When selecting ion exchange resins, many engineers and purchasers will fall into the same confusion: should they prioritize the cross-linking degree or the exchange capacity? These two parameters are very conspicuous on the resin datasheet, and it seems that both of them directly determine whether the resin is good or not. In fact, the answer is not so absolute, there is a certain which is more important, the core depends on your actual application scenario.
1.What is Crosslinking Degree?
Definition
Crosslinking Degree usually refers to the percentage of the mass of the crosslinking agent (generally DVB) in the resin skeleton, and there are three common specifications in the industry: 4% DVB, 8% DVB, 10% DVB.
It is essentially reflecting: how tight the molecular structure of the resin is. It essentially reflects how tight the molecular structure of the resin is. This tightness directly determines the subsequent durability and operational stability of the resin, and is the core indicator for determining whether the resin "carries manufacturing".
The Impact of Crosslinking Degree on Resin Performance
The degree of crosslinking will directly affect the core performance of the resin, the difference between the two can be clearly differentiated:
High crosslinking degree of the resin, the structure of the more dense, the corresponding advantages are also very obvious - high mechanical strength, not easy to be broken in the operation; temperature resistance, oxidation resistance is better, and can adapt to more complex working conditions; resistance to organic pollutants and oxidation resistance, and can be used in the production of the resin, and can be used in the production of the resin. Complex working conditions; resistance to organic pollution is also stronger, not easy to be blocked by organic matter in the water failure.
However, high cross-linking degree also has corresponding shortcomings: the diffusion of ions inside the resin is slower, and the speed of exchange reaction will be a little slower; at the same time, the dense structure will reduce the active exchange sites inside the resin, resulting in a low effective exchange capacity.
Resins with low cross-linking degree have relatively loose structure, which complements the high cross-linking degree resin - faster diffusion of ions and faster exchange reaction; higher effective exchange capacity and more impurity ions can be processed in a single run.
Short boards are also prominent: poor mechanical strength, easy to break in long-term operation; poor temperature resistance, oxidation resistance, narrower range of adaptability to the working conditions; easy to be contaminated by organic matter, once encountered with high organic content of water, it is easy to fail quickly.
2. What is The Exchange Capacity?
Definition
The exchange capacity refers to the unit volume or unit mass of the resin, the number of functional groups that can participate in ion exchange. Simply put, it is the upper limit of the resin's ability to work.
In industrial applications, there are two common ways of expression: mmol/g (mass exchange capacity, mainly used for laboratory testing), eq / L (volume exchange capacity, the most commonly used in engineering, directly corresponds to the actual operation of the processing capacity).
What Does Exchange Capacity Represent?
The core function of the exchange capacity is to answer the question: how much work can the resin “do”. It directly determines the treatment capacity and regeneration cycle of the resin in a single operation, which is the key index affecting the operation economy.
The advantage of high exchange capacity is very intuitive: the unit volume of the resin can handle a larger amount of water, a single run for a longer period of time; regeneration cycle is also extended, do not need to stop frequently to regenerate, can effectively improve operational efficiency.
On the contrary, the resin with low exchange capacity has low capacity utilization, limited water volume for single treatment, and needs frequent regeneration; frequent regeneration will increase the consumption of regeneration agents such as acid and alkali, and consume more water and electricity, and the long-term operation cost will be significantly higher.
Attention
Many people will step on a pit when choosing a model: take the “nominal exchange capacity” on the datasheet as the capacity in actual operation. In fact, there is a clear difference between the two, the nominal exchange capacity is the maximum capacity of the resin under ideal laboratory conditions, the actual operating capacity will be affected by a variety of factors, often lower than the nominal value.
Factors affecting the actual exchange capacity are: the degree of cross-linking (as mentioned above, a high degree of cross-linking reduces the effective capacity), the quality of the feed water (impurity concentration, pH, temperature), the speed of the flow rate, the choice of regeneration (regenerant dosage, regeneration time) and so on.
3. Crosslinking Degree Vs Exchange Capacity, Which Is More Important?
This is the most core issue when selecting, the conclusion first to everyone: no absolute who is more important, the key depends on your actual application scenarios. Under different scenarios, the priority of the two will change significantly.
Scenario 1: The Pursuit Of Water Quality, Operational Stability (Industrial Water Treatment, Condensate)
Priority: crosslinking degree is more important
The core demand of this type of scenario is to ensure that the quality of water meets the standard, while allowing the system to operate stably for a long period of time, to avoid frequent resin failure, equipment failure. For example, industrial wastewater treatment, power plant condensate fine treatment, the feed water quality is complex, often containing organic matter, oxidants, and operating pressure and flow rate is relatively high.
The reason is very simple: high cross-linking degree of the resin, pressure, oxidation resistance is stronger, not easy to break, can ensure the stability of the system pressure drop, to avoid the resin breakage caused by the bed clogging; at the same time, the ability to resist organic contamination is strong, is not prone to “poisoning” failure, longer life, long-term operation is more reliable, and effectively reduce the cost of maintenance. Typical choice: Priority is given to 8% to 8% of the total amount of water used.
Typical choice: Priority is given to 8% to 10% DVB of strong acidic cation resin or strong alkaline anion resin, which can not only ensure the stability of operation, but also take into account a certain amount of effective exchange capacity to meet the basic processing needs.
Scenario 2: Pursuit Of Treatment Capacity And Economy (Water Softening, Conventional Desalination)
Priority: exchange capacity is more important
The influent water quality of this type of scenario is relatively simple, the concentration of pollutants is not high, and there is not too much organic matter that can easily lead to resin contamination, so the core demand is to improve the treatment capacity and reduce the operating cost. For example, civil water softening, general industrial desalination treatment, the durability of the resin is not required, but the treatment efficiency and economic requirements are high.
Reason: the resin with high exchange capacity has a larger volume of treated water per unit volume, longer regeneration cycle, and does not require frequent downtime for regeneration. In this way, it not only improves the production efficiency, but also reduces the consumption of regeneration agent (acid, alkali), reduces the cost of water and electricity, and the economic advantage is very obvious in the long-term operation.
Typical choice: choose 4% to 8% DVB high-capacity type resin, not only to meet the needs of the exchange capacity, to ensure that the processing capacity, but also to ensure the basic operational stability, there will be no obvious life expectancy short board.
Scenario 3: Complex Water Quality, Organic Matter Or Oxidant
Priority: cross-linking degree > exchange capacity
The influent water conditions of this type of scenario are harsh, such as high organic wastewater treatment, industrial wastewater treatment containing oxidant, at this time, if the exchange capacity is prioritized, selecting a high-capacity resin with a low cross-linking degree is likely to cause problems.
Otherwise: the resin will be quickly contaminated by organic matter, oxidized by oxidants, the phenomenon of “poisoning”, the effective exchange capacity will rapidly decline, not a few months will need to be replaced, but increased replacement costs and downtime losses.
4.The Engineer's Idea Of Selection: Do Not Look At a Single Indicator, Look At The Combined Effect
In fact, the real professional selection of resin, never look at the high and low of a parameter, but look at the “degree of crosslinking × effective exchange capacity × operating life × regeneration efficiency” of the combination of effects.
A lot of market “high-capacity resin”, the initial operation of the performance is amazing, processing capacity is very large, regeneration cycle is also long, but after half a year to a year, the exchange capacity will fall off a cliff, the need for frequent replacement of the resin, the long-term cost is higher.
On the other hand, those resins with high cross-linking degree, although the single processing capacity is slightly lower, the regeneration cycle is a little shorter, but it is stable, and it can run continuously for 3 to 5 years, so the total processed water volume is larger and the comprehensive cost is also lower.
To share a simple selection steps, novice can also quickly get started: the first step, clear their core needs, is to run stably, or to efficiently handle; the second step, analyze the influent water quality, including pollutant concentration, organic content, temperature, pH, etc.; the third step, combined with the system's operating flow rate, regeneration, to determine the appropriate range of cross-linking degree; the fourth step, within this range, select the effective exchange capacity to match the resin; and finally, the last step, the total cost of water is greater. In the fourth step, within this range, select the resin that matches the effective exchange capacity; finally, if possible, do a small test to verify the actual operating effect of the resin.
5. Expert Perspective: How Do People In The Industry Choose?
I have communicated with many industry peers, whether it is water treatment engineers, or experts in the field of pharmaceutical purification, the consensus is: there is no standard answer to the selection, just look at the application scenario.
An engineer friend engaged in industrial water treatment for more than ten years, once told me: “in the high fouling system, crosslinking degree is the bottom line, would rather sacrifice a little exchange capacity, but also to choose a high degree of crosslinking of the resin, otherwise the maintenance cost will be ridiculously high.” He was responsible for a chemical wastewater treatment project, the initial selection of low crosslinking degree of high-capacity resin, less than half a year to replace all, and then replaced with 10% DVB resin, stable operation for more than 4 years.
There is also an expert in the field of pharmaceutical purification, his point of view is: “The pharmaceutical industry requires high purity, batch processing, exchange capacity is very critical to ensure the purity and efficiency of a single treatment, but we will also control the degree of crosslinking, to avoid resin fragmentation resulting in impurities mixed into the product.”
In the final analysis, professionals in the industry will not be entangled in “which parameter is more important”, but rather, according to their own actual working conditions, to find a balance between the two parameters, to achieve stable operation and cost optimization.
Summarize
Finally, I would like to use a common phrase to help you remember the core points: the exchange capacity determines how much work the resin can do at one time, and the cross-linking degree determines how long the resin can work and whether it can carry the construction.
When choosing ion exchange resin, you don't have to deliberately pursue the ultimate of a certain parameter, the key is to look at your application scenario: in pursuit of stability and long life, give priority to cross-linking degree; in pursuit of processing capacity and economy, give priority to exchange capacity; in most scenarios, balancing the two is the only way to realize the optimal operation effect and the lowest comprehensive cost.
If your working condition is special and you are not sure which cross-linking degree and exchange capacity resin to choose, you can also combine the specific water quality and operating parameters to further communicate the selection details to avoid selection errors!
