The selective thermochemical conversion of gaseous hydrocarbons in natural gas or renewable natural gas (such as biogas derived via anaerobic digestion of manure) to higher hydrocarbons, such as alkynes and alkenes, has been the subject of extensive investigation over the past many decades. In most cases, product hydrocarbon yields per pass are less than 40% on a carbon atom basis. One reported exception to this generalization is the multi-step conversion of natural gas to acetylene and carbon monoxide using a Ni/SrdCaO1-d catalyst/reagent with a per pass methane conversion > 70%, per pass C2+ conversion of 100%, and potential selectivities to acetylene and carbon monoxide of ca. 66% and 33%, respectively [1]. The problem investigated in this preliminary analytical study is the potential use of an analogous multi-step process for the selective conversion of gaseous hydrocarbons to propadiene/propyne and propylene.
Methods
The envisioned process requires the development of a suitable MgO-based material to serve as both catalyst and reagent in four primary unit operations:
Step 1 : Catalytic Hydrocarbon Decomposition
CxHyOz(g) → (x-z) C(s) + (y/2)H2(g) + z CO(g)
In this step, the gaseous hydrocarbon mixture (the overall composition of which is represented by CxHyOz) is catalytically decomposed (with a MgO-based catalyst) to yield solid carbon, gaseous hydrogen, and some carbon monoxide (e.g., due to the presence of some CO2 in the feed gas mixture).
Step 2 : Magnesium Carbide Formation
(x-z) C(s) + (2/5)(x-z) MgO(s) → (1/5)(x-z)Mg2C3(s) + (2/5)(x-z)CO(g)
In this step, the deposited carbon and MgO-based catalyst/reagent undergo solid-state reaction to selectively form magnesium carbide and additional carbon monoxide.
Step 3 : Magnesium Carbide Hydrolysis
(4/5)(x-z)H2O(g)or(l) + (1/5)(x-z)Mg2C3(s) → (1/5)(x-z)C3H4(g) + (2/5)(x-z)Mg(OH)2(s)
In this step the MgO-based catalyst/reagent is reacted with excess water to selectively generate propadiene/propyne and a solid hydroxide or hydroxide-hydrate [2].
Step 4 : Hydroxide Dehydration
(2/5)(x-z)Mg(OH)2(s) →(2/5)(x-z) MgO(s) + (2/5)(x-z)H2O(g)
In this step, the hydroxide-based material is thermally decomposed to regenerate the original MgO-based material, which then serves as the catalyst in Step 1.
The overall reaction chemistry is:
CxHyOz(g) + (2/5)(x-z)H2O(g)or(l) → (1/5)(x-z)C3H4(g) + (y/2)H2(g) + (1/5)(2x+3z)CO(g)
As an example, if the gaseous hydrocarbon feed consists entirely of methane (CH4), the reaction stoichiometry is:
CH4 + (2/5)H2O → (1/5)C3H4(g) + 2H2 + (2.5)CO
In this case the maximum possible yield of propadiene/propyne on a carbon-atom basis is 60%. Propylene could then be selectively produced in this process through a fifth unit operation in which excess hydrogen produced is used to hydrogenate the propadiene/propyne using an appropriate catalytic reactor.
This envisioned process was investigated using ASPEN Plus® to define the overall process, close heat and material balances, estimate equipment design and sizing, and enable a preliminary techno-economic analysis for the levelized cost of propadiene/propyne or propylene production.
Results
Preliminary process design, heat and material balances, and levelized cost of production for propadiene/propyne and propylene will be presented.
Implications
If the proposed conceptual process can be reduced to practice with a suitable material that is stable for a sufficient number of process cycles then it represents a new opportunity for the selective conversion of gaseous hydrocarbon mixtures to both acetylene [1] and propadiene/propyne, as well as ethylene and propylene.
References
[1] M.C.J.Bradford, M.Te, M.V.Konduru, A.Pollack, and D.X.Fuentes, "Alkaline earth metal-based catalyst and process for selective hydrocarbon conversion to acetylene and carbon monoxide”, Catalysis Today, 123 (2007) 23. https://doi.org/10.1016/j.cattod.2007.01.006
[2] P.Karen and H.Fjellvåg,”Hydrolysis and structure of carbides related to propadiene“ J.Alloys and Compounds, 178 (1992) 285, https://doi.org/10.1016/0925-8388(92)90270-J