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Carbon Membrane Regeneration

Carbon membranes present high thermal and chemical stability, and good separation performance, but may have durability problems related to oxygen chemisorption and water condensation inside the pore structure [55]. It should be noted that a small change in pore size dramatically influences the gas permeance. Therefore, carbon membrane aging should be well considered, and regeneration methods should be applied to recover the membrane performance over time. Possible regeneration methods include:

i. Thermal

ii. Thermochemical

iii. Chemical

iv. Electrothermal

Thermal regeneration involves heating the module with an inert gas, to desorb gases or vapors from the pores and voids. Thermochemical regeneration means heating in a reactive atmosphere like air, to increase the actual pore size distribution. Reactive surface functional groups can be made passive by reduction in, for example, H2. This method can restore or even increase the capacity of a used membrane module compared with its initial value and offers a fast route to a new module (compared with a polymeric module, which in most cases cannot be reused). The chemical method exposes the module to an agent such as propylene [56] that will dissolve some of the adsorbed gases or vapors and recover the carbon membrane porosity. Finally, electrothermal regeneration [7] releases the adsorbed molecules by applying a low-voltage current across the carbon. This requires the carbon matrix to have a certain conductivity; membranes doped with metals are suitable. For safety reasons this should be performed in a non-oxidizing gas stream.

Carbon Membrane Upscaling

Although carbon membranes have shown excellent separation performances for different applications, most of them are reported at laboratory scale. Owing to their fragility and brittleness characteristics, CMS membranes should be fabricated in free-standing hollow fibers and supported tubular configurations if considering a large scale. The membrane materials should also be considered since they relate directly to the product cost. A pilot-scale module that contains 24 medium-sized carbon hollow fiber modules (0.5-2 m2 each) has been reported by Haider et al. [57]. Their carbon membranes were produced from deacetylated cellulose acetate (CA). However, the high production cost with the extra cellulose regeneration step involved, and the difficulty of keeping fibers straight during drying after CA deacetylation, are challenging the further development and applications of CA-based carbon membranes.

This chapter discusses carbon membranes produced in a batch-wise manner (except for the spinning of fibers) on a pilot scale. Some thoughts about fabrication in a continuous way are necessary. A general challenge for batch-wise production is obtaining equal quality for each membrane inside a chamber or container. The parameters involved are usually the concentrations of different compounds, and temperature. Karvan et al. [58] reported that polyimide hollow fiber precursors came in contact with each other during carbonization and fused together at high temperatures. As a result, it is difficult to avoid defects on the membrane surface. However, cellulose hollow fibers are less prone to fusing, and large quantity of cellulose fibers can be carbonized in the same batch. Haider et al. [59] reported that 1600-4000 cellulose hollow fibers were carbonized simultaneously by using 2-m-long perforated plates with square openings. They also suggested that draining the tars and vapors that were generated during carbonization is crucial to produce uniform carbon membranes.

Membrane module design and construction for high-temperature/pres- sure applications is another challenge when upscaling carbon membranes, related to membrane mounting, potting, and sealing. Figure 1.5 (left) illustrates the upscaling of cellulose-based CHFM modules from laboratory scale

FIGURE 1.5

Upscaling of carbon membrane modules. Left: carbon hollow fiber membranes (a) laboratory- scale module, (b) medium-sized module, (c) multimodule, (d) membrane pilot plant [57]; right: tubular supported carbon membranes [60].

to a high technology readiness level; the modules have been tested successfully for biogas upgrading. For supported carbon membranes, a membrane module consisting of tubular carbon membranes with a surface area of 0.76 m2 and a packing density of 222 m2/m3 was demonstrated by Parsley et al. [60] (see Figure 1.5 (right)). A bundle of supports was fabricated before the application of carbon layers via dip-coating and carbonization. It is expected that large-scale production of supported carbon membranes will be challenging, and the production cost is still quite high, which may limit the applications to small-volume gas separation processes.

 
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