A process design for the purification of carbon contaminated with Cu-Bi alloy from catalytic methane decomposition

Abstract

Low-carbon hydrogen production by catalytic methane decomposition (𝐶𝐻4→𝐻2(𝑔)+𝐶(𝑠)) has demonstrated potential to significantly decarbonize industrial sectors. Previous research has proposed carrying out the reaction in a liquid metal bubble reactor containing catalytic molten metal alloys such as Cu0.45Bi0.55 to allow hydrogen bubbles to exit through the top while the solid carbon product floats to the top of the melt to be skimmed or removed by other mechanical means. Metal lost in the process of carbon removal can impact the plant profitability because it will need to be continually replenished. Additionally, metal contaminants in the carbon product pose limitations on its ability to be sold as carbon black, and on its ability to be stored permanently due to the risk of metal contaminants in the environment. In this work, the process design methodology for the purification of carbon contaminated with copper and bismuth from catalytic methane decomposition is presented along with the capital costs of carbon processing equipment. Bismuth has a relatively high vapour pressure, so it is removed by vacuum distillation. A mass transfer model is applied to a lift-spray apparatus wherein the molten mixture of copper, bismuth, and carbon is sprayed up through a riser into a vacuum chamber where the bismuth vapours can be condensed and recovered. The spray creates smaller droplets which help increase the amount of surface area available for bismuth to evaporate. Copper is removed by leaching in a sulfuric acid solution by 𝐶𝑢(𝑠)+2𝐻++0.5𝑂2 →𝐶𝑢2++𝐻2𝑂. The carbon product is then filtered and rinsed from the leaching solution before being dried in a rotary dryer. Copper is precipitated from the filtrate solution using scrap iron, a metal higher in the electromotive series, in a process called cementation. Copper particles can be filtered from the solution and then sold as lower grade copper due to some iron contamination in the copper. The final carbon product is 98.9 wt% carbon on a dry basis, which meets the specifications for rubber-grade carbon black. The cost of carbon processing equipment per tonne of hydrogen produced is estimated at $257/tonne H2, a value that is relatively small compared to the market price of hydrogen which ranges from $1500-8000 USD/tonne H2. This work creates a pathway to recover metal catalyst lost in the process of carbon removal from methane decomposition in a liquid metal bubble reactor. The purified carbon product can be sold on the carbon black market or safely stored underground.