Enhanced rate PSA process

Details Enhanced rate PSA process Enhanced rate PSA process

2001-10-03

2004-09-14

Spitzer, Robert H. (Department: 1724)

Gas separation: processes

Solid sorption

Including reduction of pressure

C095S102000, C095S105000, C095S130000, C095S902000

Reexamination Certificate

active

06790260

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to pressure swing adsorption (PSA) processes using adsorbents having high intrinsic adsorption rates. More particularly, the invention relates to PSA processes wherein high product recovery and low bed size factor (BSF) are achieved for fast-cycle shallow adsorbers.
BACKGROUND OF THE INVENTION
There has been significant development of the various PSA, VSA and VPSA methods for air separation over the past thirty years, with major advances occurring during the last decade. Commercialization of these processes and continued extension of the production range can be attributed primarily to improvements in the adsorbents and process cycles, with advances in adsorber design contributing to a lesser degree. Highly exchanged lithium molecular sieve adsorbents, as illustrated by Chao in U.S. Pat. No. 4,859,217, are representative of advanced adsorbents for oxygen production. Advanced adsorbents of the types mentioned above are the result of improvements in equilibrium properties.
Improving process efficiency and reducing the cost of the light component product can be accomplished by decreasing the amount of adsorbent required and by increasing the product recovery. The former is generally expressed in terms of bed size factor (BSF) in lbs adsorbent/TPDO (ton per day of contained O
2
), while the latter is simply the fraction of light component (i.e. oxygen) in the feed (i.e. air) that is captured as product. Improvement in adsorbents and reduction in cycle time are two primary methods of reducing BSF.
While shorter cycles lead to shorter beds and higher adsorbent utilization, product recovery generally suffers unless adsorption rate is increased. This phenomena can be ideally characterized in terms of the size of the mass transfer zone (MTZ), i.e. the mass transfer zone becomes an increasing fraction of the adsorbent bed as the bed depth decreases. Since the adsorbent utilization with respect to the heavy component (e.g. nitrogen) is much lower in the MTZ than in the equilibrium zone, working capacity declines as this fraction increases.
The effect of particle size upon the size of the MTZ is conceptually straightforward in a single long adsorption step where a contaminant in relatively low concentration is removed from the feed stream on the basis of its higher equilibrium affinity to the adsorbent. When the adsorbate/adsorbent combination is characterized by a favorable isotherm, a steady state transfer zone is envisioned that moves through the adsorber at a constant speed. Distinct equilibrium and mass transfer zones can be identified in the process. Under such conditions, and when the resistance to mass transfer is dominated by intraparticle pore diffusion, it has long been recognized that reducing the adsorbent particle size results in higher adsorption rates and smaller mass transfer zones. Unfortunately, pressure drop across the adsorbent bed increases with decreasing particle size and leads to difficulty in particle retention in the bed and an increased tendency for fluidization.
This ideal concept becomes blurred when the isotherms are unfavorable and/or the mass transfer zone is continuously developing or spreading throughout the adsorption step. Adding the remaining minimum steps of depressurization, desorption and pressurization to create a complete adsorption process cycle further complicates the behavior and character of the mass transfer zone. Nevertheless, the idealized concept of the MTZ has been applied in the prior art as a basis to affect improvements in process performance.
Ackley et al. (WO 99/43416) have shown increased performance in PSA air separation processes through increased adsorption rates and larger mass transfer coefficients. This was accomplished while employing adsorbents of high effective pore diffusivity (D
p
≧5×10
−6
m
2
/s) in conjunction with short cycles and shallow beds. Ackley et al. (WO 99/43418) extended these concepts to low pressure ratio cycles.
Jain (U.S. Pat. No. 5,232,474) discloses improving adsorbent utilization by decreasing the adsorbent volume and/or increasing the product purity, wherein the removal of H
2
O and CO
2
prior to cryogenic air separation is described using a pressure swing adsorption (PSA) process. The adsorber is configured entirely with alumina or with layers of alumina and 13×molecular sieve adsorbents. Smaller particles (0.4 mm to 1.8 mm) are used to achieve a smaller bed volume.
Umekawa (JP Appl. No. 59004415) shows a lower pressure drop and smaller adsorber for air purification by using a deep layer of large particles (3.2 mm) followed by a shallow layer of small particles (1.6 mm) of the same adsorbent. The bed size and pressure drop of this layered configuration are lower than for beds constructed either of all 3.2 mm or all 1.6 mm particles. The 1.6 mm particles occupy only a small fraction (low concentration part) of the mass transfer zone in the layered configuration.
Miller (U.S. Pat. No. 4,964,888) has suggested using larger particles (>14 mesh or 1.41 mm) in the equilibrium zone and small particles (<14 mesh) in the mass transfer zone. This reduces the size of the MTZ while minimizing the excessive pressure drop increase that would occur if only small particles were used in both zones. Cyclic adsorption process times greater than 30 s are indicated.
Garrett (UK Pat. Appl. GB 2 300 577) discloses an adsorption apparatus containing particles in the size range between 6 mesh (3.36 mm) and 12 mesh (1.68 mm) deployed in either discrete layers or as a gradient of sizes with the largest particles located near the feed inlet and the smallest particles located downstream near the outlet of the adsorber in both configurations.
Very small adsorbent particles (0.1 mm to 0.8 mm) are necessary for the fast cycles and high specific pressure drop that characterize a special class of processes known as rapid pressure swing adsorption (RPSA). Typical RPSA processes have very short feed steps (often less than 1.0 s) operating at high feed velocities, include a flow suspension step following the feed step and generally have total cycle times less than 20 s (often less than 10 s). The behavior of the adsorption step is far removed from the idealized MTZ concept described above. In fact, the working portion of the bed is primarily mass transfer zone with only a relatively small equilibrium zone (in equilibrium with the feed conditions) in RPSA. A major portion of the adsorber is in equilibrium with the product and provides the function of product storage. The high pressure drop (on the order of 12 psi/ft)/short cycle combination is necessary to establish an optimum permeability and internal purging of the bed which operates continuously to generate product.
RPSA air separation processes using 5A molecular sieve have been described by Jones et al. (U.S. Pat. No. 4,194,892) for single beds and by Earls et al. (U.S. Pat. No. 4,194,891) for multiple beds. Jones has also suggested RPSA for C
2
H
4
/N
2
, H
2
/CH
4
, H
2
/CO and H
2
/CO/CO
2
/CH
4
separations using a variety of adsorbents. The RPSA system is generally simpler mechanically than conventional PSA systems, but conventional PSA processes typically have lower power, better bed utilization and higher product recovery.
In somewhat of a departure from the original RPSA processes, Sircar (U.S. Pat. No. 5,071,449) discloses a process associated with a segmented configuration of adsorbent layers contained in a single cylindrical vessel. One or more pairs of adsorbent layers are arranged such that the product ends of each layer in a given pair face each other. The two separate layers of the pair operate out of phase with each other in the cycle. The intent is for a portion of the product from one layer to purge the opposing layer—the purge fraction controlled by either a physical constriction placed between the layers and/or by the total pressure drop across a layer (ranging from 200 psig to 3 psig). Particles in the size range of 0.2 mm to 1.0 mm, total cycle times of 6 s to 60 s, adsorbent layer dept

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