Plant for the chemical looping combustion of a solid hydrocarbon feedstock generating particles of unburnt residues and employing particles of an oxygen-carrying solid, said plant comprising: - a reduction reactor operating as fluidized bed in order to carry out the combustion of said solid hydrocarbon feedstock in contact with the particles of the oxygen-carrying solid, said reactor having a parallelepiped shape; - an oxidation reactor operating as fluidized bed in order to oxidize the reduced particles of the oxygen-carrying solid originating from the reduction reactor by bringing into contact with an oxidizing gas; - a solid/solid separator (120, 121, 123) surmounting said reduction reactor in order to separate the particles of the oxygen-carrying solid from the particles of unburnt residues contained in the gas/solid mixture resulting from said reduction reactor, said solid/solid separator comprising a chamber of parallelepipedal shape comprising: - a lower part operating as dense phase and an upper part operating as dilute phase; - at least one inlet of parallelepipedal shape for the gas/solid mixture originating from the reduction reactor, said inlet emerging in the upper part of the chamber and being equipped at its top with means (3, 7, 8) for distribution of said gas/solid mixture in the chamber which extend over the entire length of said inlet; - at least one drain placed in the lower part of the chamber; - at least one outlet located in the upper part of the chamber. - a reduction reactor operating as fluidized bed in order to carry out the combustion of said solid hydrocarbon feedstock in contact with the particles of the oxygen-carrying solid, said reactor having a parallelepiped shape; - an oxidation reactor operating as fluidized bed in order to oxidize the reduced particles of the oxygen-carrying solid originating from the reduction reactor by bringing into contact with an oxidizing gas; - a solid/solid separator (120, 121, 123) surmounting said reduction reactor in order to separate the particles of the oxygen-carrying solid from the particles of unburnt residues contained in the gas/solid mixture resulting from said reduction reactor, said solid/solid separator comprising a chamber of parallelepipedal shape comprising: - a lower part operating as dense phase and an upper part operating as dilute phase; - at least one inlet of parallelepipedal shape for the gas/solid mixture originating from the reduction reactor, said inlet emerging in the upper part of the chamber and being equipped at its top with means (3, 7, 8) for distribution of said gas/solid mixture in the chamber which extend over the entire length of said inlet; - at least one drain placed in the lower part of the chamber; - at least one outlet located in the upper part of the chamber. - a lower part operating as dense phase and an upper part operating as dilute phase; - at least one inlet of parallelepipedal shape for the gas/solid mixture originating from the reduction reactor, said inlet emerging in the upper part of the chamber and being equipped at its top with means (3, 7, 8) for distribution of said gas/solid mixture in the chamber which extend over the entire length of said inlet; - at least one drain placed in the lower part of the chamber; - at least one outlet located in the upper part of the chamber.