University of Groningen
Production of hydroxypropyl starch in a continuous static mixer reactor
Lammers, Geert
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Publication date:
1995
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Lammers, G. (1995). Production of hydroxypropyl starch in a continuous static mixer reactor s.n.
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Summary
Modification of starch by chemical derivation is of great industrial imporlance. A most
significant group of rnodified starches are the hydroxyalkyl starches like hydroxyethyl
and hydroxypropylstarch.The propertiesofthese starchesare determinedby:
L The native starchusedto producethe modified product
2. The Molar Substitution (MS) of the product
3. The production method
TlnreMS of a modified starch expressesthe moles of substituent introduced per mole of
anhydroglucose.Hydroxypropyl starch with a relatively low MS of about 0. I finds large
scale application in food products such as salad dressings,tomato ketchup and canned
soups.
Higlrer substitutedhydrorypropyl starchesof MS : 0.3 to 0.5 are used as adhesivesin
the paper board industry and as an additive in teÍile printing. These products aÍe
produced by reaction of propylene oxide with starch in an aqueoussolution using sodium
hydroxide as a catalyst. The high M,S products cannot be produced in an a aqueous
starch slurry becauseaÍ MS : 0.3 the modified starch becomes cold water soluble. The
production in solution has the disadvantageof a very high reaction mixrure viscosity. Of
course, viscosity can be reduced by using low starch concentrations but this decreases
overall reaction selectivity. Nowadays, high MS hydrorypropyl starch is produced in
batchprocesses.The high reactionmixture viscositylimits the scaleof operation.Even in
relatively small batch reactors, efficient mixing and optimal temperature control is
problematic, resulting in undesiredvariations in product quality.
This thesisfocuseson the developrnentofa new continuousproduction processfor high
substituted hydroxypropyl starch using a static mixer reactoÍ. Static mixers are in-line
mixing devises, especially suited for mixing of viscous fluids. Use of such mixers in a
chemicalreactor is a relatively rrew development.Potential advantagesof this reactor
type are:
1. Good mixing of viscous fluids
2. Enhancementof heat transfer relative to empty tubes
3. Good plug flow characteristics
.l
This thesis consists of 10 chapters.Chapter presents an overview of different modified
starchtlpes and production methods and gives an outline of this thesis.
Chapter 2 reviews different types of commercially available static mixers and their
properties with respect to mixing, pressure drop, heat transfer and residence time
distribution. Based on these data. the Sulzer SMX static mixer elements were selected
for the application studied here.
t75
Suntnmry
Chapter 3 presents experimentalresults on the kinetics of the four parallel reactions
irrvolvedin the production of hydroxypropylstarch:
l. Catalysedreactionwith starch:
I1OH + OH-5
ItO- + H1O
starch
(t
, .o.
/\
--+ ROCII"CIICII'l
RO- - CI12_Cll{ClLlj
o
slolv
oli
I{OCIH2CHCH3+ HrO -----+ROCH2CIHCHI + OH-
Íàst
2-hydroxypropyl
starch
2. Uncatalysedreactionwith starch:
O'..
u
ROII t CI12 CHCI13
oll
} I{OC}t2CllCFl3
3. Catalysedhydrolysis:
,ro\
?t' ?
C H 2 - C H C H 3 + O H - - - - - +C H I C H C H T
slow
oH o-
OII OII
+
CIITCIJCII.1 + II2O
CIITCIICH3 + 611
thst
propylene glycol
4. Uncatalysed
hydrolysis:
OH
,,o\
OH
I
CH2-CHCH3 + HrO
' CH2 CIHCH3
The ratesof these4 reactionswere measuredas:
-RpOt : *t,tcRO- cpo
with: É1,1 - tr
"-
Enlll't' . A : 4.28*107 m3/kmol s and E4 - 66.39kJ/mol
*
-fipOZ: nt.t CROHcpo
+
*
l tRT
with:ki, = A* n-Fo' , A* :39*1g3m3/kmol
s andoi - tt.z kJ/mol
-h*
**
-- t D
^
-r\PoJ
^l,l .oH.PO
176
rarallelreactions
witt tii
= A**
"-Ei'^'
m3llmols andEf, :,L.4kJlmol
, n**: 3.28{'108
***
-fip94: Kt,t cp2ocrc
***
with:fii-
***
-Fo* lpr
= A*** e-EA tKt,A***:449.4*103 m3/kmolsandEi* =77.15kJ/mol
These kinetic data, combined with experimental results on reaction selectivity, were used
to calculate tle acidity constant pK^ of the Sarch hydroxyl gÍoups. Ass'uming that only
one hydroxyl gÍoup per anhydroglucose unit dissociates, we found:
I
pK^:2174 - + 6.06
This relation can be used to calculate equilibrium concentÍations of dissociated starch
hydroryl gÍoups and hydroxide ions in alkaline aqueous starch solutions.
In Chapter 4 the experimental determination of the viscosity of concentrated starch
pastes using a Couette viscometer is presented. Starch paste viscosity depends on starch
concentration, shear rate and work performed on starch. A modified literature relation
describing the experimental data is prese,nted.
Basedon resultspresentedin Chapter3 and.4,Chapter5 discusses
the possibledesip of
a pilot plant scale static mixer Íeactor suitable-for the continuous production of
hyfuoxypropylstarch,seefigure.
Product
rol
I/mol
l: Starchslurry
2: Slurry pump
3: Pre-mixer
4: Heating section (microwave oven)
5: Reactor tube filled with SND( static mixers
Simplified schemeof the pilot plant scalestatic mixer reactor.
JUnmtary
The reactorwas usedfor a seriesof experirnentson starchpasteviscosity,residencetime
distribution,heat transferand for the actualproduction ofhydrorypropyl starch
In Chapter 6 the rheologicalbehaviourof starchpastesin the static mixer is discussed.
Starch paste viscosity was determinedby performing pressuredrop measurementsat
various starch concentrations.temperaturesaud flow-rates. Experimentalresults could
accuratelybe describedby:
('lT - I) It' + {n l) ltlyl
h, r a p p- o oIl) mstxcn+
for: 0.2 ( /rstarch< 0.30, 343 < T < 3ó3 K, 3.3 < W < 29.7 kJlkg and 2.7t < y <
1 0 . 8 4s - 1 .
with: rluoo - apparentviscosity
[Pa s]
ffistarclt: massfraction starch [-]
: temperature [K]
T
- work on starch
[kJ/kg]
: shearrate [s-l]
14/
i
K , B , C , D a n d n a r e c o n s t a n t s w i t h v a l u e3s .o0f ó 3 * 1 0 - + P a1s2. . 0 3[ - ] , 4 .1 3 4 * 1 0 3 K .
2.83*10-2kg,4<Jand 0.494 [-1, respectively.
Theseresults are in good agreementwith those from rheologicalexperimentspresented
in Chapter4.
Chapter7 discussesresidencetime distributionsin the static mixer reactor as determined
from step input responsemeasuÍementsusing lithium cbloride as a tracer. Experimental
resultswere modelled using the axially dispersedplug flow model. The Péclet number
1y'p6,
proved to be independerrtfiom flow rate and starch concentration.ïre average
value found was:
rt t
ND;-
" -97
t)
< v < 3 . 2 5 * 1 0 -n
f o r : 0 . 1 6í r , s t a r c<
h 0 . 2 8a n d 1 . 6 * 1 0 - 3
3d s .
with: Z (lenglh of reactor tube) - 1.58 rn., v - superficialfluid velocity and D:
dispersioncoefficient.
axial
The holding trne lirl 4>u was in good agreement with the experimetrtally observed
averageresidencetime r
An experimental study on heat transfer in a tube filled with Sulzer SMX elements is
presentedin Chapter 8. Data were obtainedfrom measuringradial temperatureprofiles
at the in- and outlet of a specially designedheat transfer section. The results were
t78
Summary
residencetime
;arch.
heat transfer model' From the onemodelled with both a one- and a two-dimensional
dimensionalmodelthefbllowingrelationfortheNusseltnumberwasforrnd:
NNut: 1.87 (NP66,)o'aa
rr is cliscussed.
)asurementsat
I results could
d 2 . 1 13 Y 3
for2'70<Neén.tS1439'
cp,1lv dt
Pn
di t d+
--^^
, NPén,t:
with: N11u,- t;
K]' 4: tube diameter [m]'
r,j,r : inside tube heat transfer coefEcient [W-2
=
= fluitl thermal conductivity [Wm K], pn fluid specific density [kC/m3]'
^fl
cpt,f7 : fluid sPecific heat [J/kg K]
data'
This res,ultis in good agreement with literature
Asanovelapproachheattransferwasalsomodelledwiththetwo-dimensionalheat
transfermodel,commonlyusedinthemodellingofpackedbedreactors.FortheefFective
insitle tube heat transfer coefficient, ai,2, the
radial thermal ,oodo"tJíty, ),",r, andthe
following relations were found:
1 , 4 . 1 3 4 * 1 0I !3
:
À*
4r"-o + Kr. NPen,t
"., :
98'5[-]
for'.2705 Neérr,t< 1439'
0**
mentspresented
lr as determined
r. Experimental
e Péclet number
rn. The aveÍage
with:/i-,0 = r.zsl-l
: t'16*10-2[-]
Kr*
*
-- - -
q,i)
qi'z dt
^
-
L1
4*
tr
è
ne,Í
Í
All
ty alLdD:
axial
rentallyobserved
SIvfr elementsis
npeÍature
Profiles
The restÍts were
scale production of hydroxypropyl starch are
Experiments involving actual pilot plant
good
conveÀon of propylene oxide was in
described in Chapter 9' Experimental
agÍeementwithresultsfrommodelcaloulationsbothfromaone-andatwo-dime,lrsional
selectivity with reaction mixture starch
reactoÍ model. The increase of reaction
the kinetic data obtained in chapter 3' The
concentration was less than expected from
experimentaldatagiveanintticationthatthisiscausedbyadecreaseintheeffective
high starch concentrations'
acidity ofthe starchhy'lrotryl groups at
design of a commercial scale statio mixer
Based on the results describedabove, a rational
kg/hr hydroxypropyl starch is discussed in
Íeactor suitable for the production of 600
Sunmmn,
Chapter 10. Based on reactiottselectivityand equiprneutcosts considerations.
adiabatic
operationis preferredfor the commercialscalereacror.
Ïre experirnentalwork describedin this thesishas shown that use
of a continuousstatic
mixer reactor for chemicalderivationof concentratedstarchsolutions
is very prornising.
Tle prototlpe reactorproved to be a robust tool due to the mechanical
simplicity of the
desigr. Relativelyhigh reaction temperaturescould be appliedwithout
undesiredbrowrr
coloudng ofthe product due to the low residencetirne irr the reactor.
relative to a batch
process. Fufthermore, the continuous well controlled production
process compares
favourableto batch processingwith respectto good manufacturing
piactices becauseit
enablescontinuous monitori'g of product quality and fits well
ili the down stream
processstepsofproduct purificationand dryang.
Ïte new production lnethod presentedhere gives incentivefor
a larger role of products
Íiom renewableresourcesin the field of adhesivesand textile print
ádditives and opens
perspectivesfor new applications.
t80