Viscose Off-Gas Cleaning via Direct Oxidation Catalyst
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Solution Overview
Problem
Current methods for treating off-gases from viscose production, particularly those containing H2S and CS2, are inefficient due to excessive caustic usage and instability in wastewater treatment, and existing catalysts do not effectively convert H2S to elemental sulfur and SO2 upstream of activated carbon filters, disrupting CS2 recycle processes.
Innovation Solution
A method utilizing a direct oxidation type catalyst, such as V2O5 on silica, to convert H2S to elemental sulfur and SO2 in a catalytic reactor, followed by removal in a condenser and caustic scrubber, respectively, while recycling unconverted CS2, using catalysts like oxides of Fe, Cr, Zn, Mn, V, Co, Ti, Bi, Sb, Cu, or Mg supported on various materials, with operation temperatures between 100 to 300°C.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a caustic scrubber is used to remove H2S from off-gases, then H2S is captured and converted to NaHS and Na2S, but excessive caustic material is consumed and the resulting wastewater is unstable and costly to treat
Solution Approach 1:
The patent changes the chemical parameters of the scrubbing system by introducing a two-stage process: first using caustic scrubber to convert H2S to sulfides (maintaining high capture efficiency), then adding an oxidation stage that converts sulfides to sulfates. This parameter change transforms the unstable NaHS/Na2S mixture into stable sulfate species, resolving the contradiction between effective H2S capture and wastewater stability.
Solution Approach 2:
The patent converts the harmful unstable sulfide byproducts into beneficial stable sulfates through oxidation. The previously problematic NaHS and Na2S that caused wastewater instability are transformed into stable sulfate species that can be easily handled and recycled, turning a harmful consequence into a beneficial outcome.
2Reliability
If H2S is present in the inlet gas to the activated carbon filter, then the activated carbon capacity is occupied and the CS2 recycle process is disrupted, but removing H2S completely requires excessive caustic usage
Solution Approach 1:
The patent applies preliminary action by introducing the oxidation step before the activated carbon filter. By oxidizing H2S to elemental sulfur and SO2 upstream, the process prevents H2S from reaching and poisoning the activated carbon, thereby protecting the CS2 recycle process while avoiding the need for excessive caustic usage.
Solution Approach 2:
The patent segments the H2S removal process into two distinct stages: chemical conversion (H2S to elemental sulfur/SO2) and physical absorption (removal of conversion products). This segmentation allows the activated carbon to focus solely on CS2 recovery without being contaminated by H2S, resolving the contradiction between protecting the recycle process and minimizing caustic consumption.
3Reliability
If the caustic scrubber operates with high pH to effectively capture H2S, then H2S removal efficiency is improved, but the wastewater becomes unstable with respect to H2S release
Solution Approach 1:
The patent changes the chemical state of sulfur compounds in the wastewater from reduced forms (sulfides) to oxidized forms (sulfates). This parameter change fundamentally alters the stability characteristics, transforming the unstable high-pH sulfide solution into a stable sulfate solution that does not release H2S, thereby resolving the contradiction between removal efficiency and wastewater stability.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach reduces caustic consumption, stabilizes sulfur compounds for easier handling, and maintains CS2 recycling efficiency by converting H2S to elemental sulfur and SO2, facilitating cost-effective and stable off-gas treatment.
Implementation Method 1
passing the off-gas through a catalytic reactor containing a direct oxidation type catalyst to convert H2S in the off-gas to elemental sulfur and SO2
Implementation Method 2
direct oxidation type catalyst to convert H2S to elemental sulfur and SO2
Implementation Method 3
removing elemental sulfur and SO2 from the effluent gas from the catalytic reactor, wherein the elemental sulfur is removed in a condenser
Implementation Method 4
the SO2 is removed in a caustic scrubber
Implementation Method 5
a regenerative activated carbon filter is applied, where the CS2 is absorbed and concentrated in the filter bed
Data Source
AI summary
A method for cleaning an off-gas from viscose production, essentially containing H2S and CS2, comprises passing the gas through a catalytic reactor containing a direct oxidation type catalyst, such as V2O5 on silica, to convert H2S in the gas to elemental sulfur, SO2 or mixtures thereof, either via the oxygen present in the gas or via oxygen added to the gas stream. Elemental sulfur and SO2 are removed from the effluent gas from the catalytic reactor, and the unconverted CS2 is recycled to the viscose production process.