Traitement des boues par friture - Institut national des ...theses.insa-lyon.fr/publication/2005ISAL0089/these.pdf · V. RUDOLPH Professeur (University of Queensland) Rapporteur G

N° d’ordre : 2005-ISAL-0089 Année 2005

Thèse

Traitement des boues par friture : Des mécanismes physiques à l’éco-conception d’un procédé

Présentée devant L’institut national des sciences appliquées de Lyon Pour obtenir Le grade de docteur Formation doctorale Sciences de l’Environnement Industriel et Urbain École doctorale de Chimie de Lyon Par Carlos-Alberto PEREGRINA-CAMBERO (Ingénieur) Soutenue le 01 décembre 2005 devant la Commission d’examen

Jury MM.

P. ARLABOSSE Maître Assistant (EMAC) R. GOURDON Professeur (INSA de Lyon) Rapporteur T. KUDRA Chercheur Scientifique Senior (CANMET) D. LECOMTE Professeur (EMAC) V. RUDOLPH Professeur (University of Queensland) Rapporteur G. TRYSTRAM Professeur (ENSIA)

Fry-drying of sewage sludge : From the physical mechanisms to the process eco-design 1

N° d’ordre : 2005-ISAL-0089 Année 2005

Thèse


Présentée devant L’institut national des sciences appliquées de Lyon Pour obtenir Le grade de docteur Formation doctorale Sciences et Techniques du Déchet École doctorale École doctorale de Chimie de Lyon Par Carlos-Alberto PEREGRINA-CAMBERO Soutenue le 01 décembre 2005 devant la Commission d’examen

Jury MM.

P. ARLABOSSE Maître Assistant (EMAC) R. GOURDON Professeur (INSA de Lyon) Rapporteur T. KUDRA Chercheur Scientifique Senior (CANMET) D. LECOMTE Professeur (EMAC) V. RUDOLPH Professeur (University of Queensland) Rapporteur G. TRYSTRAM Professeur (ENSIA)


To Cécilia… “The gods did not reveal, from the beginning all things to us; but in the course of time, through seeking, men find that which is the better…” Xenophanes


ACKNOWLEDGEMENTS

A mixture of feelings came to me when I started writing this section. On the one hand, it is always sad to say “good bye” to all the people who made my stay at LGPSD throughout the last three years so pleasant. On the other, there is a great satisfaction to end a great project and start new adventures in life.

I would like to begin my acknowledgements by expressing my thankfulness to my principal

advisor -and big FRIEND- Professor Didier LECOMTE, who gave me the chance to work in this amazing project and always stood by me. All your advice at both working and personal levels will be with me for a lifetime.

I am also thankful to my co-advisor Patricia ARLABOSSE. Thanks for all your lessons and

criticism, which sometimes were difficult to take but they always challenged me and made me think.

Thanks to Professor Victor RUDOLPH who assisted me throughout my stay in Australia.

Without your guidance this project would not have been the same. I want to express my gratitude to all the technical staff of the "Epi ENER"at LGPSD: Jean-

Marie, Jean-Claude, Dénis, Ludivine and especially to Bernard(o!), who designed the first fry-dryer for sewage sludge ever built. Thanks you all for your help and your time, feel sure that this Mexican is going to miss you!

Thanks to Jean-Michel MEOT, Philippe BOHUON and Henri BAILLERES from the

CIRAD. Thanks for sharing your knowledge with me in a so fine manner. Be sure that I will never see frying in the same way.

I am grateful for the immeasurable help received from Professor Gilles TRYSTRAM and

Senior Research Scientist Tadeusz KUDRA who accepted the responsible task of checking and assessing this thesis.

I want to show my gratitude to Sylvie PADILLA and Marlène DRESCH not only because of

the very important financial support received from the ADEME, but also for all the exchanges and discussions at the different stages of this thesis.

Thanks a lot to all the friends and colleagues from the École des Mines d’Albi Carmaux who I

shared a lot of good moments with, specially Máximo, Ana, Karim, Daniela, Naly and Anwar. I want to particularly thank Sofía (mi gran amiga Venezolana y mi colega de oficina). Thanks for all those moments when we started to talk about sewage sludge and finished discussing the sadness of being far away from home. Many thanks also to Carmen (mi Carmela) and Miguel(ón), my deepest Mexico-Albigeois friends. I will never forget all your help and support in the good, but above all, in the worst moments that I lived in Albi.

In addition, to be part of other Research groups than LGPSD was an extremely enriching

experience. First, during my meetings at the CIRAD and then during my stay at UQ, I met wonderful people who explained to me how amazing interdisciplinary work could be. I want to thank David, Aracely, Yanine and Juan in Montpellier as well as Adrian, Stephano, Federico, Rossana, Hein, Dino, Manu, Wally, Brama in Brisbane and especially Bradley LADEWIG: Thanks a lot for everything,…mate!!!. I wish I could drink a beer again with all you guys!!


Doing a PhD thesis overseas is not an every-day funny task. That is why I want to thank to all

those friends that in despite of the distance have known how to remain close to me: Quetzalcoatl, Angel, Robert, Jesús, León, Solecillo, Mariachi, Ale, Olivier, Janette, Lola, Susana, Raymundo, Dr. Nungaray and padrino Michel! I want also express gratitude to my French family who made my stay in France a lovely experience. Thanks to Monsieur Alain, Madame Marie-France, Emilie and Amaël because when you opened me the doors of your house, you did the same with your heart’s.

Furthermore, I want to show endless gratitude to my PADRE SANTO and my MADRE

SANTA who just gave me life and the knowledge to live it… without YOU this could have never been done! Thank you from the deepest of my heart (Los adoro con toda mi alma canijos!).

Thanks to my grand mother Lucrecia, I hope to see you soon!! Thanks to my cousin -almost brother- el Giorgio!. All my love and admiration go to my brothers and sisters: Cristina (Titina), Adriana (Adrianation), Miguel (Miguelito) and Vidal (Shélélé)…you have been every day -and especially THOSE days- in my mind and close to my heart from the beginning of this experience until now.

Finally I want to thank Cécilia, mi güera adorada, because of everything. How could I imagine

these three years without you? …and even less the rest of my life?! Thanks for being the perfect woman to me…celle qui m’invite à rêver en me posant bien les pieds sur terre!

Thank you all.



Résumé

Le procédé de séchage par friture consiste à mettre en contact une phase solide humide divisée (la boue d'épuration) et une phase liquide non miscible (une huile alimentaire usagée), chauffée entre 120 et 180°C, pour obtenir un solide granulaire stable, hygiénisé et valorisable notamment comme combustible. Une étude expérimentale à l’échelle du laboratoire a permis d’identifier les différents mécanismes de transfert de chaleur et de masse mis en jeu lors de l’opération de friture de boue et d’optimiser les paramètres opératoires. Aux temps courts, les phénomènes limitants sont d’origine thermique. Aux temps longs, la limitation des transferts provient du transport d’huile au sein de la matrice poreuse puis du transfert de matière en phase vapeur. Une Analyse de Cycle de Vie (ACV) a été mise en œuvre pour évaluer les performances environnementales d’une filière thermique « séchage + incinération » de valorisation des boues. Le scénario de référence fait appel à un séchage par contact avec agitation tandis que le scénario alternatif prévoit un séchage par friture. Parmi les quatre catégories d’impact retenues, le séchage par friture s’avère extrêmement performant en terme de consommation des ressources non renouvelables et d’impact sur le changement climatique. Enfin, la simulation d’un procédé continu, fonctionnant sur la base d’une production d’une tonne par heure de boues auto-combustibles, avec différents systèmes de récupération de l’énergie contenue dans les buées a été réalisée à l’aide d’un logiciel du commerce. Ce dimensionnement a servi de base à une évaluation économique des coûts d’investissement et de fonctionnement de l’installation.

Mots-Clés: séchage - friture – boues – huiles usagées alimentaires – transferts de chaleur et de matière – analyse du cycle de vie – analyse économique


Fry-drying of sewage sludge : From the physical mechanisms to the process eco-design

Abstract

Fry-drying of sewage sludge consists in bringing into contact the wet solid with a heated oil (120°C


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IONISANTS BABOUX J.C. GEMPPM*** BALLAND B. PHYSIQUE DE LA MATIERE BAPTISTE P. PRODUCTIQUE ET INFORMATIQUE DES

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Développement Urbain BOTTA-ZIMMERMANN M. (Mme)

UNITE DE RECHERCHE EN GENIE CIVIL - Développement Urbain

BOULAYE G. (Prof. émérite) INFORMATIQUE BOYER J.C. MECANIQUE DES SOLIDES BRAU J. CENTRE DE THERMIQUE DE LYON -

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PHYSIQUE DE LA MATIERE

Directeurs de recherche C.N.R.S. :

BERTHIER Y. MECANIQUE DES CONTACTS CONDEMINE G. UNITE MICROBIOLOGIE ET GENETIQUE COTTE-PATAT N. (Mme) UNITE MICROBIOLOGIE ET GENETIQUE FRANCIOSI P. GEMPPM*** MANDRAND M.A. (Mme) UNITE MICROBIOLOGIE ET GENETIQUE POUSIN G. BIOLOGIE ET PHARMACOLOGIE ROCHE A. MATERIAUX MACROMOLECULAIRES SEGUELA A. GEMPPM***

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PHYSIQUE DES MATERIAUX ****LAEPSI LABORATOIRE D’ANALYSE ENVIRONNEMENTALE

DES PROCEDES ET SYSTEMES INDUSTRIELS


PREFACE

Managing of municipal sewage sludges is of great importance to communities all over the world. Within sludge processing, it is observed that thermal drying is an important intermediate operation prior its final disposal. Thus, the enhancement of the existing drying technologies or the implementation of better adapted processes, is one important focus of the LGPSD -Laboratoire de Génie de Procédés de Solides Divisés - of the École des Mines d’Albi Carmaux (France).

In this thesis is presented the study of a novel thermal drying technology which allows transformation of indigenous sludge and recycled cooking oil into a solid fuel. In order to cover all the aspects determining the feasibility of this process, an economic and an environmental assessments were also required and carried out with the collaboration of a partner research team at the Chemical Engineering Department of the University of Queensland (Australia).

In addition, technical exchanges with the CIRAD -Centre de Coopération Internationale en Recherche Agronomique pour le Développement- (France) were necessary. Those consisted in the adaptation of the existing knowledge on heat and mass transfer phenomena taking place during frying of foods to the fry-drying of sewage sludge.

The main results and the general methodology were communicated through the following related publications:

1. PEREGRINA, C.; ARLABOSSE, P.; LECOMTE, D. RUDOLPH,V. “Heat and mass transfer during fry-drying of sewage sludge” (In Press) Drying Technology.

2. PEREGRINA, C.; LECOMTE, D. RUDOLPH,V.; ARLABOSSE, P. “Life cycle assessment (LCA) applied to the design of an innovative drying process for sewage sludge” (In Press) Process Safety and Environmental Protection.

3. PEREGRINA, C.; RUDOLPH,V.; LECOMTE, D.; ARLABOSSE, P. “A new application of immersion frying for the thermal drying of sewage sludge: An economic assessment” (Submitted) Journal of Environmental Management.

4. PEREGRINA, C.; ARLABOSSE, P.; LECOMTE, D. RUDOLPH,V. “Fry-drying: an intermediate sustainable operation for the co-disposal of sewage sludge and waste food oil”(2005) Proceedings of the 7th World Congress of Chemical Engineering, Glasgow, Scotland.

5. PEREGRINA, C.; ARLABOSSE, P.; LECOMTE, D. RUDOLPH,V “Life cycle assessment applied to the design of an innovative drying process for sewage sludge”(2005)Proceedings of the International Conference on Engineering for Waste Treatment, Albi, France.

6. PEREGRINA, C.; ARLABOSSE, P.; LECOMTE, D.. RUDOLPH,V. “The environmental performance of an alternative fry-drying process for sewage sludge: A life cycle assessment study ” (2005) Proceedings of the 4th Australian Life cycle assessment Conference, ISBN :0-9757231-0-3 Sydney, Australia.

7. PEREGRINA, C.; ARLABOSSE, P.; LECOMTE, D.. RUDOLPH,V. Optimisation energétique et environnementale d'un procédé de co-traitement huiles/boues. In : Le génie des procédés vers de nouveaux espaces (2005) Ed. Société Française de Génie des procédés No. 92.

8. PEREGRINA, C.; ARLABOSSE, P.; LECOMTE, D. “Thermal efficiency in sewage sludge fry drying”, (2004), Proceedings of the 14th International Drying Symposium, São Paulo, Brazil.

9. PEREGRINA, C.; ARLABOSSE, P.; LECOMTE, D. “ Fry-drying of sewage sludge: an alternative for the disposal of recycled food oils” (2004) , Proceedings of the 9th International Congress on Engineering and Food, Montpellier, France, pp.154-159.

10. PIRES da SILVA, D.; PEREGRINA, C.; ARLABOSSE, P.; LECOMTE, D.; PEREIRA-TARANTO, O.; RUDOLPH, V. “ Fry-drying of sewage sludge: preliminary results”,(2003), 6th Conference on Process Integration, Modeling and Optimization for Energy Saving and Pollution Reduction, Hamilton, Canada.

This PhD thesis was financially supported by the ADEME –Agence de l’Environnement et de la

Maîtrise de l’Energie- (France), the Conseil Régional Midi-Pyrénées (France) and the CONACyT –Consejo Nacional de Ciencia y Tecnología- (Mexico).

List of Contents

List of Contents


CONTENTS 1 INTRODUCTION.........................................................................................................................31 2 MATERIALS AND PRELIMINARY DEFINITIONS.....................................................43

2.1 Raw Materials ....................................................................................................................................... 44

2.1.1 Sewage sludge................................................................................................................................................... 44 2.1.2 Recycled Cooking Oil (RCO) ........................................................................................................................ 46

2.2 Fry-dried sludge .................................................................................................................................... 48

2.2.1 Oil and moisture content of the fry-dried sludge........................................................................................ 49 2.2.2 Lower heating value of the fry-dried sludge ............................................................................................... 51

2.3 Exhaust gases ........................................................................................................................................ 53

3 ANALYSIS OF HEAT AND MASS TRANSFER DURING FRY-DRYING OF SEWAGE SLUDGE..............................................................................................................................57

3.1 Experimental methods........................................................................................................................... 57

3.1.1 Determination of fry-drying curves .............................................................................................................. 57 3.1.1.1 Experimental setup..................................................................................................................... 58 3.1.1.2 Experimental protocol for discontinuous weighing ..................................................................... 61 3.1.1.3 Experimental protocol for continuous weighing ......................................................................... 61 3.1.1.4 Data adjustment ......................................................................................................................... 64

3.1.2 Data analysis..................................................................................................................................................... 69 3.1.3 Video of fry-drying of sewage sludge ........................................................................................................... 69

3.2 Results and discussions ......................................................................................................................... 70

3.2.1 Typical fry-drying curves for sewage sludge ................................................................................................ 70 3.2.1.1 Reproducibility of the fry-drying measurements.......................................................................... 70 3.2.1.2 Mapping of the external heat transfer coefficient ........................................................................ 73 3.2.1.3 Interpretation of the mechanisms involved in the fry-drying of sewage sludge ............................ 77

3.2.1.3.1 First period: Initial heating period ......................................................................................... 77 3.2.1.3.2 Second period : Fry-drying of sewage sludge as a boiling front process ................................ 79 3.2.1.3.3 Third period: oil impregnation .............................................................................................. 83 3.2.1.3.4 Fourth period: Fry-drying of sewage sludge as a boiling in a porous media process ............... 86

3.2.1.4 Discussion about the differences between the fry-drying of sewage sludge and the foods frying processes ................................................................................................................................................... 88

3.2.2 Applying the immersion frying for the thermal drying of sewage sludge ................................................ 90 3.2.2.1 Effect of some selected operational conditions on the drying curves........................................... 90

3.2.2.1.1 Oil temperature..................................................................................................................... 90 3.2.2.1.2 Sewage sludge initial moisture content .................................................................................. 93 3.2.2.1.3 Diameter of the sample......................................................................................................... 94

3.2.2.2 Fry-drying of sewage sludge, as an intensive process ................................................................... 96 3.2.2.2.1 Efficacy of the operation ...................................................................................................... 97 3.2.2.2.2 Multi-function of the operation............................................................................................. 98

3.3 Conclusions......................................................................................................................................... 100

List of Contents


4 ENVIRONMENTAL ASSESSMENT OF THE FRY-DRYING OF SEWAGE SLUDGE .................................................................................................................................................105

4.1 Disposal routes for the fry-dried sludge .............................................................................................. 106

4.1.1 Marketing of the fry-dried sludge as a solid fuel ....................................................................................... 106 4.1.2 Auto-thermal combustion ............................................................................................................................ 107

4.2 Life cycle assessment as a tool of environmental evaluation.............................................................. 108 4.3 LCA development ............................................................................................................................... 110

4.3.1 Goal and scope of the study ........................................................................................................................ 110 4.3.1.1 Goal ......................................................................................................................................... 110 4.3.1.2 Environmental impact categories.............................................................................................. 110 4.3.1.3 Boundaries of the study ............................................................................................................ 111 4.3.1.4 Other assumptions of the study ................................................................................................ 112

4.3.2 Inventory analysis .......................................................................................................................................... 115 4.3.2.1 Inventory of drying................................................................................................................... 115 4.3.2.2 Combustion balances................................................................................................................ 120 4.3.2.3 Transportation balances............................................................................................................ 121

4.3.3 Impact assessment......................................................................................................................................... 121 4.3.4 LCA interpretation ........................................................................................................................................ 128

4.4 Conclusions......................................................................................................................................... 129

5 ECONOMIC ASSESSMENT OF THE FRY-DRYING OF SEWAGE SLUDGE .... .............................................................................................................................................................133

5.1 Theory and calculation........................................................................................................................ 133

5.1.1 Basis of the assessment................................................................................................................................. 133 5.1.2 Economic assessment ................................................................................................................................... 134 5.1.3 Process Simulation ........................................................................................................................................ 136

5.1.3.1 Simulation of the fry-drying unit............................................................................................... 136 5.1.3.2 Simulation of the heat pumps ................................................................................................... 138

5.1.4 Process Design............................................................................................................................................... 141 5.1.4.1 PROCESS 1: Fry-dryer with a condenser as an energy recovery system .................................... 141 5.1.4.2 PROCESS 2: Fry-dryer with a closed heat pump as an energy recovery system ......................... 141 5.1.4.3 PROCESS 3: Fry-dryer with a open heat pump as an energy recovery system (Mechanical Vapor compression)............................................................................................................................................ 143

5.2 Results................................................................................................................................................. 145

5.2.1 Technical process comparisons ................................................................................................................... 145 5.2.2 Economic process comparisons .................................................................................................................. 148

5.2.2.1 Fixed capital cost ...................................................................................................................... 148 5.2.2.2 Manufacturing cost................................................................................................................... 150 5.2.2.3 Economic performance of frying as a thermal drying process for sewage sludge ....................... 152

5.3 Conclusions......................................................................................................................................... 156

GENERAL CONCLUSIONS AND PERSPECTIVES ..........................................................161 NOMENCLATURE............................................................................................................................169 REFERENCES......................................................................................................................................175

List of Tables and Figures

List of Figures


FIGURES Figure 2-1 Black box diagram of the fry-drying of sewage sludge............................................... 43 Figure 2-2 Schematic representation of the mass composition change from dewatered to fry-dried

sludge. ...................................................................................................................................................49 Figure 2-3 Comparison of the sewage sludge LHVsample obtained for fry-dried and air dried sludge.

........................................................................................................................................... 52 Figure 3-1 Detail of the experimental setup. .............................................................................. 59 Figure 3-2 Pneumatic jack positions during the fry-drying experiment. ...................................... 60 Figure 3-3 Detail of the sample container and micro-thermocouples. ........................................ 61 Figure 3-4 Monitored masses that are included in the continuous weighing. .............................. 62 Figure 3-5 Continuous weighing during the fry-drying of sewage sludge. ................................... 63 Figure 3-6 Comparison of the drying curves (T=160°C and D=15 mm) obtained by the

continuous ( ), adjusted ( ) and discontinuous ( ) methods. ........................................... 64 Figure 3-7 Effect of the frying temperature (a), the diameter (b) and the initial moisture content

of the sample (c) on the values of α..................................................................................... 68 Figure 3-8 Spread between the raw and fitted data of the drying (a), heating (b) and drying rate (c)

curves of the fry-drying of sewage sludge at 160°C using a sample diameter of 25 mm. ...... 71 Figure 3-9 Samples of mechanically dewatered (a) and fry-dried (b) sludge and microstructure of

the fry-dried sludge (c). ....................................................................................................... 72 Figure 3-10 Images of different boiling regimes observed during fry-drying of sewage sludge. .. 73 Figure 3-11 Mapping of the convective heat transfer coefficient for fry-drying at the reference

conditions ........................................................................................................................... 76 Figure 3-12 Co-plotted drying rate-heating curves of fry-drying at the reference conditions. ..... 78 Figure 3-13 Co-plotted drying Krischer-heating curves of fry-drying at the reference conditions.

........................................................................................................................................... 78 Figure 3-14 Regions within the cross section of a partially fry-dried sample. .............................. 79 Figure 3-15 Thermal resistances calculated during the periods 1, 2 and 3 of the fry-drying for

sewage sludge...................................................................................................................... 80 Figure 3-16 Schematized regions within the cross section of a sewage sludge sample during the

period 2 of fry-drying. ......................................................................................................... 81 Figure 3-17 Co-plotted drying-heating curves at the reference conditions. ................................. 83 Figure 3-18 Pictures of the cross section at times close to the beginning (a) and the end (b) of the

period 3 of fry-drying. ......................................................................................................... 85 Figure 3-19 Schematized regions within the cross section of a sewage sludge sample during the

period 3 of fry-drying. ......................................................................................................... 85 Figure 3-20 Schematized regions within the cross section of a sewage sludge sample during the

period 4 of fry-drying. ......................................................................................................... 86 Figure 3-21 Semi-infinite slab undergoing frying according to Farkas et al.[72]........................... 88 Figure 3-22 Effect of oil temperature on the drying curves. ....................................................... 91 Figure 3-23 Effect of oil temperature on the Krischer curves. ................................................... 91 Figure 3-24 Effect of sludge initial moisture content on the fry-drying curves. .......................... 93 Figure 3-25 Effect of sludge initial moisture content on the Krischer curves. ........................... 94 Figure 3-26 Effect of diameter of the sample on the fry-drying curves....................................... 95 Figure 3-27 Effect of the diameter of the sample on the Krischer curves .................................. 96 Figure 4-1 Boundaries of the assessment. ................................................................................ 112 Figure 4-2 Adiabatic temperature change and lower heating value versus total solids content of

the conventionally dried sludge from the WWTP of Albi. ................................................. 113

List of Figures


Figure 4-3 Adiabatic temperature change and lower heating value versus total solids content of the fry-dried sludge from the WWTP of Albi. ................................................................... 113

Figure 4-4 Oil uptake and drying curves for the fry-drying....................................................... 114 Figure 4-5 Experimental set up to recover the fry- drying exhaust vapors. ............................... 118 Figure 4-6 Immersion of the sludge sample into the reactor. ................................................... 118 Figure 4-7 Inventory of streams considered in the assessment. ................................................ 123 Figure 4-8 Inventory of streams considered in the assessment (cont.). ..................................... 124 Figure 4-9 Normalized impact categories with respect to the most important contributor. ...... 126 Figure 4-10 Normalized impact categories with respect to the most important contributor (cont.).

......................................................................................................................................... 127 Figure 5-1 Flow sheet of the simulated fry-drying unit. ............................................................ 137 Figure 5-2 PFD of a Fry-dryer with a condenser as an energy recovery system. ....................... 140 Figure 5-3 PFD of a Fry-dryer with a closed heat pump as an energy recovery system............ 142 Figure 5-4 PFD of a Fry-dryer with an open heat pump as an energy recovery system............. 144 Figure 5-5 Distribution of the direct manufacturing costs for the conventional thermal dryers,

according to Ressent [8]. ................................................................................................... 154 Figure 5-6 Distribution of the direct manufacturing costs for the simulated fry-drying processes.

......................................................................................................................................... 155

List of tables


TABLES Table 1—1 Synthesis of the specific goals adopted in this study. ............................................... 39 Table 2—1 Proximate and ultimate analysis of sewage sludge from the WWTP of Albi (France).

...............................................................................................................................................................45 Table 2—2 Comparison of the micro-pollutant contents of the sewage sludge from the WWTP

in Albi and those of the French average municipal sewage sludge[49]. ................................ 46 Table 2—3 Proximate and ultimate analysis of RCO from Sud-Recuperation (Muret, France).. 46 Table 2—4 Effect of the oil degradation on their viscosity and the convective heat transfer

coefficient. .......................................................................................................................... 47 Table 3—1 Operational parameters of the frying tests. .............................................................. 69 Table 3—2 Effect of the frying temperature on the internal and external resistances during the

second period of fry-drying of sewage sludge. ..................................................................... 92 Table 3—3 Synthesis of the identified limiting mechanisms for the different types of drying..... 98 Table 4—1 Calculated composition for the auto-thermal partially dried sludge........................ 114 Table 4—2 Proximate and ultimate analysis of the sewage sludge from the WWTP of Albi and

VilleFranche-sur-Saone [11]. ............................................................................................. 115 Table 4—3 Input and output mass streams in the two dryers. ................................................. 117 Table 4—4 Pollutant concentrations of the selected emissions for the two dryers. .................. 119 Table 4—5 Pollutant emissions during the incineration of the partially dried sludge. ............... 120 Table 5—1 Basic conditions for the simulation of each process .............................................. 145 Table 5—2 Number of main processing units required for each process. ................................ 146 Table 5—3 Description of the operating conditions of the main equipments. ...................... 147 Table 5—4 Description of utilities required for each process per hour .................................... 148 Table 5—5 Equipment and fixed capital costs of the simulated fry-drying processes. .............. 149 Table 5—6 Total manufacturing cost of the simulated fry-drying processes. ........................... 150 Table 5—7 Approximate fixed capital costs of current sewage sludge thermal drying facilities in

Europe.............................................................................................................................. 153

Introduction


1 INTRODUCTION

Sewage sludge is a by-product of wastewater treatment and represents a significant problem

in terms of its volume (850,000 tons of dry matter per year in France in 1998[1]) and of its

organic content, especially regarding final disposal. As a consequence of the dramatic increase of

the wastewater volumes treated in European countries, a generation of about 10.7 millions tons

of total dry solids of sewage sludge produced every year has been forecast for the year 2005 [2].

Sewage sludge were commonly land spread as fertilizer supplement in agriculture, used as a co-

fuel or even disposed in landfills. Since 2001 it was accepted the definitive suppression of sewage

sludge landfilling, resulting in an increase of the two other approved disposal options, namely

land spreading and incineration[3]. According to the European Environment Agency (EEA)

[4], thermal drying, which refers to the removal of moisture from a substance by evaporation or

vaporization[5], will become a major issue in the disposal of sewage sludge. In fact, it allows the

removal of the water contained in the sludge after the mechanically dewatering (i.e. 4 kg

water·kg-1 dry matter or even higher). This separation operation is positioned as an intermediate

unit operation[1, 6-9] strategic for the two available disposal routes[2], since it involves a

reduction in its volume and an increase in the calorific value[2]. Moreover, further drying serves

also to stabilize the sludge and, if the residence time and the temperature are sufficient, to

hygienize the product[2].

Current thermal dryers for sewage sludge were adapted from industrial dryers used in other

domains such chemicals, food or pharmaceutical[1]. Dryers are composed essentially of four

systems:

1. A conveying system, which moves the product into, through and out of the dryer;

2. A heating system, to rise the temperature of the drying gases or the heating surface of

the dryer;

Introduction


3. An exhaust vapors management system that extracts and treats the emissions and

sometimes recovers the energy contained in the exhaust vapors;

4. A regulation system, which controls the drying parameters such as sludge and air

flows, temperature, pressure, etc.

Three classes of dryers are reported [6]: convective or direct dryers, conductive or indirect

dryers and mixed dryers .

In the direct drying, hot gases from the combustion of oil, natural gas or the dried sludge itself,

are mixed with the dewatered cake in the dryer and transport the sludge through it, evaporating

the water off while in transit. Some examples of such equipments are drum, rotary, belt, spray

and fluidized bed dryers. For the indirect drying, heat transfer occurs through the walls of the dryer

(e.g. thin-film, discs or paddle dryers among others). The heat carrying medium, which can be

hot gas or thermal oil, is in a separate stream with respect to the vapor. Finally, mixed dryers are

a combination of the above systems using both conduction and convection. Further technical

details can be available elsewhere in the literature [1, 6, 8, 10].

Due to its high –energy consumption [1], which represents a third and a half the total running

cost [8], thermal drying is not a cost effective operation. Indeed, in order to perform this

operation, it is necessary to consume at least the latent heat of the evaporated water (some 700

kWh·ton-1 evaporated water in theoretical circumstances). Moreover, due to the thermal losses

observed in most of the current dryers, the real consumption is still higher [11]. A study [8],

which assessed the economic and thermal performance of 13 facilities treating between 1280 and

1520 kg of total solids dried per year, reveals that direct dryers are major energy consuming

process (i.e. some 1100 kWh·ton-1 evaporated water) followed by direct dryers (i.e. some 924

kWh·ton-1 evaporated water) and mixed dryers (i.e. some 770 kWh·ton-1 evaporated water).

Similar results were obtained by Ressent [11]: An average energy consumption of 905 kWh·ton-1

Introduction


of evaporated water was determined for indirect dryers while it was 1231 kWh·ton-1 of

evaporated water for the direct dryers. It is important to notice that energy consumption may

vary noticeable from one type of drying facility to another. Ressent [8] reported that energy

consumption for direct and indirect dryers may range from 847.1 to 1064.4 and 830 to 1140

kWh·ton-1 of evaporated water, respectively. This means that heat losses represent generally

between 25 to 60% of the thermal consumption. Such efficiencies suppose an opportunity to

search new drying alternatives more adapted to the sludge and also to implement energy recovery

strategies whether in the process itself or in the whole wastewater treatment plants (WWTP)

facility.

The energy cost is a parameter all the more important to control that the product has a low

added value like sewage sludge. In addition, since the dryers are major consumers of energy [12],

they are also significant contributors to non renewable natural resources consumption and to

greenhouse gases production. However, the greatest advantage in having sludge in a dry form as

compared with various other treatment methods, is the possibility of ‘marketing’ the product for

a number of applications (e.g. fertilizer, soil conditioning, fuel) and at the most suitable time [2].

Volume diminution significantly reduces the storage and transportation costs [7], although it is

economically justified only for WWTP operating in areas of large population densities, i.e. ≥100

000 EqH [9, 13]. The last reason for implementing thermal drying in a WWTP concerns

environmental and social constraints, such as the avoidance of olfactive nuisances or hygienic and

sanitary concerns [2].

In order to be considered as a sustainable intermediate step for the disposal of sewage sludge

thermal drying requires to be intensified. This means that new drying processes should be less

expensive, more efficient, and combine multiple operations into a single apparatus [14]. Another

Introduction


key point to reduce the energy consumption of the dryer is the successful integration of the unit

in the whole facility.

Fundamental strategies to integrate the drying process in the disposal of sewage sludge are:

1. Avoiding the excessive energy consumption. Since thermal drying is an intermediate operation, the

place of the dryer in the whole disposal system should be carefully defined. Energy can be

saved if the sludge, before thermal drying, is pre-dried until reach the limit of dewaterabilty

is reached [15]. Consequently, appropriate drainage processes such as thickening and

mechanical dewatering, which are minor energy consumer operations [16], should be

applied prior to drying. Concerning the final product, the degree of drying should be fixed

depending on the selected disposal route. If the marketing of the dried sludge is preferred,

it is of great importance - and cost saving - that thermally dried sludge undergoes safe

hygienisation and considerable volume reduction [2]. As a consequence full drying would

be required. However, if the sludge must be imperatively eliminated, only incineration

allows the organic matter removal that is required for its final disposal [17]. In that

situation, thermal drying must be installed on site or at a relatively short distance of the

sludge burner and performed only to reach its auto-thermal composition [17].

2. Using waste heat (Pinch analysis): Since high amounts of energy are involved in the thermal

drying, it is possible to identify integration opportunities within a process, a plant, or a total

site. In most chemical engineering processes, there exist heat sources (hot process streams

that need to be cooled) and sinks (cold streams that need to be heated). Instead of using

utilities (e.g. steam, cooling water) to bring all process streams to their desired temperatures

or conditions, a pinch analysis may be performed to exploit the heat sources and sinks in the

process before using utilities, thus reducing the operating cost of a process [18]. Using the

biogas from a digester as dryer fuel [19] or the drying condensate for heating purposes in

the plant [20] are some applications of pinch technology in the sludge thermal drying.

Introduction


3. Transforming low quality into high quality heat. Drying vapor have an energy content that may be

potentially recycled in the process reducing thus the required total heat in the operation. As

heat is not able to flow naturally from the lower temperature of the condensate to the

higher temperature that is needed in the process, the use of heat pumps, which necessitate

a relatively small amount of high quality drive energy (electricity, fuel, or high-temperature

waste heat) [21], is necessary. This application is still not widespread among the thermal

dryer constructors, since processes should be first optimized and integrated to ensure the

sound application of heat pumps in industry. Nevertheless, the efficiency of such

technologies with low energy consumption, ranging between 130 and 560 kWh·ton-1 of

evaporated water, was already demonstrated [1]. The rapid increase of oil rates as well as

the concerns related to the climatic change impacts, caused by the greenhouse emissions,

may support future implementations of heat pumps in thermal drying [12].

Within the framework of this PhD, we propose the application of immersion frying as an

innovative and intensive sewage sludge drying technique. Immersion frying, is widely used in

food processing as a cooking operation mainly because it transforms original sensory qualities of

foods [22], can also be a very effective drying and formulating method for a large variety of

products [23].

As immersion frying is an old, well established and widely used operation, little attention was

paid to the understanding of the frying process mechanisms for quite a long time. With process

and product optimization requirements, some research activity, driven by the food industries,

occurred in the mid 1970s. As a result, the heat, mass and momentum transfer mechanisms are

partially understood [22, 24-29]. Much more recently, frying has been recognized as a potentially

unit operation for drying, that can be applied in a variety of industrial processes [30].

The experiments performed essentially with food materials revealed that, after immersion

into the heated oil and initial heating of the raw material by convection, a dried crust begins to

Introduction


form at the product’s surface. Its thickness increases over the duration of the frying process until

the core region is dried. The heat is supplied by convection to the outer surface of the material

and by conduction through the solid material to the core region. As a result, water is vaporized

at the crust/core interface and flows to the outer surface. Due to the intensive heat and mass

transfers, fry-drying times are short and not longer than a ten of minutes. In addition some

advantages may be associated to the oil impregnation considering its application on the sewage

sludge processing.

Operations involving a similar solid-liquid contact for the thermal drying of sewage sludge

have been proposed some years ago [31-33], though the experimental results were not fully

published. The first diffused work about the drying of sewage sludge by immersion frying was

presented by Pires da Silva [34, 35] and conducted partially at Ecole des Mines d’Albi [36]. The

experimental tests were carried out by immersing a cylinder (about 40mm length × 20-26mm

diameter) of municipal sewage sludge into a 5 L soybean oil bath which was maintained at

temperatures above water boiling temperature (between 168°C to 213°C). In those conditions, it

was possible to reach a final total-solids content >95 % in about 600 seconds. Moreover, due to

the oil impregnation, the lower heating value (LHV) of fry-dried sludge reached 24 MJ·kg-1

which is significantly higher to that of the same air-dried sludge (i.e. 14MJ·kg-1).

Fry-drying principle gathers some characteristics of direct and indirect dryers reducing the

number of technical problems found in the thermal drying of sewage sludge. In principle, the

configuration of the frying process looks as simple as direct drying, where the product is directly

contacted with the heating mobile phase (i.e. frying oil) without needing any frictional device,

avoiding thus the plastic phase related problems [37, 38]. Moreover, the water is removed by a

boiling mechanism giving small amounts of exhaust vapors highly concentrated in water vapor.

Hence, fry-drying is suitable to be equipped with an emission management system to treat the

exhaust emission and recover the vapor latent energy.

Introduction


Then, oil uptake offers several benefits concerning the conditioning of the dried sludge. It

increases the energy value of the sludge [35]. Large size particles of the fried sludge could be

obtained due to the agglomeration of the dry solids particles that are coated by a final oily layer.

Finally, it seems that the several physico-chemical reactions taking place during fry-drying [39,

40] may stabilize the final product enabling its storage and/or transportation.

However, the oil impregnation does not allow agricultural land spreading. As a result,

incineration will remain the only valorization disposal route of the fried sewage sludge. This

apparent major constraint is not very limiting in the European context since most countries tend

to limit sewage sludge land filling and land spreading [3, 4, 17]. In addition, new incineration

technologies as well as pyrolysis and gasification processes applied to the sewage sludge [17],

which belong to the group of Energy from Waste Incineration (EfWI) [41], claim their place as

natural companion of the practicable recycling in a truly integrated waste management hierarchy.

Another key issue is that the application of this process requires the use of a co-product, the

frying oil, and thus its availability. In order to improve the environmental and economic

performance of the proposed operation, it was decided to use the recycled cooking oil (RCO) as

the frying oil in this study. RCO is the generic name for the oily phase resulting after several

stages of purification of waste vegetable oils and greasy wastewater collected in the grease-traps

of restaurants, agricultural and food industries outlets [42]. Due to the food safety problems in

1999 in Europe, the market for recycled cooking oils (RCO) has considerably decreased–

previously animal feed accounted for 85% of the oil collected in France [43]– so much that

finding new ways of disposing RCO has become a major concern for the European food

industry. As a consequence these food industry by-products are becoming available as an oil

resource with good chemical and physical stability [43]. In France, the yearly production of this

waste is estimated in 30000 tons and as a consequence new methods for the economic disposal

of RCO are required [44]. Co-valorization of RCO and sewage sludge to formulate a derived

bio-fuel provides such an opportunity.

Introduction


Although there seems to be significant advantages of this new drying process the feasibility of

any new idea or a new process should be evaluated by comparing its performance with other

equivalent or competing processes, usually assessed on a basis of technical, economical and

environmental criteria [18]. The calculation of technical and economic performances can be

made using extensive values (e.g. monetary values, energy contents) whereas the environmental

impact needs a different approach. Environmental assessment uses a set of variables that are

highly dependent on the assumptions made by the evaluators [45], a subjective input, which

raises difficulties when using any environmental assessment tool [46].

The main goal of this study is to determine the feasibility of the immersion frying operation

applied to the thermal drying of sewage sludge. The broad scope of the goal as well as the

innovative characteristics of the subject, required an original procedure based on the following

four specific goals:

1. Identification of the heat and mass transfer mechanisms involved in the operation;

2. Analysis of the effect of selected operating conditions on the fry-drying kinetics;

3. Extrapolation of the experimental results to simulate a continuous fry-dryer;

4. Assessment of the economic and environmental performance of the process

Table 1—1, summarizes the procedures derived of the specific goals adopted in this study.

Before the properly development of the objectives, this documents presents a Chapter 2, which

is devoted to give the characteristics of the materials used in this study and provide the

definitions that are used throughout the text. Afterwards, specific goals 1 and 2, which concern

the fry-drying kinetics, are developed in Chapter 3. The environmental assessment of the process

is treated in Chapter 4 and finally, the economic aspects are discussed in Chapter 5.

Introduction


Table 1—1 Synthesis of the specific goals adopted in this study.

Specific goal Procedure 1.Identification of the mechanisms involved in the fry-drying of sewage sludge

• Build up and validation of an experimental setup • Obtaining the fry-drying kinetics • Quantification of the thermal resistances

2.Study of the effect of some selected operating conditions on the fry-drying kinetics

• Obtaining the fry-drying curves varying the size and initial moisture content of the sample and the frying temperature

3.Economic assessment of the fry-drying as a intermediate step in the disposal of sewage sludge by incineration

• Proposition of a commercial scale fry-dryer • Computer simulation • Capital and operating costs estimation

4.Comparison of the environmental impacts for the disposal of sewage sludge by incineration using : -a conventional dryer -a fry-dryer

• Definition of a disposal scenario • Build up of an experimental set up to characterize the exhaust vapors • Development of a life cycle assessment (LCA)

Materials and Preliminary Definitions


2 MATERIALS AND PRELIMINARY DEFINITIONS

Figure 2-1 schematizes the sewage sludge fry-drying operation. It consists in bringing into

contact the two raw materials (i.e. the mechanically dewatered sewage sludge and the recycled

cooking oil), by immersing the wet solid into a heated deep-fat frying bath. At the end, the fry-

dried sludge is produced, which is a granular solid composed of the dried indigenous sewage solid

and the impregnated oil. In addition, an exhaust vapor stream is obtained.

Figure 2-1 Black box diagram of the fry-drying of sewage sludge.

The aim of this Chapter is to describe the characteristics of the streams of materials involved

in the sewage sludge fry-drying and provide some physical properties that are required for the

development of this thesis.

Materials and preliminary definitions


2.1 Raw Materials

2.1.1 Sewage sludge

The study was performed with the municipal sewage sludge coming from the waste water

treatment plant (WWTP) of Albi (France). In that WWTP, a primary sedimentation is carried out

followed by a biological secondary treatment. The primary and the activated sludges are mixed

and sent to a mesophilic anaerobic digester1 where a fraction of the organic matter is

decomposed into biogas. The sludge is thus stabilized to reduce pathogens, eliminate offensive

odors and lower the potential for putrefaction. Finally, the digested sludge is mechanically

dewatered with a belt filter press to obtain a pasty sludge with a final moisture content between

4.0 and 6.0 kg water·kg-1 total dry solids.

Moisture content denotes the quantity of water per unit of mass of either wet or dry product.

For most of the drying and dewatering applications, the composition of a sludge is usually

described according to the volatility of its components [47]. Thus, from a macroscopic point of

view, the composition is often presented as water mass ( Wm ), which is the removed matter after

drying of the wet sample at 105°C for 24h and total dry solids mass ( TSm ), which is the

remaining matter. Consequently, the moisture content wet basis is defined as:

TSW

WW mm

m(w.b.)ξ

+= ( 2-1 )

and the moisture content dry basis as:

TS

WW m

m(d.b.)ξ = ( 2-2 )

1 Currently, anaerobic digestion is not the most widely practiced treatment for sludge in France. However, since 1998, new policies regarding the practices of land spreading and landfill of sewage sludge, combined with recent technological progress in



A more detailed description of the sludge, from the macroscopic point of view [37], is

provided by the proximate and ultimate analyses. The proximate analysis is a thermal gravimetric

analysis that describes the total solids content (TS), total volatile solids content (TVS) and total

fixed solids content (TFS) in a sample. TS constitutes the remaining residue after drying of the

wet sample at 105°C, TVS those solids that can be volatilized and burned off when TS are ignited

at 550°C and TFS is the residue that remains after combustion. The ultimate analysis gives the

elemental (C, H, O, N, S) compositions of the total solids matter using a self-integrated and

microprocessor controlled elemental analyzer (mod: NA 2100 Protein, CE Instruments, Italy)

according to classic organic elemental analysis techniques. Table 2—1 provides the composition

of the sludge used in this study.

Table 2—1 Proximate and ultimate analysis of sewage sludge from the WWTP of Albi (France).

Composition [TS] (%) [TVS] (%TS)

[TFS] (%TS)

[C] (%TS)

[H] (%TS)

[O] (%TS)

[N] (%TS

[S] (%TS)

Sewage sludge 19±3 67±3 33±3 36.4±3 5.5±0.2 18.8±1.5 5.7±0.2 1.0±0.1

For process design and management issues, this sludge is representative of the wastewater

sludges produced in France[48]. Furthermore, the micro-pollutants contents of the Albi WWTP

are close to average values from more than 500 French WWTP sludge samples (See Table 2—2),

provided by Huyard et al. [49].

biogas production and valorization (in particular in Northern Europe and USA) give new impetus to these processes in the French context[28].



Table 2—2 Comparison of the micro-pollutant contents of the sewage sludge from the WWTP in Albi and those of the French average municipal sewage sludge[49].

Micro-pollutant Sewage sludge from the WWTP in Albi (mg·kg-1 total solids)

Average French sewage sludge[31]

(mg·kg-1 total solids) 7 polychlorinated biphenyls

(PCB’s) 0.10 0.19

Fluoranthene 0.49 0.54

Benzo(b)fluoranthene 0.23 0.34

Benzo(a)pyrene 0.18 0.32

Cd 2.2



Before its first use for fry-drying tests, RCO is preheated for 1 hour at 180°C to eliminate the

remaining water.

The transport properties of the oil are likely to have an important influence on the process

[50, 51]. Since the RCO is recovered from a waste stream, some degree of thermal degradation,

as a result of its previous use, may be expected [40, 52].

For the fry-drying process, the convective heat transfer coefficient (h) between the product

surface and the frying oil is important and is known [51] to decrease with the degree of

degradation. Viscosity of the RCO, which increases with oil degradation [50], may be used as a

convenient proxy to determine the extent of the change. Tseng et al. [51], measured

experimentally how h and viscosity changed for degraded oils and showed that the two

properties were highly correlated (R=-0.98). Using the correlation and the measured viscosity of

the RCO –according to the standard ISO3104-, the expected values of h (Table 2—4) are quite

similar to those of fresh oil, even though the RCO is quite degraded.

Table 2—4 Effect of the oil degradation on their viscosity and the convective heat transfer coefficient.

Oil Viscosity (Pa·s) at 190°C h (Wm-2K-1) at 190°C

Refined soybean oil 2.04×10-3 279.4

20h degraded soybean oil 2.17×10-3 276.2

RCO 2.46×10-3 271.6*

30h degraded soybean oil 2.57×10-3 269.8 *Calculated according to Tsen et al. [51].

Later on, in the Chapter 3 of this document, it will be necessary to evaluate the free

convection contribution to the heat transfer in fry-drying. In order to determine the free

convection coefficient ( fch ) from typical relationships [53], the Prandtl number is to be

calculated. Hence it is necessary to determine the density, specific heat and the thermal

conductivity of the RCO.



RCO specific heat at constant pressure )(CpRCO was experimentally determined using a C80

calorimeter (Setaram, Caluire, France) for a range of temperature between 60 and 200°C. A linear

expression (R2=0.98312) may be used:

C)T(2.922009.84)Ckg(JCp 11RCO °⋅+=°⋅⋅−− ( 2-3 )

Measurements of the RCO thermal conductivity ( RCOk ) were carried out with the Hot Disk

Bridge system. The radius of the chosen sensor was r=3.3mm and the power and measurement

times were 50mW and 20s, respectively. The values of the RCOk can be obtained according to

the following linear expression (R2=0.9800):

C)T(0.00050.1804)Cm(Wk 11RCO °⋅+=°⋅⋅−− ( 2-4 )

Finally, the density of the RCO was determined experimentally for temperatures close to the

ambient temperature (i.e. 15 and 20°C) using a pycnometer (mod.). The density of the RCO is

not very different to that reported for other fresh vegetable oils at the same temperature [22,

54]. Consequently, the following linear correlation (R2=0.9977), which is based on the reported

densities for vegetal oil over a temperature range from –20 to 160°C, was used to describe the

density of the RCO ( RCOρ ).

C)T(0.6379934.43)m(kgρ 3RCO °⋅−=⋅− ( 2-5 )

2.2 Fry-dried sludge

During frying, water evaporation comes along with oil uptake on the solid resulting in a final

fry-dried sludge. As a result, the final product can be seen as a fuel, where its lower heating value

(LHV) depends on the oil and moisture content achieved during the process.



2.2.1 Oil and moisture content of the fry-dried sludge

The sample of partially fry-dried sludge, at a fry-drying time t=i, will have a mass itsamplem =

including the mass of the moisture itW m = and that of the total dry solids it

TSm= . The latter is the

sum of the mass of the initial indigenous total dry solids 0tTSm= and that of the impregnated RCO

itRCOm= as schematized in Figure 2-2. In order to determine the degree of drying, the moisture

content of the sample must be referenced solely to the indigenous total dry solids of the

dewatered sludge. Consequently, it is necessary to differentiate the mass corresponding to each

fraction of the total dry solids in the sample by determining itRCOm= .

Figure 2-2 Schematic representation of the mass composition change from dewatered to fry-dried sludge.

Unfortunately, some popular methods for oil content determination in fried foods, such as

solvent extraction [22] and differential scanning calorimetry [55] were not suitable when applied

to the fry-dried sludge. Hence, itRCOm= is calculated by assuming that the indigenous total solids

are insignificantly soluble in the frying oil and are not volatilized at the fry-drying temperatures.

Thus, from the mass balance in the product,



0tTS

itW

itsample

itRCO mmmm

==== −−= ( 2-6 )

Consequently, moisture content itWξ = is defined by reference to the indigenous total solids

0tTSm= is given by:

0tTS

itTS

itsample

0tTS

0tTS

itRCO

itsample

0tTS

itWit

W m

mm

m

)m(mm

mm

ξ =

==

=

===

=

== −=

+−== ( 2-7 )

In the following, total dry solids coming from the dewatered sludge will be qualified of

“indigenous”, and their mass will be symbolized by 0tTSm= .

For the fry-dried sludge, total solids content TSχ is the mass ratio between the total solids

TSm and the partially dried sludge samplem ,

sample

TSTS m

mχ = ( 2-8 )

Oil content can be expressed on wet or dry basis, given by ( 2-9 ) and ( 2-10 ) respectively:

itsample

itRCO

RCO mm

χ=

=

= ( 2-9 )

itTS

itRCO

RCO mm

δ ==

= ( 2-10 )

or by reference to the indigenous total solids 0tTSm= , as follows:

0tTS

itF

RCO mm

=

=

=ξ ( 2-11 )



2.2.2 Lower heating value of the fry-dried sludge

For any dry material composed of inert matter and organic matter, having a generic molecular

formula of CnHmOpNq, the complete combustion reaction can be written as follows,

inertN2qOH

2mnCOO

2p

4mninertNOHC 2222qpmn +++→⎟

⎠⎞

⎜⎝⎛ −+++ ( 2-12 )

The lower heating value of a dry fuel LHV is the heat released from a complete combustion

reaction considering the water produced as vapor [56]. Since partially dried sludge is a mixture of

organic matter, inert matter and moisture, its lower calorific value sampleLHV varies as:

)χ(1∆HχLHVLHV TSvapTS0t

TSdryingair

sample −⋅−⋅==− ( 2-13 )

Where 0tTSLHV= is the lower calorific value of the indigenous dry matter and vap∆H is the

latent heat of vaporization of water.

For fry dried sludge, sampleLHV must take into account the fraction of solids corresponding to

the impregnated oil in dry basis ( RCOδ ) and the lower heating value of the oil RCOLHV :

)χ(1∆Hχ]δLHV)δ(1[LHVLHV TSvaporTSRCORCORCO0t

TSdryingfry

sample −⋅−⋅⋅+−⋅==− ( 2-14 )

0t

TSLHV= and RCOLHV are calculated from their Higher Heating Values (i.e.

0tTSHHV

= and

RCOHHV ) according to the equation ( 2-15 ).

vaporHii ∆Hδ218HHVLHV ⋅⋅−= ( 2-15 )

Where iHHV is experimentally measured using a self-contained "oxygen bomb" calorimeter

(mod. C500, IKA Analysen Technik, Germany) following the method outlined in ASTM D3286.



The factor Hδ218 ⋅ , represents the mass fraction of water produced during combustion according

to the stoichiometric proportion to the hydrogen content of the samples on dry basis.

The hydrogen fraction ( Hδ ) for the dried sludge and for the RCO are given by their ultimate

analyses ( Table 2—1 and 2-3).

After substituting the physical data into ( 2-13 ) and ( 2-14 ), the sampleLHV may be calculated

for the conventionally air-dried and fry-dried sludges, following ( 2-16 ) and ( 2-17 ) respectively:

2.26-χ18.16 )kg(MJLHV TS-1drying-air

sample ⋅=⋅ ( 2-16 )

)χ2.26(1χδ36.43χ)δ(115.90)kg(MJLHV TSTSRCOTSRCO-1drying-fry

sample −−⋅⋅+⋅−⋅=⋅ ( 2-17 )

The values calculated from equations ( 2-16 ) and ( 2-17 ) are shown in Figure 2-3. The

consequence of oil impregnation on the final sampleLHV is clearly revealing on this graph.

Figure 2-3 Comparison of the sewage sludge LHVsample obtained for fry-dried and air dried sludge.



2.3 Exhaust gases

This fry-drying by-product is mainly composed of water vapor but also contains some minor

quantities of volatile organic compound (VOC’s) belonging initially to the sewage sludge [8, 37,

38] and the oil. The presence of VOC can create an odor nuisance [57-60] or even, as it could be

for some direct dryers, a real risk of explosion [61]. These are probably the most important

complaints from the local population about WWTP facilities [8]. Due to the thermolysis and

hydrolysis reactions taking place during the frying [40], the exhaust gases are expected to have a

higher VOC content for the fry-drying process than for the conventional dryers. However it is

not possible to define a priori a constant final composition because it strongly depends on the

operating conditions [40].

Analysis of Heat and Mass Transfer during Fry-drying of Sewage Sludge


3 ANALYSIS OF HEAT AND MASS TRANSFER DURING FRY-DRYING OF SEWAGE SLUDGE

This chapter devotes to present and analyze the experimental results when applying deep-fat

frying to the thermal drying of sewage sludge using RCO as frying oil. One focus is to provide

qualitative descriptions of the mechanisms involved; another is to quantify the effect of the main

operational conditions on the fry drying kinetics. The aim is to emphasize the similarities

between fry-drying of sewage sludge and food cooking applications and also offer a basis to

further developments such as scale-up and modeling of the fry-drying process.

3.1 Experimental methods

3.1.1 Determination of fry-drying curves

Most frying heat and mass transfer studies use a discontinuous method to construct the fry-

drying curve [26, 27, 54, 62, 63]. This requires interrupting frying experiments at various times

and determining the moisture content of the sample by a destructive test. The main advantage of

this method is that a moisture-time curve can be directly constructed. However, this method is

time consuming and tedious, so that when the method is used, the drying curve is often built up

from very few experimental points [64]. Moreover, special attention is required for sample

quenching after its removal from the frying bath, especially early in the drying process when the

moisture content is high and water evaporation rate is intense. This practical difficulty in

experiments leads to overestimation of the actual drying rate curve, particularly in the initial

drying phase [26, 27, 54, 62, 63].

The alternative method, which is performed by an on-line weighing of the system formed by

the fryer, the oil bath and the sample itself, provides a dynamic measure of the moisture loss due

to water evaporation. Although this is rarely used in food frying studies, it was considered

Analysis of heat and mass transfer during fry-drying of sewage sludge


preferable for this work, because of the high initial moisture content of the sludge and the

problems related to its storage and aging. Moreover, the continuous measurement produces

larger number of experimental points to construct the moisture-time curve.

One natural difficulty of this method is the random noise in the measurement due to the

vibrations of the sample [64]. However, for the fry-drying tests, the disturbances that may be

caused by the stirring and/or the water boiling throughout the test are slight and overwhelmed

within the accuracy of the measurements (i.e. ± 0.2 g). Nevertheless, the determination of the

sludge drying curve from the mass loss of the fryer, with the sample immersed in the oil bath, is

not as straightforward as it may be for air-drying studies for instance. The major difficulty

concerns the mass balance between the recorded weight loss of the system and the actual water

losses observed in the product. For most of the drying studies using the continuous weighing,

the mass loss of the sample is directly recorded and assumed to represent the water loss of the

product [64]. Nevertheless, for the very few fry-drying works where this was applied, it was

observed that some water did not leave the frying bath as vapor and remained somewhere in the

fryer, probably as liquid water [65]. As a consequence, in order to construct the actual fry-drying

curve, the monitored mass loss must be adjusted with the liquid water losses. The implemented

strategy to compute the continuous fry-drying curve from the mass loss of the fryer consists in

adjusting this curve to the reference drying curve provided by the discontinuous method.

3.1.1.1 Experimental setup

The heart of the experimental setup is a Model Pro 500 household deep fat fryer (Magimix,

Vincennes, France) with a maximum capacity of 5 L. The fryer is heated with a 2000W electrical

resistance element, located near the bottom of the tank. The original thermostat control is

inadequate for experimental purposes and was replaced with a PID controller (Chromalox ®,

Etrex SA, France).

Analysis of heat and mass transfer during fry-drying of sewage sludge


Figure 3-1 Detail of the experimental setup. A constant speed stirrer (model: RW20DZM, IKA-Werke GmbH & Co., Germany)

operating

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