CPDNR
3339
Conceptual Process Design
Process Systems Engineering DelftChemTech - Faculty of Applied Sciences
Delft University of Technology
Subject
A novel 'green' process for cheaper production of
furfural from biomass
Authors
G .S. Andrews
M.A. Bosch
C. Nederlof
R. Nieuwstraten
R. Stemmer
Keywords
(Study nr.)
(9015078)
(9106267)
(1046705)
(9604173)
(1099086)
Telephone
06-14217351
06-41526217
06-12374280
06-14371726
06-14317055
furfural, syngas, biomass, gasification, purification,
absorption, distillation, bio-refinery, hyper-branched
polymers, biomass drying, Aspen Plus,
Final Report
A novel 'greell' pl'ocessfor cheaper produelion o.ffllJfuralfrom bio!nass
Pref ace
This report has been developed within the framework of the Conceptual Process Design course at the Delft University of Technology. This course is a part of the fourth year's curriculum ofthe Chemical Engineering Master and is coordinated by Prof. ir. J. Grievink. The assignment is issued by Dr. ir. W. de Jong from the Energy Technology department (faculty of Mechanical, Maritime and Materials Engineering) of the Delft University of Technology.
The objective of this project is to offer students arealistic experience in making a conceptual process design and achieving a high degree of integration of chemical engineering know-how. Another goal is to work effective1y as a team, develop communication skills and provide a creative environment to pursue an original design.
Photo i: CPD 3339 Members rol.t.r Michiel, Rob, Gary, Christian and Robert
Logo i: © CPD 3339
A novel 'green' process for clteaper production ojflllfliral from bio1nass
Summary
This report is the result of a conceptual design project brought forth by five Chemical Engineering master students at the Delft University of Technology. It is based on an assignment handed out by the Energy and Technology department (faculty of Mechanical, Maritime and Materials Engineering - 3ME). The duration of this project has been three months. The title ofthe assignment for this project is formulated as follows:
"Design of a novel' green' process for cheaper production of furfural from biomass" The design process has been focussed on the downstream section of an innovative reactor to convert biomass into furfural, as part of a future bio-refinery. It consists of a purification section to attain furfural at a purity of 99.5 w% at a production rate of 8 kton annually. Parallel to this a gasification section is operated, which processes residual biomass into high quality syngas. This report describes the technical and economical assessment of the above described assignment. Throughout the duration of the project the application of creativity enhancing techniques has been continuously prioritised in order to obtain an original design. The purification section has had two altematives worked out in detail. The first alternative is a distillation sequence, which makes use ofthe favourable liquid-liquid split between water and furfural. The second design is an absorption-based separation using hyper-branched polymers (HBP), a novel absorbent. This latter design is chosen as primary purification section, with the distillation design functioning as a reference.
Both alternatives have proven to be capable of separating the MTC product vapour into polymer-grade furfural and virtually pure 5-methyl furfural.
The advantage of the absorption process towards the competition lies in its highly favourable exergy utilisation of the MTC effluent and the preservation of a highly energetic amount of steam. To ensure applicability of this option, further research on acquiring a suitable HBP is essential.
The gasification section has proved to be most challenging, due to a need for an extensive pretreatment and work-up section. To function adequately, the gasifier needs a residual biomass pretreated to 70 w% solids and a minimal mineral content. To atiain a
high-grade syngas, the work-up section becomes rather complex. These factors make for an infeasible total downstream section which is pointed out by the economical evaluation in this report. To achieve a feasible operation the susceptibility of the economical margin of mainly the gasifying section plays a major role. Production cost of approximately 17 M US$ for the opted pilot plant design exceed product sales roughly by a factor 2. Optimistic estimations show that an annual furfural production in the scale of 100-500 kton, depending on development of investment cost, becomes feasible.
This report confirms atechnical viability of the proposed downstream section, and shows that this process is not economically attractive on a small scale. The knowledge and experience that will be acquired from such a pilot plant may however prove valuable in the future.
Final Report
A novel 'green· pl'ocessfor clleflper productioll oJflllflll'alfrom biOJnflSS
Table ofContents
!
INTRODUCTION ... 11.1 HISTORY AND CURRENT PRODUCTION PROCESS OF FURFURAL ... 1
1.2 ApPLICATIONS OF FURFURAL ... 2
1.3 ApPLICATIONS OF SYNGAS ••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 2 1.4 CPD PROJECT •••••••..•.•.••••••••••••••••••..•..•...••••••.•...•..•••••••...•.••...•.•.••••••.•.••••••••••••••••• 3
~ PROCESS SELECTION ... 4
2.1 PURIFICATION SECTION ... 4
2.1.1 PURIFICATION OPTIONS ... 5
2.1.2 SELECTION ... 8
2.2 PRETREATMENT OPTIONS IN THE GASIFICATION SECTION ... 9
2.2.1 INTRODUCTION ... 9
2.2.2 GENERAL ASSUMPTIONS FOR PRETREATMENT ... 10
2.2.3 WASHING THE BIOMASS ... 10
2.2.4 FEEDING AND TRANSPORTATlON OF RESIDUAL BIOMASS ... 11
2.2.5 OPTIONS FOR UNIT OPERA TI ONS ... 11
2.2.6 CHOICE PRETREATMENT SECTION ... 15
2.3 BIOMASS GASIFIER ...•.•..••..••.•...•....•....•••...••• 15
2.3.1 GASIFIER TYPES AND GASIFICATION OPTlONS ... 16
2.3.2 T AR CRACKING ... 18
2.3.3 GASIFIER PERFORMANCE ... 18
2.3.4 NEW GASIFIER DESIGN ... 20
2.4 WORK-UP SECTION ... 21
2.4.1 CLEANING ... 21
2.4.2 CONDITIONING ... 23
2.4.3 SYNGAS APPLICATIONS ... 24
2.4.4 WORK-UP OPERATION SELECTION ... 25
J
BASIS OF DESIGN ... 283.1 PURPOSE OF DESIGN .•...••...••...•...•••...•..••• 28
3.2 BATTERY LIMITS ...••...••••..••.•••...•..•...••.•....••...•...•.••...•.••... 28
3.3 DESCRIPTION OF DESIGN AND PROCESS DEFINITION ... 30
3.3.1 PROCESS CHOSEN ... 30
3.3.2 PROCESS BLOCK SCHEMES ... 31
3.3.3 THERMODYNAMIC PROPERTlES AND PURE COMPONENTS ... 31
3.4 BASIC ASSUMPTIONS .•..•..•..•••.••...•...•...••....••••...•...•. 32
3.4.1 CAPACITY ... 32
3.4.2 LOCATION ... 32
3.4.3 BATTERY LIMITS ... 32
3.4.4 IN AND OUT STREAMS ... 33
3.5 ECONOMICAL MARGIN ... 33
A novel 'green' processfor clleaper production ojflllfuralfrom bio/nass
4 THERMODYNAMIC PROPERTIES AND WINDOW OF OPERATlON ... 34
4.1 COMPONENTS AND PROPERTIES ••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 34 4.2 THERMODYNAMIC MODEL CHOICE AND V ALIDATION ... 35
4.2.1 INTRODUCTION ... 35
4.2.2 VIEWING THE PURIFICATION SECTION ... 35
4.2.3 THERMODYNAMIC MODEL OPTIONS - PURIFICATION ... 35
4.2.4 MODEL COMPARISON ... 35
4.2.5 USING POL y-NRTL ... 38
4.2.6 VIEWING THE GASIFICATION SECTION ... 39
4.3 WINDOW OF OPERATION SCHEME •••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 39 ~ PROCESS STRUCTURE AND DESCRIPTION ... 42
5.1 PURIFICATION SECTION ...••.•.••.•...•..••...•...•...•...•...•...•...• 42
5.1.1 ABSORPTION ... 42
5.1.2 DISTILLATION ... 43
5.2 GASIFICATION SECTION ... 43
5.3 TURN-DOWN RATIO ... 43
5.4 UTILITIES AND INTEGRATION ... 44
5.4.1 UTILITIES ... 44
5.4.2 INTEGRATION ... 44
2
PROCESS CONTROL ... 476.1 CONTROL OBJECTIVES ...•...•....•.••...•...•...••. 47
6.2 FURFURAL PURIFICATION - ABSORPTION ... 47
6.2.1 ABSORBER ... 47
6.2.2 DISTILLATION COLUMNS ... 48
6.2.3 COMPRESSOR ... 49
6.2.4 FLASH VESSEL V-Ol ... 49
6.3 FURFURAL PURIFICA TION - DISTILLA TION ... 50
6.3.1 DISTILLATION COLUMNS ... 50
6.3.2 DECANTER ... 50
6.4 GASIFICATION SECTION ... 50
1
MASS AND HEAT BALANCES ... 517.1 PURIFICATION SECTION •....••...•...•...•...•...•..•.•...•..•.••••••••••.•...•...•.•.•.•..••...•...••...• 51
7.2 GASIFICATION SECTION ... 52
~ PROCESS AND EQUIPMENT DESIGN ... 53
8.1 PURIFICATION SECTION ...•...••...•...•...•...•....••...•.•....•••..•.•..••••..•...•.•..•• 53
8.1.1 ABSORPTION PROCESS ... 53
8.1.2 EQUIPMENT SPECIFICATIONS - GENERAL CALCULATION PROCEDURE ... 55
8.2 GASIFICATION SECTION ...•...•...•..•...•...•...•..•...•.•.•.• 59
Final Report
A novel 'greell' processfor clzeaper production ojfmfuralfrom bioJnass
8.2.1 DRYING SECTION ... 59
8.2.2 SIMULA TION OF GASIFIER AND WORK-UP IN ASPEN PLUS ... 64
2.
W ASTE ... 679.1 SOLID WASTE ...•..••••..•••...••..•...•..•...•...•...•... 67
9.1.1 SILICA OXIDE ... 67
9.1.2 SPENT FLUIDISED BED CARRIER MATERlAL ... 67
9.2 AQUEOUS WASTE ...•••••...••...•...•.•...•...•.•..••..••...••••... 67
9.2.1 WASHFILTRATE ... 67
9.2.2 FURFURAL, ACETIC ACID & FORMIC ACID ... 67
9.3 GASEOUS WASTE ...•...••...•.•...••....•.•...•.•.•...••••.•...•.... 68
9.3.1 HYDROGEN SULFIDE ... 68
9.3.2 CARBON DIOXIDE ... 68
10 SAFETY & LOSS PREVENTION ... 69
10.1 BASIC FACTORS ••.••••••••••••••••••••••••••••••••••••••••••••••••.•••••.••••••••••••••••••••••••••••••••••••••••••••••••••• 69 10.2 HAZOP ••••••••••.•••••.•••••••••••••••••••••.••••••••••••••••••..••.•••.••..•••••••...•••••.•...•.••••••••••••••••••••.•••••••• 70
10.3 ATEX: EXPLOSIVE ATMOSPHERE •••.•••••••••••••••••••••••••••••••••••••••••••.••••••••••••••••••.••••••••••••• 70 10.3.1 ZONING GENERAL ... 70
10.3.2 ZONING UNIT SPECIFIC ... 71
10.4 LAYER OF PROTECTION ANALYSIS (LOPA) ... 73
10.4.1 FIRST LA YER: PROCESS DESIGN ... 73
10.4.2 SECOND LA YER: BASIC PROCESS CONTROL ... 73
10.4.3 THIRD LA YER: ADVANCED PROCESS CONTROL, ALARMS AND OVERRIDE ... 74
10.4.4 FOURTH LAYER: AUTOMATIC RESPONSE ... 74
10.4.5 FIFTH LAYER: PHYSICAL PROTECTION ... 74
10.4.6 SIXTH LA YER: FIRE PROTECTION ... 74
10.4.7 SEVENTH LAYER: PLANT EMERGENCY RESPONSE ... 74
10.4.8 EIGHTH LA YER: COMMUNITY EMERGENCY RESPONSE ... 74
10.5 TRANSPORTATION & STORAGE •••••••••••••••••••••••••••••••••••••••••••••••••••.•••••••••••••••••••••••••••••••• 74 10.5.1 FEEDSTOCK AND PRODUCT DELIVERY ... 74
10.6 TOXICITY ... 75 11 ECONOMY ... 76 11.1 INVESTMENT ...•...•.•••••..•••..••...•...•....•••..•..•...•..•.•...•....•••..•...•..•..• 76 11.2 OPERATING COST ••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 77 11.3 INCOME ••••..•...•.•••... 77 11.4 CASHFLOW ... 78
11.4.1 BRIDGING THE GAP ... 78
11.5 COST REVIEW ... 79
11.6 ECONOMICS PER SECTION ... 80
A novel 'green' process for clte{lper productioll ojflllfural J;'om biOI11{lSS
12 CREATIVITY AND GROUP PROCESS TOOLS ... 82
12.1 ACTIVITIES UP TO BASIS OF DESIGN MEETING ... 82
12.1.1 SWOT ANAL YSIS AND CREA TIVITY ASSIGNMENTS ... 82
12.1.2 SYMBOLISM, BRAIN WRITING AND CHALLENGING ... 82
12.1.3 DRIVING FORCE APPROACH ... 83
12.1.4 MORPHOLOGICALANALYSIS ... 83
12.2 ACTIVITIES AFTER BASIS OF DESIGN MEETING ... 83
12.2.1 CHECKLIST AND MATRIX ANAL YSIS FOR PURIFICATION CHOICE ... 83
12.2.2 WISHFUL THINKING AND BACK-CASTING FOR GASIFICA TION CHOICE ... 83
12.2.3 EXPERTS ... 83
12.2.4 ExPERIMENTs ... 84
12.3 OVERALL CREATIVITY ... 84
12.3.1 BRAINSTORMING AND OPEN DISCUSSION ... 84
12.3.2 GROUP DYNAMICS ... 84
12.3.3 SURROUNDINGS ... 85
12.3.4 VISUALISATION DUE TO WHITEBOARD AND FLIP-OVER ... 85
12.3.5 CAPTURING ... 85
12.4 CREA TIVITY RESUL TS ....•....•...•...•.•...•...•...••.•...•...• 85
12.5 SUMMARY T ABLE •.•••.•••••••••••..•••••••••••••••••••••••••••.••.•••••.•••••••••••••••••..•.•••••••••...•...•..•..•• 86
13 CONCLUSIONS AND RECOMMENDATIONS ... 87
Final Report
A 110 vel 'greell' processfor cheaper prodllctioll of/ur/liral from hiomass
List of Appendices
A CURRENT SEP ARA TION METHOD ... 1
B PURIFICATION DECISION FACTORS ... 2
C RESIDUAL BIOMASS CHARACTERISTICS ... 3
C.l RESIDUAL BIOMASS CELL STRUCTURE •••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 3 C.2 BIOMASS COMPOSITION •..••.••...•...•...••..••.••.••...••... 3
D TRANSPORTATION SELECTION CHART ... 4
E BIOMASS TESTING RESULTS ... 5
E.l SOLID CONTENT MEASUREMENTS •••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 5 E.1.1 EXPERIMENTAL PART ... 5
E.l.2 RESULTS ... 5
E.1.3 CONCLUSION ... 8
E.2 VISCOSITY TESTS ...•..•.••...•...•...•....••..•...•...•... 8
E
OPTIONS FOR BIOMASS DRYING ... 9F.l CONCEPTUAL REPRESENTATION OF A SOLID DECANTER •••••••••••••••••••••••••••••••••••••••••••••••• 9 F.2 V ACUUM DRUM FILTER ...••...•..••••••..•...•..•....•... 9
F.3 EXPRESSION CHAMBER •••..••.•....•••..•...•...•....•..•..•...•... 10
F.4 HYDROCYCLONE ...••..•..•...•...••...•..•..•....•...•... 10
F.5 V ACUUM BELT FIL TER ...••.•...•.••...•....•••.•••••..•...•...•...•....• 11
F.6 SOLID FLASH ...••.•.•..•.•...•....•••••...•....•...•.••..•..•.. 11
F.7 DIRECT STREAM DRYER ....•...•.••...•.•...•...•.••••.•...••..•...••.... 12
F.8 INDIRECT STEAM DRYER •...•...••..•....•....•.•.••... 12
G THE GASIFICATION SECTION ... 13
G.1 DIFFERENT GASIFIERS •...••...•...•....•...•.•..••..••...•...•... 13
G .1.1 FIXED BED GASIFIERS ... 13
G .1.2 FLUIDISED BED GASIFIERS ... 14
G.1.3 ENTRAINED FLOW GASIFICATION ... 14
H BA TTERY LIMITS .•...••••...•...•...•..•..•..•.•...•.... 15
H.1 OUTER BATTERY LIMITS ....•..•...•..•...•..•.••.••••••..••..•..•...••...•...•...• 15
H.2 IN·NER BATTERY LIMITS ...•....•..•••...••••••••••.•...•...•...••.•.• 16
A lwvel 'green' proce.H,/or cheaper prodllctioll of fuiflIra! from biomass
I PROCESS BLOCK SCHEMES ... 17
1.1 ABSORPTION BLOCKSCHEME .•..•.•.•.•..•..•.•..••..•..•...•... 17
1.2 DISTILLATION BLOCKSCHEME ... 18
1.3 PRETREATMENT SECTION BLOCKSCHEME ... 19
1.4 GASIFICATION SECTION BLOCKSCHEME ... 20
J
PURE COMPONENTS PROPERTY TABLE ... 21K COMPARISON OF THERMODYNAMIC MODELS ... 23
L THERMODYNAMIC DATA USED IN ASPEN PLUS SIMULATION ... 35
M STREAM TABLES ... 37
M.1 PURIFICATION PROCESS STREAM SUMMARY ... 37
M.2 STREAM TABLES GASIFICATION ... 40
N MASS& HEAT BALANCES TOT AL STREAMS ... 42
N.1 MASS & HEAT BALANCES OF THE PURIFICATION SECTION ... 42
N.2 MASS & HEAT BALANCES OF THE GASIFICATION SECTION ... 44
N.2.l BALANCES OVER THE DRYING SECTION ... 44
N.2.2 BALANCES OVER THE GASIFIER AND WORK UP ... 44
N.3 EQUIPMENT MASS & HEAT BALANCES GASIFICATION SECTION ... 45
o
ASPEN FLOWCHART ... 470.1 DISTILLATION ...•••.•...•.•.•...•...•...•...•...•....•...••••..••... 47
0.2 ABSORPTION ... 48
0.3 GASIFICATION SECTION ... 49
P CONTRIBUTORS TO COLUMN HEIGHT ... 50
Q
FLOW PROFILES TO CALCULATE COLUMN DIMENSIONS ... 51Q.1 COLUMNS IN ABSORPTION PROCESS ... 51
Q.2 COLUMNS IN DISTILLATION PROCESS ... 52
Q.3 VESSEL DIMENSIONING IN ABSORPTION PROCESS ... 53
R E UIPMENT SUMMARY ... 54
R.l COLU·MN SUMMARY ••.•.•....•...••.•.•...•.•..•...•..••.••...•...•...•... 54
R.l.l COLUMNS IN ABSORPTION ... 54
R.l.2 COLUMNS IN DISTILLATION ... 55
Final Report
A JlOvel 'greell' process for cheaper prodlictioJl of furfural from hiomass
R.2 VESSEL SUMMARY ...••...•.•.••.•.•.••••..•..•.•.•.••.••.•...•....•....•••.•••...•...•...•... 56
R.2.1 VESSELS IN ABSORPTION ... 56
R.2.2 VESSELS INDISTILLATION ... 57
R.3 PUMPS & COMPRESSORS IN ABSORPTION •••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 58 R.4 HEATERS IN ABSORPTION ...•...••..••..•..•..•...•..•••••..•... 60
~ DISTILLATION COLUMN SPECIFICATION SHEETS -ABSORPTION ... 63
S.1 C-Ol ... 63
S.2 C-02 ... 65
S.3 C-03 ... 67
T CALCULATION OF STEAM CONSUMPTION IN BIOMASS DRyING ... 69
U HAZOP ... 71
U.1 PURIFICATION EQUIPMENT •••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 71 U.l.l ABSORBER ... 71
U.l.2 DISTILLATION COLUMNS (ALL) ... 72
U.l.3 FLASH VESSEL ... 73
U.l.4 DECANTER ... 74
U.2 GASIFICATION SECTIONS ••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••••• 75 U.2.1 PRETREATMENT SECTION ... 75
U.2.2 GASIFIER ... 76
U.2.3 WORK-UP SECTION ... 77
V ECONOMY ... 78
W OVERVIEW CREATIVITY ENHANCING TECHNIQUES ... 81
X SWOT ANAL YSIS OF THE DESIGNERS GROUP PROFILE ... 82
Y ABSORBER CALCULATIONS ... 83
A novel 'green < pl'ocessfor clleflper production oJflllfllralfrom biomflS's
List ofTables
Table 2-1: Factors that influence the selection of feasible separation options, adapted from Seeder and
Henley [1] ... 4
Table 2-2: Defining aspects of each purification method ... 8
Table 2-3: Comparison of gasifiers to produce high quality syngas ... 17
Table 2-4: Gas composition data at selected temperatures ... 19
Table 2-5: rule-of-thumb specifications for impurities in syngas [20] ... 21
Table 2-6: Main Fischer-Tropsch options ... 25
Table 3-1: Biomass entering the reactor and leaving the reactor to the gasification section [1,2] ... 29
Table 3-2 Stream leaving the reactor and the furfural product stream specifications [1] ... 30
Table 4-1: All components present in the downstream section ofthe conceptual furfural/syngas plant ... 34
Table 4-2: Comparison ofthermodynamics by means ofreference literature va lues given in deviations from literature ... 38
Table 5-1: Advantages of Absorption and Distillation ... 42
Table 5-2: Overview ofthe utilities used ... 44
Table 7-1: Mass flows in and out of the purification section ... 51
Table 7-2: Deviation in units involving HBP's ... 51
Table 7-3: Inflow into gasification section ... 52
Table 7-4: Outflow from gasification section ... 52
Table 8-1: Composition of input stream ... 53
Table 8-2: Thermodynamic data ofwater/steam from steam tables [12] ... 59
Table 8-3: Composition of slurry stream entering solid flash ... 59
Table 8-4: Estimation of filter bed area necessary to achieve 30 w% solid content at continuous operation ... 61
Table 8-5: Properties of superheated steam stream from absorber in purification section ... 61
Table 8-6: Stream tab les of direct superheated steam drying option ... 62
Table 8-7: Stream tab Ie for indirect heating ... 64
Table 8-8: Overall stream mass and energy balance ... 65
Table 8-9: Equipment energy overview ... 66
Table 9-1: Summary of all wastes in the combined purification gasification process ... 67
Table 9-2: Overall carbon balance ... 68
Table 10-1: Zoning table purification section ... 72
Table 10-2: Zoning table gasification section ... 72
Table 11-1: Summary ofthe equipment cost in million US dollars (2006) ... 76
Table 11-2: Results ofthe Fixed Capital Cost ca\culations using the Lang Method in million US dollars (2006) ... 77
Table 11-3: Overview ofthe composition ofthe operating cost, expressed in million dollars per year, dollars per ton furfural and in percentages ofthe total (annual production cost) ... 77
Table 11-4: Product sales per annum ... 78
Table 11-5: Net Cash Flow ca\culation in million US dollars per annum ... 78
Table 11-6: Overview ofthe composition ofthe operating cost after adaptations ... 79
Table 11-7: Net Cash Flow ca\culation in million US dollars per annum, after adaptations ... 79
Table 11-8: Ca\culation ofsome economie index numbers, using a factor 4 investment reduction and a scale factor of 0.7, in million US dollars per year ... 80
Table 11-9: Ca\culation ofsome economic index numbers, using a factor 2 investment reduction and a scale factor of 0.7, in million US dollars per year ... 80
Table 11-10: Economie evaluation ofa furfural-only (FF-o), gasification-only(Gas-o) and combined gasification/furfural process (Gas+FF), in million US dollars ... 81
Table 12-1: Results ofthe creative sessions ... 86
Final Report
A novel 'green' pl'ocessfor clleaper production ojfwfuralji'om bioJnaS's
List ofFigures
Figure 2-1: Simplified flowchart for the purification of furfural by means of distillation ... 6
Figure 2-2: Simplified flowchart for the purification offurfural by an absorption process with HBP's 6 Figure 2-3: Sirnplified flowchart for the purification offurfural using gas-permeation ... 7
Figure 2-4: Simplified flowchart using two parallel adsorbers to remove furfural from water ... 7
Figure 2-5: Flowchart for a separation using supercritical CO2 ... 8
Figure 2-6: Block diagram of general pretreatment focus points ... 10
Figure 2-7: Unit operation options for the gasification section ... 11
Figure 2-8: Conceptual pretreatment section with solid weight percentages ofthe residual biomass slurry ... 15
Figure 2-9: Options for syngas production from biomass [15] ... 17
Figure 2-10: The amount of syngas (left) and oxygen (right) at different temperatures and water contents .. .... ... ... . . ... .. .... ... .. . . ... .. ... ... . . .. . . . ... ... . . . ... .. ... 1 8 Figure 2-11: Product distribution of autothermic biomass gasification before (Ieft) and after conditioning to a H2:CO ratio of 2 (right) ... 19
Figure 2-12: The ideal gasifier, Dual Bubbling Fluidised Bed (A), CFB with BFB (B), Dual Continuous Fluidised Bed (C) ... 20
Figure 2-13: Block scheme for syngas work-up ... 25
Figure 2-14: Selection of solvent for simultaneous removal ofH2S and CO2; ... 26
Figure 3-1: Inner and outer battery limits ... 29
Figure 4-1: (Left) Vapour-Iiquid equilibrium in the furfural-water system in mole percentages. A at 1 atm; B at 5.62 atm. Four reference points for A are shown - Kirk-Othmer [4] ... 36
Figure 4-2: (Right) Vapour-liquid equilibrium in the furfural-water system in mole percentages. A at 1 atm; B at 5.62 atm. Three reference points for Bare shown - Kirk-Othmer [4] ... 36
Figure 4-3: Vapour-Iiquid equilibrium in the furfural-water system in mass percentages at 1 atm. Three reference points shown - Zeitsch [5] ... 37
Figure 4-4: Window of operation of the purification section given in pressures and temperatures of separate unit operations ... 40
Figure 4-5: Window of operation ofthe gasification section given in pressures and temperatures of separate unit operations ... 40
Figure 5-1: Basic block diagram on steam integration ... 45
Figure 6-1: Absorber control structure ... 48
Figure 6-2: control structure for C-02 and C-03, distillation operations in absorption process ... 49
Figure 6-3: Compressor out let pressure control ... 49
Figure 6-4: Control structure for a decanter ... 50
Figure 8-1: Aspen Plus model used for the separation ofTHF and water ... 54
Figure 8-2: Schematic overview ofthe different contributing factors in determining flash vessel height ... 57
Figure 8-3: mass based division and increase ofsolid w% during solid flash vessel operation ... 60
Figure 8-4: Flow depiction ofvacuum belt filter operation, drying and washing to 30 w% solid content ... 61
Figure 8-5: Black box model of direct superheated steam drying ... 62
Figure 8-6: Black box model of indirect saturated steam drying ... 63
Figure 10-1: A TEX zoning of downstream unit operations given from most hazardous zones to normal respectively from 0 to 2 in the furfuraVsyngas plant ... 73
A novel 'green' processjor ellellper production ojflllfural jrom bioJnllSS
1
Introduction
This conceptual process design project is based on research carried out for the innovative and selective production route to furfural. In this process furfural can be produced from different biomass sources via a continuous process. This report focuses on the work-up of the product and residue stream from the reactor.
An interesting side-development of the past few years is the possibility to pro duce clean transportation fuels from the residual material by means of biomass gasification and the Fischer-Tropsch process [1]. The added value of this side-stream has the potential to make the production of furfural from biomass even more attractive. The envisaged plant using the above-described combination of process steps is novel and is thought to become part of the bio-refinery concept in the future.
1.1
History and current production process of furfural
Furfural was discovered in 1832 by Johann Wolfgang, who was a German chemist working on the distillation of de ad ants for the production of formic acid. Furfural was formed in very small amounts as a by-product. The formation of these small amounts was probably caused by biomass contained inside the ants. In 1840 John Stenhouse, a Scottish chemist, discovered that the same chemical could be produced by di stilling several crop materials like corns and oats. He also determined the chemical formula of furfural. In the early years of the 20th century the chemical structure was derived.
The production of furfural was not exploited until 1922 when the Quaker Oats Company started to produce furfural from oat hulls.
Nowadays most of the furfural in the market, about 450 kton/a, is produced in China; it represents half ofthe world capacity. The production originates from batch type processes [1]. This means there is a large potential for improvements that may make the process competitive with oil-based products.
As described by Zeitsch [2], in the reactor section the hemicellulose part of the biomass is first hydrolysed by a light acid catalysed step to form xylose (reaction (1.1
».
Under more severe acidic conditions, xylose is instantaneously dehydrated to furfural [7] (reaction (1.2». Besides furfural, also small amounts of carboxylic acids, mainly acetic acid and formic acid, are produced. These are formed by the hydrolysis of formyl and ethyl si de groups of the hemicellulose. (CSHg04t
+ n H20 pH <3 ) n CsHIOOs Cs HJO Os pH<1 ) Cs H 4 02 +3 H20 (1.1 ) (1.2)In the past, lots of research has been done with respect to furfural as possible feedstock for plastics. When oil became easily available, "oil based" polymers became cheaper and the research on furfural stopped [7] because of economic reasons. Furfural production became directly infeasible. However, with the introduction ofthe Kyoto protocol, governments started to push the use of "green" chemicals and the replacement of fossil energy by renewable energy sources.
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A novel 'green' pl'ocessfor ellellper production ojflllfuralfrom biomass
Due to the strong competition in the renewed furfural market, new processes can only become competitive when the product is ultra pure and is being produced on a continuous basis.
In the proposed process, furfural is produced in a novel reactor, a multi-turbine-column (MTC). This continuous reactor has a significantly higher potential yield (95%), Ie ss by-products and lower energy consumption compared to current batch processes. The main advantage of this reactor is that the produced furfural is directly removed from the liquid phase. This is essential in the prevention of any detrimental follow-up reactions that can take place in the liquid phase. These follow-up reactions (resinification and condensation reactions) are irreversible, proton catalysed, and produce a tar-like compound. Due to the non-dissociative behaviour of the organic acids in the gas phase these reactions are prevented by instantaneous stripping of the furfural by superheated steam.
The furfuralleaves the reactor at the top in a vapour stream. The furfural concentration is low; the vapour stream consists primarily of water (steam) and small amounts of5-methyl furfural, acetic acid and formic acid.
1.2
Applications ofFurfural
In the current industry, furfural is mainly used as a solvent to recover aromatics [4,5], which are used to make synthetic rubbers. They can also be combined with other organics for the production of different kinds of resins [3,6]. These resins have a wide application, e.g. in fibreglass, airplane components and brakes in the automotive industry (mainly cars). Also in\ the food industry some derivates of furfural al widely applied. Nowadays the largest potency for the use offurfural is in the polymer industry.
A more novel application of furfural is as an octane booster. It can be selectively hydrogenated towards furans, which have a have a very high octane number (above 200) and relatively low boiling points (31-94°C).
Moreover, furfural has the potential to be a starting material for multiple products that are now derived from petroleum. With the increasing oil prices and increasing demand for green processes, furfural is back in the picture in becoming a commodity chemical. Furfural is known as a platform chemical.
1.3
Applications of Syngas
Syngas (synthesis gas) mainly consists of carbon monoxide and hydrogen. Two types of classifications can be distinguished; high quality and low quality syngas, the latler is also referred to as producer gas. Producer gas is obtained from low temperature gasification (below 1000°C) and is comprised, next to CO and H2, of considerable amounts of CO2, CH4, H20, CxHy aliphatic hydrocarbons, benzene, toluene and tars. If gasification is operated with air as an oxygen source, also N2 will be present as an inert. The gas produced by these types of gasifiers usually contains about 50% CO and H2.
High quality syngas can directly be produced from high temperature gasification (with oxygen) at temperatures above 1200°C. This gas contains only H20 and CO2, next to CO and
H2 [8].
Producer gas is mainly used for the generation of electricity and heat. Syngas, however, is a very useful base chemical for the production of a wide range of products. Syngas nowadays is
A novel 'green' process for clzeflper production ojflll:fural from biol1UlSS
primarily produced by reforming methane. The main part of syngas is used for the manufacturing of ammonia for fertiliser production. But also the production of hydrogen and methanol for refineries is a major application.
1.4
CPD
project
During the conceptual design project the emphasis will be on the upgrading of furfural from the continuous reactor designed at the Energy and Technology section at the Delft University of Technology and simultaneously, to review certain options for gasification of the biomass residue from the reactor to produce high quality syngas for further production of transport fuels. In doing so, economical and technical feasibility will be focussed on.
References:
1. Moulijn, J.A, Makkee, M, Diepen, A.E. van, Chemical Process Technology, Wiley & Sons 2001
2. Zeitsch, K.J, The Chemistry and technology of furfural and its many by-products,
Elsevier Science 2000.
3. Swain, S.K, Sahoo, S, Mohapatra, D. K, Mishra, B. K, Lenka, S, Nayak. P.L,
"Polymers from renewable resources. V. Synthesis and characterization of thelIDosetting resins derived from cashew nut shellliquid (CNSL)-furfural-substituted aromatic compounds", J. of App Pol. Sc. 54 (10) 14l3-1421
4. Coto, B, Grieken, R van, Pena, 1.L, Espada, 1.J, "A model to pridect physical properties for light lubricating oils and its application to the extraction process by furfural", Chemical Engineering Science, 61 (2006) 4381-4392
5. Extraction of aromatics from hydrocarbon oil using a furfural -aliphatic carboxamide cosolvent extraction process. U.S. Pat. Appl. Publ. (2003)
6. Hassan, E. A.; Motawie, A. M.; Kamel, M. M, "Synthesis and study of phenols-furfural re sin from Egyptian cotton straw". Fac. Sci., AI-Azher Univ., Nasr, Egypt. Pigment & Resin Technology (1991), 20, 4-7
7. Personal communications with Dr. ir. W. de Jong, ir.H.J. Heidweiller & S. Ma MSc 8. Boerrighter, H., Rauch, R., Review of gases from biomass gasification, report
ECN-B8(--06-066,Petten(2006)
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A novel 'green' pl'ocessfor clleaper production ojflllflll'alfrom bioJnass
2
Process selection
2.1
Purification section
In the purification section furfural needs to be selectively removed from a very dilute stream in which small amounts of organic acids are present. The main component is saturated steam. In selecting feasible separation processes for the purification section, several factors must be taken into consideration. In the table bel ow the important factors are listed.
Table 2-1: Factors that influence the selection offeasible separation options, adapted from Seeder and Henley [1] A. Feed condition 1. Composition 2. Flow rate 3. Temperature 4. Pressure 5. Phase state B. Product conditions 1. Required purities 2. Temperatures 3. Pressures 4. Phase states
C. Property differences that may be exploited 1. Physical
2. Molecular 3. Thermodynamic
4. Transport
D. Characteristics of separation operation 1. Safe
2. Low cost 3. Sustainable
4. Ease of scale-up 5. Turn down ratio
6. Temperature limitations
The feed and product conditions defined in Chapter 3 offer clear objectives for the purification to be carried out. In order to find a thermodynamic basis for possible separation techniques, a small study of the presence of possible driving forces is performed. Driving forces, described as component properties (e.g. difference in density) listed below can be used for separation. The values of these component properties can be found in Appendix J.
•
Size•
Density•
Mass•
Polarity•
Volatility•
Solubility•
Pressure•
Reactivity•
Concentration•
ChargeA novel 'green' pl'ocessfor c/letlper production ojflllfuralfrom bio11lflSS
From these values the three driving forces that stand out the most are the differences in: volatility, size and polarity. Due to the presence of an azeotrope between water and furfural, the use of volatility as a driving force is hindered. Besides this ideal vapour phase, non-idealities are also encountered in the liquid phase: water and furfural mixtures show a favourable liquid-liquid split that may be used advantageously during separation.
A list can be formulated with the possible unit operations that can be used to separate the components from each other.
Phase creation Barrier Solid agent
•
Distillation•
Osmosis (reverse )•
Adsorption•
Flash vapourization•
Gas permeation•
Chromatography•
Decantation•
Pervaporation•
Ion exchange•
Absorption•
Electro-dialysis•
ExtractionFrom this list of unit operations, while keeping in mind the three most prominent driving forces, a selection is made between them. By applying these separation techniques, four possible purification schemes have been devised. These are described in the following sections.
2.1.1 Purification Options
2.1.1.1 Distillation
The first option, adapted from Zeitsch [2] is distillation. The main difference between the method described by Zeitsch, see Appendix A for a summary, and the one below can be found in the feed condition. Zeitsch operates the column at ambient conditions (the column is fed by a saturated liquid) while in the design below a saturated vapour is fed at lObara. The advantage of this slightly different approach is a reduction of energy consumption by the column's reboiler due to the exergy utilisation ofthe feed stream (see Figure 2-1).
Figure 2-1 represents a simplified flow chart of the slightly improved process, which is primarily consisting of a distillation column, a stripping column and a decanter. The top product of the first distillation column is an azeotropic vapour mixture of water and furfural, which is condensed and subsequently fed into a decanter where a liquid-liquid phase-split occurs. The furfural phase is then sent to the stripping column. This column has to be operated at reduced pressure to prevent polymerisation of furfural.
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A l10vel 'green· pl'ocessfor clleaper production o.fflllfuralfrom biomass
Water Furfural Acids 5-MF Furfural Acids Furfural 5-MF
Figure 2-1: Sim plified flowchart for the purification of furfural by means of distillation
2.1.1.2 Absorption using hyper-branched polymers (HBP 'sj
HBP's are liquid under ambient conditions and can easily absorb furfural (and 5-methyl
furfural) from a gaseous mixture [3]. This results in a liquid phase in which the furans and
some water are contained. Due to the absence of a HBP vapour pressure, a simple isothermal
flash can be used to relieve the absorbed material. The resulting, concentrated, mixture of
water and furans can subsequently be separated by means of distillation. This is depicted in
Figure 2-2. Water Furfural Acids 5-MF ,---_. Water Acids Furfural Water Acids Water Acids Furfural 5-MF water acids
Figure 2-2: Simplified flowchart for the purification offurfural by an absorption process with HBP's
2.1.1.3 Water removal using gas-permeation
U sing the exergy content of the reactor effluent, first a large percentage of the water can be
removed by means of gas-permeation. Thereafter, the resulting mixture of water, furfural and acids is sent to a decanter where the liquid-liquid equilibrium is made use of to dispose of a large amount of water. The resulting furfurallwater mixture is stripped to obtain relatively pure furfural and an azeotropic water/furfural mixture, which is fed back to the decanter. This
can be se en in Figure 2-3.
A novel 'green' process for cltellper production ojflllfural from bio11l11SS Water Furfural Acids 5-MF Acids 5-MF Water Acids Furfural 5-MF Water (Acid 5) Water Acids Furfural 5-/5 Water Furfural Acids 5-MF Furfural 5-MF water Acids
Figure 2-3: Simplified flowchart for the purification of furfural using gas-permeation
2.1.1.4
Adsorption of furfuralIn this method, the reactor effluent is condensed and fed to a column in which an adsorbent is present. This adsorbent is capable of selectively removing furfural from the liquid phase. Little acid and water will probably also be adsorbed and will be removed further downstream using distillation. Because the reactor effluent is condensed and thus lowered in exergy content, the adsorption becomes infeasible compared to the other options. Only by utilisation of this initial exergy may the unit operation prove attractive. Figure 2-4 depicts twin-column adsorption; in industry usually 4 columns are used to apply efficient pressure and temperature swing systems. Water Furfural Acids 5-/5 Steam Water Furfural acids 5-/5 Acids Furfural 5-/5 water Acids
Figure 2-4: Simplified flowchart using two parallel adsorbers to remove furfural from water
2.1.1.5
Extraction of furfural using supercritical CO2This purification design is a liquid-liquid extraction using supercritical C02 in order to transfer the furfural out of the aqueous phase. In order to make this design viabie, like the design above, heat-integration is essential. A simplified flowchart of this concept is given in Figure 2-5.
The separation is achieved by using supercritical C02 to remove the furfural from the water phase. It is then flashed resulting in liquid furfural and gaseous CO2.
Some acid might dissolve in the CO2 as well and end up in the furfural phase, which will then be separated by a small distillation column.
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A novel 'green' processfor clleaper productiol1 ojflllflll'alfrom bio!1lass
CO2 will be recompressed and recycled within the system. Other extractants have also been
considered but in view of sustainability and toxicity, the relatively inert CO2 has been chosen.
Water Furfural Acids 5-~ Fresh CO2 Water Acids ~--- Furfural 5-M' Recycle CO2 Acids water Furfural Water 5-MF
Figure 2-5: Flowchart for a separation using supercritical CO2
2.1.2 Selection
The five options mentioned above have been researched and discussed with group members as weIl as experts [3,4,5]. In Table 2-2 the defining aspects of each method are given on basis of which the decision has been made. A more thorough strengths and weaknesses analysis can be found in Appendix B. Distillation Energy requirements + Proven technology
Table 2-2: Defining aspects of each purification method Absorption lntroduction of extra chemical + Selective removalof product Gas-permeation Largest component over membrane Adsorption Loss of exergy lntroduction of extra chemical Removalof furfural trom adsorbent Extraction Loss of exergy lntroduction of extra chemical Recompression ofC02
Using negative selection, the purification methods are carefully weighed against each other after which some are discarded. This is described in the section below. It has resulted in two options that have to be examined further to prove their feasibility in more detail.
Distillation
Although distillation is proven technology and readily scalabie, the energy requirements are often substantial. In this case, the stream leaving the reactor is saturated vapour and can possibly reduce the reboiler-duty in the column. To be able to make a qualitative decision this system has to be investigated using Aspen Plus.
Absorption
Absorption using HBP's is a very novel and interesting option for the reasons mentioned in Appendix B. To prove the feasibility of this option an elaborate study in the capabilities of HBP' s for this application is required. Again, Aspen Plus can aid in clarifying the viability of this unit operation.
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A novel 'green' processfor cheaper production ojflllfilralfrom biomass
Gas-permeation
This option is found not to be viabie. When looking at the overall mass balance it can be seen that the large st component has to pass through the membrane. Membrane separations are rather expensive and only viabie for selectively removing the most valuable component.
Adsorption
This technology is currently being used to remove furfural from certain streams (primarily in biotechnology). Here, furfural is mainly considered as an impurity (toxic to microorganisms) and therefore desorption of furfural is not taken into account. After exploring different options of adsorption it can be concluded that the main challenge is the reversibility of this operation. Furfural can easily be adsorbed but is hard to desorb. This rules out adsorption as a viabie unit operation.
Extraction
The main disadvantage of using extraction is the fact that the stream from the reactor has to be condensed before the extraction can take place. When using supercritical CO2 as a solvent, a
pressure swing is encountered, requiring a lot of energy. As can be read in the recommendations (Chapter 13), using supercritical CO2 in the reactor to strip the furfural as it
is formed, and later separating it in the manner described above can be proven to be feasible. This is however beyond the scope ofthis project.
Selection
The choice was made to continue with two purification options: the distillation and the absorption with HBP's. The focus wiU be on the absorption, for the reason that it is novel and HBP' s are very promising for the future as an absorbent. Furthermore the option of an ideal recycle of absorbent complies with the need for 'green' processes. Distillation has been viewed as the conventional method of separation and will be researched as a reference with respect to the feasibility. The two options can be divided in a 'secure' option and a more novel and risky option in need of further researching.
2.2
Pretreatment options in the gasification section
2.2.1 Introduction
In order to gasify the biomass stream from the multi turbine column reactor (MTC), a pretreatment section has to be implemented. This section has two main functions; washing and drying the bottom stream coming from the reactor. The biomass slurry is very dilute; it has a 10 w% solid content, and must be dried before gasification. The more water is removed, the better the gasifier will perform. Attaining solid contents of a relatively high level, e.g. 70 WJIo, is known to be a very treacherous operation. Many industrial conceptual designs in the past have not been realised due to difficult and expensive solid handling [6]. However, a good drying section seems inherent to the gasification section of this conceptual design.
Water in the slurry is comprised of a free water part and an adherent water part. Adherent water is defined as water that is intertwined in the biomass by physical bonds or enclosed by its structure (spunge) and will be harder to remove from the slurry than the free water. This same stream also contains some minerals that might be detrimental to the gasification process, if present in too large quantities. This mineral content wiU have to be minimized. Furthermore, transportation and feeding of the solids is important to go as smoothly as possible. In theory the following block scheme can be presented for the pretreatment section.
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A novel 'green' processjor cheaper production of Jll/jural from biomflSS
bio mass slurry fro m reactor water/ minerals
i
Washr---.
wateri
, , Dry , , , , , , , , , G~)(i'd , ,---.
Feed ' , ' , , ,"
',\ ,'---
~ -~/---'~,---~ pre-treatment sectionFigure 2-6: Block diagram of general pretreatment focus points
2.2.2 General assumptions for pretreatment
treated biom to gasifier
ass
Solid handling and drying of solids has proved to be one of the most challenging parts of biomass treating in the industry. The biomass slurry leaving the reactor section is assumed to have particles of roughly 1 mm [8]. Seeing as this particle si ze is sufficient for gasification, it is assumed that no additional chipping or grinding is necessary in the pretreatment section (Figure 2-6). During furfural production, biomass is in acidic environment, which causes softening. Hemicellulose, which is mainly present in the cell walls, is released from the biomass and reacts to xylose. Because this reaction section compromises the cell structure of raw biomass, the residual biomass releases its minerals (mainly present within the cell walls) into the surrounding water. In combination with the elevated conditions in the reactor, the slurry loses rigidity, and will become more brittie or even pasty. It is important to have an
idea of this structure for secure solid handling.
It is assumed that 75 w% ofthe total water content is free water, whereas the remaining water is adherent to the biomass. Because the cell walls are compromised, it is assumed that the main part of minerals is present in the free water, e.g. 90 w% of total mineral amount. This is depicted in Appendix C.l and Appendix C.2.
Because most ofthe minerals will be present in free water, the washing step is made easier.
2.2.3 Washing the biomass
The purpose of washing is to remove mother liquor from a slurry, when this liquor is seen as a contaminant or as a valuable product. In the case of residual biomass, the present minerais, i.e. sulfuric acid and silica (ash), are contaminants for the gasification process. These have to be removed as much as possible to provide for a good gasification. Unit operations with possibilities for an easy and thorough washing step are thus more attractive to this pretreatment section than others.
The washing will occur with clean (mineral free) water because it is undesirable to add new (chemical) species to the slurry. During the washing process the wash runs through the slurry,
displacing the mother liquor partially, and causing a mix of both wash and liquor to leave in
the drying process. This can be repeated to an extent depending on the unit operation. Very little is known about the pore volumes and washing behaviour of the biomass. In literature some information is given on washing slurries [9]; which will be used to make a weIl
A novel 'green· processfor clteflperproductiol1 ojflllfllralfrom bioJnflSS
educated guess and remain within a reasonable order of magnitude for the amount of wash water. Calculations on this process can be found in Paragraph 8.2.1.
2.2.4 Feeding and transportation ofresidual biomass
The selection of a suitable transportation and feeding system is generally more troublesome and more expensive than liquid or gas handling. The best equipment to use will depend on the following factors [7]:
• Chemical & physical properties of solids • Length of transportation
• Throughput of solids • Change in conditions (P, T) • Moisture content of slurry
Some preliminary testing under realistic simulated conditions can attain prior information on the slurry handled. However, thorough testing of the residual biomass is not within the scope of this conceptual design project. Still some lab scale tests have been done (Appendix E) to at least gain some knowledge and feeling on the flowability and adherence to water of the residual biomass [10]. This has shown a very viscous biomass. Transportation of solids at elevated pressure is much more expensive than at atmospheric conditions. The pressure of the stream from the reactor will be brought down early in the process to save on operation costs, after which it will be transported over relatively short di stances (1- 20 m).
The transportation selection chart shown in Appendix D [9] can be used as a reference frame. Under high pressure, a screw feeder will suffice to transport the pressurised slurry from the reactor. Within the dryer section belt transporters are cheapest, and can handle a lot of tonnage. This makes them attractive for sc ale up reasons too. To feed the pretreated solids the gasifier a screw feeder is needed again.
2.2.5 Options for unit operations
There are numerous options for pretreating the residual biomass. Because drying a slurry stream is a very difficult operation, some thought was given to pretreatment options during creativity sessions. In Figure 2-7 a summary of options is given for the pretreatment section, and additionally the options for gasification. In pre-selecting dryer options, a few key elements were considered. These were robustness, continuous operation possibilities, washing options, high drying yield, ease of operation and scalability.
"Decanter" Entrained Flow
Wash Gasify CFB + EF Ideal Gasifier Vacuum filterbelt Piston Feed/pressurise Screw Belt
Figure 2-7: Unit operation options for the gasification section
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A novel 'greell' process for ellellper production ojflllfural j'rom bioJnllSS
Achieving a low water content of 70 wOlo solids in a biomass/water system is extremely hard. The hygroscopic behaviour of biomass causes need for intensive drying operations for attaining the non-diluted solids. It is imperative that this occurs as economically favourable as possible. This means that cheap and scalabie options of water removal should be used to their fullest extent but in constant regard with their separation performance. Other more expensive unit operations should be considered only as a necessity to attain the targeted solid weight content. In the following section all the aforementioned drying options Figure 2-7 will be conceptually touched upon and evaluated for this process.
2.2.5.1
Centrifuge decanterThis apparatus is based on centrifugal forces to split liquid medium from the solids. It is generally a relatively small application. Due to its centrifugal behaviour and its need for a precise build (closed structure ), it is quite expensive. A closed huIl makes a washing step relatively hard. This type of centrifuge decanter is known to achieve forces of 3000 g (gravitational force), which is considerable. It makes for a high, continuous capture rate. The scalability of centrifugal units is not very good, seeing as they may not become too large for operational reasons (start-up). Furthermore the decanter is a precise machine with fast moving parts. The residual biomass shows very viscous behaviour and poses a clogging threat in the solid exit. This translates to rather high maintenance needs. In Appendix F.1, a conceptual depiction of the decanter is given.
Even though the decanter will capture large amounts of water, the factors of initial and maintenance cost in combination with its washing complexity are substantial and thus the option becomes unattractive.
2.2
.
5.2
Vacuum drum filterDrum filters are very popular units in the drying industry, because they offer an easy washing step and are very compact. A rolling drum is bathed in the slurry and by applying vacuum inside the drum solids are very specifically attached to the drum rim. After optional washing, the dry cake is scraped from the drum. This is depicted in Appendix F.2.
Use of a continuous vacuum is costly, but the efficiency, low space requirements and ease of washing would weigh up against this. A disadvantage of this type of filter is the need for fluid slurry. The drum filter has to be able to run through this fluid medium. If the slurry is to viscous or even dry, the vacuum drum will not function efficiently due to lack of suction and premature caking in the slurry tank. This means that the slurry would have to be fed differently to the drum filter. This causes the drum filter to work at alesser capacity. In these prevailing circumstances the vacuum drum filter is infeasible.
2.2.5.3 Expression chamber
In expression a pressure device, e.g. a piston, places high pressure on the slurry and pushes the water through a filter cloth. The remaining cake is dropped through a retracting fIoor and the filter cloth is shaken to drop remaining solids. Quasi-continuous operation is possible but not convenient. A washing step is hard to implement. Expression units are quite expensive, and generally yield theoretical solid contents of 35 wO/o during normal operation [11]. This is a relatively high solid content. But with increase of pressure exertion the cost of operation will go up rapidly. An expres sion unit can be made so powerful to reach higher solid contents. In view of co st-bene fit analysis, the expression chamber will become much more unattractive before the yield will become interesting in comparison to other unit operation. This makes
A novel 'green' pl'ocess for clleflper productiol1 ojflllful'al fl'om biomflS'S'
expression illogical for operation. In Appendix F.3 a conceptual design of an expres sion unit operation is depicted.
2.2.5.4 Hydrocyclone
Hydrocyclones are classified as mechanical separation devices in which sedimentation takes place due to a centrifugal and an intensified gravitational force. No additional intemals are present in these types of cyclones. The slurry is tangentially introduced into the cyclone, which causes forces up to 300 g to be possible [12]. This enhances the density differences,
causing the water to split off. It is an important operational unit for separation because it has relatively low investment and operating costs, a high volume throughput, is very robust and has moderate energy expenditure. In addition, they are relatively small, have little space or foundation requirements and are easy to scale up.
A problem with hydrocyclones is that there is relatively little system data known. Especially in solid-liquid separation, opposed to solid-gas separation, theoretic foundations are scarce. In almost every case of hydrocyclone treatment, preliminary testing is done to attain information for one's specific system. By doing this testing one can gain understanding on specific conveyability and the natural split-factor between the solid and the liquid. A hydrocylone will simply augment these effects.
In theory, a hydrocyclone is a very good option for a cheap and scalabie drying operation. A washing step is very easily implemented Gust above the underflow in Appendix FA). In the sand and ore industry, dewatering of slUITies by means of hydrocyclones is a common practice. Here, dry compositions up to 70 w% solids are often achieved [12]. In comparison to biomass slurry, one cannot assume a similar separation yield as the sand/ore industry suggests. Lignocellulose has a much lower solid density, is extremely hydrophilic and shows very viscous behaviour. Thus with similar mass concentrations, biomass has a very different volumetric concentration than sand/ore. Moreover, whereas sand and ore have a weIl-defined, rather spherical form, treated biomass has a rather fibrous form. These above mentioned factors are essential in determining if a hydrocyclone will separate weIl or not. The residual biomass at 30 wOlo solids already behaves much like asolid [10], which means that in a hydrocyclone, clogging of biomass is likely to occur. Lab scale testing shows the residual biomass to be a troublesome contestant for a hydrocyclone mainly due to its ability to clog up the underflow. It is ho wever an operation to keep in mind. Based on own testing [Appendix E] and in accordance with the present day industry [12] it does not seem to do weIl enough with the residual biomass from this design.
2.2.5.5 Vacuum belt filters
Vacuum belt filters are known as a very robust drying technique. Through the belt vacuum is applied locally, much like the drum filter described above, to achieve caking of a high solid weight percentage at continuous operation. For a good vacuum to occur, the slurry must be evenly distributed over the belt filter. This cake can be collected at the end of the belt. For residual biomass a horizontal belt filter is most convenient. In Appendix F.2 a conceptual depiction is shown.
This type of operation has many advantages. Numerous washing steps can be very easily implemented, operation is cheap, it is easily scalabie and process is open to add-ons for creating better drying capacity, e.g. pressurising agents or hot air application. There are many versions of belt filters to choose from. The residual biomass will have to be present as a thin film, e.g. 5 mm, and thus can be dried and washed very easily. In preliminary lab scale tests a Büchner-filter achieved 30 wOlo solid content [10]. It is acceptable to expect at least similar, if
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A novel 'green' pl'ocessfor clteflperproduclion ojfurfuralfrom bi011lflSS
not better yields from the belt filter. This makes an option for a simplistic and thus relatively economical belt filter possible.
There is a eonsiderable equipment cost attached to any add-ons, and a continuous vacuum operation. Moreover, there is an option for pressure belt filters, whieh can go to higher solid percentages, but is even more expensive. Nonetheless belt filters remain the best option in the scope of this gasification section due to its potential to remove a lot of water from a very probiernatie slurry phase in eontinuous operation. After sealing up the process, the initial costs will have to be outweighed by the, otherwise very difficult, drying efficiency. The washing step in the gasification section is set to achieve a dilution factor 10, which is quite a lot. This will need large quantities of washing water. A belt filter ean handle such washing easily.
2.2.5.6 Solidjlash vessel
Because the stream coming from the reaction section is at elevated eonditions, i.e. 180°C and 10bara, the exergy from this stream can be used to relieve some water from the slurry by evaporation. It is also important to maintain solid handling at mild conditions due to the cost of pressurised solid transport. The amount of water flashed off will not be very substantial, but it seems to be an optimal way to lower process conditions. Flash vessels are very cheap operation units, and due to the solidity of the slurry stream, screw feeders will have to be implemented inside the vessel. The design is conceptually very simpIe. There is an option for additional heating, if this will be in favour of the thermodynamic equilibrium during flash operation. In Appendix F.6 the solid flash concept is shown. This option is very feasible and will be used in the pretreatment section.
2.2.5.7 Steam drying
As a final step in the pretreatment section, steam drying might be an option. This is generally a very energy intensive unit operation; there are little options other than drying that will reach the desired high solid content. However, the purification section releases a substantial amount of high-pressure, slightly superheated steam. Because this steam is part of the proeess, steam drying may become interesting. Drying with steam has an additional safety advantage to it. Drying with steam opposed to oxygen avoids the presence of a potential explosive environment. A distinction can be made between direct and indirect steam drying. Both techniques are shortly discussed below and are worked out further in Paragraph 8.2.1.
Direct steam drying
An advantage of direct drying is to dry the slurry phase, and additionally leave impurities from the steam stream in the biomass. This cleans the steam thoroughly. The impurities (mainly (aeetic) acid and potentially a little furfural) will be easily gasified with the residual biomass. The heat of condensation of the high-pressure steam may however not be applied, due to counterproductive wetting of the residual biomass. This will most likely not yield enough energy to heat the large water stream of the slurry. Additionally direct drying might cause more challenges, due to unwanted whirling up of biomass partieulates.
A conceptual sketch of a direct steam drying unit operation is depicted in Appendix F. 7.
This system needs a very mild vacuum undemeath the belt to collect all steam from the drying chamber.
Indirect saturated steam drying
Indirect heating to dry solid-liquid systems, e.g. biomass slurry, is used often in industry [13]. Even though the steam stream from the purification section will not be cleaned, as described in direct drying, good use can be made of the heat released by condensing the high-pressure
