Compositions – Co – s – negative element – or acid – bindant containing
Reexamination Certificate
1999-08-25
2002-01-29
Medley, Margaret (Department: 1714)
Compositions
Co, s, negative element, or acid, bindant containing
C252S190000, C423S242700, C423S243080
Reexamination Certificate
active
06342169
ABSTRACT:
TECHNICAL FIELD
The present invention pertains to improvements in the field of storage and transportation of sulfur dioxide. More particularly, the invention relates to a method of confining sulfur dioxide for storage and/or transportation under safe conditions.
BACKGROUND ART
Sulfur dioxide (SO
2
) is a widely used chemical in industries such as wood pulping and bleaching, corn wet milling, water treatment and the production of sulfuric acid. It is a colorless, nonflammable gas with a boiling point of −10° C. at atmospheric pressure. Sulfur dioxide is highly toxic by inhalation and a strong irritant to the eyes and mucous membranes. It is also a dangerous air contaminant and constituent of smog.
Currently, bulk quantities of sulfur dioxide are stored and transported as a liquid in suitable pressure vessels. The vapor pressure of liquid sulfur dioxide at temperatures that may occur in normal storage and transport operations can be up to 8 bar. Thus, in case of a leak in or rupture of the pressure vessel used to store or transport liquid sulfur dioxide, particularly if the damage occurs towards the bottom of the vessel, large quantities of sulfur dioxide can be released from the tank very rapidly. Since the sulfur dioxide at ambient temperature is above its boiling point, any liquid sulfur dioxide released to the atmosphere will vaporize rapidly, creating a vapor cloud of toxic gas that tends to stay at ground level, being heavier than air. Prevailing winds can then disperse the vapors, creating conditions hazardous to health or even lethal conditions over a large area. Concentration of 5 to 10 ppmv of sulfur dioxide in air will lead to irritation of the respiratory tract and concentrations above 400 to 500 parts per million by volume (ppmv), even for a few minutes, are dangerous to life. Areas adjacent to industrial SO
2
storage sites and railroads or roads used for the transportation of sulfur dioxide are thus at risk in the event of a release.
While it is known that sulfur dioxide dissolves in water to the extent of about 10% by weight, it is not a desirable solvent for the purpose of storing or transporting sulfur dioxide because of the expense of providing large tanks for the dilute solution. Moreover, it is not economical to transport sulfur dioxide in such a water solution because of the excessive cost of transporting nine tons of water for each ton of SO
2
. Some organic compounds such as chloroform, formic acid, acetic acid, methanol, ethanol and acetone have high solvent power for sulfur dioxide, but these have the disadvantage that they are volatile and would contaminate the regenerated sulfur dioxide with undesirable impurities. In addition, most of these compounds are flammable, thus presenting a fire hazard where none existed before.
Aqueous solutions of alkalis such as sodium hydroxide can dissolve substantial quantities of sulfur dioxide by formation of sodium sulfite (Na
2
SO
3
), sodium bisulfite (NaHSO
3
) and sodium pyrosulfite (Na
2
S
2
O
5
). However, regenerability of sulfur dioxide from these solutions is incomplete, the maximum being about 13% by weight (theoretical) from a saturated aqueous solution of the pyrosulfite. This again implies a very high effective transportation cost for the sulfur dioxide.
DISCLOSURE OF INVENTION
It is therefore an object of the present invention to overcome the above drawbacks and to provide a method of confining sulfur dioxide for storage and/or transportation under safe conditions.
In accordance with the present invention, there is thus provided a method of confining sulfur dioxide for storage or transportation under safe conditions, which comprises the steps of:
(a) contacting a sulfur dioxide-containing gas stream with an absorbing medium comprising water and a water-soluble amine absorbent having at least one amine group with a pKa value greater than about 7 and at least one other amine group with a pKa value less than about 6.5 so that the at least one amine group with a pKa value greater than about 7 irreversibly absorbs sulfur dioxide in salt form rendering the amine absorbent non-volatile and the at least one other amine group with a pKa value less than about 6.5 reversibly absorbs sulfur dioxide, to thereby saturate the absorbing medium with sulfur dioxide against a partial pressure of sulfur dioxide of no more than about 1 atmosphere at 25° C.; and
(b) charging the absorbing medium saturated with sulfur dioxide obtained in step (a) into storage or transportation means.
The expression “safe conditions” as used herein refers to conditions presenting a greatly reduced hazard to life and the environment in the case of a leak in or rupture of the storage or transportation container, in comparison to a similar leak or rupture when storing or transporting liquid sulfur dioxide. Since the absorbing medium saturated with sulfur dioxide is below its bubble point, the sulfur dioxide vapor cloud generated by a leak or spill of such a saturated absorbing medium is relatively small. With liquid sulfur dioxide, a very large vapor cloud is formed rapidly since essentially all the sulfur dioxide vaporizes. The use of an amine absorbent having at least one amine group with a pKa value greater than about 7 ensures that the amine absorbent is nonvolatile since such an amine group irreversibly absorbs sulfur dioxide to form a salt which is not regenerable under the normal operating conditions of the process.
Preferably, the amine absorbent has at least one amine group with a pKa value of about 7.5 to about 10 and at least one other amine group with a pKa value of about 4.5 to about 6.0.
Examples of suitable amine absorbents which may be used in accordance with the present invention are diamines having the general formula:
wherein R
1
is an alkylene group having 1 to 3 carbon atoms, R
2
, R
3
, R
4
and R
5
are the same or different and each represent a hydrogen atom, a lower alkyl group having 1 to 8 carbon atoms or a lower hydroxy-alkyl group having 2 to 8 carbon atoms, or any of R
2
, R
3
, R
4
and R
5
form together with the nitrogen atoms to which they are attached a 6-membered ring.
Examples of preferred diamines in free base form include:
N,N′,N′-(trimethyl)-N-(2-hydroxyethyl)-ethylenediamine,
N,N,N′,N′-tetramethyl-ethylenediamine,
N,N,N′,N′-tetramethyl-diaminomethane,
N,N,N′,N′-tetrakis-(2-hydroxyethyl)-ethylenediamine,
N,N′-dimethylpiperazine,
N,N,N′,N′-tetrakis-(2-hydroxyethyl)-1,3-diaminopropane,
N′,N′-dimethyl-N,N-bis-(2-hydroxyethyl)-ethylenediamine,
N-methyl N′-(2-hydroxyethyl)-piperazine,
N-(2-hydroxyethyl)-piperazine,
N,N′-bis(2-hydroxyethyl)-piperazine,
N-methyl-piperazine, and piperazine.
According to a preferred embodiment, step (a) is carried out in a gas/liquid contact apparatus providing countercurrent gas and liquid flows.
Where the sulfur dioxide-containing gas stream is a gaseous stream of substantially pure water-saturated sulfur dioxide, step (a) is preferably carried out under substantially atmospheric pressure and ambient temperature conditions. On the other hand, when the sulfur dioxide-containing gas stream contains less than about 90% by volume of sulfur dioxide, step (a) is preferably carried out by:
i) contacting the sulfur dioxide-containing gas stream with the absorbing medium to produce a sulfur dioxide-laden absorbing medium;
ii) dividing the sulfur dioxide-laden absorbing medium into separate first and second portions each representing a predetermined proportion of the sulfur dioxide-laden absorbing medium;
iii) removing the absorbed sulfur dioxide from the second portion of sulfur dioxide-laden absorbing medium to regenerate the amine absorbent contained therein and thereby produce a sulfur dioxide-depleted absorbing medium and a gaseous stream of substantially pure water-saturated sulfur dioxide; and
iv) contacting the gaseous stream of substantially pure water-saturated sulfur dioxide with the first portion of sulfur dioxide-laden absorbing medium, whereby the proportion of sulfur dioxide-la
Hakka Leo E.
Parisi Paul J.
Cansolv Technologies Inc.
Medley Margaret
Shanks & Herbert
Toomer Cephia D.
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