GB2067175A

GB2067175A - Process for synthesizing ammonia from hydrocarbons

Info

Publication number: GB2067175A
Application number: GB8100361A
Authority: GB
Original assignee: Montedison SpA
Current assignee: Montedison SpA
Priority date: 1980-01-08
Filing date: 1981-01-07
Publication date: 1981-07-22
Also published as: ES8204975A1; BR8100080A; AT370058B; TR21273A; DE3100359A1; NL8100003A; YU42714B; ES498352A0; IN154947B; YU981A; FR2473033B1; ATA2381A; GB2067175B; DK2481A; FR2473033A1

Abstract

A process for synthesizing ammonia, comprising generating the synthesis gas from hydrocarbons (1) by primary reforming (5) of the hydrocarbons with steam (3) and by secondary reforming (15) of the resulting gaseous mixture (16) with air (12), subjecting a part (6) of the starting hydrocarbon mixed with the steam to a tertiary reforming (7) in which the necessary heat is provided by the reaction gaseous mixture (17) leaving the secondary reforming, causing the ammonia synthesis (37) to occur at low pressure with drying (38) of the gas conveyed to the synthesis reactor by employing molecular sieves (36, 39), causing the ammonia contained in the reacted gas to be absorbed (34) by water, and subjecting the ammonia solution so obtained to distillation by means of two distillation columns (52, 53) operating at different pressures. <IMAGE>

Description

SPECIFICATION Process for synthesizing ammonia from hydrocarbons This invention relates to a process for synthesizing ammonia from hydrocarbons, and in particular to a process for synthesizing ammonia by generating the synthesis gas from hydrocarbons by reforming of the hydrocarbons with steam and oxygen.

As is known, in an ammonia synthesis process the reaction of a hydrocarbon with steam is called primary reforming and is an endothermic reaction, while the reaction of the hydrocarbon with oxygen, which is generally introduced into the process in the form of air in order to directly obtain a nitrogencontaining synthesis mixture (3H2 + N2) is called secondary reforming and is an exothermic reaction.

Ammonia is a product having a high energy content the production of which involves a considerable waste of energy. It is known that the actual energy consumption to produce 1 Kg of ammonia is considerably higher than the minimum theoretical stoichiometrically necessary consumption, even assuming that the reaction heat is recovered. The ratio between the theoretical value and the actual energy consumption indicates the degree of efficiency of the process.

The world energy shortage and the consequent increase in the cost of energy have caused the problem of reducing the energy requirements of the processes for producing ammonia to be widely felt.

The latest high-capacity ammonia plants, based on steam reforming processes of the hydrocarbons and improved with integrated heat recovery cycles and with the operation of the main machines by means of steam turbines, exhibit an efficiency of about 500/0--55%, what means that at present, in spite of the efforts made and the improvements already obtained, the actual energy consumption to produce 1 Kg of ammonia is still about double the minimum energy consumption theoretically necessary.

The present invention provides a process for synthesizing ammonia, comprising generating synthesis gas from one or more starting hydrocarbons by primary reforming of the said hydrocarbons with steam and by secondary reforming of the resulting gaseous mixture with oxygen or an oxygencontaining gas, subjecting a part of the starting hydrocarbon mixed with the steam to a tertiary reforming in which the necessary heat is provided by the reaction gaseous mixture leaving the secondary reforming, causing the ammonia synthesis to occur at low pressure with drying of the gas conveyed to the synthesis reactor by employing molecular sieves, causing the ammonia contained in the reacted gas to be absorbed with water, and subjecting the ammonia solution so obtained to distillation by utilizing two distillation columns operating at different pressures.

Preferably the tertiary reforming is of the mixing type, i.e. of the type in which the reformed gaseous mixture, which forms by flowing inside the pipes containing the catalyst of the tertiary reformer, directly mixes, before the heat exchange occurs, with the gaseous reaction mixture leaving the secondary reforming, such mixture providing, from the outside of the pipes, the heat necessary for the tertiary reforming.

In such a way the tertiary reformer provides a considerable constructive simplification, which offers the advantage of having no tube plates (i.e. plates in which the ends of the tubes are set) at high temperature and of having pipes at an optimally balanced pressure in the warmest point and therefore not subjected to mechanical stresses.

The ammonia synthesis is preferably conducted at an absolute pressure lower than 100 Kg/cm2, more preferably at a pressure ranging from 40 to 80 Kg/cm2, so considerably reducing the amount of energy required to compress the synthesis mixture.

Preferably the gas conveyed to the synthesis reactor is dried by employing molecular sieves, the regeneration of which is effected, by stripping of the water and of the residual ammonia adsorbed, by at least a portion of the reacted gas flowing from the ammonia synthesis reactor, which is successively passed to the ammonia absorption with water.

In this manner it is possible to utilize the temperature variation of the gas from the outlet of the synthesis reactor (dry at about 4200 C) to the outlet of the ammonia absorber with water (wet at about 400 C) in order to carry out, in conditions of isobaricity, both operations of gas drying and of regeneration of the adsorbing masses of the molecular sieves without requiring any external supply of heat and of refrigeration units.

Furthermore this permits the ammonia content in the gas passed to the synthesis reactor to be reduced to a minimum, such gas coming from the fresh synthesis mixture produced by the hydrocarbons to which the deammonified gas leaving the ammonia absorber with water is added prior to the drying, which is of great importance in order to obtain a good conversion yield, particularly in an ammonia production plant operating at low pressure.

The ammonia solution deriving from the ammonia absorption with water is subjected to distillation carried out by utilizing two columns operating at different pressures, in which preferably the solution to be distilled is conveyed, after mixing with the ammonia vapours flowing from the top of the column operating at the lower pressure, to the column operating at the higher pressure, from the bottom of which the partially distilled ammonia solution is passed to the column operating at the lower pressure, the liquid ammonia being recovered at the top of the column operating at the higher pressure, while the residual solution is withdrawn at the bottom of the column operating at the lower pressure.

In this way it is possible to carry out the distillation at low temperature values, for example at temperatures approximately ranging from 1 300C to 1400 C, so permitting the utilization of recovery heat at a low thermal level, which is generally available in large amounts in modern ammonia production lines.

The invention will be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 schematically shows an embodiment of the process according to the invention for synthesizing ammonia; Figure 2 schematically shows an embodiment of the drying system of the gas passed to the ammonia synthesis reactor in the process according to the invention; and Figure 3 schematically shows another embodiment of the distillation system of the ammoniacal solution in the process according to the invention.

With reference to Figure 1 , the starting hydrocarbon, consisting of natural gas, fed through a line 1, after preheating in a heater 2 and mixing with steam supplied through a line 3, is conveyed in part, through a line 4, to a primar reformer 5 while the remaining part (about 40%-50%) is conveyed, through a line 6, to a tertiary reformer 7.

Reference numeral 71 indicates air passed to a compressor 13 and to a gas turbine 9, and numeral 72 indicates fuel for the gas turbine 9. Reference numeral 8 indicates hot fumes, rich in oxygen, let off by the gas turbine 9 which operates the process air compressor 13; these fumes are the combustion medium of the furnace of the primary reformer 5. Reference numeral 10 indicates burners of fuel gas 11. Reference numeral 12 indicates air which, after compression in the compressor 1 3 and after preheating in a heater 14, is fed to a secondary reformer 15, to which secondary reformer a gaseous mixture 1 6 leaving the primary reformer 5 is fed.

Air feeding is suitably proportioned so that the final gaseous mixture may have the composition required for ammonia synthesis.

A gaseous mixture 1 7 at a high temperature leaving the secondary reformer 1 5 is made to flow to the tertiary reformer 7. The tertiary reformer 7 is of the mixing type, i.e. of the type in which the reformed mixture, which is produced by flowing inside tubes 1 8 containing a catalyst, directly mixes, prior to the heat exchange, with the gaseous mixture 1 7 which provides the heat necessary for the reforming.

A gaseous reaction mixture 1 9 leaving the reformer 7 is conveyed, after cooling in a cooler 20, to a high temperature converter 21 and from there, after further cooling in a cooler 22, to a low temperature converter 23. The known conversion reaction, between carbon monoxide and steam, to hydrogen and carbon dioxide, occurs in the reactors 21 and 23.

The process gaseous mixture is conveyed from the converter 23, after cooling in coolers 24 and 25, to an absorption column 26, where, as is known, it is purified from most of the carbon dioxide contained in the mixture by means of a suitable known solution fed through a line 27. The solution 28 flowing from the bottom of the column 26 is regenerated in a known manner, not shown in the drawing.

The process gaseous mixture, or synthesis gas, is then passed from the top of the column 26 to a methanation reactor 29, after preheating in a heat exchanger 30, using the heat of the gaseous mixture leaving the reactor, and in an exchanger 31. The known catalytic methanation reactions occur in the reactor 29; such reactions are exothermic and, as is known, lead to the elimination, by reaction with hydrogen, of the carbon monoxide and carbon dioxide still contained in the gaseous mixture.

The synthesis gas so generated is further cooled in a cooler 32. This gas, which constitutes the fresh synthesis mixture produced by the hydrocarbons, is then compressed to an absolute pressure below 100 Kg/cm2 by a compressor 33.

Such gas, to which the de-ammonified gas flowing from the top of an absorber 34 of ammonia with water is added, is conveyed, after compression by means of a circulator 35, to a molecular sieve 36 which, in the operation cycle illustrated in the figure, is in the adsorption stage. Here the gas is dried before being passed to an ammonia synthesis reactor 37.

A drying system 38 consists of two molecular sieves 36 and 39; in such a system while one molecular sieve is in the adsorption stage, the other is in the regeneration stage.

In the operation cycle shown in the figure, the water contained in the gas is adsorbed in the molecular sieve 36 and the gas, so dried, is made to flow to the ammonia synthesis reactor 37 after having been preheated in heat exchangers 40 and 41.

The reacted gas, flowing from the reactor 37, enters the exchanger 41 where it cools down to a temperature suitable for the regeneration of the molecular sieves, so preheating, as mentioned hereinbefore, the dried gas coming from the molecular sieve 36.

The reacted gas leaving the exchanger 41 is passed to the molecular sieve 39 which, in the operation cycle shown in the figure, is in the regeneration stage.

During such regeneration the gas effects the stripping of the water as well as of the residual ammonia, previously adsorbed when the molecular sieve 39 was operating in the synthesis gas drying stage.

The gas leaving the molecular sieve 39, after having being further preheated, in the heat exchanger 40, the dried gas flowing from the molecular sieve 36, is cooled in a cooler 42 and then passed to the stage of absorption of ammonia with water in the absorber 34.

Aqueous absorption solution passed through a line 43 by means of a pump 44, from the distillation of the ammonia solution -- as will be described hereinafter -- is fed to the top of the absorber 34, from the bottom of which ammonia solution is drawn through a line 45 to be sent to distillation.

As mentioned hereinabove, de-ammonified gas flows from the top of the absorber 34 through a line 46 and is added to fresh synthesis mixture at a point 47. According to such a drying cycle, all the water which during the ammonia absorption in the absorber 34, goes to saturate the de-ammonified gas, as well as the water, if any, contained in the fresh synthesis mixture, is made to flow back to absorption in a simple and effective manner.

The residual ammonia adsorbed, if that is the case, during the gas drying stage, is also conveyed back to the absorption.

The ammonia solution withdrawn through the line 45 at the bottom of the absorber 34 is passed by a pump 48 to a cooler 49, wherefrom a portion 50 is recycled to the absorber 34 and a portion 51 is sent to distillation.

Distillation is carried out in two columns 52 and 53 operating at different pressures.

The portion 51 of the ammonia solution to be distilled, after mixing with the ammonia vapours flowing from the top of the column 53 operating at a pressure lower than the one of the column 52, is expanded and conveyed to a condenser 54 (preferably of the down-flowing-film type), in which the vapours are condensed.

The resulting ammonia solution, more concentrated in ammonia than the starting solution, is passed through a pump 55 to the column 52 operating at the higher pressure, after having been preheated in a heater 56 at the expense of the residual solution drawn from the bottom of the column 53.

The maximum distillation pressure (higher pressure) depends on the temperature of the cooling water available and, on average, it is assumed to be equal to about 17-18 Kg/cm2 abs.

The minimum pressure (lower pressure) depends on the thermal level of the heat available, on the final concentration of the ammoniacal solution to be obtained, and also on the temperature of the cooling water; on average, a pressure of about 5 Kg/cm2 abs. can be considered as optimum, although such a value may vary depending on circumstances.

In the distillation column 52, preferably of the down-flowing-film type in its lower section and of the tray in its upper section, such solution is partially distilled up to an ammonia concentration consistent with the operating pressure and with the maximum temperature to be reached (for example 26% of ammonia at 1 300C and 18 Kg/cm2 abs.).

From the bottom of the column 52 the partially distilled ammonia solution is passed to the column 53 operating at the lower pressure, where its ammonia concentration is reduced to the desired value (for example 10% by weight).

The ammonia vapours rising in the lower section of the column 52 are rectified in the upper traytype section and condensed in a reflux condenser 57, in order to obtain liquid ammonia 58 at 99.9% by weight.

The column 53 operating at the lower pressure is also preferably of the down-flowing-film type.

The residual solution in the line 43, coming from the bottom of the column 53, after heat recovery in the heater 56 and final cooling in a cooler 59, is passed back to ammonia absorption at the top of the absorber 34.

The ammonia vapours released in the column 53 are mixed, as already explained, with ammonia solution to be distilled in the line 51.

By virtue of the cycle described hereinbefore it is possible to distil the ammonia solution supplying heat at a low thermal level (for example temperatures in the approximate range of 1 300C to 1400C).

The utilization of film type distillers allows not only the heat and substance exchange to occur on one surface only, but also (and this is very important) heat to be supplied in countercurrent at mean levels below the maximum distillation temperature.

Reference numerals 64 and 65 indicate the heats at low thermal value which are supplied to the columns 52 and 53 respectively.

A portion of the de-ammonified gas flowing from the top of the absorber 34 through the line 46 is purged in a line 60 and conveyed to a hydrogen recovery unit 61, which comprises a cryogenic fractionation unit. Recovered hydrogen is recycled through a line 62 to the fresh synthesis mixture, while the remaining fraction is passed through a line 63 for utilization as fuel gas.

In a plant of the type shown in the figure, having available natural gas at a pressure of 45 Kg/cm2 abs., carrying out the preparation of the synthesis gas at about 40 Kg/cm2 abs. and the ammonia synthesis at 60 Kg/cm2 abs. with production of un-refrigerated liquid ammonia at 99.9%, an efficiency degree equal to about 65% has been attained.

Figure 2 shows another embodiment of the drying system 38 illustrated in Figure 1; in such a system the gas drying is effected, before sending the gas to the synthesis reactor, by utilizing four molecular sieves, which individually, and alternately in each operation cycle, are one in the regeneration stage, one in the heating stage, one in the cooling stage and one in the adsorption stage. The molecular sieve in the heating stage is heated by using the heat withdrawn from the molecular sieve in the cooling stage.

With reference to Figure 2, where numerals 101, 102, 103 and 104 indicate the four molecular sieves, the reacted gas leaving an ammonia synthesis reactor 105 passes through a heat exchanger 106, where it is cooled down to a temperature suited to the regeneration of the molecular sieves, with simultaneous pre-heating of the dried gas flowing from the molecular sieve 1 04 which, in the operation cycle shown in the figure, is in the adsorption stage; the dried gas so pre-heated is sent to the synthesis reactor 105.

The reacted gas leaving the exchanger 106 is passed to the molecular sieve 101 which, in the operation cycle shown in the figure, is in the regeneration stage; the gas flowing from the molecular sieve 101, after having been further pre-heated in a heat exchanger 107, the dried gas coming from the molecular sieve 104, is cooled in a heat exchanger 108 and then passed to the absorption of ammonia with water in an absorber 1 09.

Water necessary for the absorption is fed through a line 110 to the top of the absorber 1 09, from the bottom of which ammonia solution is drawn through a line 111 and sent to distillation.

De-ammonified and water-saturated gas flows through a line 112 from the top of the absorber 109; such gas, after compression by means of a circulator 113, is passed to the molecular sieve 104 which, as explained hereinbefore, is in the adsorption stage.

The fresh ammonia synthesis mixture, coming from the unit for the generation of the synthesis gas from hydrocarbons, is added at 114 to the de-ammonified gas in the line 112 before conveying such gas to the molecular sieve 104.

In the molecular sieve 104 the water contained in the gas is fully adsorbed.

Most of the gas so dried, about 95%, is conveyed through a line 11 5 to the ammonia synthesis reactor 105 after having been pre-heated in the heat exchangers 107 and 106; a part of the dried gas coming from the molecular sieve 104, about 5% is passed, conversely, through a line 116 to the molecular sieve 103 which, in the operation cycle shown in the figure, is in the cooling stage, and from the sieve 103 it is passed to the molecular sieve 102 which, always in the operation cycle shown in the figure, is in the heating stage.

In fact the gas passed through the line 11 6 withdraws heat from the hot mass of the molecular sieve 103, regenerated in the preceding operation cycle as will be explained hereinafter, and transfers such heat to the cold mass of the molecular sieve 102 to be regenerated in the successive operation cycle, as will be explained hereinafter.

The gas flowing from the molecular sieve 102 is admixed, through a line 11 7, with the gas leaving the molecular sieve 101 which, as indicated hereinbefore, is in the regeneration stage, and it is passed, along with the latter gas, to the absorber 109.

In this way all the water which, during the ammonia absorption in the absorber 109, goes to saturate gas supplied through the line 112, as well as the water, if any, contained in the fresh mixture supplied through the line 114, is conveyed back to the absorption in a simple and effective manner.

Also the residual ammonia optionally adsorbed during the drying stage of the gas supplied through the line 11 2 is sent back to absorption.

In fact, as previously specified, both the gas flowing from the molecular sieve 101 and the gas flowing from the molecular sieve 102 are passed to the absorber 1 09.

After a pre-fixed time, the above-described operation cycle of the molecular sieves 101, 102, 103 and 1 04 is commuted in such a manner that, subject to the foregoing, it is the molecular sieve 102 which is in the regeneration stage while the molecular sieves 101, 103 and 104 are respectively in the cooling stage, in the adsorption stage and in the heating stage.

Such commutation is effected with known means, not indicated in the figure for the sake of simplicity, and it is carried out, always referring to the operation cycle shown in the figure, when the temperature of the molecular sieve 102 is close to that of the molecular sieve 101 and the temperature of the molecular sieve 1 03 is nearly equal to the temperature of the molecular sieve 1 04.

The commutation of the operation cycle occurs in a simple way without causing thermal unbalances. Such commutation does not cause pressure unbalances since the operation cycle is of the isobar type, as the molecular sieve which is in the adsorption stage operates on the delivery of the circulator 113, while the other three molecular sieves are balanced by the pressure existing at the outlet of the synthesis reactor 1 05. Possible pollution of the gas dried by the wet gas is therefore rendered impossible.

The frequency of commutation of the operation cycle of the molecular sieves in general is very high, for instance every hour, so as to achieve a drastic reduction of the dimensions of the molecular sieves themselves.

Nevertheless the duration of the operation cycle, as well as the regeneration temperature of the molecular sieves, are chosen as a function of the type of the adsorption mass of the molecular sieves.

With reference to Figure 2, the possible sequences of the operation cycle of the molecular sieves 101,102,103 and 104 are four, as set out in the following table:

Operation sieve in Operation cycle Molecular sieve in the stage of: A B C D adsorption 104 103 101 1P2 cooling 103 101 102 104 heating 102 104 103 101 regeneration 101 102 104 103 From the above table it can be seen that operation cycle A, which is the one shown in Figure 2 and previously described in which, while the molecular sieve 101 is in the regeneration stage, the molecular sieve 104 is in the adsorption stage and the molecular sieves 103 and 102 are respectively in the cooling stage and in the heating stage, is followed by operation cycle B, then by C and finally by D which precedes A and so on; in this way the four molecular sieves 101, 102, 103 and 104 are individually, an alternately in each operation cycle, one in the regeneration stage, one in the heating stage, one in the cooling stage and one in the adsorption stage.

In many cases it is also necessary to produce, for storage requirements, liquid ammonia refrigerated at -330C and at atmospheric pressure. In such cases the distillation cycle shown in Figure 1 is the one illustrated in Figure 3, in which the same reference numerals of Figure 1 indicate similar parts.

With reference to Figure 3, liquid ammonia 58 recovered at the top of the column 52 operating at the higher pressure is expanded in two stages, in two tanks 68 and 69, the former being balanced with the pressure of the column 53 and the latter with the atmospheric pressure.

The vapours evolved in the tank 68 are mixed with ammoniacal solution to be distilled from the line 51 and are then condensed in the condenser 54, while the vapours evolved in the tank 69 are mixed with a part of the residual solution supplied through the line 43 and then condensed in a condenser 66 wherefrom, by means of a pump 67, they too are mixed with ammoniacal solution to be distilled supplied through the line 51.

The refrigerated liquid ammonia is recovered at 70.

The term "molecular sieve" as used throughout this specification relates to a known drying agent, see for example Kirk-Othmer, vol. 7, 1965, "Encyclopaedia of Chemical Technology", pages 393-394.

Claims

1. A process for synthesizing ammonia, comprising generating synthesis gas from one or more starting hydrocarbons by primary reforming of the said hydrocarbons with steam and by secondary reforming of the resulting gaseous mixture with oxygen or an oxygen-containing gas, subjecting a part of the starting hydrocarbon mixed with the steam to a tertiary reforming in which the necessary heat is provided by the reaction gaseous mixture leaving the secondary reforming, causing the ammonia synthesis to occur at low pressure with drying of the gas conveyed to the synthesis reactor by employing molecular sieves, causing the ammonia contained in the reacted gas to be absorbed with water, and subjecting the ammonia solution so obtained to distillation by utilizing two distillation columns operating at different pressures.

2. A process as claimed in Claim 1, wherein the tertiary reforming is of the mixing type.

3. A process as claimed in Claim 1 or 2, wherein the ammonia synthesis is conducted at an absolute pressure lower than 100 Kg/cm2.

4. A process as claimed in Claim 3, wherein the ammonia synthesis is conducted at an absolute pressure of from 40 to 80 Kg/cm2.

5. A process as claimed in any of Claims 1 to 4, wherein the gas conveyed to the synthesis reactor is dried by employing molecular sieves, the regeneration of which is effected by at least a part of the reacted gas flowing from the ammonia synthesis reactor, such gas being then passed to the absorption of ammonia with water.

6. A process as claimed in Claim 5, wherein the said drying is obtained by employing four molecular sieves which individually, and alternately in each operation cycle, are one in a regeneration stage, one in a heating stage, one in a cooling stage and one in an adsorption stage.

7. A process as claimed in Claim 6, wherein the reacted gas leaving the ammonia synthesis reactor, before being passed to the molecular sieve in the regeneration stage, is cooled down by preheating the dried gas coming from the molecular sieve in the adsorption stage.

8. A process as claimed in Claim 6 or 7, wherein the reacted gas leaving the molecular sieve in the regeneration stage, before being passed to the absorption of ammonia with water,pre-heats the dried gas flowing from the molecular sieve in the adsorption stage.

9. A process as claimed in any of Claims 6 to 8, wherein heating of the molecular sieve in the heating stage is effected by using the heat withdrawn from the molecular sieve in the cooling stage.

10. A process as claimed in Claim 9, wherein a part of the dried gas coming from the molecular sieve in the adsorption stage is passed to the molecular sieve in the cooling stage and therefrom to the molecular sieve in the heating stage, transferring to the latter sieve the heat withdrawn from the preceding one.

11. A process as claimed in Claim 10, wherein the said gas flowing from the molecular sieve in the heating stage is conveyed to the ammonia absorber.

12. A process as claimed in any of Claims 6 to 11 , wherein the gas leaving the absorption of ammonia with water is conveyed, after compression by means of a circulator and after addition of fresh synthesis mixture, to the molecular sieve in the adsorption stage.

13. A process as claimed in any of Claims 1 to 12, wherein the ammonia solution obtained from the absorption of ammonia with water is subjected to distillation by using two columns operating at different pressures, in which the said solution to be distilled, after mixing with the ammonia vapour flowing from the top of the column operating at the lower pressure, is conveyed to the column operating at the higher pressure, from the bottom of which the partially distilled ammonia solution is passed to the column operating at the lower pressure, and wherein the liquid ammonia is recovered at the top of the column operating at the higher pressure while the residual solution is withdrawn at the bottom of the column operating at the lower pressure.

14. A process as claimed in Claim 13, wherein the ammoniacal solution to be distilled mixed with the ammoniacal vapour flowing from the top of the column operating at the lower pressure is subjected to condensation before being passed to the column operating at the higher pressure.

1 5. A process as claimed in Claim 14, wherein the said condensation is effected in a condenser of the down-flowing-film type.

16. A process as claimed in any of Claims 13 to 15, wherein the ammoniacal solution to be distilled mixed with the ammoniacal vapour flowing from the top of the column operating at the lower pressure is pre-heated, before being passed to the column operating at the higher pressure, by the residual solution withdrawn from the bottom of the column operating at the lower pressure.

1 7. A process as claimed in any of Claims 13 to 1 6, wherein the liquid ammonia recovered at the top of the column operating at the higher pressure is caused to expand, so obtaining refrigerated liquid ammonia, while the vapours evolved by the said expansion are mixed with the ammoniacal solution to be distilled.

1 8. A process as claimed in Claim 17, wherein the vapours evolved by the said expansion are mixed after condensation with the ammoniacal solution to be distilled.

1 9. A process as claimed in any of Claims 1 3 to 16, wherein the liquid ammonia recovered at the top of the column operating at the higher pressure is caused to expand in two stages, the first being balanced with the pressure of the column operating at the lower pressure and the second being balanced with the atmospheric pressure, and wherein the vapours evolved in the first stage are mixed with the ammoniacal solution to be distilled, while the vapours evolved in the second stage, mixed with a part of the residual solution withdrawn from the bottom of the column operating at the lower pressure, are condensed and mixed with the ammoniacal solution to be distilled.

20. A process as claimed in any of Claims 1 3 to 19, wherein the column operating at the lower pressure is of the film type, while the column operating at the higher pressure is of the film type in its lower section and of the tray type in its upper section.

21. A process according to Claim 1 for synthesizing ammonia, substantially as herein described with reference to the accompanying drawings.