AAPS PharmSciTech

, Volume 10, Issue 1, pp 303–309 | Cite as

Psychrometric Analysis of the Environmental Equivalency Factor for Aqueous Tablet Coating

Research Article

Abstract

Process control of aqueous tablet coating depends on a number of thermodynamic and psychrometric variables. Since many of these variables are interdependent, the choice of parameters by which to control the process or designate a design space is not necessarily obvious. Several mass or heat conservation models for aqueous tablet coating can be found in the literature, varying in approach and proposed method for controlling the coating process. A commonly used first-principles model built upon the coupled heat and mass transfer in evaporative mass transfer derives an “Environmental Equivalency” (EE) factor as an indicator of the relative rate of water evaporation from the tablet bed surface and as a relevant scaling factor for aqueous coating. The EE factor is expressed by an equation involving ten individual parameters; however, if the derivation of EE is extended further under the context of an adiabatic process, a much-simplified yet equivalent expression for EE emerges consisting of only three parameters, each directly measurable or obtainable from a psychrometric chart and which bear direct significance to the gross thermodynamic conditions of the coating. The psychrometric model herein is presented as a more physically evocative description of the coating process, enhancing process understanding and potentially playing a key role in a Quality by Design approach to defining an aqueous coating design space.

Key words

aqueous tablet coating environmental equivalency psychrometric chart scale up thermodynamics 

Nomenclature

A

surface area (m2)

cp

constant pressure specific heat (J/kg·K)

h

enthalpy of moist air (J/kg)

hfg

latent heat of evaporation (J/kg)

Hloss

heat loss (J/s)

k

mass transfer coefficient (kg/m2·s) or heat transfer coefficient (J/m2·K·s)

K

empirical heat loss coefficient (J/K·s)

\(\mathop m\limits^. \)

rate of mass transfer (kg/s)

M

molecular weight (mol/kg)

p

pressure (Pa)

q

rate of heat transfer (J/s)

RH

relative humidity (%)

R

ideal gas constant (J/mol·K)

T

temperature (°C or K)

Y

mass fraction in vapor phase

Greek

ρ

density (kg/m3)

ω

humidity ratio (kg water per kg dry air)

Subscripts

1

point 1

2

point 2

free stream

a

dry air

a+w

moist air

amb

ambient

B

tablet bed

ex

exhaust conditions

film

film conditions

h

heat transfer

i

species i

in

inlet conditions

m

mass transfer

na

nozzle air

pa

process air

w

water

wb

wet bulb conditions

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Copyright information

© American Association of Pharmaceutical Scientists 2009

Authors and Affiliations

  1. 1.Global Formulation Sciences, Global Pharmaceutical Research and DevelopmentAbbott LaboratoriesAbbott ParkUSA

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