NL8100003A - METHOD FOR PREPARING AMMONIA. - Google Patents

Description

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Process for preparing ammonia.

This invention relates to a process for preparing ammonia consisting of a first reaction in which synthesis gas is formed from hydrocarbons and steam and a second reaction of the gas mixture thus formed with oxygen.

As is known, the reaction of hydrocarbons with steam is called the first conversion and is endothermic, and the reaction of hydrocarbons with oxygen, which is usually used in the form of air, is used directly to provide the nitrogen-containing synthesis mixture (3 ^ +, the second conversion is mentioned and is exothermic.

As is also known, ammonia is one of the mass products with a high energy content, the formation of which consumes a great deal of energy. However, the actual energy consumption for the preparation of ammonia is very much higher than the theoretical minimum amount, even if one recovers energy. The ratio between the theoretical need and the actual energy consumption is the measure of the efficiency of the method.

The energy shortage in the world and the ever increasing energy price make people feel the need to limit the consumption of energy sharply, also in the preparation of ammonia.

The recent large ammonia plants, based on steam hydrocarbon conversion and built integrally to recover this energy with steam turbines driving the main machines, exhibit an efficiency of 50-55%, which means that despite the effort put in and the improvements already made, the actual energy consumption is still about double what is theoretically required at least 30 times. It remains therefore important to further reduce the energy consumption of ammonia plants.

The present invention provides a method of high efficiency in which the energy consumption in the production of ammonia from hydrocarbons is further reduced. This, as well as other purposes, is accomplished by a process characterized in that it comprises a third reaction in which part of the starting hydrocarbons are mixed with steam, 5 and the required heat being provided by the gaseous reaction mixture from the second reactor is because the ammonia synthesis at low pressure takes place with gas that has been dried by molecular sieves, because the ammonia from the converted. gas is absorbed with water, and in that the ammonia solution thus obtained is distilled in two distillation columns operating at different pressures.

Preferably, the third reaction is of the mixed type, that is of the type in which the reacted gas mixture formed in the catalyst-containing pipes of the third reactor is mixed immediately before heat exchange occurs with the gaseous reaction mixture from the second reactor. conversion, which mixing provides the heat needed for the third conversion. In this way, the third conversion represents a constructional simplification that offers the advantage of no high temperature plate bundles and with pipes having a perfect pressure equilibrium at their hottest points and thus not exposed to mechanical stresses.

The ammonia synthesis takes place at an absolute pressure below 100 atmospheres, preferably between 40 and 80 atmospheres, which represents a considerable energy saving for compressing the synthetic mixture.

The gas to be fed to the synthesis reactor is dried by means of molecular sieves which are regenerated by expelling absorbed water and residual ammonia through at least part of the converted gas coming from the ammonia synthesis, and then will go to the ammonia absorption.

In this way it is possible to utilize the temperature difference between the from the synthesis reactor (dry and about 420 ° C) and the ammonia absorption (wet, about 40 ° C) to, in the absence of pressure differences, both operations of gas drying and regeneration of the absorbent without having to supply heat or cool from outside 8100003-3. Furthermore, this makes it possible to minimize the amount of ammonia that goes with the gas to the synthesis reactor; such gas is a fresh gas mixture from the synthesis created from hydrocarbons mixed with ammonia-free gas coming from the ammonia absorption with water; this is very important to obtain a good degree of conversion, especially in a low pressure ammonia synthesis.

The ammonia solution emerging from the ammonia absorption with water is distilled into two columns operating at different pressures, the solution to be distilled after mixing with the ammonia vapors from the top of the lower pressure column at the top of the higher pressure column. operating column, and the higher pressure column passes partially distilled ammonia solution to the lower pressure column, dry ammonia recovered from the top of the higher pressure column and residual solution under the lower pressure column is removed. In this way it is possible to carry out the distillation at low temperatures, for instance between 130 ° and 140 ° C, so that the waste heat can be used at the low temperature level that occurs on a large scale in modern ammonia plants.

The invention will now be explained in more detail with reference to the accompanying figures, of which figure 1 schematically represents an embodiment of the method according to the invention, figure 2 schematically represents an embodiment of the drying system of the gas to be fed to the ammonia synthesis, and 30 in which figure 3 schematically represents another embodiment of the distillation of the ammonia solution.

According to figure 1, the starting hydrocarbon, usually natural gas, is supplied at 1, and after preheating mixed in 2 at 3 with steam, it is partly led via line 4 to the first conversion 5, while the rest (40 to 50%) via line 6 to the third conversion 7. 71 indicates the air 8100003 - 4 - which goes to compressor 13 and gas turbine 9, and 72 indicates the fuel for this gas turbine 9. Number 8 indicates the hot, oxygen-rich gases coming from the compressor 13 driving gas turbine 9, these the combustion medium for the first conversion 5, while 10 indicates the burners for gas 11. 12 also represents the air, which after compression in 13 and before heating in 14, goes to the second conversion 15, to which also goes the mixture 16 coming from the first reactor 5. The amount of air is chosen such that the final gas mixture 10 has approximately the composition required for the ammonia synthesis.

The gas mixture 17 coming out of the second reactor 15 at high temperature goes to the third reactor 7. This is of the mixing type, that is of the type in which the converted mixture, 15 formed in the catalyst-containing tubes 18, immediately preceding some heat exchange is mixed with the gas mixture 17 which provides the heat required for this conversion.

The gas mixture 19 coming from reactor 7, after cooling in 20, goes to the high temperature converter 21 and from there 20, after further cooling in 22, to the low temperature converter 23. In 21 and 23 the "preservation" takes place. instead, that is the reaction of carbon monoxide with steam to hydrogen and carbon dioxide.

After cooling in24 and 25, the gas mixture goes from converter 23 to absorption column 26, in which it is freed from the major part of the carbonic acid in a known manner using a suitable, known per se solution, which is supplied via 27. This solution comes at the bottom of the column at 28, and is regenerated in known manner, not shown in this drawing.

The gas mixture, or "synthesis gas", then passes at the top of column 26 to the methanation reactor 29, and after preheating in heat exchanger 30 (utilizing the heat from the gas mixture that will leave that reactor) and then through heat exchanger 31.

The known "methanation" takes place in ereactor 29, which is exothermic and, by reaction with hydrogen, leads to the removal of residual carbon monoxide and carbon dioxide residues in the gas mixture. The synthesis gas thus obtained is introduced in 32 This gas, the fresh starting material for the synthesis, is then compressed to an absolute pressure below 100 atmospheres by compressor 33. To this is added the supernatant column 34, liberated from ammonia, and they are added together. , after compression by pump 35, is passed to molecular sieve 36, which is in the working position at the stage shown in the figure, in which the gas is dried before it goes to the synthesis reactor 37. The drawing shown in the figure drying system 38 consists of two molecular sieves 36 and 39, one of which is working and the other is being regenerated in the meantime The thus dried gas, after preheating in heat exchangers 40 and 41, goes to the synthesis reactor 37. The gas coming from the ammonia synthesis 37 is first cooled in heat exchanger 41 so that it is suitable for regenerating the molecular sieves, and thus heats up the just dried gas. It then passes through one of the molecular sieve columns (39 in the drawing phase), from which the absorbed water and residual ammonia expels. Then it passes through heat exchanger 40, in which it is further cooled, and in which the dried gas receives its first preheating.

Then it enters the ammonia absorption, which takes place in column 34. At the top of column 34, aqueous solution 43 is supplied by means of pump 44, which originates from the ammonia distillation to be discussed below.

Ammonia solution 45 is removed from the bottom of the absorption column to the distillation. As already said, gas 46 freed from ammonia is discharged from the top of column 34, which is mixed with fresh starting mixture 47.

In this cycle, all the water that ends up in that gas through saturation of the gas from which ammonia is absorbed is returned to the starting mixture in a simple and effective manner. Any unabsorbed ammonia residues are also included.

The ammonia solution 45 drained from the bottom of column 34 is pumped by pump 48 to cooler 49, from which one part 50 is returned to absorption column 34 and another part 51 goes to the distillation. The distillation is done in two columns 52 and 53 operating at different pressures.

The ammonia solution 51.5 to be distilled is allowed to relax after mixing with the ammonia vapors exiting from the column 53 operating at a lower pressure than column 52 and then placed in condenser 54 (which is preferably of the falling film type is), in which the vapors are condensed again. The ammonia solution thus formed is stronger than the starting solution, 10 and is brought by pump 55 to column 52 operating at a pressure higher than 53, but is preheated there first in 56 at the expense of the residual heat of the solution which is column 53 is added.

The higher distillation pressure depends on the temperature of the available cooling water, but will usually be 17-18 atmospheres. The lower pressure depends on the level at which the heat is available, the final concentration of the ammonia solution and the temperature of the cooling water; in general, 5 atmospheres will be optimal, but this can vary from case to case.

In distillation column 52, preferably at the bottom of the falling film type and at the top of the tray type, the solution is partially distilled to an ammonia concentration corresponding to the operating pressure and the maximum temperature to be reached (e.g. 26% ammonia at 130 ° C and 18 atmosphere absolute). From the bottom of column 52, the partially distilled ammonia solution is discharged and passed to column 53 operating at lower pressure, in which the ammonia concentration is reduced to the desired value (e.g. 10% by weight). The ammonia vapors rising in the lower part 30 of column 52 are rectified in the upper part, with the dishes, and condensed in reflux condenser 57, so that liquid ammonia of 99.9% by weight can be discharged via 58.

Also the column 53 operating at lower pressure is preferably of the descending film type.

Residual solution 43, which is discharged from the bottom of column 53, 8100003 * - 7 -, after heat has been extracted in 56 and after cooling in 59, goes back to the ammonia absorption in column 34.

The ammonia vapors released from the top of column 53 return, as already stated, to the distillation with the ammonia solution 51.

Thanks to the cycle described above, it is possible to distill the ammonia solution with heat offered at a low level (for example from only 130 ° to 140 ° C). The use of film-type distillation columns allows not only exchange of heat and substances 10 on one and the same surface, but also (which is very important) to supply the heat in countercurrent at a level below the maximum distillation temperature. The heatings of columns 52 and 53 with low value heat are indicated by 64 and 65.

A portion of the ammonia-free gas 46 released from the top of column 34 is vented through 60 and passed to the hydrogen recovery unit 61, which comprises a cryogenic fraction. Recirculated hydrogen is mixed in part (62) with fresh synthesis gas, while the remaining part 63 serves as fuel.

In a device of the type described, synthesis gas was prepared from natural gas at a pressure of 45 atmospheres at a pressure of 40 atmospheres and ammonia at a pressure of 60 atmospheres, whereby uncooled ammonia of 99.9% was obtained and an efficiency degree of 65% reached.

Figure 2 shows a different embodiment of the drying system than what is shown in figure 1; in this system, the gas is dried before it goes to the synthesis reactor, using 4 molecular sieves, 30 of which are always regenerated, one is heated, one is cooled, and one is working. During heating, heat is used which is extracted from the molecular sieve that is just cooling down.

In Fig. 2, 101, 102, 103 and 104 represent four columns of molecular sieve and the gas from the synthesis reactor 105 first passes through heat exchanger 106, where it is cooled to a temperature suitable for the regeneration of molecular 8100003-8.

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sieving with simultaneous preheating of the gas dried in another tower (in this case from no. 104).

The converted gas coming from heat exchanger 106 goes to molecular sieve 101 which is in regeneration; the gas coming out of molecular sieve 101, after further heat release in heat exchanger 107, goes to heat exchanger 108, in which it is cooled, and then to the ammonia absorption in column 109. Via 110, the water required for absorption is supplied, tower, ammonia solution 111 is drawn off, which goes to the distillation. Gas 112, liberated from ammonia and saturated with water, is delivered to the top of absorption column 109, and this gas, after compression by means of a fan 113, goes to molecular sieve 104, where absorption is now occurring.

The ammonia synthesis starting mixture, which comes from the synthesis gas preparation section, is mixed at 114 with ammonia-free gas from line 112, and then goes to molecular sieve 104. The water present is completely adsorbed.

Most of the thus dried gas, about 95%, passes through line 115 to the synthesis reactor 105 after it has passed heat exchangers 107 and 106. However, some of this, about 5%, is passed through line 116 to molecular sieve 103 which is cooling at the separated phase and from there the gas passes to molecular sieve 102 which is just heating up. The gas passing through line 116 extracts heat from the hot molecular sieve 103, which was regenerated in the previous phase, and transfers this heat to the cold molecular sieve 102, which will be regenerated in the next cycle.

The gas that has passed through molecular sieve 102 passes through line 117 to a point where it is mixed with the gas that passes through molecular sieve 101 (just in regeneration), and goes there together to the ammonia absorption.

In this way, all the water that saturates the gas 112 in the absorption tower 109, as well as any water in the starting mixture 114, is recycled into the operation in a simple and effective manner. Any ammonia residues adsorbed during drying thus also return to the absorption. In fact, both the main portion goes through 101 and the minor amount through 102 and 103 to the absorption in 109.

After a while, the feeds to molecular sieves 101, 102, 103 and 104 are changed in such a way that now molecular sieve 102 enters the regeneration, and no. 101, 103 and 104 enter cooling, adsorption and heating phases, respectively. .

Such an exchange of feeds takes place with aids known in the art which are not shown in the drawing for the sake of simplicity. This happens when (at the indicated mode of connection) when the temperature of molecular sieve 102 has come close to that of molecular sieve 101 and the temperature of molecular sieve 103 is almost equal to that of molecular sieve 104.

The connection is changed in a 15-way manner without disturbing the thermal balance. It also does not disturb the pressure equilibrium since the whole operation is isobaric for the molecular sieve in adsorption, while the other three molecular sieves remain balanced by the pressure at the outlet of synthesis reactor 105; this makes possible contamination of the dry gas by the wet gas impossible.

Switching the feed to the molecular sieves generally happens very often, for example every hour, so that the dimensions of the molecular sieves can be taken very small. Nevertheless, the duration of the operating cycle and the regeneration temperature of the molecular sieves are selected depending on the nature of the adsorbent in the molecular sieves.

A possible diagram of connections in the arrangement according to figure 2 is as follows: 30 _

Molecular sieve _Operation cycle_ in the phase of: _A_B_C_D_ adsorption 104 103 101 102 cooling 103 101 102 104 35 heating 102 104 103 101 regenerating 101 102 104 103 8100 003 - 10 -

This shows that during processing cycle A shown in Figure 2, molecular sieve 101 is in regeneration, molecular sieve 104 in adsorption, and molecular sieves 103 and 102 are cooling and heating, respectively. This is followed by cycle B, cycle C and finally cycle D, after which cycle A returns. In this way, all molecular sieves go through the regeneration, heating, cooling and adsorption successively.

In many cases it is also necessary for good storage to store liquid ammonia cooled to -33 ° C at ordinary pressure. In those cases a distillation according to figure 3 is used, in which the reference numerals are the same as in figure 1. It is seen that liquid ammonia is discharged in two phases through 58 from the higher pressure column 52, namely in barrels 68 and 69, the former of which is under the same pressure as column 53 and the latter under atmospheric pressure. The vapors released in vessel 68 are mixed with ammonia solution from 51, distilled and then condensed in 54, while the vapors released from vessel 69 are mixed with part of the remaining solution 53 and then condensed in condenser 66, from which they are also removed to the distillation with the aid of pump 67. The cooled liquid ammonia is vented through 70. 1 81 0000 3

Claims (18)

1. A process for the preparation of ammonia, consisting of a first reaction in which synthesis gas is formed from hydrocarbons and steam and a second reaction of the gas mixture thus formed with oxygen, characterized in that it comprises a third reaction in which a part of the starting hydrocarbons are mixed with steam, and the necessary heat is provided by the gaseous reaction mixture coming from the second reaction, the ammonia synthesis is carried out with gas 10 which has been dried by molecular sieves, the ammonia from the reacted gas mixture is absorbed with water, and the ammonia solution thus obtained is distilled in two distillation columns operating at different pressures. 2. Method according to claim 1, characterized in that the third conversion is of the mixing type. A method according to claim 1 or 2, characterized in that the ammonia synthesis takes place at a pressure below 100 atmospheres absolute, preferably between a pressure between 40 and 80 atmospheres. 4. Process according to any one of the preceding claims, characterized in that the gas fed to the synthesis reactor is dried by means of molecular sieves, the regeneration of which takes place at least in part with converted gas coming from that synthesis reactor, which gas then goes to the ammonia absorption. Method according to claim 4, characterized in that the drying is carried out with four molecular sieves which alternate through the four stages of regeneration, heating, cooling and adsorption. Process according to claim 5, characterized in that the converted gas from the synthesis reactor is cooled by exchange with dried gas from a molecular sieve before it goes to the molecular sieve to be regenerated. 7. Process according to claim 5, characterized in that the converted gas coming from the regenerating molecular sieve heats the dried gas from the molecular sieve by heat exchange 8100003-12 before it goes to the ammonia absorption. 8. A method according to claim 5, characterized in that the heating of the molecular sieve in the heating stage is done with heat which is extracted from the molecular sieve which is cooling. 9. Process according to claim 8, characterized in that part of the dried gas from the molecular sieve which is in the absorption stage is sent to the cooling molecular sieve and from there to the warming molecular sieve, so that it transfers heat to the latter which is withdrawn from the penultimate. Method according to claim 9, characterized in that this gas stream, after it has left the warming molecular sieve, goes to the ammonia absorption. Process according to any one of claims 5 to 10, characterized in that the gas which has passed through the ammonia absorption after compression and further addition of fresh starting mixture is introduced into the molecular sieve which is in the absorption stage. Method according to any one of the preceding claims, characterized in that when the ammonia solution obtained is distilled in the two columns operating at different pressures, the solution to be distilled after mixing with ammonia vapors from the top of the column lower pressure, are fed to the higher pressure column, and ammonia solution is passed from the bottom of the higher pressure column to the lower pressure column, and liquid ammonia is added to the higher pressure column column and residual solution from the bottom of the lower pressure column. Method according to claim 12, characterized in that the ammonia solution to be distilled is subjected to condensation together with the ammonia vapors from the top of the column operating at lower pressure, preferably in a condenser of the falling film type before being placed in the higher pressure column 35. . 14. A process according to claim 12, characterized in that the ammonia solution to be distilled, together with the ammonia vapors from the top of the column operating at lower pressure, with the residual heat from the remaining solution at the bottom of the lower pressure column is preheated before being placed in the higher pressure column. 15. Process according to claim 12, characterized in that the liquid ammonia discharged from the top of the column operating at a higher pressure is relaxed so that liquid ammonia is cooled and the vapors generated during this relaxation are mixed with the ammonia solution to be distilled, possibly after condensation thereof. 16. A method according to claim 12, characterized in that the liquid ammonia discharged from the top of the higher-pressure column is relaxed in two phases, the first of which is under the same pressure as the lower-pressure column and the second of which is under normal pressure. pressure works, that the vapors formed in the first relaxation are mixed with the ammonia solution to be distilled and that the vapors generated in the second relaxation are condensed together with a part of the residual solution discharged from the bottom of the column operating at a lower pressure and with the distillation ammonia solution are mixed. 17. A method according to claim 12, characterized in that the lower pressure column is of the falling film type and the higher pressure column is of the falling film type at the bottom and the tray type at the top. 18. Method mainly according to description and / or examples. 1 8100003