Aspects of fermentation and distillation in ethanol production

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ASPECTS

OF

FERI1ENTATION

AND

DISTI LLATION

IN

ETHANOL PRODUCTION

Thesis subnritted

for the

degree

of

Doctor

of

^philosophy

at

the

School

of

Engineering, The

University of

Auckland,

New Zealand

by

[vl. G,

^WEEKS

December, I983

Page

4 10 11 20 24 35

42 64 69 70 80

81

Corrections Replace

Line

22 10 11 2

2L

equation 1.19

16

Figure

2.1

9 2L

Figure

3'2 Table

3-1, for stainless steel Figure

3.4,Ln 2

Figure 3.8rln

²

L2

with

insert

^(Othmer,

f98I)

84 91 184

10r

operation insoluble

pthalate

dY.l_

dy

directionally

fermenter

calibrated + o.loc

Iron

Sand Concentration

O.26 trun/s

0.19 ^nmr/d

initial

o.2 g/L o.5

g/L Relmolds Nr:rnber

IO

v/v

⁴

operating soluble phthal.ate

dY.) dy

directly

fermentor graduated

+

o.tzsoc

Iron

Sand Concentration (as received)

2.6

nn/s

1.9

run/s

nominal

initial

o.5 s/L o.2

^g/L

length

Reynolds Nr:mber

1-

ABSTRACT

The fermentation

of

sugars from biomass

to

^produce

liquid fuels is receiving

widespread

attention

as

a

renewable source

of energry. For

suctr processes

to

^become competitive

with current alternatives,

technology must

be improved

to

increase

the efficienqg

and

productivity of the

operation.

Using

ethanol

fermentation

by

Sacchatomgces

cereyisiae yeast

as

a

^model system, two aspects

of the

process were considered

in det,ail.

The

first

aspect concerned

the

use

of cell recycle in a

continuous

fermentation.

A new technique was developed

for ttre rapid settling of yeast cells in ttre fernentation

redium and

involved the addition of

dense,

inert particles to a yeast

suspension

at pll 4.5, followed by a

rapid change

in

^pH

to 8.0 - 9.0.

^Large

flocs

forned imnediateJ-y and

settled rapidly' leaving a clear supernatant.

Separations

of 99.9t

were possible, even

at yeast

concentrations

of

⁵⁰

g/L

^(dr:y

weight)

and increases

in

settling rate of

up

to

¹⁵⁰⁰

fold

were

observed.

lrlhen

the

^pH was returned

to 4.5rthe flocs

were destroyed.

Seeded

settling at

constant pH was

possible

although

the flocs

were

smaller, the settling rates

were lower and

sigrnificantly

more seed was

required. Flocculation

was

also

found

to

be

influenced to a

greater

extent by certain

components

in solution.

Nickel

powder was used

extensively in

^ijrese experiments although severar

other materials

were

tested, with

ground

iron

sand showing

potential for application

on

a larger

scale.

The pH

switching

technique

for

seeded

settling

was used

to

recycle

yeast cells in a

semi-continuous

fenentation. Application of the

tech- nique

to this

and

similar

systems

is

discussed.

The

factors affecting yeast/lnert

^porrvder

flocculation is

discussed and

a

model

is

proposed

to explain the

observed experimental behaviour

for flocculation, both at

constant pH and

with rapid

pII switching.

The second aspect

of ethanol production

considered

in ttris thesis

was

the distillation stage.

Equipment and techniques were developed

to obtain basic

mass

transfer information in binary or

multi-component

systems.

A new design

of

evaporation

cell

^{was used}

to

measure the evaporation

of ethanol

and water

mixtures into

an

air

stream

in

a wind

tr:nnel. fhis

^enabled

the effect of liquid

concentration on evaporation

rate to

be

studied

dynarnically from batch

tests.

Radiochemical

labelling

was used

to

measure

liquid

concentrations and proved

to

be

a relatively simple, rapid

and

precise analytical technique.

^Coupled

with the direct

measurement

of liquid

displacement,

precise information

on

the

evaporation

Ioss of

bottr

ethanol

and

water

components was obtained.

The pure component evaporation data agreed

well wittr literature correlations and, for ttre binary Iiquid mixtures,

good agreement ^was found between

the

experimental.Iy determined mass

transfer flux ratios

^and

those

predicted

from

Gil1iland''s solution to the

multi-component ^gas

diffusion

equations.

The

velocity

dependence

of the overall

mass

transfer coefficients

enabled estimates to be rade

of

the d:

stribution of diffusional

^resistance

between

the

gas and

liquid

^phases.

Lt-L

For

the

ethanol-wat€r system

diffusion

was gas

film controlled

and

the overall

mass

transfer driving forces

could

best

be represented

in

terms

of

^vapour concentration.

ACKNOWLEDGEMENTS

r

wourd

like to

thank my

supervisor, Dr p.A.

_Munro,

for his

guidance, advice and continued encouragement ttrroughout

all

aspects

of this investigation. r

would

also like to

^thank my co-supervisor,

Associate-Professor

P.L.

Speddingr and Associate-professor

D.J.

Spedding

for their

advice and assistance.

The

financial

support

of a university

Grants committee post-Graduate Scholarship

is gratefully

acknowledged.

I

would al-so

like to

thank:

The

technical staff of ttre

chemicar and Materials

Engineering Department

for their willing

and

able

assistance,

in particular

_Mr

T.N.

_{Gray, Mr}

J.p. Batt

and Mr

T.A.

Snape;

Mrs Anne Marie McGlashan

for her excellent typing

and

her

patience _and

skill in

deciphering

the script;

My

fellow

graduates

for providing a

congeniar atmosphere

in

which

to

work; Mark Jabronka

for his

computer software, Pare Keiha

for help in proof-reading,

_{and Mark Tayror}

for his

rhelpfulr

_comments;

Last, but not least,

my

parents for their

support and

encouragerent throughout

the

course

of this

work.

TABLE OF CONTENTS

ABSTRACT

ACKNOWLEDGEIT{ENTS

TABLE OF CONTENTS

I,IST OF FIGURES

LIST OF TABLES

SECTION

1.

INTRODUCTION

1.1 tiquid

Fuel Production

I.I.1

^The Global Energy

Crisis 1.1.2 Liquid

Fuels from Biomass

1.I.3

Energy Fanaing

1.I.4

Choice

of

^Areas

for

Experinentation

L.2

Ethanol Fenrcntation

L.2.L

Current Ethanol Fermentation Technology

L.2.2

Continuous

Fen€ntation

Concepts

I.2.3

Techniques

for

Retaining

or

Resycling Micro-Organisms

L.2.4

The Concept

of Cell Flocculation with

an

Inert

^Seed

1.3

Ethanol-Water Separation

I.3.1

Current Ethanol-Water

Distillation

Tectrnology

L.3.2 Alternatives to Distillation for

Ethanol-Water Separation

1.3.3

The Relevance

of

Fundamental Mass Transfer Research

1.3,4

^Mass

Transfer

Theory

I.3.4.1

General

L.3.4.2 Fickrs

law

of diffusion

I.3.4.3

Multi-component

diffusion in

^gases

1.3.4.4

^Mass

transfer

across

the gas-liquid

phase boundary

1.3.4.5

The mass

transfer coefficient

i

1v

v

ix xiii.

1

1

1

2

5 7 9

I

11 T4

19

2t 2t

23 26 29 29 30 35 39

43

I.3.5

Fundamental Research on Gas-Liquid Mass Transfer

1.3.5.1 Gas-Iiquid

mass

transfer in

ducts

I.3.5.2

Mass

transfer correlations

I.3.5.3

The wetted

waII

column

for

research

into gas-liquid transfer

1.3.5.4

Scope

for furttrer gas-liquid

nass

transfer

research

SECTION

2.

EXPERII4ENTAI,

2.L

Yeast

Settling

Experirents

2.1.1

Seed

Material Characteristics

2.L.2

^Seeded Yeast

Settling at

^{Constant pH}

2.L.3

Seeded Yeast

Settling with plf

Switching

2.L.4 lficroscopic Analysis of

Flocs

2.L.5

Seeded

Settling with

Bacteria

2.2

Ethanol Fernentation

2.2.L

Batch Fermentation

2.2.2

Semi-Continuous Fernrentation

2.2.3

Assay Techniques

2.3

Wind Tunnel Evaporation

of

Ethanol and Water

2.3.L

General Description

2.3.2 Air Velocity

Measurement

2.3.3

Ethanol Concentration Measuren€nt

2.3.4

Procedure

for

Evaporation Runs

SECTION

3.

NO\TEL I,IETIIODS OF YEAST ^RECYCLE

3.1 Selection of

^Seed Materials

3.2

Seeded

Settling with

^Nicke1

PAGE

45 45 51 54

57

59 59 59 60 6T 62 62 62 62 63 67 67 67 72 74 76

78 78 85

3.3

Batch Fermentation

3.4

Factors

Influencing

Yeast Separation

3.5 Prelininary Investigation of a

pH Switctring Technique

3.6

^Seeded

Settling wittr

^pH Switching

3.7

Seni-Continuous Fernentation

3.8 Settler

^Designr

3.9

Detennination

of Flocculation

Mechanism

3.10

Acid

and Base Requirements

for

^pH Switching

3.lL

Seeded

Settling wittr

Bacteria

3.12 A Proposed Mechanism

for

Seeded

Settling

3.13

Itre Potential for Industrial Application of

^Seeded

Settling

3.14 Conclusions

SECTION

4.

ETIIANOL.WATER MASS TRANSFER

4.I Velocity

and Temperature

Calibration 4.2 Prelininary

Experiments

4.3

Mass Transfer Results

4.3.1

Evaporation

of the

Pure Components

4.3.2

Evaporation

of

Ethanol-Water Mixture

4.4

Data

Analysis

Using Multi-Component

Diffusion

Equations

4.5 Selection of ttre Overall

Mass

Transfer Driving

Force

4.5.I Liquid

Concentration

as the Driving

Force

4.5.2

Vapour Concentration

as the Driving

Force

4.5.3

Sununation

4.5

The

Effect of Air Velocity

on

the Overall

^{Mass Transfer}

Coefficient

4.5.I

The Pure Component Mass

Transfer Coefficients

vii

PAGE

96 98 r02

108 116

t22 r25 t28

129 130 1,40 L44

t47 r47

t47 1s0 150 154

r62

163 164

t7r

t82

182

782

4.6.2

The Mass Transfer

Coefficients for

Ethanol-Water Mixtures

4.7

The Phase

Distribution of the

^Mass

Transfer

Resistance

4.8 Theoretical Prediction of the

^Mass

Transfer

Flux

4.8.1

^{The Ternary}

Diffusion

Eguations

4.8.2

Comparison

of

Theory

with

Experiment

4.9

Conclusions

SECTION

5.

RECOMMENDATIONS

5.1

Yeast Recycle

5.2

Ethanol-Water Mass Transfer

SECTION

6.

^APPENDICES

6.1

^{Raw Data}

for

Ethanol-Water Evaporation

6.2 Effect of

Sampling on Evaporation Data

6.3

Determination

of the

Gas Phase

Diffusion Coefficients 6.4

Vapour-Liquid

Equilibrium

Data

for

Ettranol-Water Notation

Referenc-es

Publications

PAGE

187

189 195 195

r96 20t

203 203 204

205 205 230 236 238 239 242 247

1X

FIGURE

1.r

2.L 2.2 2.3 2.4 2.5 2.6 3.1 3.2 3.3

I,IST OF FIGURES

Concentration

Profile at a

Gas-Liquid phase ^Boundary Semi-Continuous Fermentation Equiprent

The

Ferrentor

and

Cell

Separator

in

Operation Schematic Diagram

of

Wind Tunne1

overall

View

of

Wind Tunnel

A Cross-Sectional View

of the

Evaporation CeII The Evaporation Cel-l

Initial Settling

^{Rate versus Seed} Concentration

Initial Settling

Rate versus

Iron

^Sand Concentration Supernatant Yeast Concentration versus added Seed

Concentration

for

nonrinally ²⁰

g/L (dry

weight)

yeast

suspensions

in distilled

$tater

Supernatant Yeast Concentration versus

Iron

^Sand

Concentration

Supernatant Yeast Concentration ^(1og

scale)

versus added

Nickel

Concentration

for a

range

of initial yeast

concentrations

Batch

Settling

Curves

for various

concentrations

of yeast

seeded

with nickel

Initial Settling Velocity

versus Yeast Concentration

for yeast

^seeded

with sufficient nickel to give

^very

Iow supernatant

yeast

concentrations

Nickel

Concentration

required to give

Supernatant Yeast Concentrations

of 0.2 S/L

and

0.5 g/l for various Initial

^Yeast Concentrations

Concentration

Profiles in

^Batch Fennentation ^using 20

g/L yeast at

^35oc

Yeast Supernatant Concentration versus added Nickel Concentration

in clistitled water

^and

in the

^presence

of various

fermentation ^{medium components}

PAGE

40 64 65 68 69 7L 72 79 80 83

3.4

3.5

3.6

3.7

3.8

3.9

3.10

84

87

88

89

9t

97

99

3.r2

3. r3

3. 14

3. 15

3. 16

3. 17

3. r8

3. ¹⁹

3 .20

3.2L

3.22

3.23

3.24

Concentration

after

^seeded

settling

Supernatant Yeast Concentration

after switching

^the

pH from

acidic to basic

conditions

Yeast and

Nickel Flocs

fornred

after

pH Switching A 20

g/L

Yeast Suspension

before

^and

after

^pH

Switching

with Nickel

^Seed

Scanning

Electron

Micrograph

of Yeast/Nickel

^Flocs formed

at

Constant PH

Scanning

Electron

Micrograph

of

Yeast/Nickel ^Flocs formed bY PH Switching

Supernatant Yeast Concentration ^(1og

scale) versus

¹⁰⁹

Nickel

concentration

after

^pH

switching

^from

4'5 to

9.0 in distilled water, followed by gravity settling

Batch

Settling

^Curves

for various yeast concentrations

¹¹¹

seeded

with nickel

^{using pH}

switching

^{from about}

4.5 to

9.0

lnitial Settling Velocity

^{versus Yeast}

Concentration tlz for yeast

^seeded

with sufficient nickel to give

a

low supernatant

yeast

concentration ^and

with

^pI{

switching

^{from about}

4.5 t0

^9.0

Nickel concentration required to give a Supernatant

¹¹⁵

Yeast Concentration

of

^O-2

g/L (after

^{pH switching}

from about

4.5 to 9'0) for various lnitial

^Yeast

Concentrations

Settler in

^Operation

with

^{Yeast and}

Nickel Suspended tI?

in Distilled

^Water

Glucose Concentration ^and Ethanol Concentration

in the settler overflow

^and

Total

Ferrnentor Output versus

Tine, for operation of the

serni-continuous

fermentation ^equiPment

Settler in

^Operation

during a

Seni-Continuous Fermentation

Settling

^Curve

for Yeast/Nickel

^{Flocs produced by}

switching the

pH

of

^{t-he fermentor}

outflow

^{from 4'5}

to 8.0 after a

fermentation

time of

49 hours

Solids

Sedimentation

FIux

Curve obtained from batclt

settling of

⁵⁰

g/l yeast

and 200

g/1 nickel

103

ro7

118

LT9

L2r 105 106

L07

3.25 ^{L Z.+}

xl

PAGE FIGURE

3.26 4.1 4.2 4.3

4.4

4.5 4.6

4.7 4.8

4.9

4 .10

4 .11

4.L2

4 .13

4.L4

Schematic Representation

of Flocculation

^Processes

Vertical Velocity Profiles at ttre centre of the

^Winil

TunneI

A

Cross'sectional Velocity Profile at a bulk velocity

^I48

of

^{4.05 m,/s}

The Evaporation

of

^Pure Ethanol versus ^Time

at air

¹⁵²

velocities of

^4.OS

iZ",

^4.91

m/s,

^{5.64 m/s and 6 '47 n/s}

lltre Evaporation

of

Pure Water versus ^Time

at air

¹⁵³

vetocities of 4.0; ;/;,

^4.9L

m/s,

^{5.54 m/s and 6.47 m/s}

Evaporation

with

^Time

of

^{an aqueous}

ethanol solution

^t55

(initially 0.2 v/v \) at

^an

air velocity of

^{4'9L m/s}

Evaporation

with

^Time

of

^{an aqueous}

ethanol solution

^t57

iiiiai.uy

⁵

v/v t) at

^an

air verocity of 4'9r

^m'/s

Evaporation

wittr

^Time

of

^{an aqueous}

ethanol solution

¹⁵⁸

(initially

25

v/v t) at

^an

air velocity of 4'9I

^rnls

Evaporation

with

^Time

of

^{an aqueous}

ethanol solution

¹⁵⁹

(initially

50 v,/v

B) at

an

air velocity of

^{4'9L m/s}

Ethanol Evaporative

Flux

versus

Liquid Concentration

¹⁶⁵

at four air velocit'ies

Ethanol Evaporative FIux versus

Liguid Concentration

¹⁶⁶

at

an

air velocitY of 4'05

^n/s

Ethanol Evaporative FIux versus -Liguid

Concentration'

^t69

corrected

to a

temPerature

of lToc for air velocities of

^{4.05 m/s and 5.64 m/s}

Ethanol Evaporative

Flux

^versus

liquid Concentration'

^t7O

corrected to a

temperature

of

^17oc

for air velocitj'es

of

^{4.91 m/s and 5.47 m/s}

Evaporative FIux versus vaPour Mole

Fraction Driving

^t72

Force

for the ethanol

^component

at

^an

air velocity of

4.05 m/s

Evaporative FIux versus Vapour Mole

Fraction Driving

^t74

Force

for the ethanol

^component

at air velocities of

4.05 ^{rn/s and} 5.64 ^m/s

EvaporativeFluxversusVapourMoleFractionDrivingtTs

Force

for the ethanol

^component

at air velocities of 4.gt

^{m/s and 6.47 m/s}

136 148

4.15

FIGURE PAGE

4.16

^Ttre Evaporative

Flux

versus vapour MoIe

Fraction

^t77

Driving

Force

for the water

cornponent

at

^an

air

velocitY of

^{4-05 m/s}

4.17

Evaporative FIux versus Vapour Mole

Fraction Driving

¹⁷⁸

Force

for

^ttre

water

^component

at

^an

air velocity of 4.9I

m/s

4.IB

Evaporative FIux versus Vapour MoIe

Fraction Driving

¹⁷⁹

Forceforthewatercomponentatanairvelocityof

5.64 m/s

4.I9

Evaporative FIux versus Vapour Mole

Fraction Driving

¹⁸⁰

Force

for the water

^component

at

an

air velocity of

6.47 m/s

4.20

^The

Effect of Air Velocity

^on

the

Pure

Component

¹⁸³

MassTransferCoefficientforettranolandwater

4.2L iI^

^{versus Re__}

for the

pure components

of this study

¹⁸⁶

"Rd

for ttre

^xdata

of

wade ⁽¹⁹⁴²⁾

4.22

^The

Effect of

Aj.r

Velocity

^on

the Overall Mass

¹⁸⁸

Transfer Coefficients for bottr the ethanol

^and

vrater comPonents

4.23

A Conparison between Ttreory

a'd

Experiment

of ttre

^I97

l{o].arFluxRatioofWatertoEthanolinamixture

initiallY 5 I

ethanol

4.24

A Comparison between Theory and Experinent

of the

¹⁹⁸

MolarFluxRatioofWatertoEthanolinamixture

initiallY

^{25* ethanol}

4.25

^A Comparison between Theory and

Experirent of ttre

^L99

MolarFluxRatioofWatertoEthanolinamixture

initiallY

50* ethanol

6.I tiquid

^{Volune versus Time}

6.2 Liguid

^{Volune versus Time}

231 232

5.3

Ethanol ^and water vapour Pressures versus

Liquid

²³⁸

Concentration

at

^{20oc and}

I

^atm

TABI,E

LIST OF ^TABLES

Properties of

^{Seed Materials}

conparison

of

^seed

llaterials in settling a

^{2O g/L}

Yeast Suspension

Characteristics of Yeast/Nickel

^Flocs

Effect of

^Seed

Addition

^t'lethod

characteristics of Yeast/ Nickel Flocs

(pH Switctring) Saryrle Data

for 50.v/v t

^Ethanol

at 5'5

rn/s

Correction Factor for

^{Equation 1'25}

Raw Data

for

^{Ethanol and} Water Evaporation

Effect of

^SamPling

PAGE

81 82

xili

93 95

tt4

L54 162 206-229 234 3.1

3.2

3.3 3.4 3.5 4.1 4.2 6.L-24 6.25