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Distillation Absorption 2010
A.B. de Haan, H. Kooijman and A. Górak (Editors)
All rights reserved by authors as per DA2010 copyright notice
211
INNOVATIVE BIODIESEL PRODUCTION BY HEAT-INTEGRATED REACTIVE
ABSORPTION
Anton A. Kiss
AkzoNobel – Research, Development and Innovation, 6824 BM Arnhem, The Netherlands
Email: tony.kiss@akzonobel.com
Abstract
This study proposes an innovative biodiesel technology based on heat-integrated
reactive absorption. Rigorous simulations embedding experimental results were
performed in AspenTech AspenONE engineering suite to design this novel process
and evaluate the technical and economical feasibility. The main results are given
for a plant producing 10 ktpy biodiesel from waste vegetable oil with high free fatty
acids content, using solid acids as green catalysts. This innovative process
eliminates all conventional catalyst related operations, and efficiently uses the raw
materials and the reactor volume in an integrated setup that allows significant
reduction of both capital and operating costs – with up to 85% energy savings.
Keywords: biodiesel, reactive absorption, reactive distillation, energy integration
1. Introduction
As a non-petroleum-based diesel fuel, biodiesel consists of fatty acid methyl esters (FAME), currently
produced by acid/base-catalyzed (trans-)esterification, followed by several neutralization and
purification steps1. Fatty esters are key products of the chemical process industry, involved not only in
the production of biodiesel but also in specialty chemicals. However, the main interest has shifted
nowadays to the larger scale production of biodiesel (Figure 1) – hence the market requirements for
more innovative processes. Accordingly, the development of an efficient continuous process for
biodiesel is essential and the use of solid catalysts is highly desirable in order to suppress costly
downstream processing steps such as neutralization, recovery and waste treatment.
Due to obvious economic incentives, process integration and intensification are increasingly applied
also in the biodiesel production2-5. This study proposes an innovative heat integrated process based
on reactive absorption (RA) thus further improving previous work on reactive-separation technology for
biodiesel production. RA offers significant benefits over reactive distillation (RD), such as: reduced
capital investment and operating costs due to the absence of the reboiler and condenser, higher
conversion and selectivity as no products are recycled in form of reflux or boil-up vapors, as well as no
occurrence of thermal degradation of the products due to a lower temperature profile in the column.
Rigorous simulations embedding experimental results were performed in AspenTech Aspen Plus to
design this novel reactive-absorption process and evaluate the technical and economical feasibility.
The main results are given for a plant producing 10 ktpy biodiesel from waste vegetable oil with high
free fatty acids content (up to 100%), using solid acids as green catalysts. By adding heat integration,
the heating/cooling requirements are drastically reduced by 85% and 90%, respectively.
0
2000
4000
6000
8000
10000
12000
14000
16000
2004 2005 2006 2007 2008 2009
Central & South America
Central & Eastern Europe
North America
Asia
Western Europe
Biodiesel production per region (thousands tons / year)
0%
20%
40%
60%
80%
100%
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
Asia
Central & Eastern Europe
Central & South America
North America
Western Europe
Figure 1. Biodiesel production in the world, and global market share per region.
A. A. Kiss
212
2. Problem statement
Conventional biodiesel processes are all plagued among others by the use of an excess of alcohol to
push the chemical equilibrium of (trans-)esterification towards fatty esters formation, and costly
downstream processing steps associated to the use of homogeneous catalysts. Due to the increased
costs of raw materials, the current trend is to use less expensive alternatives such as animal fat, waste
cooking oil, or waste vegetable oil1. Moreover, although the raw materials represent now the major
part of the variable costs associated with the biodiesel production, the reduction of energy
requirements remains still a very strong incentive.
The problem with waste oils is the very high content of free fatty acids (FFA) that lead to soap
formation in a conventional base catalyzed process. Moreover, the homogeneous catalysts require
neutralization, washing, separation, recovery, and waste disposal operations with severe economical
and environmental penalties. To solve these problems we propose a novel fatty acids esterification
process based on reactive absorption using water-tolerant solid acids as catalysts and therefore
eliminating the additional separation steps and the salt waste streams. In this work we selected
sulfated zirconia as acid catalyst due to its thermal stability and high activity. Nevertheless, ion-
exchange resins6 are also suitable due to the moderate temperatures used in this particular process.
Previously reported experimental results7 are embedded in the process simulations performed here.
3. Results and discussion
High purity products are possible in a reactive distillation setup, but the high temperature in the
reboiler – caused by the high boiling points of the fatty esters – is in conflict with the thermo-stability of
the biodiesel product. This problem can be avoided by either working at lower pressure or by allowing
some methanol in the bottom product2-3.
By using the reactive absorption process proposed in this work, the drawbacks of the reactive
distillation can be completely avoided and fatty esters (biodiesel) can be produced at moderate
temperatures and ambient pressure. Moreover, the water by-product is not returned as reflux in the
reactive absorption column, hence the detrimental effect of water on the equilibrium reaction and the
solid acid catalyst is also completely avoided.
The physical properties of the components present in this process were previously determined
experimentally or estimated using state-of-the-art contribution methods such as UNIFAC – Dortmund
modified. The ternary map shows that liquid phase split is possible, while the residue curve map
(RCM) reveals no azeotropes (Figure 2). Kinetic data for esterification of dodecanoic acid with
methanol is available from previous work2-5. Since the reverse hydrolysis reaction is negligible, a
simple kinetic expression can be used for simulation purposes. The reaction rate is given by r = k ·
Wcat · CAcid · CAlcohol, where CAcid and CAlcohol are the mass concentration of reactants and k =
250·exp(−55000/RT). Note that the reaction rate could be similar for different catalysts, just by
changing the weight amount of catalyst.
WATER
ACID
METHANOL
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0.90.80.70.60.50.40.30.20.1
Molefrac WATER
Molefrac ACID
Molefrac METHANOL
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
0.90.80.70.60.50.40.30.20.1
Figure 2. Ternary map and Residue Curve Map for the mixture methanol-acid-water.
Innovative Biodiesel Production by Heat-Integrated Reactive Absorption
213
F-ALCO
F-ACID
REC-TOP
REC-BTM
BTM
TOP
ACID
ALCO
F-ESTER
FAME
TOP-LQ
WATER
RAC
FLASH
COOLER
DEC
COMP
HEX1
FEHE2
FEHE1
130 140 150 160 170
0
3
6
9
12
15
Temperature / °C
0 0.5 1 1.5 2 2.5 3
Ester formation / kmol/hr
Reaction rate
Temperature
Figure 3. Simulated HI-RA flowsheet (left). Temperature and reaction rate profiles (right).
Figure 3 (left) presents the developed flowsheet of a biodiesel production process based on a reactive
absorption column (RAC) as the central operating unit. The reactants feed streams are pre-heated
using the product streams. The pre-heated fatty acid is fed in the top of the reactive column while a
stoichiometric amount of pre-heated alcohol is injected as vapor into the bottom of the column, thus
creating a counter-current flow regime over the reactive zone. Water by-product is removed as top
vapor, then condensed and separated in a decanter from which the fatty acids are recycled back to
the column while water by-product is recovered at high purity – hence this stream could be reused as
industrial water on the same site. The fatty esters are delivered as high-purity bottom product of the
reactive column. The hot FAME product is flashed first to remove the remaining methanol, used then
to pre-heat the alcohol feed and then it is cooled down and stored. The complete process was
simulated in AspenTech Aspen Plus – using the rigorous RadFrac unit with RateSep model for the
RAC – then, the sensitivity of the key operating parameters was also evaluated.
Figure 3 (right) illustrates the temperature and reaction rate profiles along the column. The reactive
separation column is operated in the temperature range of 135–160 °C, at ambient pressure, with a
maximum reaction rate in the middle of the reactive zone – rate profile similar to a RD column.
Table 1. Mass and energy balance of a 10 ktpy biodiesel process based on reactive-absorption.
F-ACID F-ALCO BTM REC-BTM REC-TOP TOP WATER FAME
Temperature C 160 65.4 136.2 146.2 51.8 162.1 51.8 30
Pressure bar 1.051.051.031.2161110.203
Vapor Frac 0 1 010100
Mass Flow kg/hr 1166.755 188.306 1261.295 11.295 9.369 114.43 105.061 1250
Volume Flow cum/h
r
1.492 157.565 1.417 8.949 0.011 213.042 0.109 1.259
Enthalpy Gcal/hr -0.94 -0.279 -0.904 -0.013 -0.009 -0.337 -0.395 -0.957
Mass Flow kg/hr
METHANOL 0 188.304 9.125 7.544 0.002 0.103 0.101 1.581
ACID 1166.739 0 trace trace 9.218 9.233 0.016 trace
WATER 0 0 trace trace 0.24 105.166 104.926 trace
ESTER-M 0 0 1252.183 3.764 0.846 0.846 trace 1248.419
Mass Frac
METHANOL 0 1 0.007 0.667 172 ppm 894 ppm 965 ppm 0.001
ACID 1 0 trace trace 0.894 0.08 148 ppm trace
WATER 0 0 trace 10 ppb 0.023 0.912 0.999 trace
ESTER-M 0 0 0.993 0.333 0.082 0.007 513 ppb 0.999
A. A. Kiss
214
Table 2. Design parameters for simulating the reactive absorption column.
Parameter Value Units
Total number of theoretical stages 15 –
Number of reactive stages 10 (from 3 to 12) –
Column diameter 0.4 M
HETP 0.6 M
Valid phases VLL –
Volume liquid holdup per stage 18 L
Mass catalyst per stage 6.5 Kg
Catalyst bulk density 1050 kg/m3
Fatty acid conversion >99.99 %
Fatty acid feed (liquid, at 160 °C) 1167 kg/h
Methanol feed (vapor, at 65 °C) 188 kg/h
Production of biodiesel (FAME) 1250 kg/h
RA column productivity 19.2 kg FAME /kg cat. h-1
The production rate considered for the biodiesel plant designed in this work is 10 ktpy fatty acid methyl
esters (FAME). The complete mass and energy balance is given in Table 1, while the main process
design parameters – such as column size, catalyst loading, and feed condition – are listed in Table 2.
High purity products are possible, the purity specifications exceeding 99.9 %wt for the final biodiesel
product (FAME stream). Note that the total amount of the (optional) recycle streams is not significant,
representing only ~1.5% of the biodiesel production rate. High conversion of the reactants is achieved,
at a remarkable productivity of ~19 kg fatty esters/kg catalyst/h.
Figure 4 shows the molar composition profiles in both liquid and vapor phase. The concentration of the
fatty acid and water increases from the bottom to the top of the column, while the concentration of fatty
ester and methanol increases from the top to bottom. Consequently, in the top of the reactive
separation column there are vapors of water and liquid fatty acids, while in the bottom there are
vapors of the methanol feed and liquid fatty esters product (biodiesel). Note that the concentration of
methanol in the liquid phase could be further increased by working at higher pressure.
0 0.2 0.4 0.6 0.8 1
0
3
6
9
12
15
Stage
Molar fraction (liq)
0 0.2 0.4 0.6 0.8 1
Molar fraction (liq)
Wate
r
Methanol Este
r
Acid
0 0.2 0.4 0.6 0.8 1
0
3
6
9
12
15
Stage
Molar fraction (vap)
0 0.2 0.4 0.6 0.8 1
Molar fraction (vap)
Wate
r
MethanolEste
r
Acid
Figure 4. Liquid and vapor composition profiles along the reactive column.
Innovative Biodiesel Production by Heat-Integrated Reactive Absorption
215
80%
85%
90%
95%
100%
60 80 100 120 140 160 180 200
Temperature / [C]
Purity / [wt%
]
Fatty esters
Water
50%
60%
70%
80%
90%
100%
0.80.9 1 1.11.21.31.4
Molar ratio alcohol:acid / [-]
Purity / [wt%
]
Fatty esters
Water
Figure 5. Purity of top (water) and bottom (FAME) products versus temperature of the fatty acid feed
stream (left) or versus molar reactants ratio (right).
Afterwards, sensitivity analysis was used as a powerful tool to evaluate the optimal range of the
operating parameters, such as: temperature of feed streams, reactants ratio, recycle rates, decanting
temperature, flashing pressure. The main results are shown in Figure 5. An optimal range exists for
the temperature of the fatty acid feed stream (100-160 °C), in order to obtain concentrated fatty esters
(min. 95%) in the bottom of the column. Remarkably, the optimal molar ratio of the reactants
(alcohol:acid) is the stoichiometric value of one. Higher values lead to complete conversion of the fatty
acids, but the excess of methanol becomes a significant impurity in the top stream and thereafter in
the water by-product. On the contrary, a lower ratio leads to incomplete conversion of the fatty acids
that will contaminate the bottom product. Due to the difficult separation of fatty acids from fatty esters,
this situation should be avoided. In practice, using a small excess of methanol (~0.5%) or an efficient
control structure8 that can ensure the stoichiometric ratio of reactants, is sufficient for the complete
conversion of the reactants.
Table 3 shows a head-to-head comparison of the novel heat-integrated RA process proposed in this
study against the previously reported state-of-the-art RD process,3,4 in terms of number of stages,
operating parameters, productivity, as well as heating and cooling requirements – figures that
ultimately translate into equipment size and cost. Remarkably, the energy usage is less than 22
kW/ton biodiesel, with significant reduction of the heating (–85%) and cooling (–90%) requirements
compared to the non-integrated process.
Table 3. Comparison between the integrated reactive-absorption vs reactive-distillation.
Equipment / Parameter / Description Units RD HI-RD RA HI-RA
Reactive column – reboiler duty (heater) kW 136 136 n/a n/a
HEX-1/FEHE heat duty (fatty acid heater) kW 95 0 108 27
HEX-2/FEHE heat duty (methanol heater) kW 80 65 0
Reactive column – condenser duty (cooler) kW – 72 – 72 n/a n/a
HEX-3/FEHE water cooler/decanter kW – 6 – 6 – 77 0
COOLER heat duty (biodiesel cooler) kW – 141 – 38 – 78 – 14
FLASH heat duty (methanol recovery) kW 00 0 0
Compressor power (electricity) kW 0.6 0.6 0.6 0.6
Reactive column, number of reactive stages – 10 10 10 10
Feed stage number, for acid / alcohol streams – 3 / 10 3 / 10 1 / 15 1 / 15
Reactive column diameter m 0.4 0.4 0.4 0.4
Reflux ratio (mass ratio R/D) kg/kg 0.10 0.10 n/a
n/a
Boil-up ratio (mass ratio V/B) kg/kg 0.12 0.12 n/a n/a
Productivity (kg ester per kg catalyst per h) kg/kg.cat.h-1 20.4 20.4 19.2 19.2
Energy requirements per ton FAME product kW.h/ton 191.2 108.8 138.4 21.6
A. A. Kiss
216
4. Conclusions
This study presented a heat-integrated biodiesel process based on FFA esterification in a reactive
absorption column using solid acids as green catalysts. CAPE tools, such as AspenTech Aspen Plus,
were successfully applied in the development and evaluation of this novel process. Sensitivity analysis
was also used as a powerful tool to determine the optimal process parameters and explore the effect
of the reactants ratio or the feed streams temperature on the purity of products. The most favourable
results were obtained near the stoichiometric reactants ratio and relatively high temperature of the
fatty acids feed stream. At optimal operation, the highest yield and purity can be achieved by using
stoichiometric reactants ratio, with practically negligible amount of methanol lost in top (ppm level) and
complete conversion of the fatty acids.
The innovative process proposed in this work significantly improves the biodiesel production and
considerably reduces the number of downstream processing steps. Compared to conventional
processes, the major benefits of this unique process are:
• Multifunctional plant suitable for a large range of light alcohols and fatty raw materials with very
high FFA content, such as frying oils, animal tallow and wvo.
• Simple and robust process with no catalyst-related waste salt streams, no soap formation, and
sulfur-free biodiesel as the solid catalysts do not leach into product.
• Elimination of all conventional catalyst-related operations such as: catalyst neutralization,
separation and disposal of waste salts, waste water treatment.
• Efficient use of raw materials: stoichiometric reactants ratio, high conversion, no products recycled
as reflux or boil-up vapors, and no thermal degradation.
• Effective use of the reactor volume leading to significantly high unit productivity, up to 6-10 higher
than conventional biodiesel processes.
• Reduced CapEx and OpEx due to the integrated design with no reboiler or condenser. Compared
to similar reactive distillation processes, about 20% savings on the total capital investment and
30% less operating costs are possible.
• Significant reduction of the heating (–85%) and cooling requirements (–90%), compared to the
non-integrated alternatives based on reactive separations.
Acknowledgements
We thank Alexandre Dimian and Gadi Rothenberg (University of Amsterdam) for valuable discussions.
References
1. M. G. Kulkarni, A. K. Dalai, Ind. Eng. Chem. Res., 45 (2006) 2901-2913
2. A. A. Kiss, G. Rothenberg, A. C. Dimian, F. Omota, Top. Catal., 40 (2006) 141-150
3. A. A. Kiss, A. C. Dimian, G. Rothenberg, Eng. Fuels, 22 (2008) 598-604
4. A. A. Kiss, Sep. Pur. Technol., 69 (2009) 280-287
5. A. C. Dimian, C. S. Bildea, F. Omota, A. A. Kiss, Comput. Chem. Eng., 33 (2009) 743-750
6. S. Steinigeweg, J. Gmehling, Ind. Eng. Chem. Res., 42 (2003) 3612-3619
7. A. A. Kiss, A. C. Dimian, G. Rothenberg, Adv. Synt. Catal., 348 (2006) 75-81
8. C. S. Bildea, A. A. Kiss, Comput. Aided Chem. Eng., 28 (2010) 535-540
