Thermal swing adsorption process

Details Thermal swing adsorption process Thermal swing adsorption process

2000-08-28

2002-08-13

Spitzer, Robert H. (Department: 1724)

Gas separation: processes

Solid sorption

Inorganic gas or liquid particle sorbed

C095S123000, C095S129000, C095S139000, C095S143000, C095S144000, C095S902000

Reexamination Certificate

active

06432171

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a novel thermal swing adsorption (TSA) process for the removal of hydrocarbons and oxides of nitrogen from air. More particularly, the present invention relates to a TSA process for use in pre-purification units (PPU) prior to cryogenic distillation of air to separate it into oxygen and nitrogen.
BACKGROUND OF THE INVENTION
Prior to cryogenic air separation by distillation to produce oxygen and nitrogen, various trace impurities must be removed from feed air to avoid formation of solids in the heat exchanger equipment and resultant high pressure and safety issues in the cryogenic process. The most obvious trace air impurities that must be removed include carbon dioxide, CO
2
, and water, H
2
O. There are many references which disclose the use of pressure swing adsorption, PSA, and thermal swing adsorption, TSA, to remove these types of impurities from air in pre-purification units prior to cryogenic separation of air by its distillation into oxygen nitrogen and argon.
The importance of the removal of nitrogen oxides from air prior to entering into a cryogenic air separation plant has only recently been recognized. The removal of dinitrogen oxide, N
2
O , which is also known as nitrous oxide, is particularly important because of its increase in concentration in the atmosphere. It is well known that N
2
O is a greenhouse gas and the concentration of N
2
O in the atmosphere (currently about 0.3 ppm) has been increasing steadily (by about 0.2 to 0.3% annually), over the last decade. This increase is mainly caused by anthropogenic activities as well as by emissions from various chemical processes. An excess of N
2
O in cryogenic air separation units may lead to tube p contamination of the products. The fact that N
2
O is very stable in air, viz., its “lifetime” in the atmosphere comprises about 150 years, makes the removal of N
2
O in an air pre-purification unit absolutely essential in both present time and the future. It is envisioned that in the future the removal of N
2
O will become as important as the removal of water and CO
2
. As the concentration of N
2
O in air increases further, the current regime of PPU processes will become inadequate because N
2
O cannot be removed easily by existing PPU processes. Accordingly, there is a clear need to develop an approved adsorption process suitable for use in PPU units to remove not only water and CO
2
but also the trace amounts of nitrogen oxides, in particular N
2
O, which are present in the air being sent to th addition, great care should be taken for the removal of traces of hydrocarbons, such as low-molecular weight hydrocarbon gases ethane, propane, n-butane, iso-butane as well as any non-saturated species, such as acetylene, ethylene, propylene, the n-butylene isomers and iso-butylene, from air in PPU processes. It is also important that both hydrocarbons and plugging components such as N
2
O and CO
2
be simultaneously removed in the air pre-purification process. Deposits of plugging components may create an opportunity for hydrocarbons to collect and concentrate in those deposits. The present invention is directed to such a solution, which aims, in particular, to a simultaneous, highly effective pre-purification of air.
N
2
O is not the only hazardous trace impurity present in atmospheric air. N
2
O belongs to a class of hazardous impurities collectively referred to as plugging/plating components. Water and CO
2
are the other plugging/plating components removed in the PPU. These impurities freeze at cryogenic temperatures and plug passages and plate out on cold surfaces. Atmospheric air also contains several hydrocarbon impurities that must not be allowed to come in contact with liquid oxygen. Typically, hydrocarbon impurities enter the cold box, and accumulate in the liquid oxygen sump in the reboiler of the low-pressure column. The air separation process also concentrates the levels of these impurities, such that their concentrations in the low-pressure column are several times greater than their concentrations in the PPU product.
When the solubility and flammability limits of these compounds in liquid oxygen are exceeded, these compounds may be combusted, leading to small explosions and in extreme cases, to the combustion of the distillation column internals. The accumulation of hydrocarbon impurities can also take place inside the passages of heat exchangers and reboilers causing the same problems as seen in the distillation column. This accumulation in passages of heat exchangers is aggravated by the presence of plugging/plating components. Plated N
2
O and CO
2
may create pockets of stagnant liquid oxygen in which hydrocarbons can accumulate and concentrate beyond flammability limits.
It is clear that plugging components and hydrocarbons pose a combined threat to ASU safety, in addition to their individual deleterious effects. These observations highlight the need to remove in the PPU process not only N
2
O but hydrocarbons as well. The present invention is directed to such a solution that aims, in particular, to a simultaneous, highly effective pre-purification of air in a thermal swing adsorption process.
SUMMARY OF THE INVENTION
The present invention provides for a novel process for the removal of impurities from a gas stream by thermal swing adsorption. The process comprises passing a gas stream containing impurities through a first adsorbent that is capable of removing water from the gas stream, then passing the stream through a second adsorbent layer which is capable of removing carbon dioxide from the gas. Lastly, the gas stream is passed through a third layer capable of removing hydrocarbons and oxides of nitrogen from the gas.
The third adsorbent is a composite adsorbent that contains at least one adsorbent that will adsorb preferentially oxides of nitrogen and a series of hydrocarbons, ie., “N
2
O removal adsorbent”, and at least one adsorbent that will adsorb preferentially those hydrocarbons that were adsorbed by the first adsorbent to lesser an extent, i.e., “hydrocarbon removal adsorbent”. Preferably, the “N
2
O removal adsorbent” is a CaLSX, CaMSX or CaX type zeolite and the “hydrocarbon removal adsorbent” is a CaA type zeolite.
A composite adsorbent as to this invention is understood as a physical mixture of primary particles (micro-particles) of at least two types of adsorbents, e.g., zeolites mentioned, that stem from separate syntheses, and shaped by extruding or beading or another known method into macro-particles of the final type, using a binder material.
In a preferred embodiment of the present invention, there is provided a process comprising passing a gas stream containing impurities through a first adsorbent which is capable of substantially removing carbon dioxide and moisture from the gas stream and then passing the gas stream through a second adsorbent layer which is capable of removing hydrocarbons and oxides of nitrogen from the gas. In this embodiment, the first layer is a zeolite and the second layer is the composite adsorbent.
In a different embodiment of the present invention, there is provided a process comprising passing a gas stream containing impurities through a first adsorbent which is capable of substantially removing moisture from the gas stream and then passing the gas stream through a second adsorbent layer which is capable of removing carbon dioxide, hydrocarbons and oxides of nitrogen from the gas. In this embodiment, the first layer is activated alumina and the second layer is the composite adsorbent.
In another preferred embodiment, there is provided a process comprising passing a gas stream containing impurities through the composite adsorbent which will substantially remove water, carbon dioxide and hydrocarbons and oxides of nitrogen from the gas. In this embodiment, the composite adsorbent comprises a first adsorbent and a second adsorbent.
The advantage of the present invention resides in its relative simplicity compared to using a fourth adsorbent layer. The use of the composite adsorbent will convert the

Affiliated with

Also associated with

Rating

Thermal swing adsorption process does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Thermal swing adsorption process, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Thermal swing adsorption process will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2930314