DE2128450A1 - Urea process from ammonia and carbon dioxide - improved by stripping recycle carbamate with feed carbon dioxide - Google Patents

Abstract

In a high pressure synthetic of urea, aq. NH4 carbamate (I) is recycled by pressurising and stripping with feed CO2, pref. at about synthesis reactor pressure. This is carried out in a heat exchanger, externally heated to decompose (I), giving a high pressure off-gas contg. CO2 and NH3 and a discardable liquid effluent. Feed NH3 is pref. added to the off-gas to give an NH3:CO2 ratio of 2:1 and this is passed through a heat-exchanger to heat effluent from the reactor, thus decomposing (I) in the effluent and giving an off-gas which is converted to the aq. (I) soln. This results in the condensation of molten (I), to produced a gas-liquid mixture of low water content. This condensed (I) is passed to the synthesis reactor. Alternatively, the enriched off-gas is cooled in heat-exchange with liq. water to produce steam used to heat the synthesis effluent and decompose (I). Advantages are avoidance of urea decomposition by stripping operations, reduced water content of recycled (I), conversion rate raised from 67-75%, reduction in reactor vol.% a 25% reduction in energy required for evaporation.

Description

Method of Making Urea The invention relates to on a process for the production of urea.

In particular, it relates to a urea synthesis process of the return type, in which urea is produced by the reaction of ammonia with carbon dioxide is produced at elevated pressure, forming a urea synthesis effluent will, the urea, water, ammonium carbamate and usually excess ammonia contains and in which the exhaust gas from the heating of the urea solution Synthesis effluent used to evaporate unreacted ammonium carbamate and ammonia is processed further} around an aqueous ammonium carbamate solution : to form for the return to urea synthesis.

Urea is generally made up of ammonia and carbon dioxide, typically synthesized at 210 kg / cm2 and 190 0C, with normally a total - ammonia - to carbon dioxide molar ratio of about 4: 1 is used. The reaction goes over the initial formation of ammonium carbamate will take place, with a portion of the same is dehydrated to urea. The effluent from the synthesis reactor thus contains urea, Water, ammonium carbamate - and excess ammonia.

The reactor effluent is heated at reduced pressure to produce ammonium carbamate to decompose and a mixed exhaust gas that contains ammonia, carbon dioxide and water vapor contains, together with an aqueous urea product solution to produce which for the production of urea crystals or the like is further processed. A portion of the unreacted mixed exhaust gas is dissolved in an aqueous ammonium carbamate solution recycled with such a water content that the feed ratio of Water to carbon dioxide in the total synthesis reactor ventilation usually Q, 6 to 1 on a molar basis. Because water is one of the products of the urea synthesis reaction is, the synthesis conversion-to urea-decreases from the theoretical value from about 77 to about 67%.

The pressure of the reactor effluent is reduced, typically to 21 kg / cm² and the effluent is then usually heated to about 1600C. About 90 ffi of not implemented Ammonia and carbon dioxide can evaporated at 21 kg / cm2. The exhaust gas can be through an adiabatic flash evaporation due to the pressure reduction, as in U.S. Patent 3 172 911. Some energy is released during the condensation of the vapors obtained, which typically according to the procedures of US Patents 3 147 304 and 3 137 725. Typical uses for the energy are as Heat source for the decomposition in the second stage at about 2.1 kg / cm², for heating the urea solution in the crystallizer or for preheating the feed ammonia.

The partially condensed vapors or the exhaust gas from the 21 kg / cm² evaporation or the decomposition of the carbamate and the exhaust gas from the 2.1 kg / cm2 evaporation often collected in water in two absorbers and placed in the reactor as concentrated recirculated aqueous ammonium carbamate solution. Much of the ammonia from the evaporation stages, especially from the first evaporation stage at 21 kg / cm² is freed from carbon dioxide in a tower and condensed as almost pure ammonia. The exhaust gas treatment procedures are described in U.S. Patents 3,155,722; 3,155 723 and 3,191,916.

The separation of the unreacted components from the urea synthesis effluent using ammonia or carbon dioxide as a denuding medium to get a denuded Preparing aqueous product urea solution is described in U.S. Patents 2,056 283, 3 o46 307, 3 049 563, 3 301 897 and 3 356 725 as well as in the Canad. Patent specification 787 960. The use of the mixed exhaust gases for the Bare is in Canada. U.S. Patent 736 520 and U.S. Patent 3,072,721. The use of an inert gas or air for peeling is described in US patents 2,267,133, 2,087,325 and 3,120,563.

The invention provides a method for urea synthesis for Provided at which ammonia and carbon dioxide at a pressure of 100 up to 350 kg / cm² are reacted with each other, the product at a pressure of less than 100 kg / cm2 is heated to an exhaust gas from a liquid urea effluent separate, an aqueous ammonium carbamate solution is obtained from the exhaust gas and is pressurized to 30 to 350 kg / cm², the solution in an externally heated Zone is exposed by carbon dioxide to decompose ammonium carbamate and ammonia and removing carbon dioxide, the resulting gas with a flowable medium is subjected to heat exchange, and in which at least a portion of the ammonium carbamate and the resulting stream with the condensate is returned to the urea synthesis will.

This denuding process is applicable to urea processes, which use an aqueous recycle ammonium carbamate solution. The energy recovery working methods, how they are practiced in the present litigation theme can deal with changes of the ammonia to carbon dioxide ratio in the reactor feed is significant change. The energy recovery operations control the overall energy requirements. The features of the present invention include the combination high pressure carbon dioxide stripping of the aqueous ammonium carbamate solution and the condensation of the resulting mixed exhaust gas to form ammonium carbamate melt at a high pressure to heat other parts of the process to drive the rudder. The method of the invention provides for the selection and high pressure stripping of one Carbamate-containing, urea-free process stream, which compared to the Process conditions is relatively insensitive. The present invention avoids thus-those problems encountered with the denuding of urea-containing solutions are connected where a dwell at the required temperatures and reactor pressures critical with regard to the hydrolysis of urea and / or the formation of biuret is.

The aqueous recycle ammonium carbamate solution derived from the 4: 1 liquid Recirculation urea process stems from ammonium carbamate and ammonia using the carbon dioxide feed stream, which is preferably takes place at or slightly above the reactor pressure and at temperatures such that such relatively complete bare can be obtained. The pressure of the Water vapor in the water vapor jacket around the exposure zone generally exceeding 14 kg / cm² and is only made by the economical ones The conditions of the jacket construction and the corrosion of the bare is limited, when the wall temperature goes over 1900C. Water vapor pressures that are currently practicable are 20 kg / cm2 to 40 kg / cm2, with the water vapor of the usually associated Ammonia system is available.

The head vapors are close to the maximum condensation temperature mixture, which is about 2 moles of ammonia per mole of carbon dioxide, by adding ammonia set.

In general, the majority of the vapors prior to entering the Reactor condenses. The heat of condensation is preferably used in part to to decompose ammonium carbamate in the reactor effluent; and carbon dioxide, ammonia and evaporating some water from the reactor effluent at reduced pressure, wherein the preferred pressure range is about f5 kg / cm2 to about 55 kg / cm2. The rest of excess heat required from carbamate condensation in the overhead vapor stream above dE Amount to decompose the ammonium carbamate in the reactor effluent is used, to produce 3 to 6 kg / cm² water vapor.

Alternatively, a larger part of the heat can also be derived from the carbamate condensation be converted into water vapor, with part of the water vapor decomposing of the ammonium carbamate is used in the reactor effluent.

The liquid undercurrent of the debuster, which is mainly water generally contains some ammonia and carbon dioxide, these amounts depend on local economic conditions. This material is through Distillation obtained in a still.

In a preferred embodiment of the invention, the entire water removed from the aqueous product urea solution is condensed and used, all ammonia and carbon dioxide released from the urea systemic effluent at low pressure is removed to absorb. This solution also becomes the same still passed to distillation. In an alternative option, the high pressure stripping floor products in the high pressure unit will be exposed to the point where only carbon dioxide and water remain in the liquid phase and the effluent can then be discarded. In any case, the top product of the distillation ultimately reaches the high pressure absorber, which forms the aqueous ammonium carbamate solution.

A significant part of the heat content of the mixed exhaust gas or the vapors resulting from the decomposition of ammonium carbamate in the reactor effluent, is obtained by heating the high pressure ammonia feed to the reactor will. These vapors and / or the above-mentioned distillation vapors can also can be used to provide the heat demand for the urea crystallizer to deliver. These vapors can also be used as a heat source in a low pressure stripper can be used to remove the residual ammonia in the reactor effluent.

The mixing of the high pressure stripper coproduct with the ammonia can be used to measure the extent of carbamate decomposition and evaporation to control in the reactor effluent. By adjusting the ammonia feed rate, to move away the 2: 1 molar ratio, the condensation temperature is decreased.

This lowering of the temperature driving force reduces the amount of Carbamate decomposition in the reactor effluent and the accompanying evaporation or formation of the mixed exhaust gas.

This is an advantage when using this decomposer for the soiled Condition sized, however, is consistent with a near clean condition.

In an alternative possibility of the invention, the high pressure denuding stage be modified by adding to the aqueous ammoniacal ammonium oarbamate solution enough carbon dioxide is mixed in to react with some of the free To yield ammonia in the solution and to bring the solution to the feed tray temperatures to heat. The heat of the reaction due to a further excess of the carbon dioxide addition and the formation of ammonium carbamate can be attributed to the heat exchange with absorbed by the ammonia feed stream.

The end result is improved system behavior, however with a higher high pressure steam consumption and a higher export of low pressure steam. C. This modification can be based on the local market for such low pressure steam be justified as a heat source for heating the solution or the like can be used.

The method of the invention has numerous advantages. the Reduction of the water content of the recycled ammonium carbamate and the corresponding resulting reduction in the water content during urea synthesis increases the reactor conversion from 67% to 75%. This reduces the energy requirements for evaporation by about 25-ß.

The device for the high pressure denuding process is determined to a large extent based on what is available.

Temperature Driving Force .-- The advantage of processing the aqueous Recycle ammonium carbamate solution in the stripper is that the film temperatures can be significantly higher than when processing the reactor effluent, since the Decomposition of the urea is not a problem.

Currently the permissible water vapor pressure is around 35 kg / cm2 the corrosion of titanium or stainless steel is limited by the ammonium carbamate. The use of modified titanium or stainless steel or other alloys however, would allow operation with much hotter heating media, with the selection would be primarily based on economic considerations.

The stripping operating pressure can be varied over a wide range. The operating pressure is preferably just above the reactor pressure> by necessity to avoid pumping hot condensed ammonium carbamate and to avoid any To avoid constraints in place of both the stripper and the condenser, since the pressure is available to drive the liquid into and around the reactor the skin temperature of the stripper is reduced to a minimum in order to use it of alloys such as 316 L. The condensation temperature allows the Formation of 7 kg / cm2 water vapor.

Higher operating pressures are also possible, especially at higher pressures Stripping jacket temperatures and the water vapor regeneration can be higher at such Press.

The recycling of the recycle carbamate solution offers itself for the repeated reuse of energy. At very high condensation temperatures the residence time of the liquid must be reduced to a minimum value, to avoid by-product formation outside the urea stage.

In most cases there is a direct heat transfer between the condensing ammonium carbamate and the reactor effluent in one Heat exchanger provided. This relationship can be modified, e.g. the water vapor could be formed first and then the water formed there pf could be used to remove the ammonia and carbon dioxide from the reactor effluent to evaporate. In this latter stage define the residence time of the effluent liquid in the exchanger, the film temperature and the free ammonia partial pressures to one large extent the formation of blurite and the urea hydrolysis ratio. As a result At high carbamate condensation temperatures, the intermediate formation of water vapor can occur as an alternative to designing a special heat exchanger for the condensing effluent from the carbamate reactor should be preferred.

The flexibility in the condensation temperature of the ammonium carbamate simplifies the design of the stage of high pressure evaporation and absorption, there the high recycling temperatures an equivalent evaporation at high pressures allow the second recycling of the high pressure steam and the final Condensation of the excess ammonia simplified. A final benefit of repatriation of near dry molten ammonium carbamate is a significant reduction the reactor volume.

The following is an embodiment of the method according to the invention are explained in more detail using the attached flow chart.

A preheated ammonia feed stream 1 and a stir-back stream 2, the ammonium carbamate at least partially in the liquid state contains, is introduced into the urea synthesis reactor 3, which in general is a high pressure autoclave or the like. In the unit 3 an increased pressure, generally in the range from 100 to 350 kg / cm2, preferably in the range from 135 kg / cm2 to 270 kg / cm2. The temperature is generally in the range held from 150 to 2200C. The total molar feed ratio of ammonia carbon dioxide becomes in the unit 3 in a range generally between 2: 1 and maintained about 8: 1.

The synthesis of urea takes place under these operating conditions by splitting off water from the ammonium carbamate in unit 3. The splitting off of water reaches equilibrium and the total conversion to urea is in the unit 3 is not obtained in conventional practice, so that accordingly the effluent 4, the is discharged from the urea reactor or zone 3, urea, water, ammonium carbamate and contains excess ammonia. The stream 4 is through a pressure reducing valve 5 passed and the resulting effluent 6, which did not decrease with a Pressure is below 100 kg / cm² and generally in the range of 15 kg / cm² to 55 kg / cm² is passed into the shell of a heat exchanger decomposer 7 and rises outwards to the pipes 8, which are provided with suitable outer baffles can be provided. The synthesis effluent process stream is in the shell of the unit 7 is heated and some of the ammonium carbamate in the process stream is decomposed, whereby a gaseous phase is formed that contains ammonia, carbon dioxide and water vapor.

The resulting gas-liquid mixture formed in the shell of the unit 7 becomes from the upper part of the unit 7 discharged as stream 9, in the gas-liquid separator 10 to separate the gaseous phase from the remaining urea-containing liquid phase arrives. The unit 10 is a gas-liquid separator of conventional design, e.g. one with baffles provided or cyclonic unit. The separated gaseous phase is removed from unit 10 as stream 11, which leads to the formation of an aqueous Ammonium carbamate solution, as described below, is worked up.

The remaining urea-containing liquid phase turns out of unit 10 as stream 12, which passes through a pressure reducing valve 13 is passed, which the resulting stream 14 with a reduced pressure in Range from 1 to 5 kg / cm². Due to the reduced pressure of stream 14 a gaseous phase is released in the stream 14, which changes into a gaseous phase Phase and a remaining liquid phase is separated by the stream 14 is passed into a gas liquid separtor 15. The unit 15 is the unit 10 similar. It separates the released exhaust gas stream 16 from the remaining one liquid stream 17 from. Stream 16 contains ammonia and carbon dioxide along with a small amount of water vapor. He is, as described below, responsible for the repatriation and recovery of the ammonia values further processed.

Stream 17 now mainly consists of an aqueous urea solution, which contains lower dissolved amounts of ammonia and carbon dioxide. The stream 17 is used in any suitable manner for the recovery of solid product urea further processed. Stream 17 is preferably combined with recycle stream 18, which, as below described, derives, combined and the resulting combined stream 19 is passed into vacuum crystallizer 2O to form. of solid urea crystals by removing water as water vapor passed with remaining amounts of ammonia and carbon dioxide. The resulting Slurry stream 21 withdrawn from the bottom of unit 20 now exists of solid urea crystals contained in a saturated aqueous mother liquor are. Stream 21 is now preferably divided into two parts, with the recycle stream 22 is heated in the heat exchanger 23 and returned as stream 18.

The remainder of the total stream 21 reaches the centrifuge or as stream 24 the crystal filter 25, from which solid product urea 26 arrives for product utilization. In some cases, the stream 26 can be used directly for product utilization as fertilizer, for the production of plastics or the like. The stream 26 but can also be dried, melted and sprayed for fertilizer purposes be subjected to talllsation. The remaining mother liquor pulp 27, which from the unit 25 is removed, can be returned to the unit 20 or in derl Process, as described below, can be recycled.

In some cases, the stream 21 can initially lead to a dense slurry insulation, which-is used as stream 24, and a clear liquid which is used as stream 22 is returned, can be further processed.

In the unit 20 is an overhead vapor stream 28, which is mainly consists of water vapor together with residual ammonia and carbon dioxide, in the Wasserdampfdüsenexhauster 29 initiated, through which a water vapor stream 30 is directed and expanded to the Pull in stream 28 and to create a vacuum in the unit 20. The one discharged from unit 29 resulting combined stream 31 is cooled and can at least partially be condensed in the heat exchanger 32, which with cooling water or a other suitable coolant. The resulting cooled stream 33 is combined with the stream 16 'in the cooler-condenser 34, which the entire combined streams or a major part thereof are condensed to the liquid state. The resulting liquid stream 35 is used for recovery as described below the ammonia values further worked out.

In the unit 10, the top mixed gas flow 11 with the return flow 36 combined, which is obtained in the manner described below and the ammonia, Contains carbon dioxide and water, these components in the stream 36 at least partially in a condensed liquid phase. The resulting hot combined stream 37, which due to the formation of ammonium carbamate in the liquid phase has elevated temperature, is cooled and placed in the heat exchanger partially condensed further into the liquid state. The resulting mixed gas liquid flow 39 now enters the condenser bare unit 40, which is the device of U.S. Patents 3,155,723 and 3,191,916 and performs a similar function having. The liquid part of stream 39 descends and joins the liquid Body in the bottom part of the unit 40, while the gaseous part of the stream 39 after flows above through the gas washing and condensation zone 41, which is typically a Bed with suitable spherical, annular or saddle-like Packing bodies for effective gas-liquid contact is. In some cases it can the zone 41 consist of sieve trays, bubble caps, plates or the like. In each Fall is the cold liquid stream 42, which consists of an aqueous ammoniacal Ammonium carbamate solution is introduced into the unit 40 above the zone 41 and it flows in countercurrent to the rising gases through zone 41 downwards, whereby the carbon dioxide was washed out of the gas phase as condensed ammonium carbamate will. The resulting aqueous ammonium carbamate solution settled at the bottom of the unit 40 accumulates and which usually contains residual free ammonia is called a stream 43 removed. A portion of the stream 43 is returned as stream 42, which in an external heat exchanger, not shown, can be cooled before moving into the unit 40 above the zone 41 is returned.

The remainder of the stream 43, which consists of the stream 44, is according to FIG Invention returned to urea synthesis. Stream 44 is pumped and in the pump 45 is pressurized, which the aqueous ammonium carbamate solution on compresses an elevated pressure which is usually slightly greater than the urea synthesis pressure in unit 3. It is generally in the range of about 30 in each case to 350 kg / cm2, preferably from 100 to 350 kg / cm2. The resulting under pressure The set stream 46 discharged from unit 45 is now fed into the delamination heat exchanger 47 for removing the ammonium carbamate by high pressure delamination with the feed carbon dioxide passed, which removes the water from the system and according to the invention a High pressure mixed gas flow generated.

Stream 46 flows down through tubes 48 in unit 47. The tubes 48 are externally heated, typically by placing a hot, flowable one Medium flow 49, which usually consists of high pressure steam, outside the tubes 48 is passed, wherein the cooled liquid or the condensed Water as stream 50 is removed. The external heating of the tubes 48 accelerates the internal decomposition of the ammonium carbamate resulting from stream 46 and the formation an aqueous phase containing ammonia and carbon dioxide at elevated pressure within the tubes 48 contains. A temperature typically in the range of about 200 to about 250 ° C. is maintained in unit 47 by stream 49. In the ground feed carbon dioxide gas is introduced into unit 47, typically by feed carbon dioxide gas stream 51 in compressor 52 to an elevated level Pressure is compressed, which is usually higher than the urea synthesis pressure and which is generally comparable to the pressure of stream 46 and in which the resulting compressed carbon dioxide gas stream 43 is directed into the bottom of unit 47, so that the feed carbon dioxide gas rises up through tubes 48 and that Ammonia and the carbon dioxide from the downward flowing aqueous liquid solution in the tubes 48 takes out a residual aqueous phase, which is mainly consists of water, accumulates at the bottom of unit 47 and is called stream 54 removed. In some cases, the stream 54 can be negligible of dissolved ammonia and is discarded. In this preferred embodiment of the invention however, the stream 54 is further processed to recover the dissolved ammonia and carbon dioxide levels in it.

The stream 54 is passed through the pressure reducing valve 55 and the resulting aqueous stream 56, which is now typically at reduced pressure is in the range of 15 to 55 kg / cm² is fed into the ammonia recovery distillation apparatus 57 passed along with the stream 35, which, as described above, from the processing originates. In most cases stream 35 will be at a pressure below 15 kg / cm2 formed. Therefore, the stream 35 is pressurized by being through a pump, not shown, is directed before it flows into the unit 57. the Distillation device 57 is a distillation column or retort that the vapors from aqueous feed streams 56 and 35 at one pressure typically in the range of 15 to 55 kg / cm² and a temperature in the range of 150 to 230 ° C boils up and brings to reflux, to a steam overhead, which is enriched in ammonia is to be separated from the waste water.

The depleted waste water stream 58 is from the bottom of the unit 57 taken. A portion of stream 58 is discarded as waste water stream 59. In In some cases, heat can be obtained from the hot water stream 59 by the Stream 59 in heat exchange with stream 35 before the final rejection of the stream 59 is directed. The remainder of stream 58 is used as stream 60 in the reboiler heat exchanges 61 for heating, partial evaporation in the heat exchange with water vapor is directed and the resulting mixed steam liquid stream 62 becomes unit 57 below returned to the upper reflux sieve zone. A final hot overhead steam stream 6) is from the top of purity 57 removed. The current 63 is rich in recovered ammonia levels and also contains carbon dioxide and water vapor. The stream 63 is first through the heat exchanger 23 in heat exchange with the Stream 22, as described above, passed. At least a portion of stream 63 condenses in the unit 23 to a liquid and the resulting cooled stream 36, which contains a condensed liquid phase is combined with stream 11 and, as described above, for conversion into an aqueous ammonium carbamate solution and recycled overhead purified ammonia vapor in unit 40.

In the condenser stripping unit 40, the one above the zone 41 rising gas phase is now depleted in carbon dioxide and consists mainly of Ammonia vapor. This gas phase rises through reflux deletion zone 64, such as it is described in U.S. Patents 7,155,723 and 3,191,916. Of the, Top of the unit 40 becomes a resulting purified residual ammonia vapor stream 65 taken. Stream 65 is combined with feed ammonia stream 66 and the combined ammonia stream 67 is preferably cooled and in the heat exchanger 68 totally condensed by exchanging heat with cooling water. The resulting liquid ammonia stream 69 is fed by pump 70 to an elevated pressure, typically brought in the range of 100 to 350 kg / cm2 and the pressurized liquid Ammonia stream 71 discharged from unit 70 - is in the heat exchanger 38 heated by heat exchange with process stream 37. The heated high pressure ammonia stream 72 is now preferably into stream 1, which, as described above, is passed into the urea synthesis, and the flow 77 divided, which according to the invention is exploited.

The stream 73 is now preferably mixed with the hot mixed gas stream 74 resulting from the tubes 48 of the unit 47, like the resulting one hot gas stream, which by delamination of the aqueous ammonium carbamate solution with gaseous Carbon dioxide has been formed in unit 47. In most cases that is added amount of ammonia stream 73 to the mixed gas stream 74 so dimensioned that the resulting combined process stream 75 has a molar ratio of ammonia to carbon dioxide in the range of about 1.5: 1 to about 3: 1. To achieve optimal results with regard to the following maximum temperature values during the ammonium carbamate condensation the molar ratio of ammonia to carbon dioxide in stream 75 is essentially 2: 1. The urea stream 76 is now preferably added to the stream 75 to the initial Promote condensation processes. In most cases stream 76 consists of an aqueous urea solution and it can be from part of the streams 22 or 27 exist.

The final combined recycle stream 77 eliminated by addition of the stream 76 is formed to the stream 75 is now passed into the unit 7 and flows through the tubes 8 downwards. At least some of the ammonia and carbon dioxide content of stream 77 condenses into molten ammonium carbamate of low water content within the tubes 8 with the consequent formation and release of heat, which the decomposition of the ammonium carbamate in the urea synthesis effluent within the Shell of the unit 7, as described above, promotes. The downhill Recycle process stream withdrawn from tubes 8 is discharged from unit 7 as a stream 78, which is usually both a molten liquid ammonium carbamate phase as well as a residual gas phase.

Stream 78 is now used to generate further heat by carbamate condensation processed further before being returned to urea synthesis. The Stream 78 is directed down into the vertically oriented heat exchanger 79 and flows down through the tubes 8, wherein further condensation of the molten Ammonium carbamate and heat release occurs. A suitable heat exchange medium stream 81, which usually consists of condensation or boiler feed water introduced into the shell of the unit 79 and the heated heat exchange medium, the usually consisting of the high pressure water vapor formed is removed as stream 82.

The external cooling of the tubes 80 by the heat exchange medium can the total condensation of the gaseous phase in the tubes 80 to a liquid promote and educate.

In either case, it contains the downwardly discharged from the tubes 80 resulting process stream a molten liquid ammonium carbamate phase and he can consist entirely of molten ammonium carbamate with a low water content. This resulting process stream collects in the unit 79 below the tubes 80 on. It is taken as stream 2 and, as described above, from the urea synthesis fed. In most cases, as stated above, the pressure is the Process streams from which stream 2 is derived, specifically streams 53, 46 and 73 slightly higher than the urea synthesis pressure im Reactor 3, so that consequently the hot molten ammonium carbamate stream 2 goes directly into the reactor 3 flows without it being necessary to provide a pump to flow the stream 2 into the Press reactor 3. This fact is of great advantage because a special pump for pumping the hot stream of ammonium carbamate, which is a molten salt, thus is not required.

Numerous alternatives fall within the scope of the invention. In some In some cases, the method of the invention can also be used outside of the specified process variables, e.g. the temperatures and pressures as mentioned above, but these areas represent preferred embodiments of the invention. The stream 14 can be heated before it is fed into the unit 15 for heat exchange with streams 11 or 36 final ammonium carbamate decomposition and formation to promote of exhaust gas. The stream 17 can alternatively be produced by multiple evaporation processed to produce an essentially anhydrous urea melt for the direct spray drying to form. In this context, the working methods U.S. Patents 3,211,788 and 7,147,174 apply.

In some cases it is advantageous to stream 46 with carbon dioxide at high pressure as stream 46 usually has excess free ammonia contains. In this case the added carbon dioxide settles with the free ammonia in the stream 46 um, with ammonium carbamate in situ with correspondingly occurring Temperature increase is formed. The resulting hot process stream can then be conducted in heat exchange with stream 72, another Provide heating of the feed ammonia or it can go directly into the unit 47 are conducted at elevated temperatures, reducing the heating requirements for the unit 47 can be reduced.

In some cases, the delamination pressure in the tubes 48 of the unit 47 lie below the urea synthesis reactor pressure. In this case, the Stream 74 may be compressed prior to combining with stream 77. In some cases, e.g., if there is a high level of free ammonia in stream 46, then the stream 73 can be omitted and the stream 75 merely comes from the stream 74 ago. In most cases, however, the current 73 is provided to the required Molar ratio of ammonia to carbon dioxide in stream 75 for optimal temperature values and to achieve heat generation. As already stated above, the feed speed of stream 73 can be modified from the value which is the optimum molar ratio of Ammonia to carbon dioxide in stream 75 delivers 2: 1. This modification would be e.g.

useful in initial periods of operation when the heat exchanger surfaces in units 7 and 79 are clean and tubes 8 and 80 are clear. If during pollution occurs during operation and the heat transfer rate are decreased, then the feeding speed of the stream 73 can be relatively to stream 74 are modified so that a molar ratio of ammonia to carbon dioxide is obtained in stream 75 of 2: 1, which is the subsequent ammonium carbamate condensation and promotes heat release at maximum temperature values.

Eventually, condensation of the molten ammonium carbamate can occur as it takes place in the method of the invention, are only used to to form water vapor or some other heat exchange besides heating the Provide urea synthesis effluent 6.

In this case the stream 77 would be fed directly into the unit 79 and he would bypass Unit 7. The hot stream 82 flowing in the shell of the unit 79 is formed and which usually consists of high pressure steam, would then partly or wholly used for heating purposes in the unit 7 and he would through the pipes 8 passed. This alternative option is advantageous if parts of the Current 77 initially increased to molten ammonium carbamate at very large Temperatures condense, causing biuret formation and urea hydrolysis would be effected in the urea synthesis effluent3 if a direct heat transfer from the stream 77 is provided to the stream 6.

The following is an example of the application of the invention Process to be explained on a technical urea plant.

Examples The following are the compositions and operating conditions of the main process streams of a technical urea plant, with which the Method of the invention was used. The compositions are in moles per mole Urea expressed in the reactor effluent 4.

Electricity Carbon Ammonia Water Urine Absolute. Tempe no. dioxide substance Pressure retur kg / cm² ° C 53 0.953 - - - 228.2 176 66 - 1.921 - - 19.3 38 0.333 3.332 1.087 1.000 224.7 190 11 0.308 2,970 0.399 - 21.05 165 12 0.025 0.362 0.688 l, ooo 21.05 165 16 o, olg 0.260 0.129 - 1.12 98 17 0.006 0.102 0.559 1.000 1.12 98 28 0.006 0.102 0.554 - 0.105 66 35 0.025 0.362 1.019 - 21.6 38 63 0.072 0.451 0.130 - 21.4 165 59 0.003 0.021 1,352 - 21.4 210 39 0.380 3,421 0.529 - 21.05 77 46 0.380 1.210 0.555 - 228.3 87 74 1,283 1,100 0.092 228.1 176 54 0.050 0.110 0.463 - 228.2 202 76 - - 0.025 0.050 228.1 66 73 - 1,466 0.007 - 228.1 127 2 1.283 2.566 0.124 0.050 228.1 127 1 - 2.676 0.013 - 228.1 127 26 - - 0.005 0.950 1.03 38

Claims (10)

Claims process for the synthesis of urea, in which Ammonia and carbon dioxide are converted at elevated temperature and pressure, the product is heated at reduced pressure to remove an exhaust gas from the liquid Form urea effluent and an aqueous ammonium carbamate solution from the exhaust gas is recovered for recycling in the urea synthesis, thereby g e -k e it is noted that the solution is pressurized to 30 to 350 kg / cm2, the solution in an externally heated zone with gaseous carbon dioxide for decomposition bare of ammonium carbamate and removal of ammonia and carbon dioxide that resulting gas with a medium subjected to heat exchange to partially To condense ammonium carbamate and that one can use the resulting stream in the urea synthesis returns. 2. The method according to claim 1, characterized in that g e k e n n -z e i c h n e t that one distills the remaining solution from the denudation to a gaseous Produce ammonia stream, which is separated from the liquid urea effluent Exhaust gas is added. 3. The method according to claim 1 or 2, characterized in that g e -k e n n z e i c h n e t 0 that the heat exchange medium, which with the resulting gas one Is subjected to heat exchange, the urea product is used for separation an exhaust gas is heated. 4. The method according to any one of claims 1 to 3, characterized g e k e n n indicate that the heat exchange medium is in at least part of the heat exchange system Water is that which is converted to water vapor, which is used to support heating of the urea product can be used to form the exhaust gas. 5. The method according to any one of the preceding claims, characterized g e k It is noted that prior to the heat exchange to the gas from the stripping Ammonia adds to form a stream that has ammonia and carbon dioxide in proportion 1.5: 1 to 3: 1. 6. The method according to any one of the preceding claims, characterized g e k It is noted that one can reduce the pressure of the liquid urea effluent after separation of the exhaust gas is reduced again and that further exhaust gas is separated off from which one an aqueous ammonium carbamate solution wins and in the carbon dioxide denudation returns. 7. The method according to any one of the preceding claims, characterized g e k e It is noted that urea can be used, preferably as mother liquor from a Crystallizer, from which solid urea is recovered, to the gas from the stripper before the heat exchange with the medium there. 8. The method according to any one of the preceding claims, characterized g e k Note that aqueous ammonium carbamate is obtained from the exhaust gas, to obtain an ammonia stream that is free of carbon dioxide3, which liquefies and partly the urea reaction and partly the gas from the peeling is fed. 9. The method according to claim 8, characterized in that g-e k e n n -z e i c h n e t j that the exhaust gas is first carried out before the aqueous ammonium carbamate is obtained Heat exchange with the liquid ammonia cools. 10. The method according to any one of the preceding claims, characterized g e k It should be noted that the aqueous ammonium carbamate is added prior to bare Carbon dioxide adds. L e r s e i t e