Gas separation: processes – Liquid contacting – Gaseous fluid mixture discharged against or beneath surface...
Reexamination Certificate
2002-06-18
2004-06-15
Smith, Duane S (Department: 1724)
Gas separation: processes
Liquid contacting
Gaseous fluid mixture discharged against or beneath surface...
C095S237000, C095S240000, C549S538000, C549S542000
Reexamination Certificate
active
06749668
ABSTRACT:
INTRODUCTION AND BACKGROUND
The present invention relates to a process for the recovery of combustible components of a gas stream comprising the combustible components and oxygen by selective absorption of the combustible components in a solvent. The present invention in particular refers to a process for recovery of combustible components from an exit gas stream from an oxidation reaction with a peroxide component, whereby due to the decomposition of the peroxide, oxygen is accumulated during the oxidation reaction. The present invention is particularly suitable for a working-up stage in a process for epoxidation of olefins.
In numerous processes involving oxidation with peroxide compounds, particularly hydrogen peroxide, gas mixtures are formed that comprise considerable amounts of organic combustible components in addition to molecular oxygen resulting from the decomposition of the peroxide compounds. For safety reasons, these processes have to be conducted to ensure that the oxygen content is below the explosion limit. For economic reasons, it is often necessary to recover the combustible components of an exit gas stream, since they may contain valuable compounds like, for example, product compounds or components that can be recycled to the initial reaction stage. Therefore, it has been proposed to recover the combustible components by selective absorption in a suitable solvent.
The oxygen content of the gas phase increases within the absorption unit due to the absorption of organic combustible components in the solvent. This may have the consequence that even if the gas stream entering the absorption unit is not an ignitable composition, it may become ignitable during the process of absorption. Therefore, for safety reasons, an inert gas is introduced in the absorption unit to avoid, under any condition, the formation of an ignitable composition within the absorption unit.
For example, EP-A-719 768 describes a process for the epoxidation of an olefin with hydrogen peroxide, wherein a gas mixture of the olefin and oxygen from hydrogen peroxide decomposition is separated from the liquid epoxidation reaction mixture and the olefin is absorbed from the gas mixture in a liquid absorbent and inert gas is added to the oxygen in an amount sufficient to prevent formation of a flammable gas composition. Referring to the example in EP-A-719 768 the absorption liquid is introduced into the upper section and the gaseous purge stream into the lower section of the absorption zone to ensure a counter-current flow. But there is no indication whether the liquid phase or the gas phase is the continuous phase.
Similarly, European Patent Application 00102542.8 discloses a process for the working-up of an exit gas stream from the epoxidation of an olefin with hydrogen peroxide, comprising olefin oxide, unreacted olefin, and oxygen, whereby the exit gas stream is brought into contact in an absorption unit with the same solvent as used in the epoxidation stage, and a solvent stream loaded with olefin and olefin oxide is drawn off from the absorption unit, and an exit gas stream containing oxygen is discharged. Additionally, an inert gas stream is introduced into the absorption unit, wherein the inert gas leaves the absorption unit together with the oxygen in the exit gas stream. The quantity of inert gas introduced is preferably selected as a function of the quantity and composition of the exit gas stream leaving the reaction stage, such that the exit gas stream leaving the absorption unit is no longer an ignitable composition. Thereby, measures are taken to avoid at any stage within the absorption unit the presence of an ignitable composition. There is a considerable drawback associated with this procedure. Due to the addition of inert gas, the total gas stream within the absorption unit is drastically increased. Accordingly, the efficiency of the absorption is reduced and larger absorption units and larger amounts of absorbing solvents are necessary to achieve the desired separation of valuable organic components from the exit gas stream of a reaction zone.
EP-A-583 828 discloses a process for ethylene recovery in direct-oxidation ethylene oxide processes. According to the disclosure starting from column 4, lines 46 to column 5, line 18 the ethylene-containing stream enters an absorber column, wherein ethylene is preferentially absorbed in a high molecular weight organic liquid. There is no explicit disclosure in that reference whether the liquid phase of the absorbent solvent is the continuous phase and the gas phase is dispersed in that continuous phase. The information given in column 5, lines 6 to 18 is insufficient for a person skilled in the art to determine which of the two phases is continuous and which one is dispersed in the continuous phase. But from the disclosure that the contacting surface may be created by trays, structured packing or random dump packing (column 5, lines 12 to 14 of EP-A-583 828) a person skilled in the art would conclude that the absorption column is run as a trickle bed, i.e. with a liquid phase dispersed in a continuous phase.
Thus, an object of the present invention is to provide a process for the recovery of combustible compounds of a gas stream comprising combustible components and oxygen that does not have the above discussed disadvantages while ensuring safety of the overall process.
SUMMARY OF THE INVENTION
The above and other objects of the invention can be attained by a process for the recovery of combustible components of a gas stream comprising the combustible components and oxygen by selective absorption of the combustible components in a solvent, whereby during the absorption the gas phase is dispersed in a continuous liquid phase of the solvent.
The present inventors have surprisingly discovered that if the gas phase comprising combustible components and oxygen is dispersed in a continuous liquid phase of the solvent during absorption, even if due to the depletion of combustible components from the gas phase the oxygen concentration in the gas phase rises above the explosion limit, the gas phase still cannot be ignited within the absorption unit because the gas phase is finely dispersed in the continuous liquid phase of the solvent. Consequently, addition of inert gas to the gas phase prior to entering the absorption unit or within the absorption zone in the absorption unit is not necessary any longer. Thus, it is preferred that neither prior to entering the absorption unit nor within the absorption zone an inert gas is added. This way, the absorption process is very efficient since the gas phase contains little or no inert gas. Consequently, the dimensions of the absorption unit can be reduced, thereby saving investment costs, and a reduced volume of the absorption fluid, i.e., the solvent, can be used, with the result that the amount of solvent to be recycled or processed in working-up stages can be considerably reduced, thereby improving the overall economics of the process.
But although no inert gas is added prior to entering the absorption zone or in the absorption zone, safety of the process is ensured since the gas phase, although the oxygen content may be above the explosion limit, cannot be ignited.
Preferably, the absorption unit is run as a bubble column.
According to a preferred embodiment, the gas bubbles dispersed in the continuous phase of the absorption solvent have a diameter of 10 millimeters or less, preferably 2-10 millimeters, most preferred 5 millimeters at most.
According to a specifically preferred embodiment of the present invention, the gas stream is introduced into the absorption unit at a lower section of the absorption unit, and the liquid solvent phase enters the absorption unit at a position upwards with respect to the location the gas stream enters the absorption unit, and the liquid solvent phase exits the absorption unit at a position below the entry of the gas stream into the absorption unit. In that way there may be achieved that the gas stream and the solvent pass through the absorption unit in a countercurre
Hofen Willi
Thiele Georg
Wöll Wolfgang
Degussa - AG
Smith Duane S
Smith , Gambrell & Russell, LLP
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