Process Gases and Mass Transfer

Process Gases and Mass Transfer

Mass transfer between gas/liquid phases

•
Oxygenation of wort

•
Carbonation of beer

•
Deaeration of dilution water (high gravity brewing)

•
Nitrogen for blanketing gas in beer storage, mixed
gas dispense systems and dissolved into beers

Must control level of CO
2

in beer

Must exclude O
2

from beer

Principles: equilibrium, solubility, hydrodynamics of
gas/liquid systems

Liquid/Gas mix in closed system at constant temperature
–

dynamic equilibrium

Rate of transfer liquid to gas = Rate of transfer gas to liquid

Process Gases and Mass Transfer

At dynamic equilibrium, amount of gas dissolved in liquid
is proportional to the partial pressure in gas phase

p = H X or

Partial pressure = (Henry’s Constant) (Mole Fraction)

Henry’s constant increases with increasing temperature

Solubility of CO
2

measured on vol/vol basis

That is, volume occupied by the dissolved gas at STP if it
were removed from 1 m
3

of beer

Conditions in fermenter determine amount of CO
2

in green
beer

•
Open square fermenter
–

1 vol/vol

•
Cylindroconical fermentor
–

much greater

All brewery processes
–

must avoid gas breakout!

Process Gases and Mass Transfer

Pasteurization
–

Solubility decreases, must raise pressure

If gas breakout occurs, heating uneven, unreliable
pasteurization

Rate of dissolution of a gas in a liquid



dm/dt = rate of mass transfer

K
g

and K
L

= overall mass transfer coefficients

P
g

and P
E

= gas and equilibrium partial pressures of the
gas in the gas phase

C and C
E

= liquid and equilibrium concentrations of the gas
in the liquid phase

A = interfacial surface area for mass transfer

)
(
)
(
C
C
A
K
P
P
A
K
dt
dm
E
L
E
G
G




Process Gases and Mass Transfer

If partial pressure of CO
2

above the beer in storage is
greater than that required to keep CO
2

in solution, up
carbonation or pick
-
up occurs




dC/dt = rate of CO
2

concentration change w/ respect to time

V = Volume of the vessel contents

K
L

= overall liquid mass transfer coefficient

C and C
E

= liquid and equilibrium concentrations of the gas
in the liquid phase

A = interfacial surface area for mass transfer

)
(
C
C
A
K
dt
dC
V
E
L


Process Gases and Mass Transfer

Decarbonation can occur in rough pipes

•
Bubbles form in pits

•
Serve as nucleation centers for more bubbles

•
Use smooth pipes and avoid constrictions

•
Can be problematic in beer dispensing systems

Oxygenation
–

similar mass transfer processes

Equilibrium O
2

concentrations < CO
2

concentrations

Nitrogen blanketing can prevent excessive O
2

pick
-
up


Process Gases and Mass Transfer

Beer containing 1.8 volumes of CO
2

per volume of beer at
stp is pasteurized at 73

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carbon dioxide in the beer and the pressure required to
maintain it in solution at pasteurization temperature.
Explain why the pressure on the beer passing into the
pasteurizer unit must be significantly greater than the
pressure required at the pasteurization temperature


Density of water = 1000 kg/m
3

Gas constant for carbon dioxide = 0.189 kJ/kg.K.

Molecular weigh of carbon dioxide = 44

Molecular weight of water = 18

Henry’s constant at 73


= 440 MPa/mol fraction

Process Gases and Mass Transfer

A beer keg of 50 x 10
-
3

m
3

capacity stored in a cellar at a
temperature of 12

䌠捯C瑡楮猠㐰砠㄰
-
3

m
3

of beer whose
CO
2

concentration is 1.4 volumes per volume of beer at
stp. The head space in the keg is filled with CO
2

which is
in equilibrium with the beer. Stating any assumptions
made, calculate the pressure of the CO
2

in the head
space and the total mass of CO
2

in the keg.


Gas constant for carbon dioxide = 0.189 kJ/kg.K.

Molecular weigh of carbon dioxide = 44

Molecular weight of water = 18

Henry’s constant at 12

C = 120 MPa/mol fraction

1 kmol gas = 22.4 m
3

at stp.

Assume MW and density are same as that of water

Instrumentation

Accuracy
–

“Freedom from Error”

Process Industry
–

Accuracy = Inaccuracy…?

1% accuracy indicates that measured value
should be within 1% of true value

Accuracy vs. Precision

Random Error (Precision)

Systematic Error (Bias)

Accuracy of instrument


% of full scale


% of measurement


Instrumentation

Selection and siting of remote sensors

•
Product composition, temperature, pressure

•
Cleaning and sterilization

•
Accuracy and repeatability

•
Reliability and maintenance

If instruments to not meet user requirements

•
Extra design/engineer effort for new plant

•
Delay in construction and start
-
up

•
Extra cost (man hours) for commissioning

•
Less than optimum performance

•
Adverse consequences on personnel, plant
and/or environment

Pressure Measurement

Manometer
–

Measure height of liquid (Calibrate)

Pressure Measurement

Mechanical
–

Bourdon Tube

Pressure Measurement

Electrical
–

Strain, capacitive or piezoresistive

•
Strain
–

small deflection changes resistance

•
Capacitive
–

High freq. oscillator, plates vary gap

•
Piezoresistive
–

Monocrystalline silicon

Temperature Measurement

Thermometer

•
Liquid/gas

•
Bi
-
metallic

Resistance Temperature Detector

Thermocouple

Infrared Temperature Detector

Level Measurement

Bubble Tube
–

Pressure required to inject air into
a tank indicates height (bulk sugar tanks)

Force balance
–

Measure pressure at bottom of
tank, indicates height

Ultrasonic
–

Sound waves reflect off of liquid/gas
interface

Float

Single position

Flow Measurement

Magnetic
–

Faraday’s Law (Conductor moving
through magnetic field, voltage produced)

40:1 Rangability


〮0%潦⁦o献s慣捵牡捹

乯扳N牵捴楯湳

Fluid must conduct

No good for

•
Pure water

•
Gases

•
Hydrocarbon fuels

External elec/mag fields

Flow Measurement

Non
-
Linear
–

Venturi, orifice, nozzle meters

4:1 Rangability


2% f.s. accuracy


Flow Measurement

Turbine
–

magnetic pulse as turbine wheel spins

20:1 Rangability,

〮05%昮f⸠慣.畲慣y

Easy to interface with control system


Gas Measurement

Sensor Performance Parameters:

•
Sensitivity (or signal magnitude)

•
Response time

•
Repeatability (or precision)

•
Linearity

•
Background current or voltage

•
Temperature, pressure effects

•
Stability of span and background

•
Expected lifetime

•
Interference

Gas Measurement

CO
2

Infrared Sensor

•
Non
-
dispersive infrared (NDIR)

•
IR source at one end of tube

•
Variable filter (Fabry
-
Perot Interferometer)

•
IR detector

•
FPI varied to adsorption band of gas

•
Ratio of two signals indicates concentration

O
2

Measurement in Solution

•
Chemical processes or electrochemical

•
Membrane separates fluid and electrode

•
Oxygen diffuses through membrane

•
Split into ions and electrons, goes to anode

•
Signal amplified

Drying and Psychrometrics


Reasons for drying


-

Reduce mass of material


-

Reduce volume of material


-

Change handling characteristics


-

Material preservation


Moisture in solids


-

Bound
–

water retained in capillaries, absorbed

on surfaces or in solution in cell walls


-

Free
–

water in excess of equilibrium content


Drying and Psychrometrics







Free water first

Saturated surfaces

Rate slows, diffusion

Equilibrium reached

T
2

T
1

Material being
dryed

Temp (

C)

T
2

T
1

Time

Drying Rate

Moisture Content

Drying and Psychrometrics

Barley


-

Harvested (20%), Storage (10%)


-

Max. drying temp: 46

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
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

-

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

-

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
䵡汴


-

Drying to stop enzyme activity after malt

modification is complete


-

Moisture content reduced from 45% to <4.5%


-

Stage 1: T
in

= 65

䌬⁔
out

= 30

䌬⁗䌠㴠C4%


-

Stage 2: T
in

= 80

䌬⁔
out

= 50

䌬†坃‽‵%


-

Stage 3: T
in

= 100

䌬CT
out

= 100

䌬C坃‽′%


-

佶敲慬a灲潣敳猠
–

18 to 48 hours

Drying and Psychrometrics

Hops


-

Picked at 80% moisture, dryed to 10%


-

T
in

55 to 65

䌠


Y敡獴


-

䑲D楮朠牥煵楲i搠慦瑥爠扥湧慵瑯汹獥d


-

Sprayed onto rotating, steam heated drums


-

Removed with a “knife” after one rotation


Spent grains


-

75% moisture after being pressed


-

Must be dryed at low temperature


-

Air heated with steam or hot water

Drying and Psychrometrics

Humidity ratio: mass of water vapor / mass of air

Saturation humidity: saturation mass / mass of air
at a given temperature

Relative humidity: humidity / saturation humidity

Dew point: Temperature at which a given mixture
would become saturated at given humidity ratio

Wet bulb temperature: equilibrium temperature at
interface of water and air


See psychrometric chart.

Drying and Psychrometrics

Example 1: Determine the humidity ratio and
relative humidity in the room today

Example 2: Air at 50% relative humidity and 22

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
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
Humidifying with water spray


m
w,in

+ m
w,spray

= m
w,out

Dehumidifying
–

2 step process (cool, then heat)

Evaporative cooling
–

Water spray into air,
evaporation cools water (latent + sensible)


Approaches wet bulb temperature (+3

䌩

What we’ve covered so far…

Dimensions and Units

Systems, Phases and Properties (Steam tables)

Conservation of Mass and Energy

Newtonian Fluids and Viscosity

Reynolds Number, Laminar and Turbulent Flow

Entrance Region, Velocity Profiles

Friction Loss in Pipes and Fittings

Flow Meter and Valve Types
–

Relative Merits

Pumping Power, Types, Sizing, Cavitation/NPSH

Filtration, Solids Settling

Heat Transfer (Conservation of Energy)

Conduction, Convection and Radiation

What we’ve covered so far…

Heat Transfer Equipment

LMTD and Overall Heat Transfer Coefficient

Heat Exchanger Sizing, Heat Losses

Combustion and Steam Generation

Refrigeration Cycle

Pasteurization
–

Flash and Tunnel

Drying and Psychrometrics

Primary and Secondary Refrigerant Applications

Wort Boiling

Process Gases and Mass Transfer

Instrumentation and Control

Materials and Corrosion

Analysis “Tools”

•
Mass and energy balance

•
Properties, phases, steam/refrig. tables

•
Pressure drop in pipework (Re, Moody)

•
Pump sizing technique

•
Heat transfer, resistance analysis

•
Heat exchanger sizing

•
Refrigeration cycle and tables

•
Psychrometric chart (for drying)

•
Gas
-
liquid mass transfer

Where we are Going

The next 6 weeks
–

mornings only


-

Fluid Flow


-

Heat Transfer


-

Steam


-

Refrigeration


-

Materials (of Construction)


-

Process Control and Instrumentation


-

Sterile Filtration and Pasteurization

Readings, Tests, Class Discussions