A chemical conversion reactor contains a substantially vertical catalytic bed between an upper end and a lower end and comprises, in combination: close to its upper end, at least one means for introducing a solid catalyst, means for introducing and evacuating said feed allowing its flow in a substantially horizontal direction through the catalytic bed, means for heating said feed integrated into said reactor, said reactor comprising, close to its lower end, at least one means for extraction of the catalyst, which extraction is differentiated between an upstream portion and a downstream portion of said catalytic bed, with respect to the direction of flow of said feed.