Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2001-06-12
2002-12-10
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S642000, C062S908000
Reexamination Certificate
active
06490883
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates to the cryogenic removal of volatile compounds (“VC”), especially volatile organic compounds (“VOCs”), from a process gas and has particular, but not exclusive, application to the removal of contaminants from a waste process gas stream to meet environmental requirements. It provides processes for removing volatile compounds and apparatus for use in those processes.
BACKGROUND OF THE INVENTION
Modern industrial processes often produce a gaseous waste stream containing one or more volatile contaminants such as, for example vaporized reactant, product or solvent. Environmental legislation limits the extent to which these contaminants can be released into the atmosphere and several technologies exist to remove them from waste streams. However, with new maximum concentrations being set by environmental bodies as well as more pressure on enforcement, existing technologies often cannot clean waste steams to the desired level in a cost-effective manner.
Process gas streams are cleaned of contaminant(s) chemically by, for example, burning or reaction with another chemical added to, or present in, the process stream, or physically by, for example, condensation. Cryogenic condensation is particularly suitable for removal of a volatile compound, especially a VOC because it permits of the recovery and re-use of the compound. In cryogenic condensation, the process gas stream is cooled to temperatures at which the VC condenses out to form a liquid phase of high VC concentration and a gaseous phase of low VC concentration. The amount of VC left in the gas stream is dependant upon several factors, especially temperature, pressure, process stream composition, and the identity of the VC. In order to meet the relevant legislation, the outlet temperature may have to be well below that required to condense the VC in order to ensure that the remaining VC content of the waste gas is reduced to the required level. With the tightening of legislation, it is probable that, in many cryogenic condensation processes, the outlet temperature will have to be decreased from those currently used. For example; to meet an emission requirement of 20 mg/m
3
to recover methyl chloride (CH
3
CI; freezing point −97.6° C.; boiling point −23.7° C.) an outlet temperature of −150° C. is required and for methylene chloride (CH
2
CI
2
; freezing point −97° C.; boiling point +40.1° C.) an outlet temperature of −120° C. is required.
Further, it may be necessary for a cryogenic condensation process to deal with process gas streams having small differences in composition and from which a VC is substantially entirely removed at a common temperature but for which streams there are significant differences in the extent of VC removal at higher temperatures.
The requirement for colder temperatures often causes the VC to freeze forming so-called “VC ice”. Normally shell-and-tube heat exchangers are used to remove a VC by indirect condensation with liquid nitrogen, or another cryogen, passing on the tube side (inside) of the exchanger, and the VC condensing on the shell side. When freezing takes place, VC ice builds up on the surface of the exchanger tubes, which over time reduces the effectiveness of the exchanger. The normal solution to the problem of VC ice build-up is to have two heat exchangers so that, when one heat exchanger requires regeneration, the process stream can be diverted to the other heat exchanger. Regeneration is by warming to melt the VC ice for removal as a liquid. This ‘freeze-thaw’ type of system is also required when moisture (water vapour) or other compound with a relatively high freeze point is present within the process stream. However, the capital and operating costs are relatively high because of the duplication of equipment and there is a problem of VC ice entrainment as discussed below.
The problem of freezing also can be mitigated by the use of two or more condenser units arranged in series with decreasing operating temperatures. In particular, a first condenser can be provided to pre-cool the waste stream to, for example, +1° C., to remove the majority of water before it freezes, and the resultant gaseous stream further cooled in a second condenser with the outlet temperature set to remove the VC. An alternative arrangement is shown in U.S. Pat. No. 5,083,440 where an intermediate heat transfer fluid cooled by the cryogen is used to maintain the process stream temperature above the VC freeze point. U.S. Pat. No. 5,533,338 describes a special cryogenic heat exchanger that utilises a cold, re-circulating, vaporized nitrogen fluid as the refrigeration medium inside the condenser tubes permitting, by careful control of the fluids circulation rate and temperature, the effect of freezing to be minimized.
When cryogenic condensation is used to remove contaminants from a process stream and especially when it is required to cool a solvent to well below its dew-point, fogging can occur within the process stream. When the rate of cooling of a gas exceeds the rate of mass transfer, the bulk of the gas quickly cools to below the dew point of the condensable vapour forming droplets which condense in the process stream without making any contact with a cold surface of the condenser. This fogging can be minimized by using a successive series of cooling stages with increasing temperature control across each step or by using a de-mister to capture the droplets within the condenser, thereby preventing droplets being than entrained in the process gas stream.
When operating at very low temperatures, small particles of VC ice often are entrained in the cleaned process gas exiting a cryogenic condenser, thereby increasing the retained VC content above the design level of the condenser. As VC ice builds up on exchanger tubes, the outer layer of the ice is like a fine powder and is easily picked up and entrained by the process gas stream passing over it. If the process gas velocity is high, some VC ice can become entrained in the gas stream, especially if the residence time in the exchanger is too short. Further, the strength of adhesion of VC ice to the exchanger tube can be low if, for example, the rate of ice formation is so rapid that, as the ice forms on a cold surface, contraction takes place and the bond that causes the ice to stick to the surface is broken.
Systems installed to prevent the entrainment of VC ice in the cleaned process stream typically have been liquid separation devices such as wire mesh de-misters and/or liquid separators. Although these systems work well for removal of liquid droplets, they are ineffective with fine (less than 100 &mgr;m) particles and cause pressure drop problems in the system due to VC ice blockage. Further, de-misters are installed in the end of the condensers, after the tube bundle, and cannot be easily kept cold or defrosted.
It has long been well known to produce coffee aroma frost by use of a cryogenic condenser to condense volatiles from an aroma-bearing gas produced during coffee processing (see, e. g. U.S. Pat. No. 2,680,687 published Jun. 8, 1954). The gas typically is mainly carbon dioxide (up to 90 wt % or more) together with water vapour and the aromatic constituents responsible for the aroma.
GB-A-1,339,700 (published Dec. 5, 1973) disclosed the use of a scraped-wall condenser in which aroma frost is continuously or periodically scrapped from a cryogenically cooled condenser wall on which it builds-up.
GB-A-1,480,997 (published Jul. 27, 1977) disclosed using a filter element to accumulate sublimated aroma frost particles entrained in the gaseous phase remaining after condensation. The resultant layer of aroma frost particles built-up on the filter element is stated to minimize passage of uncondensed aromatics directly to the atmosphere as well as minimising the loss of frost particles. The element can be located internally or externally of the condenser and can be freed of frost by shaking or vibrating the filter.
U.S. Pat. No. 5,182,926 (published Sep. 16, 1991) dis
Griffiths John Louis
Trembley Jean-Philippe
Air Products and Chemicals Inc.
Capossela Ronald
Wolff Robert J.
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