Biomass conversions in subcritical and supercritical water: driving force, phase equilibria, and thermodynamic analysis

Chemical Engineering and Processing 43 (2004) 1459–1467

Biomass conversions in subcritical and supercritical water: driving force, phase equilibria, and thermodynamic analysis Wei Feng∗ , Hedzer J. van der Kooi, Jakob de Swaan Arons Physical Chemistry and Molecular Thermodynamics, Delft University of Technology, Julianalaan 136, 2628 BL, Delft, The Netherlands Received 5 September 2003; received in revised form 26 January 2004; accepted 26 January 2004 Available online 12 March 2004

Abstract Two biomass conversion processes have been studied: hydrothermal upgrading (HTU) under subcritical water conditions; supercritical water gasification (SCWG) in supercritical water. For the design of the two biomass conversion processes, the following contributions of thermodynamics have been presented: phase behavior and phase equilibria in the reactor and separators; indication of the favourable operation conditions and the trends in product distribution for the conversion reactions; construction of heat exchange network and exergy analysis. A wide variety of fluids have been dealt with, from small molecules to large molecules, including non-polar and polar substances. The statistical association fluids theory (SAFT) equation of state has been applied to calculate the mass distribution in different phases and to estimate the entropy and enthalpy values for different mass streams. © 2004 Elsevier B.V. All rights reserved. Keywords: Hydrothermal upgrading; Supercritical water gasification; Driving force; Biomass conversion; The SAFT equation of state; Thermodynamic analysis

1. Introduction Biomass fuels are potential substitutes for fossil fuels. Biomass conversion is gaining more attention. To produce biomass fuels, pyrolysis, gasification, and liquefaction processes are under development. In this work, we pay attention to the hydrothermal upgrading (HTU) [1] and supercritical water gasification (SCWG) [2–4] processes. The HTU process is a promising liquefaction process, it can be used for the conversion of a broad range of biomass feedstocks, which is an advantageous characteristic. The process is especially designed for wet materials, the drying of feedstock is not necessary. Treated in water in a temperature range of 300–350 ◦ C and a pressure range of 120–180 bar, biomass is depolymerised to a hydrophobic liquid product so called “biocrude”. Gases are also produced, consisting of CO2 , H2 , methane and CO. Other product includes water and organic compounds. In supercritical water gasification, the reaction generally takes place at the temperature over 600 ◦ C and a pressure higher than the critical point of water. With temperature ∗

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higher than 600 ◦ C, water becomes a strong oxidant, and oxygen in water can be transferred to the carbon atoms of the biomass. As a result of the high density, carbon is preferentially oxidized into CO2 but also low concentrations of CO are formed. The hydrogen atoms of water and of the biomass are set free and form H2 . The gas product consists of hydrogen, CO2 , CH4 and CO. For the design of the reactor and separators, the information of phase equilibria is very important. To construct a heat exchange network in order to recover the process heat and further to perform a thermodynamic analysis, we need to know some thermodynamic properties, like enthalpy and entropy. For both processes, we have carried out following work: the calculation of driving forces for the conversion reactions to indicate the trends in product distribution and favourable operation conditions; the descriptions of phase equilibria in the reactor and separators; the thermodynamic analysis to see how to improve the process efficiency. 2. Statistical association fluid theory (SAFT) equation of state The SAFT equation of state [5] expresses the residual Helmholtz energy as a sum of segment, chain, and

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association terms. The general expression for the Helmholtz energy is given by ares = aseg + achain + aassoc    u i  η j 2  aseg 4η − 3η  = m + Dij RT kT τ (1 − η)2 i

3. Hydrothermal upgrading (HTU) process (3)

3.1. Biomass conversion under subcritical water conditions

(4)

In aqueous environment, in the temperature range of 300–350 ◦ C and pressures up to 180 bar [1], as shown in Fig. 1, biomass, such as wood, with a lower energy density is converted to biocrude with a higher energy density, organic compounds (OD) including mainly alcohols and acids, gases mainly including CO2 . Water is also a by-product. In the products, CO2 , the main component of the gas product, can be used to represent all gas produced, and methanol and ethanol represent organic compounds. However, it is impossible to know the biocrude exactly. The biocrude has to be characterized using the representative components.

The constants, Dij , can be found in reference [5]. The mole fraction of molecules not bonded at site A, XA , is expressed as −1

 A B AB X = 1 + NAV (5) ρX ∆ B

(6)

where g(d)hs and σ are calculated using the Eqs. (7) and (8)  13 6τυ00 σ= (7) πNAV g(d)hs =

1 − 0.5η (1 − η)3

(8)

In the above expressions, the reduced density is calculated by   3 −3u0 00 η = τρmυ 1 − C exp (9) kT and the temperature-dependent dispersion energy of interaction between segments, u/k, is expressed as u u0  e = 1+ (10) k k kT C and e/k are constants set to 0.12 and 10.0, respectively. For each fluid, three parameters are needed for a non-associating component, segment number m, segment volume υ00 , segment-segment interaction energy u0 /k. And two additional parameters are needed for an associating component, the association energy εAB /k and volume κAB . For mixtures, the following mixing rule [6] is used  uij  0   u i j Xi Xj mi mj kT (υ )ij =   (11) 0 k i j Xi Xj mi mj (υ )ij where

   3 1 0 1/3 + (υ ) (υ0 )13 (υ )ij = i j 2  u u 1/2 uij i j = (1 − kij ) k k k 0

water

(2)

A

where ∆AB , the association strength, is expressed as  AB ε ∆AB = g(d)hs exp − 1 (σ 3 κAB ) kT

(wood)

biocrude gases organic compounds(OC) water

Fig. 1. Biomass conversion in subcritical water.

j

achain 1 − 0.5η = (1 − m) ln RT (1 − η)3  aassoc XA 1 = ln XA − + M RT 2 2

biomass

(1)

300-350 0C 100-180 bar

(12) (13)

3.2. Characterizing and modeling of the biocrude The Biocrude produced from the HTU process has its own characteristics compared to fossil fuel, such as a higher oxygen content, being a mixture with a wide molecular weight distribution, consisting of various types of molecules, and being in solid state around room temperature. By vacuum flash operation, hundreds of components can be produced from the biocrude [7]. The information of elemental composition, molecular weight, molecular weight distribution [1,7] is very useful to determine the representative components for the biocrude. Combining the information on the biocrude mentioned above, methyl-n-propyl ether and polycarbonate (PC) polymers with different molecular weight are used to represent HTU biocrude. The elemental composition of PC agrees well with that of biocrude. The molecular weight of biocrude is distributed in a wide range, sometimes they could be larger Table 1 Representatives for the products from the HTU process Product

Component

Molecular weight

Weight fraction (%)

Biocrude

PC760 PC500 PC380 PC260 Methyl-n-propyl ether

760.00 500.00 380.00 260.00 74.123

7.5 10.0 17.5 12.5 2.5

Gas

CO2

44.010

25.0

Organic compounds

Methanol Ethanol

32.042 44.010

5.0 3.5

Water

Water

18.015

16.5

PC: polycarbonate.

W. Feng et al. / Chemical Engineering and Processing 43 (2004) 1459–1467 400

350

Pressure /bar

300

Exp. [8] Cal. SAFT Mw = 5000 T=235 ˚C

250

200

150

100

50 0.016

0.018

0.020

0.022

0.024

0.026

0.028

0.030

Weight fraction of CO2 Fig. 2. Swelling of polycarbonate using supercritical CO2 .

than 2000 depending on the operating conditions and the feedstocks. By changing the number of repeat units of PC, the molecular weight of modeled biocrude can be easily adapted to that of biocrude. In Table 1, the weight fraction of each component is assigned on the basis of the data of the vacuum flash of biocrude and the data of a pilot plant [7]. The products from HTU are biocrude, gases, and organic compounds such as alcohols and acids, and water. These products have the following characteristics: • They include non-associating and associating components. • Both small and large molecules are present. • Both vapour or liquid phases are present. The operating conditions are:

The selected thermodynamic model should accommodate the above. The SAFT equation of state is selected as the thermodynamic model to describe phase equilibria. The SAFT model needs three parameters for a non-associating component, segment number m, segment volume υ00 , segment-segment interaction energy u0 /k, and two additional parameters for an associating component, the association energy εAB /k and volume κAB . By fitting the binary data of polycarbonate/CO2 [8] and pressure-volume-temperature (PVT) data of PC [9] to the SAFT model, the parameters of polycarbonate are determined. The segment number m has a relationship with molecular weight (Mw) of m = 0.0353 Mw, u0 /k is equal to 244.70, and υ00 is set to 12.0. With these parameters, the calculated relative deviation of liquid density for PC is 4.33% for 107 PVT data points. The binary data of polycarbonate/CO2 [8] and polycarbonate/ethanol [10] have been well correlated as shown in Figs. 2 and 3. As the molecular weights of PC are smaller than 1000 in Table 1, we make use of the relationship m = 0.0353 Mw to get the parameter u0 /k for each PC component with different molecular weight, by fitting the PVT data of PC. The parameters needed by SAFT are listed in Table 2. As shown in Fig. 4, the experimental results of vacuum flash operation for the biocrude have been reproduced well. 3.3. Calculation of the enthalpy and Gibbs energy for the conversion reaction The enthalpy and entropy can be calculated using Eqs. (14) and (15).  T id H = H0 + CPid dT + H R (14) 

• Temperatures up to 350 ◦ C. • Pressures up to 180 bar.

1461

S=

S0id

+

T0

T T0

CPid P + SR dT − R ln T P0

0.14

0.12

Pressure /bar

0.10

Exp. [10] Cal. SAFT Mw = 24000 T= 305.15K

0.08

0.06

0.04

0.02

0.00 0.00

0.02

0.04

0.06

0.08

Weight fraction of ethanol Fig. 3. Vapor–liquid equilibria for polycarbonate/ethanol.

0.10

(15)

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Table 2 SAFT parameters Component

Mw

m

υ00 (cm3 /mol)

u0 /k K

εAB /k K

κAB

Nass

PC760a

760 500 380 260 74.123 44.010 32.042 46.069 18.015 28.01 30.07 16.043 2.016 9000

26.828 17.650 13.410 9.178 4.069 1.417 1.776 2.457 1.179 1.221 1.941 1.000 1.0004 518.30

12.0 12.0 12.0 12.0 10.224 13.578 12.0 12.0 10.0 15.776 14.460 21.576 13.625 12.0

260.331 268.461 275.930 290.204 208.13 216.08 216.3 213.48 528.17 111.97 191.44 190.29 39.171 322.604

0 0 0 0 0 0 2714 2759 1809 0 0 0 0 0

0 0 0 0 0 0 0.04856 0.0292 0.01593 0 0 0 0 0

0 0 0 0 0 0 2 2 3 0 0 0 0 0

PC500a PC380a PC260a MPEb CO2 b Methanolb Ethanolb Waterb COb Ethaneb Methaneb Hydrogenc Woodb

MPE: methyl-n-propyl ether; Nassoc : number of association sites. a This work. b Huang [1]. c Kreglewski [11].

40

Vaporized portion of biocrude, wt %

where, CPid is the heat capacity of an ideal gas, H0id and S0id are the enthalpy and entropy of an ideal gas at 298.15 K and 1 atm, repectively. HR and SR are residual enthalpy and entropy, respectively, and R is gas constant. For small molecules, we can find the values of H0id and S0id , and the functions for CPid from literature [12,13]. As wood and polymers do not have a gas state, we have to estimate CPid , H0id and S0id using a group contribution method of Benson [14]. Table 3 lists the CPid funtions and the values of H0id and S0id . HR and SR can be calculated using Eqs. (16) and (17), respectively. res

∂(a /RT) HR = −T + (Z − 1) (16) RT ∂T ρ,xi

Exp. [7] Cal. by SAFT 32

700Pa 24

16

8

168

196

224

252

280

308

Temperature / ˚ C Fig. 4. Reproduced result for vacuum-flash operation of biocrude.

Table 3 Enthalpy, entropy and heat capacity of an ideal gas at the reference state Component

T (K)

CPid (J/mol K)

H0id (kJ/mol)

S0id (J/mol K)

Wooda PC760a PC500a PC380a PC260a MPEb Ethanolb CO2 b Methanolb Waterb COb Ethaneb Methaneb Hydrogena

298–800 298–800 298–800 298–800 298–800 298–1000 298–1000 273–1500 298–1000 273–1500 273–1500 298–1200 273–1500 273–1500

245.076 + 44.533T − 2.386 × 10−2 T 2 −238.394 + 4.170T − 2.15 × 10−3 T 2 −173.174 + 2.838T − 1.48 × 10−3 T 2 −140.74 + 2.178T − 1.14 × 10−3 T 2 −107.953 + 1.506T − 8 × 10−4 T 2 7.919 + 0.387T − 1.518 × 10−4 T 2 29.249 + 0.1663T − 4.99 × 10−5 T 2 26 + 43.5 × 10−3 T − 148.3 × 10−7 T 2 15.557 + 0.103T − 2.927 × 10−5 T 2 30.36 + 9.61 × 10−3 T +11.8 × 10−7 T 2 26.86 + 6.97 × 10−3 T − 8.20 × 10−7 T 2 7.1769 + 0.1672T − 5.1556 × 10−5 T 2 14.15 + 75.5 × 10−3 T − 180 × 10−7 T 2 29.07 − 0.836 × 10−3 T + 20.1 × 10−7 T 2

−45267.50 −1067.95 −711.41 −534.08 −354.87 −238.02 −253.3 −393.52 −201 −241.83 −110.53 −84.67 −74.87 0.0

41925.08 1515.41 1080.88 860.10 646.53 352.00 290.09 213.79 237.60 188.84 197.66 229.49 186.25 130.68

a b

Group contribution method of Benson [14]. References [12,13].

W. Feng et al. / Chemical Engineering and Processing 43 (2004) 1459–1467 Table 4 Product distribution

case 3

Wt.%

Biocrude OC + water CO2

SR = −T R

400



Case 1

Case 2

Case 3

49 21 30

50 25 25

51 29 20

∂(ares /RT) ∂T

ρ,xi

ares /RT + T

 + ln(Z)

(17)

Driving force, kJ/kg DB

Component

1463

CO2 OC + water 0

300

T = 330 C P = 150 bar case 2

case 1

200

The Gibbs energy can be calculated as Eq. (18). 'G = 'H − T 'S

15

(18)

20

25

30

35

weight percent, %

3.4. Driving force to indicate the product distribution The oxygen in biomass is removed as CO2 , water and organic compounds. A higher content of oxygen removed as CO2 is desirable, as the separation of CO2 is easier than that of water and organic compounds. Based on the balance of carbon, hydrogen and oxygen, three cases are considered as shown in Table 4, in order to see if CO2 is favourably produced. We achieve this goal by calculating the driving force for the conversion reaction. Driving force is the decrease in Gibbs energy on going from the reactants to the products, i.e. driving force = −'G. The reaction with larger driving force or lower Gibbs free energy has more chance to occur. As shown in Fig. 5, the less CO2 and more organic compounds and water produced, the larger is the driving force of the reaction. In Fig. 5, the case 3 has the largest driving force among the three cases at 330 ◦ C and 150 bar. As demonstrated in Fig. 6, by reducing the reaction pressure to 120 bar for the

Fig. 5. The effect of wt.% of CO2 and the mixture of OD and water on the driving force. 400 case 3 150 bar

Driving force, kJ/kg DB

As shown in Fig. 1, water is the reaction medium, it has been taken into account for the calculation of the reaction enthalpy and Gibbs free energy. The ratio of water to biomass in the reactant mixture is 2.8. On the reactant side, the enthalpy is that of the mixture of water and wood, on the product side, the enthalpy is that of the mixture of water and the products as listed in Table 1. The same approach has been used for the caculation of Gibbs free energy of the conversion reaction. The calculated reaction enthalpy at 330 ◦ C and 180 bar is about −850 kJ/kg biomass, the experimental value [1] is about −800 kJ/kg biomass.

case 2 120 bar

300 case 1 100 bar

CO2 OC + water 0

T = 330 C

200

15

20

25

30

35

weight percent, % Fig. 6. The effect of operating pressure on the driving force.

case 2 and 100 bar for the case 1, respectively, the driving forces of the cases 1 and 2 are approaching that of the case 3. Based on this result, we can say lower reaction pressure is favoured to produce more CO2 . 4. Phase equilibria and thermodynamic analysis Corresponding to the operation conditions in Fig. 7, the results of phase behavior and phase equilibria are shown in Table 5, which are predicted using the statistical association

Table 5 Results of phase behavior and phase equilibria Unit

Mass flow (kg/min) Vapor phase

Reactor (330 ◦ C, 180 bar) HPS (180 ◦ C, 100 bar) LPS (150 ◦ C, 1 bar) VFS (230 ◦ C, 0.01 bar)

Water phase

Biocrude phase

Others

Biocrude

Others

Biocrude

Others

Biocrude

73.70 0.00 24.51 24.51

2.22 0.00 2.54 29.83

548.46 561.67 0.00 0.00

74.42 1.29 0.00 0.00

37.84 24.63 0.00 0.00

23.36 96.49 27.29 66.76

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W. Feng et al. / Chemical Engineering and Processing 43 (2004) 1459–1467

Biomass

grinding

180 0C

pumping

180 0C

gas

MV

210 0C

80 0C

recycled water 180 0C

HE1

HE2

water + OD

HE3 200 0C HE4 275 0C

230 0C 0.01bar

Q1

VFS heavy biocrude

HPS Q2

Reactor L/L

180 0C 100bar

330 0C 180bar

gas+ water LPS

HE5

150 0C 1bar

hydrogen upgrading light biocrude

upgraded product

300 0C 120bar

Fig. 7. The heat exchange network of the HTU process: MV: mixing vessel; HE: heat exchanger; HPS: high-pressure separator; LPS: low-pressure separator; VFS: vacuum-flash separator.

fluid theory (SAFT) equation of state. For the vacuum-flash separator (VFS) and low-pressure separator (LPS), the operating temperatures and pressures are proposed in terms of the calculation results for separating the biocrude efficiently. The operating conditions for other units correspond to the conditions of a pilot plant. As shown in Fig. 7, a heat exchange network is constructed in order to heat up the feedstock stream to a temperature as high as possible by recovering the available process heat as much as possible. In the heat exchanger 1 (HE1), the heat exchanging occurs between the feedstock, the mixture of water and organic compounds, and the biocrude streams. The feedstock can be heated up to 80 ◦ C. After the heat exchanging between the liquid–liquid mixture from the reactor and the feedstock in HE2, the temperature of the feedstock can reach 180 ◦ C. The feedstock can be heated up to 200 ◦ C after HE3, further heated up to 275 ◦ C after HE4. Extra heat of Q1 is needed to heat the feedstock up to the reaction temperature of 330 ◦ C. Q2 is required to heat the biocrude stream to 230 ◦ C in the vacuum-flash separator. Based on the calculation, the heat exchanging in HE5 can make the light-biocrude temperature reach the desired temperature 300 ◦ C. Wood is assumed to be dry and ash free. The chemical exergy of wood and biocrude is calculated using Eq. (19) as that by Szargut [15]. Exch = β(CF − 9L ZH ) + bchw ZW

(19)

L is the enthalpy of water vaporization at 25 ◦ C, bchw the chemical exergy of water, ZW the water content. {1.0412 + 0.2160(ZH /ZC ) − 0.2499(ZO /ZC ) [1 + 0.7884(ZH /ZC )] + 0.045(ZN /ZC )} β= [1 − 0.3035(ZO /ZC )]

(20)

ZH , ZO , ZO , ZN are the elemental composition of hydrogen, oxygen, carbon and nitrogen, respectively, in biomass. CF is estimated by Eq. (20) as that in reference [16]. CF = 33.5(% C) + 142.3(% H) − 15.4(% O)

(21)

The biocrude-upgraded product includes water, C1–C4 hydrocarbons, naphtha, and gasoil [17]. The mixture containing 23 wt.% of anthracene, 36 wt.% of hexadecane, 20 wt.% of hexane, 8 wt.% of ethane, and 13 wt.% of water is used to represent the upgraded product. The values of chemical exergy for the representative components can be found in reference [15]. The calculated heat and chemical exergy flows are listed in Table 6. The required process heat is calculated after the construction of the heat exchange network, The electricity required is calculated on the basis of the data of a pilot plant [7]. The amount of hydrogen used for upgrading the light biocrude is calculated on the basis of the data [7]. We assume that all these energies are provided by biomass. The corresponding biomass exergy flows are calculated as listed in Table 6. Without taking into account the energy

W. Feng et al. / Chemical Engineering and Processing 43 (2004) 1459–1467

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Table 6 Heat and exergy flows in the HTU process

Input Output ηE

Heat flows, (kJ/kg DB)

Exergy flows (kJ/kg DB)

Q1

Q2

Wooda

Woodb

Woodc

Woodd

505

191

21800

920

2875

2803

HB

UP

12221

4778

59.85% (without taking into account the energy cost for the treatment of waste water)

HB: heavy biocrude; UP: upgraded product; ηE = exergy out/exergy in: exergy efficiency. a Feed stock. b Providing process heat by wood combustion with an efficiency of 75%. c Generating electricity by wood conversion with an efficiency of 40%. d Producing hydrogen with an efficiency of 40%.

for wastewater treatment, the calculated exergy efficiency is 59.85%.

The gas product consists of hydrogen (55.56%), CO2 (33.33%), CH4 (7.41%), and CO (3.70%) in mole fraction. Hydrogen is the target product. 5.2. Driving force for biomass conversion reaction in supercritical water For the calculation of the reaction enthalpy and the Gibbs free energy for the biomass conversion in supercritical water, the same approach as that in HTU has been used. The ratio of water to biomass in the reactant mixture is 4. In Fig. 8, the calculated driving force is based on Eq. (22). The operating pressure does not have much effect on the driving force of this reaction. The driving force increases with the reaction temperature. We assume two other cases as shown in Eqs. (23) and (24). In Eq. (23), less hydrogen is produced, while in Eq. (24), more hydrogen is produced, compared to that in Eq. (22). As shown in Fig. 9, at reaction temperatures 600 and 650 ◦ C and pressure of 350 bar, the case of Eq. (23) leads to the smallest driving force compared to other cases. That means producing other gas, like ethane, is not favoured. The maximum points have been indicated as the solid circles. The

1800

1600 600

650

700

750

Temperature / ˚C

Fig. 8. The effect of pressure and temperature on the driving force of the conversion reaction.

maximum driving force corresponds to the maximum production of hydrogen. C6 H10 O5 + 3.5H2 O = 4CO2 + 5.75H2 + CH4 + 0.5CO + 0.25C2 H6

(23)

C6 H10 O5 + 5.5H2 O = 5CO2 + 9.5H2 + 0.5CH4 + 0.5CO (24) 2000 600 ˚C, 350 bar 650 ˚C, 350 bar maximum

1900

Eq.(24)

(22)

2000

Eq.(22)

C6 H10 O5 + 4.5H2 O = 4.5CO2 + 7.5H2 + CH4 + 0.5CO

350 bar 240 bar

2200

Eq.(23)

We suppose that the feedstock contains 20% of cellulose and 80% of water on a weight basis. Cellulose is assumed to be completely converted into gas product at the temperature over 600 ◦ C and a pressure of 350 bar. Based on the experimental data [2–4], we have Eq. (22) to express the conversion reaction.

Dirving force, kJ/kg DB

5.1. Biomass conversion in supercritical water

Driving force, kJ/kg DB

5. Supercritical water gasification process

2400

1800

1700

1600

50

55

60

Mole percent of hydrogen in gas product, %

Fig. 9. The change of driving force with the hydrogen production.

65

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W. Feng et al. / Chemical Engineering and Processing 43 (2004) 1459–1467 fuel gas: CH4 + CO + CO2 +H2

H2

membrane SEP3

furnace

1 bar 25 0C

SEP1

25 0C 350 bar

cooler HE

100 0C 350 bar

SEP2

1 bar 25 0C

pumping

H2+CH4+CO+CO2+H2O

recycled water

H2O +CO2

reactor 600 0C 350 bar

recycled water

water pumping

feedstock

Fig. 10. Supercritical water gasification process.

Table 7 Heat and exergy flows in the SCWG process Heat flows (106 kJ/kmol biomass) 'HF Input Output ηE

Exergy flows (106 kJ/kmol biomass)

'HP

'HR

2.111

0.250

2.597

Biomassa

Biomassb

Biomassc

3.078

1.044

0.256

hydrogen 1.777

40.6%

'HF heat for heating feedstock; 'HP heat released from product stream; 'HR heat of reaction;ηE the exergy efficiency. a Cellulose as feedstock. b Providing process heat by wood combustion with an efficiency 75%. c Generating electricity by wood conversion with an efficiency 40%.

5.3. Thermodynamic analysis A schematic flow sheet is shown in Fig. 10. The operating conditions correspond to that of a pilot plant [4]. Before entering the reactor, the feedstock is heated up by the product stream via a heat exchanger. The conversion reaction is taking place at 600 ◦ C and 350 bar. The product stream, including fuel gas and water, is cooled down to 100 ◦ C after the heat exchanging. A preliminary separation of fuel gas and water occurs at 25 ◦ C in separator 1 (SEP1). Further separations take place in SEP2 and SEP3 at a low pressure of 1 bar. We assume that a membrane can make pure hydrogen by separating it from other fuel gas. As presented in Table 7, the process heats have been calculated, including that for heating up the feedstock, the heat released from product stream, and the heat of reaction. The exergy flows include that of the required biomass being

feedstock, providing process heat and generating electricity needed. The exergy efficiency is about 40.6%.

6. Conclusions For the HTU and supercritical water gasification processes, based on some limited data, we have applied a thermodynamic model, the SAFT equation of state, to describe phase behavior and phase equilibria and to calculate the enthalpy and entropy values, which are the basis of the thermodynamic analysis for the processes and the calculations of Gibbs energy for the conversion reactions. The driving-force results can give some indications for the trends in product distribution and favourable reaction conditions for the conversion reactions. Lower reaction pressure is favoured to produce more carbon dioxide in HTU. These results are

W. Feng et al. / Chemical Engineering and Processing 43 (2004) 1459–1467

useful information for the reactor design. The results of product separation have shown the possible route and operation conditions for separating the product efficiently. By performing an exergy analysis, we have obtained process efficiency. If taking into account the wastewater treatment in the HTU process, we can expect that both processes have comparable exergy efficiency about 40%, which is comparable to the non-biomass process. The combination of the calculations for product separation and heat recovery, allows to improve the process in terms of separating the product efficiently and recovering the process heat as much as possible.

Acknowledgements We acknowledge financial support of The Netherlands Science Foundation (NWO) within the scientific cooperation project between the Netherlands and Japan on biomass conversion.

Appendix A. Notation a CPid Dij DB g(d)hs G H H0id HR k kij m M NAV P P0 R S S0id SR T T0 u/k u0 /k υ0 υ00

Helmholtz energy heat capacity of an ideal gas square-well energy constants in Eq. (2). dry biomass hard sphere distribution function Gibbs free energy enthalpy enthalpy of an idea gas at reference state residual enthalpy Boltzmann constant binary interaction parameter in Eq. (11) segment number number of association sites on a molecule Avogadro’s number pressure pressure at reference state gas constant entropy entropy of an idea gas at reference state residual entropy temperature temperature at reference state temperature-dependent dispersion energy of interaction between segments (K) temperature-independent dispersion energy of interaction between segments (K) temperature-dependent segment volume (cm3 /mol) temperature-independent segment volume (cm3 /mol)

V wt X XA Z

1467

molar volume (cm3 /mol) weight fraction mole fraction mole fraction of molecules not bonded at site A compressibility in Eqs. (15) and (16)

Greek symbols κAB volume of interaction between sites A and B ∆AB strength interaction between sites A and B εAB /k association energy interaction between sites A and B η reduced density ρ density √ τ constant, 2π/6 References [1] F. Goudriaan, D. Peferoen, Liquid fuels from biomass via a hydrothermal process, Chem. Eng. Sci. 45 (1990) 2729–2734. [2] I. Lee, M. Kim, S. Ihm, Gasification of glucose in supercritical water, Ind. Chem. Eng. Res. 41 (2002) 1182–1188. [3] M.J. Antal, S. Allen, D. Schulman, X. Xu, R. Divilio, Biomass gasification in supercritical water, Ind. Chem. Eng. Res. 39 (2000) 4040–4053. [4] W. Prins, Pilot Plant of Supercritical Water Gasification, Personal Communication, 2003. [5] S.H. Huang, M. Radosz, Equation of state for small, large, polydisperse, and associating molecules, Ind. Chem. Eng. Res. 29 (1990) 2284–2294. [6] S.H. Huang, M. Radosz, Equation of state for small, large, polydisperse, and associating molecules: extensions to fluid mixtures, Ind. Chem. Eng. Res. 30 (1991) 1994–2005. [7] F. Goudriaan, Vacuum-flash Operation for Biocrude and a HTU Pilot Plant, Personal Communication, 2001. [8] S.M. Gross, R.D. Givens, M. Jikei, J.R. Royer, S. Khan, J.M. DeSimone, Synthesis and swelling of poly(bisphenol A carbonate) using supercritical CO2 , Macromolecules 31 (1998) 9090–9092. [9] P. Zoller, A study of the pressure–volume–temperature relationships for four related amorphous polymers: polycarbonate, polyarylate, phenoxy, and polysulfone, J. Polym. Phys. Edu. 20 (1982) 1453. [10] S. Hwang, J. Kim, K.P. Yoo, Vapor liquid equilibrium data of binary polymer solutions by vacuum electromicrobalance, J. Chem. Eng. Data. 43 (1998) 614. [11] A. Kreglewski, Equilibrium Properties of Fluids and Fluid Mixtures, AM University Press, College Station, Texas, 1984. [12] J.M. Smith, H.C. Van Ness, M.M. Abbott, Introduction to Chemical Engineering Thermodynamics, McGraw-Hill, New York, 1996. [13] G.M. Barrow, Physical Chemistry, third ed., McGraw-Hill, New York, 1973. [14] R.C. Reid, J.M. Prausnitz, B.E. Poling, The Properties of Gases and Liquids, McGraw-Hill, New York, 1987. [15] J. Szargut, D. Morris, F. Steward, Exergy Analysis of Thermal, Chemical, and Metallurgical Processes, Hemisphere, New York, 1988. [16] A. Demirbas, D. Gullu, A. Çaglar, F. Akdeniz, Estimation of calorific values of fuels from lignocellulosics, Energy Sources 19 (1997) 765. [17] F. Goudriaan, The Upgraded Product from Biocrude, Personal Communication, 2001.