When the mixture liquid containing two components a and B to be separated is added to the chromatographic column loaded with fixed phase (generally resin) from the upper part, under the driving force of the continuously added mobile phase (generally water) from the top to the bottom, the components a and B in the material liquid will be separated due to their different affinity with the two phases.
This process is similar to the race between tortoise and rabbit. Tortoise represents the components with strong and strong affinity (slow component), while rabbit represents the components with weak and weak affinity (fast component). With the extension of time, tortoise and rabbit will finally be separated at the end line.
Fig. 1 Schematic diagram of batch chromatography process
In the 1960s, the industrial chromatography equipment realized the upgrading from batch processing to continuous operation, which greatly expanded the application field and scope of chromatography separation technology.
1. Principle of continuous operation of chromatography
How to realize continuous production? The research and development scientists of chromatography technology come up with a way. Its basic idea can be simply described as making tortoises and rabbits race on the conveyor belt. The rotation direction of the conveyor belt is opposite to the running direction of tortoises and rabbits, and the rotation speed is between tortoises and rabbits, so that tortoises (representing slow components) and rabbits (representing fast components) can realize continuous production To separate.
Fig. 2 continuous chromatography process
How to achieve the effect similar to the reverse rotation of conveyor belt, the most intuitive idea of scientists is to make the fixed phase (packing) reverse cycle, which is actually difficult to achieve in industry, because it will bring many problems such as packing wear and uneven distribution of liquid phase.
A chromatographic tower can be divided into several sections, and the inlet and outlet valves are set between the sections. Through the periodic switching of these valves, the effect of reverse circulation of fixed phase can be simulated, which is called SMB. Novartis applexiontm SC technology is developed on this basis.
The application of chromatography separation technology in industrial desalination
2.1 industrial desalination
In the chemical production process, when there is salt generation in the main reaction process of the production unit or salt generation or addition in the intermediate process, no matter inorganic salt or organic salt, these salts need to be removed as by-products or impurities finally.
In most cases, these salts in the system are difficult to be effectively separated from products due to their high content and complex components, or the separation process will bring more difficult equipment problems and environmental problems.
With the gradual improvement of national safety and environmental protection standards, it has become the direction of many enterprises to explore clean and efficient desalination process. In the process of chromatography desalting, there is no need to add any additional chemicals, and the operation is safe and stable with obvious operating cost advantages. These characteristics are particularly valuable in today's economic policy environment.
2.2 industrial desalination process
The desalination processes of chemical products mainly include: ion exchange, electrodialysis and chromatography. According to different working conditions, these processes have their own applicability.
Ion exchange process: it is suitable for desalting of low salt materials, and the conductivity is usually within 2000us.
Advantages: the desalting process is very thorough, and the electrical conductivity of materials after desalting can be as low as 1-2us.
Disadvantages: when the salt content is high, because the process needs a lot of acid-base regeneration resin, it will produce a lot of waste water, so it is not the best choice.
Electrodialysis process: it is suitable for desalination of medium salt content materials, and the conductivity of general materials is 10000us-20000us.
Advantages: the electrodialysis process can remove salt from the material through the action of electric field and the selectivity of ion exchange membrane. In the process, only electrical energy is consumed without any chemicals.
Disadvantages: compared with the separation process, desalting is not very thorough. Usually, it can only be desalted to a conductivity of about 1000 US, and then the separation process is used for deep desalting. In addition, when the salt content of electrodialysis process is very high, not only the power consumption is high, but also the yield of the target product will be reduced.
Chromatography separation process: suitable for desalination of large-scale high salt products.
Advantages: for some products of chemical synthesis, the proportion of target product and salt in the synthetic solution can be as high as 1:1 or even 1:2. At this time, both the separation process and electrodialysis process are no longer applicable, and the advantages of chromatography separation process will become more obvious.
The process does not need any chemicals or electric energy. Only a certain amount of deionized water is added as the eluant in the separation process, the separation of the target product and salt can be realized.
At present, the process has been widely used in large-scale devices in biochemical and food industries, such as the production of 10000 tons of amino acids, organic acids and molasses recovery.
Fig. 3 chromatographic desalination plant with a diameter of 5m
2.3 principle of industrial desalination by chromatography
The mechanism of chromatographic separation is based on the different affinity between the stationary phase (generally resin) and the mobile phase (generally water) in the chromatographic tower due to the different physical and chemical properties of different components in the mixture. The separation of salts and neutral molecules (such as amino acids, organic acids, polyols, etc.) can be said to be a typical application of the separation mechanism, because salts are dissolved in the mobile phase The separated charge will repel the ions loaded on the stationary phase, while the neutral molecules will not (as shown in Fig. 4), so that the salts can flow out of the chromatographic tower faster, thus realizing the effective separation from the neutral molecules.
Fig. 4 separation mechanism of neutral molecule and salt
Source: chemical 707
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