-
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)
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Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 1
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
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)
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Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 3
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
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Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 5
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!!
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Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 6
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.
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Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 7
Traitement des boues par friture : Des mécanismes physiques à
l’éco-conception d’un procédé
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
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Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 9
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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Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 11
ECOLES DOCTORALES SIGLE ECOLE DOCTORALE NOM ET COORDONNEES DU
RESPONSABLE
CHIMIE DE LYON Responsable : M. Denis SINOU
M. Denis SINOU Université Claude Bernard Lyon 1 Lab Synthèse
Asymétrique UMR UCB/CNRS 5622 Bât 308 2ème étage 43 bd du 11
novembre 1918 69622 VILLEURBANNE Cedex Tél : 04.72.44.81.83 Fax :
04 78 89 89 14 [email protected]
E2MC
ECONOMIE, ESPACE ET MODELISATION DES COMPORTEMENTS Responsable :
M. Alain BONNAFOUS
M. Alain BONNAFOUS Université Lyon 2 14 avenue Berthelot MRASH
M. Alain BONNAFOUS Laboratoire d’Economie des Transports 69363 LYON
Cedex 07 Tél : 04.78.69.72.76 Alain.bonnafous∂ish-lyon.cnrs.fr
E.E.A.
ELECTRONIQUE, ELECTROTECHNIQUE, AUTOMATIQUE M. Daniel
BARBIER
M. Daniel BARBIER INSA DE LYON Laboratoire Physique de la
Matière Bâtiment Blaise Pascal 69621 VILLEURBANNE Cedex Tél :
04.72.43.64.43 Fax 04 72 43 60 82 [email protected]
E2M2
EVOLUTION, ECOSYSTEME, MICROBIOLOGIE, MODELISATION
http://biomserv.univ-lyon1.fr/E2M2 M. Jean-Pierre FLANDROIS
M. Jean-Pierre FLANDROIS UMR 5558 Biométrie et Biologie
Evolutive Equipe Dynamique des Populations Bactériennes Faculté de
Médecine Lyon-Sud Laboratoire de Bactériologie BP 1269600 OULLINS
Tél : 04.78.86.31.50 Fax 04 72 43 13 88
E2m2∂biomserv.univ-lyon1.fr
EDIIS
INFORMATIQUE ET INFORMATION POUR LA SOCIETE
http://www.insa-lyon.fr/ediis M. Lionel BRUNIE
M. Lionel BRUNIE INSA DE LYON EDIIS Bâtiment Blaise Pascal 69621
VILLEURBANNE Cedex Tél : 04.72.43.60.55 Fax 04 72 43 60 71
[email protected]
EDISS
INTERDISCIPLINAIRE SCIENCES-SANTE http://www.ibcp.fr/ediss M.
Alain Jean COZZONE
M. Alain Jean COZZONE IBCP (UCBL1) 7 passage du Vercors 69367
LYON Cedex 07 Tél : 04.72.72.26.75 Fax : 04 72 72 26 01
[email protected]
MATERIAUX DE LYON http://www.ec-lyon.fr/sites/edml M. Jacques
JOSEPH
M. Jacques JOSEPH Ecole Centrale de Lyon Bât F7 Lab. Sciences et
Techniques des Matériaux et des Surfaces 36 Avenue Guy de Collongue
BP 163 69131 ECULLY Cedex Tél : 04.72.18.62.51 Fax 04 72 18 60 90
[email protected]
Math IF
MATHEMATIQUES ET INFORMATIQUE FONDAMENTALE
http://www.ens-lyon.fr/MathIS M. Franck WAGNER
M. Franck WAGNER Université Claude Bernard Lyon1 Institut Girard
Desargues UMR 5028 MATHEMATIQUES Bâtiment Doyen Jean Braconnier
Bureau 101 Bis, 1er étage 69622 VILLEURBANNE Cedex Tél :
04.72.43.27.86 Fax : 04 72 43 16 87
[email protected]
MEGA
MECANIQUE, ENERGETIQUE, GENIE CIVIL, ACOUSTIQUE
http://www.lmfa.ec-lyon.fr/autres/MEGA/index.html M. François
SIDOROFF
M. François SIDOROFF Ecole Centrale de Lyon Lab. Tribologie et
Dynamique des Systêmes Bât G8 36 avenue Guy de Collongue BP 163
69131 ECULLY Cedex Tél :04.72.18.62.14 Fax : 04 72 18 65 37
[email protected]
mailto:[email protected]:[email protected]://biomserv.univ-lyon1.fr/E2M2http://www.insa-lyon.fr/ediismailto:[email protected]://www.ibcp.fr/edissmailto:[email protected]://www.ec-lyon.fr/sites/edmlmailto:[email protected]://www.ens-lyon.fr/MathISmailto:[email protected]://www.lmfa.ec-lyon.fr/autres/MEGA/index.htmlhttp://www.lmfa.ec-lyon.fr/autres/MEGA/index.htmlmailto:[email protected]
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Fry-drying of sewage sludge : From the physical mechanisms to
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INSTITUT NATIONAL DES SCIENCES APPLIQUEES DE LYON Directeur :
STORCK.A Professeurs :
AUDISIO S. PHYSICOCHIMIE INDUSTRIELLE BABOT D. CONT. NON DESTR.
PAR RAYONNEMENT
IONISANTS BABOUX J.C. GEMPPM*** BALLAND B. PHYSIQUE DE LA
MATIERE BAPTISTE P. PRODUCTIQUE ET INFORMATIQUE DES
SYSTEMES MANUFACTURIERS BARBIER D. PHYSIQUE DE LA MATIERE
BASTIDE J.P. LAEPSI**** BAYADA G. MODELISATION MATHEMATIQUE ET
CALCUL SCIENTIFIQUE BENADDA B. LAEPSI**** BETEMPS M. AUTOMATIQUE
INDUSTRIELLE BIENNIER F. PRODUCTIQUE ET INFORMATIQUE DES
SYSTEMES MANUFACTURIERS BLANCHARD J.M. LAEPSI**** BOISSON C.
VIBRATIONS-ACOUSTIQUE BOIVIN M. (Prof. émérite) MECANIQUE DES
SOLIDES BOTTA H. UNITE DE RECHERCHE EN GENIE CIVIL -
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 -
Thermique du bâtiment BREMOND G. PHYSIQUE DE LA MATIERE BRISSAUD
M. GENIE ELECTRIQUE ET FERROELECTRICITEBRUNET M. MECANIQUE DES
SOLIDES BRUNIE L. INGENIERIE DES SYSTEMES
D’INFORMATION BUREAU J.C. CEGELY* CAVAILLE J.Y. GEMPPM*** CHANTE
J.P. CEGELY*- Composants de puissance et applications CHOCAT B.
UNITE DE RECHERCHE EN GENIE CIVIL -
Hydrologie urbaine COMBESCURE A. MECANIQUE DES CONTACTS COUSIN
M. UNITE DE RECHERCHE EN GENIE CIVIL -
Structures DAUMAS F. (Mme) CETHIL – Energétique et Thermique
DOUTHEAU A. CHIMIE ORGANIQUE DUFOUR R. MECANIQUE DES STRUCTURES
DUPUY J.C. PHYSIQUE DE LA MATIERE EMPTOZ H. RECONNAISSANCE DES
FORMES ET VISION ESNOUF C. GEMPPM***
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Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 13
EYRAUD L. (Prof. émérite) GENIE ELECTRIQUE ET
FERROELECTRICITEFANTOZZI G. GEMPPM*** FAVREL J. PRODUCTIQUE ET
INFORMATIQUE DES
SYSTEMES MANUFACTURIERS FAYARD J.M. BIOLOGIE APPLIQUEE FAYET M.
MECANIQUE DES SOLIDES FERRARIS-BESSO G. MECANIQUE DES STRUCTURES
FLAMAND L. MECANIQUE DES CONTACTS FLORY A. INGENIERIE DES
SYSTEMES
D’INFORMATION FOUGERES R. GEMPPM*** FOUQUET F. GEMPPM*** FRECON
L. INFORMATIQUE GERARD J.F. MATERIAUX MACROMOLECULAIRES GERMAIN P.
LAEPSI**** GIMENEZ G. CREATIS** GOBIN P.F. (Prof. émérite)
GEMPPM*** GONNARD P. GENIE ELECTRIQUE ET FERROELECTRICITEGONTRAND
M. CEGELY*- Composants de puissance et applications GOUTTE R.
(Prof. émérite) CREATIS** GOUJON L. GEMPPM*** GOURDON R.
LAEPSI****. GRANGE G. GENIE ELECTRIQUE ET FERROELECTRICITEGUENIN G.
GEMPPM*** GUICHARDANT M. BIOCHIMIE ET PHARMACOLOGIE GUILLOT G.
PHYSIQUE DE LA MATIERE GUINET A. PRODUCTIQUE ET INFORMATIQUE
DES
SYSTEMES MANUFACTURIERS GUYADER J.L. VIBRATIONS-ACOUSTIQUE
GUYOMAR D. GENIE ELECTRIQUE ET FERROELECTRICITEHEIBIG A. LAB.
MATHEMATIQUE APPLIQUEES LYON JACQUET RICHARDET G. MECANIQUE DES
STRUCTURES JAYET Y. GEMPPM*** JOLION J.M. RECONNAISSANCE DES FORMES
ET VISION JULLIEN J.F. UNITE DE RECHERCHE EN GENIE CIVIL -
Structures JUTARD A. (Prof. émérite) AUTOMATIQUE INDUSTRIELLE
KASTNER R. UNITE DE RECHERCHE EN GENIE CIVIL -
Géotechnique KOULOUMDJIAN J. INGENIERIE DES SYSTEMES
D’INFORMATION LAGARDE M. BIOCHIMIE ET PHARMACOLOGIE LALANNE M.
(Prof. émérite) MECANIQUE DES STRUCTURES LALLEMAND A. CENTRE DE
THERMIQUE DE LYON -
Energétique et thermique LALLEMAND M. (Mme) CENTRE DE THERMIQUE
DE LYON -
Energétique et thermique LAREAL P. UNITE DE RECHERCHE EN GENIE
CIVIL -
Géotechnique LAUGIER A. PHYSIQUE DE LA MATIERE LAUGIER C.
BIOCHIMIE ET PHARMACOLOGIE
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Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 14
LEJEUNE P. GENETIQUE MOLECULAIRE DES MICROORGANISMES
LUBRECHT A. MECANIQUE DES CONTACTS MAZILLE H. PHYSICOCHIMIE
INDUSTRIELLE MERLE P. GEMPPM*** MERLIN J. GEMPPM*** MIGNOTTE A.
(Mle) INGENIERIE, INFORMATIQUE
INDUSTRIELLE MILLET J.P. PHYSICOCHIMIE INDUSTRIELLE MIRAMOND M.
UNITE DE RECHERCHE EN GENIE CIVIL -
Hydrologie urbaine MOREL R. MECANIQUE DES FLUIDES MOSZKOWICZ P.
LAEPSI**** MOURA A. GEMPPM*** NARDON P. (Prof. émérite) BIOLOGIE
APPLIQUEE NIEL E. AUTOMATIQUE INDUSTRIELLE NORTIER P. DREP ODET C.
CREATIS** OTTERBEIN M. (Prof. émérite) LAEPSI**** PARIZET E.
VIBRATIONS-ACOUSTIQUE PASCAULT J.P. MATERIAUX MACROMOLECULAIRES
PAVIC G. VIBRATIONS-ACOUSTIQUE PELLETIER J.M. GEMPPM*** PERA J.
UNITE DE RECHERCHE EN GENIE CIVIL -
Matériaux PERRIAT P. GEMPPM*** PERRIN J. ESCHIL – Equipe
Sciences Humaines de l’Insa de
Lyon PINARD P. (Prof. émérite) PHYSIQUE DE LA MATIERE PINON J.M.
INGENIERIE DES SYSTEMES
D’INFORMATION PONCET A. PHYSIQUE DE LA MATIERE POUSIN J.
MODELISATION MATHEMATIQUE ET
CALCUL SCIENTIFIQUE PREVOT P. GRACIMP – Groupe de Recherche en
Apprentissage,
Coopération et Interfaces Multimodales pour la Productique
PROST R. CREATIS** RAYNAUD M. CENTRE DE THERMIQUE DE LYON -
Transferts
Interfaces et Matériaux REDARCE H. AUTOMATIQUE INDUSTRIELLE
REYNOUARD J.M. UNITE DE RECHERCHE EN GENIE CIVIL -
Structures RIGAL J.F. MECANIQUE DES SOLIDES RIEUTORD E. (Prof.
émérite) MECANIQUE DES FLUIDES ROBERT-BAUDOUY J. (Mme) (Prof.
émérite)
GENETIQUE MOLECULAIRE DES MICROORGANISMES
ROUBY D. GEMPPM*** ROUX J.J. CENTRE DE THERMIQUE DE LYON –
Thermique de l’Habitat
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Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 15
RUBEL P. INGENIERIE DES SYSTEMES D’INFORMATION
RUMELHART C. MECANIQUE DES SOLIDES SACADURA J.F. CENTRE DE
THERMIQUE DE LYON - Transferts
Interfaces et Matériaux SAUTEREAU H. MATERIAUX MACROMOLECULAIRES
SCAVARDA S. AUTOMATIQUE INDUSTRIELLE SOUIFI A. PHYSIQUE DE LA
MATIERE SOUROUILLE J.L. INGENIERIE INFORMATIQUE
INDUSTRIELLETHOMASSET D. AUTOMATIQUE INDUSTRIELLE UBEDA S. CENTRE
D’INNOV. EN TELECOM ET
INTEGRATION DE SERVICES THUDEROZ C. ESCHIL – Equipe Sciences
Humaines de l’Insa de
Lyon UNTERREINER R. CREATIS** VELEX P. MECANIQUE DES CONTACTS
VIGIER G. GEMPPM*** VINCENT A. GEMPPM*** VRAY D. CREATIS**
VUILLERMOZ P.L. (Prof. émérite)
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***
Directeurs de recherche I.N.R.A. :
FEBVAY G. BIOLOGIE APPLIQUEE GRENIER S. BIOLOGIE APPLIQUEE RAHBE
Y. BIOLOGIE APPLIQUEE
Directeurs de recherche I.N.S.E.R.M. :
PRIGENT A.F. (Mme) BIOLOGIE ET PHARMACOLOGIE MAGNIN I. (Mme)
CREATIS**
* CEGELY CENTRE DE GENIE ELECTRIQUE DE LYON ** CREATIS CENTRE DE
RECHERCHE ET D’APPLICATIONS EN
TRAITEMENT DE L’IMAGE ET DU SIGNAL ***GEMPPM GROUPE D'ETUDE
METALLURGIE PHYSIQUE ET
PHYSIQUE DES MATERIAUX ****LAEPSI LABORATOIRE D’ANALYSE
ENVIRONNEMENTALE
DES PROCEDES ET SYSTEMES INDUSTRIELS
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Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 17
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
Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 21
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
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Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 22
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
Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 25
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
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List of Figures
Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 26
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
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List of tables
Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 27
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
-
Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 31
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;
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Introduction
Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 32
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
Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 33
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
Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 34
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.
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Introduction
Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 35
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
Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 36
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.
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Introduction
Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 37
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
Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 38
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
Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 39
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
-
Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 43
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
Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 44
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
-
Materials and preliminary definitions
Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 45
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].
-
Materials and preliminary definitions
Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 46
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
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Materials and preliminary definitions
Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 47
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.
-
Materials and preliminary definitions
Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 48
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.
-
Materials and preliminary definitions
Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 49
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,
-
Materials and preliminary definitions
Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 50
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 )
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Materials and preliminary definitions
Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 51
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.
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Materials and preliminary definitions
Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 52
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.
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Materials and preliminary definitions
Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 53
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
-
Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 57
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
Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 58
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).
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Analysis of heat and mass transfer during fry-drying of sewage
sludge
Fry-drying of sewage sludge : From the physical mechanisms to
the process eco-design 59
Figure 3-1 Detail of the experimental setup. A constant speed
stirrer (model: RW20DZM, IKA-Werke GmbH & Co., Germany)
operating