problem 4 thermodynamics

8/3/2019 Problem 4 Thermodynamics

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Problem #4Chemical-Reaction Equilibria

Solution to Problem #4 No.1 and No.2

Rindang Isnaniar Wisnu Adji

Fransiska Citra Mariana

Dicka A.Rahim

Urly Agustina


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Table of ContentsNumber 1 ................................................................................................................................................ 2

Solution ............................................................................................................................................... 3

Synthesis gas preparation using methane .......................................................................................... 3

Steam methane reforming .............................................................................................................. 3

CO2 Methane reforming .................................................................................................................. 3

CPO (catalytic partial oxidation) ..................................................................................................... 4

Factors-factors affecting equilibrium .................................................................................................. 5

Effect of temperature ..................................................................................................................... 5

Effect of pressure ............................................................................................................................ 5

Catalyst used in the reaction .......................................................................................................... 5

Evaluating Catalyst Work on the Problem .......................................................................................... 5

Calculating K .................................................................................................................................... 6

Reason(s) not to change the catalyst ................................................................................................ 16

Reason(s) to change the catalyst ...................................................................................................... 16

Number 2 .............................................................................................................................................. 17

Solution ................................................................................................................................................. 17

Bibliography .......................................................................................................................................... 21


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Pemicu 4

Number 1Setelah bekerja selama lima tahun di PT RBL, sebagai process engineer, Temon dipromosikan

menjadi manager seksi gas sintesis. Baru-baru ini, ia dikunjungi oleh seorang perwakilan dari sebuah

perusahaan ang menawarkan mereka katalis baru mereka katalis oksidasi parsial disebut super-CPO.

Katalis oksidasi representatif yang dipromosikan ini memiliki keaktifan yang sangat tinggi.

Karena tertarik pada penawaran tersebut Temon berdiskusi dengan Sule, seorang enjiner muda

lulusan Universitas Indonesia Depok (UID) yang enerjik dengan kemampuan analitis yang tinggi.

Temon meminta Sule mengevaluasi keaktifan katalis Ni/Al2O3 yang digunakan PT RBL sehigga

keputusan mengenai penggantian katalis dapat dibuat. Temon juga memberikan Sule hasil tes

terbaru yang menunjukkan kinerja katalis mereka. Plot konversi dan selektivitas vs temperature

ditunjukkan pada Gambar 1.

Berdasarkan beberapa informasi berikut ini:

Batch katalis Ni/Al2O3 di dalam reactor adalah batch katalis baru. Biaya Ni/Al2O3 katalis sebanding dengan super CPO. Produk reaksi hanya gas sintesis dengan reaksi samping. Maka Sule merekomendasikan untuk tidak mengganti katalis yang telah biasa digunakan

kecuali jika katalis baru memiliki aktifitas yang lebih tinggi dengan harga yang ekivalen.

Mengapa Sule berpendapat katalis Ni/Al2O3 yang digunakan tidak perlu diganti?

Dapatkah anda menggunakan Gambar 1 sebagai dasar melakukan beberapa perhitungan untuk

memverifikasi data termodinamika yang ditunjukkan pada Gambar 1?


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Solution

Synthesis gas preparation using methane

At this time, the only economically available route for the conversion of methane into more valuable

chemicals is via synthesis gas. Several synthesis gas production methods are available, depending on

the purpose of industrial application. Synthesis gas can come from steam reforming, oxyreforming or

decomposition of methanol (mainly used in hydrogen production for fuel cells, because methanol is

easy to transport and has a high energy density); methanol is synthesized form synthesis gas

produced from coal or natural gas (York et al., 2003).

Using methane to prepare synthesis gas can be realized through three reactions, i.e., steaming

reforming, dry reforming and partial oxidation.

Steam methane reforming

Among the most important commercial methods of synthesis gas manufacturing is the steam

reforming of methane:

Equation 1

This process, utilizing a Ni/Al2O3 catalyst promoted with CaO and/or K2O, necessitates large amount

of energy and also suffers from limitations like poor selectivity for CO and high H2/CO product ratio.

Because of the energy crisis and the considerable expenses needed for maintaining the steam

reforming reaction, it is great practical importance to develop new routes for the synthetic gas

production from methane (Chu et al., 1996).

CO2 Methane reforming

CO2 reforming of methane shows a growing interest from both industrial and environmental

viewpoint. From an environmental perspective, CO2 and CH4 are undesirable greenhouse gases and

both are consumed by the proposed reaction. From the industrial point of view, the reaction allows

to transform these invaluable gases into synthesis gas with a low H2/CO ratio, adequate for

hydroformylation and carbonylation reactions as well as for both methanol and Fischer-Tropsch

syntheses (Juan-Juan et al., 2004).

The CH4/CO2 reforming reaction is

Equation 2

(Kang et al., 2011)

Associated problems with this process are energy-intensive process, low H2/CO ratio, more H2

needed for follow-up Fischer-Tropsch or methanol process, and easy carbon deposition (York et al.,

2003).


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CPO (catalytic partial oxidation)

In recent years, catalytic partial oxidation of methane to synthesis gas (POM) has been extensively

investigated for natural gas utilization. The POM process has great promise to replace the current

strongly endothermic and slower steam reforming for the production of synthesis gas. POM process

can greatly enhance the production of syngas since it can be operated at very high space velocities

(Jin et al., 2000).

About the reaction mechanism, some authors pointed out that partial oxidation of CH 4 to syngas

proceeds via indirect oxidation mechanism namely: complete combustion of CH4 to CO2 and H2O and

subsequently reforming reaction of the residual CH4 with CO2 and H2O to CO and H2.

Figure 1 Syngas production from CH4

However other authors claimed that the reaction proceeds via direct oxidation mechanism: H 2 from

the decomposition of CH4 and CO is the product of the reaction between surface carbon species and

surface oxygen species (Li et al., 2002).

Equation 3

The figure below is a thermodynamic representation of the partial oxidation of methane:

Figure 2 Thermodynamic representation of the partial oxidation of methane

(York et al., 2003)


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Factors-factors affecting equilibrium

Effect of temperature

Equation 4

(Smith et al., 2001)

According to Eq, the effect of temperature on the equilibrium constant K is determined by the sign

of. Thus when is positive, i.e., when the standard reaction is endothermic, and increase inT results in an increase in K. An increase in K at constant P results a shift of the reaction to the right

and an increase in e. Conversely, when is negative, i.e., when the standard reaction isexothermic, an increase in T causes a decrease in K at constant P. This implies a shift of the reaction

to the left and a decrease in e (Smith et al., 2001).

Effect of pressure

Equation 5


If the total stoichiometric number is negative, Eq shows that an increase in P at constantT causes an increase in

, implying a shift of the reaction to the right and an increase in e. If v

is a shift of the reaction to the left, and a decrease in e.

Catalyst used in the reaction

It has been reported that high methane conversion, H 2 and CO selectivities can be obtained over a

number of metal catalysts, such as Rh, Pt, Ru, Ir, Ni, and Co. Rh catalysts have been found to be the

most selective and stable for methane oxidation. Ni shows similar activity and selectivity but

deactivates by volatilization of metal nickel and carbon deposition. However, because of the very

high cost of Rh catalysts, Ni is more promising than Rh for industrial utilization (Jin et al., 2000). So

the studies on supported Ni catalyst have attracted a great number of researchers. One of thecatalyst used for the PO reaction is Ni/Al2O3 which is used in this problem.

Evaluating Catalyst Work on the Problem

Assume the reaction is the direct reaction

Equation 3


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This reaction is a single homogeneous gas-phase reaction. CPO of methane gas is conducted on P =

1 atm and in this calculation we took the temperature range from 300 C to 1000 C. Calculation based

on feedgas with the ratio of CH4/O2 = 1.8 (Dissanayake et al., 1991)

Calculating K

When all the participants in a reaction are gaseous, activities become numerically equal tofugacities, and the equilibrium constant can be written as

Equation 6

In terms of fugacity coefficients we have

( )Equation 7

For low to moderate pressures , and

Equation 8

Equation 9

For this reaction

Equation 10

For the given number of moles of species initially present

Equation 11

The mole fractions yi of the species present are related to by:

Equation 12

For the given reaction, P = 1 atm then Eq. 14 becomes


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Equation 13

Equation 14

Equation 15

Equation 16

Equation 17

K is calculated using Eqs. 6-10

Equation 18

Equation 19

*

+

Equation 20

Equation 21

,*

+-

Equation 22

The meaning of is indicated by:


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Values of and at 298.15 K for the partial oxidation of methane are found from the heatof formation and Gibbs-energy-of formation data of Table C.4 (Smith et al., 2001). The values of A, B,

and C, for Cp values of each component were obtained from the heat-capacity data of Table C.1


Table 1 Delta calculations

Reaktan Produk Cp C2H4 O2 H2 CO J/mol J/mol

A 1,702 3,64 3,249 3,376 6,3525

B 9,08x10-3 5,06x10-4 4,22x10-4 0,000557 -0,00793

C -2,16x10-6 0 0 0 2,16E-06

D 0 -2,27x104 8,30x103 -3100 24850

-7,45x104 0 0 -1,11x105 -36005 -5,05x104 0 0 -1,37x105 -86709

Table 2 K calculations

T ref T/oC = T/Tref K0 K1 K2 K

298,15 298,15 1 1,55x1015 1 1 1,55 x1015

298,15 300 1,006205 1,55 x1015 9,14 x10-01 1,00008504 1,42 x1015

298,15 400 1,341607 1,55 x1015 2,48 x10-02 1,179163747 4,54 x1013

298,15 500 1,677008 1,55 x1015 2,84 x10-03 1,570811397 6,94 x1012

298,15 600 2,01241 1,55 x1015 6,71 x10-04 2,106939257 2,20 x1012

298,15 700 2,347812 1,55 x1015 2,39 x10-04 2,756193689 1,02 x1012

298,15 800 2,683213 1,55 x1015 1,10 x10-04 3,489766674 5,99 x1011

298,15 900 3,018615 1,55E x1015 6,05 x10-05 4,277965354 4,02 x1011

298,15 1000 3,354016 1,55E x1015 3,74 x10-05 5,091701442 2,96 x1011

The next step to calculate is by substitution of K Eq. 13. For example substitution of T= 400 oC.


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The value of K for temperature 298.15 until 1000 oC is very big so we can conclude that the

conversion for the temperature range is 100% or =1. The product gas composition is then

calculated by Eq. 13 16.

= 0

0,116279 = 0,697674

The K value for T 298.15 oC is 1,55 x 1015 and decrease with the increase of temperature to 1000oC.

Consequently the value of will also decrease with increase of temperature.

This reaction is exothermic, this is according to the theory when an increase in T causes a decrease in

K, and implies a shift of the reaction to the left, decrease in and decrease in conversion.

From literature study, the CH4 conversion over Ni/Al2O3 is as follows:

Figure 3 Effect of the catalyst bed temperature on CH4 conversion and CO selectivity over precalcined

Ni/Al2O3. Conditions: CH4/O2=1.8; Space velocity= 750 h-1

(Dissanayake et al., 1991)

Calculation using direct reaction shows doesnt fit the data and graph on the literature. T herefore

the conversion of CPO reaction cannot be calculated using direct reaction. Thermodynamic

calculations indicated that the CPO reaction could be described by the initial combustion of methane

to carbon oxides and water, in combination with steam reforming and water-gas shift reactions.(Smet, 2000)


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The indirect reactions are as follows: 1. Total oxidation

Equation 23

2. Reforming CO2 dan H2O

Equation 24

Equation 25

Calculation steps:

1. Calculate the number of mole CH4, O2, CO2, and H2O after total oxidation reaction using thesame steps as single reaction. Assume the values after the total oxidation reaction are as

follows

2. Calculate the equilibrium constants for each of reforming reaction (Eq. 24 and Eq. 25) as K I

and KII

3. Form the equations relating the product composition

Equation 26


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Equation 27

Equation 28

Equation 29

4. Combine with the equilibrium relations yield

Equation 30

Equation 31

5. Combine Eqs 26-29 with the equilibrium relations on Eqs 30-316. Solve the values for and

Calculations conducted using Aspen Plus 7.1 Simulator with the process flow diagram as follows:

Figure 4 Process Flow Diagram For Simulation


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Reactors used for the simulations are REquil reactors. The first reactor is for the total oxidation while

the second reactor is for the multireaction during methane reforming.


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Figure 5 Total oxidation reaction on the 1st reactor

Figure 6 Methane reforming reaction on the 2nd reactor

Figure 7 Simulation Result for T: 500 C


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Stream 1 is the stream of reactant in into the 1st reactor. Stream 4 is the stream of the product out

from the 2nd reactor.

Methane conversions are calculated by

Equation 32

CO selectivities arecalculated by

Equation 33

Calculation result:

Figure 8 Calculation Results for Conversions and Selectivity

Convmetshows the value of methane conversion (%), ConvO2 shows the value of oxygen conversion

(%), select shows the value of CO selectivity (%). Plot of the values obtained vs temperature are

given bellow.


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Table 3 Calculation Resulf for Methane and Oxygen Conversion , and CO selectivity

Temperature (oC)

Methane

conversion (%)

Oxygen

conversion (%) CO Selectivity (%)

1 400 36,5422 89,9999 10,1466

2 450 43,8681 90 26,0213

3 500 54,5432 90 49,6193

4 550 67,8047 90 71,9248

5 600 80,3691 90 86,5431

6 650 89,3247 90 94,0422

7 700 94,4701 90 97,3961

8 750 97,1182 90 98,8268

9 800 98,4461 90 99,4445

10 850 99,1247 90 99,7222

11 900 99,4845 90 99,8535

Figure 9 Methane and Oxygen Equilibrium Conversion

Figure 10 CO Selectivity

0

20

40

60

80

100

120

300 400 500 600 700 800 900 1000

Methane and Oxygen Conversion

Methane conversion Oxygen conversion


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.

Figure 9 shows the plot of Methane and Oxygen conversion on the equilibrium state vs temperature

for the temperature range of 400-900oC.

Reason(s) not to change the catalyst

1. According to calculations done using Aspen Plus Simulator v. 7.1 and also from the journalused for literature study, CH4 conversion for 400

oC is around 36 % and for 850oC is 99,12 %,

almost 100%. The CH4 conversion of Ni/Al2O3 as CPO catalyst used in PT Reaktor Bagus

Lancar is 35% for 400oC and for 850oC is around 97%.

These data shows that the catalyst used in PT RBL works well enough because it provides

high methane conversion and CO selectivity similar with the equilibrium calculations

conducted using Aspen Plus Simulator and from the journal.2. No coke deposition reported for the catalyst.

Reason(s) to change the catalyst

It was stated in the problem that the product of methane CPO only produce syngas with sidereaction and no coke deposition reported for the catalyst. This is probably because the catalyst isnew.

The tendency for coke deposition is generally considered to be the main drawback in theapplication of supported Ni catalysts. Carbon deposition can lead to rapid catalyst deactivation,whereas whisker-carbon formation can result in reactor plugging. Claridge et al. (1993) indeedreported that Al2O3 supported Ni catalysts displayed considerably higher carbon deposition rates

compared to supported noble metal catalysts. Hence, many studies have been conducted in the pastdecade to develop supported Ni catalysts with high resistance to coke formation (Smet, 2000).

The super-CPO catalyst offered might be one of the newly developed Ni catalysts withenhanced properties against coke deposition.

0

20

40

60

80

100

120

300 400 500 600 700 800 900 1000

CO selectivity

CO selectivity


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Number 2

Problem

Data given in the problem:

Initial composition(%)

N2 = 15% mol

H2O = 60% mol

C2H4 = 25% mol

Constant temperature = 527 K

Constant pressure = 264.2 atm

Reaction

Equation 34

Calculate the equilibrium composition of a mixture of the following species!

Solution

Assumptions:

N2is an inert gas, therefore it doesnt react with the other species. All components are pure Calculation basis : 100 mole

Initial number of moles

Initial Number of Moles

nN2, 0 = 15.0% x 100 g mol = 15 g mol

nH2O, 0 = 60.0% x 100 g mol = 60 g mol

nC2H4, 0 = 25.0% x 100 g mol = 25 g mol

Total number of moles present after the reaction = The mole fractions yi of the species present are calculated by Eq. 12


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Equation 35

Equation 36

Equation 37

Equation 38

Equation 39

Thevalue of K is calculated by utilizing Eqs. 18- 22. The meaning of is indicated by:

Values of and at 298.15 K for the partial oxidation of methane are found from the heatof formation and Gibbs-energy-of formation data of Table C.4 (Smith et al., 2001). The values of A, B,

and C, for Cp values of each component were obtained from the heat-capacity data of Table C.1


Table 4 Delta calculations

Reaktan Produk Cp C2H4 H2O C2 H5OH J/mol J/mol

A 1,424 3,27 3,518 -1,176

B 1,44x10-2 1,45x10-3 2,00x10-2 0,004157

C -4,39x10-6 0 -6,00x10-6 -1,6E-06

D 0 1,21x104

0 -12100


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5,25x104 -241818 -2,35x105 -45792 6,85x104 -2,28x105 -1,68x105 -8,38x103

Table 5 K calculation

T ref T/oC = T/Tref K0 K1 K2 K

298,15 527 1,7676 29,3659 3,28x10-4 1,0046 9,68 x10-3

Calculating fugacity coefficient for each component

Values for accentric factor, Tc, Pc, are taken from Appendix B (Smith et al., 2001)

Table 6 Calculation of

Component Tc (K) Tr Pc (atm) Pr B0 B1

H2O(g) 647,3 0,8142 218,2 1,2108 0,3451 -0,50339 -0,2689 0,4120

C2H4(g) 283,1 1,8615 50,5 5,2317 0,0872 -0,07314 0,1264 0,8398

C2H5OH(g) 516,3 1,0207 63,0 4,1937 0,6480 -0,32537 -0,0188 0,2499

Equation 40

Substitution from Eq (40) to Eq (39)

Using solver in Microsoft Excel, = 16,2273

Substitution into Eqs 35-38 to yields the vapor fraction of each component.


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BibliographyCHU, Y., LI, S., LIN, J., GU, J. & YANG, Y. (1996) Partial oxidation of methane to carbon monoxide and

hydrogen over NiO/La2O3/-Al2O3 catalyst.Applied Catalysis A: General, 134, 67-80.DISSANAYAKE, D., ROSYNEK, M. P., KHARAS, K. C. C. & LUNSFORD, J. H. (1991) Partial oxidation of

methane to carbon monoxide and hydrogen over a Ni/Al2O3 catalyst. Journal of Catalysis,132, 117-127.

JIN, R., CHEN, Y., LI, W., CUI, W., JI, Y., YU, C. & JIANG, Y. (2000) Mechanism for catalytic partialoxidation of methane to syngas over a Ni/Al2O3 catalyst. Applied Catalysis A: General, 201,71-80.

JUAN-JUAN, J., ROMN-MART NEZ, M. C. & ILLN-GMEZ, M. J. (2004) Catalytic activity andcharacterization of Ni/Al2O3 and NiK/Al2O3 catalysts for CO2 methane reforming. AppliedCatalysis A: General, 264, 169-174.

KANG, K.-M., KIM, H.-W., SHIM, I.-W. & KWAK, H.-Y. (2011) Catalytic test of supported Ni catalysts

with core/shell structure for dry reforming of methane. Fuel Processing Technology, 92,1236-1243.

LI, C.-Y., ZHANG, Z.-B., YU, C.-C. & SHEN, S.-K. (2002) Temperature-Programmed Studies on PartialOxidation of CH4 to Syngas over a Ni/Al2O3 Catalyst Fuel Chemistry Division Preprints, 47,123.

SMET, C. R. H. D. (2000) Partial Oxidation of Methane to Synthesis Gas:Reaction Kinetics and ReactorModelling. Eindhoven, Technische Universiteit Eindhoven.

SMITH, J. M., NESS, H. C. V. & ABBOTT, M. M. (2001) Introduction to Chemical EngineeringThermodynamics, New York, McGraw-Hill Inc.

YORK, A. P. E., XIAO, T. & GREEN, M. L. H. (2003) Brief Overview of the Partial Oxidation of Methaneto Synthesis Gas. Topics in Catalysis, 22, 345-358.

problem 4 thermodynamics

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