Study On The Preparation Of Quaternary Ammonium Hydroxides From Its Halide Via Bipolar Membrane Electrodialys

As a new type of membrane separation technique, resulting from the combination of specific properties of bipolar membranes and electrodialysis (ED), bipolar membrane electrodialysis (BMED) has been used to directly achieve salt conversion into the corresponding acid and alkali under the direct current field. Remarkablely, there are no other substances introduced and no byproducts produced in the process. In comparison to the conventional preparation process, BMED is environmentally friendly and economically competent. Indeed, as a kind of organic alkali, quaternary ammonium hydroxide (R4NOH) is usually acquired from its halide solution. Accordingly, BMED was considered to be adopted to treat quaternary ammonium salt (R4NX) solution in view of the R4NOH production.Firstly, a study on the preparation of highly pure tetrapropyl ammonium hydroxide (TPAOH) via BMED was performed. The operation parameters such as stack configuration, current density, the types of cation membrane C, and the TPABr concentration, were investigated in the process. Furthermore, special attention was paid to scale-up experiments and industrial operation modes. Based on the feasibility of lab scale experiments in TPAOH preparation, a continuous operation was conducted in pilot tests. The results showed that a highly pure TPAOH was produced from a0.4M TPABr solution via a four-compartment BMED stack equipped with cation membranes C at current density of200A·m-2. Additional, the TPABr conversion rate achieved the highest value of98.22%; the process energy consumption was2.867kWh·kg-1; the average current efficiency was above76%; the Br-content detected in a concentrated25%TPAOH solution could be as Iow as219.12ppm.Since the four-compartment BMED technique was feasible and economic to synthesis TPAOH, it was also applicable to other R4NOHS’ preparation. Therefore, the process feasibility of another three quaternary ammonium hydroxides including tetramethylammonium hydroxide (TMAOH), methyltriethylammonium hydroxide (MTAOH) and benzyltrimethylammonium hydroxide (BTAOH) via BMED technique were studied and compared. The effects of operation parameters including current density, R4NX concentration and flow rate etc. on the preparation process were analyzed on the basis of the properties of R4N+, such as molecular weight, molecular volume, vander Waals volume, as well as the diffusion coefficient and electrophoretic mobility in their aqueous solution. The results showed that the impurity ion X" content in their corresponding R4NOH solution was all below500ppm. That suggested BMED technique was also suitable to synthesis the three R4NOHS. Besides, it was found that the change of the process energy consumption and average current efficiency have a certain relation to R4N+s’ structure, the more complex the structure of R4N+s were, the higher the process energy consumption were, and the lower the average current efficiency were.