Processes and systems for pyrolysing a hydrocarbon. In some examples, the process can include mixing a cooled hydrocarbon effluent and a cooled de-coking effluent to produce a combined effluent. The combined effluent can be introduced into an inlet conduit of a separator under conditions that provide >80 wt. % of the plurality of coke particles with a Stokes number of >10. From a first exit conduit of the separator >55 wt. % of the plurality of coke particles in the combined effluent can be removed, and from a second exit conduit of the separator a coke-lean hydrocarbon effluent that includes