Drying

Question 1

Why is drying performed?

Answer 1

• Prevents chemical degradation, physical aggregation or precipitation
• Aids in formulation (can remove volatile contaminants)
• Helps with storage (biological products are thermally labile so cannot use high T)

Question 2

2 forms of water in biological solids:

Answer 2

Unbound water:

• Water in equilibrium with vapour phase (same vapour pressur)
• Held mainly in voids of the solid

Bound water:

• In fine capillaries
• Very low vapour pressure

Question 3

Dry Basis vs Wet Basis

Answer 3

Dry:
x_a = m_a/m_b

Wet:
w_a = m_a/m_t
=m_a/(m_a + m_b)

Question 4

Define Relative Saturation

Answer 4

Phi = p_i/p_i*

Question 5

Define Humidity

Answer 5

Mass of H2O per mass of dry air:

H = m_w/m_a = M_wp_w/(M_a(P-p_w))

Question 6

Relationship between concentration and humidity

Answer 6

c_w = p_w/RT

c_w = H(M_a/M_w)/(1+H(M_a/M_w))*P/RT

Question 7

Relative Saturation

Answer 7

Phi = (pw/pw*) * 100

Question 8

Describe the two drying periods

Answer 8

Constant Drying Period
- r.d.s: removal of liquid from surface to bulk

Falling Drying Period
- r.d.s: internal diffusion of water to cake surface

Question 9

Describe the two falling drying periods

Answer 9

First falling period

• wet surface
• r.d.s: transfer of moisture to surface

Second falling period

• dry surface: plane of separation in the solid
• r.d.s: molecular diffusion through solid (largely independent of conditions outside solid)

Question 10

Define Hygroscopic

Answer 10

Strong interaction with water

No constant drying period

Question 11

Constant drying period equation and assumptions

Answer 11

w = k_gA(p_s - p_w)

Assumes drying takes place from a saturated surface

In case of wind:
w = k_gA(p_s - p_w)*u^0.8

Question 12

Heat transfer equations required for vacuum shelf dryer with moving wet solid level

Answer 12

Heat Transfer:
q = k(T_0 - Ts)/y

Heat in = Heat to vaporise water:
qA = -LambdaroA(dy/dt)

• Insert for q
• Integrate and solve (y=d at t=0)

Question 13

Falling rate period: drying mechanism

Answer 13

Diffusion Theory: Movement of water is governed by diffusion (Fick’s Law)

Capillary Theory: Movement of water due to Capillary forces

Question 14

Sherwood and Newman solution to Fick’s Law

Answer 14

Truncated to only the first term for long drying times (Dt/d^2 > 0.1):

-dXm/dt = (pi^2 D)/(4d^2) *(Xm - Xe)

Question 15

Constant dying period empirical equation

Answer 15

Assume rate of dring is proportional to free moisture content:

Rc = -1/A *dX/dt

tc = (X1 - Xc)/(A*Rc)

Question 16

Falling rate drying period

Answer 16

dX/dt = -mA (X - Xe)

tf = 1/(m*A) *ln((Xc - Xe)/(X - Xe))

Question 17

Solving for Total drying time

Answer 17

Falling and constant drying rates are the same at Xc

Rc = mA(Xc -Xe)

Question 18

Means of heat supply in drying processes

Answer 18

• Direct heat transfer (hot gas)

- Indirect heat transfer (heat exchange surface, microwave, radiative)

Question 19

Means of vapour removal in drying processes

Answer 19

• Cross flow of carrier gas
• through flow of carrier gas
• suction

Question 20

Means of solid motion in drying processes

Answer 20

• Static (fluidised bed) vs agitated (e.g. tumble drum or agitated vessel)

Question 21

Freeze Drying process description

Answer 21

• Decrease T below triple point Temp to freeze moisture
• Decrease P to below triple point pressure
  (First 2 steps on the order of hours)
• Increase T & decrease P to cause sublimation (primary drying of unbound liquid)
• Further increase of T and decrease P (secondary drying of bound liquid)
  (time of these steps on the order of days)

Question 22

When is freeze drying used

Answer 22

Thermally labile proteins or hygroscopic solids