any one of claims 11 to 13, characterized in, that ammonia and carbon dioxide or ammonium carbamate to said liquid phase from an external source is added and the resulting Gesaitgemiseh from aqueous ammoniacal solution of ammonium carbamate is returned to the urea reactor (1). 209821/1059 Blank page