Chemistry of inorganic compounds – Carbon or compound thereof – Oxygen containing
Reexamination Certificate
1999-09-13
2001-05-01
Langel, Wayne (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Oxygen containing
C423S245100, C423S481000, C423S24000R, C423S24000R
Reexamination Certificate
active
06224843
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the catalytic purification of carbon dioxide and water condensate contaminated with saturated, unsaturated and chlorinated hydrocarbons from ethylene glycol plants. The invention finds application where CO
2
off-gas produced by ethylene glycol plants is to be used in applications requiring high purity CO
2
, such as in methanol production where it must meet a total allowable chlorides specification of 0.01 ppm, and in carbonated beverages. Additionally, the invention enables recovered waste water to be recycled as Boiler Feed Water (BFW).
Chlorine as HCl can be removed from many vent gases with conventional adsorbents. The removal of organic chlorides, however, has been problematic. A small quantity of CO
2
for food grade uses is presently purified by adsorption on different kinds of carbon beds; however, no viable scheme has been available to convert chlorinated hydrocarbons catalytically to HCl followed by adsorption. The present invention relates to a combined process for complete catalytic combustion of saturated, unsaturated and chlorinated hydrocarbons and mono ethylene glycol (MEG) to CO
2
, H
2
O and HCl. The inventive process offers purification of CO
2
off-gas produced by ethylene glycol plants by oxidation of saturated, unsaturated and chlorinated hydrocarbons on precious metal based catalyst e.g., platinum, in the presence of excess oxygen; condensation of pure saturating water; removal of HCl on an adsorbent, followed by final removal of excess oxygen by reaction with hydrogen on a commercial Deoxo catalyst. The resulting CO
2
stream meets the specifications of food grade, oxyalcohols and methanol industries and the water condensate produced meets BFW requirements.
BACKGROUND OF THE INVENTION
There is no technology currently available for the combined conversion of saturated, unsaturated and chlorinated hydrocarbons as is provided by this invention. However, some work has been done on individual schemes such as oxidation of saturated and unsaturated hydrocarbons alone on precious metal based catalyst, i.e., palladium or platinum. It is also the practice in some ethylene glycol industries to convert hydrocarbons in the carbon dioxide off-gas to CO
2
and H
2
O followed by carbon dioxide vent to the atmosphere or further purification of a small stream of good grade CO
2
by adsorption on carbon. Relevant patents and published applications are discussed below.
German Patent No. DE 4305386 of H. Meye, et al. describes catalyst development in the oxidative purification of saturated hydrocarbon from CO
2
with platinum or palladium based catalyst supported on alpha-Al
2
O
3
with a specific surface area less than 50m
2
/g. The content of platinum is specified to be under 0.2 wt %. The technology described in this patent was developed for food grade purification of CO
2
and is different from the present invention because DE 4305386 deals only with development of a catalyst for the removal of organic impurities, especially saturated hydrocarbons from a CO
2
stream. The present invention enables the removal of chlorinated and unsaturated hydrocarbons through a combination of oxidation, condensation, adsorption and deoxidation steps to obtain CO
2
of high purity.
Japan Patent No. 60086015 of H. J. Ishikawajima describes a method to purify CO
2
by physical separation. Since there is no chemical reaction involved in this patent, it is completely different than the present invention.
The present invention is also distinguished from the prior art in that along with oxidation of unsaturated hydrocarbons on a platinum based catalyst, the invention also deals with the catalytic conversion of chlorinated hydrocarbons to HCl, the adsorption of the HCl and the oxidation of MEG to CO
2
and H
2
O to produce not only carbon dioxide of high purity, but also water which meets the specification of Boiler Feed Water (BFW).
SUMMARY OF THE INVENTION
The present invention provides a complete process for purifying CO
2
off-gas from ethylene glycol plants to produce high purity carbon dioxide through catalytic conversion. This high purity carbon dioxide can be used in food grade applications, or in the production of methanol, urea, oxy-alcohols, etc., and the condensate water from the process can be used as boiler feed water (BFW).
Carbon dioxide off-gas from ethylene glycol plants is contaminated with saturated, unsaturated and chlorinated hydrocarbons. Traces of these chlorides, together with ethylene, oxygen and other contaminants, are usually dissolved in the solvent employed in the carbon dioxide removal system. Conventionally, after intermediate flash, the final traces are stripped and discharged with the CO
2
off-gas saturated with water at 93° C. Therefore carbon dioxide, containing almost 100% by wt. water, produced by many ethylene glycol plants around the world is presently being vented to the environment. However, some glycol plants condense the CO
2
off-gas water prior to oxidation and then carry out individual oxidation of hydrocarbons present in the off-gas in the presence of excess oxygen for environmental purposes. Other glycol plants purify a small stream of CO
2
off-gas, through carbon adsorption, to provide food grade CO
2
for beverage industries. Water condensate produced during the off-gas cooling is usually sent to a biological treatment system prior to sewage discharge, since it has as much as 2000-3000 ppm of monoethylene glycol (MEG). It is believed that the catalytic elimination of organic chlorides and the purification of wastewater from ethylene glycol plant off-gas is not currently practiced. Therefore, we believe there is a need for a process that could purify this off-gas and its saturating water, so these could be utilized commercially rather than being vented. The degree of purification required depends on the type of end use for this off-gas and associated condensate.
Typical analysis of the off-gas from ethylene glycol plants is as follows:
TABLE 1
a
Composition
O
2
0.24
ppmv
Ar
(less than) 0.05
ppmv
CH
4
0.07
ppmv
C
2
H
4
0.13
ppmv
C
2
H
6
(less than) 0.05
ppmv
CO
2
99.46
ppmv
EO (ethylene oxide)
(less than) 0.05
ppmv
H
2
O
Saturated
at 93°
C.
Chlorides
Methyl chloride
<0.1
ppmv
Vinyl chloride
<0.1
ppmv
Ethyl chloride
2.5
ppmv
Allyl chloride
0.1
ppmv
Ethylene dichloride
<0.1
ppmv
a
Units are volume percent on a dry basis.
In accordance with the present invention, the off-gas is first mixed with oxygen in excess of the amount required for complete combustion and heated to approximately 300° C., then passed over a platinum based catalyst for oxidation. In this step, all of the MEG and unsaturated hydrocarbons present in the off-gas are oxidized to CO
2
and H
2
O and the chlorinated hydrocarbons are converted to HCl. The outlet stream from oxidation is then cooled to about 45° C. to condense substantially all of the water in the stream and the remaining gas passed through a bed of adsorbent, such as aluminum oxide impregnated with sodium, to separate HCl, followed by the catalytic reaction of excess oxygen with hydrogen.
Although some of the individual steps proposed in the present invention, such as oxidation of saturated and unsaturated hydrocarbons with excess oxygen on a platinum based catalyst, have been previously demonstrated, a viable process for the purification of ethylene glycol CO
2
off-gas and all the specified trace contaminants in it for the production of high purity CO
2
and BFW grade condensate has not previously been demonstrated.
The present invention was tested in a unit installed on-line on an ethylene glycol production unit. Results of this test were continuously analyzed and estimated the total chlorides level in the CO
2
off-gas feed to be between 3 to 6 ppm. At the outlet of the CO
2
off-gas purification unit, total chlorides analyzed were less than 0.01 ppm. Chlorides in the outlet stream were analyzed to be as follows:
TABLE 2
Methyl chloride
<0.01
(ppmv)
Vinyl chloride
<0.01
(ppmv)
Ethyl chloride
<0.01
(ppmv)
Allyl chloride
<0.01
(ppmv)
E
Ahmed Ijaz
Al-Jodai Abdulaziz M.
Al-Khamees Abdulmohsin M.
Al-Saif Shaker A.
Kramer Levin Naftalis & Frankel LLP
Langel Wayne
Saudi Basic Industries Corporation
Strickland Jonas N.
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