A particle loading assembly for loading particles into a vessel to form a particle bed comprising an inner layer of particles and an outer layer of particles, the inner layer of particles arranged radially and concentric with the outer layer of particles, the assembly comprising: a vessel, into which said particles are loaded; a sliding barrier for segregating particles for the inner layer from the particles for the outer layer during loading, the sliding barrier having a top end and a bottom end; an inner particle distributor configured to distribute particles for the inner layer, the inner layer containing at least a first type of particle of different granulometry or range of composition or both granulometry and range of composition from a second type of particle contained in the outer layer, the inner particle distributor in fixed spaced relation to the sliding barrier, the inner particle distributor having a lowermost particle discharge position, wherein a horizontal plane passing through the lowermost particle discharge position of the inner particle distributor passes through the sliding barrier at a position between the top end and the bottom end of the sliding barrier; an outer particle distributor configured to distribute particles for said outer layer, the outer particle distributor in fixed spaced relation to the sliding barrier, the outer particle distributor having a lowermost particle discharge position, wherein a horizontal plane passing through the lowermost particle discharge position of the outer particle distributor passes through the sliding barrier at a position between the top end and the bottom end of the sliding barrier; and a hoist for lifting the sliding barrier, the inner particle distributor, and the outer particle distributor during loading of the particles for the inner and outer layers; wherein the lowermost particle discharge position of the inner particle distributor is maintained from 70 mm to 2000 mm above the upper surface of the inner layer when discharging particles for the inner layer, wherein the lowermost particle discharge position of the outer particle distributor is maintained from 70 mm to 2000 mm above the upper surface of the outer layer when discharging particles for the outer layer; and wherein the bottom end of the sliding barrier is maintained at a position ranging from below the upper surface of the inner layer to no more than 20 mm above the upper surface of the inner layer and ranging from below the upper surface of the outer layer to no more than 20 mm above the upper surface of the outer layer when lifting the sliding barrier. a vessel, into which said particles are loaded; a sliding barrier for segregating particles for the inner layer from the particles for the outer layer during loading, the sliding barrier having a top end and a bottom end; an inner particle distributor configured to distribute particles for the inner layer, the inner layer containing at least a first type of particle of different granulometry or range of composition or both granulometry and range of composition from a second type of particle contained in the outer layer, the inner particle distributor in fixed spaced relation to the sliding barrier, the inner particle distributor having a lowermost particle discharge position, wherein a horizontal plane passing through the lowermost particle discharge position of the inner particle distributor passes through the sliding barrier at a position between the top end and the bottom end of the sliding barrier; an outer particle distributor configured to distribute particles for said outer layer, the outer particle distributor in fixed spaced relation to the sliding barrier, the outer particle distributor having a lowermost particle discharge position, wherein a horizontal plane passing through the lowermost particle discharge position of the outer particle distributor passes through the sliding barrier at a position between the top end and the bottom end of the sliding barrier; and a hoist for lifting the sliding barrier, the inner particle distributor, and the outer particle distributor during loading of the particles for the inner and outer layers; wherein the lowermost particle discharge position of the inner particle distributor is maintained from 70 mm to 2000 mm above the upper surface of the inner layer when discharging particles for the inner layer, wherein the lowermost particle discharge position of the outer particle distributor is maintained from 70 mm to 2000 mm above the upper surface of the outer layer when discharging particles for the outer layer; and wherein the bottom end of the sliding barrier is maintained at a position ranging from below the upper surface of the inner layer to no more than 20 mm above the upper surface of the inner layer and ranging from below the upper surface of the outer layer to no more than 20 mm above the upper surface of the outer layer when lifting the sliding barrier.