(479c) Hydrogen Production with a Membrane Steam Reformer

Composite palladium membrane technology has matured considerably in the past decade and is now poised for commercial applications in H₂ manufacture in the near future. The use of membrane reactors for H₂ manufacture is particularly attractive. Membrane reactors can combine steam reforming, water gas shift and hydrogen separation into a single unit operation. Pd-alloy membranes separate hydrogen from CO₂ at very high selectivity. The continuous withdrawal of hydrogen along the length of the reactor removes the thermodynamic limitations enabling conversions of 95 % or higher. The removal of equilibrium constraints on conversion enables the membrane reactor to operate at much lower temperatures (500^oC) than conventional steam reforming reactors. In addition to pure hydrogen, the membrane reactor outputs a high pressure (30 Bara) retentate stream that is concentrated in CO₂, and is low in CO. This stream is advantaged for CO₂ sequestration as compared to low pressure CO₂ output from conventional SMR.

Shell has pursued a program in the development and commercialization of high temperature membranes, and membrane steam reforming (MSR) process technologies for H₂ production [1,2]. CRI/Criterion is in the process of commercializing Pd and Pd-alloy membranes on sintered porous metal supports. The scope of the CRI/Criterion effort includes the development of membranes for sale to third parties in addition to the support of the Shell MSR efforts. A 1"OD by 6"L Pd membrane from CRI/Criterion, was used in the MSR experiments presented below. Moreover, CRI has produced membranes of this type as large as 2"OD by 48"L, by welding two separate 24"L sections. These membranes can be produced with a hydrogen permeance in the range of 50-70 [Nm³/m².h.bar^0.5]. Both the hydrogen permeance and the separation selectivity are stable at temperatures up to 500^oC.

MSR experiments were conducted at 500^oC with a reaction pressure of 30 Bara in an electrically heated shell and tube reactor. The annulus between the membrane and the reactor outer shell was packed with a commercial reforming catalyst. The hydrocarbon feed to the reactor was a blended gas that resembled natural gas (93 % CH₄, 4 % C₂H₆, 1% C₃H₈, 2% N₂) and feed steam to carbon ratio was 3.0. Figure 1 shows hydrogen output, purity and conversion data for an MSR run lasting 51 days.

The H₂ permeance was 56.1 [Nm³/m².h.bar^0.5] at 500^oC. The H² output of the unit was dependent on MSR feed flow rate. At the points labeled a and b the hydrocarbon feed was increased, resulting in increases in the H₂ flux and slight decreases in conversion. When operating conditions were held constant the H₂ flux and conversion were very stable. The highest output rate demonstrated for this trial was 125 SLPH and conversion varied between 94.0 and 92.4 %. The retentate stream composition from the reactor, measured at maximum H₂ output, was 91 mol% CO₂ and 0.7 mol% CO (dry basis). The membrane in this experiment yielded 98+ mol% hydrogen over the life of the test. The use of a high reaction pressure is advantageous because it increases the driving force for H₂ transport allowing less membrane area, and it provides advantages for CO₂ sequestration. Efforts are currently under way to improve the membranes to achieve higher permeate purities while retaining a high reaction pressure.

These results show that the CRI/Criterion membrane technology, is capable of maintaining stable H₂ flux and stable hydrogen product purity for extended periods at conditions of temperature and pressure which are both among the highest demonstrated for a metal supported Pd membrane. Further MSR evaluations are currently underway on a 2"OD by 24" long version of this membrane technology. Following the results shown in Figure 2, the membrane was provided as an evaluation sample to a third party with potential interest in purchasing membranes from CRI/Criterion.

[1] Matzakos, A.N.; Wellington, S.L.; Mikus, T.; Ward, J.M.. U.S. Patent 6,821,501 B2, Nov. 23, 2004.

[2] Andreas N. Matzakos and Scott L. Wellington. "Novel Membrane Steam Reforming (MSR) Reactor for Pure Hydrogen Production". 2004 Spring AIChE Meeting