Melt pyrolysis of hydrocarbon feedstock containing nitrogen and/or hydrogen sulphide

Abstract

The present invention relates to a method for molten metal pyrolysis of a feed comprising hydrocarbons and nitrogen and/or hydrogen sulphide to produce solid carbon and one or more of liquid sulfur, hydrogen gas and ammonia gas. The molten salt layer contains two reaction zones of different temperatures, a high temperature zone for pyrolysing the hydrocarbon and a low temperature zone for pyrolysing the hydrogen sulphide and/or forming the ammonia. Liquid salt is used to separate produced solid carbon and optionally the produced liquid sulphur from the molten metal and to facilitate isolation of produced carbon. The invention further relates to a reactor for performing the method according to the invention.

Claims

1. Method for producing of solid carbon and one or more of liquid sulphur, hydrogen gas and ammonia gas by molten metal pyrolysis of hydrocarbons, the method comprising: (i) feeding a stream comprising hydrocarbon and nitrogen and/or hydrogen sulphide into a pyrolysis reactor comprising a catalytic layer of molten metal, wherein the reactor comprises two reaction zones, a first zone for pyrolysing the hydrocarbon into solid carbon and hydrogen gas, and a second zone at a lower temperature than the first zone, for reacting the hydrogen with the nitrogen to form ammonia, and/or for pyrolysing hydrogen sulphide into sulphur and hydrogen gas; (ii) feeding a stream of molten salt into the pyrolysis reactor to separate the solid carbon from the molten metal; (iii) collecting a product gas containing ammonia gas and/or hydrogen gas that evolves from the reactor; (iv) collecting a mixture comprising solid carbon and molten salt; (v) optionally collecting liquid sulphur; (vi) separating the mixture obtained in step (iv) to obtain a product comprising solid carbon and separated salt.

2. The method according to claim 1, which is for producing solid carbon and ammonia gas by molten metal pyrolysis of hydrocarbons, the method comprising: (i) feeding a stream comprising hydrocarbon and nitrogen into a pyrolysis reactor comprising a catalytic layer of molten metal, wherein the reactor comprises two reaction zones, a first zone for pyrolysing the hydrocarbon into solid carbon and hydrogen gas, and a second zone for reacting the hydrogen with the nitrogen to form ammonia, and wherein the second zone is kept at a lower temperature than the first zone; (ii) feeding a stream of molten salt into the pyrolysis reactor to separate the solid carbon from the molten metal; (iii) collecting a product gas containing ammonia gas that evolves from the reactor; (iv) collecting a mixture comprising solid carbon and molten salt; (vi) separating the mixture obtained in step (iv) to obtain a product comprising solid carbon and separated salt.

3. The method according to claim 1, which is for producing solid carbon, liquid sulphur and hydrogen gas by molten metal pyrolysis of hydrocarbons, the method comprising: (i) feeding a stream comprising hydrocarbon and hydrogen sulphide into a pyrolysis reactor comprising a catalytic layer of molten metal, wherein the reactor comprises two reaction zones, a first zone for pyrolysing the hydrocarbon into solid carbon and hydrogen gas, and a second zone for pyrolysing hydrogen sulphide into liquid sulphur and hydrogen gas, and wherein the second zone is kept at a lower temperature than the first zone; (ii) feeding a stream of molten salt into the pyrolysis reactor to separate the solid carbon and the liquid sulphur from the molten metal; (iii) collecting a product gas containing hydrogen gas that evolves from the reactor; (iv) collecting a mixture comprising solid carbon and molten salt; (v) collecting liquid sulphur; (vi) separating the mixture obtained in step (iv) to obtain a product comprising solid carbon and separated salt.

4. The method according to claim 1, wherein the metal in the molten metal is selected from the group consisting of Mg, Pd, In, Bi, Sn, Ga, Pb, Ag, Cu, Sn, Pt, Ni, Fe, Co, Au, Mo, Cr, W and V, preferably wherein the metal in the first zone is selected from Mg, Pd, In, Bi, Sn, Ga, Pb, Ag, Cu, Sn, Pt, Ni, Fe, Co and Au and/or in the second zone is selected from In, Co, Fe, Ni, Mo, Cu, Cr, W and V.

5. The method according to claim 1, wherein the salt has a heat capacity of at most 2 J/K, more preferably at most 1.7 J/K, most preferably at most 1.6 J/K, and/or wherein the salt comprises at least one of KNO.sub.3, NaNO.sub.3, NaCl, KCl, LiCl, MgCl.sub.2, CuCl, NiCl.sub.2, ZnCl.sub.2, ZnBr.sub.2 and NaBr.

6. The method according to claim 1, wherein the hydrocarbon comprises a C.sub.1-C.sub.4 hydrocarbon, preferably methane.

7. The method according to claim 1, further comprising: (vii) separating the product gas obtained in step (iii) into unconverted hydrocarbon gas, unconverted nitrogen gas, hydrogen gas and ammonia gas, preferably using an adsorbent material, to obtain purified hydrogen gas, purified ammonia gas, recovered hydrocarbon and recovered nitrogen gas.

8. The method according to claim 1, wherein the metal for the catalytic layer of molten metal in the first zone differs from the metal used for the catalytic layer in the second zone, preferably wherein the metal of the catalytic layer in the first zone is Ni and/or the metal of the second zone is Fe or Co.

9. The method according to claim 1, wherein the first zone and second zone are in separate reactors.

10. The method according to claim 1, wherein step (i) typically involves bubbling of the hydrocarbon feed though the molten metal, wherein the diameter of the bubbles is in the range of 0.1-1000 μm, preferably in the range of 1-500 μm, most preferably in the range of 10-100 μm.

11. Reactor for performing molten metal pyrolysis of hydrocarbons, the reactor comprising: (a) a vessel for holding a catalytic layer of molten metal and a layer of molten salt, (b) an inlet for receiving the feedstock comprising hydrocarbon and nitrogen and/or hydrogen sulphide at or near the bottom end of the vessel, a first outlet for discharging a mixture of solid carbon and molten salts in a side wall of the vessel, and a second outlet for discharging a product gas at the top end of the vessel; (c) catalytic layer of molten metal comprising two reaction zones with different temperatures; (d) means for separating a mixture of solid carbon and molten salts discharged from the first outlet; (e) means for heating the reactor to a first temperature in a first zone and to a second temperature in a second zone, wherein the first temperature is higher than the second temperature; (f) a recycle for recycling molten salts from the separator to the vessel.

12. The reactor according to claim 11, which is for performing molten metal pyrolysis of hydrocarbons, wherein the reactor comprises: (a) a vessel for holding a catalytic layer of molten metal and a layer of molten salt, (b) an inlet for receiving the hydrocarbon and nitrogen at or near the bottom end of the vessel, a first outlet for discharging a mixture of solid carbon and molten salts in a side wall of the vessel, and a second outlet for discharging a product gas comprising ammonia at the top end of the vessel; (c) catalytic layer of molten metal comprising two reaction zones with different temperatures; (d) means for separating a mixture of solid carbon and molten salts discharged from the first outlet; (e) means for heating the reactor to a first temperature in a first zone and to a second temperature in a second zone, wherein the first temperature is higher than the second temperature and the first zone is located upstream of the second zone; (f) a recycle for recycling molten salts from the separator to the vessel.

13. The reactor according to claim 11, which is for performing molten metal pyrolysis of hydrocarbons, wherein the reactor comprises: (a) a vessel for holding a catalytic layer of molten metal and a layer of molten salt, (b) an inlet for receiving the hydrocarbon and hydrogen sulphide at or near the bottom end of the vessel, a first outlet for discharging a mixture of solid carbon and molten salts in a side wall of the vessel, and a second outlet for discharging a product gas comprising hydrogen at the top end of the vessel, and a separate outlet for discharging a mixture of liquid sulphur and molten metal in a side wall which is positioned below the outlet for discharging a mixture of carbon and molten salts; (c) a catalytic layer of molten metal comprising two reaction zones with different temperatures; (d) means for separating a mixture of solid carbon and molten salts discharged from the first outlet; and (e) means for heating the reactor to a first temperature in a first zone and to a second temperature in a second zone, where the first temperature is higher than the second temperature, (f) a recycle for recycling molten salts from the separator to the vessel.

14. The reactor according to claim 11, wherein the reactor is a bubble column reactor.

15. The reactor according to claim 11, wherein the reactor is heated using the hydrocarbon, the hydrogen gas, or electricity.

Description

DESCRIPTION OF THE DRAWINGS

[0186] FIG. 1 (A) State of the art reactor for hydrocarbon conversion, drawn here as CH.sub.4, to H.sub.2 and carbon. The hydrocarbon is bubbled through a layer of molten metal catalyst (hatched) after which gaseous H.sub.2 product evolves from the reactor. Solid carbon product as a lower density than the molten metal and accumulates at the top, where it can be collected. Carbon that is not collected can clog the reactor. Collected carbon is easily contaminated with metal. (B) Use of molten salt in a method according to the invention. The molten salt separates the produced solid carbon from the molten metal, preventing accumulation of solid carbon on the catalyst. Collected carbon is not contaminated with metal, while any potential residual salt can be conveniently washed away. Not depicted here is the use of two reaction zones within the layer of molten metal.

[0187] FIG. 2 depicts a preferred reactor for continuous process for production of solid carbon and NH.sub.3 from hydrocarbon and nitrogen gas using molten salt. A stream of hydrocarbon and nitrogen (1) is fed, optionally using a pump compressor (2), towards an inlet (3) for receiving the hydrocarbon at the bottom of a pyrolysis reactor (4). During operation, two layers of molten metal catalyst (high temperature layer (5) and low temperature layer (6)) and a layer of molten salt (7) are present in the reactor (4). In layer (5), the hydrocarbon is pyrolysed into solid carbon and hydrogen gas, which move up to layer (6), where the hydrogen gas reacts with nitrogen gas to form NH.sub.3. Produced ammonia gas evolves from the molten layers and can collect in a headspace (8), along with possible unconverted hydrocarbon, hydrogen and/or nitrogen gas. The product mixture can be collected via an outlet (9) for discharging a product gas comprising ammonia at the top end of the reactor (4), after which it can be transported with an optional pump compressor (10) towards means (11) for separating pure ammonia gas (12) from the unconverted gases (13). These recovered gases can be fed into the original stream of hydrocarbon (1) for instance at a junction (26) before the feed enters the reactor (4). The reactor has an outlet (14) for discharging a mixture of carbon and molten salts in a side wall, which mixture can be passed through separation means (15) such as a filter, after which separated salt (16) can be conveyed into a salt vessel (17) and recycled back into the reactor (4), optionally after increasing the temperature, via an inlet (18) for replenishing the molten salt layer. Carbon can be further treated in a washing vessel (20), to which it is transported via a pump (19), to remove residual traces of salt. The washing vessel is supplied by a stream of aqueous solution (27) after which the suspension comprising water, salt, and carbon is separated using separation means (21) such as a filter. Separated carbon is optionally dried using drying means (22) after which pure solid carbon (23) is obtained. The washed and dissolved salts can be dried using drying means (24) after which the salt can be transported back to a salt vessel (17) using a pump (25).

[0188] FIG. 3 depicts a preferred reactor for continuous process for production of solid carbon and liquid sulphur from hydrocarbon and hydrogen sulphide gas using molten salt. A stream of hydrocarbon and hydrogen sulphide (1) is fed, optionally using a pump compressor (2), towards an inlet (3a) for receiving the hydrocarbon at the bottom of a first pyrolysis reactor (4a), containing a first layer of molten metal catalyst (low temperature layer (6)) and a layer of molten salt (7a). During operation, a layer of liquid sulphur (28) accumulates on top of salt layer (7a). In layer (6), the hydrogen sulphide is pyrolysed into liquid sulphur and hydrogen gas, which move up through reactor (4a). The liquid sulphur is discharged via an outlet (14a) in the side wall of the reactor. The gaseous product, hydrogen gas and unreacted hydrocarbon, are led via inlet (3b) to pyrolysis reactor (4b), containing a second layer of molten metal catalyst (high temperature layer (5)) and a layer of molten salt (7b). In layer (5), the hydrocarbon is pyrolysed into solid carbon and hydrogen gas, which move up through reactor (4b). Produced hydrogen gas evolves from the molten layers and can collect in a headspace (8), along with possible unconverted hydrocarbon and/or hydrogen gas. The product mixture can be collected via an outlet (9) for discharging a product gas comprising hydrogen at the top end of the reactor (4b), after which it can be transported with an optional pump compressor (10) towards means (11) for separating pure hydrogen gas (12) from the unconverted gases (13). These recovered gases can be fed into the original stream of hydrocarbon (1) for instance at a junction (26) before the feed enters the reactor (4a). The reactor has an outlet (14b) for discharging a mixture of carbon and molten salts in a side wall, which mixture can be passed through separation means (15) such as a filter, after which separated salt (16) can be conveyed into a salt vessel (17) and fed back into the reactor (4a) and/or (4b) via recycle (18), optionally after increasing the temperature, via an inlet for replenishing the molten salt layer. Carbon can be further treated in a washing vessel (20), to which it is transported via a pump (19), to remove residual traces of salt. The washing vessel is supplied by a stream of aqueous solution (27) after which the suspension comprising water, salt, and carbon is separated using separation means (21) such as a filter. Separated carbon is optionally dried using drying means (22) after which pure solid carbon (23) is obtained. The washed and dissolved salts can be dried using drying means (24) after which the salt can be transported back to a salt vessel (17) using a pump (25). The liquid sulphur discharged via outlet (14a) may be subjected to separation of salt. Conveniently, the mixture is cooled to below the melting point of the salt while being transported to separation means (29) such as a filter, after which liquid sulphur (31) can be collected. The separated salt is preferably recycled to the reactor, for example via pump (30) conveyed into salt vessel (17). Optionally, the reactor contains two salt vessels, one for reactor (4a), which is fed from (29) and replenishes salt layer (7a), and one for reactor (4b), which is fed from (15) and replenishes salt layer (7b).

EXAMPLES

Example 1—Molten Metal Hydrolysis of a Hydrocarbon Stream

[0189] Conventional molten metal pyrolysis employs a setup as depicted in FIG. 1A. The method of the invention is depicted in FIG. 1B, which uses a reactor wherein liquid salt is present. Natural gas (NG) is fed to the molten metal bubbling column reactor in which the methane pyrolyses into C and H.sub.2. The H.sub.2 and un-converted CH.sub.4 is passed through a pressure swing adsorption (PSA) unit to separate high purity H.sub.2. Unconverted CH.sub.4 is recycled back to the natural gas input. The bubbling column reactor consists of two liquid layers, separated by density differences. The bottom layer is the molten metal, which catalysis the pyrolysis reaction. Floating on top is the molten salt layer. The produced carbon, due to a significant density difference with the molten metal layer, floats through the molten metal into a molten salt layer (assisted by the produced hydrogen and unconverted hydrocarbon gas bubbles). The molten salt works as a washing solution for the carbon particles. The skimmed off solid carbon/molten salt slurry which is formed in the reactor is further separated with the help of a filter. The filtered carbon can be subsequently washed with water to remove traces of the salt, dried, and collected and sent to carbon storage. The salt stream is recycled back to the molten metal reactor to collect new carbon formed.

Example 2—Separation of Carbon from Molten Metal and Molten Salt

[0190] The following procedure was followed: [0191] 1. A predefined amount (see table below) of starting mixture comprising metal (gallium), carbon (carbon black with a particle size of at most 100 μm), and salt (a 1:1 by weight mixture of NaNO3 and KNO3) were added to a glass test-tube. Carbon was placed at the bottom and metal at the top. [0192] 2. The test tube was heated to 350° C. in an electric oven in two configurations, (a) without bubbling, and (b) with bubbling. The bubbling was induced by an immersed steel tube to replicate conditions during molten metal pyrolysis, where a hydrocarbon stream is bubbled through the molten system. [0193] 3. The mixture was maintained in the above defined conditions for 15 minutes up to eight hours. The results shown in table 1 represent samples after 15 minutes. [0194] 4. After the duration of predefined time (here 15 minutes) the test tube was taken out of the oven and allowed to cool down. Liquid layers solidified. [0195] 5. After cooling down, the carbon (in powered state) was retrieved from the top. The molten metal was taken from the bottom by breaking the test tube. The salt (solid) with carbon embedded in it and was taken from the middle of the test tube. [0196] 6. Some salt got stuck to fragments of the broken test tube. This salt was retrieved by washing the fragments in water and collecting the water. This water was added to the mixture of salt and carbon. Any fragments of glass were decanted from the solution, and the carbon was then filtered out and combined with the collected carbon, which was subsequently dried. [0197] 7. Water was evaporated to provide the initial salt.

[0198] The table below shows the measured mass of carbon, salt and metal (in grams) before and after the separation tests. In the beginning, there are distinct layers of carbon, salt and metal in the test-tube. At high temperature, the layers were reordered by density of the material, and after the test, the separate layers were collected. It was found that almost all of the carbon is separated from the metal, but the collected carbon and salt samples have cross-contamination, which is resolved by washing of the carbon.

TABLE-US-00001 Material Start mixture Separated mixture Recovery (%) * Without bubbling Salt 2.91 2.86 98 Carbon 0.52 0.48 92 Molten metal 8.6 8.6 100 With N.sub.2 bubbling Salt 3.45 3.21 93 Carbon 0.67 0.64 96 Molten metal 17.2 17.2 100

[0199] Recovery percentages are determined as follows: Salt is recovered from the salt layer (determined after removal of the carbon), carbon is recovered from the carbon layer and the salt layer (determined after removal of the salt), and metal is recovered from the molten metal layer. Thus, carbon was efficiently separated from the molten metal and recovered from the carbon and salt layers with high yields of over 90%. Residual salt was readily rinsed away and no contamination with molten metal was observed.

Example 3—Separation of Carbon and Ammonia from Molten Metal and Molten Salt

[0200] To a reactor comprising a molten metal (gallium) and a molten salt (NaCl+KCl in a 50:50 molar ratio) was added CH.sub.4, Ar and N.sub.2. CH.sub.4 was pyrloysed to form solid carbon and H.sub.2 gas at a temperature of 1000° C. at a time of T1. H.sub.2 and N.sub.2 were then reacted together in the presence of Ar at a temperature of 500° C. at a time of T2 to form NH.sub.3. The solid carbon and evolved ammonia were collected. Ar did not react during the process. The outlet gas mainly comprised N.sub.2, H.sub.2, Ar and NH.sub.3.

[0201] The pH of the solution after pyrolysis was 5.45 and following reaction of N.sub.2 and H.sub.2 was 6.85. This increase in alkalinity indicated the presence of NH.sub.3.

[0202] Table 1 shows a high % conversion of CH.sub.4 at a temperature of 1000° C. at T1.

TABLE-US-00002 TABLE 1 Methane N.sub.2 Ar H.sub.2 Total Volume Volume Volume Volume Volume volume flow (ml/min) (ml/min) (ml/min) (ml/min) (ml/min) Inlet gas 30 20 50 100 Outlet gas 1.5 20 50 57 128.50 Molar Methane N.sub.2 Ar H.sub.2 concentration mol % mol % mol % mol % Inlet gas 30.00 20.00 50.00 Outlet gas 1.17 15.56 38.91 44.36 Inlet gas volume 100 ml Conversion 95%

[0203] The data of Table 1 indicates that almost all CH.sub.4 was converted to carbon and H.sub.2. N.sub.2 and Ar did not react. The outlet gas mainly comprised N.sub.2, H.sub.2 and Ar.

CH.sub.4C+2H.sub.2

[0204] Based on above reaction, 1 mole of CH.sub.4 provided two moles of H.sub.2. The overall volume increased from 100 ml (inlet) to 129 ml (outlet). Thus the relative concentration of gases (Ar, N.sub.2) decreased by 77% (100/129). Thus, N.sub.2 concentration decreased from 20% to 15.5% and Ar concentration decreased from 50% to 39%.

[0205] Table 2 shows the % conversion of N.sub.2 and H.sub.2 at a temperature of 500° C. at T2.

TABLE-US-00003 TABLE 2 Methane N.sub.2 Ar H.sub.2 NH.sub.3* Total Volume Volume Volume Volume Volume volume Volume flow (ml/min) (ml/min) (ml/min) (ml/min) (ml/min) (ml/min) Inlet gas 0 33 50 17 100 Outlet gas 32.5 50 15.42 1.58 99.5 Molar Methane N.sub.2 Ar H.sub.2 NH.sub.3* concentration mol % mol % mol % mol % mol % Inlet gas 0.00 33.00 50.00 17.00 100.00 Outlet gas 0.00 32.65 50.26 15.50 1.59 100.00 *NH.sub.3 was estimated by the decrease in concentration of H.sub.2 in the experiment.

[00001]$\begin{array}{cc}\mathrm{Moles}\mathrm{of}{H}_2\mathrm{converted}& =1.58\mathrm{ml}/\min \\ & =7.5357^{-5}\mathrm{mol}/\min \\ N_2+3H_{2}2{\mathrm{NH}}_3& \end{array}$

[0206] Based on the above reaction, 3 moles of H.sub.2 and 1 mole of N.sub.2 provided 2 moles of NH.sub.3. Thus, there was an overall decrease in volume and a relative increase of the molar % of unreacted components. After taking NH.sub.3 production into account this decrease was not considered to be significant.

[0207] NH.sub.3 production is estimated via: [0208] (a) measuring the decrease in H.sub.2 concentration in the outlet gas—provided estimated production of 1 ml/min NH.sub.3. [0209] (b) measuring the pH increase in the water wash at the exit of the column—provided estimated absorption of 2.1 ml of NH.sub.3 gas in liquid water.