On-line monitoring and/or control of headspace in a fluid bed reactor system

A method of detecting and/or dynamically controlling the expansion of a fluid bed reactor system comprising

• (i) a reactor vessel having an inlet and an outlet, the reactor vessel containing
• (a) a headspace volume of carrying medium substantially void of fluidised particles, and
• (b) a bed of fluidised particles in a carrying medium, said bed comprising a first volume where the particles are substantially homogeneously fluidised, and, between said first volume and said headspace volume, a second volume in which the fluidised particles form a particle density gradient, and,
• (ii) particle density detection means for detecting a pre-determined density of the fluidised particles at one or more positions in said second volume, and operationally connected thereto
• (iii) flow controlling means capable of being activated or deactivated by a signal from the density detection means generated by said detection means at the pre-determined particle density,
• the method comprising pre-setting the density of particles at said one or more positions in the second volume at which the density detection means generates a signal activating or deactivating the flow control means, whereby the expansion of a fluid bed reactor system is detected and/or dynamically controlled.