Armfield Engineering Education
BASIC PROCESS PRINCIPLES
CERa/CERb Mass Transfer and Diffusion Coefficients - Issue 10
Two separate items of laboratory equipment have been designed to allow measurement of molecular diffusivities and, in so doing, to familiarise students with the basic notions of mass transfer theory. The gaseous diffusivity apparatus (CERa) involves diffusion with bulk flow, whilst the liquid diffusivity apparatus (CERb) relates to an equi-molar counter-diffusion process.
ORDERING SPECIFICATION
Ordering Specification CERa
Bench mounted apparatus for the determination of diffusion coefficients of a vapour in air, which uses the method of measuring the rate of evaporation of a liquid through a stagnant layer into a flowing air stream, comprising:
i) A precision bore capillary tube, which may be filled from a syringe, and at the top of which means are provided to pass air (or an inert gas) stream to remove vapour.
ii) An air pump.
iii) A travelling microscope with accurate focus adjustment and mounted for vertical axis movement against a Vernier scale having 0.1mm graduations.
iv) A thermostatically controlled water bath, in which to place the capillary tube, capable of accurate temperature control within the range ambient to 60 degrees centigrade to ±1 degree centigrade.
Experimental Capabilities
> Direct measurement of mass transfer rates in the absence of convective
effects.
> Use of gas laws to calculate concentration differences in terms of
partial pressures.
> Use of Fick’s Law to measure diffusion coefficients in the presence
of a stationary gas.
> Measurement of the effect of temperature on diffusion coefficients.
> Gaining familiarity with the use of laboratory instruments to achieve
accurate measurements of data required for industrial process design.
Ordering Specification CERb
Bench mounted apparatus for the determination of diffusion coefficients of components in the liquid phase. The method employs a diffusion cell of capillary tubes so constructed to permit equi-molar counter diffusion between liquids of differing concentration each side of the cell without convective effects being present.
Concentration changes on one side of the cell with respect to time are measured with the conductivity cell and the meter provided, and a magnetic stirrer keeps the bulk solution well mixed.
The resolution of the conductivity sensor and meter is 0.1 x 10-6 S for a 1M salt solution diffusing into pure water.
Possible to obtain reproducible and accurate values of diffusivity within a period of 1.5 hours of practical laboratory time.
Experimental Capabilities
> Accurate measurement of mass transfer rates in the absence of convective
effects.
> Use of gas laws to calculate concentration differences in terms of
partial pressures.
> Use of Fick’s Law to deduce diffusion coefficients from measurement
of mass transfer rate and concentration difference.
> Simple analysis of a first order unsteady state process.
> Effect of concentration on diffusion coefficients.
> Gaining familiarity with the use of laboratory instruments to achieve
accurate measurements of data required for industrial process design.
Data logging accessory (supplied). Allows the signals from the conductivity meter used in the diffusion cell to be displayed, logged and recorded on a customer supplied PC, using a RS232 Serial interface.
TOPMeasurement and Instructional Capabilities
CERa Gaseous Diffusion Coefficient Apparatus
> direct measurement of mass transfer rates in the absence of convective
effects
> use of gas laws to calculate concentration differences in terms of
partial pressures
> use of Fick's Law to measure diffusion coefficients in the presence
of a stationary gas
> measurement of the effect of temperature on diffusion coefficients
> gaining familiarity with the use of laboratory instruments to achieve
accurate measurements of data required for industrial process design.
CERb Liquid Diffusion Coefficient Apparatus
> accurate measurement of mass transfer rates in the absence of convective
effects
> use of Fick's Law to deduce diffusion coefficients from measurements
of mass transfer rate and concentration difference
> simple analysis of a first order unsteady state process
> effect of concentration on diffusion coefficients
> gaining familiarity with the use of laboratory instruments to achieve
accurate measurements of data required for industrial process design
> option of using on-line microcomputer data logging and subsequent analyses.
Description a
CERa Gaseous Diffusion Coefficients Apparatus
The diffusion of a vapour 'A' from a volatile liquid into another gas 'B' can be conveniently studied by confining a small sample of the liquid in a narrow vertical tube, and observing its rate of evaporation into a stream of gas 'B' passed across the top of the tube. Normally, for simple instructional purposes, 'B' is air and 'A' is an organic solvent such as acetone or methyl alcohol.
The apparatus consists essentially of a glass capillary tube placed in a transparent-sided temperature controlled water bath. A horizontal glass tube is fixed to the upper end of the capillary tube and air is blown through this by a small air pump included within the unit. This arrangement allows the maintenance of a partial pressure difference within the capillary tube between the evaporating liquid surface and the flowing air stream. A travelling microscope, with sliding vernier scale, is mounted on a rigid stand alongside the thermostatic bath and is used to measure the rate of fall of the solvent/air meniscus within the capillary.
The relation between the measured molar mass transfer rate ('NA ' per unit area), the partial pressure gradient and the diffusion coefficient D is deduced from the one dimensional steady state version of Fick's Law with bulk flow:
| NA=-D | [CA+CB] | dCA |
| ________ | ____ | |
| [ CB ] | dy |
where CA and CB are the molar concentrations of the vapour 'A' and air 'B' respectively.
Description b
CERb Liquid Diffusion Coefficients Apparatus
Armfield has developed a unique diffusion cell which overcomes the traditional problem of slow diffusion rates in liquids requiring long observation times, but without sacrificing accuracy or introducing convective effects. Essentially, the cell consists of a honeycomb of accurately dimensioned capillaries, positioned between two liquids of differing concentration of the solute whose diffusion coefficient is to be determined.
In practice, a small volume of concentrated solution is placed on one side of the honeycomb, whilst the other side consists initially of a large volume of pure solvent (water). As diffusion of the solute occurs, the concentration within the larger volume increases, and is monitored with a conductivity sensor and meter. The mixture is continually stirred with a magnetic stirrer to ensure uniform concentration within the bulk liquid. Whilst the conductivity sensor may be readily calibrated for any required aqueous system, for introductory studies, dilute solutions of sodium chloride are recommended, for which conductivity data are provided.
TOPTechnical details -A
CERa Gaseous Diffusion Coefficients Apparatus
Technical Details
Thermostatic bath: capacity 4.0 litres
Water heater element: 500 Watts
Temperature controller: range 0 to 60°C, on/off type
Temperature sensor: PTC
Vernier range: 0 to 70 x 0.10 resolution (mm)
Technical details -b
CERb Liquid Diffusion Coefficients Apparatus
Diffuser vessel: capacity 1.0 litre
Conductivity meter: range 200 µS, 2mS, 20mS
Recommended Accessories
Stop clock
Cartridge deioniser
Complementary Products
CES: Wetted Wall Gas Absorption Column
UOP3BM: Batch Distillation Column
UOP3CC: Computer Interfaced Distillation Column
UOP4Mkll: Solid-Liquid Extraction Unit
UOP5: Liquid-Liquid Extraction Unit
UOP7: Gas Absorption Column
requirements
Electricity supply:
CERa-A: 220-240V/1ph/50Hz
CERa-B: 120V/1ph/60Hz
Data logging accessory requires a customer supplied PC, with RS232 Serial
interface, running Windows 98, 2000, ME or XP.
Shipping specification
| ITEM | Volume | Gross weight |
|---|---|---|
| CERa | Volume: 0.20m3 | Gross Weight: 30kg |
| CERb | Volume: 0.10m3 | Gross Weight: 10kg |
Overall dimensions
| ITEM | Height | Width | Depth | Diameter |
|---|---|---|---|---|
| CERa | 355mm | 450mm | 390mm | - |
| CERb | 305mm | - | - | 190mm |
| Conductivity Meter | 130mm | 150mm | 220mm |
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