December 2024
Process Optimization
Distillation of a mixture containing CO2, CH4 and H2—Part 1
This article describes a method for the simultaneous separation of a mixture containing hydrogen, methane and carbon dioxide, which may result from certain shifted synthesis gas (syngas) mixtures.
This article describes a method for the simultaneous separation of a mixture containing hydrogen (H2), methane (CH4) and carbon dioxide (CO2), which may result from certain shifted synthesis gas (syngas) mixtures. This type of stream could result from steam methane reforming (SMR) followed by water-gas shift (WGS), followed by methanation of the residual carbon monoxide (CO), for example. The resulting mixture is introduced into a distillation column to generate:
- An overhead distillate stream very rich in H2 \
- An intermediate-volatility stream off the reboiler that is very rich in CH4
- A bottoms stream comprising liquid CO2.
The CH4-rich stream can be recycled to the upstream SMR, increasing the utilization of the CH4 and thereby increasing the total product yield.
In an alternative case, the mixture also contains nitrogen (N2), which is obtained in the distillate stream along with the H2. Preferably, the H2 and the N2 are present in the distillate stream in a molar ratio of 3:1 (H2:N2). This way, the distillate overhead may be used directly for the synthesis of ammonia.
The distillation column was modeled for the ammonia case on the process simulator UniSim Design v. R370, build 13058 from Honeywell.
Background. The term synthesis gas, also known as syngas, is widely used to refer to a gas mixture containing CO2 and/or CO, and H2 generated by the gasification/reforming of a feedstock containing carbon (and often H2), often in the presence of added water, to a gaseous product containing CO and H2. Syngas is produced, for example, by SMR of natural gas or liquid hydrocarbons, by the gasification of coal and, in some cases, waste-to-energy gasification.
Depending on the application, the desired product may be a H2/CO mixture. In others, water vapor may be added so that the WGS reaction can be used to react water with CO to maximize H2 production (Eq. 1):
Syngas is used, for example, as an intermediate for creating synthetic natural gas, or for producing ammonia or methanol. Syngas can also be used to produce synthetic fuels, waxes or lubricants via the Fischer-Tropsch (F-T) reaction.
In the synthesis of ammonia from air (as a source of N2) and hydrocarbons (as a source of H2), the hydrocarbon, such as CH4, is made to react with water vapor at elevated temperatures to generate H2, CO and CO2. This produces a raw syngas that contains—in addition to these compounds—residual unreacted hydrocarbon and water vapor, as well as N2 and other trace air constituents. A N2/H2 mixture must then be separated from the other components of the syngas for the generation of ammonia.
The production of ammonia presents two main challenges. One is that excessive amounts of hydrocarbon, typically CH4, remain unreacted in the conversion of the hydrocarbon to H2 and CO. Thus, the ammonia yield is far less than the theoretical maximum. This so-called “hydrocarbon slip” can be reduced by using elevated reforming temperatures [i.e., temperatures higher than those required merely to obtain an acceptable rate of reaction, > 850°C (> 1,562°F)]. The other is that CO2 must be removed from syngas to prevent poisoning of the catalyst used in the ammonia conversion. CO2 removal involves high capital costs and energy consumption.
Significant work has been applied to the development of methods for the removal of CO2 from syngas, and there is rejuvenated interest in the area1 with at least two cryogenic processes commercialized.2,3
If the syngas is fully shifted to H2, and the CO2 is removed and sequestered, this is known as “pre-combustion capture.”4 This is a potential tool for reducing anthropogenic CO2 emissions. In fact, H2 is now assigned a color that relates to how much CO2 is emitted during its production. In general, lighter colors like green or blue indicate lower CO2 emissions than brown or black.
CO2 removal processes can be separated into four general classes; absorption by physical solvents, absorption by chemical solvents (e.g., amines), adsorption by solids, and distillation. Each of these types of separation has advantages and disadvantages; this article will focus on distillation.
The high relative volatility of CH4 with respect to CO2 (TABLE 1) at pressures below the critical pressure of CH4 (roughly 640 psia) makes cryogenic distillation theoretically very attractive.
However, CH4/CO2 distillation has a significant disadvantage in that solid CO2 exists in equilibrium with vapor mixtures of CO2 and CH4 over a broad range of temperatures, pressures and compositions that would be required to effect the desired separation under cryogenic conditions. Obviously, the formation of solids in a distillation tower has the potential for plugging the tower and its associated equipment and thus should be avoided.
Increasing the operating pressure of the tower will result in warmer operating temperatures and a consequent increase in the solubility of CO2, thus narrowing the range of conditions at which solid CO2 forms. At sufficiently high pressure (~700 psia) and temperature, the CO2/CH4 mixture reverts to a vapor-liquid system. However, this pressure is above the critical pressure of CH4-rich gas. Upon reaching criticality, distinct vapor and liquid phases cannot be produced, and therefore cannot be separated. A single-tower operating in the vapor-liquid equilibrium region above CO2 freezeout conditions will produce a product CH4 stream containing 10% or more CO2, which is less pure than desired for CH4 recycle to the reformer. The distillate stream should contain only ppm levels of CO2.
Since raw natural gas frequently contains significant amounts of CO2, a number of methods have been proposed to avoid the conditions at which CO2 freezes and yet obtain an acceptable CH4 purity. Some of these methods may have application to syngas separation.
One proposed method involves adding either H2 or helium (He) to the feed stream to increase the critical pressure of the mixture.5,6 For example, adding 20 mol%–30 mol% of H2 to the mixture will allow the separation to take place at 1,025 psia–1,070 psia. The separation is said to take place without the formation of solid CO2, as will be discussed further below.
Since syngas generally contains substantial amounts of H2, it has been suggested that it be purified by using cryogenic distillation.7 In this particular case, refrigeration for the distillation is obtained from waste fluid expansion using a liquid expander to recover mechanical work from the waste fluid. This method reduces pressure loss in the syngas stream and reduces compression and power relative to similar ammonia-generating processes.
Proposed way to separate syngas mixtures and application to ammonia process. A method for the separation of a mixture containing H2, hydrocarbon and CO2 is proposed here. FIG. 1 shows the system for separating a mixture containing H2, hydrocarbon (CH4) and CO2, in one version of the process.
FIG. 1. An ammonia syngas fractionation system (SGFS): the side stream is withdrawn from the reboiler vapor.
In this case, the mixture is fed into a distillation column from which a portion of the reboiler vapor stream is withdrawn. Distillation of the mixture in the column generates three streams:
- An overhead distillate stream rich in H2
- An intermediate volatility side stream rich in CH4
- A bottoms stream containing liquid CO2.
The hydrocarbon in the system is usually, although not necessarily, CH4. For ammonia synthesis, the syngas mixture contains H2 and N2, ideally present in a molar ratio of 3:1. This may be achieved by mixing the hydrocarbon with a stoichiometric amount of air. Thus, in the ammonia-synthesis application, the separation allows for an improved process for the production of ammonia. In accordance with this aspect, a mixture containing H2, N2, hydrocarbon, CO2 and Ar is introduced into a distillation column to produce the three streams described above. The middle (CH4-rich) stream can be recycled to the reformer, leading ultimately to near 100% conversion. The top stream comprising H2 and N2 is then used to generate ammonia by any known method. Recycling the hydrocarbon to the upstream reformer (e.g., SMR) increases the utilization of the hydrocarbon, thus increasing the ammonia yield.
The process offers several degrees of freedom to allow flexibility in operating the system, such as adjusting the CH4 slip. This is controlled by the reforming temperature and the side stream composition,8 and can improve the efficiency of the raw syngas generating process, while involving fewer process steps than traditional approaches.
Liquid CO2, which is obtained as the column’s bottoms stream, may be stored, sequestered or used [e.g., for enhanced oil recovery (EOR), or for urea or methanol production]. Emissions reductions from some uses may be traded in some carbon markets, resulting in both economic benefits and a more sustainable process for the production of syngas.
Distillation feasibility and thermodynamic analysis. Experimental data for the five-component mixture described here is not available in the literature. However, examining phase diagrams of the corresponding binary system (CO2/CH4) illustrates the core separation problem, as seen in FIGS. 2A and 2B.9
FIGS. 2A and 2B. Diagrams for CO2-CH4 at (A) 600 psig and (B) 800 psig.
FIG. 2A shows a wide vapor-liquid envelope at 600 psig across the full composition range [note that the y-axis (temperature) is inverted to match how the temperature in a distillation column is warm at the bottom and cold at the top]. This means that separation by distillation should be relatively easy. However, the “solid CO2 curve” intrudes on much of the bubble point curve. That means solid CO2 will be present over much of the composition range, though it reverts to a vapor-liquid system at high (> 0.9), and low (< 0.25) CH4 mole fractions. Note that all three phases (S, L and V) can be present at the two points at which the bubble point line crosses the solid CO2 curve in FIG. 2A. Otherwise, the system is (V) or (V + L) or (V + S) or (L + S) at this pressure.
FIG. 2B shows the same system at 800 psig. Note how the bubble point curve has moved to a higher temperature (downward), away from the CO2 solidification curve (which does not change much with pressure). However, note also how the bubble and dewpoint curves converge around a 0.9 CH4 mole fraction. As mentioned earlier, this is a consequence of exceeding the critical pressure of CH4-rich gas. Further separation is not possible.
High concentrations of H2 lead to significant increases in the mixtures’ pseudo-critical pressures, and also to decreases in the freeze-out temperature of the CO2. It can be concluded that for the “real-life” multi-component mixture at hand, the pressure range for operation in which CO2 freeze-out is prevented may be wider than for the binary CO2/CH4 mixture.10 Conveniently, the H2 is already present, and need not be separated from the overhead stream afterwards. It is noted that the rest of the gases of the multi-component mixture have lower critical temperatures than CH4. In addition, it is worth noting that for the multi-component mixture with no CO2 (column overhead conditions), the vapor-pressure line will rise above 700 psia, and thus not limit separation at low temperatures in a single distillation column (see P-T diagram in literature11).
For the limiting case of a CO2/CH4 mixture (no H2), the following can be asserted:
- CO2 freezing is temperature- and concentration-dependent. It may occur by freezing out of the vapor phase like snow [solid/vapor equilibrium (SVE), the basis for the controlled freeze zone (CFZ) separation technology],12 or by crystallization out of the liquid [solid/liquid equilibrium (SLE)].
- The lower the CO2 concentration (technically, fugacity) is relative to CH4, the lower the temperature (and pressure, if in the VLE envelope) is required to form solid CO2.13 Eggeman and Chafin14 provide thermodynamic bases for predicting CO2 crystallization from liquid, and CO2 desublimation from gas.
- Adding a diluent will enable the operation of a distillation column at the same pressure (and approximate temperature profile), but less CO2 solid will form. Sufficient diluent will eliminate CO2 solidification altogether, as illustrated with the addition of H2S in a large-scale distillation process.9
- Other lighter gases such as H2 may significantly contribute to the success of the separation of CH4 from CO2. (It increases the mixture’s pseudo-critical pressure, so that the separation is no longer constrained by the critical pressure of pure CH4.)
- If every tray in the column is kept above ~700 psia, CO2 solid formation will be prevented, as seen in FIG. 3. However, the necessary presence of distinct liquid and vapor phases will be not be attained at high concentrations of CH4. Consequently, high-purity CH4 cannot be achieved, as mentioned above.
- The presence/concentration of liquid H2, liquid N2 or liquid Ar, or a mixture thereof, will require working at lower temperatures, moving operation to the left on the x-axis in FIG. 3. This means that the concentration of CO2 in the vapor phase will have to be lower, or else the CO2 will form frost. The liquid phase of a given composition can generally dissolve more CO2 on a molar basis than the vapor phase at a given temperature. Sufficient liquid can thus prevent crystallization, but at the cost of additional cold reflux (i.e., more refrigeration horsepower).
- The addition of liquid (usually butane) to dissolve solid CO2 is the basis of the Ryan-Holmes process.11 It also breaks the CO2-ethane azeotrope, but the process requires at least three distillation columns to effect the separations and recover the solvent. This increases the cost and complexity of the overall process.
FIG. 3. A representation of the interrelationship between temperature and partial pressure of CO2 + CH4 wherein solid phase formation can occur.5
Specifying a Syngas composition, which is the feed to the column as used for the ammonia-synthesis process (TABLE 2), offers the opportunity to combine all of the above.
The ammonia syngas is hydrogen-rich (~0.55 mol fraction). Comparing this to Column 6 in the literature,5 it is seen that, for a mixture containing mole fractions of 0.4 CH4, 0.28 CO2 and 0.24 H2, freeze-out is prevented due to the presence of H2. This is because the tower operates at 1,025 psia, which shifts the phase envelope away from solid CO2 formation; however, the H2 keeps the CH4-rich side of the phase envelope from collapsing.
The partial pressure of the CH4 + CO2 is (0.28 + 0.4) x 1,025 psia = 688 psia; this is < 705 psia, which is the upper pressure of the Donnely & Katz binary CO2/CH4 diagram (at the tray temperature of –62°C to –78°C).15 Above that CH4 + CO2 partial pressure, freeze-out should not occur. Freeze-out may still potentially occur, if applying only that criterion.
The fact that the partial pressure of CO2 + CH4 is lower, and yet freezing is prevented, suggests that H2 inhibits freeze-out due to dilution. In the mixture presented in TABLE 2, which is fed to the column, the CO2 + CH4 mole fraction is lower, and the H2 mole fraction is significantly higher, which may point towards increased/further inhibition of freeze-out, perhaps at pressures even lower than 688 psia. Hence, supporting the above-mentioned argument that for the multi-component mixture at hand, the pressure range for operation in which CO2 freeze-out is prevented may be wider than for the binary CO2/CH4 mixture.10
Part 2 of this article (January 2025) will detail process simulation and provide detailed lab results.
ACKNOWLEDGEMENTS
The authors which to acknowledge Thermo Dynamico’s staff members Rubin J. McDougal, Lane R. Gardner and Seth T. Herway for performing the lab measurements.
LITERATURE CITED
1 Berstad, D., P. Nekså and G. A. Gjøvåg, “Low-temperature syngas separation and CO2 capture for enhanced efficiency of IGCC power plants,” Energy Procedia, Vol. 4, 2011.
2 Air Liquide, “World premiere: Air Liquide inaugurates its CO₂ cold capture system, Cryocap™,” November 5, 2015, online: https://www.airliquide.com/group/press-releases-news/2015-11-05/world-premiere-air-liquide-inaugurates-its-co2-cold-capture-system-cryocaptm#:~:text=Cryocap%E2%84%A2%20is%20the%20first%20CO2%20capture%20technology%20using,improving%20efficiency%2C%20leading%20to%20an%20increased%20hydrogen%20production.
3 Honeywell, “Honeywell technology enables large U.S. carbon capture and storage project,” April 12, 2021, online: https://www.honeywell.com/us/en/press/2021/04/honeywell-technology-enables-large-us-carbon-capture-and-storage-project
4 U.S. Department of Energy, “Pre-combustion carbon capture research,” online: https://www.energy.gov/fecm/pre-combustion-carbon-capture-research
5 Eakman, J. M. and H. A. Marshall, “Separation of carbon dioxide and other acid gas components from hydrocarbon,” U.S. Patent No. 4,149,864, 1979.
6 Valencia, J. A. and R. D. Denton, “Method of separating acid gases, particularly carbon dioxide, from methane by the addition of a light gas such as helium,” U.S. Patent No. 4,511,382, 1985.
7 Malhotra, A., T. Ahmad and B. Richard, “Low-delta P purifier for nitrogen, methane and argon removal from syngas,” U.S. Patent No. 7,090,816, 2006.
8 Riaz, A., A. Farsi, G. Zahedi and A. Manan, “Investigation of inert gas injection in steam reforming of methane: Energy,” Sultan Qaboos University, 2011.
9 Northrop, P. S., A. K. Nagavarapu and J. A. Valencia, “Sour gas treating using Controlled Freeze Zone™ technology: Demonstrated commercial readiness,” Gas Processors Association Convention, San Antonio, Texas, 2014.
10 Dr. Ilya Polyshuk, personal communication, 2008.
11 Holmes, A. S. and J. M. Ryan, “Cryogenic distillative separation of acid gases from methane,” U.S. Patent 4,318,723, 1982.
12 Northrop, P. S. and J. A. Valencia, “The CFZ™ process: A cryogenic method for handling high-CO2 and H2S gas reserves and facilitating geosequestration of CO2 and acid gases,” Energy Procedia, Elsevier, pp. 171–177, February 2009.
13 Ke, J., et al., “The phase equilibrium and density studies of the ternary mixtures of CO2 + Ar + N2 and CO2 + Ar + H2, systems relevance to CCS technology,” International Journal of Greenhouse Gas Control, Vol. 56, pp. 55–66, 2017.
14 Eggeman, T. and S. Chafin, “Beware the pitfalls of CO2 freezing prediction,” Chemical Engineering Progress, pp. 39–44, March 2005.
15 Donnely, H. G. and D. L. Katz, "Phase equilibria in the carbon dioxide–methane system,” Industrial & Engineering Chemistry, Vol. 46, pp. 511–517, 1954.
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