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. ([email protected])
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
Pernis, Rotterdam, Netherlands
New Orleans H2 Plant, LA
Pasadena Hydrogen Plant
Houston, TX
Wilton Hydrogen Plant
Teekside, UK
Paraguana H2 Plant, Venezuela
Amuay H2 Plant, Venezuela
Geismar HYCO Plant
Geismar, LA
Convent, LA, USA
ANSDK I, El-Dikheila, Egypt
ISPAT DR3
Point Lisas, Trinidad & Tobago
Saldanha Steel, Saldanha Bay,
South Africa
Main product
H2/CO
H2
H2
Capacity, MMSCFD
80
70
80
Year
994
1994
1996
H2
18
1997
H2
H2
H2+CO+ Syngas
50
50
45
1997
1997
1999
Direct reduction
Iron (DRI)
DRI
DRI
1.2 MT/yr
1998
0.80
1.36
2000
1999
DRI
0.804
1999
Table 2: Typical applications for Syngas production
H2/CO
>3
2-3
2-2.5
1.7-2
1.5
1
<1
Application
Hydrogen, Ammonia
Methanol
Fischer Tropsch (gasoline and light
olefins)
Fischer Tropsch (Fixed Bed, for waxes or
diesel)
Aldehydes, Isobutane, Isobutanol, Higher
Alcohols (C1-C6)
Acetic Acid
Polycarbonates
3
Process Used/Proposed
Steam reforming and water gas shift
Steam reforming
Partial oxidation, steam reforming
Steam reforming –Partial Oxidation
Hydroformylation
Autothermal Reforming
CO2/CH4 (dry) reforming
Table 3: Typical Conditions of Downstream Processes 4,5
Process
T (°C)
P (MPa)
Hydrogen
generation
815-870
2.5
Ammonia
Synthesis
775-835
Methanol
Synthesis
350-420
9.7-24.9
11.8-16.0
Fischer Tropsch
Fix Bed
Synthol
215-250
355-415
2.72
2.3
Hydroformylation
160-200
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
>3
2-3
2-2.5
1.7-2
1.5
1
<1
Application
Hydrogen, Ammonia
Methanol
Fischer-Tropsch synthesis
Fischer Tropsch (Fixed Bed)
Aldehydes, alcohols
Acetic Acid
Polycarbonates
Maximum CO2 Currently allowed
Total carbon oxides (CO+ CO2) must be reduced
to less than 15 ppm. 7
2-8 % vol.
Removed in previous stages
Removed in previous stages
CO2/CH4 = 2.2/1.0 5
CO2/CH4 = 0.54/1.0 5
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
Comp.
CH4
CO2
C2H6
N2
Terrell Gas
Composition
45.7 %
53.9 %
0.2 %
0.2 %
California Gas
composition
89.4 %
0.8 %
9.8 %
---
9
California Gas after
ethane removal
98.0 %
0.9 %
1.1 %
---
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 Æ 2 CO +2 H2
Reverse water gas shift (RWGS) reaction:
(2) CO2 + H2
Æ H2O (g) + CO
Steam reforming reaction:
(3) H2O + CH4 Æ 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 Æ CO2 + C(S)
Methane cracking reaction:
(5)
CH4
Æ 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
0.6
1 atm
10 atm
CO
0.5
25 atm
Equilibrium composition
CO
CO
0.4
H2
0.3
CO2
H2
CH4
CO2
CO2
H2
0.2
CH4
CH4
H2O
0.1
H2O
0
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
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.
0.6
900°C
800°C
Equilibrium composition
0.5
700°C
CO
CO
0.4
H2
H2
0.3
0.2
CH4
CH4
CO2
CO2
0.1
H2O
H2O
0
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
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.
4.5
1 atm
10 atm
25 atm
4.0
3.5
1/3
CO2/CH4 ratio
3.0
1/2
2.5
1/1 No carbon
deposition
2.0
1.5
1.0
carbon
deposition
region
Terrell gas
0.5
0.0
500
600
700
800
900
1000
1100
1200
1300
Tem perature, °C
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.
6.0
CO2/CH4 feed ratio
5.0
carbide
formation
4.0
3.0
2.0
carbon
deposition
1.0
0.0
750
800
850
900
950
1000
1050
1100
1150
Tem perature, K
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 FischerTropsch, 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.
0.60
0.50
H2O/feed ratio
0.40
0.30
0.20
0.10
0.00
0.0
0.5
1.0
1.5
2.0
2.5
3.0
CO2/CH4 feed ratio
800 °C
1000°C
900 °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.
3.0
10
H2O/CH4
2.5
9
Carbon
formation
curve, 1 atm
8
2.0
1.5
7
H2:CO 3:1
1.0
6
H/C ratio
0.7
H2:CO 2.5:1
0.4
5
H2:CO 2:1
B
4
H2:CO 1.5:1
0.2
3
0.5
H2:CO 1:1
0.75
2
A
1.0
1.5
1
2.0
CO2/CH4
3.0
5.0
0
0.0
0.5
1.0
1.5
2.0
O/C ratio
Figure 7: Equilibrium Chart at 1 atm
12
2.5
3.0
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
3.0
10
2.5
H 2O/CH 4
9
Methane
Conversion
8
2.0
99.8%
carbon
form ation line
99.95%
1.5
7
99.5%
99%
1.0
97%
95%
6
H/C ratio
0.7
5
0.4
H2:CO 3:1
80%
50%
H2:CO 2.5:1
30%
4
H2:CO 2:1
3
H2:CO 1.5:1
0.2
0.5
2
0.75
H2:CO 1:1
1.0
1.5
1
2.0
CO2/CH 4
3.0
5.0
0
0.0
0.5
1.0
1.5
2.0
2.5
O/C ratio
Figure 8: Constant methane conversion lines at 850°C and 1 atm
14
3.0
3.0
10
2.5
H2O/CH 4
9
8
3.999
2.0
Constant Yield
mole Syngas
per mole CH4
3.99
1.5
3.98
7
3.95
1.0
3.8
6
H/C ratio
H 2 :C O 3 :1
0.7
0.4
5
3.5
3
H 2 :C O 2 .5 :1
2.5
2
H 2 :C O 2 :1
4
H 2 :C O 1.5 :1
3
0.2
0.5
2
H 2 :C O 1:1
0.75
1.0
1.5
1
2.0
CO2/CH4
3.0
5.0
0
0.0
0.5
1.0
1.5
O/C ratio
2.0
2.5
3.0
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
3.0
10
0.3
H2O/CH4
9
Constant CO2 at outlet
stream
m oles CO2/m ole CH4
2.5
0.4
2.0
0.2
8
0.5
carbon
form ation
line
7
0.1
H/C ratio
0.005
0.7
0.7
0.075
H 2 :C O 3 :1
0.05
1.0
6
0.6
1.5
0.9
0.02
H 2 :C O 2 .5 :1
0.4
5
H 2 :C O 2 :1
4
H 2 :C O 1.5 :1
3
0.2
0.5
2
H 2 :C O 1:1
0.75
1.0
1.5
1
2.0
CO2/CH4
3.0
5.0
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
O/C ratio
Figure 10: Lines of constant CO2 in the outlet at 850°C and 1 atm.
OUTLET
GAS
M1
REACTOR
F1
S8
COOLER
HEAT-EXCH.
CO2
S3
S10
INLET
H2O
S7
Amb. Temperature
S11
Figure 11: Single reactor flow sheet for duty calculations
16
H2O/CH4
Constant duty
BTU/Lb-mol CH4
9
2.5
(KJ/Kg-mol CH4)
2.0
8
114,000 (54,556)
1.5
7
4
3
113,000 (54,078)
0.4
5
100,000 (47,857)
0.7
80,000 (38,285)
H/C ratio
6
110,000 (52,642)
1.0
115,000 (55,035)
3.0
10
H2:CO 3:1
H2:CO 2.5:1
H2:CO 2:1
H2:CO 1.5:1
0.2
0.5
2
H2:CO 1:1
0.75
1.0
1.5
2.0
1
CO2/CH 4
3.0
5.0
0
0.0
0.5
1.0
1.5
O/C ratio
2.0
2.5
3.0
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.
3.0
10
H2O/CH4
2.5
9
1 atm
Carbon formation
curve,
1 atm - 25 atm
8
2.0
25 atm
1.5
7
H2:CO 3:1
1.0
H/C ratio
6
0.7
H2:CO 2.5:1
0.4
5
H2:CO 2:1
4
H2:CO 1.5:1
0.2
3
0.5
2
H2:CO 1:1
0.75
1.0
1.5
1
2.0
CO2/CH4
3.0
5.0
0
0.0
0.5
1.0
1.5
2.0
2.5
O/C ratio
Figure 13: Influence of pressure on equilibrium Syngas lines @ 850°C
18
3.0
3.0
10
H2O/CH4
2.5
9
8
Carbon formation
curve,
1 atm - 25 atm
2.0
1.5
7
80 % Methane
Conversion
1 atm - 25 atm
1.0
H/C ratio
6
0.7
0.4
5
H2:CO 2:1
4
0.2
3
0.5
2
H2:CO 1:1
0.75
1.0
1.5
1
2.0
3.0
CO2/CH4
0
0.0
0.5
5.0
1.0
1.5
2.0
2.5
O/C ratio
Figure 14: Influence of pressure on methane conversion @ 850°C
19
3.0
3.0
10
Syngas yield
3.0 at
25 atm
H2O/CH4
9
8
Carbon formation
curve,
1 atm - 25 atm
2.5
2.0
1.5
7
1.0
H/C ratio
6
0.7
0.4
5
H2:CO 2:1
4
0.2
3
2
1
Syngas
yield
3.0 at
1 atm
0.5
H2:CO 1:1
0.75
1.0
1.5
2.0
3.0
CO2/CH4
0
0.0
0.5
5.0
1.0
1.5
2.0
2.5
3.0
O/C ratio
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
3.0
10
H2O/CH4
2.5
7
0.7
0.4
5
4
0.2
3
90,000 (43,071) @ 1 atm
1.0
6
H/C ratio
2.0
Carbon formation curve
1 atm - 25 atm
113,000 (54,078) @ 1 atm
8
1.5
25 atm
H2:CO 2:1
H2:CO 1.5:1
0.5
2
90,000 (43,071) @25 atm
9
H2:CO 1:1
0.75
1.0
1.5
1
2.0
CO2/CH4
3.0
5.0
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
O/C ratio
Figure 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).
3.0
10
H2O/CH4
2.5
9
2.35
2.0
8
carbon form ation line
850°C - 920 °C
1.8
1.5
7
1.0
6
H/C ratio
0.7
0.4
5
920 °C
H2:CO 2:1
850 °C
4
750 °C
0.2
3
0.5
0.75
2
A
1.0
1.5
2.0
1
3.0
CO2/CH4
5.0
0
0.0
0.5
1.0
1.5
O/C ratio
2.0
2.5
Figure 17: Influence of temperature on equilibrium Syngas lines @ 1 atm
22
3.0
3.0
10
H2O/CH4
X CH4
2.5
9
carbon
form ation line
at:
850°C - 920°C
8
2.0
1.5
7
99.8 %
920 °C
1.0
6
99.8 %
850°C
H/C ratio
0.7
0.4
5
C 920 C
850 C
H2:CO 2:1
4
B
0.2
3
0.5
0.75
2
A
1.0
1.5
2.0
1
3.0
CO2/CH4
5.0
0
0.0
0.5
1.0
1.5
O/C ratio
2.0
2.5
Figure 18: Influence of temperature on methane conversion @ 1 atm.
23
3.0
3.0
10
H2O/CH4
2.5
9
Syngas
Yield
carbon
form ation line
at:
850°C - 920°C
8
3.99
850°C
1.5
7
1.0
6
2.0
3.99
920°C
H/C ratio
0.7
0.4
5
C 920 C
850 C
H2:CO 2:1
4
B
0.2
3
0.5
0.75
2
A
1.0
1.5
2.0
1
3.0
CO2/CH4
5.0
0
0.0
0.5
1.0
1.5
O/C ratio
2.0
2.5
Figure 19: Influence of temperature on yield @ 1 atm.
24
3.0
3.0
10
H2O/CH4
Duty, BTU/m ol CH4
(KJ/kg-m ol CH4)
carbon form ation
line at:
850° C- 920 °C
1.5
7
1.0
H/C ratio
6
0.7
0.4
5
114,000 (54,556) @920°C
2.0
114,000 (54,556) @ 850°C
8
2.5
114,000 (54,556) @750°C
9
C
H2:CO 2:1
850 C
B
4
920 C
0.2
3
0.5
0.75
2
A
1.0
1.5
2.0
1
3.0
CO2/CH4
5.0
0
0.0
0.5
1.0
1.5
O/C ratio
2.0
2.5
Figure 20: Influence of temperature on reactor duty @ 1 atm
25
3.0
3.0
10
H2O/CH4
2.5
9
carbon
form ation line
at:
850°C - 920°C
8
2.0
CO2
mole CO2
mole CH4
1.5
7
0.7
850°C
1.0
0.7
920°C
6
H/C ratio
0.7
0.4
5
C 920 C
850 C
H2:CO 2:1
4
B
0.2
3
0.5
0.75
2
A
1.0
1.5
2.0
1
3.0
CO2/CH4
5.0
0
0.0
0.5
1.0
1.5
O/C ratio
2.0
2.5
Figure 21: Influence of temperature on CO2 content @ 1 atm
26
3.0
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
H 2O/CH 4
9
Constant Yield
Kg-mol
Kg- mol CH4
8
Constant duty
KJ/Kg-mol CH4
Constant
methane
conversion
2.0
3.99
7
H/C ratio
6
3.999
99.8%
99.95%
54,078
1.0
2.5
54,556
1.5
0.7
H 2 :C O 3 :1
0.4
5
H 2 :C O 2 .5 :1
4
B
C
3
55,035
3.0
10
H 2 :C O 2 :1
H 2 :C O 1.5 :1
0.2
B1
0.5 A1
2
H 2 :C O 1:1
0.75
1.0
A
1.5
1
2.0
CO2/CH 4
3.0
5.0
0
0.0
0.5
1.0
1.5
O/C ratio
2.0
2.5
3.0
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
3.0
10
Constant duty
KJ/Kg-mol CH4
9
Constant
methane
conversion
2.0
3.99
Constant Yield
Kg- mol Syngas
Kg-mol CH4
1.5
3.95
1.0
2.0
33,500
0.4
5
30%
52,642
43,071
0.7
H/C ratio
99%
3.8
6
H 2 :C O 3 :1
H 2 :C O 2 .5 :1
D1
D
4
3.98
54,078
7
3.999
54,556
8
2.5
55,035
H 2O/CH 4
H 2 :C O 2 :1
C
H 2 :C O 1.5 :1
3
0.2
0.5
2
H 2 :C O 1:1
0.75
1.0
1.5
2.0
1
3.0
CO2/CH 4
5.0
0
0.0
0.5
1.0
1.5
O/C ratio
2.0
2.5
3.0
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
2.0
H2O/CH4
Constant CO2
Syngas Yield
carbon
form ation
line
7
Duty,
BTU/m ol CH4
(KJ/kg-m ol CH4)
3.999
3.99
0.3
114,000 (54,556)
8
1.5
0.4
1.0
0.5
6
0.6
0.7
0.7
H/C ratio
5
B
H2:CO 2:1
B2
B1
0.9
4
115,000 (55,035)
3
0.75
1.0
2
A
1.5
A1
CO2/CH4
A2
2.0
2.5
1
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2
2. 4
O/C ratio
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.
3.0
10
Constant CO2 at outlet stream
Kg-m ol CO2/Kg-m ol CH4
Duty
BTU/m ol CH4
(KJ/kg-m ol CH4)
9
2.5
Syngas
yield
3.999
0.2
2.0
8
carbon
form ation line
1.5
114,000 (54,556)
3.99
7
0.1
0.075
0.05
1.0
6
H/C ratio
0.3
115,000 (55,031)
H2O/CH4
0.02
0.01
0.005
0.7
0.5
0.6
0.7
H 2 :C O 3 :1
0.9
H 2 :C O 2 .5 :1
0.4
5
0.4
H 2 :C O 2 :1
B
4
C
B1
C1
H 2 :C O 1.5 :1
0.2
3
0.5
H 2 :C O 1:1
0.75
2
1.0
A
1.5
A1
2.0
1
3.0
CO2/CH4
5.0
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
O/C ratio
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
1.0
5.5
70,000 (33,500)
5
0.4
4.5
D
4
C
D1
C1
0.1
H2:CO 3:1
0.005
0.7
carbon
form ation
line
H/C ratio
0.01
Constant CO2 at outlet stream
Kg-m ol CO2/Kg-m ol CH4
0.05
0.02
110,000 (52,642)
H2O/CH4
90,000 (43,071)
6
H2:CO 2.5:1
E
H2:CO 2:1
3.5
0.2
3
H2:CO 1.5:1
0.5
2.5
CO2/CH4
0.75
2
0.0
0.2
0.4
0.6
0.8
H2:CO 1:1
1.0
1.0
1.2
1.4
O/C ratio
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.).
3.0
10
H2O/CH4
Duty
9
Syngas
yield
2.5
0.2
2.0
8
0.4
3.999
0.5
carbon
form ation line
1.5
0.1
0.02
0.01
0.7
0.4
5
0.2
H 2 :C O 2 .5 :1
H 2 :C O 2 :1
90,000 (43,071)
C
3
H 2 :C O 3 :1
D2
D1
D
4
110,000 (52,642)
6
113,000 (54,078)
0.05
1.0
114,000 (54,556)
3.99
7
H/C ratio
0.3
Constant CO2 at outlet
stream
Kg-m ol/Kg-m ol CH4
C1
0.5
2
H 2 :C O 1.5 :1
H 2 :C O 1:1
C2
0.75
1.0
1.5
1
2.0
3.0
0
5.0
CO2/CH4
0.0
0.5
1.0
1.5
2.0
2.5
O/C ratio
Figure 27: CO2 recycle on Single reactor, California gas
34
3.0
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
Methane conversion
Syngas Yield, (Kg-mol/Kg-mol CH4)
CO2 outlet (Kg-mol/Kg-mol CH4)
Water addition (Kg-mol/Kg-mol CH4)
Duty
Reactor pressure, atm
Reactor Temperature, °F
Value
99.9 %
3.99
The minimum possible
The minimum possible
The minimum possible
1.
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
H2
CO
ratio
1/1
1.5/1
2/1
Terrell gas processing – optimal conditions from equilibrium chart
Syngas yield
Recycle
Duty
XCH4 H2O
CO2 outlet,
%
CH4
Kg-mol CO2
Kg-mol
/Addition
(KJ/Kgmol CH4)
Ratio Kg-mol CH4 Kg-mol CH4
99.0
0.22
0.15
3.96
Addition/recycle
53,599
99.85 1.42
0.55
3.993
No recycle
54,509
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
1/1
1.5/1
2/1
XCH4
%
H2O
CH4
99.95
99.95
99.95
0.22-0.76
1.42-2.6
2.7-3.6
CO2 out.
Kg-mol
Kg-mol CH4
0.15-0.57
0.55-0.9
0.85-1.42
Syngas
Kg-mol
Kg-mol CH4
3.99
3.999
3.999
Recycle
Or
Addition
Both
Recycle
Recycle
Duty
(KJ/Kgmol
CH4)
54,561
54,509
55,274
Temp
Rx.
°C
898
829-862
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
H2/CO
ratio
1/1
1.5/1
2/1
3/1
5/1
California gas, adding water to the feed to meet H2/CO ratio
Rx.
H2O
H2+CO,
Duty
%
CO2 outlet,
Temp.
CH4
Kg-mol
Kg-mol
KJ
XCH4
°C
Kg-mol CH4
Kg-mol CH4 Kg-mol CH4
Infeasible
Infeasible
Infeasible
1078
99.95
1.1
0.01
3.999
53,659
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
H2/CO
ratio
%
Recycle
1/1
1/1
1/1
1/1
1.5/1
1.5/1
1.5/1
1.5/1
2/1
2/1
2/1
2/1
20
40
60
100
20
40
60
100
20
40
60
100
Terrell gas, recycling CO2 from simulator
Rx.
%
H2O
H2+CO,
CO2 out.
Temp.
XCH4
CH4
Kg-mol
Kg-mol
°C
Kg-mol CH4 Kg-mol CH4
946
930
917
898
862
846
829
806
824
806
790
767
99.95
99.95
99.95
99.95
99.95
99.95
99.95
99.95
99.95
99.95
99.95
99.95
0.39
0.49
0.58
0.76
1.93
2.30
2.61
3.22
3.30
3.89
4.43
5.43
0.25
0.33
0.41
0.57
0.84
1.09
1.36
1.87
1.23
1.61
1.98
2.74
3.99
3.99
3.99
3.99
3.999
3.999
3.999
3.999
3.999
3.999
3.999
3.999
Duty
KJ
Kg-mol CH4
54,829
54,882
54,935
55,146
55,780
56,097
56,414
56,942
56,625
57,047
57,470
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
H2/CO
Ratio
2/1
Rx 1
Temp.
°C
927
1/1
1.5/1
2/1
898
829-862
814-850
California gas, one reactor with fixed temperature.
CO2
X CH4
Syngas
H2O
CO2 out
CH4
Kg-mol CO2
Kg-Mol
CH4 Total, %
Kg-mol CH4 Kg-Mol CH4
0.32
99.95
0.66
0.004
3.87
Terrell gas, one reactor configuration
1.19
99.95 0.22-0.76
0.15-0.57
3.990
1.19
99.95
1.42-2.6
0.55-0.9
3.999
1.19
99.95
2.7-3.6
0.85-1.42
3.999
Duty
KJ
Kgmol CH4
50,163
54,561
54,509
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.
H 2O
California/
Terrell
H 2 O /CO 2
Rx-1
Rx-2
H 2 O /CO 2
H2
CO
H 2O
CO2
CH4
C 2H 6
N2
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.
H2O
H2O
California/Terrell
California/Terrell
H2O
H2O
Rx-1
H2/CO=2
Rx-1
Rx-2
CASE A
H 2O
California/Terrell
California/Terrell
H2O
CO2
H2O
CO2
H2/CO=2
CO2
Rx-2
CASE B
H2O
Rx-1
CO2
H2/CO=2
Rx-1
Rx-2
CO2
Rx-2
CASE D
CASE C
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.
3.0
10
Constant duty
BTU / mole CH4
9
(KJ/Kg-mol CH4)
7
90,000 (43,071)
0.4
70,000 (33,500)
5
4
3
110,000 (52,642)
0.7
113,000 (54,078)
3.95
1.0
114,000 (54,556)
3.99
1.5
3.98
6
3.999
2.0
Constant Yield
Kg-mol/Kg-mol CH4
8
H/C ratio
2.5
115,000 (55035)
H2O/CH4
H 2 :C O 3 :1
H 2 :C O 2 .5 :1
B1
H 2 :C O 2 :1
B
A2
H 2 :C O 1.5 :1
0.2
A1
0.5
2
H 2 :C O 1:1
0.75
1.0 A
1.5
1
2.0
CO2/CH4
3.0
5.0
0
0.0
0.5
1.0
1.5
O/C ratio
2.0
2.5
Figure 30: Series reactors cases a and b - Terrell gas, 850 °C, 1 atm
40
3.0
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
850
1
99.95
3.999
0.82
2.0
2.65
56,088
1
Final
Reactor Temperature, °C
Reactor Pressure, atm.
Methane conversion, %
Syngas yield, Kg-mol /Kg-mol CH4
CO2 outlet, Kg-mol CO2/Kg-mol CH4
Final H2/CO ratio
Water added, total H2O/CH4 ratio
Reactor duty, total KJ/Kg-mol CH4
Reactors involved
CO2 separation
Reactors in series
Case b
850
1
99.82
3.993
0.44
2.0
2.63
57,777
2
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
3.0
10
Constant duty
BTU / mole CH4
9
(KJ/Kg-mol CH4)
2.0
Constant Yield
Kg-mol/Kg-mol CH4
3.99
1.5 3.98
7
3.95
110,000 (52,642)
70,000 (33,500)
5
4
A
90,000 (43,071)
0.7
0.4
H 2 :C O 3 :1
H 2 :C O 2 .5 :1
B1
A2
3
113,000 (54,078)
1.0
6
3.999
114,000 (54,556)
8
H/C ratio
2.5
115,000 (55,035)
H2O/CH4
H 2 :C O 2 :1
H 2 :C O 1.5 :1
A1
0.2
0.5
2
H 2 :C O 1:1
0.75
1.0
1.5
1
2.0
CO2/CH4
3.0
5.0
0
0.0
0.5
1.0
1.5
O/C ratio
2.0
2.5
3.0
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
3.0
H2O/CH4
Constant CO2
Constant duty
BTU /mole CH4
9
2.0
0.4
5
4
3.95
0.6
H 2 :C O 3 :1
0.9
H 2 :C O 2 .5 :1
B
H 2 :C O 1.5 :1
0.2
0.5
2
H 2 :C O 1:1
0.75
1.0
A
1.5
1
0
B1
H 2 :C O 2 :1
A1
3
114,000 (54,556)
3.98
113,000 (54,078)
1.0
3.8
3.5
110,000 (52,642)
0.7
1.5
70,000 (33,500)
H/C ratio
6
3.999
3.99
Constant Yield
Kg- mol/Kg- mol CH4
7
2.5
Kg-mol KG-/mol CH4
(KJ/Kg-mol CH4 )
8
115,000 (55,035)
10
A2
2.0
CO2/CH4
0.0
0.5
3.0
5.0
1.0
1.5
O/C ratio
2.0
2.5
3.0
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:
• Constraint:
• Specifications:
• Variables:
Minimization of the total duty (KJ/Kg-mol CH4)
Total methane conversion = 99.95%
H2/CO ratio Reactor 1= 1.25. Reactor 2= 2.0
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.
H2O
CO2 out
Kg-mol CO2
Syngas
Kg-mol
Duty
KJ
°C
Total, % CH4
Kg-mol CH4
Kg-mol CH4
Kg-mol CH4
850
99.95%
0.84
3.999
58,222
H2/CO
Rx1
Temp.
Rx2
Temp.
Ratio
°C
2/1
701
X CH4
2.63
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
H2/CO
Ratio
2/1
Rx 1
Temp.
°C
747
Terrell Gas, CO2 removal after first reactor.
Rx 2
Syngas
CO2 out
Duty
Temp. X CH4 H2O Kg-mol CO2
Kg-mol
KJ
°C Total, % CH4 Kg-mol CH4 Kg-mol CH4 Kg-mol CH4
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
Syngas
CO2 out
Kg-mol CO2
Kg-mol
H2/CO Rx 1 temp. Rx 2 temp. X CH4
H2O
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%
1.25/1
Specifications:
H2/CO reactor 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
Rx 1
H2/CO Temp.
Ratio
°C
2/1
809
California gas and addition of CO2 = 0.6 CO2/CH4
Rx 2
Syngas
H2O
CO2 out
Temp. X CH4
CH4
Kg-mol CO2
Kg-Mol
°C
Total, %
Kg-mol CH4 Kg-mol CH4
927
99.95%
1.49
0.28
3.999
Duty
KJ
Kg-mol CH4
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
3.0
Constant Duty, BTU/mole CH4
(KJ/Kmol CH4)
H2O/CH4
2.5
Constant Yield
mole Syngas/mole CH4
9
2.0
8
3.99
1.5
115,000 (55,035)
10
3.999
A
3.98
7
3.95
110,000 (52,642)
3.5
0.7
H/C ratio
3
0.4
5
2.5
2
M
4
114,000 (54,556)
3.8
6
H 2 :C O 3 :1
113,000 (54,078)
1.0
H 2 :C O 2 .5 :1
H 2 :C O 2 :1
C
H 2 :C O 1.5 :1
0.2
3
0.5
B1
H 2 :C O 1:1
0.75
2
1.0
B
1.5
1
2.0
3.0
CO2/CH4
5.0
0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
O/C ratio
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
3.0
Constant Duty, BTU/mole CH4
(KJ/Kg-mol CH4)
H2O/CH4
Constant Yield
mole Syngas/mole CH4
9
2.0
3.999
3.99
8
1.5
7
3
2.5
2
114,000 (54,556)
0.4
5
3.5
110,000 (52,642)
0.7
113,000 (54,078)
3.8
6
3.98
3.95
1.0
H/C ratio
2.5
115,000 (55,035)
10
H 2 :C O 3 :1
B
H 2 :C O 2 .5 :1
H 2 :C O 2 :1
4
H 2 :C O 1.5 :1
3
M
0.2
C
0.5
2
H 2 :C O 1:1
0.75
1.0
A
1.5
1
2.0
CO2/CH4
3.0
5.0
0
0.0
0.5
1.0
1.5
O/C ratio
2.0
2.5
3.0
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.
OUT-STEAM-RX
TERRELL-1
S3
INLET-RX1
WATER-RX1
E1
REACTOR-1
M1
COOLER-1
S9
SYNGAS
S1
M3
OUT-CO2RX
S13
TERRELL-2
S10
INLET-RX2
WATER-RX2
COOLER-2
E3
M2
REACTOR-2
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
10
8
3.99
1.5
110,000 (52,642)
3.5
0.7
0.4
114,000 (54,556)
113,000 (54,078)
3.8
6
3.98
3.95
1.0
5
3.999
2.0
7
H/C ratio
2.5
(KJ/Kg-mol CH4)
Constant Yield
mole Syngas
per mole CH4
115,000 (55,035)
H2O/CH4
9
3.0
Constant duty
BTU / mole CH4
H 2 :C O 3 :1
H 2 :C O 2 .5 :1
Reactor 1
H 2 :C O 2 :1
4
M
H 2 :C O 1.5 :1
3
0.2
0.5
2
H 2 :C O 1:1
0.75
1.0
Reactor 2
1.5
1
2.0
3.0
CO2/CH4
5.0
0
0.0
0.5
1.0
1.5
O/C ratio
2.0
2.5
3.0
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
2.3/43.0
4.5/40.8
9.0/36.3
Fixed
56,643
57,624
59,605
Water to
reactor 1 only
56,614
57,572
59,538
51
Water to both
reactors
56,610
57,533
59,452
Single reactor
configuration
56,007
55,944
55,844
60,000
59,500
Temp. Both Reactors changing
KJ/kg-mol CH4
59,000
58,500
Temp. Both Reactors Constant
58,000
57,500
minimizing duty
57,000
Temp. Both reactors changing
56,500
and addition of water to Reactor 2
56,000
0.9965
0.9970
0.9975
0.9980
0.9985
0.9990
X CH4
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
Ratio Reactor 1 Reactor 2
1.5/1 760.0
788.3
2/1
808.3
760.0
3/1
812.8
760.6
X CH4
0.0081
0.0250
0.6460
Flow to reactors,
H2O/CH4 CO2 outlet H2+CO
Kg-mol/hr
In feed Mole frac. Kg-mol/hr Steam Ref.
CO2 ref
0.01
0.0000
3.1
45.4
0.0
0.02
0.0000
5.2
15.9
29.5
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
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