## 1 Introduction  
For efficient and effective utilization of the solid phase in process industries, the use of small particles is often recommended because of their high surface areas. A large surface area invariably ensures high heat and mass transfer rates and better utilization of catalyst particles where the internal mass transport resistance controls the overall mass transport process. However, gas-solid contacting involving small particles in process industries poses a major challenge irrespective of whether it is implemented in a fixed or a fluidized mode of operation. When a fluid flows through a fixed bed of solid particles in the laminar flow region, the pressure drop inversely varies with the square of the particle diameter. Thus, the high pressure drop associated with small particles would eventually translate into a high energy cost, thus compromising the cost effectiveness of the process. An otherwise preferred mode of gas-solid contacting for small particles is fluidization. However, fine powders consisting of small particles (less than 30 \(\upmu\)m in size), often cohesive in nature, are difficult to fluidize as they develop channels and cracks through which the fluidizing gas tend to bypass the solid materialspresent in the bed, thus resulting in poor contact and inefficient mixing. [PERSON] classified such powders as group C [1]. Similarly, powders typically in the range of hundreds of micrometers, classified as group B, exhibit bubbling behavior, in which a substantial fraction of the fluidizing gas tend to escape as bubbles without coming into contact with the resident solid phase of the fluidized bed. By contrast, powders in the intermediate size range, classified as group A, show smooth and particulate fluidization behavior, resulting in intimate contact and efficient mixing of the gas and solid phases. To improve the fluidization behavior of solids with poor fluidization hydrodynamics, various techniques and strategies have been proposed in the literature [2; 3; 4; 5; 6; 7; 8; 9; 10]. The main objective of these techniques, commonly known as assisted fluidization techniques, is to provide additional energy to the fluidized bed to overcome interparticle forces that are responsible for the cohesive nature and other non-homogeneities during fluidization. Of particular interest in this connection is the fluidization of fine cohesive powders due to their intrinsically high surface area to volume ratios. An extensive review on this topic has recently been presented by [PERSON] et al. [11]. They pointed out the dominance of interparticle forces as being primarily responsible for the non-homogeneities observed during fluidization. Several studies highlighted the effect of the interparticle force on the fluidization hydrodynamics [12; 13; 14; 15; 16] and proposed techniques for their measurement [17]. Among the assisted fluidization technique is the pulsation of the inlet flow to the fluidized bed, which has been shown to improve the fluidization hydrodynamics of ultrafine powders [18; 19; 20; 21]. Another extensively employed technique is the introduction of acoustic vibrations during fluidization [22; 23; 24]. This technique helps to lower the minimum fluidization velocity and eliminate non-homogeneities in a bed of ultrafine powder [22]. [PERSON] et al. [24] used this technique for drying lignite by varying the sound pressure level and frequency and reported the fluidization quality. The rate of drying and drying completion time improved at high sound pressure levels. Despite being effective in improving the quality of the fluidization, this technique is energy intensive and requires modification in the fluidization setup for introducing the acoustic vibration in the bed. Another simple and cost-effective assisted fluidization technique that does not require input of energy is the addition of external particles. A careful selection of external particles even in small proportions can substantially improve the fluidization hydrodynamics [7; 25; 26].