acid but also for the conversion of iron chlorides to iron hydroxides.

Transcription

1 Minimization of Spent Pickle Liquor Dhiru Patel and Steven J. Pratt - ayqj-7 PJf acid but also for the conversion of iron chlorides to iron hydroxides. 2. Acid Recovery by Diffusion Dialysis This technique involves use of specially formulated ion-exchange membranes in a dialysis unit to separate HC1 from a mixture of IC1 and iron chlorides. Typically, the concept of dialysis involves the separation of solute molecules through a porous membrane. However the dialysis unit considered here separates acid and salt by the difference in diffusion rates of individual species across the membrane. A unique feature in the system is the ability of the membranes to selectively let hydrogen ions pass through along with the anions (like chlorides). Traditional anion-exchange membranes would let only the anions transfer across the membrane, which does not help separate the acid from the salts. The concentration of the recovered acid is less than that in the feed liquor. The preliminary analysis led us to believe that this technique requires further evaluation. 3. Acid Recovery Using Bipolar Membranes This technique involves the neutralization step, followed by the conversion of salts and water back to acid and alkali using specially formulated bipolar 3

2 Minimization of Spent Pickle Liquor membranes capable of splitting water molecules in to hydrogen (H') and hydroxyl (OH-) ions. The recovered acid is recycled back to process while the caustic is utilized for the neutralization step. While this is a powerful technique for acid recovery, the relatively low cost of recovered HC1 is difficult to justify against the large use of electrical energy. The preliminary analysis showed that this technique is not cost effective for the current application. This method is useful when the unit value of recovered chemicals is high and dilution of the recovered acid is not desirable. i MEMBRANE SEPARATION FOR WASTE MINIMIZATION Increased public awareness of wastes along with the tighter environmental regulations and unavailability of disposal sites have made waste treatment and minimization an important issue. Membrane separation systems have a lot to offer because significant technological advances have resulted in lower capital and lower operation and maintenance costs. Modularity and space efficiency are the other advantages offered by these systems. While membranes have been used in the water treatment industry for almost three decades, 4

3 Minimization of Spent Pickle Liquor Dhiru Patel and Steven J. Pratt primarily for the desalination applications, newer applications are emerging steadily as the technology advances. By varying the material, design, size and shape of the modules containing membranes, specific applications can be tailored. In addition to the traditional polymeric materials, inorganics such as steel, carbon and ceramics are finding applications for membrane construction to meet specific industrial environments. While most membrane applications for the waste management industry are site specific, there is a tremendous potential to handle applications such as recovery of valuable resources, volume reduction and remediation. The membrane systems can be applied within the process or at the end of the process towards waste minimization. As waste minimization gains more importance due to increased environmental awareness, the membrane innovations and applications are likely to grow to meet the challenge. Diffusion dialysis is one such technique that is found to be a superior alternative towards cost minimization at Capitol Metal. DIFFUSION DIALYSIS FOR ACID RECOVERY Diffusion dialysis is a concentration gradient governed transport process. The transfer through the 5

4 Minimization of Spent Pickle Liquor Dhiru Patel and Steven J. Pratt membrane is by diffusion, that is, the progress of individual molecules, rather than by the hydrodynamic flow that would occur through a porous membrane. Figure 1 shows the principle of the dialysis process. The feed containing material to be recovered flows upwards on the side of the vertical membrane while water flows downwards on the other side of the membrane. The acid component diffuses to the water side of the membrane due to the concentration gradient across the membrane boundary. An important feature of this system is the ion selectivity of the membrane. The membrane is anion selective, allowing only negatively charged ions to transfer across the boundary. However, the membrane also allows positively charged hydrogen ions to transfer across along with the anions. Figure 2 illustrates this point. For the application that was considered at Capitol, this is very important as the objective was to separate both hydrogen and chloride ions, to meaningfully separate HC1 from the mixture of HC1, water and iron chlorides. The unique property of allowing the transfer of anions and hydrogen ions results in a good application for separation. The ion exchange membrane considered for this system is a 6

5 3 Minimization of Spent Pickle Liquor patented design of Asahi Glass Company of Japan. The membrane system does not utilize electrical energy as is the case in electrodialysis systems, resulting in cost effective operation. Mass transfer of the acid across the boundary can be represented by the following equation: M = (U) (A) (LMDC) M = Molar transfer rate in unit time, mol/hr A = u = Effective area, square meters Dialysis coefficient, mol/(hr-sq.m.-mol/l) LMDC = Logarithmic mean concentration difference, g/1 The dialysis coefficient (U) is dependent on the membrane material and the type of acid and salt in the mixture. Higher water rates can increase the acid recovery but reduce the concentration of the recovered acid. On the other hand, lowering of the water rates can give higher concentration of acid but the acid recovery rate or dialysis rate is smaller. Acid recovery can also be increased by having a larger area per unit treating rate. However, that can also 3 increase the leakage rate of salts, which is undesirable. 7

6 Minimization of Spent Pickle Liquor Dialysis System - Equipment Description The dialysis system involves a multi-cell dialysis chamber in a plate and frame type arrangement, with alternating chambers of water and feed solution separated by ion exchange membranes. The unit looks similar to typical plate and frame filter presses. The feed is introduced at the bottom of the feed chamber (dialysate chamber) and flows upwards. Water flows downwards from the top of the water chamber (diffusate chamber). Figure 4 illustrates the basic arrangement of the system. The cell frame is made of synthetic resin with high chemical resistance. The unit suitable for application at Capitol contains about 800 sheets of 44-inch by 44-inch membranes for the treatment of 5 gallons per minute of spent liquor. The inlet concentration of HC1 is about 9 percent with a salt concentration of about 20 percent. The unit is designed to recover 80 percent of the acid in feed stream. The water flowrate in the diffusate chamber is matched with the flow rate of feed 1 iquor. Higher recovery can be achieved with two identical dialysis units resulting in overall acid recovery of over 95 percent. However, optimization analysis is necessary to justify the increased cost of a

7 '3 Minimization of Spent Pickle Liquor capital investment against the reduction in purchased acid costs and the savings in spent acid neutralization costs. Figure 3 shows the process flow diagram of the dialysis system along with the peripheral equipment like tanks and pumps. As can be evident, the system is relatively simple and free of complicated equipment. The pumps are the only major moving equipment resulting in ease in maintenance. The system is designed to operate at a maximum of 104 degree F. At higher than design temperatures, the thermal stability of the polymeric membranes becomes questionable. A cartridge filter is required in the feed line to the dialysis unit to remove suspended solids and oily matter for membrane protection. The head tanks at the dialysis unit are used to maintain constant flowrates during the operation. The efficiency of the membranes decreases as a result of contamination inside the unit over a period 3 time. The contaminants cause poor flow distribution inside the unit, resulting in loss of mass transfer efficiency. The unit requires cleaning about every six months. Water used in the process also requires pretreatment to ensure proper mass transfer 9

8 Minimization of Spent Pickle Liquor efficiencies. The unit is designed to operate continuously for best results. Maintenance requirements include cleaning of the unit about twice a year and replacement of about 25 percent of the membranes. The membranes have a projected life of about 3 years. The membrane replacement is the most expensive maintenance cost for the system. Normal maintenance and regular replenishment of the membranes should result in long useful life for the system. The system including the dialysis unit, tanks, pumps, instrumentation and other related equipment require plot space of about 30 feet by 30 feet. The entire system can be designed and installed as a skid mounted package unit. Economic Analysis The economics of the diffusion dialysis system are compared against the alternative of on site neutralization. The spent liquor contains HCl and iron chlorides. The neutralization process requires caustic not only for neutralizing acid, but also for the conversion of iron chlorides to iron hydroxides. The economic analysis is based on the chemical cost savings in HC1 and sodium hydroxide (NaOH) for the treatment of 10

9 Minimization of Spent Pickle Liquor the acid portion of the pickle liquor only. Table 1 below shows the results of the economic analysis for the installation of the diffusion dialysis system at Capitol. The simple payback period for the system investment is little less than two years. Table 1. Economic Analysis for Diffusion Dialysis System Description Cost (dollars) A. Installed cost 650,000 B. System operating costs 1. Membrane replacement 2. Utilities and maintenance Annual operating costs C. Savings in chemical costs 1. HC1 recovered and reused 35,000 7,000 42, , NaOH savings Annual chemical cost savings D. Net annual cost savings E. Simple payback period 237, , ,000 2 years Potential For Further Cost Savings Under investigation is the feasibility of installing two dialysis modules for an acid recovery by over 95 percent. This scenario allows the company to 11

10 . Minimization of Spent Pickle Liquor eliminate most of the acid content in the pickle liquor stream. It needs to further investigate the feasibility of converting the residual acid content into iron chlorides to raise the ph of the solution above 6. Iron chloride solution without contamination by HC1 is a commercially marketable chemical used in the municipal wastewater treatment plants. This would enable Capitol to not only recover most of the acid in the pickle liquor stream, but also completely eliminate the need for caustic used for neutralization and precipitation of iron hydroxides. Also eliminated are the needs for sludge disposal and the associated sludge disposal costs and sludge dewatering equipment costs. This solution, if found feasible, would almost completely eliminate the waste disposal problem that exists at the present time, and make a valuable product. CONCLUSION Tougher environmental regulations have made the problem of waste disposal prohibitively expensive for many operating companies. However, the necessity is the mother of invention as someone has wisely said. Escalating disposal costs and liabilities are turning 12

11 3 Minimization of Spent Pickle Liquor companies to eliminate the generation of the waste by innovative practices. Diffusion dialysis for recovery of hydrochloric acid and reduction of waste disposal costs is a significantly attractive option for Capitol Metals. More significant is the possibility of completely eliminating the waste disposal problem by a cost effective approach of process modification along with the efficient utilization of diffusion dialysis systems. Diffusion dialysis is just one of the several membrane separation techniques available to process industry for waste minimizaiton. The other techniques include reverse osmosis, ultrafiltration, electrodialysis, gel permeation chromatography and liquid permeation. The potential applications include industrial gas separations; aqueous waste treatment; and replacement of energy intensive evaporation, distillation and cryogenic operations. For all engineers involved in process engineering, this is definitely a technology to watch. 13

12 Minimization of Spent Pickle Liquor REFERENCES: 1. Webbcr, W.F. & Bowman, W.; Membranes Replacing Other Separation Technologies; Chemical Engineering Progress, 82 (ll), p.23 (1986). 2. Cross, R. A.; AICHE Symposium; "Recent Advances in Separation Techniques"; 120 ( 68) (1972). 3. Handbook of Separation Process Technology; Ed. Rousseau, R.W.; p.954, Wiley-Interscience (1987). 4. Lynch, James D.(HPD Incorporated); Personal Communication (1988). 5. McArdle,John C. (AQUATECH systems); Personal Communication (1988). 14

13 Minimization of Spent Pickle Liquor using Membrane Separation. Mr. Dhiru Patel and Mt. Steven Pratt. Ha Fec13 MEMBRANE Figure 1 Principle of Dialysis Process 3

14 Minimization of Spent Pickle Uquor d ng Membrane Separation, Mr. Dhiru Patel and Mr. Steven Pratt. El DIALYZATE E WATER SALT 0 ACID D SELEMION MEMBRANE i pgj FEED LIQUOR Figure 2 Mechanism of Ion-exchange Membrane

15 "3 MWmttalh of Spent PlckJe Uqm using Membrane Separation, Mr. ohtu Patel and Mr. Steven Pratt RRE WATER TANK DIFFUSION DlALnER RECOVERED AUD TANK RECOVERED SOUrrlON Figure 3 Acid Recovery Flow Diagram

16 Minimization of Spent Pickle Liquor using Membrane Separation, Mr. DNru Patel and Mr. Steven Pratt. " RECOVERED ACID SPENT UQUOR PUR E w AT ER FEED UQUOR Figure 4 Schematic Diagram of Dialysis Process