General Frequently Asked Questions
1. General Considerations
- What is an ion exchange resin?
- What is a chelating resin?
- What is the nature of insoluble polymer?
- What are gel-type resins?
- What are macroreticular ion exchange resins?
- What are polymeric adsorbents?
2. Physical and chemical properties
- Typical particle shape and size
- Total capacity and Operating capacity
- Moisture Holding Capacity
- Porosity
- Bulk density, shipping weight, true wet density
- Swelling and Shrinking
- Physical stability
- Pressure drop
- Chemical stability
- Thermal stability
3. Safety, Packaging, Storage
4. Handling and Disposal
1. General Considerations
What is an ion exchange resin?
It is an insoluble polymeric matrix containing labile ions capable of exchanging with ions in the surrounding medium. They can be grouped into four general categories: strong acid, weak acid, strong base and weak base.
Chelating resins have special functional groups which contain two or more electron donor atoms that can form coordinate bonds to a single metal atom. Classes of chelating functional groups of industrial importance are phosphonic acids, amino-, carboxylic acids and sulfur compounds. Resins that have metal chelating capabilities include those containing aminophosphonic acid or iminodiacetic acid or thiol sites. Resins containing an N-methylglucamine functionality are selective for boron.
What is the nature of insoluble polymer?
The most common backbone is made of crosslinked polystyrene, polyacrylate esters and polymethacrylate esters. These polymers are crosslinked with a poly-functional monomer to give insolubility in solvents, physical and chemical stability. Divinylbenzene is the most common crosslinking agent or crosslinker.
There are no permanent pore structures for the gel-type resins. These pores are generally considered to be quite small, usually not greater than 30A, and are referred as gelular pores or molecular pores. The pore structures are determined by the distance between the polymer chains and crosslinks which vary with the crosslink level of the polymer, the polarity of the solvent and the operating conditions. The gel type resins are generally translucent.
What are macroreticular
ion exchange resins?
The macroreticular resins are made of two continuous phases - a continuous pore phase and a continuous gel polymeric phase. The polymeric phase is structurally composed of small spherical microgel particles agglomerated together to form clusters, which, in turn, are fastened together at the interfaces and form interconnecting pores. The surface area arises from the exposed surface of the microgel glued together into clusters.
Macroreticular ion exchange resins can be made with different surface areas ranging from 7 to 1500 m2/g, and average pore diameters ranging from 50 to 1,000,000 A.
What are polymeric adsorbents?
In contrast to macroreticular ion exchange resins, the polymeric adsorbents are truly non-ionic. They are have a macroporous structure and are dimensionally and chemically stable and inert in virtually all environments Their adsorption properties are totally dependent on their chemical structures and their surface characteristics. Three classes of XAD polymeric adsorbents are available: styrenic, acrylic and phenolic. The hydrophobicity behavior is most pronounced with styrenic adsorbent.
2. Physical and chemical properties
Typical particle
shape and size
Generally, resins are supplied water wet in the form of spherical
beads having a particle diameter between 0.30 and 1.2mm. Powder resins are
also available for specific requirements.
Standard grade resins obtained by suspension polymerization have a gaussian
distribution of particle size and resins by jetting process have a more uniform
particle size. The uniformity coefficient number indicates closeness of all
beads to the same size. Typical values of uniformity coefficient are 1.5-1.7
for standard grade and 1.1-1.25 for jetted grade.
Total capacity and operating capacity
The total available capacity or total exchange capacity is a measure of all the functional groups on a resin and is reported on a unit volume of wet resin or a unit weight of dry resin. Test methods are available to measure the total exchange capacity of each type of ion exchange resin. The total exchange capacity is not necessarily the same as the operating capacity.
The actual operating capacity is usually lower than the total capacity for an ion exchange process such as in recovery or purification applications. Some of these factors affecting the operating capacity are total and relative ion concentrations, charge density of ions, flow rate, temperature, pH, regeneration efficiency and equipment design.
Each resin has a characteristic water content associated with the functional groups and adhering to the outer surface of the resin particles. This moisture content, which we refer to as Moisture Holding Capacity, does not include excess water or free water which is removed by filtration before discharging into shipping containers. This equilibrium water depends on the resin backbone, the nature of the functional groups and the ionic form.
There is no measurable porosity for the gel type resin when it is dry. Macroreticular resins have permanent pores which can be measured by nitrogen BET.
Bulk density, shipping weight, true wet density
The bulk density is the weight of wet resin per unit volume. This density is measured in a calibrated glass column after backwashing the resin, allowing it to settle and draining off the water. This density is specific for each resin and is dependent on the type of resin and ionic form. For each wet resin, Rohm and Haas has set a value of bulk density, called shipping weight.
The true wet density or specific gravity is determined on the wet resin. The resin sample is weighed and the volume is found by water displacement using a pycnometer. Values range from about 1.04 to about 1.25. Cation exchangers have a greater true wet density than anion exchangers.
Water wet ion exchange resins shrink or swell when they change from one ionic form to another and they shrink when they are in contact with non-polar solvents.
The polymeric beads are physically very stable and can be used in different kinds of columns or reactors. The mechanical stress happens:
- When the catalyst bed is severely compressed or under a high pressure drop (typically beyond 2 or 3 bars depending on the type of resin). In that case, the resin beads are likely to be deformed, flattened and ultimately fragmented. We usually recommend an operating pressure drop below 1 bar.
- When the resin is transferred by the means of a pump (this is not recommended) or crushed in the body of the valves.
With few exceptions, we usually recommend not to exceed 1 bar of pressure drop across a bed of ion exchange or chelating resins. The pressure drop is directly proportional to the flow rate, the viscosity of the feed, and the depth of the resin bed, and is inversely proportional to the square of the diameter of the resin beads.
Ion exchange and chelating resins are stable over the full range of pH. They are virtually stable to most inorganic or organic chemicals except for strong oxidants such as dissolved chlorine, ozone or peroxides. The resin matrix is decrosslinked and becomes soft. The rate of oxidation is enhanced by the increase of the concentration of oxidants and by the presence of metals such as iron and copper which serve as catalysts by higher temperatures.
Functional groups of resins are progressively lost when the recommended temperature limit is exceeded. The rate of loss increases exponentially as the temperature rises.
- Sulfonic acid resin catalysts are resistant to heat up to 120°C with some special grades exhibiting a heat resistance up to 170°C.
- Styrenic strong base resins having trimethyl quaternary ammonium (type 1) structure can be used up to 60°C in hydroxide form, 80°C in chloride form.
- Strong base resins having dimethylethanol quaternary ammonium (type 2) can be used up to 60°C.
- Acrylic anion exchangers are limited to 40°C for strong base functionality and to 50°C for weak base functionality.
- Acrylic cation exchangers can be used up to 100°C.For special resins: 60°C with thiol functionality, 75°C with N-methylglucamine functionality, 80°C with aminophosphonic acid functionality, 90°C with iminodiacetic acid functionality.
3. Safety, Packaging, Storage
Ion exchange resins are not hazardous.
The wet resins are packed either in bags or in drums. The size of the bags is either 25L or 1000L (big bag or super-sack). The size of the drums is either 50L or 200L.
Wet ion exchange and chelating resins should be protected from freezing and from temperatures above 40°C. Do not expose resin bags to direct sunlight for a long time to avoid drying out the resins.
How to unfreeze
ion exchange resins?
Frozen resin containers should be thawed gradually, avoiding thermal shock.
The shelf life can be more than 10 years if the resins are stored properly but the recommended shelf life depends on the type of resin and the container type. For specific product recommendations, please contact your Rohm and Haas sales representative.
4. Handling and Disposal
See our "Startup procedure - loading and washing" page.
The backwash operation is important to remove from the resin bed any particles, debris and air pockets and to classify resin particles by size. Water is injected at the bottom of a bed of resin at a flow rate sufficient to fluidize the resin to 60% expansion minimum, for a duration of about 15 to 30 minutes. Hydraulic expansion curves are available for each resin. Please contact our technical department.
Disposal
Disposal of the used resin must be done in accordance with all applicable
local, state, and federal regulations. Incineration and land filling are two
options that have been used by customers.
