On-line analysis of process gas during the production of ketene

Details On-line analysis of process gas during the production of ketene On-line analysis of process gas during the production of ketene

2000-04-27

2001-09-04

Warden, Jill (Department: 1743)

Chemistry: analytical and immunological testing

Nuclear magnetic resonance, electron spin resonance or other...

C073S863840

Reexamination Certificate

active

06284547

ABSTRACT:

The invention relates to a process for the on-line analysis of a reactive gas mixture, in particular of a gas mixture comprising ketene and/or diketene and/or acetic anhydride and/or acetic acid, which gas mixture is situated in a vessel at a first pressure.
The catalyzed pyrolysis of acetic acid is carried out on an industrial scale for the production of acetic anhydride, diketene and ketene. Ketene is further processed directly to secondary products, e.g. sorbic acid and dimethylacrylolactone. The reaction proceeds at elevated temperatures in the range from 400 to 800°C. and at reduced pressure. In addition to the main products water and ketene, under said reaction conditions, a number of byproducts are formed, e.g. carbon monoxide, carbon dioxide, ethene, ethyne, methane, propadiene and hydrogen. After the reaction, in the further course of the process, unreacted acetic acid and water are condensed out and the process gases are scrubbed with acetic anhydride. The following process steps depend on the target product.
It has not been possible hitherto to analyse such process gases quantitatively. This is due to the high reactivity of the components, which excludes representative sampling and analysis by conventional methods (e.g. GC). The ketene and water formed in the process form acetic acid in a very short time in a reverse reaction, and ketene and acetic acid reform acetic anhydride. In addition, there is the risk of ketene dimerization to form diketene.
The object therefore underlying the invention was to provide a process which permits the on-line analysis of the gas mixture mentioned at the outset. It has now surprisingly been found that the reactive gas mixture described can be analyzed on-line and continuously if a probe having a small orifice diameter is used for drawing off a sample gas stream from the mixture and conducting it via an evacuated sample line to a mass spectrometer for analysis. Over the probe orifice there is thus an instantaneous expansion to a low pressure, which is maintained in the sample line to the mass spectrometer by means of vacuum pumps. There is a further pressure reduction on entry of the gas mixture into the mass spectrometer.
The invention therefore relates to a process for the on-line analysis of a reactive gas mixture in particular of a gas mixture comprising ketene and/or diketene and/or acetic anhydride and/or acetic acid, which gas mixture is situated in a vessel at a first pressure, which comprises expanding a portion of the gas mixture into a sampling line via a probe to a second pressure which is at least ten times lower than the first pressure and feeding it to a mass spectrometer for analysis.
The invention further relates to a process for the on-line analysis of a reactive gas mixture, in particular of a gas mixture comprising ketene and/or diketene and/or acetic anhydride and/or acetic acid, which gas mixture is situated in a vessel, which comprises expanding a portion of the gas mixture instantaneously into a sampling line via a probe which is integrated into the vessel and which has an orifice whose clear cross sectional area is less than or equal to 1 mm
2
and feeding it to a mass-spectrometric analysis.
Particular embodiments result from the subclaims.
The probe can be integrated into the vessel or flanged onto it and is preferably fabricated from metal, quartz or ceramic material, and the orifice preferably has a clear cross sectional area between 8×10
−4
and 0.8 mm
2
inclusive (e.g. a circular hole having a diameter of 0.01 to 1 mm). The probe is connected via the sampling line to the mass spectrometer. The vacuum can be generated by a pump which delivers a high final vacuum (e.g. a sliding-vane pump). A partial stream of the process gas drawn off is analysed by the mass spectrometer. The mass spectrometer is calibrated in the specific case with certified gas mixtures, by liquids vaporized in the carrier gas and by ketene gas mixtures which are obtained in the laboratory by thermal decomposition of diketene.
The representative sampling of the reactive gas stream is achieved in the specific case of sampling for ketene analysis by an instantaneous expansion from a process condition pressure (100-1000 mbar) to sampling pressure (1-20 mbar) across the small orifice at the probe tip. A pressure reduction by a factor of 100 slows down the reactions by a factor of 10,000 (for second order-reactions). Moreover, owing to the low heat capacity of the expanded process gas, this rapidly cools to room temperature, which leads to a further slowing down of possible reactions in the sampling line. The expansion time depends on the geometry of the probe tip orifice. The expansion proceeds instantaneously, preferably within a time period less than or equal to 0.1 s, particularly preferably within 1 ms to 1 &mgr;s.
Adaptation to higher process pressures, to over 30 bar for example, is possible without difficulty. Heating the sampling line to avoid condensation of components from the process gas is not necessary in the specific case owing to the low pressure in the sampling line. The sampling apparatus used, essentially comprising probe, sampling line and mass spectrometer, can be equipped with pressure sensors, filters, valves and control valves for monitoring and controlling the sampling. A two-stage pressure reduction within the mass spectrometer facilitates control, installation and, in particular, simultaneous operation of a plurality of measurement points.


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