1-s2.0-S0360319911009608-main_2

8/13/2019 1-s2.0-S0360319911009608-main_2

1/20

Thermodynamic study of the supercritical water reforming

of glycerol

F.J. Gutierrez Ortiz*, P. Ollero, A. Serrera, A. Sanz

Departamento de Ingeniera Qumica y Ambiental, Universidad de Sevilla, Camino de los Descubrimientos, s/n, 41092 Sevilla, Spain

a r t i c l e i n f o

Article history:Received 15 March 2011

Accepted 14 April 2011

Available online 18 May 2011

Keywords:

Reforming

Supercritical water

Thermodynamic analysis

Equation of state

Glycerol

Biodiesel

a b s t r a c t

Hydrogen can be produced by steam reforming, partial oxidation, autothermal, or aqueous-phase reforming processes using various noble metal based catalysts, but also by super-

critical water (SCW) reforming. Using AspenPlus, a systematic thermodynamic analysis

of glycerol reforming using supercritical water has been carried out by the total Gibbs free

energy minimization method, which computes the equilibrium composition of synthesis

gas (syngas). The predictive SoaveeRedlicheKwong equation of state (EOS) has been used

as thermodynamic method in the simulation of the supercritical region, after evaluating it

against other EOS methods. A sensitivity analysis has been conducted on supercritical

water reforming of pure and pretreated crude glycerol, as obtained from biodiesel

production. The effect of the main operating parameters (temperature, concentration of

glycerol feed, glycerol purity in the feed of crude glycerol, and pressure) aimed to the

hydrogen production has been investigated in the reforming process, by obtaining the mole

fraction and molar flow-rate of components in syngas, as well as the hydrogen yield.

Selectivity to the different compounds has been also calculated. By this way, the ther-modynamic favorable operating conditions at which glycerol may be converted into

hydrogen by SCW reforming have been identified. The simulation results agree well with

some few experimental data from the literature. This study is the first of a series addressed

to glycerol reforming using SCW.

Copyright 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction

In recent years there have been intensive efforts toward the

development of novel technologies for the production of

hydrogen from renewable resources, mainly biomass. Among

the various biomass-derived compounds proposed as feed-

stock for hydrogen production, glycerol (C3H5(OH)3) is of

special interest because it is produced in large amounts (10 wt

%) as by-product of the chemical reaction (transesterification)

in which vegetable oil is processed into biodiesel. By-product

glycerol comprises a mixture of several other constituents,

such as methanol, water, inorganic salts, free fatty acids,

unreacted mono-, di-, and triglycerides, and methyl esters.

Conventional options for crude glycerol consist of refining it to

a higher purity. Unfortunately, the rapidly expanding market

for biodiesel cannot accommodate the excess amounts of

glycerol generated altering thus the cost and availability of

glycerol. However, glycerol production and utilization has

a great impact on both the economic stability and sustain-

ability of biodiesel production that will continue to increase as

the industry grows. As such, for utilizing the glycerol by-

product it is crucial to develop innovative processes.

The valorization of the crude glycerol, while avoiding the

application of expensive purification processes, will allow

* Corresponding author. Tel.:34 95 448 72 68; fax: 34 95 446 17 75.E-mail address:[email protected](F.J. Gutierrez Ortiz).

A v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c om / l o c a t e / h e

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 8 9 9 4 e9 0 1 3

0360-3199/$ e see front matter Copyright 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

doi:10.1016/j.ijhydene.2011.04.095
mailto:[email protected]://www.sciencedirect.com/http://www.elsevier.com/locate/hehttp://dx.doi.org/10.1016/j.ijhydene.2011.04.095http://dx.doi.org/10.1016/j.ijhydene.2011.04.095http://dx.doi.org/10.1016/j.ijhydene.2011.04.095http://dx.doi.org/10.1016/j.ijhydene.2011.04.095http://dx.doi.org/10.1016/j.ijhydene.2011.04.095http://dx.doi.org/10.1016/j.ijhydene.2011.04.095http://www.elsevier.com/locate/hehttp://www.sciencedirect.com/mailto:[email protected]

8/13/2019 1-s2.0-S0360319911009608-main_2

2/20

augmented profitability of the biodiesel plants. One promising

and economical alternative is the transformation of glycerol

into an energy derivative as, for instance, to use it as

a renewable source of hydrogen, which is often defined as the

future energy carrier. This can be done with the use of several

methods including some reforming processes. Besides,

establishing a technology for hydrogen production from this

waste is desirable from the perspective of reduction of fossilfuel consumption for power generation.

By now, glycerol reforming has been extensively studied

and an evaluation of these studies appears attractive. Three

types of glycerol reforming processes e steam reforming

[1,2], aqueous-phase reforming[3], and autothermal reform-

ing [4] e have primarily been investigated, but the glycerol

reforming by using supercritical water (SCW) has been barely

studied from a thermodynamic point of view, and even less

using not pure but crude glycerol.

Reforming reactions are generally endothermic, and

a reforming process may be characterized depending on the

source of heat and types of reactants. A general equation to

describe glycerol reforming is shown in reaction (1).

C3H8O3 xH2O aCO2 bCO cH2O dH2 eCH4 . (1)

The equilibrium composition depends upon the reactant

ratios as well as the reaction temperature and pressure.

Reforming products include hydrogen and carbon monoxide

in addition to carbon dioxide and methane. A catalyst is nor-

mally used to accelerate the reactions in the reforming

process. Ni, Co, Ni/Cu, and noble metal (Pd, Pt, Rh) based

catalysts all favor hydrogen production, with Ni being the

most commonly used[5]. Catalysts boost the reforming reac-

tion rates at the molecular level and many thorough discus-

sions of the topic are available in the literature.Glycerol steam reforming is the more popular reforming

process, and it can be represented by the overall reaction (2):

C3H8O3 3H2O 3CO2 7H2 (2)

Thus, 7 mol of H2are produced per mol of glycerol on the

reaction stoichiometry. Major concerns are by-product

formation (e.g., CO), catalyst deactivation, and high energy

consumption. There are more reforming processes and this

work is focused on reforming using supercritical water (SCW),

which is defined as water that is heated and compressed over

its critical temperature (374 C) and pressure (22.1 MPa).

Supercritical water (SCW) has properties very different

from those of liquid water. The dielectric constant of SCW ismuch lower, the number of hydrogen bonds is much lower

and their strength is much weaker. As a result, SCW behaves

like many organic solvents so that organic compounds have

complete miscibility with SCW. Moreover, gases are also

soluble in SCW, thus an SCW reaction environment provides

an opportunity to conduct chemistry in a single fluid phase

that would otherwise occur in a multiphase system under

conventional conditions[6].

Gases like CO2, CH4, H2, and CO are completely miscible in

supercritical water[7,8]. The polar inorganic compounds like

KCl, NaCl, CaSO4etc., which have high solubility in subcritical

water, shows very low solubility in supercritical water. Thus,

it is relatively easy to separate them from the product. This

allows the product of SCWG to leave the system free from the

salt. More details are available in the literature[9].

Supercritical water is characterized by its high ion product,

which implies high [H] or [OH] concentration in supercrit-

ical water. This allows SCW act like an acid or base catalyst in

the reactions. Many organic chemicals that do not react in

water without the presence of strong acid or base catalyst may

readily react under the hydrothermal condition of SCW.Reactivity of water increases in the neighborhood of the

critical point with or without a catalyst. Thus, the reforming of

glycerol to synthesis gas or syngas (SG) using supercritical

water under a catalyst-free process arises as a very interesting

alternativeand it will be studiedin a futureexperimental work.

Due to the unique properties of SCW, thermodynamic

equilibrium and high chemical reaction rates are possible. In

fact, using SWC may be an excellent means for extraction of

energy from biomass, and allows high hydrogen concentra-

tion in the product gas with suppression of char and tar

formation[10]. At temperature higher than 600 C and pres-

sure higher than that of its critical point, water becomes

a strong oxidant. As a result, carbon is preferentially oxidizedinto CO2although low concentrations of CO are also formed.

The hydrogen atoms of water and glycerol, as biomass, are set

free and form H2. The gas product (syngas) consists of H2, CO2,

CH4 and CO.Thus, for the design of thereactorand separators,

the knowledge of phase equilibria is very important.

2. Aims and scope

The objective of a reforming process of crude glycerol is to

produce hydrogen; however selectivity to hydrogen remains

challenging due to subsequent reactions in the gas. Thermo-

dynamic studies are very important because they provideinformation on conditions that are advantageous for

hydrogen production. Thus, the aim of this study is to

examine hydrogen production by SCW reforming of pure and

crude glycerol,which comprises impurities that cause catalyst

deactivation. Firstly, a discussion about the most suitable

thermodynamic method to be used for the simulation of the

supercritical state is carried out. Then, by predicting the

synthesis gas composition (hydrogen, carbon monoxide and

others) at equilibrium condition in the reforming reactor,

a sensitivity analysis is performed to know the effect of the

main operating parameters on hydrogen yield, so as to ach-

ieve optimal conditions for glycerol SCW reforming that

maximize hydrogen production.

3. Equations of state and simulation of thesupercritical state

When the operating temperature is beyond the critical point,

the simulation tool used in this work, AspenPlus, considers

a gaseous behavior for the stream. Likewise, for lower

temperatures the properties taken by this software corre-

sponds to a liquid. Therefore, in the supercritical region, the

error of some thermodynamic properties, like enthalpy and

entropy, should be quoted depending on their reference to the

liquidorvaporstate,asconsideredbyAspenPlus .Bychecking

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 6 ( 2 0 1 1 ) 8 9 9 4 e9 0 1 3 8995
http://dx.doi.org/10.1016/j.ijhydene.2011.04.095http://dx.doi.org/10.1016/j.ijhydene.2011.04.095http://dx.doi.org/10.1016/j.ijhydene.2011.04.095http://dx.doi.org/10.1016/j.ijhydene.2011.04.095

8/13/2019 1-s2.0-S0360319911009608-main_2

3/20

and comparing the results provided by different thermody-

namic methods, previously discriminated based on their

rightness of use in supercritical state, the most suitable one to

be chosen is that providing minima deviations in properties

from liquid and vapor state. Besides, this is also essential as

a prior step for achieving a conceptual design of the processto

bestudied with confidence,by carryingout energyanalysisand

using a heat exchangers network for recovering the processheat[11].

The most computationally straightforward and thermody-

namically consistent method for calculating high pressure

phase behavior is to select an equation of state (EOS) to model

both the liquid and vapor or supercritical fluid phases, in

opposition to those thermodynamic methods based on activity

coefficients, which cannot be used for phase equilibrium with

supercritical fluids due to the different manner that they treat

each phase, and hence they cannot represent the changes that

occur in the critical region in continuous form. The thermo-

dynamicmethod finally chosenfor fugacitycalculation was the

predictive SoaveeRedlicheKwong (PSRK) equation of state

[12,13], which is an extension of the SRK equation of state, anduses the generalized MathiaseCopeman a-function. This

model uses the HolderbaumeGemehling mixing rules, which

can predict the binary interactions at any pressure. Using

UNIFAC, the PSRK method is predictive forany interaction that

can be predicted by UNIFAC at low pressure. The main advan-

tage of using PSRK equationof state is that it is more accurate in

the prediction of the binaryinteraction parameters andit gives

more satisfactory results for mixtures of non-polar and polar

components, as the case of the crude/pure glycerol and water

mixture. This choice has been weighed against other thermo-

dynamic methods such as the original SoaveeRedlicheKwong

(SRK), Peng-Robinson (PR) and Peng-Robinson with the Boston-

Mathias afunction (PR-BM).The PSRK method represents quite accurately the super-

critical state for the glycerol and the CO2 (taking as a reference

since it is probably the fluid more studied in supercritical

state), as shown inFig. 1, although 240 atm is a pressure much

higher than critical pressure of glycerol (74.02 atm) and CO2(72.86 atm). PR and PR-BM also give a very good fit and the SRK

shows small deviations, although these are not shown.

On the other hand, for the water, the PSRK method shows

the minimum deviation between the liquid and vapor curves

in the supercritical state (Fig. 2) of all the thermodynamic

methods tested. The absolute error for specific enthalpy(difference between liquid and vapor curves) has been quoted

between 3 and 6 kJ/mol in the critical region, showing the

maximum deviation around 565 C. Although 6 kJ/mol may

not be considered as a large difference, it should be noted that

the glycerol should be very dilute in the reforming with SCW,

as after explained, so an error of 6 kJ/mol could become

a significant divergence from the energy standpoint. The PR

and PR-BM methods showed behaviors similar to that depic-

ted inFig. 2for PRSK, but the deviations are slightly higher.

Then, these methods could be also used with relative confi-

dence. However, the SRK method showed large errors.

For the glycerol, it has beenverified thatin the supercritical

state,the vaporizationheat is saved,which is a very significantenergy saving andagreeswell with data from literature [14,15].

This is illustrated inFig. 3, where a specific energy saving of

47.3 kJ/mol is obtained when operating at 600 C and 240 atm,

which is lower than theglycerol vaporizationheat (58.2 kJ/mol)

at its boiling point (1 atm, 290 C). However, the energy saving

continuously decreases as the temperature rises.

In the case of water, depicted in Fig. 4, there is an uncer-

tainty relative to the error due to the mismatch between the

enthalpies referred to vapor andliquid states fortemperatures

higher than the critical temperature. This inconvenient may

be overcome by an average curve between the vapor and

liquid curves, with a maximum error of 3 kJ/mol, or by

directly taking the vapor curve as the valid one, since thisoption corresponds to the minimum energy saving. The

increase in enthalpy is huge when changing from liquid to

vapor at atmospheric pressure. However, for the liquid at

Fig. 1e

Simulation of the supercritical state for glycerol and CO2at 240 atm (PSRK method).


8/13/2019 1-s2.0-S0360319911009608-main_2

4/20

240 atm, it can be seen that when temperature rises to the

critical value, the slope of the curve rapidly increases. Beyond

this zone, the specific energy saving becomes 10.7 kJ/mol at

400 C(Fig. 4). Although this value is quite lower than that for

glycerol, there will be a significant energy saving due to the

heat up of the large amount of water required for the SCW

reforming of glycerol to maximize hydrogen yield, as below

exposed. However, by increasing the temperature from 400 to

800 C, the specific energy saving would decrease from 10.7 to

4.5 kJ/mol. Therefore, the energy saving is lower for temper-

atures much higher than 374 C. Besides, the energy analysisshould account for the mechanical energy necessary to raise

the pressure from atmospheric to supercritical values.

Fig. 5illustrates the effect of pressure on simulation of the

supercritical state for the water, it can be seen that when the

pressure increases over the critical value, the vapor curve

exhibits a decrease in enthalpy at temperatures slightly under

the critical one in such a way that the liquid and vapor

enthalpies matchesjust in the critical point. Besides, when the

pressure is twoethree times higher than critical pressure, the

vapor and liquid states match well for supercritical tempera-

tures. InFig. 6a more narrow range of temperatures has been

used in order to get better accuracy in thecited vapor behavior

around the critical point under different pressures. Thus, itwas carried out an analysis between 350 and 400 C using

temperature increments of 0.10 C (500 point data versus the

Fig. 2 e Evolution of the water trend in liquid and vapor state at 240 atm (PSRK method) - each enthalpy interval is 2 kJ/mol.

Fig. 3e Vaporization heat (l) and energy saving for the glycerol at 240 atm. The blue curve represents the vapor state at

atmospheric pressure and it is taken as a reference in the computation of the energy saving. (For interpretation of the

references to colour in this figure legend, the reader is referred to the web version of this article.)


8/13/2019 1-s2.0-S0360319911009608-main_2

5/20

previous figures that is 100 point data). These analyses weredone for both subcritical and supercritical conditions. At 210

and 215 atm, the enthalpy curves for liquid and vapor phases

are not still concurrent. At 218 atm, the curves match each

other just at 374 C. At pressures higher than the critical one,

the enthalpy curve tends to abruptly diminish just at 374 C,

where it matches with theliquidcurve. Next, a unique curve is

shown. The sudden enthalpy drop of the vapor curve at the

critical temperature for pressures higher than the critical is

quite sound. The reason is because the thermodynamic

method used is based on an EOS where, by definition, thefugacity of liquid and vapor in the critical point must be equal.

This implies that all the properties must be equal in this point,

and so the enthalpy. By this way, it has been obtained

a decrease in enthalpy as the temperature increases inside

a given range just before reaching the critical point, which is

not thermodynamically reasonable. The explanation of this

simulated behavior is due to the vapor curve for water is not

real at temperatures lower than the critical under pressures

higher than the critical. That is, at pressures as high as those

Fig. 4 e Vaporization heat (l) and energy saving for the water (supercritical pressure of 240 atm).

Fig. 5e

Enthalpy variation for water in liquid and vapor phase at 220, 300, 600 and 800 atm.


8/13/2019 1-s2.0-S0360319911009608-main_2

6/20

used in these analyses, the water must be in liquid phase. As

a consequence, although AspenPlus

provides results for thevapor curve in any interval of temperatures, the part of the

vapor curve to be rightly usedis that just over the critical point,

i.e., the part relative to the supercritical region. Likewise, the

rapid increase of the liquid enthalpy around the critical

temperature found for pressures around the critical pressure

becomes negligible at pressures higher than 400 atm, where

supercritical state is more clearly distinguished from the

subcritical state.

Finally, a sensitivity analysis for the case of water was

carried out to study the transition from the subcritical to

supercritical state starting at a given pressure. Results are

shown inFig. 7, in an HeP diagram, where it can be seen that

there is a decrease in the enthalpy corresponding to thevaporization heat due to phase change from vapor to liquid for

all the isothermal curves, except at 400 and 500 C (values

higher than the critical temperature). This vaporization heat

decreases when temperature approaches to the critical one

(374 C), where becomes null. Beyond the critical point,

enthalpy slightly decreases as pressure rises.

4. Methodology of the thermodynamicanalysis

For given operating conditions, the equilibrium compositions

in the reforming reactor have been calculated. The

computation has been made with the aid of AspenPlus

version 2006.5[16]. In this study, an R-Gibbs reactor has beenused to calculate the products composition and the heat of

overall reaction in a system under the conditions that mini-

mize of Gibbs free energy. This is a non stoichiometric

approach, where a selection of the possible set of reactions is

not necessary and does not require any initial estimation of

the equilibrium. Indeed, the method of minimizing the Gibbs

free energy is normally preferred in fuel-reforming analysis,

especially when the reaction temperature and pressure are

specified. The R-Gibbs reactor does not take reaction kinetics

into account and allows individual reactions to be at

a restricted equilibrium.

The thermodynamic analysis ignores and does not provide

kinetics effects of reactions taking place, and it assumesequilibrium. Although a practical situation may diverge, the

results of the analysis provide a valuable reference on optimal

conditions for hydrogen production so as to design experi-

mental tests and compare the results from these.

4.1. Process simulated

The simulated process is illustrated inFig. 8. The flow-sheet

consists of a high pressure pump for the glycerol-water

mixture, obtained in a mixer and a heater in order to respec-

tively get the pressure and temperature specified for the

reforming reactor. It is assumed that reactor also operates by

providing the energy needed to hold the endothermic

Fig. 6e Enthalpy variation between 350 and 400 C for water in liquid and vapor phase at 210 and 215 atm (subcritical

conditions), 218 atm (critical conditions), and 220, 240, 400 and 600 atm (supercritical conditions).


8/13/2019 1-s2.0-S0360319911009608-main_2

7/20

reactions so as to operate at the specified temperature.

Hydrogen, carbon monoxide, carbon dioxide, methane,

ethane, propane, water, methanol, ethanol and glycerol as

well as pure carbon were manually added forbeing considered

as the possible species in SCW reforming of glycerol. The

simulation did not predict coke formation for any of the

experimental conditions in this paper, and it did not add any

other compound (mole fractions lower than 1020). Next, an

expander (turbine) is included to reduce the pressure in an

efficient way. Downstream from the expander, a cooler

diminishes the temperature to 200 C. Then, an Equilibriumreactor (Requil) is usedto model the wateregas shift reaction to

increase the conversion to hydrogen in the syngas and to

reduce the CO content, which poisons the anode of proton

exchange membrane fuel cells (PEMFC). Requiloperates with-

drawing the energy released from the exothermic reaction so

as to operate at the specified temperature. The outlet stream is

after cooled to 60 C in another cooler to drive, e.g., the gas to

a PEMFC where hydrogen would be converted into electrical

energy. Finally, a flash separator is included to separate liquid

condensates from the gaseous phase. Pressure reduction

should not be made after cooling to 200 C because another

separator should be added, prior to expansion, where there

would be a significant fraction of hydrogen removed in the

liquid outlet stream and, hence, an important yield lost.

Specifications of elements used in the simulation are shown

inTable 1. As aforementioned, the thermodynamic method

used has been the predictive SoaveeRedlicheKwonge(PSRK).

This method was contrasted against UNIFAC and Ideal

methods for the low pressure zone of flow-sheet downstream

from the expander (operating at about atmospheric pressure),but results are undisturbed, so for the sake of simplicity, PSRK

method was used for all the simulation.

4.2. Crude glycerol simulated

Two feeds have been considered: pure and pretreated crude

glycerol. Normally, 1:6e1:9 M ratios of oil-to-methanol are

used in the transesterification of vegetable oilto biodiesel with

catalyst NaOH. After the reaction is finished, large amount of

Fig. 7e

Enthalpy of water as function of pressure at constant temperature (100, 200, 300, 350, 400 and 500 C).

05 SG1

01

02 03

04

SG2

8

SG3

W

GLY

SG4

W2

COOL1

PUMP

HEAT1R1

R2

COOL2

B1

B2 SEP

Fig. 8e

Flow-sheet of the simulated process.


8/13/2019 1-s2.0-S0360319911009608-main_2

8/20

the methanol is unreacted. Since methanol and glycerol both

have carbon-to-alcohol ratios of 1:1 with associated high

degrees of hydrogen bonding, the methanol preferentially

distributes into glycerol phase, and only a relatively small

amount dissolves in biodiesel, as experimentally verified[17].

Indeed, the main components of crude glycerol are glycerol

and methanol [18], and also water [19], which is used to reduce

its viscosity and allow the crude glycerol to be pumped.

Anyway, in order to better perform the reforming process, an

economic solution for the partial purification of crude glycerol

stream should be done, especially to reduce the negativeimpact on catalyst and reactor material under supercritical

conditions. Thus, in this study it is assumed that de crude

glycerol is pretreated to remove high salt (it may be very

corrosive) and free fatty acid content as well as methyl esters

impurities (these two latter may be converted into undesired

tar and coke [10]), e.g., by neutralization. The salt formed

during this phase may be recovered for use as fertilizer, after

precipitation and filtration. Furthermore, the methanol may

be stripped from thisstreamto be recoveredand reused. Thus,

this pre-treatment would produce a stream with >80 wt%

pure glycerol. Therefore, a pretreated crude glycerol consists

of glycerol (80 wt%), methanol (20 wt%) and no waterhas been

considered in this study. The possible water content in thepretreated crude glycerol feed is included inside the water

stream, and thus an anhydrous crude glycerol feed is referred

in this work.

The upper concentration of MeOH has been limited to 30 wt

% in the crude glycerol, as a realistic value, since the MeOH

recovery is essential for the economy of a biodiesel production

plant.

5. Results and discussion

A sensitivity analysis has been carried by varying operating

pressure and temperature as well as the glycerol feed

concentration for both the pure and the pretreated crude

glycerol. In addition, for this latter, a sensitivity analysis has

been performed for the MeOH content in the crude glycerol,

as a measure of the glycerol purity of crude feed. The total

molar flow-rate fed to the system is always 1000 mol/h. The

analyses were carried out over the following variable ranges:

temperature of 400e1000 C, glycerol to feed (glycerol plus

water) mole ratio of 1e

16 (990 mol/h water and 10 mol/hglycerol e 840 mol/h water and 160 mol/h glycerol) and

pressure ranging from 200 to 300 atm, for both pure and

pretreated crude glycerol. MeOH content in the crude glycerol

was changed from 10 to 30 wt%. Selectivity results are

analyzed for the reactor outlet stream (4) and the hydrogen

yield is referred to this stream as well as to the outlet stream

of the WGS reactor (stream SG2), so as to include the global

process.

In the simulation results, the contents of ethane,

propane and ethanol have been grouped as the content in

organic carbon different of methane (namely Others), due to

in all the cases the concentration of those compounds at

the reactor outlet are quite lower than those correspondingto the other compounds. In addition, at glycerol to feed

mole ratios used from 1 to 16, carbon formation is predicted

to be thermodynamically inhibited at any temperature

analyzed in this study, due to the high proportion of water,

and so the carbon (graphite) production is insignificant and

not shown.

5.1. Effect of the reaction temperature

Reaction temperature is perhaps the most important param-

eter that influences the performance of SCW reforming of

glycerol. It is expected to have a significant effect on the

process yield, especially in absence of catalyst[15]. To avoidthe use of a catalyst, which becomes essential for low

temperature processes, the present study only considers high

temperatures. So, the uncatalyzed reforming will need high

temperature (500e800 C, and even more), although it is less

efficient than that at low temperature from the energy and

exergy point of view, since external energy may be needed to

sustain the process.

The overall reforming yield depends on chemical reactions

involved and their rate. The product gas composition would be

governed by the chemical equilibrium of the reactions

involved. Since the reaction rate constant would increase with

temperature, the overall reforming yield would be higher and

its rate of increase with time would also increase as temper-ature does so. On the other hand, the reaction of complete

conversion of glycerol to hydrogen is endothermic while the

reaction that completely converts glycerol to methane is

slightly exothermic. Thus, on equilibrium conditions and

according to the Le Chatelier principle, the methane forma-

tion increases at lower temperature. CH4 competes against H2,

and obviously, CH4is not a desirable product if the ultimate

goal is hydrogen production. As a consequence, it is clear that

high reforming temperatures are recommended.

Since the minimum operating temperature is the critical

(374 C), the effect of the temperature has been analyzed on

the equilibrium of the different reactions taking place in the

reactor by varying in increments of 50 C, between 400 and

Table 1e Specifications of the components for thesimulation.

Code Equipment Specifications

B1 Mixer

PUMP Pump Outlet pressure: Variable

HEAT1 Heat exchanger Pressure drop: 0.2 atm.

Outlet temperature VariableR1 Reactor Inlet temperature: Variable

Outlet temperature Inlet

temperature

Pressure drop: 0.22 Pa

B2 Expander Isentropic turbine

Outlet Pressure: 1.4 atm.

COOL1 Heat exchanger Outlet temperature: 200 C

Pressure drop: 0.2 atm.

R2 Reactor Inlet temperature: 200 C

Outlet temperature Inlet

temperature

Reaction: H2O CO H2 CO2COOL2 Heat exchanger Outlet temperature: 60C

Pressure drop: 0.2 atm.

SEP Gase

Liquidseparator Outlet temperature Inlettemperature 60 C


8/13/2019 1-s2.0-S0360319911009608-main_2

9/20

1000 C. The pressure was kept at 240 atm and the glycerol

concentration in the feed was always 1 mol % (5 wt%).

Results are shown inFigs. 9 and10relative to streams 4 and

SG2 (outlet of the reformer and WGS reactor respectively) for

both pure and pretreated crude glycerol. These figures

represent the equilibrium mole fraction of the gaseous

products in dry basis as a function of temperature. For the

pure glycerol, it is depicted that the mole fraction ofhydrogen increases with the increase in the temperature up

to a maximum value and then it is approximately constant

(about 0.689) in R1, and 0.696 at the outlet of R2, very next to

maximum value according to the stoichiometry of the reac-

tion(3), as CH4and CO are insignificant, due to the operating

conditions that does not promote the methane production

and to the huge excess of water, which consumes near all of

carbon monoxide. In addition, the reaction (3) is endo-

thermic, so it is very shifted to the right side at high

temperatures.

overall glycerol reforming:

C3H5OH33H2O 3CO2 7H2 (3)glycerol decomposition:

C3H5OH3 3CO 4H2 (3a)

wateregas shift:

CO H2O CO2 H2 (3b)

As expressed by reactions (3a) and (3b), overall glycerol

reforming may be represented as a first stage of glycerol

decomposition followed by a wateregas shift reaction.

In Figs. 9 and 10, it can be also observed that the mole

fraction of CH4 decreases with the increase in the tempera-

ture, mainly because the high temperatures favors its thermaldecomposition forming hydrogen and CO2in the presence of

water, as it can be seen in the reaction(4).

methane steam reforming:

CH4 H2O 3H2 CO (4)

Likewise, the mole fraction of CO2 decreases with the

increase in the temperature because at high temperature the

reaction between CO2 and CH4 is promoted thus producing CO

and H2, on the reaction(5).

methane dry reforming:

CO2 CH4 2H2 2CO (5)

At higher temperatures, this endothermic reaction is favoredand therefore CO2produced is after consumed. Indeed, reac-

tions(3)e(5)are endothermic so an increase in temperature

will provide a shift of the equilibrium toward the products

(right side of reactions).Table 2shows the reaction enthalpy

for these three reactions(3)e(5), obtained for both standard

conditions (298 K and 1 atm) and nominal operating condi-

tions (800 C and 240 atm).

The rest of organic compounds (grouped in Others) present

a concentration much lower than 0.01 mol %, so these

compounds can be considered as insignificant and be

neglected. As a result, the more likely expected products are

CO, CO2, CH4and H2. In addition, CO will practically disappear

in the watere

gas shift (WGS) reactor (R2).

The reactions of methanol reforming(6) and decomposi-

tion(7)are the following:

methanol reforming:

CH3OH H2O CO2 3H2 (6)

methanol decomposition:

CH3OH CO 2H2 (7)

For pretreated crude glycerol reforming the results are very

similar to those obtained for pure glycerol, especially relative

to the mole fraction of hydrogen, although the mole flow-rates

are lower according to reactions(3) and (6).

On reactions(3) and (6), hydrogen yield are computed for

pure and pretreated crude glycerol as follows (Eq.(8) and (9)):

hpure mol=h H2

mol=h C3H5OH3

17

(8)

hcrude mol=h H2

mol=h C3H5OH37 mol=h CH3OH 3 (9)

Thus,Figs. 9 and 10also illustrate the hydrogen yield in the

reforming reactor (R1) and for the overall process. The glycerol

and methanol conversion were always 100%, at equilibrium

condition. Between 750and 800 C, thehydrogenyieldbecomes

approximately constant and equal to 95%, at the R1 outlet,

achieving99% in theWGS reactor at 200 C and1 atm. Likewise,

a maximum value is reached for 900 C in R1 (95.9%), and the

hydrogen content scarcely changes for temperatures greater

than 900 C. Anyway, for a practical purpose, beyond800 Cthe

yield can be considered as constant, and thus a temperature

higher than 800 C would be unnecessary. For the pretreated

crudeglycerol, thehydrogen yieldshould be referred tothe sum

of glycerol and methanol, so as to avoid to get efficiencies

higher than 100%. With respect to the pure glycerol, for pre-

treated crude glycerol relatively higher hydrogen yields are

obtained although hydrogen production (molar flow-rate) is

lower, due to the methanol presence, which reforms better but

produces less hydrogen than glycerol.

A case study was performed (but not shown) by consid-

ering a feed with a 16 mol % of pretreated crude glycerol with

a 10 wt% methanol, and very low mole fraction of hydrogen as

well as very high increase in methane production were ob-

tained. Moreover, it was verified that at temperatures lower

than 750 C, hydrogen yield is lower than 20%, and almost the

main production is methane and carbon dioxide.

5.2. Effect of glycerol concentration in the feed

An interval from 1 to 16 mol % (about 5e50 wt%) of glycerol

concentration in the feed, with water being the rest, has been

studiedat240atmand800 C.Forthepretreatedcrudeglycerol,

20 wt% is methanol. Results are represented in Fig. 11 and 12,

which depict gas concentration at the outlet of the reforming

reactor (R1) and the WGS reactor (R2) as a function of glycerol

feed concentration for pure and pretreated crude glycerol. It

can be observed that the mole fraction andthe molar flow-rate

of methane increase as well as the mole fraction of hydrogen

decreasesand itsmolarflow-rate increases as the glycerol feed

concentration rises. This is an expected result since a higher


8/13/2019 1-s2.0-S0360319911009608-main_2

10/20

Fig. 9 e Mole fraction (a), molar flow-rate (b) of the outlet gases and hydrogen yield (c) for a pure glycerol concentration in the

feed of 1 mol % at 240 atm. Streams 4 and SG2.


8/13/2019 1-s2.0-S0360319911009608-main_2

11/20

Fig. 10e Mole fraction (a), molar flow-rate (b) of the outlet gases and hydrogen yield (c) for a crude glycerol (20 wt% MeOH,

80 wt% C3H8O3) concentration in the feed of 1 mol % crude glycerol at 240 atm. Streams 4 and SG2.


8/13/2019 1-s2.0-S0360319911009608-main_2

12/20

glycerol concentration lessensthe shift towardthe right side of

reaction(3) due to the lower water surplus. Accordingly, the

reforming reaction to produce hydrogen compete against the

methanation reactions, in such a way that the produced

hydrogen is partiallyconsumed by reacting with CO andCO2 to

produce CH4, as shown in reactions(10) and (11):

methanation of CO:

CO 3H2 CH4 H2O (10)

methanation of CO2:

CO2 4H2 CH4 2H2O (11)

The temperature also affects these two equilibria, moving

them to the left side when the temperature increases, because

of reactions(10) and (11) are exothermic. Thus, CH4 produc-

tion is inhibited by operating at high temperatures, as

aforementioned.

Mole fractions of CO and CO2increase as glycerol concen-

tration increases in the feed, although not as much as CH4.

Anyway, CO2mole fraction is quite higher than that of CO due

thewateregas shiftreaction, which promotes the reaction(3b)

leading thus to CO2 production. Results for pretreated crude

glycerol are qualitatively similar to those obtained for pure

glycerol, and values of mole fraction for the product gases are

practically the same. The difference is found in the molar flow-

rate that is lower for a crude glycerol with higher content in

MeOH, since the number of carbon,hydrogen andoxygenatoms

is lower in the methanol for a given total feed molar flow-rate.

Moreover,Figs. 11 and 12also illustrate that higher meth-

anol content in the crude glycerol relatively enhances the

hydrogen yield defined by Eq.(9), although hydrogen produc-

tion (molar flow-rate) is lower. If instead of using Eq.(8) and

(9), the hydrogen yield is expressed by the produced H2molar flow-rate to (pure or crude) glycerol feed molar flow-

rate ratio, the values obtained for pure glycerol would be

higher than those for pretreated crude glycerol. Thus, with

regarding to R1 reactor, and in case of feeding pure glycerol,

6.66 mol of H2are produced per mol of glycerol for a glycerol

concentration of 1 mol % (5 wt%), while at 16 mol % (49 wt%)

glycerol, the number of H2moles produced per mol of glycerol

decreases to 1.44. For the crude glycerol with 20 wt% meth-

anol, the ratios are 5.17 and 1.39 mol of H2are produced per

molof pretreated crude glycerol, respectively. When the yields

are referred to R2 reactor, the ratios are 6.86 and 1.99 for pure

glycerol; 5.90 and 1.66 for crude glycerol, respectively.

Therefore, the process moves far from the optimum for

hydrogen production as the glycerol feed concentration

increases, but for low glycerol feed concentration the

unreacted water (as steam) dilutes hydrogen product. These

issues should be further inspected for an energy analysis of

the process, as it will be done in the next work.

Because of the high CO2content in the syngas produced,

the dry reforming of methane (reaction(5)) will also proceed.

Dependent upon the water to methane ratio and CO2content

in the syngas, CH4is more or less reformed with H2O or CO2.

Thereby, the equilibrium of the wateregas shift reaction and

H2and CO mole fraction in the synthesis gas are affected.

5.3. Effect of the methanol concentration in the crude

glycerol

Fig. 13 shows how the increase in MeOH concentration

(decrease in C3H5(OH)3 purity) of the crude glycerol leads to

a reduction of the molar flow-rate of hydrogen and carbon

dioxide for streams 4 and SG2. This effect is stronger if the

crude glycerol feed concentration was higher, in such a way

that hydrogen produced from the reforming of methanol is

less than that obtained from glycerol reforming, on reactions

(3) and (6). Fractions of MeOH in the crude glycerol should be

controlled at low values, not because of inefficient methanolreforming, but to save it thus recovering and returning to the

biodiesel production process.

An increase of methanol fraction in crude glycerol makes

the concentration of CO and CO2decrease very slightly as well

as does with the molar flow-rates of those compounds. There-

fore, the concentrations of carbon monoxide and carbon

dioxide are not practically affected by the variation of MeOH in

the feed concentration. The simulation results showed good

agreement with the theoretical results obtained by previous

studies regarding with the SCW reforming of methanol [20,21].

Moreover, in order to contrast the effect of methanol by mini-

mizingthe effectof thehigh water dilution,another simulation

was also performed (but notillustratedin a figure)using 16 mol

% pretreated crude glycerol with variable MeOH content at

800 C and240 atm. Apart from thelow mole fraction andyields

of hydrogenas wellas highproductionof methane, as expected,

the effect of MeOH as such was disclosed as insignificant (flat

curves) for the range tested (10e30 wt% MeOH).

5.4. Effect of the reaction pressure

According to some authors, the effect of pressure is negligible

in the supercritical region [14,21,22]. However, when the

pressure is below thecritical pressure, the special physical and

chemical properties of water disappear. Likewise, higher

pressure causes trouble for the design and maintenance of the

system andalso increases the operating andinvestmentcosts.

Forit,apressurerangefrom200to300atmhasbeenconsidered

by taking increments of 20 atm. By this way, the process is

Table 2e Reaction enthalpies for the main reactions taking place inside the reforming reactor under standard (298.15 K,1 atm) and operating (1073.15 K, 240 atm) conditions.

Reaction Enthalpy standard cond. (kJ/mol)(25 C, 1 atm)

Enthalpy operating cond. (kJ/mol)(800 C, 240 atm)

C3H8O3 3H2O43CO2 7H2 358.01 192.42

CH4 2H2O44H2 CO2 252.85 197.64

CO2 CH44

2H2 2CO 247.00 260.87


8/13/2019 1-s2.0-S0360319911009608-main_2

13/20

compared under subcritical conditions and the possible

advantagesof increasing the operating pressure are evaluated,

from the thermodynamic point of view. The temperature was

kept in 800 C and the glycerol concentration in the feed has

been of 1 mol%. The simulation for crude glycerol is almost

equal to that of pure glycerol, and it has not been included in

this paper.Fig. 14depicts the results of the simulation, for the

pure glycerol. In general, it can be observed that the pressure

barely affects the gas composition at the reformer outlet, even

for subcritical region. H2, CO2, CO and CH4change all of them

Fig. 11e Mole fraction (a), molar flow-rate (b) of the outlet gases and hydrogen yield (c) as a function of the glycerol

concentration in the feed (pure glycerol) at 800 C and 240 atm. Streams 4 and SG2.


8/13/2019 1-s2.0-S0360319911009608-main_2

14/20

very slightly. Consequently, hydrogen yield and glycerol

conversion are almost constant for the overall range of pres-

sures tested. Thus, 240 atm can be considered as a suitable

pressure, taking into account that it is convenient to have

a margin over the critical pressure (218 atm) thus accounting

for pressure drops in the equipment and through the pipes.

However, values slightly lower than 240 atm should be exper-

imentally inspected.

Fig. 12 e Mole fraction (a) and molar flow-rate (b) of the outlet gases and hydrogen yield (c) as a function of the glycerol

concentration in the feed (crude glycerol, 20 wt% MeOH and 80 wt% C3H8O3) at 800 C and 240 atm. Streams 4 and SG2.


8/13/2019 1-s2.0-S0360319911009608-main_2

15/20

5.5. Selectivity computation

Finally, in order to assess the reactions taking place inside

the reforming reactor (R1), selectivity to main compounds

(H2, CO, CO2 and CH4) were computed, for both pure and

pretreated crude glycerol, just at the reforming reactor outlet

(stream 4). For the pure glycerol, the selectivity computation

to component x was made by using Eq. (12), where the

Fig. 13e Mole fraction (a) and molar flow-rate (b) of the outlet gases and hydrogen yield (c) as a function of the methanol

concentration (wt%) in the feed (crude glycerol, 1 mol %) at 800 C and 240 atm. Streams 4 and SG2.


8/13/2019 1-s2.0-S0360319911009608-main_2

16/20

calculation for H2and CO2are shown as examples (Eq.(13)).

This equation was modified for hydrogen by referring it to

CO2in reaction(3)that produces 7 mol of H2per each 3 mol of

CO2.

Sx no:C atoms inxno:total C atoms

stream 4

(12)

SCO2 nCO2

nCO nCO2 nCH4 100 andSH2

nH2

nCO nCO2 nCH4 17=3 100 (13)

For a crude glycerol, a revised equation of selectivity to

hydrogen was derived from reactions (3) and (6) and then

used, as follows (Eq.(14)):

SH2 nH2

nCO nCO2 nCH4

xC3H8O3

7=3

xCH3OH3=1

100 (14)

where xC3H8O3 and xCH3OHare the mole fraction of glycerol and

methanol in the feed, respectively. Thus, it is weighted the

mole ratio between H2and CO2in the reforming of glycerol (7/

3) and methanol (3/1), on reactions (3) and (6).

First, the selectivity analysis was performed for a nominal

glycerol feed concentration of 1 mol% at 240 atm, and

temperature was changed from 400 to 1000 C. Then, the

glycerol concentration was changed from 1 to 16 mol %, at800 C and 240 atm.

InFig. 15, it can be observed as, for a pure glycerol feed,

hydrogen selectivity increases with the increase in the reac-

tion temperature, and the methane selectivity decreases.

Fig. 14e

Mole fraction of the outlet gases for a glycerol concentration in the feed (pure glycerol) of1 mol% glycerol at 240 atmand an operating temperature of 800 C. Stream 4.

Fig. 15 e Selectivity of H2, CO, CO2and CH4as a function of the temperature for a glycerol concentration in the feed (pure

glycerol) of 1 mol% glycerol at 240 atm and an operating pressure of 240 atm.


8/13/2019 1-s2.0-S0360319911009608-main_2

17/20

Beyond 800 C, CH4production is insignificant. For a temper-

ature higher than 750 C, CO2selectivity begins to decrease,

probably due to the reformation of CH4 with CO2following the

reaction(5). Similar trends are observed for pretreated crude

glycerol (Fig. 16), and main differences between this latter and

pure glycerol are in selectivity to hydrogen, about 850e900 C:

93.1 and 95.9%, respectively.

Fig. 17 and 18 illustrate the decrease in selectivity to

hydrogen as well as the rapid increase of selectivity to

methane when the glycerol feed concentration is increased.

Beyond the glycerol concentration of 8 mol %, CH4selectivity

becomes higher than H2 selectivity, for pure glycerol. So, if the

product desired is hydrogen the glycerol feed concentration

should be reduced as much as possible. Likewise, CO2

selectivity decreases as glycerol feed concentration rises

while CO selectivity increases to reach a maximum at about

2e3 mol % pure glycerol and then decreases to 20%, remain-

ing constant in this value from about 6 to 16 mol %, due to the

reforming reaction is inhibited and promoted the methana-

tion reactions (8) and (9). For pretreated crude glycerol,

selectivity to CO rises to maximum (12.3%) at 2.8 mol % pure

glycerol, remaining constant for higher concentration of

glycerol.

Finally, selectivity evolution versus pressure was also

studied but it was verified a lack of dependence between both

variables, and so the simulation results are not shown.

Fig. 19depicts a very weak effect of the methanol concen-

tration in the crude glycerol for all the compounds. Although

Fig. 16e Selectivity of H2, CO, CO2and CH4as a function of the temperature for a glycerol concentration in the feed (crude

glycerol, 20 wt% MeOH and 80 wt% C3H8O3) of 1 mol % crude glycerol at 240 atm.

Fig. 17 e Selectivity of H2, CO, CO2and CH4as a function of the glycerol concentration in the feed (pure glycerol) at 800C and

240 atm.


8/13/2019 1-s2.0-S0360319911009608-main_2

18/20

this figure shows the simulation results obtained by using

1 mol % crude glycerol, this is not due to the high proportion

of water because another simulation for 16 mol % glycerol

feed concentration was performed (but not shown) where

the concentration of MeOH was changed from 10 to 30 wt%

and no noticeable effect due to MeOH concentration was

observed.

5.6. Optimal conditions for maximum hydrogen

production

The maximum hydrogen yield is achieved at 900 C and for

a glycerol feed concentration of 1 mol %, combining thus low

carbon monoxide and methane yields. Carbon monoxide and

methane are considered undesirable products. CO affects the

overall size of the reforming process, especially the water-

egas shift reactors, and CH4 contains hydrogen, decreasing

thus the overall hydrogen production. The higher the mole

ratio of water to glycerol, the higher mole fraction of

hydrogen (in dry basis) is. The carbon monoxide is nearly

converted in the WGS reactor. However, the energy

consumption and the size of the units increase as water flow-

rate increases.

All the thermodynamic analyses agree well with the few

experimental results found in the literature [22], where at

dilutefeed concentration(5 wt%glycerol), 6.5mol of hydrogen/

Fig. 18e

Selectivity of H2, CO, CO2and CH4as a function of the glycerol concentration in the feed (crude glycerol, 20 wt%MeOH and 80 wt% C3H8O3) at 800 C and 240 atm.

Fig. 19 e Selectivity of H2, CO, CO2and CH4as a function of the methanol concentration (wt%) in the feed (crude glycerol,

1 mol %) at 800 C and 240 atm.


8/13/2019 1-s2.0-S0360319911009608-main_2

19/20

mol of glycerol was obtained at a temperature of 800 C and

pressure of 241 bar, over Ru/Al2O3catalysts in a tubular fixed-

bed flow reactor. Therefore, if the real yields come close to

those calculated assuming equilibrium is because of a right

approach to equilibrium in real reactors is achieved.

6. Conclusions

Thermodynamic equilibrium calculations were done by

minimizing Gibbs free energy using the predictive Soa-

veeRedlicheKwong method in AspenPlus, after evaluating

against other equation of state based methods and discussing

about the simulation of the supercritical state. The aim was to

identify the operating conditions that maximize hydrogen

production from a mixture of water and glycerol. The effects

of reaction temperature and pressure as well as the glycerol

feed concentration have been studied for pure glycerol and

pretreated crude glycerol consisting mainly of glycerol and

methanol, by varying this latter from 10 to 30 wt%. The

reforming reactor and the watere

gas shift reactor work atisothermal condition.

From a thermodynamic point of view, and under equilib-

rium conditions, the best conditions to optimize hydrogen

production are 900 C and 1 mol % glycerol in the feed. By this

way, a hydrogen yield of about 95% for the pure glycerol and

97.2% for pretreated crude (20 wt% methanol) glycerol in the

reforming reactor are achieved. These values rise to 99.7% and

99.9% in the wateregas shift reactor, respectively. However,

compared to pure glycerol, the use of crude glycerol to produce

hydrogen gives a lower performance regarding with the molar

flow-rate of hydrogen produced in the reformingreactor dueto

the lower number of atoms present in methanol, i.e., the

hydrogen production per mol of (pure or crude) glycerol ishigher for pure glycerol. The water to crude glycerol ratio is

a key factor to reduce the CO content in the WGS reactor, and

the hydrogen yield decreases and methane productionrises as

theglycerol feed concentrationincreases, so a highwaterflow-

rate is required. The operating pressure does not affect the

results in the studied range (200e300 atm), and the analysis

showsthatasuitableoperationpressuremaybeabout240atm.

For practical purposes, it is recommended to operate at

temperatures from 750 to 800 C, depending on the hydrogen

yield specified, since the glycerol conversion is total in all the

range. Operating at not too high temperatures reduces the

energycostandextendsthedurabilityofmaterials.Thisisvery

important since under operating conditions so severe (combi-nation of high pressure and temperatures) the special mate-

rials required significantly increase the cost of the plant.

The next work will consist of a conceptual design of the

overall process including an energy and exergy analysis of

the SCW reforming of glycerol. Likewise, a facility is being

assembled to test the performance of this process. One of

the foreseeable features is that the catalyst will not be

strictly necessary. This is an important issue to be checked

since, so far, it is crucial to identify catalysts and design

reactors that maximize the yields of desired products and

minimize undesired by-products formed in series and/or

parallel reaction pathways. Thus, next the process will be

tested by a tubular reactor with and without using catalyst.

In addition, it will be assessed the effect of reaction time by

changing the feed flow-rate and performed studies on

chemistry kinetics.

Acknowledgment

This research is supported by the Science and Technology

Ministry of Spain under the research project ENE2009-13755,

as a Project of Fundamental Research inside the framework

of the National Plan of Scientific Research, Development and

Technological Innovation 2008e2011.

r e f e r e n c e s

[1] Czernik S, French R, Feik C, Chornet E. Hydrogen by catalyticsteam reforming of liquid byproducts from biomassthermoconversion processes. Ind Eng Chem Res 2002;41:

4209e15.[2] Adhikari S, Fernando S, Haryanto A. Production of hydrogen

by steam reforming of glycerin over alumina-supportedmetal catalysts. Catal Today 2007;129:355e64.

[3] Cortright RD, Davda RR, Dumesic JA. Hydrogen from catalyticreforming of biomass-derived hydrocarbons in liquid water.Nature 2002;48:964e7.

[4] Dauenhauer PJ, Salge JR, Schmidt LD. Renewable hydrogenby autothermal steam reforming of volatile carbohydrates.

J Catal 2006;244:238e47.[5] Douette AMD, Turn SQ, Wang W, Keffer VI. Experimental

Investigation of hydrogen production from glycerinreforming. Energ Fuel 2007;21:3499e504.

[6] Savage PE. Organic chemical reactions in supercritical water.Chem Rev 1999;99:603e21.

[7] Japas ML, Franck EU. High pressure phase equilibria andPVT-data of the water-nitrogen system to 673K and 250 MPa.Ber Bunsenges Phys Chem 1985;89:793e800.

[8] Seward T, Franck E. The system of hydrogen-water up to440 C and 2500 bar pressure. Ber Bunsenges Phys Chem1981;85:2e7.

[9] Peterson AA, Vontobel P, Vogel F, Tester JW. In situvisualization of the performance of a supercritical water saltseparator using neutron radiography. J Supercrit Fluid 2008;43:490e9.

[10] Calzavara Y, Dubien CJ, Boissonnet G, Sarrade S. Evaluationof biomass gasification in supercritical water process forhydrogen production. Energ Convers Manage 2005;46:615e6731.

[11] Feng W, van der Kooi HJ, de Swaan A. Biomass conversions

in subcritical and supercritical water: driving force, phaseequilibria, and thermodynamic analysis. Chem Eng Process2004;43:1459e67.

[12] Holderbaum T, Gmehling J. PSRK, A group contributionequation of state based on UNIFAC. Fluid Phase Equilib 1991;70:251e65.

[13] Gmehling J. Present status and potential of groupcontribution methods for process development. J ChemThermodyn 2009;41:731e47.

[14] Matsumura Y, Minowa T, Potic B, Kersten SRA, Prins W, vanSwaaij WPM, et al. Biomass gasification in near- and super-critical water: status and prospects. Biomass Bioenerg 2005;29:269e92.

[15] Lu Y, Guo L, Zhang X, Yan Q. Thermodynamic modeling andanalysis of biomass gasification for hydrogen production in

supercritical water. Chem Eng J 2007;131:233e44.


8/13/2019 1-s2.0-S0360319911009608-main_2

20/20

[16] Aspen Technology Inc.,http://www.aspentech.com.[17] Chiu CW, Goff MJ, Suppes GJ. Distribution of methanol and

catalysts between biodiesel and glycerin phases. AIChE J2005;51:1274e8.

[18] Valliyappan T, Ferdous D, Bakhshi N, Dalai AK. Production ofhydrogen and syngas via steam gasification of glycerol ina fixed-bed reactor. Top Catal 2008;49:59e67.

[19] Dou B, Dupont V, Williams PT, Chen H, Ding Y.

Thermogravimetric kinetics of crude glycerol. BioresourTechnol 2009;100:2613e20.

[20] Tang H, Kitagawa K. Supercritical water gasification ofbiomass: thermodynamic analysis with direct Gibbs freeenergy minimization. Chem Eng J 2005;106:261e7.

[21] Voll FA, Rossi CC, Silva C, Guirardello R, Souza RO,Cabral VF, et al. Thermodynamic analysis of supercriticalwater gasification of methanol, ethanol, glycerol,glucose and cellulose. Int J Hydrogen Energ 2009;34:9737e44.

[22] Byrd AJ, Gupta RB, Pant KK. Hydrogen production from

glycerol by reforming in supercritical water over Ru/Al2O3catalyst. Fuel 2008;87:2956e60.

http://www.aspentech.com/http://dx.doi.org/10.1016/j.ijhydene.2011.04.095http://dx.doi.org/10.1016/j.ijhydene.2011.04.095http://dx.doi.org/10.1016/j.ijhydene.2011.04.095http://dx.doi.org/10.1016/j.ijhydene.2011.04.095http://www.aspentech.com/

Date post:	04-Jun-2018
Category:	Documents
Upload:	aquared-lexus
View:	217 times
Download:	0 times

1-s2.0-S0360319911009608-main_2

Documents