Fuel Cell Post-Processing for Diaphragm Caustic Purity
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Solution Overview
Problem
Chloralkali processes using diaphragm cells produce caustic solutions with high chloride content, making them less marketable, and membrane cells require costly membranes and stringent impurity control, while converting diaphragm cells to membrane cells is costly and inefficient.
Innovation Solution
A method involving the use of diaphragm cells to produce a cell liquor stream, which is then treated in a fuel cell to create a high-concentration alkali metal hydroxide stream substantially free of chlorides and a second stream with appreciable alkali metal chloride, allowing for the production of both membrane-grade and diaphragm-grade caustic solutions without the need for expensive membrane purification and under mild conditions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If diaphragm cells are used to produce caustic solutions, then the process is robust and tolerates impurities at ppm level, but the resultant catholyte solution contains high chloride content (15% alkali metal chloride) making it less marketable
Solution Approach 1:
A fuel cell is introduced as an intermediary device between the diaphragm cell and the market. The fuel cell accepts the high-chloride catholyte from the diaphragm cell and converts it into two streams: one with high caustic concentration and low chloride content, and another with recovered chloride. This mediator resolves the contradiction by transforming the harmful high-chloride stream into valuable products.
Solution Approach 2:
The patent changes the concentration parameters of the catholyte stream by passing it through a fuel cell. The fuel cell selectively converts alkali metal hydroxide to alkali metal chloride, thereby changing the ratio of caustic to chloride in the stream. This parameter transformation allows the production of marketable caustic solutions with controlled chloride content.
2Manufacturing precision
If membrane cells are used to produce caustic solutions, then the catholyte solution has low chloride content (<100 ppm), but the membranes are costly and sensitive to impurities requiring purification to ppb level
Solution Approach 1:
The patent replaces expensive, sensitive membrane cells with a combination of robust diaphragm cells and fuel cells. The diaphragm cell, though producing high-chloride catholyte, is much more robust and tolerant of impurities. The fuel cell then processes this catholyte to achieve the desired low chloride content in the final product, avoiding the need for costly membrane purification infrastructure.
Solution Approach 2:
The fuel cell serves as an intermediary that decouples the robustness requirement from the purity requirement. Instead of using a single membrane cell that must simultaneously be robust and produce pure product, the system uses a diaphragm cell for robust operation followed by a fuel cell for purification, separating these two functions.
3Manufacturing precision
If diaphragm cells are converted to membrane cells to reduce chloride content, then membrane-grade caustic solution is produced, but the conversion is cost prohibitive
Solution Approach 1:
The patent maintains the existing diaphragm cell infrastructure rather than investing in expensive membrane cell conversion. The diaphragm cell continues to operate as before, and the fuel cell is added as a relatively inexpensive post-processing unit to produce membrane-grade caustic solution, avoiding the prohibitive conversion costs.
Solution Approach 2:
Instead of changing the entire cell type from diaphragm to membrane (a costly structural change), the patent changes the chemical parameters of the catholyte stream by processing it through a fuel cell. This parameter transformation achieves the desired product specification without expensive infrastructure replacement.
4Use of energy by moving object
If intense energy is applied to convert dilute caustic solution to concentrated solutions, then diaphragm-grade caustic solutions at 50% concentration are produced, but solid alkali metal salts precipitate requiring additional processing
Solution Approach 1:
The fuel cell performs a partial conversion of alkali metal hydroxide to alkali metal chloride, producing a stream with intermediate caustic concentration (1-20 wt%) rather than immediately concentrating to 50%. This partial action allows subsequent concentration with less severe conditions and reduced salt precipitation, as the chloride content is already reduced by the fuel cell conversion.
Solution Approach 2:
The fuel cell changes the chemical composition parameters of the catholyte before concentration, reducing the chloride content and altering the solubility characteristics. This parameter change enables concentration to proceed with less salt precipitation, as the reduced chloride content changes the crystallization behavior during evaporation.
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 enables the production of commercially viable caustic solutions with reduced energy requirements and extended membrane life by regulating alkali metal hydroxide conversion in the fuel cell, avoiding severe conditions that shorten membrane lifespan and allowing for the production of membrane-grade solutions without purifying the brine feed.
Implementation Method 1
Chloralkali processes electrolytically convert brine solutions to caustic solutions and chlorine gas
Implementation Method 2
treating the first catholyte stream within a fuel cell to produce a first anolyte stream including from about 1 to about 20 wt % alkali metal hydroxide and from about 1 to about 25 wt % alkali metal chloride and a second catholyte stream including from about 25 to about 40 wt % alkali metal hydroxide
Data Source
AI summary
A method of the type where a brine solution is converted to an alkali metal hydroxide solution within a diaphragm cell, and the resulting cell liquor from the diaphragm cell is introduced to one or more fuel cells for the conversion of the alkali metal hydroxide to form electricity, the improvement comprising regulating the conversion of alkali metal hydroxide within the fuel cell to a conversion of less than 90%, and then subsequently concentrating the alkali metal hydroxide concentration from the anolyte stream of the fuel cell.