Method for cleaning viscose production off-gases and catalysts for use in the method

Abstract

A method for cleaning an off-gas from viscose production, essentially containing H.sub.2S and CS.sub.2, comprises passing the gas through a catalytic reactor containing a direct oxidation type catalyst, such as V.sub.2O.sub.5 on silica, to convert H.sub.2S in the gas to elemental sulfur, SO.sub.2 or mixtures thereof, either via the oxygen present in the gas or via oxygen added to the gas stream. Elemental sulfur and SO.sub.2 are removed from the effluent gas from the catalytic reactor, and the unconverted CS.sub.2 is recycled to the viscose production process.

Claims

1. A method for cleaning off-gases from viscose production, said off-gases containing air and sulfur components, including H.sub.2S and CS.sub.2, comprising: passing the off-gas through a catalytic reactor containing a direct oxidation type catalyst to convert H.sub.2S in the off-gas to elemental sulfur and SO.sub.2, either via oxygen present in the gas or via oxygen added to the gas stream, removing elemental sulfur and SO.sub.2 from the effluent gas from the catalytic reactor, and recycling an amount of unconverted CS.sub.2 to the viscose production process, wherein the elemental sulfur is removed in a condenser and the SO.sub.2 is removed in a caustic scrubber.

2. The method according to claim 1, wherein the CS.sub.2 is absorbed and concentrated in a regenerative activated carbon filter downstream from the caustic scrubber, and the absorbed CS.sub.2 is desorbed and returned as a raw material to a viscose production plant.

3. The method according to claim 2, wherein the absorbed CS.sub.2 is desorbed and returned, optionally after being upconcentrated, as a raw material to the viscose production plant.

4. The method according to claim 1, wherein the off-gas from viscose production contains H.sub.2S in a concentration of 200 to 2000 ppm and CS.sub.2 in a concentration of 200 to 1000 ppm.

5. The method according to claim 1, wherein the operation temperature of the catalytic reactor is in the range from 100 to 300° C.

Description

(1) The idea of the present invention is to use a direct oxidation type catalyst upstream from the activated carbon filter and thus convert H.sub.2S to elemental sulfur, SO.sub.2 or mixtures thereof. Elemental sulfur can be removed in a condenser, and SO.sub.2 is easily removed in a caustic scrubber using standard caustic agents, such as NaOH or CaCO.sub.3, in a mild excess compared to the stoichiometric demand, thereby creating sulfates that can easily be handled in a liquid discharge system. Direct oxidation type catalysts comprising a catalytically active material, such as V.sub.2O.sub.5, on a support, such as silica, are known in the art and described in e.g. WO 2015/082351, U.S. Pat. No. 4,277,458 and US 2012/0014854.

(2) More specifically, the present invention concerns a method for cleaning off-gases from viscose production, said off-gases essentially containing air and sulfur components, mainly H.sub.2S and CS.sub.2, comprising the steps of passing the off-gas through a catalytic reactor containing a direct oxidation type catalyst to convert H.sub.2S in the off-gas to elemental sulfur, SO.sub.2 or mixtures thereof, either via the oxygen present in the gas or via oxygen added to the gas stream, removing elemental sulfur and SO.sub.2 from the effluent gas from the catalytic reactor, and recycling the unconverted CS.sub.2 to the viscose production process.

(3) The off-gas from viscose production generally contains H.sub.2S in a concentration around 200 to 2000 ppm and CS.sub.2 in a concentration around 200 to 1000 ppm.

(4) The catalytic reactor can be operated so as to convert H.sub.2S to elemental sulfur, SO.sub.2 or mixtures thereof and simultaneously leave CS.sub.2 substantially unconverted.

(5) Preferably the elemental sulfur is removed in a condenser, and the SO.sub.2 is removed in a caustic scrubber. The CS.sub.2 is absorbed and concentrated in a regenerative activated carbon filter downstream from the caustic scrubber.

(6) Suitable catalysts for use in the method include catalysts comprising oxides of Fe, Cr, Zn, Mn, V, Co, Ti, Bi, Sb, Cu or Mg or mixtures thereof supported on silica, alumina, titania, ceria, silicium carbide or activated carbon or mixtures thereof and optionally promoted by an alkali metal, preferably Na.sub.2O. An especially preferred catalyst comprises V.sub.2O.sub.5 supported on silica.

(7) Operation temperatures are in the range from 100 to 300° C., preferably from 160 to 260° C.