Heat and mass transfer in spray drying

Abstract

ACKNOWLEDGEMENTSThe author wishe s to express his tha.nks to ail tho se who, in various ways, have cmtributed to this work:To the staff and graduate students of the Chemical :Engineering Department, for their willing contributions ani suggestions.To the Pulp and Paper Research Institute of Canada, for the loan and construction of certain pie ces of equipnent.To his wife, for her assistance wi.th the spray dryer operation and a.nalytical work.To the National Research Council, .for the award of a Bursary and two studentships which greatly facilitated the carrying out of this re se arch. 3.4. 3In i ts broader aspects the sprey drying operation can be di vided into three major operations:1. atomization of the solution, slurry or colloidal suspension;2. evaporation a.n:l. drying of the liquid droplets so produced;3. separation of toe dried particles from the drying gaseous mediwn and their collection.Recent studies have made it abundantly clear that atomization has far reaching consequences mt only on the rate of drying but also on the physical properties of the product. The first section of this survey is therefore devoted to a brief description of the more common atornizing nozzles presently available, particl.Üarly from the point of view of the particle size ani size distribution that càn be expected from them.Following atomization, the liquid droplets are mixed with the hot gases in the drying chamber. The design of this chamber must provide adequate residence time for the droplets to be dried, and the various flow patterns encountered in drying chambers constitute the next section.Finally a section is also devoted to the various methods of separation ani collection of the dried product.The evaporation of the liquid sprey involves simultaneous consideration of heat, mass and momentwn transfer, and although a considerable amount of work has been done on the evaporation and drying of single droplets, only limited data are available for the evaporation and drying of clouds of particles, particularly in turbulent gas streams. These considerations are discussed separately am in considerable detail in the second chapter of this review.I. THE SPRAY DRYING PROCESS ATillHZATIONThe most :ilnportant single step in acy sprey drying process is, without doubt, atomization. Under given operating conditions, the initial partiel& size will have a direct bearing on the product 1 s moisture content, recovery, appearance, bulk density ani wetting properties, and must therefore be closely controlled.Atomization is, moreoever, of great importance in IIl8.l\Y' other fields out side the process industries. In agriculture the spraying of liquid chanicals such as insecticides, herbicides, fungicides and defoliants is achieved by me ans of atomization. The use of liquid fuels for combustion in boilers, fuma ces, internal combustion piston angines, jet engines and rocket angines has also led to the increased use of atomizers and, consequerrt.ly, further stimulated research in the field of atomization. a) Theory of Atom...

Spray‐Dried Particle Morphologies

“…Morphologies of multicomponent mixtures were found to be complex with the respective migration rates of the solids being dependent on drying temperature. Dlouhy and Gauvin (1960) studied the evaporation and drying rate of spray-dried calcium lignosulphate (1 8% w/w) solutions. Photomicrographs of the spray-dried particles reveal mainly discrete particles with clear, smooth skins.…”

“…A greater proportion of hollow particles was also produced at the higher drying temperature, although a gas pressurised feed tank was used to transfer the feed to the atomiser, possibly promoting particle vacuolation. Dlouhy and Gauvin (1960) reported no significant falling-rate period when drying the lignosulphate and suggested this was due to the concentration Left: codried egg and skimmed milk (shrivelledparticles), middle: dyestuffs (blowhole), right: coffee (thin shell wall). Figure 13.…”

Evaporation and Humidification

“…(25) While equation (6.41) is derived for the evaporation of an isolated drop into still air, the expression also describes the evaporation from a cloud of droplets co-currently entrained in turbulent air if the droplets are very fine (14 < d p < 30 μπι). (26) Whenever the droplet size is much smaller than the scale of turbulence, the droplet appears to follow faithfully the continuous-phase motion with a negligible slip velocity ; (27) moreover, the atomized droplets swiftly disperse, often to occupy less than 0-1% of the available space, the reported threshold for significant droplet interaction. <28 ' 29) Measured integral scales of turbulence in one cocurrently worked spray-dryer, 0-2 m in diameter, ranged between 90 and 520 μιη, (30) and thus complete entrainment in that unit is likely with particles below 50 μιη in diameter.…”

Experimental investigation on the migration of sub‐microscopic droplets from the evaporating surface

“…---(Fo-Ho")/T = (H"-Ho")/T = ideal gas heat capacity a t constant pressure, cal/mole "K ideal gas free energy function, caI/nioIe "I< ideal gas enthalpy function, where Flu" is l-Io a t O'K, cal/mole "I( product of principal iiionients of inertia, g 3cms principal moment of inertia with the ith principal axis, where i = A , B , C ; gcniz moment of inertia of rotating top, gcin2 reduced moment of inertia, gcmz free internal rotation partition function gas constant, 1.987 cal/inole "I< ideal gas entropy, e.u. ctbsolu te tempera t 11 re, "I( potential barrier for internal rotation, cal/ inole number of minima in one rotation of a group of atoms with internal rotation directional cosine between axis of rotation and the ith principal axis, where i = A,B,C fundamental vibrational frequency, cni-1 molecular symmetry number a m b a a e d . This discrepancy was ascribed to the formation of very emall droplets which are entrained by the vapor generated at the pores of the surface into the boundary layer where they evaporate.…”