Use of molten salt to separate carbon from a molten metal catalyst

Abstract

The present invention relates to a method for molten metal pyrolysis of hydrocarbons to produce hydrogen gas and carbon. Liquid salt is used to separate produced carbon from the molten metal and to facilitate isolation of produced carbon.

Claims

1. Method for producing solid carbon and hydrogen gas by molten metal pyrolysis of hydrocarbons, the method comprising: (i) feeding a stream of hydrocarbon into a pyrolysis reactor through a catalytic layer of molten metal to pyrolise the hydrocarbon into solid carbon 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 hydrogen gas that evolves from the reactor; (iv) collecting a mixture comprising solid carbon and molten salt; (v) separating the mixture obtained in step (iv) into a product comprising solid carbon and separated salt.

2. The method according to claim 1, wherein the metal in the molten metal is selected from the group consisting of In, Bi, Sn, Ga, Pb, Ag, Cu, Sn, Pt, Ni, and Au.

3. 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 KNO3, NaNO3, NaCl, KCl, LiCl, MgCl2, CuCl, NiCl2, ZnCl2, ZnBr2 and NaBr.

4. The method according to claim 1, wherein the hydrocarbon comprises a C1-C4 hydrocarbon, preferably methane.

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

6. The method according to claim 5, wherein the recovered hydrocarbon is recycled back into the pyrolysis reactor as part of step (i).

7. The method according to claim 1, wherein the reactor has an inlet for receiving the hydrocarbon at or near the bottom end of the reactor, an outlet for discharging a mixture of carbon and molten salts in a side wall, and an outlet for discharging a product gas comprising hydrogen at or near the top end.

8. The method according to claim 1, wherein a layer of molten salt is present in the pyrolysis reactor, and wherein step (iv) involves skimming to collect the solid carbon and part the layer of molten salt, such that substantially all of the solid carbon is removed from the reactor.

9. The method according to claim 1, wherein step (v) involves separating solid carbon from the separated salt by filtering and/or washing the mixture with an aqueous liquid, preferably using a metal filter or a ceramic filter, to obtain a product comprising pure solid carbon and a separated salt.

10. The method according to claim 1, wherein the separated salt is recycled into the reactor as part of step (ii).

11. The method according to claim 1, wherein the reactor is kept at a temperature in the range of 250-1500° C.

12. 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 hydrocarbon 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, (c) means for separating a mixture of solid carbon and molten salts discharged from the first outlet; and (d) a recycle for recycling molten salts from the separator to the vessel.

13. The reactor according to claim 12, wherein the reactor is a bubble column reactor.

14. The reactor according to claim 12, wherein the reactor is heated using the hydrocarbon, the hydrogen gas, or electricity.

15. (canceled)

Description

DESCRIPTION OF THE FIGURES

[0101] 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.

[0102] FIG. 2: Preferred reactor for continuous process for production of carbon and H.sub.2 from hydrocarbon, depicted here as CH.sub.4, using molten salt. A stream of hydrocarbon (1) is fed, optionally using a pump compressor (2), towards an inlet (4) for receiving the hydrocarbon at the bottom of a pyrolysis reactor (5). During operation, a layer of molten metal catalyst (6) and a layer of molten salt (7) are present in the reactor (5). Produced hydrogen gas evolves from the molten layers and can collect in a headspace (8), along with possible unconverted hydrocarbon 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 (5), after which it can be transport with an optional pump compressor (10) towards means (11) for separating pure hydrogen gas (12) from unconverted hydrocarbon gas (13). The recovered hydrocarbon gas can be fed into the original stream of hydrocarbon (1) for instance at a junction (3) before the stream enters the reactor (5). The reactor preferably has an outlet (14) for discharging a mixture of carbon and molten salts in a side wall, which can be passed through separation means (15) such as a filter, after which separated salt can be conveyed via a recycle (16) into a salt vessel (17) and back into the reactor (5) via an inlet (18) for replenishing the molten salt layer. Carbon, or a mixture of carbon and salt, can be further treated in a washing vessel (20) to which it is optionally transported via a pump (19). The washing vessel is supplied by a stream of aqueous solution (26) 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. Aqueous solution comprising salt obtained via separation means (21) can be dried using drying means (24) after which the salt can be transported back to a salt vessel (17) optionally using a pump (25).

EXAMPLES

Example 1—Molten Metal Hydrolysis of a Hydrocarbon Stream

[0103] 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 pyrolyzes 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

[0104] The following procedure was followed: [0105] 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. [0106] 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. [0107] 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. [0108] 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. [0109] 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. [0110] 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. [0111] 7. Water was evaporated to provide the initial salt.

[0112] 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

[0113] 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.