## 1 Introduction  
Reacting an active bed material solid with a gaseous component is a common application of fluidized beds, e.g., chemical looping combustion, combustion, gasification, fluid catalyst cracking, and chemical looping reforming. In many of these processes, mass transfer between the reacting gas and the active particle can prove a limiting step towards conversion, as demonstrated by e.g., [PERSON] et al. [1]. According to the two-phase theory [2] only gas in the emulsion phase can interact with the bed material. The challenge of limited mass transfer between the bubble and emulsion phases can be addressed by means of using confined fluidized beds, i.e., inserting inert spheres of a size much larger than the bed material to break down the bubble-emulsion flow structure. It has been shown that doing so provides smooth fluidization and increases bed expansion [3.4]. This will also have other effects that can be beneficial to mass transfer, such as locally increased relative gas-particle velocity. The spheres, here called packing solids, form a stacked lattice from the bottom of the reactor, with fluidization of bed material (here referred to as fluidizing solids) occurring in the packing voids (Figure 1). In the fuel reactor of a chemical looping combustion unit, combustible gas (e.g., gasification products of solid fuel, or gaseous fuel such as methane) reacts with a metal oxide bed material, to form water and carbon dioxide. It has been shown by modelling that mass transfer between bubbles, containing the bulk of the fuel, and the emulsion phase, containing reactive bed material, is a severe limitation to conversion [5]. This is especially true when looking at scaled-up units such as the proposed 1000 MW CLC design by Lyngelt & Leckner [6] where the dense bed required can be several meters tall. Three major models for bubble-emulsion mass transfer are those by [PERSON] [7], [PERSON] and [PERSON] [8], and they all indicate that bubble size is inversely proportional to mass transfer. If Geldart type B particles are used for bed material the bubbles can grow to tens of centimetres in diameter [7], and limiting this size should improve mass transfer performance. A system such as seen in Figure 1 was described by [PERSON] and [PERSON] [4; 9; 10] as a fluidized-packed bed, while [PERSON] et al. [11] named it confined fluidization. [PERSON] and [PERSON] [4] first investigated confined fluidized beds for the application of using the fluidizing solids as a heat transport medium in fluorination of spent uranium pellets, which acted as the packing solids. They investigated and formulated expressions for several basic fluidization properties such as pressure drop, solids mixing and bed expansion. Among other things it was found that the addition of fluidizing solids decreased the heat transfer coefficient by a factor two at the reactor walls, and eight at the top of the packing; reducing the need for in-bed cooling. [PERSON] & [PERSON] [11; 12] looked at expansion behaviour and pressure drop correlations, and [PERSON] & [PERSON] [13] investigated pressure drop and minimum fluidization Reynolds numbers. More recently, [PERSON] and [PERSON] [14] investigated the hydrodynamics of confined fluidization with non-spherical packing solids. [PERSON] and [PERSON] [15] investigated kinetics and mass transfer for catalytic hydrogenation of ethylene, where the packing solids had a catalytic surface. Figure 1: (**a**) Conceptual representation of a confined fluidized bed. (**b**) Photo of a confined fluidized bed using aluminium silicate ball packing solids and silica gel fluidizing solids. Mass transfer was measured between the gas and catalytic packing material and was found to be affected by factors such as size and amount of packing. Other methods for improving mass transfer in fluidized beds exist, such as adding wedges to reduce the reactor aperture [16] or the use of spouted beds [17].