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Ind. Eng. Chem. Res. 2005, 44, 7218-7227
Carbon Molecular Sieves for Hydrocarbon Separations by Adsorption Carlos A. Grande, Simone Cavenati, Francisco A. Da Silva,† and Alı´rio E. Rodrigues* Laboratory of Separation and Reaction Engineering (LSRE), Department of Chemical Engineering, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias s/n, 4200-465 Porto, Portugal
The use of vacuum pressure swing adsorption (VSA-PSA) with carbon molecular sieves (CMS) as selective sorbents is evaluated as an alternative technology for methane-nitrogen and propane-propylene separations. The larger molecules, methane and propane, are very lowdiffusing species, resulting in processes where nitrogen and propylene are retained in the bed. For the methane-nitrogen separation, a Skarstrom cycle (pressurization, feed, blowdown, and purge) was used with the advantage of recovering methane in the feed step with a low-pressure drop. At ambient temperature, from a mixture with 20% of nitrogen balanced by methane, a purity >93% was obtained. For the propane-propylene mixture, a five-step cycle was useds pressurization, feed, rinse, intermediate depressurization, and countercurrent blowdownswhere purified propylene is obtained as a low-pressure product. Starting with an equimolar mixture at 373 K, the purity of propylene was 83% with a product recovery of 84%. 1. Introduction The splitting of propane-propylene and methanenitrogen streams are two major challenges in separation technology. Both systems involve the separation of two molecules of very similar properties. This work presents the use of carbon molecular sieves (CMS) as selective kinetic adsorbents to use in vacuum pressure swing adsorption (VSA-PSA) units as an alternative process to traditional distillation. Propane-propylene separation is the most difficult separation practiced in the petrochemical industry, commonly performed in columns containing over 200 trays with reflux ratios over 12.1 Adsorption was already proposed for this separation.2 The aid of a specific massseparating agent may increase the separation factor, diminishing energy requirements.3 Much of the literature published in this area was devoted to adsorbent characterization. Most of the work dealt with zeolites and π-complexation adsorbents,4-6 and only a few dealt with carbonaceous materials. Activated carbons,7 as well as CMS,8,9 had shown poor equilibrium selectivity toward propylene. Another sample of CMS (Bergbau-Forschung) did not adsorb either propane or propylene.10 We have previously reported the adsorption equilibrium and kinetics of pure propane and propylene onto Takeda CMS 4A.11 This adsorbent has small equilibrium selectivity toward propylene, but a considerable difference in the micropore diffusivity coefficients was observed, indicating the possibility of kinetic separation to be performed. The adsorption equilibrium of the binary mixture was also reported,12 and it was wellpredicted with the multisite Langmuir model.13 Propylene is the most adsorbed species, with faster diffusion, constraining the system to recover propylene in the desorption step. * To whom correspondence should be addressed. Tel.: +351 22 508 1671. Fax: +351 22 508 1674. E-mail:
[email protected]. † Present address: Department of Chemistry, University of Aveiro, Portugal.
Table 1. Average Composition of Natural Gas in Different Countries composition, molar fraction country of origin
CH4
C2H6
others
N2
Netherlands Germany United States Algeria Canada Australia Indonesia Russia North Sea Venezuela Argentina Brazil
81.2 74.0 81.8 76.0 88.5 76.0 84.9 97.8 94.7 78.1 95.0 89.4
2.9 0.6 5.6 8.0 4.3 4.0 7.5 0.5 3.0 9.9 3.8 6.7
1.5 17.9 5.7 9.6 4.6 18 5.8 0.4 1.0 10.8 0.2 3.1
14.4 7.5 6.9 6.4 2.6 2.0 1.8 1.3 1.3 1.2 1.0 0.8
Previous work on kinetic VSA-PSA for propanepropylene separation by the partial exclusion of propane was carried out using zeolite 4A.14-16 In these works, purity ranging from 96 to 99% was obtained, but recovery decreases dramatically when purity is >99%. To the knowledge of the authors, no work was previously published using CMS 4A in a VSA-PSA unit to carry out this separation. Methane-nitrogen separation appears in the context of natural gas for fuel usage. For transportation of methane through pipelines, the content of inert gas has to be 85%. In all these simulations, it was observed that nitrogen breaks through in the feed step, even in the first cycle. This is a consequence of the very small amount of nitrogen adsorbed and means that too much nitrogen is entering per cycle and its quantity must be diminished. The only way to substantially diminish the amount of nitrogen per cycle is to perform the pressurization step with purified product. When purified product is used in the pressurization step, it is normally used in countercurrent flow to further displace nitrogen molecules from the region near the exit of the column.25 For this reason, the behavior of the VSA-PSA was studied with countercurrent pressurization with pure methane as an example. Experimental and simulated results obtained using different step times are shown in Table 7. As an example of an experimental run obtained, we show in Figure 10 the molar flowrate of methane and nitrogen exiting the column in cycles 1 and 52. It can be seen in this plot that nitrogen breaks through in the feed step, which is the reason a purity >94% was not obtained. On the other side, to avoid nitrogen break-
Figure 10. Molar flowrate of methane and nitrogen in (a) cycle 1 and (b) cycle 52 for a 4-step VSA-PSA for the CH4 (80%)-N2 (20%) separation experiment at 304 K using countercurrent pressurization with pure methane (conditions are detailed in Table 7, run 6). Solid points are experimental data, and lines correspond to the simulation using the bidisperse model.
through, the feed step should be reduced and methane recovery will drop to smaller values that the ones shown in Table 7. In all the experiments performed with this cycle configuration, isothermal behavior was also observed in the column. It can be seen in this case that the purity of product is much higher, >90% in all the cases. On the other side, the recovery of methane is
