ag onomy
Communica ion
A New Technique o Imp o ed Use o The mal
Ene gy om Was e E luen s
JoséManuel Magide-Ameijide 1,*, Hi am Va ela-Rod íguez 2,* and Adol o López-Fabal 1
1
Escuela Poli
é
cnica Supe io de Ingenie
í
a, Uni e sidad de San iago de Compos ela, Campus Uni e si a io,
27002-Lugo, Spain; [email p o ec ed]
2Facul ad de In o má ica, Uni e sidad de La Co uña, 15001 A Co uña, Spain
*Co espondence: [email p o ec ed] (J.M.M.-A.); [email p o ec ed] (H.V.-R.);
Tel.: +34-600-940-052 (J.M.M.-A.)
Recei ed: 3 Decembe 2019; Accep ed: 7 Janua y 2020; Published: 9 Janua y 2020


Abs ac :
Ene gy sus ainabili y and en i onmen al p o ec ion in gene al a e a he hea o enginee ing
and indus y discussions. Coun less e o s ha e been de o ed o imp o ing he ene gy e iciency o
indus ial p ocesses and speci ically o ha nessing hei was e ene gy sou ces. One such sou ce is
was e om ag o-indus ial p ocesses, which is equen ly cha ac e ized by inc eased empe a u es
and high pollu ing po en ial. The e a e mul iple a ailable choices o exploi ing ene gy om such
was e, bu his pape p oposes a new al e na i e echnique ha subs an ially imp o es he e iciency.
Based on he echnology o le e aging a ho liquid e luen o hea ing a p ocess luid, his sys em
in oduces a hi d liquid o be e alo ized by d ying ha is placed in be ween he ho and cold liquids.
By adding s i e s inside he hea exchange , he he mal esis ance o he hi d luid is educed o a
negligible le el. Thus, his sys em has almos he same ad an ages as he p e ious one, bu wi h he
added bene i ha i allows d ying o a hi d luid. One o he speci ic applica ions o his p oposed
echnology is using hea om was e e luen s o ob ain d ied ood p oduc s. In he p esen wo k,
i was used o d y slaugh e house blood o ob ain so-called “blood meal”, a p oduc wi h a high added
alue ha is used as pe ood o o ganic e ilize , and also has many o he indus ial applica ions.
As shown he e, he new echnique ou pe o ms exis ing al e na i es in e ms o ene gy e iciency and
economic p o i abili y.
Keywo ds:
esidual ene gy; alo iza ion; d ying; acuum; mul iple-e ec e apo a ion; blood; blood
meal; slaugh e house
1. In oduc ion
Ene gy e iciency o indus ial p ocesses is a majo issue due o i s economic and en i onmen al
implica ions. Ene gy e iciency can be achie ed h ough he use o ene gy om esidual e luen s,
such as hose o ag oindus y, which a e o en cha ac e ized by high empe a u es. Ene gy con ained in
was e e luen s can be ex ac ed wi h a numbe o echniques o he chemical o he mal ype. Chemical
echniques use e luen s o ex ac subs ances wi h a high alue such as uel gas (biogas, biosolids,
syngas, and bio-oil), whe eas he mal echniques in ol e hea ans e om e luen s o a cold luid.
The o me is mo e commonly used wi h highly di y, sludge- ype e luen s, whe eas he la e a e
p e e en ially applied o cleane , mo e dilu e e luen s bu can also be used wi h sludge [1].
The mal echniques o ene gy ex ac ion di e depending on he empe a u e o he e luen o
be p ocessed. We deemed empe a u es below 80
◦
C o be low and empe a u es abo e his le el o be
high. Tempe a u es a ound his alue s ongly in luence he design o he mal eco e y equipmen [
2
].
Mos eco e y echniques, howe e , ha e been designed o ope a ion a high empe a u es.
Ag onomy 2020,10, 97; doi:10.3390/ag onomy10010097 www.mdpi.com/jou nal/ag onomy
Ag onomy 2020,10, 97 2 o 14
E luen s a empe a u es highe han 80
◦
C a e e y easy o alo ize and easily ha e an indus ial
applica ion [2]. One immedia e use o e luen s is as hea ing luids o condi ioning o dwellings and
he mal com o . An u ban applica ion is he mal eco e y om ho household was ewa e [
3
], using
hea exchange s o deli e eco e ed hea in he o m o hea ing o households. One o he highly
in e es ing use is o hea ing animal a ms, such as pig a ms, which can bene i om adian loo
hea ing p o ided by ag a ian was e e luen s [4].
One o he use o e luen s is o gene a ing elec ici y by means o de ices based on o ganic
Rankine o Kalina cycles, in which he al e na i e e apo a ion and condensa ion o luids esul in he
mo emen o a mechanical sha connec ed o an elec ic gene a o . Rankine and Kalina cycles p o ide
ene gy as elec ical powe wi hou eleasing oxic gases, albei a he expense o e y low yields and
high ope a ing empe a u es [5]. These sho comings make hem useless in many cases.
One widesp ead choice o he e icien exploi a ion o he mal ene gy om was e e luen s is
hea ing cold luids p io o en e ing an indus ial p ocess. Unlike he Rankine and Kalina cycles,
his app oach allows ene gy o be eco e ed om e luen s a empe a u es well below 70
◦
C [
5
]. P ocess
luids a e usually ed a oom empe a u e o sligh ly low (10–25
◦
C) [
6
] and equi e la ge amoun s
o ene gy o be hea ed. I can be ob ained ee om ho was e e luen s. This is an ad an ageous
al e na i e o he Rankine cycle, e en when luids a e o be hea ed by a ew deg ees only.
Hea -pump echnology allows e luen s a empe a u es as low as 15
◦
C o be highly e icien ly
used and has been deemed he bes choice o p ocessing cooled e luen s [
7
]. A hea pump consis s
o a closed ci cui whe e a cooling luid con inuously e apo a es and condenses a a pai o hea
exchange s [
2
]. I uses a comp esso and ope a es like a e ige a o , albei in he opposi e di ec ion.
Hea pumps a e he mos e icien machines; a pump ed wi h was ewa e a abou 15
◦
C can easily
p o ide a coe icien o pe o mance (COP) exceeding 3.5 (i.e., he pump will p o ide 3.5 uni s o
ene gy o each uni i ecei es). A hea pump uses ene gy mainly o ope a e he comp esso . Thus,
a comp esso using 10 kW o elec ic powe will allow a pump o deli e 35 kW in he o m o hea .
The empe a u e o household was ewa e ypically anges om 10 o 25
◦
C, depending on he ime
o yea [
6
]. Because his ype o was e is p oduced massi ely and expec ed o inc ease in he u u e,
i p o ides a majo ene gy niche ha could be highly e icien ly exploi ed wi h hea pumps.
A need clea ly exis s o u he de elopmen o echniques enabling he e icien ex ac ion
o ene gy om was e e luen s—pa icula ly low- empe a u e e luen s, which accoun o 66% o
all indus ial e luen s p oduced on a global scale [
7
]. The amoun o he mal ene gy eleased in
was ewa e by a ci y o a e age size could mee abou 7% o i s ene gy demand in he o m o hea ing
and ho wa e [
3
]. A numbe o esea che s ha e concluded ha he ypical imp o emen schemes o
was ewa e ea men plan s a e la gely ine icien and should be supplemen ed wi h be e exploi a ion
o he mal ene gy [
8
]. In ac , a ypical ea men plan can easily p oduce 35 m
3
o biogas pe inhabi an
pe day [
9
] bu can only ex ac 10%–14% o he chemical ene gy con ained in he e luen s [
8
]. Unlike
he mal eco e y echnology (pa icula ly ha o hea pumps), was ewa e ea men plan s ha dly
achie e ene gy su pluses.
In his wo k, a no el echnique o imp o ing he e iciency wi h which ene gy can be ex ac ed
om was e e luen s was de eloped. Ope a ionally, i in ol es ans e ing ene gy om was e e luen s
o p ocess luids o be p ehea ed; howe e , one o mo e acuum chambe s a e placed be ween he
ho and cold spo s, o boil a luid o be e alo ized by dehyd a ion. This gene a es a wo old bene i ,
as i allows a p ocess luid o be p ehea ed and a hi d luid o aw ma e ial, such as milk, b ine, juice,
o blood, o be d ied. This new echnique can be o especial in e es o ag i- ood indus ies, which
usually handle liquid p oduc s o be e alo ized by d ying, ei he by emo ing mos o he mois u e
hey con ain o only pa o i , o educe i s luidi y and ob ain a concen a e [
10
]. One po en ial use o
g ea economic in e es is d ying slaugh e house blood o ob ain blood meal, a p oduc wi h a mois u e
con en o 8%–12% and a high ma ke alue. Blood meal has a numbe o indus ial uses [
11
], including
he p oduc ion o pe ood, o ganic e ilize s, gas-p oo il e s, asphal ic emulsions, cosme ics, accines,
bake y p oduc s, chemicals, and pha maceu icals.
Ag onomy 2020,10, 97 3 o 14
2. Ma e ial and Me hods
2.1. Layou o he P oposed Technique: Hea ing o a P ocessed Fluid by Means o a Was e E luen and an
In e posed Thi d Fluid
The p oposed echnique is based on exis ing sys ems o using ene gy ex ac ed om was e
e luen s (such as cleaning was ewa e o sewage wa e ) o p ehea indus ial p ocess luids (as boile
eed wa e o supply wa e used o cleaning). As can be seen in Figu e 1, he p ocess luid and he
was e e luen a e isola ed by a me al pla e (1), ac oss which hea is ans e ed om he o me o he
la e . Hea a els h ough h ee he mal esis ances, namely (3) he bounda y laye o he ho luid
(3), he hickness o he me al pla e (1), and he bounda y laye o he cold luid (6). The bounda y
laye s a e hin luid ilms ha s ick i ually s a ically o he me al pla e by e ec o iscosi y [
12
]. Hea
ans e ac oss he bounda ies is much mo e di icul han in he emainde o he luid, so he hicke a
bounda y laye is, he highe i s esis ance o hea ans e will be [13].
Ag onomy 2020, 10, 97 3 o 14
2. Ma e ial and Me hods
2.1. Layou o he P oposed Technique: Hea ing o a P ocessed Fluid by Means o a Was e E luen and an
In e posed Thi d Fluid
The p oposed echnique is based on exis ing sys ems o using ene gy ex ac ed om was e
e luen s (such as cleaning was ewa e o sewage wa e ) o p ehea indus ial p ocess luids (as boile
eed wa e o supply wa e used o cleaning). As can be seen in Figu e 1, he p ocess luid and he
was e e luen a e isola ed by a me al pla e (1), ac oss which hea is ans e ed om he o me o
he la e . Hea a els h ough h ee he mal esis ances, namely (3) he bounda y laye o he ho
luid (3), he hickness o he me al pla e (1), and he bounda y laye o he cold luid (6). The
bounda y laye s a e hin luid ilms ha s ick i ually s a ically o he me al pla e by e ec o
iscosi y [12]. Hea ans e ac oss he bounda ies is much mo e di icul han in he emainde o he
luid, so he hicke a bounda y laye is, he highe i s esis ance o hea ans e will be [13].
Figu e 1. Hea exchange by use o 3 he mal esis ances. (1) Me al pla e. (3) Ho - luid bounda y laye .
(6) Cold- luid bounda y laye . The bounda y laye s a e depic ed as dashed lines o exagge a ed
hickness o cla i y.
The main no el y is placing a liquid p oduc ( hi d luid) o be d ied in be ween he was e
e luen (ho spo ) and he luid o be p ehea ed (cold spo ). The luid is d ied unde acuum-boiling,
and wo hea exchange s a e used o his pu pose (Figu e 2). In his way, he hi d luid is e apo a ed
in a boiling chambe unde acuum (7) and condensed on he walls o he second exchange , o ob ain
a condensa e laye (5). This sys em p esen s 6 se ially a anged he mal esis ances a he han 3 o
he p e ious one. Such esis ances a e as ollows: he bounda y laye o he was e e luen (3), he
hickness o he me al pla e (1) in he i s exchange , he bounda y laye o he hi d luid (4) in he
i s exchange , he apo condensa e laye (5) in he second exchange , he hickness o he me al
pla e (1) in he second exchange , and he bounda y laye o he cold luid (6) passing h ough i .
The d awback o p esen ing so many esis ances can be a oided by s i ing in bo h exchange s.
The s i e s o be used should sc ub he me al pla es (1) in o de o mechanically b eak he bounda y
laye s o he was e e luen in he i s exchange (4) and he condensa e laye in he second (5). Hea
ans e in he con inuously b oken bounda y laye s is go e ned by conduc ion in a no-s eady s a e.
This esul s in e y high hea - ans e coe icien s and hence in negligible he mal esis ance. Figu e
3 shows a h ee-dimensional iew o a hea exchange wi h s i e s. Fo ope a ional easons, s i e s
a e only placed in he bulk o he hi d luid; howe e , hey migh also be placed in he was e e luen
and p ocess luid.
Figu e 1.
Hea exchange by use o 3 he mal esis ances. (1) Me al pla e. (3) Ho - luid bounda y
laye . (6) Cold- luid bounda y laye . The bounda y laye s a e depic ed as dashed lines o exagge a ed
hickness o cla i y.
The main no el y is placing a liquid p oduc ( hi d luid) o be d ied in be ween he was e e luen
(ho spo ) and he luid o be p ehea ed (cold spo ). The luid is d ied unde acuum-boiling, and wo
hea exchange s a e used o his pu pose (Figu e 2). In his way, he hi d luid is e apo a ed in a
boiling chambe unde acuum (7) and condensed on he walls o he second exchange , o ob ain
a condensa e laye (5). This sys em p esen s 6 se ially a anged he mal esis ances a he han 3
o he p e ious one. Such esis ances a e as ollows: he bounda y laye o he was e e luen (3),
he hickness o he me al pla e (1) in he i s exchange , he bounda y laye o he hi d luid (4) in he
i s exchange , he apo condensa e laye (5) in he second exchange , he hickness o he me al pla e
(1) in he second exchange , and he bounda y laye o he cold luid (6) passing h ough i .
The d awback o p esen ing so many esis ances can be a oided by s i ing in bo h exchange s.
The s i e s o be used should sc ub he me al pla es (1) in o de o mechanically b eak he bounda y
laye s o he was e e luen in he i s exchange (4) and he condensa e laye in he second (5). Hea
ans e in he con inuously b oken bounda y laye s is go e ned by conduc ion in a no-s eady s a e.
This esul s in e y high hea - ans e coe icien s and hence in negligible he mal esis ance. Figu e 3
shows a h ee-dimensional iew o a hea exchange wi h s i e s. Fo ope a ional easons, s i e s a e
only placed in he bulk o he hi d luid; howe e , hey migh also be placed in he was e e luen and
p ocess luid.
Ag onomy 2020,10, 97 4 o 14
Ag onomy 2020, 10, 97 4 o 14
Figu e 2. Hea exchange wi h six he mal esis ances. (1) Me al pla es in he exchange s. (2) B idge.
(3) Was e e luen bounda y laye . (4) Thi d- luid bounda y laye in i s exchange . (5) Condensed
apo laye in second exchange . (6) Cold- luid bounda y laye . (7) E acua ed boiling chambe . The
bounda y laye s a e depic ed as dashed lines o exagge a ed hickness o cla i y.
Figu e 3. Pa ial c oss-sec ional iew o a hea exchange , including s i e s. (1) Me al pla es o
he mal exchange. (8) Ex e nal mo o . (9) Ro a y axle ac ua ing he s i e s. (10) Chassis. (11) S i e s.
B eaking he bounda y laye s allows he numbe o esis ances o be educed om 6 o 4, which
is close o he 3 o he sys em wi h no in e media e luid. Bo h sys ems include he esis ances o he
bounda y laye s o he ho (3) and cold luid (6), and he me al pla e o pla es (1)— wo o his sys em
and a single one o he p e ious app oach. Howe e , he esis ance o he luid bounda y laye s in
he mal- ans e p ocesses is usually much highe han ha o he me al pla es [10]. Consequen ly,
he mal esis ance is simila in bo h sys ems (wi h o wi hou a hi d luid).
The acuum in he boiling chambe (7) is main ained by e ec o apo condensa ion in he
second exchange and he aid o a acuum pump. No pump is needed in he ideal si ua ion because
condensing he gas p esen in an ai igh chambe educes he inne p essu e [13]. In indus ial
p ac ice, howe e , all boiling subs ances elease some noncondensing gas a he usual ope a ing
empe a u es [12]. The e o e, he condense mus be supplemen ed wi h a acuum pump, o
wi hd aw uncondensed gases [10]. One disad an age o using a acuum pump is ha i also
wi hd aws some wa e apo and esul s in a p opo ional loss o i s condensa ion ene gy in he
Figu e 2.
Hea exchange wi h six he mal esis ances. (1) Me al pla es in he exchange s. (2) B idge.
(3) Was e e luen bounda y laye . (4) Thi d- luid bounda y laye in i s exchange . (5) Condensed
apo laye in second exchange . (6) Cold- luid bounda y laye . (7) E acua ed boiling chambe .
The bounda y laye s a e depic ed as dashed lines o exagge a ed hickness o cla i y.
Ag onomy 2020, 10, 97 4 o 14
Figu e 2. Hea exchange wi h six he mal esis ances. (1) Me al pla es in he exchange s. (2) B idge.
(3) Was e e luen bounda y laye . (4) Thi d- luid bounda y laye in i s exchange . (5) Condensed
apo laye in second exchange . (6) Cold- luid bounda y laye . (7) E acua ed boiling chambe . The
bounda y laye s a e depic ed as dashed lines o exagge a ed hickness o cla i y.
Figu e 3. Pa ial c oss-sec ional iew o a hea exchange , including s i e s. (1) Me al pla es o
he mal exchange. (8) Ex e nal mo o . (9) Ro a y axle ac ua ing he s i e s. (10) Chassis. (11) S i e s.
B eaking he bounda y laye s allows he numbe o esis ances o be educed om 6 o 4, which
is close o he 3 o he sys em wi h no in e media e luid. Bo h sys ems include he esis ances o he
bounda y laye s o he ho (3) and cold luid (6), and he me al pla e o pla es (1)— wo o his sys em
and a single one o he p e ious app oach. Howe e , he esis ance o he luid bounda y laye s in
he mal- ans e p ocesses is usually much highe han ha o he me al pla es [10]. Consequen ly,
he mal esis ance is simila in bo h sys ems (wi h o wi hou a hi d luid).
The acuum in he boiling chambe (7) is main ained by e ec o apo condensa ion in he
second exchange and he aid o a acuum pump. No pump is needed in he ideal si ua ion because
condensing he gas p esen in an ai igh chambe educes he inne p essu e [13]. In indus ial
p ac ice, howe e , all boiling subs ances elease some noncondensing gas a he usual ope a ing
empe a u es [12]. The e o e, he condense mus be supplemen ed wi h a acuum pump, o
wi hd aw uncondensed gases [10]. One disad an age o using a acuum pump is ha i also
wi hd aws some wa e apo and esul s in a p opo ional loss o i s condensa ion ene gy in he
Figu e 3.
Pa ial c oss-sec ional iew o a hea exchange , including s i e s. (1) Me al pla es o he mal
exchange. (8) Ex e nal mo o . (9) Ro a y axle ac ua ing he s i e s. (10) Chassis. (11) S i e s.
B eaking he bounda y laye s allows he numbe o esis ances o be educed om 6 o 4, which is
close o he 3 o he sys em wi h no in e media e luid. Bo h sys ems include he esis ances o he
bounda y laye s o he ho (3) and cold luid (6), and he me al pla e o pla es (1)— wo o his sys em
and a single one o he p e ious app oach. Howe e , he esis ance o he luid bounda y laye s in
he mal- ans e p ocesses is usually much highe han ha o he me al pla es [
10
]. Consequen ly,
he mal esis ance is simila in bo h sys ems (wi h o wi hou a hi d luid).
The acuum in he boiling chambe (7) is main ained by e ec o apo condensa ion in he
second exchange and he aid o a acuum pump. No pump is needed in he ideal si ua ion because
condensing he gas p esen in an ai igh chambe educes he inne p essu e [
13
]. In indus ial
p ac ice, howe e , all boiling subs ances elease some noncondensing gas a he usual ope a ing
empe a u es [
12
]. The e o e, he condense mus be supplemen ed wi h a acuum pump, o wi hd aw
uncondensed gases [
10
]. One disad an age o using a acuum pump is ha i also wi hd aws some
wa e apo and esul s in a p opo ional loss o i s condensa ion ene gy in he second exchange [
14
].
Ag onomy 2020,10, 97 5 o 14
By iden i ying whe e uncondensed p oduc s accumula e in he boiling chambe (7) and placing he
pump suc ion he e, we can ypically educe he ene gy loss o 10% [10].
The si ua ion whe e he empe a u e o he ho luid is below he h eshold o e ec i e applica ion
o he p oposed echnique poses special p oblems. Howe e , luids can s ill be exploi ed by using
hea -pump echnology. Thus, a cooling luid is pumped h ough a closed ci cui , whe e i unde goes
successi e e apo a ion and condensa ion in o de o acili a e abso p ion o he mal ene gy om a
cool luid and elease a a poin whe e he empe a u e exceeds ha o he luid [12].
To imp o e i s e iciency, he p oposed sys em can be modi ied by spli ing he e apo a ion
chambe in o a numbe o se ially connec ed chambe s o o m a mul iple-e ec e apo a o [
10
].
This subs an ially inc eases ene gy sa ings because he ene gy needed o e apo a e a gi en amoun o
wa e is being educed in p opo ion o he numbe o se ial e apo a o s used [
14
]. Thus, using wo
mul i-e ec e apo a o s can educe e apo a ion cos s by one-hal , and using h ee by wo- hi ds [
10
].
In o de o enable he mal exchange be ween each e apo a o and he nex , hei boiling empe a u es
mus be made di e en by e acua ing hem o a di e en ex en . The boiling empe a u e a each
e apo a o in a ypical h ee-e apo a o sys em could be 55, 45, and 35
◦
C, espec i ely, he espec i e
p essu es also being ob iously di e en and dec easing wi h dec easing empe a u e [12].
Figu e 4shows a lowcha o he sys em, including wo exchange s and h ee se ially a anged
e apo a o s. Each e apo a o comp ises an in e nal chambe o condensa ion and an ex e nal chambe
o boiling he p oduc o be d ied o concen a ed. Each ci cui is depic ed in a di e en colo : ed
o he hi d luid o be d ied (slaugh e house blood); da k blue o he wa e apo eleased by
boiling blood; ligh blue o he cold supply wa e o be hea ed (cold spo ); b own o he ho was e
e luen ; g een o he hea pump (which is equi ed because he e luen can en e he sys em a as
low a empe a u e as 45
◦
C); and black o d ained condensa es and wi hd awn noncondensing gases.
The pump ecycles cooling luid R-134a, which is e apo a ed a 10
◦
C by abso bing ene gy om he
e luen in he i s hea exchange ; hen i s p essu e is aised by a comp esso o acili a e condensa ion
a 45 ◦C in he i s e apo a o .
Ag onomy 2020, 10, 97 5 o 14
second exchange [14]. By iden i ying whe e uncondensed p oduc s accumula e in he boiling
chambe (7) and placing he pump suc ion he e, we can ypically educe he ene gy loss o 10% [10].
The si ua ion whe e he empe a u e o he ho luid is below he h eshold o e ec i e
applica ion o he p oposed echnique poses special p oblems. Howe e , luids can s ill be exploi ed
by using hea -pump echnology. Thus, a cooling luid is pumped h ough a closed ci cui , whe e i
unde goes successi e e apo a ion and condensa ion in o de o acili a e abso p ion o he mal
ene gy om a cool luid and elease a a poin whe e he empe a u e exceeds ha o he luid [12].
To imp o e i s e iciency, he p oposed sys em can be modi ied by spli ing he e apo a ion
chambe in o a numbe o se ially connec ed chambe s o o m a mul iple-e ec e apo a o [10]. This
subs an ially inc eases ene gy sa ings because he ene gy needed o e apo a e a gi en amoun o
wa e is being educed in p opo ion o he numbe o se ial e apo a o s used [14]. Thus, using wo
mul i-e ec e apo a o s can educe e apo a ion cos s by one-hal , and using h ee by wo- hi ds [10].
In o de o enable he mal exchange be ween each e apo a o and he nex , hei boiling empe a u es
mus be made di e en by e acua ing hem o a di e en ex en . The boiling empe a u e a each
e apo a o in a ypical h ee-e apo a o sys em could be 55, 45, and 35 °C, espec i ely, he espec i e
p essu es also being ob iously di e en and dec easing wi h dec easing empe a u e [12].
Figu e 4 shows a lowcha o he sys em, including wo exchange s and h ee se ially a anged
e apo a o s. Each e apo a o comp ises an in e nal chambe o condensa ion and an ex e nal
chambe o boiling he p oduc o be d ied o concen a ed. Each ci cui is depic ed in a di e en
colo : ed o he hi d luid o be d ied (slaugh e house blood); da k blue o he wa e apo eleased
by boiling blood; ligh blue o he cold supply wa e o be hea ed (cold spo ); b own o he ho was e
e luen ; g een o he hea pump (which is equi ed because he e luen can en e he sys em a as
low a empe a u e as 45 °C); and black o d ained condensa es and wi hd awn noncondensing gases.
The pump ecycles cooling luid R-134a, which is e apo a ed a 10 °C by abso bing ene gy om he
e luen in he i s hea exchange ; hen i s p essu e is aised by a comp esso o acili a e
condensa ion a 45 °C in he i s e apo a o .
Figu e 4. Flowcha o a sys em o ex ac ing ene gy om was e e luen s including a hea pump and
h ee e apo a o s.
2.2. Desc ip ion o a P o o ype Appa a us
To es and alida e he p oposed echnology, a unc ional p o o ype was buil whose main
componen s we e a p op ie a y design cus om e ical pla e hea exchange ( e e ed o as i s
exchange ), a g a ime ic decan e , a condense ( e e ed o as second exchange ), and a acuum
Figu e 4.
Flowcha o a sys em o ex ac ing ene gy om was e e luen s including a hea pump and
h ee e apo a o s.
2.2. Desc ip ion o a P o o ype Appa a us
To es and alida e he p oposed echnology, a unc ional p o o ype was buil whose main
componen s we e a p op ie a y design cus om e ical pla e hea exchange ( e e ed o as i s

Ag onomy 2020,10, 97 6 o 14
exchange ), a g a ime ic decan e , a condense ( e e ed o as second exchange ), and a acuum pump.
The g a ime ic decan e is a de ice in e posed be ween he i s and second exchange ; i s pu pose
is so sepa a e gases om liquid pa icles. Inside he i s exchange , blood is boiling unde acuum
condi ions a a empe a u e no highe han 43
◦
C. This boiling cons i u es a dewa e ing p ocess ha
u ns liquid blood in o blood meal. The boiling gene a es wa e apo , which lea es he i s exchange ,
goes h ough he g a ime ic decan e , and inally eaches he second exchange , whe e i condensa es
in o liquid wa e . I he decan e we e supp essed, he apo would d ag li le blood pa icles upwa d
and would lead hem up o he condense . Tha would ha e a de as a ing e ec , since i means losing
p oduc . Blood mus emain inside he i s exchange in o de o be ans o med in o blood meal.
The e ical pla e hea exchange consis s o h ee cylind ical chambe s o nominal diame e
DN500 (508 mm ou e diame e and 495.2 mm inne diame e ), he me ically sepa a ed. The pla es
be ween chambe s (in o he wo ds, he hea -exchange su aces), a e 6 mm hick. The cen al chambe ,
which is 50 mm hick, is in ended o he luid o be dehyd a ed, which is manually ed. Bo h side
chambe s, each one 35 mm hick, ecei e he ho esidual e luen h ough a pipe o nominal diame e
DN15, which is pumped wi h a a iable low o 500–1600 L h
−1
, p o iding an in e nal p essu e in he
side chambe s o +0.6 ba . Inside he cen al chambe , he e is a 4-blade s i e ha eaches he ull
olume o he chambe o sc ub he walls. I is mo ed by a cen al axle d i en by a 0.75 kW ex e nal
mo o , which can o a e a a iable speed be ween 0 and 240 pm.
The one-s ep condense is a shell-and- ube ype, 2 m long, and e ically a anged. I consis s
o 9 ubes wi h in e nal ba les e e y 1/5 o diame e . In he gas ou le o he condense , he e is he
g a ime ic decan e .
The liquid ing acuum pump is equipped wi h a 4-kW engine, and i s nominal low is
120 m3h−1.
I is connec ed o he cen al chambe o he hea exchange , p o iding a acuum anging om
−
0.8 o
−0.94 ba s
2.3. Hea Exchange Pe o mance
The measu emen o hea - eco e y e iciency was made by calcula ing he o e all hea - ans e
coe icien in ela ion o he o a ion speed o he hea -exchange s i e , as i is he mos condi ioning
a iable due o he b eaking o he bounda y laye s. Twen y minu es o es ing was conduc ed, a ying
he o a ion speed be ween 0 and 100 pm, wi h h ee epe i ions pe condi ion.
The wo side chambe s o he hea exchange we e ed a low a e o 1500 L h
−1
o ho wa e .
The inpu empe a u e was 63–80
◦
C, and he ou pu empe a u e was 74–58
◦
C. In he acuum
chambe , 9 L o wa e was in oduced and subjec ed o a acuum o
−
0.91 ba . A he end o each es ,
he e apo a ed amoun was measu ed and he ans e ed hea (Q) was calcula ed, aking in o accoun
he la en hea o apo iza ion.
The o e all hea - ans e coe icien was calcula ed wi h Equa ion (1):
U=Q
A×∆Tml
(1)
whe e Uis he o e all hea - ans e coe icien (wi h uni s o W m
−2
K
−1
), Qis he ene gy eco e ed
pe uni ime, Ais he hea -exchange su ace (0.385 m
2
, aking in o accoun he wo exchange pla es o
he p o o ype), and ∆Tml is he loga i hmic mean empe a u e di e ence, acco ding o Equa ion (2):
∆Tml =
∆Tb−∆Ta
ln ∆Tb
∆Ta
(2)
whe e
∆
T
b
is he he mal gap be ween he ho was e e luen a he inle and he boiling blood, and
∆
T
a
is he he mal gap be ween he ho was e e luen a he ou le and he boiling blood.
Ano he exp ession (3) o he o e all hea - ans e coe icien was also employed, he one ha
akes in o accoun he local hea - ans e coe icien s. They a e named as h
3
,h
1
, and h
2
and ep esen a
Ag onomy 2020,10, 97 7 o 14
local hea - ans e coe icien h ough he me allic wall (h
3
) and h ough he bounda y laye s on bo h
i s sides (h
1
and h
2
). Uni s a e he same as hei o e all coun e pa . The exp ession based on he h
i
is
as ollows:
U=1
1
h1
+1
h2+1
h3
(3)
whe e he local coe icien h
2
canno be imp o ed a all, unless he me allic wall is hinned, and he
o he wo can be imp o ed ia highe low a es o highe o a ional speed. Fo he calcula ion o h
2
,
since i is due o he he mal conduc ion in a me allic su ace, he ollowing exp ession (4) is used:
h2=k
(4)
whe e kis he he mal conduc i i y (14 W m
−1
K
−1
o s ainless s eel), and is he wall hickness (in his
expe imen 6 mm).
2.4. D ying Blood in o Blood Meal
The p o o ype was es ed o d ying blood ( hi d luid), o ob ain blood meal in eal condi ions in
a slaugh e house nea Lugo ci y (NW Spain), whe e cows a e slaugh e ed daily (abou 25,000 heads a
yea ). Gu -washing equi es wa e o e 80
◦
C, so supply wa e is hea ed in a boile . The ho was e
e luen esul ing om his washing is disca ded di ec ly o a was ewa e ea men plan . Blood is
deli e ed o a was e manage who disposes o i , a a cos o he slaugh e house.
The p o o ype was ins alled in he acili y. Fo he expe imen s, blood was ob ained manually
om he slaugh e ed animals by using a ampi e kni e; he blood was collec ed in a d um and used
immedia ely, o p e en clo ing. I was manually ed in o he hea exchange , illing he o al olume
o 9 L o he boiling chambe . A acuum o
−
0.91 ba was applied, co esponding o a blood boiling
empe a u e o 43.4 ◦C.
The hea exchange was ed wi h washing was ewa e p e iously il e ed (ho was e e luen ).
I was used 550–800 L h
−1
o e luen a a a iable empe a u e be ween 60 and 80
◦
C. Supply
wa e o he washing (p ocess luid) was p ehea ed, which en e ed a 10–15
◦
C, wi h a low a e o
1500–2000 L h−1.
The a e age he mal gap be ween he ho e luen and he boiling empe a u e o he
blood was 31.6 ◦C.
The inne s i e o he hea exchange o a ed a 60 e olu ions pe minu e ( pm), o abou 75%
o he ime, while he emaining 25% had o be lowe ed o 15 pm, so as no o o e load he engine
when he liquid became sludge. The ea men was main ained un il he blood meal was ob ained,
when he sludge was disagg ega ed. The ime equi ed was a ound 1 h, depending on he wo king
condi ions. Finally, he amoun o blood meal ob ained was weighed and la e i s mois u e con en
was de e mined in labo a o y by o en d ying a 105 ◦C o a cons an weigh .
2.5. Economic Feasibili y
One key ac o ha de e mines economic easibili y is how much hea can be ex ac ed om he
ho was e e luen . In o he wo ds, he maximum empe a u e d ops a ainable wi h some echnology.
I he was e e luen comes wi h 80
◦
C, and echnology A is capable o cooling i down o 20
◦
C,
is ex ac ing mo e ene gy and being mo e p o i able han echnology B ha only dec eases empe a u e
o 40 ◦C.
The 40
◦
C example is easible when hea ing dwellings o p ehea ing p ocess luids, bu o achie e
u he cooling ( o example, 20
◦
C), a hea pump is usually needed. This is because he pump,
wo king wi h he R134a coolan , can sus ain a cons an boiling empe a u e o no mo e han 10
◦
C in i s
e apo a o , which makes hea ans e om a sou ce a 20
◦
C possible. On he o he hand, a dwelling
will hea up o empe a u es much highe han 10 ◦C.
Ag onomy 2020,10, 97 8 o 14
To calcula e he ene gy sa ings, an e luen a 80
◦
C was conside ed o be cooled o he minimum
usable by he echnology (20
◦
C i i includes hea pump, 40
◦
C i no ). I was assumed ha , in all cases,
90% o he ene gy ceded by wa e is eco e ed, which is alued as he equi alen amoun o diesel
hea ing oil. Fo he calcula ions, he ene gy densi y o he diesel uel was conside ed
32.93 MJ L−1,
and i s ma ke p ice 0.7
€
L
−1
. To know he amoun o blood meal ob ained pe on o was e e luen
unde some cooling ange, he a io o blood meal pe on o aw blood mus be aken in o conside a ion.
F om ou expe imen s, ha a io is 15%. The i s s ep is o assess he amoun o aw blood ha is
possible o d y wi h he ene gy ex ac ed om he hea was e e luen . Fo ha pu pose, he ex ac ed
ene gy is di ided by he blood la en hea o apo iza ion a
−
0.9 ba , 2.39 MJ kg
−1
. The esul ing
weigh is mul iplied by 15/85 o ob ain he mass o blood meal.
Fo he economic alo iza ion, i was supposed blood meal unde he assump ion o a ma ke
p ice o 400 € −1and a p oduc ion cos o 287 € −1wi h a single e apo a o and 115 € −1wi h h ee.
3. Resul s
3.1. Hea Exchange Pe o mance
The mos in e es ing p ope y o he hea exchange is he dependence o i s pe o mance wi h
espec o he deg ee o s i e mo emen , measu ed in pm. The pe o mance has he uni o W/(m
2·
K),
meaning he amoun o hea ha en e s he exchange pe second, squa e me e o exchange su ace,
and Kel in deg ee o he mal di e ence.
Figu e 5shows he expe imen al da a. The ela ionship be ween bo h a iables o in e es looks
linea , a leas up o 100 pm. The ac ha aising he le el o agi a ion om none o 38 pm doubles
he hea -exchange coe icien is ema kable.
Ag onomy 2020, 10, 97 8 o 14
L−1, and i s ma ke p ice 0.7 € L−1. To know he amoun o blood meal ob ained pe on o was e
e luen unde some cooling ange, he a io o blood meal pe on o aw blood mus be aken in o
conside a ion. F om ou expe imen s, ha a io is 15%. The i s s ep is o assess he amoun o aw
blood ha is possible o d y wi h he ene gy ex ac ed om he hea was e e luen . Fo ha pu pose,
he ex ac ed ene gy is di ided by he blood la en hea o apo iza ion a −0.9 ba , 2.39 MJ kg−1. The
esul ing weigh is mul iplied by 15/85 o ob ain he mass o blood meal.
Fo he economic alo iza ion, i was supposed blood meal unde he assump ion o a ma ke
p ice o 400 € −1 and a p oduc ion cos o 287 € −1 wi h a single e apo a o and 115 € −1 wi h h ee.
3. Resul s
3.1. Hea Exchange Pe o mance
The mos in e es ing p ope y o he hea exchange is he dependence o i s pe o mance wi h
espec o he deg ee o s i e mo emen , measu ed in pm. The pe o mance has he uni o
W/(m2·K), meaning he amoun o hea ha en e s he exchange pe second, squa e me e o
exchange su ace, and Kel in deg ee o he mal di e ence.
Figu e 5 shows he expe imen al da a. The ela ionship be ween bo h a iables o in e es looks
linea , a leas up o 100 pm. The ac ha aising he le el o agi a ion om none o 38 pm doubles
he hea -exchange coe icien is ema kable.
Figu e 5. Hea exchange pe o mance: hea -exchange coe icien in ela ion o o a ion speed o he
s i e .
3.2. D ying Blood in o Blood Meal
The p ocess o con e ing a luid in o a d ied powde (blood meal in he case o blood) goes
h ough ou clea ly iden i ied s ages: hin liquid, hick liquid, sludge, and inally powde , ollowing
ha o de , as hea goes h ough. One quali a i e analysis o g ea impo ance is o de e mine i he
d ying p ocess has come o he powde s age in a easonable ime. The expe imen s we conduc ed
showed ha aw blood was u ned in o a powde in no mo e han one hou . Howe e , he inal
mois u e con en was di e se, depending upon ope a ional condi ions, bu o ou case expe imen ,
i was 17% o humidi y on a we basis (one hou o d ying).
On he o he hand, he a io o blood meal e sus aw blood also a ied wi h he ope a ional
condi ions. The maximum amoun o blood meal is limi ed o 19% o aw blood weigh in he case o
cows, as is ou case. Unde ou expe imen , we go 15% o aw blood weigh ans o med in o blood
meal.
y = 11.137x + 428.33
R² = 0.8697
0
200
400
600
800
1000
1200
1400
1600
1800
0 20406080100
O e all Hea T ans e Coe icien
W m-1 K-1
pm
Figu e 5.
Hea exchange pe o mance: hea -exchange coe icien in ela ion o o a ion speed o
he s i e .
3.2. D ying Blood in o Blood Meal
The p ocess o con e ing a luid in o a d ied powde (blood meal in he case o blood) goes
h ough ou clea ly iden i ied s ages: hin liquid, hick liquid, sludge, and inally powde , ollowing
ha o de , as hea goes h ough. One quali a i e analysis o g ea impo ance is o de e mine i he
d ying p ocess has come o he powde s age in a easonable ime. The expe imen s we conduc ed
showed ha aw blood was u ned in o a powde in no mo e han one hou . Howe e , he inal
mois u e con en was di e se, depending upon ope a ional condi ions, bu o ou case expe imen ,
i was 17% o humidi y on a we basis (one hou o d ying).
Ag onomy 2020,10, 97 9 o 14
On he o he hand, he a io o blood meal e sus aw blood also a ied wi h he ope a ional
condi ions. The maximum amoun o blood meal is limi ed o 19% o aw blood weigh in he case
o cows, as is ou case. Unde ou expe imen , we go 15% o aw blood weigh ans o med in o
blood meal.
3.3. Economic Feasibili y
Table 1shows he empe a u e anges o e which he was e e luen can be used wi h he
ene gy-exploi a ion echniques desc ibed he e. As can be seen, elec ical powe p oduc ion sys ems
a e a a disad an age, as hey a e imp ac ical wi h e luen s a empe a u es below 70
◦
C—Thei
e iciency a hose le els is oo low and no shown in he able. P ehea ing and condi ioning sys ems
a e subs an ially be e , wi h hea pumps enabling ope a ion a he lowes possible empe a u es [
6
].
The de ice p oposed in his pape comp ises he use o a hea pump supplied wi h a condense
e apo a ing below 10
◦
C and can hus ope a e wi h e luen s a empe a u es abo e 20
◦
C. The i h
column displays economic pe o mance pe on o was e e luen .
Table 1.
Compa ison o economic easibili y among majo echniques o exploi ing ene gy om was e
e luen s and p oducing blood meal ( e e eed o a on o was e e luen ).
Technique Tempe a u e (◦C) Reco e ed Ene gy
(MJ −1)
Blood Meal
(kg −1)
Valo iza ion
(€ −1)
Condi ioning o Dwellings 40–80 167.47 - 3.20
P ehea ing o P ocess Fluids 40–80 167.47 - 3.20
D ying wi h a Hea Pump 20–80 251.21 16.69 6.70
D ying wi h a Hea Pump and
Th ee E apo a o s 20–80 251.21 16.69 9.57
4. Discussion
4.1. Hea Exchange Pe o mance
The hea p oposed exchange seeks o sol ing a main p oblem in he ag i- ood indus y: o ans e
hea o sludge in an e icien and as way, so ha i can be d ied in a small amoun o ime. Usually,
sludge d ying is e y slow, despi e he ac ha i ypically con ains no mo e han 30% o he wa e ha
came wi h he aw liquid om which i is p oduced [
15
]. Howe e , in usual condi ions in indus y,
ha 30% o wa e would ake mo e han 60% o o al d ying ime, which e idences ha he d ying o a
liquid is much as e [16].
The hea exchange p esen ed in his pape has a geome y ha allows high hea - ans e a es,
e en wi h sludge, due o i s pa icula geome y. As p e iously s a ed, i consis s o na ow e ical
chambe s wi h s i e s inside. Na ow means a wid h ha allows hea o ge o he bulk o he d ying
ma e ial in a sho ime by he mechanism o he mal conduc ion. This is a e y impo an ea u e,
since sludge ends o agglome a e and is di icul o unbundle and o be p ope ly mixed, which means
ha hea ans e by con ec ion is se e ely educed, and hus conduc ion akes he main ole [
17
].
This phenomenon occu s e en wi h he s i e s, hough o a lesse ex en . In any case, na ow chambe s
gua an ee good pe o mance e en in he wo s scena io. Fo he exchange he e disclosed, a wid h o
50 mm means ha hea should a el a maximum dis ance o 25 mm o each he bulk by conduc ion.
This dis ance is educed enough o p o ide a easonable hea - ans e speed [18].
The a o emen ioned geome y makes he exchange all in o he pla e-hea -exchange ca ego y [
19
],
hough i has se e al di e ences. The main one is ha he chambe s om he ypical pla e hea
exchange s om he cu en s a e o he a ha e a much sho e wid h, anging om 1 o 10 mm.
Ano he di e ence is ha he wo pla es ha con o m e e y chambe make con ac no only o e hei
bo de s, bu also on he inside [
19
]; his has se e al implica ions, he mos impo an being ha i
would block any s i e o agi a o o any kind, making i s p esence impossible [
19
,
20
]. Tha is no he