# Process Gases and Mass Transfer

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22 Φεβ 2014 (πριν από 4 χρόνια και 10 μήνες)

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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

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

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

through magnetic field, voltage produced)

40:1 Rangability

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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

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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䌠景爠C0%

-

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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‽′%

-

18 to 48 hours

Drying and Psychrometrics

Hops

-

Picked at 80% moisture, dryed to 10%

-

T
in

55 to 65

Y敡獴

-

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-

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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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

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)

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