The definition of a reversible thermodynamic process requires in any instant the mechanical equilibrium (equal pressures) and thermal equilibrium (equal temperatures) of the system in a quasi-static processr.
But there are cases of processes in which one of the two kinds of equilibrium cannot be reached.
Can these processes be considered "reversible" anyway?
I'll make two examples
- Quasi-static process in a completely adiabatic tank with two different gases at different temperatures: mechanical equilibrium always present, but thermal equilibrium (between the two gases) not necessarily reached.
- Isochoric quasi-static process of a gas in rigid and diatermic tank: thermal equilibrium always present, but mechanical equilibrium (between the gas and the environment) not necessarily reached.