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LIQUID HYDROGEN PRODUCTION VIA

HYDROGEN SULFIDE METHANE REFORMATION

Cunping Huang and Ali T-Raissi

Florida Solar Energy CenterUniversity of Central Florida

1679 Clearlake RoadCocoa, FL 32922

IntroductionHydrogen sulfide is a common contaminant in many of the

worlds natural gas (NG) wells. Approximately one-third of U.S. NGresources can be considered low or sub-quality gas not suited for

pipeline shipment.1 Hydrogen sulfide concentration in NG variesfrom traces to 90% by volume.2 In natural gas processing hydrogensulfide is viewed as a pollutant requiring treatment and removal.Presently, hydrogen sulfide is separated from hydrocarbon gases byamine adsorption and regeneration, producing acid gas containing10-90% by volume hydrogen sulfide. When hydrogen sulfideconcentration exceeds 40% the natural gas must be treated in theClaus plant, where a portion of hydrogen sulfide is burned to formsulfur dioxide. The gas then reacts with the main hydrogen sulfidestream to produce elemental sulfur and water. In this process,

hydrogen in the hydrogen sulfide is converted into water vapor andwasted instead of being extracted as a clean energy source.

As an alternative to the Claus process, hydrogen sulfide can beremoved by methods that produce elemental sulfur and hydrogeninstead. For example, pyrolysis, thermochemical cycles,electrochemical and photochemical methods can be used todecompose H2S directly. However, there are two drawbacks to directdecomposition of hydrogen sulfide for hydrogen production. Firstly,in the case of a hydrogen sulfide contaminated NG, the treatment

process requires that H2S be recovered from the sub-quality gas priorto its decomposition. In many cases, H2S separation process requiresconsiderable energy input, making utilization of the sour gaseconomically unfavorable. Secondly, H2S decomposition producesonly one mole of hydrogen per mole of H2S reacted. A betterapproach is to reform H2S and CH4 already present in natural gas in a

process analogous to steam reformation of methane as follows:

2H2S + CH4 = CS2 + 4H2 H0

298K=232.4 kJ/mole

Hydrogen sulfide reformation of methane (HSRM) can generatetwo moles of hydrogen per mole of H2S reacted. The main objectiveof this paper is to assess the merits of HSRM process with respect tothe thermodynamics and chemical equilibrium considerations. Priorto the development of flowsheets, pinch point analyses are conductedto determine the necessary process conditions for zero carbon laydown based on the underlying HSRM chemistry. In addition, certainaspects of the process relevant to the liquid hydrogen production bycryogenic separation of constituent gases are discussed and comparedto hydrogen membrane separation processes. The process energyrequirements and efficiencies are also calculated using Aspen

Technologies HYSYSTM chemical process simulator.

Thermodynamics of hydrogen sulfide reforming of methaneThe thermodynamic analyses were carried out using Aspen

Technologies HYSYSTM chemical process simulator. Since methanedecomposition can result in carbon lay down that deactivates catalystused for HSMR process, the analyses intended to find conditions thatminimize carbon formation. This is done by pinch-point analyses.The yield of carbon, carbon disulfide and hydrogen are defined as themoles of product divided by the moles of methane input. The yield

of carbon and carbon disulfide as a function of temperature during

HSRM process are given in Figure1 and2.

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

100.0

Figure 1. Carbon yields at various temperatures and CH4 to H2Smolar feed ratios (Region I - CH4 pyrolysis; Region II - excess CH4

pyrolysis and HSRM zone; Region III - purely HSRM process).

In region III (HSRM), the hydrogen sulfide to methane feedratio (x) does not have appreciable effect on the extent of CH4conversion. The major factor affecting methane conversion is

temperature. When reaction temperatures exceed 800oC, methaneconversion is practically 100% at all x values. The amount of carbonformed increases with increasing temperatures and has maxima as

depicted in Figure 1. The temperature at which pinch-point occursdecreases with the increasing feed ratio x. For example, when x = 4,

pinch point temperature is 1500oC, while it is 1000oC at x = 10.Carbon lay down can't be avoided if the hydrogen sulfide to methanefeed ratio is lower than 4.

Total enthalpy (H) change of HSMR process is calculatedbased on the enthalpy differences of products and reactants shown in

Figure 2 for one mole methane system.

Figure 2. Total heat flow for HSRM process as a function oftemperature and CH4 to H2S molar feed ratios.

It can be seen that H is strongly dependent on both the temperatureand hydrogen sulfide to methane feed ratio x. The required energiescan be separated into two categories. Process energies required tocarry out HSRM are much higher than those needed for hydrogensulfide decomposition. At pinch-point temperature, the requiredenergy drops from that for HSRM to a much lower value associated

with the H2S pyrolysis. As Figure 2 indicates, HSRM is a highly

400 600 800 1000 1200 1400 1600 1800 2000 2200

Temperature (oC)

Carbonyield(%)

CH4:H2S = 1:1

1:2

1:3

1:41:51:61:81:101:12

I

II

III

P = 1 atm

0.E+00

2.E+04

4.E+04

6.E+04

8.E+04

1.E+05

1.E+05

400 600 800 1000 1200 1400 1600 1800 2000 220

Temperature (oC)

Heatflow

(kJ/kgm

ole)

CH4:H2S=1:1

1:51:6

1:81:101:12

H2S pyrolysis

1:3

1:4

1:2

P = 1 atm

Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 2004, 49(2), 904

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endothermic process and as such can be employed for storing energyin the form of chemical energy (e.g. hydrogen) from hightemperature sources such as solar resource. The pinch-pointtemperatures and energy requirements for HSMR process aredepicted in Figure 3. Increasing the hydrogen sulfide to methaneratio, x, reduces the pinch-point temperature, as does the total energyrequired for the HSRM process

Figure 3. Pinch points temperature and total heat flow at various

CH4 to H2S molar feed ratios.

Process flowsheet and efficiency calculationsBased on the thermodynamic analyses discussed above, three

flowsheets based on cryogenic and membrane separation techniqueswere established for producing both liquid and gaseous hydrogen viaHSRM process. All three flowsheets were set up at the pinch-pointconditions for the HSRM process.

Figure 4. Process flowsheet for liquid hydrogen production viaHSRM process.

A HYSYSTM-based process flowsheet for liquid hydrogen

production by cryogenic separation is shown in Figure 4. Hydrogen,sulfur diatomic gas and carbon disulfide are produced in a Gibbsreactor that operates under no carbon lay down condition. Thegaseous mixture at the outlet of the Gibbs reactor is cooled toseparate out sulfur, and the remaining gases are sent to a cryogenicdistillation tower to recover hydrogen and carbon disulfide from theunreacted hydrogen sulfide and methane. The unreacted hydrogensulfide and methane are mixed with fresh feed in a manner tomaintain the initial H2S to CH4 feed ratio x and subsequently heatedand sent back to the Gibbs reactor. Low temperature (-236.3 oC)gaseous hydrogen that exists distillation tower can be liquefied to byadding a small amount of cryogenic energy to reduce its temperatureto -240.6 oC. One important advantage of the process is that

hydrogen sulfide serves as both a reactant and as a working fluid forthe cryogenic separation process. With H2S recirculation, no otherworking fluid, such as nitrogen or helium, is necessary.

0

200

400

600

800

1000

1200

1400

Figure 5. Gaseous hydrogen production via HSRM process paired witha membrane separation.

The HSRM process can be combined with a membraneseparation technology for gaseous hydrogen production. Two typesof membrane separation processes are developed based on thesequences of carbon disulfide and hydrogen separation. In one

version depicted by the flowsheet of Figure 5, hydrogen separationoccurs before that of carbon disulfide. After quenching the gasmixture exiting the Gibbs reactor, sulfur separates out of the mainstream and the remaining gas mixture compressed to 12 atm(necessary for hydrogen separation through the membrane). Themembrane separation efficiency assumed to be 100%. Afterhydrogen separation, the remaining gas mixture, consisting ofhydrogen sulfide, methane and carbon disulfide, is sent to adistillation tower to separate the carbon disulfide from the mixture.The gas mixture is then expanded to 1 atm, mixed with fresh feedand sent back to the Gibbs reactor. The total required energy for

both the cryogenic and membrane systems portions of the process are

show in Figure 6.

Figure 6. Overall energy requirements for hydrogen production viaHSRM process

Acknowledgments. The authors are grateful for the financialsupport provided by the U.S. Department of Energy and NASAGlenn Research Center.

References1. Hugman, R.H., Vida, E.H., Springer, P.S., Chemical composition

of discovered and undiscovered natural gas in the United States.

1993 update. GRI-93/04561993.2. T-Raissi, A., Technoeconomic analysis of area II hydrogen

production Part 1. Final report fro DOE, 2002.

1600

2 4 6 8 10 12 14 16 18 20 22

H2S/CH4

Tem

perature

(oC)

2.8E+05

2.9E+05

2.9E+05

3.0E+05

3.0E+05

3.1E+05

3.1E+05

3.2E+05

3.2E+05

3.3E+05

Heatflow(

kJ/hr)

Pinch Point Temperature

Total Reaction Heat

P = 1 atm

4.0E+05

4.5E+05

5.0E+05

5.5E+05

6.0E+05

6.5E+05

7.0E+05

7.5E+05

8.0E+05

2 4 6 8 10 12 14 16 18 20 22

Ratio of H2S/CH4

Overallenergyflow(

kJ/hr)

Memb rane Separtion for GH2

Membrane Separation for LH2

Cryogenic Separation for LH2

Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 2004, 49(2), 905