THERMODYNAMIC ANALYSIS OF STEAM AND CO2 REFORMING OF METHANE

José F. Cancino and Miguel Bagajewicz†

Department of Chemical Engineering and Materials Science University of Oklahoma

Norman, OK, USA † To whom all the correspondences should be addressed. (bagajewicz@ou.edu)

Abstract A process thermodynamics analysis of natural gas reforming is performed to identify optimal processing conditions. Two types of natural gases, one with almost no CO2 and the other with a high content of CO2 were chosen as representative raw materials. When H2/CO syngas ratios of 2/1 or smaller are desired, addition of water and CO2 are required. The paper investigates the use of CO2 recycles, single, series, and parallel reactors on top of temperature and pressures to establish optimal conditions.

2

Introduction Steam reforming of methane is the most common process to produce Syngas. Table 1, summarizes some of the commercial plants recently built. In turn, dry reforming that is, reforming using CO2, has been claimed to be a viable economical alternative route for the production of Syngas1,2,3. Based on such claims a large number of investigators have started research programs to identify good catalysts and appropriate operating conditions under which dry reforming reactions would proceed for a variety of applications. Such typical processes requiring different H2/CO feed ratios are shown in Table 2. Finally, a short summary of temperature and pressure conditions of these processes is given in Table 3.

Table 1: Industrial production of Hydrogen and Syngas through steam reforming [1,2]

Plant/Location Main product Capacity, MMSCFD YearPernis, Rotterdam, Netherlands H2/CO 80 994 New Orleans H2 Plant, LA H2 70 1994 Pasadena Hydrogen Plant Houston, TX

H2 80

1996

Wilton Hydrogen Plant Teekside, UK

H2 18

1997

Paraguana H2 Plant, Venezuela H2 50 1997 Amuay H2 Plant, Venezuela H2 50 1997 Geismar HYCO Plant Geismar, LA

H2+CO+ Syngas 45

1999

Convent, LA, USA Direct reduction Iron (DRI)

1.2 MT/yr 1998

ANSDK I, El-Dikheila, Egypt DRI 0.80 2000 ISPAT DR3 Point Lisas, Trinidad & Tobago

DRI 1.36 1999

Saldanha Steel, Saldanha Bay, South Africa

DRI 0.804 1999

Table 2: Typical applications for Syngas production

H2/CO Application Process Used/Proposed >3 Hydrogen, Ammonia Steam reforming and water gas shift 2-3 Methanol Steam reforming

2-2.5 Fischer Tropsch (gasoline and light olefins)

Partial oxidation, steam reforming

1.7-2 Fischer Tropsch (Fixed Bed, for waxes or diesel)

Steam reforming –Partial Oxidation

1.5 Aldehydes, Isobutane, Isobutanol, Higher Alcohols (C1-C6)

Hydroformylation

1 Acetic Acid Autothermal Reforming <1 Polycarbonates CO2/CH4 (dry) reforming

3

Table 3: Typical Conditions of Downstream Processes 4,5

Fischer Tropsch Process Hydrogen generation

Ammonia Synthesis

Methanol Synthesis Fix Bed Synthol

Hydro-formylation

T (°C) 815-870 775-835 350-420 215-250 355-415 160-200

P (MPa) 2.5 9.7-24.9 11.8-16.0 2.72 2.3 4.92-10.24

In addition, some conditions such as residual methane are also important because they dictate lower limits on conversion for the reforming stage. For example, a few reports indicate that some CH4 (e.g. up to 2 percent) may be economically advantageous for ammonia synthesis. Likewise, the presence of CO2 can be tolerated and even desired in some cases 6. For example, in methanol synthesis, some CO2 in the feed is allowed (9% vol. is typical) 5. However, in ammonia synthesis the removal of CO2, H2O, and CO (compounds that contain oxygen) are of critical importance. Table 4 summarizes the maximum level of CO2 allowed in different Syngas uses.

Table 4: Maximum content of CO2 allowed in the synthesis step

H2/CO Application Maximum CO2 Currently allowed

>3 Hydrogen, Ammonia Total carbon oxides (CO+ CO2) must be reduced to less than 15 ppm. 7

2-3 Methanol 2-8 % vol. 2-2.5 Fischer-Tropsch synthesis Removed in previous stages 1.7-2 Fischer Tropsch (Fixed Bed) Removed in previous stages 1.5 Aldehydes, alcohols CO2/CH4 = 2.2/1.0 5

1 Acetic Acid CO2/CH4 = 0.54/1.0 5

<1 Polycarbonates 1-300 ppm 8

With the exception of the SPARG process, the removal of sulfur compounds is essential for downstream processes because they poison the catalysts. For example, in the case of methanol synthesis, the maximum allowable sulfur level is 1 to 2 ppm by volume5. In this study, the issue of sulfur will not be discussed.

To a great extent, all existing research aimed at the identification of catalysts is mostly concerned with activity and stability of the catalyst and it has been seldom linked with temperature, pressure and feed composition conditions for optimal process economics. This article aims at the identification of optimal processes conditions that are rooted on economical considerations. To do so, it is assumed that industrial reactors are operated in such a way that equilibrium holds in the outlet (one practical exception for this are fluidized reactors). Finally, a link between inlet and outlet conditions, as well as the corresponding energy requirements for the process provides target-operating conditions (temperatures, inlet compositions, etc) for fundamental catalytic studies.

4

Goals and Constraints of this Study The purpose of this study is to identify under which conditions (pressure, temperature, amount of water in the feedstock, and reactors scheme) the reforming of natural gas using steam and/or CO2 is thermodynamically and economically optimal. The constraints are:

• High methane conversion. • The lowest possible CO2 content in the product. • The highest possible Syngas production rate. • A low usage of steam is preferred.

Although the results of this study can be used for any feedstock, two different types of gases are picked to illustrate certain points. Table 5 shows the composition of these two gases, one with a high content of CO2 (Terrell, TX) and the other corresponds to a typical natural gas with almost no CO2 (California). The latter contains a high amount of ethane, and it was assumed that it could be removed prior to reforming.

Table 5: Natural Gas Composition 9

Comp. Terrell Gas Composition

California Gas composition

California Gas after ethane removal

CH4 45.7 % 89.4 % 98.0 % CO2 53.9 % 0.8 % 0.9 % C2H6 0.2 % 9.8 % 1.1 % N2 0.2 % --- ---

Single reactor and multiple reactor configurations in series and parallel arrangements are considered. Addition/removal of CO2 and water to/from the inlet and intermediate streams is also considered. Preliminary Equilibrium Considerations

To a great extent, most papers related to CO2 reforming focused on finding a suitable

catalyst with high activity and able to prevent carbon formation. When dealing with kinetics rate expressions, reaction mechanism is important to establish the rate limiting step and which reaction is predominant. When using equilibrium data, reactions are not important. Because calculations are based on the minimization of the Gibbs free energy, distinctions between steam and dry reforming are therefore confined to determine how much water and CO2 are present in the feed. In the presence of both water and CO2 in the initial mixture, the distinction becomes a little harder.

5

The set of reactions involved in reforming are: CO2 reforming reaction: (1) CO2 + CH4 2 CO +2 H2 Reverse water gas shift (RWGS) reaction: (2) CO2 + H2 H2O (g) + CO Steam reforming reaction: (3) H2O + CH4 CO + 3 H2 The steam reforming reaction is a linear combination of the other two. In turn, the reactions considered for carbon forming are: Boudouard reaction: (4) 2 CO CO2 + C(S)

Methane cracking reaction: (5) CH4 2 H2 + C(S)

Although charts showing explicitly equilibrium composition have been published elsewhere1,2,10, for completeness such plots are included as a function of initial composition. The effect of pressure and temperature on the equilibrium and consequently, on the yield of Syngas is investigated first.

Effect of Pressure: Figure 1 shows the equilibrium conditions as a function of the CO2/CH4 ratio and parametric in pressure. From this graph, it can be seen that the methane conversion, as well as the H2/CO ratio decreases when increasing the pressure for different ratios of CO2/CH4 in the feed. On the other hand, the CO2 content in the outlet stream is lower at lower pressure. Thus atmospheric pressure should be the preferred condition for both dry and steam reforming.

6

0

0.1

0.2

0.3

0.4

0.5

0.6

0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.00

CO2/CH4 ratio

Equi

libriu

m c

ompo

sitio

n

H2

CO

CO2

H2O

CH4

CO

CO

CH4

CH4

H2

H2

CO2

CO2

H2O

1 atm10 atm

25 atm

Figure 1: Effect of pressure on equilibrium compositions. T= 800°C

Effect of temperature: Figure 2 shows the equilibrium conditions at atmospheric pressure as a function of the CO2/CH4 ratio and parametric in temperature. Clearly, high temperatures favor a higher methane conversion and a lower amount of CO2 in the outlet stream.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.00

CO2/CH4 ratio

Equi

libriu

m c

ompo

sitio

n

H2O

CO

CO

H2

H2

CO2

CH4CH4

900°C

800°C

CO2 H2O

700°C

Figure 2: Effect of Temperature on equilibrium compositions. P =1 atm

7

While all these charts are useful to make general conclusions, the identification of water and CO2 inlet concentration to achieve desired outlet conditions is not straight forward. This determination is performed later in connection with a specific set of reactors. Carbon Formation Carbon formation can easily plug catalyst beds and heat exchangers and cause cracking of high-pressure fittings even by penetrating through tiny crevices. Therefore, it is desirable to predict temperature, pressure, and composition conditions under which carbon precipitation is not thermodynamically favorable.

Rostrup-Nielsen11 analyzed conditions for carbon-free operation considering the Boudouard and the methane cracking as reversible reactions. For any gas of fixed composition, there is a temperature Tb below which there is a thermodynamic affinity for the exothermic Boudouard reaction, and a temperature Tm above which the endothermic methane cracking reaction is favored. The real ∆G of reaction is not based on graphite, but on a whisker-like structure of the carbon deposited on the catalyst. Thus, the predictions using graphite are conservative. Instead of looking at the affinity in each reaction independently, one can obtain equilibrium compositions using the degree of advance of each reaction (ξi) and predict favorable conditions for carbon formation more accurately. This procedure was performed for CO2/CH4 feed ratios ranging from 1/1 to 4/1 and evaluated at pressures of 1, 10, and 25 atmospheres and temperatures ranging between 500°C and 1300°C. The results are shown in Figure 3 and show good agreement with previous work 2.

1/3

1/2

1/1

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

500 600 700 800 900 1000 1100 1200 1300Temperature, °C

CO

2/CH 4 r

atio

25 atm

carbon deposition region

No carbon deposition

1 atm 10 atm

Terrell gas

Figure 3: Carbon deposition regime for dry reforming

8

Figure 3 shows the minimum temperature at which one can operate without having thermodynamic affinity for carbon deposition on the catalyst. Carbides formation temperatures are shown in Figure 4. Conditions to the left of each curve indicate the region at which carbon deposition is favored. Temperature conditions to the right of the curve for a fixed CO2/CH4 ratio mean working on a region where there is no carbon formation. Feed compositions (with a high amount of methane) like in most natural gas are located in the diagram where the carbon formation is present at any temperature.

0.0

1.0

2.0

3.0

4.0

5.0

6.0

750 800 850 900 950 1000 1050 1100 1150

Temperature, K

CO

2/CH 4 f

eed

ratio carbide

formation

carbondeposition

Figure 4: Carbon deposition and carbide formation on Ni catalysts. Adapted from Wang

and Lu 1

For example, feedstock with CO2/CH4 ratios ranging from 2:1 to 1:1 require temperature conditions higher that 720°C to 1000°C to avoid carbon formation conditions. Using Terrell gas as feedstock at atmospheric pressure, the temperature below which carbon formation is favored is between 820°C - 840°C. This temperature increases up to 1000°C when the pressure is increased to 10 atm. While the above curves are informative, they do not take into account the initial composition of all three components (CH4, CO2 and H2O). Rostrup-Nielsen developed such a graph (Figure 5).

9

H/C

O/C

Figure 5: Temperatures for carbon limits on Ni catalyst. ( Rostrup Nielsen 11) All the equilibrium studies on reforming are focused on initial conditions of the mixture that exclude water or CO2, never including both water and CO2. In this article such conditions are explored. Role of Water

Typically, steam reforming is used to produce hydrogen using natural gas with very low CO2 content. However, to obtain Syngas with higher content of CO (for Fischer- Tropsch, H2/CO=2:1, or acetic acid production, H2/CO=1:1, etc), pure steam reforming produces excess hydrogen. Thus, the question is: given a certain gas, with a particular CH4/CO2 ratio, what is the amount of water needed to achieve a Syngas with a given ratio. Moreover, what is the conversion in such case?

To answer the question, the amount of water needed to achieve the H2/CO ratio of 1/1

is depicted in Figure 6, for several feed ratios of CO2/CH4 from 0.2 to 3.0. For example, for the case of a composition close to Terrell natural gas (1.2/1 CO2/CH4), one can obtain the desired ratio of H2/CO of 1/1 using in the feed a percentage of water in the feed up to 10%, and that amount of water increases as the ratio of CO2/CH4 increases. Each curve

10

represents one temperature. At higher temperatures the amount of water needed also increases.

0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.0 0.5 1.0 1.5 2.0 2.5 3.0

CO2/CH4 feed ratio

H 2O

/feed

ratio

800 °C 900 °C 1000°C 1100 °C

Figure 6: Water requirements for CO2 reforming at 1 atm. For a typical composition of natural gas (close to California natural gas), which has

very little CO2, almost no water is needed. However, due to the low amount of CO2 present, methane conversions obtained are very low. Handling so many variables at the same time motivates the search for a different type of diagram to represent a feedstock defined by the composition of CO2, CH4, and H2O, which at the same time would give information on methane conversion, Syngas yield, duty, and CO2 content in the outlet stream. Equilibrium Chart Rostrup-Nielsen11 proposed the diagram shown in Figure 7. It has the atomic ratios of O/C in the abscissa and the atomic ratio H/C in the ordinate. The straight line that connects the point H/C=4.0, O/C=0 (which represents methane component) with the point H/C=0, O/C=2.0 (which represents CO2 component), represents all feed mixtures of CH4 and CO2 (no water), while the one starting at methane and increasing represent all feed mixtures steam-methane (no CO2).

11

The points inside the region defined by these lines represent different mixtures with the ratios H2O/CH4 and CO2/CH4. It is important to note that the same point represents initial and final equilibrium conditions in this diagram. Thus no trajectories in a reactor can be constructed. Initial equilibrium conditions (corresponding to feed compositions) are read on the non-rectangular axis, while final equilibrium conditions are read on the O/C and H/C axis. The figure contains the lines of constant Syngas ratio (3:1, 2:1, 1:1, etc) built to indicate what feed composition leads to this desired H2/CO ratio. This equilibrium diagram was originally plotted at a fixed temperature of 900°C (1650°F) and a pressure of 6 atm (72 psig). Here, since atmospheric pressure has been set as the best condition, Figure 7 has been built at 1 atm and 850°C (1562°F). The carbon formation line was obtained from early work of J.R. Rostrup-Nielsen11 at different temperatures.

0

1

2

3

4

5

6

7

8

9

10

0.0 0.5 1.0 1.5 2.0 2.5

O/C ratio

H/C

rati

3.0

o

H2O/CH4

CO2/CH4

0.4

0.7

1.0

1.5

2.0

2.5

3.0

0.2

0.50.75

1.0

5.03.0

1.52.0

H2:CO 2.5:1

H2:CO 2:1

H2:CO 1.5:1

H2:CO 1:1

H2:CO 3:1

Carbon formation curve, 1 atm

A

B

Figure 7: Equilibrium Chart at 1 atm

12

For example, when using Terrell gas to produce a Syngas H2/CO ratio of 2:1, first the feed has to be located in the graph (Point A). Since the same point represents the equilibrium Syngas, it is seen that the H2/CO ratio obtained is lower than 1:1 and that the point is located inside the region favorable for carbon formation (at the left of the carbon formation curve). Therefore, water needs to be added to the feed to reach two objectives: to move the feed location point to reach a desired line of H2/CO and to work outside of the region favorable for carbon formation. For example, if a ratio of 2:1 is desired, 2.65 moles of H2O per mole of CH4 is added to reach point B, which is outside the carbon formation region.

The simulator ProII TM (Simulation Sciences Inc.) was used to perform the calculations using an equilibrium reactor. According to recommendations for natural gas systems at high temperatures and low pressures, the thermodynamics packages suitable for this system are: SRK (Soave, Redlich & Kwong), PR (Peng Robinson), or BWRS (Bennedict-Webb-Rubin-Starling), which give good accuracy in the calculations performed. The SRK equation of state was chosen to perform the equilibrium calculations. As it is shown in the rest of this article, the diagram of figure 7 can be used to completely characterize reactor conditions and to make an analysis of a variety of alternatives. Constant methane conversion, yield, reactor duty, and CO2 content at the outlet stream curves were added to the equilibrium chart to complete the information required to define a reactor performance. Figure 8 shows the constant conversion lines, which was obtained changing the ratio of CO2/CH4

in the feed and adding water until reaching the desired fixed methane conversion or the fixed yield of Syngas (H2+CO). Figure 9 shows the constant yield (moles of Syngas/mole of methane). The lines were obtained by changing the CO2/CH4 ration in the feed and adding water to reach a constant Syngas yield expressed as moles of Syngas per mole of methane.

13

0

1

2

3

4

5

6

7

8

9

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0

O/C ratio

H/C

ratio

H2O/CH4

CO2/CH4

0.4

0.7

1.0

1.5

2.0

2.5

3.0

0.2

0.5

0.751.0

5.03.0

1.52.0

95%

99%

99.5%

99.8%

99.95%

H2:CO 3:1

H2:CO 2.5:1

H2:CO 2:1

H2:CO 1.5:1

H2:CO 1:1

80%

50%

30%

97%

carbon formation line

Methane Conversion

Figure 8: Constant methane conversion lines at 850°C and 1 atm

14

0

1

2

3

4

5

6

7

8

9

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0O/C ratio

H/C

ratio

H 2 :C O 3:1

H 2 :C O 2.5:1

H 2 :C O 2:1

H 2 :C O 1.5:1

H 2 :C O 1:1

0.4

0.7

1.0

1.5

2.0

2.5

3.0

0.2

0.5

1.00.75

2.0

5.0

1.5

3.0

3.99

3.95

3.83.5

32.5

2

3.98

Constant Yield mole Syngas per mole CH4

3.999

H2O/CH4

CO2/CH4

Figure 9: Constant yield at 850°C and 1 atm.

Figure 10 shows lines of constant CO2 in the outlet stream (expressed as moles CO2

at the outlet stream per mole of CH4 in the feed), which is an important value that gives information on the amount of CO2 that can eventually be recycled to the reactor. They were calculated by changing the CO2/CH4 feed ratio and adding water to the feed to reach the desired fixed CO2 content in the outlet steam.

One of the most important economic indicators is the duty. To make a value of duty

meaningful, the net amount of heat that needs to be supplied to the equilibrium reactor was calculated. It was assumed that most of the energy requirements for the reactor are recovered through steam generation and preheating of the feed, using a minimum approach temperature of 30°F in all heat exchange. Figure 11 shows schematically the flow sheet used and.Figure 12 shows the constant duty lines.

15

0

1

2

3

4

5

6

7

8

9

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0

O/C ratio

H/C

ratio

H2O/CH4

CO2/CH4

0.4

0.7

1.0

1.5

2.0

2.5

3.0

0.2

0.5

0.751.0

5.03.0

1.52.0

0.02

H 2:C O 3:1

H 2:C O 2.5:1

H 2:C O 2:1

H 2:C O 1.5:1

H 2:C O 1:1

0.05

carbon formation line

Constant CO2 at outlet stream moles CO2/mole CH4

0.005

0.075

0.1

0.3

0.6

0.2

0.4

0.5

0.7

0.9

Figure 10: Lines of constant CO2 in the outlet at 850°C and 1 atm.

M1 REACTORF1

HEAT-EXCH.COOLERGAS

CO2 S3 INLET

OUTLET

S7

S8

H2O

S11

S10

Amb. Temperature

Figure 11: Single reactor flow sheet for duty calculations

16

0

1

2

3

4

5

6

7

8

9

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0O/C ratio

H/C

ratio

H2:CO 3:1

H2:CO 2.5:1

H2:CO 2:1

H2:CO 1.5:1

H2:CO 1:1

0.4

0.7

1.0

1.5

2.0

2.5

3.0

0.2

0.5

1.00.75

2.0

5.0

1.5

3.0

H2O/CH4

CO2/CH4

Constant duty BTU/Lb-mol CH4

(KJ/Kg-mol CH4)

115,

000

(55,

035)

114,

000

(54,

556)

113,

000

(54,

078)

110,

000

(52,

642)

100,

000

(47,

857)

80,0

00 (3

8,28

5)

Figure 12: Constant duty lines at 850°C and 1 atm Effect of Pressure On Figure 13 the constant H2/CO ratio lines at 1 and 25 atm are shown to see the effect of pressure on the Syngas lines. Combined with the carbon formation line these curves help identify steam or CO2 needed to get a Syngas with a specific ratio of H2/CO. While there are some differences in the lines at different pressures, for producing Syngas with H2/CO ratio 2:1, the line is almost the same. However, the main disadvantage of working at high pressure is that more water is required to reach the desired H2/CO ratios (lower than 2:1). In addition, at high pressure the region favorable to carbon formation covers a wide range

17

of feed composition conditions. At high pressure (25 atm), conversion and yield curves are shifted to the right that is they decrease with increasing pressure (Figures 14 and 15). This is due to the increasing water requirements to reach a fixed methane conversion at higher pressure.

0

1

2

3

4

5

6

7

8

9

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0

O/C ratio

H/C

ratio

H2O/CH4

CO2/CH4

0.4

0.7

1.0

1.5

2.0

2.5

3.0

0.2

0.50.75

1.0

5.03.0

1.52.0

H2:CO 2.5:1

H2:CO 2:1

H2:CO 1.5:1

H2:CO 1:1

H2:CO 3:1

25 atm

1 atmCarbon formation curve, 1 atm - 25 atm

Figure 13: Influence of pressure on equilibrium Syngas lines @ 850°C

18

0

1

2

3

4

5

6

7

8

9

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0

O/C ratio

H/C

ratio

H2O/CH4

CO2/CH4

0.4

0.7

1.0

1.5

2.0

2.5

3.0

0.2

0.50.75

1.0

5.03.0

1.52.0

H2:CO 2:1

H2:CO 1:1

Carbon formation curve, 1 atm - 25 atm

80 % MethaneConversion 1 atm - 25 atm

Figure 14: Influence of pressure on methane conversion @ 850°C

19

0

1

2

3

4

5

6

7

8

9

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0

O/C ratio

H/C

ratio

H2O/CH4

CO2/CH4

0.4

0.7

1.0

1.5

2.0

2.5

3.0

0.2

0.50.75

1.0

5.03.0

1.52.0

H2:CO 2:1

H2:CO 1:1

Carbon formation curve, 1 atm - 25 atm

Syngas yield3.0 at1 atm

Syngas yield3.0 at25 atm

Figure 15: Influence of pressure on yield @ 850°C and 1 atm

Figure 16 shows the influence of pressure on reactor duty. At high pressure (25 atm) the duty lines shift right (that is, the duty decrease with increasing pressure). When operating at higher pressures the importance of this difference can be assessed by the following example: For an industrial size plant processing 4,000 lb-mol/hr of methane, the savings are 1,730,000 $/year. However, the yield reduces by 10 %, thus reducing the revenues. The CO2 content at the outlet of the reactor also increases, bringing higher costs to the separation step. All this confirms the notion that lower pressures favor dry reforming.

20

0

1

2

3

4

5

6

7

8

9

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0

O/C ratio

H/C

ratio

H2O/CH4

CO2/CH4

0.4

0.7

1.0

1.5

2.0

2.5

3.0

0.2

0.50.75

1.0

5.03.0

1.52.0

H2:CO 2:1

H2:CO 1.5:1

H2:CO 1:1

Carbon formation curve1 atm - 25 atm

25 atm

90,0

00 (4

3,07

1) @

1 a

tm

113,

000

(54,

078)

@ 1

atm

90,0

00 (4

3,07

1) @

25 a

tmFigure 16: Influence of pressure on duty @ 850°C and 1 atm

Effect of Temperature This effect is depicted in Figure 17. An increase in temperature shifts the Syngas ratio lines up, meaning that for a feedstock of constant composition, at high temperature more water is required to reach that ratio. The temperature also has a direct influence on the conversion, yield, and CO2 at the outlet stream. However, its influence on duty is not straightforward.

Figure 18 shows the influence of temperature on CH4 equilibrium conversion, while Figure 19 shows the effect on yield. Increasing temperature increases conversion and

21

yield. No clear tendency on the duty lines (Figure 20) with respect of temperature is observed, however. Finally, the CO2 content decreases (Figure 21).

0

1

2

3

4

5

6

7

8

9

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0O/C ratio

H/C

ratio

H2O/CH4

CO2/CH4

0.4

0.7

1.0

1.5

2.0

2.5

3.0

0.2

0.50.75

1.0

5.03.0

1.52.0

H2:CO 2:1

carbon formation line850°C - 920 °C

920 °C

750 °C

850 °C

A

1.8

2.35

Figure 17: Influence of temperature on equilibrium Syngas lines @ 1 atm

22

0

1

2

3

4

5

6

7

8

9

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0O/C ratio

H/C

ratio

H2O/CH4

CO2/CH4

0.4

0.7

1.0

1.5

2.0

2.5

3.0

0.2

0.50.75

1.0

5.03.0

1.52.0

H2:CO 2:1

carbon formation line at:850°C - 920°C

920 C850 C

A

99.8 %920 °C

99.8 %850°C

B

C

X CH4

Figure 18: Influence of temperature on methane conversion @ 1 atm.

23

0

1

2

3

4

5

6

7

8

9

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0O/C ratio

H/C

ratio

H2O/CH4

CO2/CH4

0.4

0.7

1.0

1.5

2.0

2.5

3.0

0.2

0.50.75

1.0

5.03.0

1.52.0

H2:CO 2:1

carbon formation line at:850°C - 920°C

920 C850 C

A

3.99920°C

3.99850°C

B

C

SyngasYield

Figure 19: Influence of temperature on yield @ 1 atm.

24

0

1

2

3

4

5

6

7

8

9

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0O/C ratio

H/C

ratio

H2O/CH4

CO2/CH4

0.4

0.7

1.0

1.5

2.0

2.5

3.0

0.2

0.50.75

1.0

5.03.0

1.52.0

H2:CO 2:1

carbon formation line at:850° C- 920 °C

920 C

850 C

A

B

C

114,

000

(54,

556)

@ 8

50°C

114,

000

(54,

556)

@92

0°C

114,

000

(54,

556)

@75

0°C

Duty, BTU/mol CH4(KJ/kg-mol CH4)

Figure 20: Influence of temperature on reactor duty @ 1 atm

25

0

1

2

3

4

5

6

7

8

9

10

0.0 0.5 1.0 1.5 2.0 2.5O/C ratio

H/C

rati

3.0

o

H2O/CH4

CO2/CH4

0.4

0.7

1.0

1.5

2.0

2.5

3.0

0.2

0.50.75

1.0

5.03.0

1.52.0

H2:CO 2:1

carbon formation line at:850°C - 920°C

920 C850 C

A

B

C

0.7850°C

0.7920°C

CO2

mole CO2

mole CH4

Figure 21: Influence of temperature on CO2 content @ 1 atm

26

It is clear that increasing temperature can increase conversion and yield and lower the CO2 content at the outlet reactor stream. The negative effect on costs is caused by the increase on the amount of water needed or the more favorable conditions for carbide formation. Regarding the effect of pressure, the main point is the reduction of duty obtained when operating at high pressure (25% when increasing pressure from 1 to 25 atm), which has to be compared to the reduction in revenues caused by a lower Syngas yield and the higher costs of the CO2 separation step caused by the increase in CO2 content at the outlet stream. This is performed in the next section, when analyzing a single reactor scheme. Use of a Single Reactor The first gas feedstock to investigate is Terrell natural gas, which contains high proportion of CO2/CH4 (1.2:1). This feedstock is located at the point A in Figure 22. The point is inside the region favorable for carbon formation (to the left of the carbon formation line). If the production of a Syngas ratio of 2/1 is desired, water needs to be added until the ratio H2O/CH4 is about 2.7/1 (point B). The addition of water prevents carbon deposition. Methane conversion is 99.95%, the yield is almost 4.0 Kg-mol of Syngas/Kg-mol CH4, and the duty is 55,131 KJ/kg-mol CH4. That amount of water can be reduced to a H2O/CH4 ratio of 1.52/1 represented by point C by introducing a CO2 separation step and moving the CH4/CO2 ratio to point A2. The methane conversion lowers to 99.8%, the Syngas yields to 3.992 Kg-mol/Kg-mol CH4, and the duty to 54,461 KJ/kg-mol CH4. The change in duty is less than 1.2%, and the change in Syngas yield is less than 0.20%. In an economy of scale these savings are significant, especially the one related to the reduction in steam consumption. For example, for a typical size of plant (methane feed = 1,814 kg/hr) the steam reduction is of 37,500 kg/hr, while the amount of CO2 to be removed is equivalent to 36,700 kg/hr. Regarding the duty, although it decreases, the amount is negligible. Regarding the CO2 content at the outlet stream, point B gives a value of 0.85 kg-mol of CO2/Kg-mol CH4 in the feed, while point C gives 0.39 kg-mol of CO2/kg-mol CH4. Therefore, the influence of a CO2 recycle can be analyzed on the equilibrium chart and determine whether recycling is beneficial in terms of methane conversion, Syngas yield, and reactor duty. This is studied later.

27

0

1

2

3

4

5

6

7

8

9

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0O/C ratio

H/C

ratio

H 2 :C O 3:1

H 2 :C O 2.5:1

H 2 :C O 2:1

H 2 :C O 1.5:1

H 2 :C O 1:1

0.4

0.7

1.0

1.5

2.0

2.5

3.0

0.2

0.5

1.00.75

2.0

5.0

1.5

3.0

H2O/CH4

CO2/CH4

Constant Yield Kg-molKg- mol CH4

B

A

B1

3.999

3.99

Constant methane conversion

99.8%

99.95%

C

A1

Constant duty KJ/Kg-mol CH4

55,0

35

54,5

56

54,0

78

Figure 22: Terrell gas, single reactor configuration (1atm, 850 °C) If the desired Syngas ratio using Terrell gas is 1.5/1 (point B1 in Figure 22), the required water addition corresponds to a H2O/CH4 ratio of 1.48/1 (at the same pressure and temperature conditions as above). It is observed that a process to obtain a lower syngas H2/CO ratio of 1.5/1 needs less water, has a lower reactor duty (54,509 KJ/kg-mol CH4), has a slightly lower syngas yield (3.993 Kg-mol/Kg-mol CH4), lower methane conversion (99.85%), and has a lower CO2 content (0.58 Kg-mol/Kg-mol CH4) at the outlet stream, when compared with a process to obtain the Syngas H2/CO ratio of 2/1. If a different temperature is analyzed on the equilibrium chart, the whole equilibrium diagram has to be built for the new temperature conditions, making in this particular case

28

the process simulator more useful. However, since it is known how temperature affects the equilibrium, a qualitative analysis is still valid. We now turn to analyze the processing of California gas in a single reactor (Figure 23). Such gas is represented by point C, which contains a very low CO2/CH4 ratio (typical for most natural gas feedstock). Two options are available to get Syngas H2/CO ratios of 2/1. By adding a little amount of water, one obtains point D. However, the values for methane conversion (2.16 %), Syngas yield (0.13 Kg-mol Syngas per Kg-mol of CH4), and the favored carbon formation are the price to pay, making this option not worthwhile. From here, to move to the right side of the carbon formation line, H2O and CO2 have to be added to go from point D to its right, increasing conversion and yield. Depending on the desired yield, desired H2/CO ratio, and on the availability of CO2, one can add CO2

and H2O to read for example point D1. Thus, an addition of water of 0.9/1 H2O/CH4 and 0.45 CO2/CH4 renders a methane conversion of almost 99.0%, a Syngas yield of 3.95 Kg-mol/Kg-mol CH4, and a duty of 52,642 KJ/Kg-mol CH4. The best option is based on the constraints imposed on the particular case (high methane conversion, high yield) and the economics of each alternative.

29

0

1

2

3

4

5

6

7

8

9

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0O/C ratio

H/C

ratio

H 2 :C O 3:1

H 2 :C O 2.5:1

H 2 :C O 2:1

H 2 :C O 1.5:1

H 2 :C O 1:1

0.4

0.7

1.0

1.5

2.0

2.5

0.2

0.5

1.00.75

2.0

5.0

1.5

3.0

3.95

3.8

2.

H2O/CH4

CO2/CH4

3.98

Constant YieldKg- mol Syngas Kg-mol CH4

C

D

3.999

3.99

Constant methane conversion

30%

99%

D1

3.0

0

Constant duty KJ/Kg-mol CH4

33,5

00

54,0

78 54,5

56

55,0

35

43,0

71 52,6

4 2

Figure 23: California gas on a single reactor configuration Recycle of CO2 The recycle of CO2 can also be studied using a procedure illustrated in Figure 24 for a final desired syngas ratio of 2:1. For a given composition of natural gas represented by point A, a desired Syngas ratio of 2/1 is reached by adding water to get point B. Through that equilibrium point, the line of constant CO2 corresponding to 0.55 Kg-mol of CO2/Kg-mol of CH4 intersects.

30

1

2

3

4

5

6

7

8

0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.

O/C ratio

H/C

ratio

4

H2O/CH4

CO2/CH4

0.7

1.0

2.0

1.5

0.75

1.0

H2:CO 2:1

carbon formation line

Constant CO2

B

A1

B1

A

2.5

Syngas Yield1.5

2.0

B2

A2

3.99

3.999

0.3

0.4

0.5

0.6

0.7

0.9

Duty, BTU/mol CH4(KJ/kg-mol CH4)

114,

000

(54,

556)

115,

000

(55,

035)

Figure 24: Partial and total recycle on the equilibrium chart The operation using partial recycle is obtained by adding to the feed a constant CO2

value (a fraction or all the value from the outlet stream, represented by point A1). This new feed renders a Syngas represented by point B1. The total recycle operation is obtained when the increment in CO2 to the feed reaches point A2, which renders point B2 through which a constant CO2 line intersects with a value that is equal to the increment of CO2 in the feed. Since the production goal is a 2/1 H2/CO ratio, the new equilibrium syngas over the 2/1 lines is point B2. It is clear that an increase of CO2 recycle increases methane conversion, syngas yield, duty, the amount of water needed and the CO2 content of the outlet stream. Since the yield at point B is already close to the maximum theoretical of 4, no clear gain is obtained. In addition, more water is needed to keep the same Syngas ratio of 2/1 and the duty increases. Finally, the cost of compression is higher and the increasing amount

31

of CO2 at the outlet stream increases the cost of the CO2 separation stage. In other words, there is no incentive to recycle CO2 in this case.

0

1

2

3

4

5

6

7

8

9

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0

O/C ratio

H/C

ratio

H2O/CH4

CO2/CH4

0.4

0.7

1.0

1.5

2.0

2.5

3.0

0.2

0.5

0.751.0

5.0

3.0

1.52.0

0.02H 2 :C O 3:1

H 2 :C O 2.5:1

H 2 :C O 2:1

H 2 :C O 1.5:1

H 2 :C O 1:1

0.05

carbon formation line

Constant CO2 at outlet stream Kg-mol CO2/Kg-mol CH4

A

C

B B1

0.010.005

3.999

3.99

Syngas yield

C1

A1

0.075

0.1

0.3

0.6

0.2

0.4

0.5

0.7

0.9

DutyBTU/mol CH4(KJ/kg-mol CH4)

114,

000

(54,

556)

115,

000

(55,

031)

Figure 25: Recycling CO2 with Terrell gas For the case in which a lower Syngas ratio (1.5/1) is desired, Terrell gas represented by point A (Figure 25) needs the addition of water to reach point C. The amount of CO2 in the outlet stream is 0.55 Kg-mol/Kg-mol of CH4. The values of Syngas yield and methane conversion obtained on the diagram are still high not justifying the recycle either.

32

2

2.5

3

3.5

4

4.5

5

5.5

6

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

O/C ratio

H/C

ratio

H2O/CH4

CO2/CH4

0.4

0.7

1.0

0.2

0.5

0.75

1.0

0.02

H2:CO 3:1

H2:CO 2.5:1

H2:CO 2:1

H2:CO 1.5:1

H2:CO 1:1

0.05

carbon formation line

Constant CO2 at outlet stream Kg-mol CO2/Kg-mol CH4

C

0.010.005

D D1

C1

E

0.1

70,0

00 (3

3,50

0)

90,0

00 (4

3,07

1)

110,

000

(52,

642)

Figure 26: Recycling CO2 with California gas

When processing California gas without CO2 recycling in a single reactor (point C in Figure 26), any desired ratio of H2/CO (for example 2/1) can be reached at point D with very low conversions and yield. As a consequence, when recycling all the CO2, its flowrate at the outlet stream is too low making it impossible to reach the H2/CO ratio of 2/1 with a high syngas yield and methane conversion and away from the carbon formation region, as desired. Indeed, the total recycle gives a new feed composition represented by point C1. Point D1, however, it is still located inside of the region favorable for carbon formation. Therefore, any point at the right of the carbon formation curve cannot be obtained just by recycling. As in the previous case, increasing the CO2 recycle increases the syngas yield and methane conversion. Therefore, to reach point E (outside of the carbon formation region), addition of external CO2 and H2O to the reactor has to be considered.

33

Consider the addition of external CO2 (Figure 27) to obtain a syngas ratio of 2/1 with high methane conversion (95%) and high syngas yield. This is reached by adding CO2 to the feed to get 0.35 CO2/CH4 (Point C1). The new feedstock also needs the addition of water (0.7 H2O/CH4) to reach point D1, which is located on the borderline of the carbon formation line. Since the operation on the safe side of the carbon formation line is desired, more addition of both CO2 and H2O has to be considered, because the recycling option is not enough to move the equilibrium away from the carbon formation region. Finally, the effect of recycling CO2 on the duty of the reactor for the case of California gas with external CO2 addition results in an increase of duty increases, but at a very low rate (0.4% to 0.6 % max.).

0

1

2

3

4

5

6

7

8

9

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0

O/C ratio

H/C

ratio

H2O/CH4

CO2/CH4

0.4

0.7

1.0

1.5

2.0

2.5

3.0

0.2

0.5

0.751.0

5.0

3.0

1.52.0

0.02

H 2 :C O 3:1

H 2 :C O 2.5:1

H 2 :C O 2:1

H 2 :C O 1.5:1

H 2 :C O 1:1

0.05

carbon formation line

Constant CO2 at outlet stream Kg-mol/Kg-mol CH4

C

0.01

D D

3.999

3.99

Syngas yield

C1

C2

D2

0.1

0.3

0.2

0.4

0.5

1

Duty

114,

000

(54,

556)

113,

000

(54,

078)

110,

000

(52,

642)

90,0

00 (4

3,07

1)

Figure 27: CO2 recycle on Single reactor, California gas

34

Concluding, for Terrell gases processing the use of CO2 recycle does not have an advantage over the no-recycling option (more addition of water, more duty required, and a marginal increase in Syngas yield). On the other hand, when recycling CO2 the separation and compression step has to be considered. For California gas processing, the CO2 recycling option does not help to obtain good syngas yield or a high methane conversion, and the addition of CO2 has to be considered. In such case, recycling can be of value, especially if the generation of CO2 proves to be more costly than its generation. Optimal conditions for Single Reactors After analyzing the single reactor configuration on the equilibrium chart, it is desired to establish a set of optimum operating conditions for the processing of both Terrell and California gas. To obtain the optimum temperature, minimum flowrate of water, higher conversion, higher yield possible, and minimum duty, the simulator Pro II TM (Simulation Sciences) has been used. The fixed range for the constraints of the problem is shown in Table 6.

Table 6: Set of constraints for single reactor configuration

Constraint Value Methane conversion 99.9 % Syngas Yield, (Kg-mol/Kg-mol CH4) 3.99 CO2 outlet (Kg-mol/Kg-mol CH4) The minimum possible Water addition (Kg-mol/Kg-mol CH4) The minimum possible Duty The minimum possible Reactor pressure, atm 1. Reactor Temperature, °F 650°C-930°C

From the equilibrium chart at a fixed temperature of 850°C, these optimum values can be obtained. These values are depicted for the case of Terrell gas processing in Table 7. The addition of external CO2 is considered for the case of Syngas ratio 1/1 to meet our set of constraints regarding Syngas yield as it was seen from previous equilibrium chart analysis.

Table 7: Optimal conditions Terrell gas from Equilibrium chart at 850°C, 1 atm

Terrell gas processing – optimal conditions from equilibrium chart H2

CO ratio

XCH4 %

H2O CH4

Ratio

CO2 outlet, Kg-mol CO2

Kg-mol CH4

Syngas yield Kg-mol

Kg-mol CH4

Recycle /Addition

Duty (KJ/Kgmol CH4)

1/1 99.0 0.22 0.15 3.96 Addition/recycle 53,599 1.5/1 99.85 1.42 0.55 3.993 No recycle 54,509 2/1 99.95 2.70 0.85 3.999 No recycle 55,274

35

Only for the case of required 2/1 ratios, the conditions chosen have fulfilled the constraints of methane conversion and syngas yield. Therefore, to reach the constraints it is required an increase in the reactor temperature for the other ratios. Since this effect is not easily visualized using the equilibrium chart, a process simulator is used. To find the optimal temperature required to meet the constraints, the methane conversion has been fixed at 99.95% and a fixed ratio of H2/CO has been set as constraint. The amount of water added to the feed and the temperature of the reactor were adjusted to achieve this goal. Since conversion and H2/CO syngas ratio are set only one temperature satisfies this goal. The results are given in Table 8.

Table 8: Optimal conditions Terrell gas and recycling CO2

H2 CO ratio

XCH4 %

H2O CH4

CO2 out. Kg-mol

Kg-mol CH4

Syngas Kg-mol

Kg-mol CH4

Recycle Or

Addition

Duty (KJ/Kgmol

CH4)

Temp Rx. °C

1/1 99.95 0.22-0.76 0.15-0.57 3.99 Both 54,561 898 1.5/1 99.95 1.42-2.6 0.55-0.9 3.999 Recycle 54,509 829-862 2/1 99.95 2.7-3.6 0.85-1.42 3.999 Recycle 55,274 814-850 When processing California gas at H2/CO ratios ranging from 1/1 to 2/1, a reasonable syngas yield cannot be obtained unless external CO2 addition is considered. However, under the set of constraints imposed, for H2/CO ratios from 5/1 and above, its processing is convenient. In addition, when a constant high CH4 conversion is desired under the same scheme of Terrell gas processing, the system evaluated turns out to be infeasible for the cases when the goal was a syngas ratio from 1/1 to less than 3/1, as one can infer from the equilibrium chart. When the syngas ratio increases to values of 3/1 and higher, there is convergence for all the set of data. This is shown in Table 9.

Table 9: Temperature to get constant XCH4, California gas-No recycle

California gas, adding water to the feed to meet H2/CO ratio H2/CO ratio

Rx. Temp.

°C

% XCH4

H2O CH4

CO2 outlet, Kg-mol

Kg-mol CH4

H2+CO, Kg-mol

Kg-mol CH4

Duty KJ

Kg-mol CH4

1/1 Infeasible 1.5/1 Infeasible 2/1 Infeasible 3/1 1078 99.95 1.1 0.01 3.999 53,659 5/1 860 99.95 3.48 0.35 3.999 56,715

36

It is observed that the temperature required to get fixed methane conversion decreases as the recycle of CO2 increases but the duty goes up. This effect plays a very important role in deciding to recycle CO2. For example for the case with an H2/CO ratio of 2/1, when recycling all the CO2 the temperature can be reduced from 850°C to 767°C, allowing the catalyst to work under less severe conditions. The H2O/CH4 ratio also increases significantly. However, the duty of the reactor increases in approximately 5 % when going from 0% to 100% CO2 recycle. Table 10, shows similar calculations for Terrell gas processing.

Table 10: Terrell gas and recycling CO2

Terrell gas, recycling CO2 from simulator H2/CO ratio

% Recycle

Rx. Temp.

°C

% XCH4

H2O CH4

CO2 out. Kg-mol

Kg-mol CH4

H2+CO, Kg-mol

Kg-mol CH4

Duty KJ

Kg-mol CH4

1/1 20 946 99.95 0.39 0.25 3.99 54,829 1/1 40 930 99.95 0.49 0.33 3.99 54,882 1/1 60 917 99.95 0.58 0.41 3.99 54,935 1/1 100 898 99.95 0.76 0.57 3.99 55,146

1.5/1 20 862 99.95 1.93 0.84 3.999 55,780 1.5/1 40 846 99.95 2.30 1.09 3.999 56,097 1.5/1 60 829 99.95 2.61 1.36 3.999 56,414 1.5/1 100 806 99.95 3.22 1.87 3.999 56,942 2/1 20 824 99.95 3.30 1.23 3.999 56,625 2/1 40 806 99.95 3.89 1.61 3.999 57,047 2/1 60 790 99.95 4.43 1.98 3.999 57,470 2/1 100 767 99.95 5.43 2.74 3.999 58,421

Conclusions for Single reactor configurations Assuming that a catalyst able to work under the predicted conditions exists (high temperature, presence of water, carbon deposition, etc.), the following conclusions are made:

• When using Terrell gas, recycling CO2 increases the yield, the water required, the

duty, and the methane conversion at constant desired H2/CO ratios. The cost of CO2 separation and recycling needs to be considered.

• For California gas processing, and working at low desired Syngas H2/CO ratios,

recycling CO2 is not enough to get high yields and conversion. Thus, external CO2 is needed.

37

• The best conditions for Single reactor configuration are given in Table 11.

Table 11: One reactor-California gas, addition of CO2 and H2O to the feed

California gas, one reactor with fixed temperature. H2/CO Ratio

Rx 1 Temp.

CO2

CH4

X CH4

Total, % H2O CH4

CO2 out Kg-mol CO2

Syngas Kg-Mol

Duty KJ

°C Kg-mol CH4 Kg-Mol CH4 Kgmol CH4

2/1 927 0.32 99.95 0.66 0.004 3.87 50,163 Terrell gas, one reactor configuration

1/1 898 1.19 99.95 0.22-0.76 0.15-0.57 3.990 54,561 1.5/1 829-862 1.19 99.95 1.42-2.6 0.55-0.9 3.999 54,509 2/1 814-850 1.19 99.95 2.7-3.6 0.85-1.42 3.999 55,274

Reactors in Series We first consider the case of reactors in a series configuration. Removal of H2O and/or CO2 after the first reactor and an eventual addition of water to the second reactor to meet the desired H2/CO ratio at the outlet syngas (Figure 28) are considered. The goal is to find a reactor scheme that might competitively produce syngas (high conversion, high yield, low CO2 at the outlet stream, and with better economics) when compared to the single reactor scheme.

California/ Terrell

H2O/CO 2

H 2 CO H 2O CO 2 CH 4

C 2H 6 N 2 Rx-1 Rx-2

H2O

H2O/CO 2

Figure 28: Generalized series reactor scheme Several arrangements of two series reactors were evaluated; some for simple illustration of the way the equilibrium chart can be used.

a. Both reactors working at the same temperature, the first one to get an

intermediate Syngas ratio H2/CO, while in the second one the objective ratio of H2/CO of 2/1 is reached. There is no CO2 or water removal after the first reactor.

38

b. Addition of water to the first reactor, removal of CO2 from the outlet stream, and addition of water to the second reactor. This scheme is advantageous for a gas like Terrell gas.

c. Addition of CO2 and water to the first reactor, removal of CO2 from the outlet stream, and addition of water to meet the desired H2/CO ratio. This scheme is not good for a Terrell gas, so it is used for a California type gas.

d. Same as case b but recycling the CO2 from the separation stage. These cases are summarized in Figure 29.

California/Terrell

H2O H2/CO=2

Rx-1 Rx-2

H2O

CO2

CO2

CASE D

California/Terrell

H2O H2/CO=2

Rx-1 Rx-2

H2O

CO2

CO2

CASE C

California/Terrell California/Terrell

H2O

Rx-1 Rx-2

H2O

CASE A

H2O H2/CO=2

Rx-1 Rx-2

H2O

CO2

CASE B

Figure 29: Layout of cases study with series reactors

For case a, in the first reactor an intermediate syngas H2/CO ratio is obtained adding water to the feed to operate outside of the region favorable for carbon formation, whereas in the second reactor the final H2/CO ratio of 2/1 is obtained. Using Terrell gas represented by point A (Figure 30), one can, for example, obtain a syngas ratio of 1.25/1 by adding water to move to point A1. This point represents the feed to the second reactor, which in turn needs additional water to reach the desired final syngas H2/CO ratio of 2/1 (point B). Clearly, the performance of two reactors in series can easily be reached with just one step (single reactor scheme), using the temperature and feedstock conditions the second reactor. As it will be shown later case a can be optimized using different reactor temperatures.

In case b, there is an intermediate step of CO2 removal before the second reaction step. As long as the temperatures in both reactors are the same, the equilibrium chart can

39

still accurately represent the behavior of this reactors scheme (Figure 30). When performing the CO2 removal of the mixture of composition of point A1, point A2 is obtained. This point lies directly on a constant water line. From here, since water has to be added to get the final ratio, the point is moved following the non-rectangular equilibrium coordinates until it reaches the 2/1 Syngas ratio line represented by point B1 (the total water required per mol of methane fed to the set of reactors is 1.70 H2O/CH4). The final yield and methane conversion is lower than in a single reactor configuration (point B), but the total amount of water used is lowered by 0.22 Kg-mol H2O/Kg-mol CH4.

0

1

2

3

4

5

6

7

8

9

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0O/C ratio

H/C

ratio

H 2 :C O 3:1

H 2 :C O 2.5:1

H 2 :C O 2:1

H 2 :C O 1.5:1

H 2 :C O 1:1

0.4

0.7

1.0

1.5

2.0

2.5

3.0

0.2

0.5

1.00.75

2.0

5.0

1.5

3.0

3.99

3.95

3.98

Constant Yield Kg-mol/Kg-mol CH4

Constant duty BTU / mole CH4

(KJ/Kg-mol CH4)

B

A

3.999

H2O/CH4

CO2/CH4

A1

A2

B1

115,

000

(550

35)

114,

000

(54,

556)

113,

000

(54,

078)

110,

000

(52,

642)

70,0

00 (3

3,50

0)

90,0

00 (4

3,07

1)

Figure 30: Series reactors cases a and b - Terrell gas, 850 °C, 1 atm

40

Table 12 shows a comparison between a single reactor and two reactors in series using Terrell gas as feedstock and the configuration of case b. The only advantage of the series reactors scheme is therefore the reduction in steam usage. However, there is an increase in duty (3%).

Table 12: Comparison of reactors in series (case b) with single reactor at 850 °C

Single Reactor

Reactors in series Case b

Reactor Temperature, °C 850 850 Reactor Pressure, atm. 1 1 Methane conversion, % 99.95 99.82 Syngas yield, Kg-mol /Kg-mol CH4 3.999 3.993 CO2 outlet, Kg-mol CO2/Kg-mol CH4 0.82 0.44 Final H2/CO ratio 2.0 2.0 Water added, total H2O/CH4 ratio 2.65 2.63 Reactor duty, total KJ/Kg-mol CH4 56,088 57,777 Reactors involved 1 2 CO2 separation Final Intermediate and final. We illustrate case c using California gas because it is the appropriate option for this

gas. In the single reactor configuration analysis, it was concluded that this gas needs the addition of both CO2 and H2O as well as a recycle of CO2 in order to obtain a mixture outside of the region favorable for carbon formation with relevant conversions or yields. It is clear that, unless syngas ratios lower than 1:1 are desired, adding CO2 to a Terrell gas does not make sense. Figure 31 shows California gas represented by point A, which with addition of CO2 and H2O in the first reactor reaches the point A1 (0.65 H2O/CH4. and 0.75 CO2/CH4). After the first reactor, a CO2 separation step is performed reaching point A2, which is followed by water addition to reach the final 2/1 Syngas ratio (Point B1). The total amount of water is 1.3 H2O/CH4.

41

0

1

2

3

4

5

6

7

8

9

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0O/C ratio

H/C

ratio

H 2 :C O 3:1

H 2 :C O 2.5:1

H 2 :C O 2:1

H 2 :C O 1.5:1

H 2 :C O 1:1

0.4

0.7

1.0

1.5

2.0

2.5

3.0

0.2

0.5

1.00.75

2.0

5.0

1.5

3.0

3.99

3.953.98

Constant Yield Kg-mol/Kg-mol CH4

Constant duty BTU / mole CH4

(KJ/Kg-mol CH4)

A

3.999

H2O/CH4

CO2/CH4

A1A2

B1

115,

000

(55,

035)

114,

000

(54,

556)

113,

000

(54,

078)

110,

000

(52,

642)

70,0

00 (3

3,50

0)

90,0

00 (4

3,07

1)

Figure 31: Series reactors with California gas-case c

Case d, where recycling of CO2 is performed after the first reactor, can also be represented on the equilibrium chart (Figure 32). Starting with Terrell gas as feedstock, if no recycle exists, the reactor requires water to go from A to A1 to reach a desired partial ratio of 1.5/1. At that point, there is an intersection line of constant CO2 (0.6 moles/mol CH4). The segment A2 to B represents the partial recycling operation

42

0

1

2

3

4

5

6

7

8

9

10

0.0 0.5 1.0 1.5 2.0 2.5O/C ratio

H/C

ratio

3.0

H 2:C O 3:1

H 2:C O 2.5:1

H 2:C O 2:1

H 2:C O 1.5:1

H 2:C O 1:1

0.4

0.7

1.5

2.0

2.5

3.0

0.2

0.5

1.00.75

2.0

5.0

1.5

3.0

3.99

3.95

3.83.5

H2O/CH4

CO2/CH4

3.98

Constant YieldKg- mol/Kg- mol CH4

B

A

A1

A2

3.999

0.6

Constant CO2

Kg-mol KG-/mol CH4

0.9

1.0

B1

70,0

00 (3

3,50

0)

Constant duty BTU /mole CH4

(KJ/Kg-mol CH4)

110,

000

(52,

642)

113,

000

(54,

078)

114,

000

(54,

556)

115,

000

(55,

035)

Figure 32: Series reactor with CO2 recycle-Terrell gas

It is very important to point out that to reach the final H2/CO ratio, water has to be added to the second reactor to reach point B1 and as a consequence a higher reactor duty is needed. Another option to consider is the separation and recycling of CO2 at each step of reaction, however, as in the previous case d, the usage of water and the duty increases.

In the single reactor analysis it is shown that there is no monotone dependence of duty with temperature. However, the requirements of water increases when increasing temperature, showing a H2O/CH4 ratio increase from 2.67 to 3.01 (12.4%), thus identifying savings on steam when working at lower temperatures. Therefore, to accurately predict the behavior of series reactors arrangement, simulations using ProII

43

(Simulation Sciences) have to be performed to obtain the values of temperatures and corresponding duties, yield, conversions, and CO2 content at the outlet stream. In addition the effect of recycling CO2 needs to be analyzed.

Optimal conditions for two reactors in series

Case a is now optimized varying the temperature of both reactors. The following

objective function, variables and constraints have been set up in the simulator: • Objective Function: Minimization of the total duty (KJ/Kg-mol CH4) • Constraint: Total methane conversion = 99.95% • Specifications: H2/CO ratio Reactor 1= 1.25. Reactor 2= 2.0 • Variables: Temperature reactor 1 (593-927°C)

Temperature reactor 2 (760-927°C) Flowrate of water to reactor 1 and 2

When there is no specification for the syngas ratio at the outlet of reactor 1, the

system converges to a solution using only one reactor. Therefore a syngas ratio specification for reactor 1 was introduced to force the system to choose one non-optimal alternative. This is done just to assess the differences.

Table 13 shows the results for Terrell gas. When comparing with data for single

reactor (Table 12) one can realize that higher duty is required for this case (an additional 2,134 KJ/Kg-mol CH4), which represents an increase of 3.8%. The Syngas yield, conversion, and total flowrate of water are almost the same when comparing to single reactor performance.

Table 13: Series reactors at two temperatures-Terrell gases Terrell gas, one reactor with fixed temperature.

H2/CO Rx1

Temp. Rx2

Temp. X CH4

H2OCO2 out

Kg-mol CO2

Syngas Kg-mol

Duty KJ

Ratio °C °C Total, % CH4 Kg-mol CH4 Kg-mol CH4 Kg-mol CH4 2/1 701 850 99.95% 2.63 0.84 3.999 58,222

Therefore, the use of Terrell gas in series reactors configuration is not worthwhile

because of the higher duty required when compared to single reactor operation. In addition, the use of CO2 recycle increases the duty at higher levels and also involves the cost of separation and compression.

44

Results for the optimization of case b are shown in Table 14 (also using Terrell gas as feedstock). The same values for the temperature range in reactor 1 were chosen, and, in addition the temperature range in reactor 2 is allowed to vary between 760°C to 930°C. Since a high conversion is set as constraint, both conversion and Syngas yield are at its maximum value. However, in this case the amount of water used is considerably lower when comparing to the single reactor configuration. The duty is still higher (an additional 1665.4 KJ/Kg-mol CH4, or 3.0 %).

Table 14: Series Reactors with CO2 removal – Terrell gas case b

Terrell Gas, CO2 removal after first reactor.

H2/CO Rx 1

Temp. Rx 2

Temp. X CH4

H2O CO2 out

Kg-mol CO2

Syngas Kg-mol

Duty KJ

Ratio °C °C Total, % CH4 Kg-mol CH4 Kg-mol CH4 Kg-mol CH4

2/1 747 899 99.95% 1.79 0.84 3.999 57,753

The CO2 recycling option is not analyzed in the simulator since it is already known

that an increase in recycle increases the duty and the water required to reach the fixed Syngas H2/CO ratios.

We now turn to the case of processing California gas without any addition of CO2

to the feed (using case b). The results of the simulation studies when there is no addition of CO2 to the feed are shown in Table 15. As it was confirmed using the equilibrium chart, it is possible to produce the syngas ratio of 2/1 but the yield of Syngas as well as the conversion are too low. Clearly, California gas without CO2 addition is not an option to be considered to produce Syngas with a ratio lower than 3/1.

Table 15: Simulation results for series reactors – California gas

CALIFORNIA NATURAL GAS – no CO2 addition to the feed

H2/CO Rx 1 temp. Rx 2 temp. X CH4

H2O CO2 out

Kg-mol CO2

Syngas Kg-mol

Ratio °C °C Total, % CH4 Kg-mol CH4 Kg-mol CH4

1/1 650 816 1.9 0 0 0.00 2/1 650 760-816 2.0 0.026-0.029 0 0.001 3/1 650 760-816 86.5 0.73-0.93 0.01 0.03

When comparing these results with a single reactor configuration for California gas

shown in Table 7 it is seen that there is no advantage in this alternative scheme.

45

The results of using case c for California gas are shown in Table 16. For this case the objective function, constraints and variables are as follows:

Objective Function: Minimizing Cost Constraint: Methane conversion: 99.95% Specifications: H2/CO reactor 1: 1.25/1 H2/CO reactor 2: 2/1 Variables: Temperature reactor 1: 593°C-927°C Temperature reactor 2: 593°C-927°C Water to reactor 1 Water to reactor 2 CO2 to reactor 1

The cost is based on duty and water usage. With the addition of CO2 and H2O in the feed, methane conversion as well as the Syngas yield increase for a fixed H2/CO ratio. The amount of CO2 at the outlet stream also increases. However, the maximum yield obtained is close to the stoichiometric value of 4.0 moles Syngas/lb-mol CH4, which can be obtained using just a single reactor.

Table 16: Simulation results for series reactors: California gas with CO2 addition

California gas and addition of CO2 = 0.6 CO2/CH4

H2/CO Rx 1

Temp. Rx 2

Temp. X CH4 H2O CH4

CO2 out Kg-mol CO2

Syngas Kg-Mol

Duty KJ

Ratio °C °C Total, % Kg-mol CH4 Kg-mol CH4 Kg-mol CH4

2/1 809 927 99.95% 1.49 0.28 3.999 55,983 Conclusions for two reactors in series

• Using Terrell gas the series reactors scheme has no advantage over the single reactor configuration because the duties obtained with series reactors are in all cases higher. When minimizing the duty, and, at the same time the usage of water in both reactors, the system gives a solution with lower steam consumption. However, the duty is still high in the order of 3.0 to 3.5 %.

• The alternative of recycling CO2 when using Terrell gas as feed, is not a good

option for any configuration because the duty and the water requirements increases with increasing recycle.

46

• California gas is better suited for hydrogen processing through the Steam reforming process. For producing lower Syngas ratios, it needs CO2 addition. Only when the cost of external CO2 is zero or very low, this alternative can be used effectively. No configuration of reactors in series is advantageous.

Analysis of Parallel Reactors using the equilibrium chart

Consider a parallel arrangement of reactors, one under steam reforming conditions and the other under CO2 reforming conditions. Water is separated from the outlet streams, which are mixed to form the final outlet Syngas stream. CO2 recycling from the CO2 reforming reactor has also been explored. The goal is to find the best set of operating conditions (maximum yield and methane conversion) under which CO2 reforming of CH4 is a good production scheme for several fixed H2/CO ratios, while keeping at a minimum the amount of water added and the CO2 flowrate at the outlet stream. Both extreme feedstock are evaluated using this arrangement.

In Figure 33, two options have been represented: the first consists of a reactor working under steam reforming (A), and the other working only under CO2 reforming conditions (B). Both reactors operate using California gas feedstock. Point B is obtained by adding CO2 to the feed and performing dry reforming. Any mixture of both reactors is located at point M, over the line joining B and A. (Mixtures follow the lever rule). The duty also follows the lever rule and is equal to 54,556 KJ/Kg-mol CH4, a Syngas yield of 3.994 Kg-mol /Kg-mol CH4, a methane conversion of 99.6 %, and a CO2 content in the outlet stream of 0.19 Kg-mol CO2/Kg-mol CH4 is obtained. It is important to point out that the operation of point B is inside the region favorable for carbon formation. Therefore water addition is needed to move the feed composition to point B1. Now the point M represents mixture of A and B1 reactors.

47

0

1

2

3

4

5

6

7

8

9

10

0.0 0.5 1.0 1.5 2.0 2.5

O/C ratio

H/C

ratio

3.0

H 2:C O 3:1

H 2:C O 2.5:1

H 2:C O 2:1

H 2:C O 1.5:1

H 2:C O 1:1

0.4

0.7

1.0

1.5

2.0

2.5

3.0

0.2

0.5

1.00.75

2.0

5.0

1.5

3.0

3.95

3.83.5

32.5

2

H2O/CH4

CO2/CH4

3.98

C

M

A

B

Constant Yieldmole Syngas/mole CH4

3.99

3.999

B1

Constant Duty, BTU/mole CH4(KJ/Kmol CH4)

110,

000

(52,

642)

113,

000

(54,

078)

114,

000

(54,

556)

115,

000

(55,

035)

Figure 33: Parallel reactors California gas When processing Terrell gas (Figure 34), the operation of the CO2 reforming

reactor falls inside of the carbon formation region (Point A). Therefore, with addition of water, the point should be moved to point C. Point B corresponds to steam reforming operation. Mixtures of B and C follow the lever rule.

48

0

1

2

3

4

5

6

7

8

9

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0O/C ratio

H/C

ratio

H 2:C O 3:1

H 2:C O 2.5:1

H 2:C O 2:1

H 2:C O 1.5:1

H 2:C O 1:1

0.4

0.7

1.0

1.5

2.0

2.5

3.0

0.2

0.5

1.00.75

2.0

5.0

1.5

3.0

3.95

3.83.5

32.5

2

H2O/CH4

CO2/CH4

3.98

M

B

A

Constant Yieldmole Syngas/mole CH4

3.99

3.999

C

Constant Duty, BTU/mole CH4 (KJ/Kg-mol CH4)

110,

000

(52,

642)

113,

000

(54,

078)

114,

000

(54,

556)

115,

000

(55,

035)

Figure 34: Parallel reactors, Terrell gas Optimal conditions for parallel reactors configuration

To accurately predict the behavior of parallel reactors a simulator is used with the following objective function, variables and constraints:

49

Objective function: Minimize cost (duty + water usage) Constraints: H2/CO syngas ratio = 2/1 Total XCH4= 0.98, 0.99, 0.995, 0.9995 Gas flow to both reactors = 45.3 Kg-mol/hr (100 lb-mol/hr) Variables: Water to Reactor 1 Temperature of reactor 1

Temperature of reactor 2 Flowrate of gas to reactor 1 Flowrate of gas to reactor 2

Terrell gas processing

For the parallel reactors case studies, both gas feedstock have been evaluated using the flowsheet from Figure 35. The first approach to perform the simulations is to work under same temperature conditions and fixing the flow of CH4 to the CO2 reforming reactor. This is necessary because otherwise, the optimization chooses one reactor.

REACTOR-1

REACTOR-2

M1

E1COOLER-1

M2E3

COOLER-2

M3

TERRELL-1

WATER-RX1

TERRELL-2

WATER-RX2

S3 INLET-RX1

OUT-STEAM-RX

S9

S10 INLET-RX2

OUT-CO2RXS13

SYNGASS1

S2

Figure 35: Parallel reactors optimization flowsheet – Pro II

This simulation has been plotted on Figure 36. For this case point M represents the mixture corresponding to 95% of reactor 2 and 5% of reactor 1. When the duty is minimized by varying reactor temperatures and by keeping the flowrate to each reactor fixed and adding water only to the second reactor, or to both reactors, the results shown in Table 17 are obtained.

50

0

1

2

3

4

5

6

7

8

9

10

0.0 0.5 1.0 1.5 2.0 2.5 3.0O/C ratio

H/C

ratio

H 2 :C O 3:1

H 2 :C O 2.5:1

H 2 :C O 2:1

H 2 :C O 1.5:1

H 2 :C O 1:1

0.4

0.7

1.0

1.5

2.0

2.5

3.0

0.2

0.5

1.00.75

2.0

5.0

1.5

3.0

3.99

3.95

3.83.5

3.98

Constant Yield mole Syngas per mole CH4

Reactor 1

3.999

H2O/CH4

CO2/CH4

Reactor 2

M

Constant duty BTU / mole CH4

(KJ/Kg-mol CH4)

110,

000

(52,

642)

113,

000

(54,

078)

114,

000

(54,

556)

115,

000

(55,

035)

Figure 36: Parallel reactors – Terrell gas

For a fixed flowrate, the duty of the parallel reactors arrangement is always higher than the case of one reactor operation (for any combination of flows). Table 18 provides additional results.

Table 17: Duties calculated for the different approaches of Parallel reactors

Flow reactor 1/2 Kg-mol/hr

Fixed Water to reactor 1 only

Water to both reactors

Single reactor configuration

2.3/43.0 56,643 56,614 56,610 56,007 4.5/40.8 57,624 57,572 57,533 55,944 9.0/36.3 59,605 59,538 59,452 55,844

51

56,000

56,500

57,000

57,500

58,000

58,500

59,000

59,500

60,000

0.9965 0.9970 0.9975 0.9980 0.9985 0.9990

X CH4

KJ/

kg-m

ol C

H 4 Temp. Both Reactors Constant

Temp. Both Reactors changing

Temp. Both reactors changing

and addition of water to Reactor 2

minimizing duty

Figure 37: Reactor duty as a function of methane conversion at fixed H2/CO ratios

Table 18: Parallel reactor simulation data-Terrell gas

Reactor 2, kg-mol/hr 2.268 4.536 9.072 Reactor 1, kg-mol/hr 43.092 40.824 36.288

H2/CO ratio 2.0 2.0 2.0 Duty, KJ/kg-mol CH4 56,610 57,533 59,452

Total X CH4 0.9987 0.9979 0.9966 Yield, kg-mol/kg-mol CH4 3.999 3.999 3.999 (H2O/CH4) reactor 1 ratio 2.77 2.91 3.37 Reactor 1 temperature, C 841 828 811 Reactor 2 temperature, C 824 829 831 (H2O/CH4) reactor 2 ratio 0.17 0.15 0.15

California gas processing

When processing California gas with the parallel arrangement, it has been shown, using the equilibrium chart, that the CO2 reforming reactor requires additional CO2 to increase the Syngas yield and make the process profitable. The results are shown in table 19. The table clearly shows that California gas is not suitable for Syngas ratios in the range of 2.5/1 and below.

52

Table 19: California gas processing

California gas - Parallel Reactors -No CO2 addition to the CO2 Reforming Reactor

H2/CO Temp, °C X CH4 H2O/CH4 CO2 outlet H2+CO Flow to reactors,

Kg-mol/hr Ratio Reactor 1 Reactor 2 In feed Mole frac. Kg-mol/hr Steam Ref. CO2 ref 1.5/1 760.0 788.3 0.0081 0.01 0.0000 3.1 45.4 0.0 2/1 808.3 760.0 0.0250 0.02 0.0000 5.2 15.9 29.5 3/1 812.8 760.6 0.6460 0.70 0.0037 117.3 32.9 12.5

Conclusions for parallel reactors

• The parallel scheme of processing the two feedstocks (Terrell and California gas) does not offer advantages over the single reactor configuration. Lower duty, and the same level of yield are obtained using a single reactor for both cases. The recycling option was not included in the simulations because the recycling increases the usage of steam and also the duty of the reactor.

Conclusions In this paper, a thermodynamic analysis of the steam and dry reforming of methane is performed. Single and multiple reactor configurations are investigated to conclude that single reactor configurations are the optimum. A previously developed equilibrium chart was expanded to include constant conversion and yield, as well as constant duty and constant outlet CO2 lines. The use of this chart proves to be useful to pick the right processing conditions. Acknowledgements This work was supported by the DoE/EPSCOR program of the Department of Energy (DE-FG02-99ER45759).

53

54

REFERENCES 1 Wang, S and Lu, G.Q., Carbon Dioxide Reforming of Methane to produce synthesis

gas over Metal-supported catalysts, Energy and Fuels, Vol. 10 No. 4, p. 899 (1996)

2 Edwards, J.H., Maitra A.M., The Chemistry of methane reforming with carbon dioxide and its current and potential applications, Fuel Processing Technology, 1995, 42, 269-289.

3 Nicolaas, A., Van Keulen, J., Hegarty, E.S., Ross Julian R.H., Van den Oosterkamp, Paul F., The development of Platinum-Zirconia catalysts for the CO2 reforming of methane, Faculty of Chemical technology, University of Twente, P.O.Box 217, 7500 AE Enschede, The Netherlands.

4 Dibbern, H.C.; Olesen, P.; Rostrup-Nielsen, J.R.; Tottrup P.B.; Udengaard, N.R.; Hydrocarbon Processing, January 1986, 71-74, (1986).

5 Wender, Irving, Reactions of Synthesis Gas, Fuel Processing Technology, Vol 48, 189-297 (1996).

6 Meyers, R.A., Handbook of Petroleum Refining Processes, 2nd Edition, Mc Graw-Hill 1997.

7 Satterfield Charles, Heterogeneous Catalysis in industrial Practice, 2nd Edition, Krieger Publishing Company, Malabar Florida, 1996

8 Harold Gunardson, Industrial Gases in Petrochemical Processing, Marcel Dekker, Inc, New York, 1998.

9 Bitter, J.H., Platinum based bifunctional catalysts for carbon dioxide reforming of methane, Enschede, The Netherlands, 1997

10 Bradford, M.C., Vannice, M.A., CO2 reforming of methane, Catal.Rev. Sci.Eng., 41(1), 1-42 (1999).

11 Rostrup-Nielsen, J.R., Catalytic Stem Reforming, Catalysis Science and Technology, Vol. 5, 1-117, 1984