GB2109361A - Process for the production of ammonia - Google Patents

Abstract

In an improved ammonia production a purge taken from a recycle stream of a primary reactor is fed without compression over an ammonia synthesis catalyst comprising a transition metal, eg ruthenium, promoted by alkali or alkaline earth metal on a carbon support having a basal plane surface area of at least 100 m<2>/g, a ratio of BET surface area to basal plane surface area of not more than 5:1 and a ratio of basal plane surface area to edge surface area of at least 5:1

Description

Broad Range Preferred Range Temperature C 250-600 250-400 Pressure bars 50-450 100-350 (PaX 10 0.5-4.5 1-3.5) Space velocity v/v.hr 100-30,000 500-10,000 The second Stage process can be operated as a single pass or with a recycle loop in a manner analogous to that of a conventional process.

In the latter case, the concentration of inert materials tends to build up as before but naturally to a higher level, e.g. about 40-80%, a further build up being prevented by withdrawing a purge stream.

In one preferred embodiment of the present invention the fresh feed to the primary ammonia synthesis reactor has an inert content of greater than 1.4% mol and the purge gas stream is increased suficiently to maintain the concentration of inerts in the feed to the primary ammonia synthesis catalyst at nor more than 13% mol. The advantages of using an increased inerts content in the feed are discussed below.

The invention will now be described with reference to the accompanying drawings in which Fig. 1 is a diagrammatic representative of a conventional purge ammonia recovery stage of a conventional plant, and Fig. 2 is a diagrammatic representation of a purge gas treatment plant with a secondary ammonia synthesis step in accordance with the present invention.

In the conventional purge gas treatment plant purge gas from a conventional primary ammonia synthesis reaction system (not shown) enters through line 1. It gives up heat in a heat exchanger 2 and is further cooled in heat exchanger 3 to about - 33"C.

Ammonia condensed from the purge gas is separated from the purge gas in a gas-liquid separator 4, and is taken off to the ammonia recovery system of the primary ammonia synthesis step (not shown) through line 5. The purge gas leaves separator 4 through line 6 and is fed to heat exchanger 2 where it takes up heat from incoming purge gas. The purge gas may be passed through line 7 to a furnace where it is burnt.

In the purge gas treatment plant in accordance with the invention purge gas from a primary ammonia synthesis plant enters through line 21, and gives up heat in heat exchangers 22 and 23. Ammonia condensing from the purge gas is separated in gas liquid separator 24 and taken off to the ammonia recovery system of the primary plant (not shown) through line 25. Purge gas leaves the separator 24 through line 26 and is fed through line 26 to heat exchanger 22 where it cools incoming gas. A portion of the purge gas is taken off through line 27 and disposed of eg by burning as fuel. The remainder of the purge gas is fed to heat exchangers 28 and 29 where it is heated to reaction temperature before being fed to an ammonia synthesis reactor 30 containing the special transition metal/carbon catalyst required in the process of the present invention.The exit gas from the reactor 30 is then passed to heat exchanger 28 where it heats incoming gas. (Heat is suppled from an external source for heat exchanger 29). The gas is then fed through line 31 to a blower 32 which serves to recycle the gas through line 33 to meet gas entering the purge gas plant through line 21. The mixed gases are then fed to the ammonia recovery section of the plant (22, 23, 24) when ammonia produced in the reactor 30 as well as ammonia in the incoming purge gas is recovered.

It should be noted that the blower 32 is only provided to maintain gas circulation and to compensate for the pressure drop across the various pieces of equipment in the purge gas treatment plant. Gas is fed through line 21 at the pressure at which it is received from the primary synthesis reactor recycle loop and there are no compressors required to raise the pressure above the pressure used in the primary ammonia synthesis reaction.

A catalyst for use in the purge gas treatment plant of the present invention was prepared as follows. An active carbon was heated slowly in helium to a maximum temperature of 1700 C and then cooled. It was then heated in air at 540"C till a weight loss of 20% was obtained. It was then heated slowly in helium gas to 1800"C and then cooled.The resulting carbon had the following characteristics: BET surface area 531 m2/g basal plane surface area 337 m2/g edge surface area 1.3 m2/g BET surface area: basal plane surface area 1.56:1 basal plane surface area:edge surface area 259:1 A catalyst was prepared from this carbon support as follows: Catalyst Preparation a) The ruthenium was added to the carbon as follows: i) the weight of ruthenium trichloride required to give 10% weight ruthenium on the carbon was dissolved in a 50/50 MeOH/5% HCI mixture sufficient to just cover the carbon ii) The solvent was carefully removed by rotary evaporation at approx 60"C/60 torr (7999 Pa) pressure.

iii) The resulting material was dried in a vacuum oven at 110"C for 24 hours.

iv) The dry ruthenium/carbon was reduced in flowing hydrogen (1 Bar (01 5 Pa) gauge, 200 GHSV) for 5 hours at 200"C and subsequently cooled back to 20 C in 200 GHSV nitrogen (GHSV = gas hourly space velocity).

v) The potassium promoter was added from an aqueous solution of potassium nitrite (sufficient to give 20% weight of potassium based on the original carbon) by rotary evaporation.

vi) The resulting catalyst after drying in a vacuum oven at 100"C for 24 hours was ready for use.

In the following examples the compositions of various streams are given both for a primary or main ammonia synthesis plant and for a purge gas treatment plant in accordance with the invention. The flow sheet for the mainammonia synthesis plane is essentially the same as for the purge gas treatment plant described in Fig. 2. Thus in both plants there will be the following streams: Stream 1: The initial gas feed (corresponding to the gas entering in line 21 in Fig. 2).

Stream 2: The combined recycle and initial gas feed passed to the ammonia recovery step (corresponding to the gas feed through line 22 in Fig. 2).

Stream 3: The ammonia recovered from the ammonia recovery unit (corresponding to that taken off through line 25 in Fig. 2).

Stream 4: The gas leaving the ammonia recovery unit (corresponding to the gas in line 26 in Fig. 2).

Stream 5: The gas taken as purge (corresponding to the gas in line 27 in Fig. 2). In the main ammonia synthesis plant stream 5 is the feed to the purge gas treatment plant and corresponds to the feed through line 2 ie stream 1 in the purge gas treatment plant.

Stream 6: The feed to the ammonia synthesis reactor.

Stream 7: The gas leaving the ammonia synthesis reactor (corresponding to the gas in line 31 of Fig. 2).

Example I In a typical conventional main ammonia syntesis plant using a conventional iron based catalyst operated under conventional conditions of temperature and pressure (pressure about 2 X 107Pa) and with a capacity of 1180 tonne/day the composition of the streams mentioned above is as given in Table 1 in Kg mol X 102/hour.

When stream 3 is fed to a purge gas treatment plant using a catalyst as described above the results are given in Table 2 where stream 1 corresponds to stream 5 of the main ammonia synthesis plant.

In a conventional purge gas treatment plant as in Fig. 1, only the ammonia contained in the main plant purge gas stream (stream 5 of Table 1) is recovered. The nitrogen (1031 X 102 kg mol/hour) is wasted by discharge to atmosphere. The hydrogen (3.085 x 102 kg mol/hour) may be recovered cryogenically or molecular sieves at considerable expense, or may be burnt as fuel. In the process according to the present invention only 0.15 X 102 kg mol/hour of nitrogen and 0.48 kg X 102 kg mol/hour of hydrogen is discharged from the process through line 27, ammonia production from the gas treatment plant (stream 3, Table 2) is 1.80 X 102 kg mol/hour. The ammonia available for recovery by the conventional technique of Fig. 1 in stream 5 of Table 1 is only 0.08 X 102 kg mol/hour.

The extra production of ammonia is ca 74 tonnes/day. This is approximately equal to $ 5 X 108 p.a., with no increase in energy consumption. The catalyst charge in the reactor system is ca 7.6 m3.

Example A The ammonia synthesis gas (mixture of N2 and H2) used in conventional ammonia synthesis processes is usually produced by a process involving one or more reforming steps in which a hydrocarbon eg methane is converted to a mixture of carbon monoxide and hydrogen.

If it were possible to carry out these reforming stages under slightly less severe conditions the efficiency of the overall process would be improved. Unfortunately reducing the severity of the reforming stages increases the content of inert gases (inerts) in the feed to the ammonia synthesis reactor.

Table 3 shows the results obtained for the various streams in the main ammonia synthesis reactor if normal purge levels are maintained. An increase in inerts in the initial feed from 1.03% to 1.58% mol results in an increase in inerts at the reactor inlet from 13.4% to 16.24% mol at constant total ammonia production. An increase in ammonia concentration at the outlet from 13.2% to just in excess of 13.5% mol is necesary to maintain plant throughput.

Example B The increase in inerts described in Example A can be prevented by increasing the purge gas flow from the reactor, as shown in Table 4. The recycle flow is decreased by about 7%. The reactor feed inerts concentration is reduced to 12.8% mol but the ammonia production is reduced by ca 50 tonne/day and the purge gas flow is increased from ca 1 9 to ca 32 tonne/day.

If this purge stream is fed to a conventional purge gas treatment plant as in Fig. 1 the reduction in ammonia production and the loss of purge gas make the increased purge economically unacceptable.

Example 2 If a purge gas treatment plant as in Fig. 2 is used to treat the main plant purge gas stream the flows in the purge gas plant are shown in Table 5. The hydrogen loss in the purge gas is very considerably reduced (compare stream 5, Table 4 and stream 5, Table 5). Ammonia production in- the purge gas treatment plant is 1 20 tonne/day, which more than offsets the reduced ammonia production in the main ammonia production plant. The substantially increased purge gas recovery unit flows necessitate a larger synthesis reactor 30 in the purge gas treatment plant containing ca 1 3 m2 catalyst in a 3 stage adiabatic reactor.

Example 3 Economies may be made by reducing the severity of ammonia recovery in the main ammonia plant, so that there is increased ammonia in the purge gas stream. The flows in the purge gas recovery stream are shown in Table 6.

Example 4 The effect of increasing the purge stream in the main ammonia synthesis plant can be increased without changing the inerts content of the feed to the main ammonia synthesis reactor. This can be done by decreasing the recycle flow at constant feed rate. The results are shown in Table 7. The flows in a purge gas plant according to Fig. 2 are shown in Table 8. The decreased ammonia yield in the main ammonia synthesis plant is compensated for by increased production from the purge gas treatment plant. The total ammonia yield from the two plants is now 1245 tonne/day compared with 1180 tonne/day initially.

TABLE 1 Stream flows for main recycle unit 1180 te/DAY-kg mol X 102/hour Stream H2 N2 NH3 I Total 1 46.610 15.540 .000 .650 62.800 2 185.364 61.988 34.003 39.044 320.400 3 .000 .000 28.931 .000 28.931 4 185.364 61.988 5.071 39.044 291.468 5 3.085 1.031 .084 .650 4.852 6 182.278 60.956 4.987 38.394 286.616 7 138.754 46.448 34.003 38.394 ^ 257.600 REACTOR OUTLET AMMONIA % MOL = 13.2 TABLE 2 Stream flows in purge recovery loop kg molX 102/hour Stream H2 N2 NH3 I Total 1 3.085 1.031 .084 .650 4.850 2 7.450 2.527 2.334 10.539 22.850 3 .000 .000 1.801 .000 1.801 4 7.450 2.527 .533 10.539 21.049 5 .460 .156 .033 .650 1.298 6 6.991 2.371 .500 9.889 19.750 7 4.365 1.496 2.250 9.889 18.000 REACTOR OUTLET AMMONIA % MOL = 12.5 TABLE 3 Stream flows in main loop-increased feed in ens kg mol X 102/hour W : r Stream H2 N2 NH3 I Total 1 46.610 15.540 .000 1.000 63.150 2 173.169 57.875 33.345 45.759 310.150 3 .000 .000 28.443 .000 28.443 4 173.169 57.875 4.901 45.759 281.706 5 3.784 1.264 .107 1.000 6.156 6 169.385 56.611 4.794 44.759 275.550 7 126.559 42.335 33.345 44.759 247.000 REACTOR OUTLET AMMONIA % MOL = 13.5 TABLE 4 Stream flows in main loop-increased inerts/increased recycle kg molX 102/hour Stream H2 N2 NH3 I Total 1 46.610 15.540 .000 1.000 63.150 2 176.490 58.947 32.400 35.311 303.150 3 .000 .000 27.605 .000 27.605 4 176.490 58.947 4.794 35.311 275.544 5 4.998 1.669 .135 1.000 7.803 6 171.492 57.278 4.658 34.311 267.741 7 129.880 43.407 32.400 34.311 240.000 REACTOR OUTLET AMMONIA % MOL = 13.5 TABLE 5 Stream flows in purge recovery loop-High purge flow kg mol X 102/hour Stream H2 N2 NH3 I Total 4.998 1.669 .135 1.000 7.802 2 12.195 4.119 3.585 17.904 37.802 3 .000 .000 2.979 .000 2.979 4 12.196 4.119 .606 17.904 34.823 5 .681 .230 .034 1.000 1.945 6 11.513 3.889 .572 16.904 32.878 7 7.107 3.450 ~ .345 16.904 30.000 REACTOR OUTLET AMMONIA % MOL = 11.5 TABLE 6 Stream flows in purge loop-increased feed ammonia kg mol X 102/hour Stream H2 N2 NH3 I Total 1 3.085 1.031 .640 .650 5.406 2 6.544 2.221 2.720 9.920 21.406 3 .000 .000 2.389 .000 2.389 4 6.544 2.221 .331 9.920 19.017 5 .429 .146 .022 .650 1.246 6 6.114 2.076 .309 9.270 17.771 7 3.459 1.191 2.080 9.270 16.000 REACTOR OUTLET AMMONIA % MOL = 13 TABLE 7 Main unit-increased purge by decreased recycle kg mol X 102/hour Stream H2 N2 NH3 I Total 1 46.610 15.540 .000 .650 62.800 2 188.127 62.812 31.680 20.180 302.800 3 .000 .000 26.878 .000 26.878 4 188.127 62.812 4.801 20.180 275.921 5 6.059 2.023 .154 .650 8.887 6 182.067 60.789 4.646 19.530 267.030 7 141.517 47.272 31.680 19.530" 240.000 REACTOR OUTLET AMMONIA % MOL = 13.2 TABLE 8 Stream flows in purge recovery unit-high purge for main unit kg molX 102/hour Stream H2 N2 NH3 I Total 1 6.059 2.023 .154 .650 8.886 2 18.069 6.102 4.354 15.360 43.886 3 .000 .000 3.654 .000 3.654 4 18.069 6.102 .700 15.360 40.232 5 .765 .258 .030 .650 1.702 6 17.305 5.844 .670 14.711 38.523 7 12.010 4.079 4.200 14.711 35.000 REACTOR OUTLET AMMONIA % M6L = 12 CLAIMS 1.A process for the production of ammonia in which a feedstock comprising inert gases, nitrogen and hydrogen is fed in a primary ammonia synthesis step over an ammonia synthesis catalyst at elevated temperature and pressure, ammonia is recovered, the gas is recycled to the ammonia synthesis catalyst, and a purge gas stream is taken from the recycled gas stream, characterised in that at least a portion of the purge gas is fed at a pressure not greater than the main ammonia synthesis step to a secondary ammonia synthesis step over an ammonia synthesis catalyst which comprises i) as support a graphite-containing carbon having a) a basal plane surface area of at least 100m2/g, b) a ratio of BET surface area to basal plane surface area of not more than 5:1, and c) a ratio of basal plane surface area to edge surface area of at least 5::1, and iii) as active component (a) 0.1 to 50% by weight of a transition metal of the 4th, 5th and 6th horizontal periods of Groups VA, VIA, VIIA and VIII of the Periodic Table expressed by percentage by weight of total catalyst and b) 0.1 to 4 times by weight of (a) of a promoter selected from Groups IA or lIA of the Periodic Table under conditions of temperature, pressure and space velocity such that conversion to ammonia is effected.

2. A process according to claim 1 wherein the carbon support has a basal plane surface area of at least 1 50m2/g, a ratio of BET surface area to basal plane surface area not more than 4:1 and a ratio of basal plane surface area to edge surface area of at least 10:1.

3. A process according to any one of the preceding claims wherein the carbon support has a basal plane surface area of at least 200m2/g, a ratio of BET surface area to basal plane surface area of not more than 3:1 and a ratio of basal plane surface area to edge surface area of at least 100:1.

4. A process according to any one of the preceding claims wherein the BET surface area is at least 300 m2/g, the ratio of BET surface area to basal plane surface area is not more than 2:1 and the ratio of basal plane to edge surface area is at least 200:1.

5. A process according to any one of the preceding claims wherein the carbon support is prepared by heating carbon having a BET surface area in the range 100 to 3000m2/g in an inert atmosphere to a temperature in the range 900 to 3000 C then in an oxidising atmosphere to a temperature in the range 300 to 1200"C and subsequently in an inert atmosphere to a temperature in the range 900"C to 3000"C.

6. A process according to any one of the preceding claims wherein the transition metal is ruthenium.

7. A process according to any one of the preceding claims wherein the Group IA or IIA metal is rubidium and/or potassium.

8. A process according to any one of the preceding claims wherein the primary ammonia synthesis step is carried out at a pressure not exceeding 4.5 X 107 Pa.

9. A process according to any one of the preceding claims wherein fresh feed to the primary ammonia synthesis reactor has an inerts content greater than 1.4% mol and a purge stream is taken sufficient to maintain the inerts concentration in the feed to the primary ammonia synthesis catalyst at not more than 13% mol.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (1)

1. **WARNING** start of CLMS field may overlap end of DESC **.