Applications of Carbon Membranes in Hydrogen Separation

Most previous work has focused on developing high-performance CMS membranes on the basis of laboratory-scale gas-permeation measurement for H2 separation, and few commercial CMS-membrane H2-separation processes have been reported. CMS membranes provide great potential for selected gas separation, especially of H2, because of their mechanical and chemical stabilities with superior separation performances. A potential application for CMS membranes in the recovery of H2 that is transmitted with natural gas was evaluated by Grainger and Hagg [38]. Performance data for cellulose- derived CMS membranes were applied to a techno-economic evaluation of H2 recovery and the results were compared with a commercial polyimide membrane as a benchmark. The CMS membranes produced higher-purity H2, consumed less energy during separation, and achieved competitive specific separation costs under certain conditions. The CMS membranes easily achieved single-stage separation specifications at higher permeate pressures compared with the polyimide membrane because the H2 selectivity of CMS membranes was higher. He [39] conducted a techno-economic feasibility analysis on carbon membranes for H2 purification from a biomass fermentation process. A novel, energy-efficient, two-stage carbon membrane system for H2 purification based on a combination of PR-selective CMS membranes in the first stage and C02-selective carbon membranes in the second stage was proposed. The designed two-stage carbon-membrane system can capture C02 >95 vol% in the first-stage membrane unit at 20 bar and 120°C, and the high-purity H2 (>99.5 vol%) with a high H2 recovery (>95%) was produced in the second-stage membrane unit that was operated at a feed operation pressure of 20 bar and 20°C.

CMS membranes can be considered as possible alternatives to Pd-based membranes for use in MR H2 production. Pd-based membranes are expensive and experience H2-embrittlement cracking during thermal cycling and surface contamination by sulfur-containing species. CMS membranes have clear advantages in terms of their chemical stability, lower production costs, and relatively high selectivity for H2. However, they are fragile and cannot be used in oxidizing atmospheres, so only a few applications of carbon membranes in MRs have been reported. Table 7.3 summarizes examples of the use of CMS MRs for H2-related reactions.

The first experimental CMS MR was reported by Itoh and Haraya [40]. The MR for cyclohexane dehydrogenation consisted of a bundle of 20 hollow carbon fibers that were produced by pyrolysis of hollow polyimide fibers that contained 0.5 wt% Pt/AhO, pellets as a catalyst. Figure 7.1 provides a schematic that shows the CMS MR that was developed in this study. Cyclohexane dehydrogenation to benzene was carried out at 195°C under atmospheric pressure. The CMS MR produced a conversion that was somewhat better than the equilibrium conversion; the findings were supported by a mathematical model for a limited range of reaction conditions.

Sznejer and Sheintuch [41] tested a carbon MR (CMR) for isobutane dehydrogenation at high temperatures (450-500°C) on chromia/alumina catalyst

TABLE 7.3

Examples of hydrogen-related carbon molecular sieve membrane reactors.

Precursor

Catalyst

Reaction

Temperature

Reference

Polyimide

Pt/AIA

Cyclohexane

dehydrogenation

195°C

Itoh and Haraya [40]

Cellulose

Cr203/ai2o3

Isobutane

dehydrogenation

~500°C

Sznejer and Sheintuch [41]

Phenolic resin + PEG

Cu/ZnO/Al,03

Methanol steam reforming

~250°C

Zhang et al. [42]

Not disclosed

CuO/ZnO/AhO,

Water-gas-shift

reaction

250°C

Harale et al. [43]

PEI

Sulfided Co/Mo/

ai2o3

Water-gas-shift

reaction

~300°C

Abdollahi et al. [44]

Cellulose

derivative

CuO/ZnO/АЩ,

Methanol steam reforming

~200°C

Sa et al. [24]

Matrimid

CuO/ZnO/A1203

Methanol steam reforming

~250°C

Briceno et al. (2012b)

PFA (FFA)

Pt/Ah03

Me thy lcyclohexane dehydrogenation

~220°C

Hirota et al. [30]

Phenolic

resin-based

polymer

CoS,/MoS2

Water-gas-shift

reaction

~250°C

Parsley et al. [45]

FIGURE 7.1

Schematic of a carbon membrane reactor [40].

pellets. The membrane module, which consisted of 100 hollow carbon fibers (Carbon Membranes Ltd.), had a H2-to-isobutene permeability ratio above 100. Although the results obtained were better than those achieved with a corresponding fixed-bed reactor, the authors concluded that the improvement resulted because of sweeping N2 transport and dilution. Simulations of the MR behavior showed poor agreement with the experiments [46]. Zhang et al. [42] studied the use of carbon membranes in methanol steam-reforming reactors. The carbon membrane was prepared from a novolac-type phenolic resin and poly(ethylene glycol) on a green support. Methanol steam reforming was performed at 200-250°C with a Cu/Zn0/Al203 catalyst, and the CMR was compared with a conventional fixed-bed reactor. A higher methanol conversion and lower carbon monoxide yield were achieved by enhancing the potential of the carbon membrane.

Harale et al. [43] used a CMR in the water-gas-shift reaction for H2 production. The authors proposed a hybrid adsorbent MR (HAMR) system that combines the reaction and membrane-separation steps with adsorption. Cu0/Zn0/Al203 was used as a catalyst, and a layered double hydroxide was selected as the C02 adsorbent. The carbon membranes, which were 25.4 cm long and had an outside diameter of 0.57 cm, showed high H2 permeation fluxes at 250°C, but their preparation methods were not discussed. The experimental results agreed well with the model predictions, and the HAMR system can provide improved yields of H2 with reduced CO concentrations. Abdollahi et al. [44] proposed the "one-box" process, which combines reaction and membrane separation in the same unit for the water- gas-shift reaction. It includes a catalytic MR, which makes use of a H2- selective CMS membrane and a sulfur-tolerant Со/Mo/A1203 catalyst. The MR performance was investigated for a range of pressures and sweep ratios and showed higher CO conversions and a higher H2 purity compared with the traditional packed-bed reactor. This result was extended to a field test by using a multitubular-supported CMS module with 86 single tubes and an effective membrane area of 0.76 m2, shown in Figure 7.2, fabricated by Parsley et al. [45]. This module was tested above 250°C, and the membrane

FIGURE 7.2

Photographs of a single tube and pilot-scale and full-scale bundles of carbon molecular sieve membranes [45].

performance remained unchanged over several hundred cumulative hours in the field test.

Sa et al. [24] studied the methanol steam-reforming reaction in a CMR over a commercial CuO/ZnO/AhO, catalyst at 150 and 200°C. CMS membranes prepared from cellulose derivative were supplied by Carbon Membranes Ltd. The CMR was operated at atmospheric pressure and with vacuum at the permeate side. It was found that methanol conversion, H2 recovery, and H, yield were enhanced by lower feed flow rates because of higher residence times, with the drawback of higher carbon monoxide production. The simulation study showed that using water as sweep gas brings several advantages. In addition to an increase in methanol conversion and H2 recovery, carbon monoxide production decreases significantly. Briceno et al. [47] applied CMS membranes to a methanol steam-reforming reaction in a MR at 550°C. The CMS membranes that were derived from Matrimid were pyrolyzed at 550°C with a H2/N2 ideal separation factor of 2.67-2.77 in the range of 23-150°C. There was little difference between a conventional reactor and MR because of the low H2 selectivity; however, the total yield and methanol conversion were higher in the MR.

Hirota et al. [30] applied an activated FFACMS membrane to methylcyclohex- ane dehydrogenation in a MR. The reaction temperatures were 200 and 220°C and 0.5 wt% Pt/A1203 was used as a catalyst. The methylcyclohexane conversion exceeded the equilibrium values because of the selective H2 permeation.

Sa et al. [48] evaluated the potential advantages of a CMR by using a onedimensional mathematical model compared with a Pd-MR for H2 production by methanol steam reforming. The study focused on an analysis of the methanol conversion, the selectivity of the H2/CO reaction, the CO concentration at the permeate side, and the H2 recovery, and concluded that the CMR gave a higher H, recovery than the Pd-MR at high H2 concentrations, but the Pd-MR showed more advantages at lower H2 production rates. For successful use in MRs, CMRs require a high separation selectivity and a high permeability so that the permeation rate is comparable with the catalytic reaction rate. The key challenges in this context are to reduce the membrane thickness without introducing defects and to scale up techniques for carbon-membrane fabrication with large surface areas. For commercial applications, carbon membranes should be prepared as monoliths or hollow-fiber modules to provide the additional benefits of a low pressure drop and a high surface-to-volume ratio. It would also be advantageous to shift the thermodynamic equilibrium by improving the porous structure of the carbon membranes.

 
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