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NF Membranes Key to Desalting/Concentrating Dye-stuffs  Nanofiltration membranes are used to desalt and concentrate dye-stuffs. A typical membrane desalting process is performed by a combined concentration/diafiltration procedure: concentration is performed first and a diafiltration step follows. During the diafiltration step, fresh water is added to the feed tank and forced across the membranes together with salts and impurities. The dye is retained and purified from the minerals. The diafiltration process must be optimized with respect to the following parameters: • Investment, operation, and maintenance costs • Time required to obtain a purified and concentrated dye for a given batch size • Quantity of water required to reach the required dye purity Each of these factors is dependent upon membrane type, configuration, and performance. In evaluating which membrane is best, one should examine a combination of several key criteria: • Salt permeability, which should be as high as possible (minimal retention) • Dye retention, which should be as high as possible to minimize dye losses • High membrane flux, which is needed to reduce plant size and operating costs • Availability of proven cleaning procedures In the case of manufacturing liquid dye formulations, it is very important to obtain a liquid of good shelf stability. In many cases such stability is reached only when organic impurities are effectively removed by the membrane from the reaction mixture of the dye. This study presents the results of a membrane experiment that focused on the desalting efficiency of the SelRO® membrane. The results have been quantified in terms of investment and process costs. Efficiency of Desalting - Membrane Experiment Purpose of Experiment The purpose of this experiment was to quantitatively study the ability of the SelRO® membrane, to desalt a solution containing dye and sodium chloride. A sequence of concentration/diafiltration steps was applied in the study. No attempt was made to reach a particular salt concentration in the final dye concentrate; rather, the desalination efficiency was compared. The amount of diafiltration water used in the process was fixed at 2.5 times the initial volume of dye solution; i.e., if the initial volume of the solution was 30 liters, the diafiltration step used 75 liters of water. Experiment Protocol Membranes SelRO® membranes were used in the experiment. The membranes were installed in pilot scale modules, containing roughly one square meter membrane area (~10 ft2). Membranes were characterized prior to the experiment to ensure that their performance was within specifications. Operating Conditions The operating conditions during the test were: • Inlet Pressure: 30 bar • Temperature: 25-30° C The initial solution was: • Volume: 30 liters • Dye Concentration: 3-4% • Salt Concentration: ~ 1% The Process The sequence of steps that was applied is shown in Table 1. Experimental Results and Conclusions Feed Salt Concentration The main purpose of a concentration/diafiltration process is to reduce as much salt in the dye solution as is possible for a given removed permeate volume. A good desalination membrane should give low salt concentration in the concentrate (feed) with small dye loss and reasonable flux. The results obtained for the membranes are shown in Figure 1. This figure shows the concentration of NaCl that remains in the feed after each treatment step. Table 1: Sequence of Steps for Membrane Tests
Figure 1 - Salt Concentration in the Dye Solution as a Function of the Process Stage Number  The final sodium chloride concentration in the dye solution is shown in Table 3. Spot NaCl Rejections The spot sodium chloride rejections that were measured are shown in Table 2. Please refer to Table 1, as necessary, for a description of each Process Stage Number. Table 2: Spot NaCl Rejection as a Function of the Process Stage Number
These results reinforce the conclusions mentioned above. In most cases negative values are attained due to the "Donnan effect." Negative salt rejection means that salt concentration in the permeate is higher than salt concentration in the feed (see equation).  Mass Balance of Salt Table 3 shows the mass balance passage of NaCl - the equivalent to the extent of desalination achieved at the end of the concentration/diafiltration process. The calculation was made using the feed salt concentrations and volumes. Table 3: Mass Balance NaCl Passage and Final NaCl Concentrations in the Dye (after Stage #8)
Dye Rejections The spot rejections measured = 99.9% ± 0.05%. Removal of Organic Impurities In addition to the desalination process itself, non-reacted intermediates must be removed from the concentrated dye for better dye purity and stability of dye formulation. The membrane processing should allow optimal passage of these non-reacted intermediates. Cleaning and Lifetime The membrane life is typically one year or longer. The required frequency of membrane washing depends on the specific dye: from once a week to once per dye batch. Washing techniques have been developed with a standard cleaning agent. Dye Waste Treatment SelRO® membranes may also be specified for concentrating dye wastes and intermediates. Plants using SelRO® membranes have been successfully operating for a decade. The membranes have achieved long lifetimes. For additional information about these plants and membranes, contact us. **************************** Literature Downloads Call us first! We want to help you choose the most effective solution for your application. Take advantage of 35 years of membrane treatment expertise. Performance is everything! Please call +1-978-657-4250 or e-mail us.
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