Lecture 2, Spray Drying Flashcards
definition id spray Atomization?
it is a process of converting fluid into a fine spray comprining high surface area droplets
What are the equilibrium conditions for the Breakup of drops?
write down the equation
the internal pressure at any point on drop surface balances out external aerodynamic pressure and the surface tension pressure
what influence the breakup of drops?
the external areodynamic pressure and surface tension pressure
When do we speak of a stable droplet?
When the surface tension pressure is not higher enough to compensate the external aerodynamic pressure
what is the Critical drop size?
it is the smallest droplet size at a given external aerodynamic pressure
Which kind of distribution do we have
Numberic base (laser diffraction) and volumeric distribution (sieve analysis)
Which distrbution are used to describe the droplet size distribution?
Normal and Log-norma distribution used for model distribution
Log-hyperbolic distribution for bi.modal distribution
What’s the advantages of useing the Log-hyperbolic distribution?
used to calculated the real mean value of the droplet size distribution
Name some empirical distributions
Weibull /Rosin-Rammler
Nuyiyama-Tanasawa
what’s the equation of the Weibull emiorcal distribution?
1-Q= exp- (D/X)^q/n
Where is is preferable to used the weibull distribution?
when having larger particles sizes
learn how to used the equation
do you know it?
Where is is preferable to used the Nukiyama-Tanasawa distribution?
when having multiple peak distribution
which kind of Spray guns/ nozzles do we have?
Hydraulic and Pneumatic
what’s the main different between the Hydraulic and Pneumatic nozzles?
in the Pneumatic, there we additional have the atomization air
Name the two types of hydraulic nozzles
how how to draw them
Plain and V-Orifice
What’s the disavantages o the hydraulic nozzle?
it requires high pressure. the higher the pressure, the higher the velocity, obatining samller droplet size
Limited flexbility
Where should be the hydraulic nozle not be used?
for water base system because it will lead to overwetted product.Therefore better for organic solvent system
What’s the problem when using the hydraulic V orific?
due to the tin diamter of the orific, it’s pronts to clumping
Name the two types of Pneumatic nozzle
Know how to draw them
Internal and External mix
Advantage of the Pneumatic nozzle
low pressure (1-2 bar) more degree of freedow (change of the atomizing gas, liquid feed rate and the position of the infeed of the liquid of the nozzless
What’s the main difference between the internal and external mix?
the atomizing gas is mixed internal with the liquid in the internal mix, while in the eterna is vice vers
other difference between the internal and external mix?
Internal mix required higher velocity of the atomizing gas, which should lead to clumps
External mix, the spray cone is very wide
Which nozzle design is prefered for spray drying
Hollo cone
How does the rotary atomizer works?
under mechanical conditions, with the hepl of centrifugal energy the droplets are breakdown into smaller droplets
What can be adjusted in the rotary atomizer?
the feeding rate of the fluid an dthe rotation speed.
how does the vicosity influence the breakup of the droplets?
high vicosity at low rotion speed, leads to less breakup of the droplets
the gravitaion force and velosity has to be increse
What’s the main problem with the rotary atomizers?
the chnage of one parameter (density) leads to the change of every other paramter
what are the two different kind of liquid break up?
Subcritical and supercritical liquid breakup
what’s the main different between the Subcritical and supercritical liquid breaup?
Subcritical is when the surface tension is sufficiently strong relative to film/liquid inertial to draw liquid into a singe ligament (stream of bigger droplet size)
Supercritical iw when inertial effects casue ligament break up into many smaller ligamnets very close to the rotary atomizer
the higher the viscosity the more favored is ligament formation–>Subcritical atomization
what are other factors which infleunces the atomization?
Surface tension Viscosity Rheological proerties Temperature Properties of second liquid
How does the Surface tension influence the atomization process?
if the surface tension it to high it leasd to igher droplet and therefore bigger angle
How does the Viscosity influence the atomization process?
higher the viscosity is leads to a decrease of the Reynold number–> Laminar flow is dominant
When do we have a Turbulent flow?
at low viscosity which increase the Reynold number
Rheological properties influences
Definition of shear rate
Temperature influences
Viscosity and surface tension
Which kind of Liquid supply pump do we have?
Peristaltic pump, internal gear pump and lobe pump
Which influence does the pump has?
it greatly influence spray homogeneity
applicatio of spray drying
Solution, suspension, emulsions
why do we use atomization
to oncrease the surface area and therefore decrease the drying time
Learn to draw the Spray dryer
Do you know it?
what are the process of Spray drying?
Atomization, flasj drying and separation
What are the CPP in Spray dyring?
Drying air temperature
drying air flow rate
liquid feed rate
atomizing air pressure
what are the dependent parameters
outlet air temperature
droplet size
dyring efficiency
How does the Spray dried products looks like?
Draw and name each parts
Do you know it?
What are the two driving forces in component distribution in SD’d particles
- Surface actvity may lead to preferred adsorption of components to surface
- Surface area shrinks during evaporation from surface, leading to increase solute concentration at surface (decrease radius)
Which equation are used to describe the component distribution in Sd’d particles?
Evaporation rate, Characteristic droplet drying time
Surface entrichment
What does the evaporation od solution droplet spray dyring described
cen be described as a coupled heat and mass transport problme.
what’s the driving force for the drying process?
diference between the vapor pressure o the solvents and their partial pressure in the gas phase
what does the evaporation rate determines?
it determines the surface recession rate of the droplet
what is the equation or the evaporation rate?
diameter (d)^2 (t)= diameter at t(0)^2 - kt
why can the evaporation rate be described with the giving equation?
due to absence of internal converction, the distribution of the chemical components in an veaporating droplets is dscribed by the non-linear diffusion equation whihc becomes Fick’s second law of diffusion if no interactions between the solutes and constant diffusion coefficients are assumed
What’s the equation for the charcteristic droplet drying time?
t(d)= d(0)^2/k
What is the surface enrichment equation and from what does it derived from?
know the equation
it expresses the concentration C as a function of ceoncentration at the centr of the droplet
for what id the peclet number used?
it is used to explain the low density of particles spray dried, which is being pointed to the importance of the ratio between evaopration rate and diffusional motion of the solutes a
why is the surface enrichmnet important?
to decrease the speed of evaporation
what is the alternatively expression for the surface enrichment?
do you know the equation?
what does the constant evaporation model assumes?
that during most of the evaporation the gas temperature and thus the droplet temperature are constant
When does the constant evaporation model doesn’t count?
for the non-stationary evaoration at the beginning of the evaporation process (chnage of droplet temperature) and for any changes in the evaporation rate immediately prior to solidification of the particles
On what does the diffusion coefficient depends?
on the cencentraion of the solutes and change significantly during the drying process, especially when solue concnetrations reach super saturation or the liquid phase viscosity increase
why do we used the simplification model?
to prevent the application of the analytical model during the solidification of the particles
What does a low peclet number mean (< 1)
it means the diffusional velocity of the solute is faster or on the same order as the recession rate of the surface
Low surface enrichment
What does a low peclet number mean (< 1)
give the range of the low paclet number
it means the diffusional velocity of the solute is faster or on the same order as the recession rate of the surface
Low surface enrichment
0.44-1.0
What happend to the characteristic time when having a low peclet numbers
the tome of solute tp reach saturation at surface is lose to droplet lifetime, leading to very dense particles, potenially true density level
what happend if small peclet numbers ae coupled with large solute solubility?
precipitation is expected to apprear late in the evaporation process and relatively homogeneously throughout the droplet
What is the results of the precipitation?
the dry particles are expected to have little or no void space and a density close to the true density
What is the effect of a high peclet number?
it causes an enrichment of the solute at the surafce which likely leads to shell or skin formation.
even small molecules with high solubility (saccarrides) are expected to form hollo particles
range of a high peclet number
2-5
how does the density drops influences the palet number?
it increases the paclet number which can be seen in the increase void size
what does a larger peclet indicates?
it indicates that the rescession f the surface is fast compared to the diffusional motion of the dissolved molecules
How does the larger peclet number influence the concentration of the solute?
it increase sharply at the surface as the evaporation progresses
how does the high surface concentration influenve the viscosity?
the high surface concentration leads to a local inceae in viscosity with subseqent skin or shell formation
how the peclet number changes?
diffusion coefficents change with chnaging concentration
Pe number changes also with phase transition
which number affect the particle formation?
peclet number and the saturation of the solute
what does the peclet number described?
the surface accumulation due to differences in the diffusional motion of the solute and the surface recession rate
which othe parameters are important?
nucleation, crystal grwoth rate or bucking or crumpling of the shell
what does the contept of Pe and S allows
the prediction of radial concentration profiles in droplets which are about to solidify
Know how to draw the cyclone seperator and explain the motion
have you done so?
what are the difference typ of spray drying chamber?
cocurrent, counter-current and mixed flow dryers
when is secondary drying implemented?
when organic solvents (achive acceptable liquid contents) , QTPP, ASDs, vaccume desiccation
Application od spray drying?
Amorphous solid dispersion, Microencapsulation, density control
