Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
2002-07-30
2004-05-11
Lawrence, Frank M. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S143000, C095S144000, C095S902000, C095S115000, C585S824000
Reexamination Certificate
active
06733572
ABSTRACT:
FIELD
This invention relates to a process for separating propylene and dimethylether from mixtures of low molecular weight hydrocarbons.
BACKGROUND
The separation of low molecular weight hydrocarbons in mixed hydrocarbon streams is an extremely important and large volume operation in the chemical and petrochemical industries. Catalytic cracking and steam cracking are among the most common and large-scale processes leading to these mixed hydrocarbon streams. The chemical conversion of oxygenates to olefins, such as the conversion of methanol to olefins (MTO), is another potential source of these hydrocarbon streams that require purification before final use or to improve overall process economics via recycle.
The MTO process typically uses low acidity silicoaluminophosphate catalysts to drive the transformation of methanol to ethylene and propylene in high yields. Typical pilot plant data indicates that, excluding water by-product and unreacted methanol, the major components in the reactor effluent are ethylene (approximately 40 wt %), propylene (approximately 40 wt %), C
4
+ (approximately 14 wt %), ethane (approximately 1 wt %), propane (approximately 1 wt %), and dimethylether (approximately 1 wt %). The process is accompanied by coke formation (approximately 3 wt %) and by lesser amounts of CO, CO
2
, CH
4
, ethers, ketones, acids, alcohols, and aldehydes. Even though the composition of the MTO product differs from that obtained in conventional steam crackers, the post-processing equipment required to produce polymer grade ethylene and propylene is largely the same. Ongoing process development studies indicate that one of the main differences lies in the C
3
splitter, which is the final step in the generation of a high purity propylene stream.
In steam cracking, the feed to the C
3
splitter contains primarily propylene and propane because prior to entering the splitter, this stream is selectively hydrogenated to transform residual methylacetylene and propadiene (MAPD) into additional propylene (and propane). In MTO, due to the milder reaction temperatures, the amount of MAPD is negligibly small but dimethylether is readily formed by dehydration of methanol. Paralleling the situation in steam cracking, efforts are underway to selectively react the dimethylether to prevent the potential contamination of the high purity propylene stream. However, recent vapor-liquid equilibrium calculations indicate that dimethylether will end up with propane in the bottoms of the C
3
splitter and thus no feed pretreatment will be necessary. Moreover, since the propylene recovery in the splitter is less than 100%, this bottoms stream will also contain propylene.
Under this scenario, the C
3
splitter bottoms stream will contain valuable dimethylether and propylene, but unless these are separated from the propane, the stream is currently expected to have fuel value only. However, if dimethylether could instead be recovered, recycled, and converted into olefin products, an additional 2 wt % ethylene equivalent could be produced. For a large-scale plant of 1000 kTa ethylene this would amount to about 54.8 T/day and could increase further if methanol conversion in the MTO reactor were to decrease by design or by other unit constraints. Similarly, the recoverable propylene represents an additional 2.17%, which amounts to 59.5 T/day for a 1000 kTa propylene plant.
The close proximity in boiling points between propylene and propane, in particular, suggests that conventional distillation may not provide an economically viable method of separating dimethylether and propylene from propane. An object of the present invention is therefore to provide an alternate process for selectively recovering dimethylether and propylene from a mixture containing propane.
Some of the leading alternatives to fractional cryogenic distillation involve the use of adsorbents that exploit their ability to adsorb some of the components selectively. This has given rise to various forms of pressure or temperature swing adsorption (PSA/TSA) processes in which the mixture is first passed through an adsorbent material under conditions where one or more of the components are selectively removed. The loaded material is then typically exposed to a lower pressure and/or higher temperature environment where the adsorbed components are released and recovered at a higher purity level. Economic viability requires adsorbent materials that can deliver high selectivity, high adsorption capacity, and short duration cycles. An additional and critically important requirement is that the material should not catalyze chemical reactions that might lower the recovery of the desired components and/or render the adsorbent inactive. This is a particularly demanding condition when dealing with olefins and oxygenates.
Among the adsorbents which have been proposed for the recovery of propylene from hydrocarbon mixtures are ion exchange resins, mesoporous solids, activated carbons, and zeolites. Ion exchange resins and mesoporous solids usually exploit equilibrium adsorption properties in which one of the components is selectively adsorbed over suitably dispersed chemical agents. They principally rely on the adsorption affinity of cationic active centers such as Ag and Cu ions for the double bond in propylene (&pgr;-complexation). The duration of the adsorption cycle is that required to bring the mixture close to thermodynamic equilibrium with the adsorbent. The relative rates of diffusion of the various components within the adsorbent are of secondary importance but the total time for equilibration is preferably kept low for economic reasons.
Unlike traditional equilibrium separations that rely on the preferential adsorption of some of the components, kinetic-based separation processes rely on the property of some of the components to diffuse more rapidly than others into the adsorbent material. Two related cases of diffusion control are of interest. In one extreme case, the separation is achieved by excluding the diffusion of some of the components into the adsorbent. The second case exploits a sufficiently large difference in diffusion rates to allow the preferential uptake of some of the components within a predetermined adsorption time.
Activated carbons and zeolite adsorbents typically resort to a combination of adsorption affinity and diffusion control. Thus, carbons are usually activated to very high surface area forms in order to provide textural properties and pore sizes that maximize adsorption while selectively controlling diffusion. Crystalline microporous materials have become even more attractive than activated carbons because of the ever increasing possibilities afforded by new synthetic routes, which allow for a more flexible and precise control of chemical composition, pore size, and pore volume. The tetrahedrally coordinated atoms in these microporous materials form ring structures of precise dimensions that selectively control the diffusional access to the internal pore volume.
8-membered ring zeolites, in particular, have been actively investigated for the separation of small molecular weight hydrocarbons because their, window sizes are very comparable to molecular dimensions and because they can provide high adsorption capacities. A typical example is the Linde type A zeolite, which is characterized by a set of three-dimensional interconnected channels having 8-membered ring window apertures. The effective size of the windows depends on the type of charge-balancing cations. This has given rise to the potassium (3A), sodium (4A), and calcium (5A) forms, which have nominal window sizes of about 3 Å, 3.8 Å, and 4.3 Å, respectively.
Thus, for example, EP-B-572239 discloses a PSA process for separating an alkene, such as propylene, from a mixture comprising said alkene and one or more alkanes by passing the mixture through at least one bed of zeolite 4A at a temperature above 323° K to preferentially adsorb said alkene and then desorbing the alkene from the bed. EP-A-943595 describes a similar process in which t
DeMartin Gregory J.
Reyes Sebastian C.
Santiesteban Jose Guadalupe
Sinfelt John H.
Strohmaier Karl G.
ExxonMobil Chemical Patents Inc.
Lawrence Frank M.
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