Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
2002-03-27
2003-10-28
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S106000, C095S117000, C095S135000, C095S136000, C095S138000, C095S139000, C095S140000
Reexamination Certificate
active
06638340
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the removal of carbon dioxide and other contaminants from gas streams, and more particularly to the pre-purification of air by the removal of contaminants from air prior to air separation. More specifically, the invention relates to adsorbents, having at least three components, that provide enhanced performance in the pre-purification of air.
BACKGROUND OF THE INVENTION
Gases that occur in nature or which are produced in industrial processes often contain carbon dioxide in small amounts. Atmospheric air generally contains about 0.035% or 350 parts per million (ppm) carbon dioxide. It is sometimes desirable or necessary to remove the carbon dioxide from a gas because of certain process constraints or the intended end use for the gas. For example, air that is separated into various component products by cryogenic separation techniques (cryogenic air separation), such as cryogenic distillation or cryogenic adsorption, must be substantially free of both carbon dioxide and moisture. These operations are carried out at temperatures below the freezing points of water and carbon dioxide and consequently, if they are not removed, they will freeze and eventually clog the air separation process equipment.
Relatively pure oxygen (i.e. an oxygen-containing gas having an oxygen content of 88% or more) has a number of desirable industrial and medicinal applications at various pressures and purities. The Earth's atmosphere, typically comprising nearly twenty-one percent oxygen gas, is the natural candidate for use as an economical oxygen source. Many of the most practical and economical oxygen production plants employ air separation systems and methods.
One of the more common systems for producing oxygen in relatively large volumes incorporates cryogenic technology to liquefy and separate a desired oxygen component of a predetermined purity from the air mixture. While the design works well for high-volume oxygen production, the specialized cryogenic hardware and associated high capital startup expenditures make such systems cost-prohibitive when used for production in low to moderate volumes, e.g. from about 30 to about 200 tons per day of an oxygen containing gas with an oxygen concentration higher than about 88% and up to about 95%.
Traditionally, higher volumes of oxygen have been produced via the well-known cryogenic rectification of air in which air is cooled to a temperature near the normal boiling point of the components and treated in fractionation columns. The significant capital and operating costs of the cryogenic separation systems are justified only when large quantities and/or extremely high purities (such as 97% to 99.999%) are required.
Before air can be introduced into a cryogenic air separation process in which oxygen and nitrogen are separated from one another, CO
2
in the air at low levels must be removed in order to avoid the CO
2
solidifying in the components of the air separation plant. Two methods generally used for such CO
2
removal include temperature swing adsorption (TSA) and pressure swing adsorption (PSA).
In each of these techniques, a bed of adsorbent is exposed to a flow of feed air for a period of time to adsorb CO
2
from the air. Thereafter, the flow of feed air is shut off from the adsorbent bed and the adsorbent is exposed to a flow of purge gas which strips the adsorbed CO
2
from the adsorbent and regenerates it for further use. In TSA processes, the CO
2
is driven off from the adsorbent by heating the adsorbent in the regeneration phase. In PSA processes, the pressure of the purge gas is lower than that of the feed gas and the change in pressure is used to remove the CO
2
from the adsorbent.
Other components can be removed from the feed air by these processes, including hydrocarbons and water. These adsorption techniques can also be applied to feed gases other than air or be applied to air to be purified for purposes other than use in an air separation plant.
The choice of adsorbent is often the key to the effectiveness of the process. Much attention has been given to the development, improvement and manufacture of adsorbents, e.g. specialized zeolite adsorbents that have been synthesized through ion exchange, lower Si/Al containing structures and improved activation procedures. These additional and/or improved manufacturing steps have resulted in higher costs for these specialized adsorbents compared to standard adsorbents, e.g. LiX compared to 5A and 13X adsorbents in PSA air separation processes. In many processes the adsorbent has become a significant fraction of the overall capital investment. Thus, there is considerable incentive to reduce the cost of the adsorbent if such reduction can be transformed into an overall reduction in the cost of the desired product of the separation.
The prior art has attempted to address the problem of thermal cycling in PSA processes, in some instances by employing mixtures of materials. Mixtures have also been applied independent of thermal cycling effects to improve specific elements of adsorption process performance such as product purity or recovery or storage capacity. Distinct materials have been combined physically (co-mixture) in an adsorber or have been integrally bound in a single composite bead or pellet.
Mixtures of adsorbents have also been utilized when multiple separations are required. An example is provided in U.S. Pat. No. 4,194,892 A for the purification of steam reformer hydrogen involving the removal of carbon dioxide, methane and carbon monoxide using a rapid pressure swing adsorption (RPSA) process. It was shown that product H
2
recovery was increased when a homogeneous mixture of activated carbon and crystalline molecular sieve was used in place of activated carbon alone.
In U.S. Pat. No. 5,779,767 A is described the use of an adsorbent mixture of a zeolite and an alumina to separate carbon dioxide, water and acetylene from a feed gas.
In U.S. Pat. No. 6,027,548 A is described a process for purifying a gas stream using a combination of a strong adsorbent such as NaX or NaY and a comparatively weak adsorbent, such as Al
2
O
3
. These adsorbents are used to separate relatively heavy components such as carbon dioxide from relatively light components, such as nitrogen.
Mixtures of fine and coarse particles have been applied to reduce interparticle void space, increase adsorbent density and increase gas storage capacity. EP 0 325 392 B1 (Kaplan et al.) provides an example of this methodology applied in PSA systems employing carbon molecular sieve (CMS) adsorbents for kinetic separation of air to produce N
2
. In Kaplan, the main CMS adsorbent is comprised of coarse particles (2.5 to 3.0 mm), while the void space between these larger particles is filled with fine particles of either an inert material or CMS adsorbent. The fine particle fraction is preferred to be an inert or non-adsorptive material (e.g. glass beads) and to occupy approximately 40% by volume of the adsorber bed. The reduction in void space was shown to improve process efficiency.
U.S. Pat. No. 4,499,208 A (Fuderer) doped activated carbon with inert dense alumina and achieved a reduced thermal swing when adsorbing C
2
at high pressure from a feed stream containing H
2
, CO
2
, CO and CH
4
. Although the specific heat of the alumina is nearly the same as the activated carbon, the high density of the inert material significantly increases the heat capacity per unit volume of the bed. Lowering the thermal swing in the process significantly improved the process recovery.
The use of PSA for removing CO
2
from air prior to separating air into its respective components by cryogenic air separation is described in numerous publications, e.g. U.S. Pat. Nos. 4,249,915 A and 4,477,264 A. Conventional processes employed a dual bed of alumina for water removal followed by a zeolite such as 13X for CO
2
removal. More recently, all alumina PSA systems have been proposed, as described in U.S. Pat. No. 5,232,474 A. The advantages of an all alumina system include lower adsorbent cost, ve
Kanazirev Vladislav I.
Latus Donald G.
Goldberg Mark
Spitzer Robert H.
Tolomei John G.
UOP LLC
LandOfFree
Composite adsorbents for air purification does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Composite adsorbents for air purification, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Composite adsorbents for air purification will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3164483