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Originally presented at Extraction 2018, August 26-29, 2018. Please visit Springer for more information about the conference or how to purchase the paper or proceedings.
Pilot Testing Ion Exchange Resin (IEX) can go from very simple beaker testing to complicated column testing. Space velocities, regenerant quantities, loading capacities, ion leakage, are all terms that get accumulated together to prove or disprove a resin’s (or resins’) ability to load a single ion (or specific set of ions). Not getting all the resin’s operating parameters correct will disqualify (or at least skew) the results.
Laboratory tests for capacity, moisture content, bead size, metals fouling, etc. – what does it all mean? At last count, there were over twenty-five separate ion exchange resin tests (analyses) that were available to end users. Standard testing procedures identify the resin properties but more specific procedures can be used to identify problems with equipment operation. Then, there are the costs to consider; simple cation resin testing (moisture content, total capacity and bead integrity) is in the $200/sample range. However, add to this Chatillon, Russian Ball Mill Test, HIAC particle size, metals and % regeneration test procedures and the lab work can increase to over $1000/sample.
In this paper, the author will explain the “must-do” pilot and test procedures and why they are important and the key to understanding how resin performs ion exchange.
KEYWORDS: Ion exchange resin, pilot study, laboratory analysis
Ion exchange resins (IEX) are synthetic resins having a chemical structure based on a cross-linked three-dimensional polymer molecule into which functional groups (sulfonic acid and quaternary ammonium) are introduced. Most of the polymer bases used for IEX are copolymers of styrene and divinylbenzene (DVB) and generally consist of spherical particles of 300 to 1200 µm diameter. The cross-linked copolymer is synthesized by mixing styrene (one vinyl group) with DVB (two vinyl groups) and carrying out a suspension polymerization in water.
The ion exchange resin is then manufactured by introducing functional groups into this copolymer matrix by means of chemical reactions. The ion exchange groups introduced are chemically bonded to the polymer, and as they cannot move freely, they are known as fixed ions. Mobile ions of opposite charge to the ion exchange groups (H+ ions in the case of R-SO3H) are known as counter ions. If the quantity of the bi-functional monomer DVB is increased in the polymerization, a tight, porous structure with considerable chain branching is obtained. If, on the other hand, the quantity of DVB is decreased, a loose, porous structure without much branching is obtained. As DVB determines the tightness of the porosity, it is known as a cross-linking agent. The proportion of DVB in % is referred to as the cross-linkage.
Since IEX contain pores, the pores are filled with liquid (for this paper we will assume water). Ions diffuse throughout these pores and give rise to ion exchanges. The more cross-linked the resin is, the more ion diffusion will be impeded. Conversely, a low degree of cross-linkage will facilitate a greater diffusion of ions. If the cross-linkage is too low, however, moisture content becomes excessive, so the resin will be softer and difficult to use, and its strength also decreases. Resins are, therefore, generally used with a cross-linkage of about 8%.
Ion exchange reactions are carried out with a diffusion of counter ions in the resin particles. The pore structure of the polymer matrix has a considerable effect on these reactions, and cross-linkage is therefore, an important factor when it comes to analyzing the properties of the resin.
The styrene-DVB copolymer does not absorb water until the functional groups are added. Water absorption is due to hydration of the fixed and counter ions. The chains of the polymer matrix elongate and swell until a balance is reached between the water absorbing capacity of the resin and the elastic forces of the polymer. This keeps the resin in its swollen state with a stable moisture content. The higher the cross-linkage, the less the polymer matrix can elongate resulting in a resin with a lower moisture content. Low cross-linkage results in a bead with a high moisture content as the swelling increases. As the resin swells during the functionalization phase, micro-pores are created in the beads.
The ion exchange resin manufactured by simple polymerization of styrene with DVB is transparent and has a gel structure; hence, it is known as a gel type ion exchange resin. Using special polymerization techniques however, IEX resins of higher porosity can be manufactured, creating a macroporous resin. The photos that follow are (1) a weak acid cation resin, (2) macro porous strong base anion resin and (3) a strong acid cation gel reins. The magnification of the photos is 100X.
Photo 1 - Weak Acid Cation Beads
Photo 2 - Macro Strong Base Anion Beads
Photo 3 - Dyed Strong Acid Cation Resin
Magnification of resin beads presents many opportunities to explore what beads physically look like. In the following photos, we observe that:
Photo 4 – 40X magnification of a macroporous anion resin with a standard Gaussian distribution (300 to 1200 µm) beads. There is a distinguished size difference in the beads, but it seems that there are more smaller beads than larger ones.
Photo 5 – 100X magnification of the same gel macro anion resin with a standard Gaussian distribution. The size difference is much clearer and there appears to be less difference in the proportion of small and large beads.
The problem with the original photo is that the photographer did not level out the mounding of the resin on to a flat surface before taking the photo. The resin sample was originally from a mixed bed (SAC+SBA) and the darker beads are strong acid gel cations.
Photo 6 is a 15K x magnification of a 600 µm gel cation resin. Even at this level of magnifciation, the mirco pores are not visible.
Photo 7 is a 25K x magnification of a 400 µm macroporous weak acid cation resin and the macro pores are clearly visible.
Photo 6 - Gelular Resin – 15Kx Magnification
Photo 7 - Macroporous Resin - 25 Kx Magnification
To make porous resins, pore formers are introduced into the process before the functionalization step. Pore formers are usually used to make very low crosslinked (< 8%) or very high crosslinked (>15%). Table 1 shows some of the characteristics of those porous types:
Table 1 - Characteristics of resin types
Ninety-nine percent of the ion exchange sites are within the bead. Photo 8 is a lab photo of 5-year-old macroporous resin from a condensate polishing system. Before the photo was taken, the sample of resin was treated with 2 N HCl to clean iron and organics from the sample. When the photo was taken, we could see that a bead had been cleaved in half (as received), clearly showing the ionic loading remaining inside the resin bead.
Photo 8 – Iron Inside the Resin Bead
The fouling condition exists due to the fact that the total regenerant chemical strength is reduced as it moves to the center of the individual beads. Two factors contribute to this condition:1.) The physical polymer structure of the bead restricts the dilute chemical flow path; allowing the dilute chemical to find an alternate path of least resistance;
2.) As the dilute chemical strength is reduced by exchange at the sites inside the resin bead, the driving force falls below the minimum strength to complete or cause the exchange.
LABORATORY PILOT TESTING OF ION EXCHANGE AND ADSORBENT RESINS
This procedure describes how to perform a lab-scale process evaluation on the effectiveness of IEX. Test results obtained and the depth of work performed may enable further optimization of operating conditions. While the test may not provide sufficient information to design a full-scale plant, it should show if a process is viable and enable the design of a larger pilot plant, if necessary.
Initial trials can be carried out using 125 mils of resin in a 25-mm diameter glass column with a sintered glass frit at the bottom is ideal. However, if a frit disk is not available a, glass or plastic column with a rubber stopper is suitable to create a basic test system. Each rubber end stoppers needs to have a glass tube running through it (see Figure 1) and a chemically resistant cloth or screen should be placed over the surface of each stopper to retain the IEX or adsorbent resin in the bed. Fill the column to a depth of 15 - 25 mm with small diameter glass beads (3 mm diameter) to help distribute the fluid and hold the resin above the bottom of the column, preventing it from blocking the outlet.
If the column does not have a valve connection to control the flow rate at the exit end of the column, rubber tubing with a screw clamp assembly can be used as an on/off valve. Depending on the laboratory set-up, additional equipment may include a funnel, tubing, a pump, chemically resistant cloth or glass wool, and a graduated cylinder. In sensitive applications, the equipment should be sterilized and then fully rinsed with high-quality demineralized water before loading the resin.
Depending on the nature of the trial, the column can be fed either by gravity or by a peristaltic or diaphragm pump. If gravity feed is used, arrange the pipework from the column in a “U” shape so that it rises to a level above the top of the bed, keeping the resin bed always flooded.
Whether gravity or pumping systems are used, it is essential to have solutions of regenerant and demineralized water prepared ahead of time and ready for use before the trial and regeneration processes begin.
Resins should be stored as received. If they are moist, they must be kept moist. If they are received dry, store them dry, but they will need to be preconditioned (see below) before use. If IEX or adsorbent resin is stored prior to testing, make sure the containers are not left open to the atmosphere or allowed to dry out. Keep storage containers away from strong sunlight and hot or cold temperature extremes.
If you are testing the sample in a non-aqueous application, the IEX or adsorbent resin should be used in the dried form for best results. This will prevent water contamination of the treated product. Resin samples received in the hydrated form should be pre-conditioned to remove the water by displacing with suitable solvents such as acetone or alcohol, if permitted in the specific process.
Prior to running a laboratory test, the sample needs to be conditioned to ensure full swelling and hydration of the polymer. Unless otherwise advised, the polymer must be soaked.
When samples are sent out from Purolite, the resin is either taken from production batches or from warehouse stock. Resin that is stored for any length of time will require rinsing with demineralized water to reduce leachable components before trials can be performed. In many applications, 5 – 10 bed volumes of rinse water should be sufficient.
Do not load IEX or adsorbent resin in a dry column. Fill 1/2 to 1/3 of the column in advance with deionized water, then transfer the pretreated IEX or adsorbent resin into the column, always making sure that the resin stays submerged and draining off excess water as needed, but never enough to expose the resin bed to air. A minimum total bed depth of 760 mm should be maintained for the trial. In most applications, the process will improve if the bed depth is increased.
Approximately 375 ml of resin is sufficient for lab scale process evaluation. If the inlet load is very low, it will require a large amount of solution to be processed through the bed to reach exhaustion, and greatly increase the time it takes to complete each test.
Once the column is loaded, the resin should be backwashed with demineralized water in an up-flow direction for 10 – 15 minutes to fully classify the bed. During settlement, the resin bed will expand and the larger particles will fall towards the bottom of the bed while the smaller beads will locate nearer the surface.
Allow the IEX or adsorbent resin to settle for approximately 5 minutes.
Following this process, and depending on the particle size range of the resin, the height of the resin will increase. This resin height and bed volume (BV = volume of wet settled resin in the column) must be noted and should be used for all test calculations going forward.
Next, drain the column to leave a maximum of 1 cm of water remaining above the bed and discard the initial run-off. Then, begin feeding the solution into the column. Adjust the opening of the bottom valve on your column set-up to control the total flow through the resin.
Table 2 - Suggested parameters for testing viability of ion exchange and adsorbent resin
For regeneration chemicals, concentrations, quantities and details for sweetening on and off, contact your local Purolite technical sales professional for guidance.
It is important to routinely sample the column effluent during service runs to enable adjustments to pH. Levels of pH can drop significantly in the treated water, affecting alkalinity, breakthrough behaviour, leakage and run lengths. Frequent analysis for the target analyte will also reveal critical information about column loading.
Depending on the application, the process conditions for the resin will vary widely. For processes that are not fully developed, discovering and maintaining the optimum specific flow rate will be one of the primary objectives of the laboratory study.
In many water applications where the loading is small and conventional ion exchange is taking place, flow rates through the resin can be high (up to 50 BV/h), and sometimes even greater flow rates are used. In special process applications, or where highly selective removal is required, flow rates can be much lower (1 – 10 BV/h).
Resin regeneration is normally carried out at relatively low flow rates (1 – 6 BV/h) to achieve maximum adsorbate removal from the beads, and is followed by a slow rinse at a similar flow rate to maximize removal of the regenerant.
The final rinse is carried out using a higher flow rate than the in-service flow rate. In non-aqueous process applications, the resin is often regenerated in the aqueous state. Under these circumstances, the process liquor must first be displaced with water. This is often referred to as “sweetening off,” and after regeneration, the water is displaced with process liquor which is called “sweetening on.” These terms derive from the sugar industry where IEX and adsorbent resins are widely used.
Once a trial run begins, the resin should be allowed to continue to operate through to exhaustion. The experiment should not be stopped mid cycle. Most ion exchange reactions are reversible, and once the solution is stopped it tries to reach equilibrium. When this happens, the ions come back off the resin into solution. This can cause premature resin exhaustion and false results. Under normal condition tests, the bed must remain covered with solution. Never drain the column and introduce air into the bed. Air bubbles are difficult to remove and will result in poor test performance.
Three consecutive cycles producing consistent results should be obtained before changing any operating conditions to optimize performance. It normally takes two or three cycles to obtain reliable test information.
LABORATORY ANALYSIS TO HELP UNDERSTAND SOME OF THE PROPERTIES OF RESIN BEADS
The periodic analysis of resin samples taken from operating equipment is generally done for two reasons:1.) Maintenance - to anticipate any problem.
Often analysis is delayed for too long (over one year) or until some malfunction occurs. In many cases, the resin may be too far gone in fouling or deterioration and the lab analysis may confirm that the resin needs replacement. Then again, lab analysis shows the resin to be "normal" in the routine analysis but the operating performance is still poor. This is because kinetically the resins are "not normal" and may be organically fouled or clogged with extraneous matter that routine lab procedures or excessive regenerations do not remove.
Standard tests are ones conducted on every batch of resin manufactured and include the resin’s moisture holding capacity, total capacity, and bead integrity.
Moisture Holding Capacity (MHC): Weighing the moist resin and then drying the sample to constant weight in an oven at 105oC determines the resin moisture content. An increase in MHC indicates a loss in DVB (cross-linkage) from oxidation, and a decrease of MHC shows fouling.
Total Capacity (TC): A known volume (weight) of resin is placed in a column and an excess of chemical solution is passed through the resin to convert to a known ionic form. The known ions are then eluted from the resin using an excess of a (different) regenerant. The concentration of known ion eluted (or exchanged) is determined quantitatively. The capacity is generally reported as milliequivalent capacity per milliliter (meq/ml) of resin, based on a reference ion form.
TC is used to judge a used resin’s rate of deterioration and to determine when resin replacement is advisable. The difference in total capacities does not always translate in operating capacity differences.
Bead Integrity (BI): Examine the resin under a microscope at 20 to 100X magnification. This is a subjective evaluation of the number of cracked or broken beads reported as a percentage of the total number of beads being viewed through the microscope. Whole beads show no flaws or cracks. Cracked beads allow for faster kinetics to the internal exchange sites of the bead, which will result in lower ion leakage and/or higher throughput capacity. Broken beads are the small fragments that can fill the void spaces between the whole beads, resulting in increased pressure drop across the bed. A photo from the microscope usually accompanies this test.
Photo 9 - Bead Integrity
With these three-analysis completed, one can usually conclude if the IEX system has a resin problem. If the MHC and TC are within the acceptable limits, then there is no resin oxidation or fouling that is interfering with the resin’s ability to work. If the BI is good, then there is no pressure stress or osmotic shock causing the bead to fail to exchange.
Since resin manufacturers have been asked to help solve IEX problems, we have developed some unique tests to check the resin’s operating conditions.
OPERATING TESTS - TESTS THAT WILL HELP YOU TROUBLESHOOT EQUIPMENT
% Strong Base (or salt splitting) Capacity: reports % Strong Base exchange sites on strong base anion resins. Most significant loss of % strong base is traceable to organic fouling and leads to poor demineralizer performance.
Extractible Organics (mg C/kg resin): reports the level of extractable organic matter stripped from 10 grams of dewatered “as recevied” resin. 10 g of resin sample is treated with 30 mils of 2 N HCl and allowed to soak overnight. High levels of organic will lead to long rinsing times. Results are interpreted as follows:
Percent (%) Regeneration: Reports level of last regeneration done by the sender on a regenerated sample. This test is like the total capacity tests except that the elution is performed without first converting the resin to the reference ionic form. A good range for samples from the field is 65-80% percent regeneration.
Metals Content (PPM): also referred to as an Ash Analysis, reports quantity of extractable metals (Fe, Cu, Zn, Mn, etc.) stripped from 10 grams of dewatered “AS RECEIVED” resin. The results are reported in ppm (mg/kg) and can help identify resin foulants or regeneration efficiency issues. Results are interpreted as follows:
HIAC Particle Size: Reports % distribution of various beads by size. HIAC is the name of the instrument used.
Cation/Anion Ratio: For mixed bed resin samples, this reports the ratio of anion to cation or stratified beds reporting the ratio of weak to strong in the mixed sample.
Hardness Fouling: reports the hardness fouling on any resin sample.
Silica Fouling: is usually done on weak base anion resin from a demineralization train where caustic from a SBA is recovered and used for the WBA.
Oil Fouling: is a blue dye test usually performed on cation resins because of their oleophilic characteristics.
% Swelling: for weak acid cation resins only, full sodium conversion, reports % bead swelling to show possible de-cross linking.
Rinse Requirements: For anion resins only, samples are fully exhausted using 1 N H2SO4 and then regenerated using 1 N NaOH. The sample is then slow rinsed for 3 BV, and then fast rinsed with DI water and the conductivity is checked every BV.
Osmotic Shock: subjects resin to successive acid and base regenerations continuously for 100 to 300 cycles. Bead integrity is reported by examining the resin after the cycles have been completed.
Chatillon testing: forty 600-micron resin beads are individually subjected to testing using a force gauge. Prior to testing the resins were soaked in de-ionized water. The results of the crush strength testing are reported as force (grams) required to crush a bead of a given sample, common know as g/bead
Russioan Ball Mill Test: test consists of milling the resin in a small ball mill for 1 hour with steel balls and then measuring the volume of the resin which is coarser than a specified size.
HIGH-RESOLUTION MICROSCOPIC PHOTOS
REFERENCESWith permission of John Soper, Process Research Manager, Archer Daniels Midland Co. Decatur, Illinois 62521-1656