In recent years, the dual-carbon background has given rise to the demand for green, low-carbon and energy-saving oriented development in all walks of life, and the chemical industry, as a major emitter of carbon emissions, urgently needs to make adjustments and improvements in all aspects from the perspective of low-carbon development. Chemical production process exists in the high energy consumption and power consumption, high purity chemicals to get difficult, low resource utilisation rate after product waste and other multifaceted problems, and separation and purification in the chemical process of energy consumption accounted for about 80%, through technical means to reduce the chemical separation of energy consumption is to reduce the chemical production of effective means of reducing the overall energy consumption, at the same time, advanced chemical separation technology to get high purity chemicals is also the downstream application of chemical low-carbon and energy-saving Key factors. The main way to improve the low carbon and energy saving of chemical separation technology is to adjust the technical route, and the second is to adopt the circular economy mode, and the multi-stage coupling of the product production process and the thermal coupling mode of the energy consumption utilisation will be the main measures and directions for the improvement and development of the current chemical separation technology.
In all chemical production, chemical separation throughout the entire production process, the acquisition of high-purity raw materials is the key to the reduction of by-products in the production process, high-purity intermediate chemicals can further reduce the chemical production of three-waste emissions, the purity of the final product is the embodiment of the market competitiveness of chemical enterprises. Distillation and crystallisation technology is the main chemical separation technology, distillation technology can get general industrial grade chemicals, but to get ultra-high purity chemicals is huge energy consumption, while melt crystallisation technology can easily get high purity chemicals from industrial grade chemicals. Therefore, the coupling of the two technologies of melt crystallisation and distillation, and the mutual use of their respective energies, to ultimately achieve high-purity chemicals at low energy consumption has become a reality.
Distillation
Distillation technology is a commonly used separation and purification technology that separates components of a mixture by taking advantage of differences in the boiling points of substances. With the advantages of higher product purity, wide applicability and sustainability, it is widely used in chemical, petroleum, pharmaceutical and food industries. However, there are also challenges of high energy consumption, complexity and cost of processes and operations when trying to obtain ultra-high purity chemicals.
The distillation process is divided into two main steps: vaporisation and condensation. First, the liquid mixture is heated to the boiling point, where the lower boiling components are first vaporised, and then the vapour is cooled and condensed to a liquid in a condenser to separate the components. Distillation techniques usually require a large supply of heat and cold and therefore have a high energy consumption. Its energy consumption depends mainly on the following factors:
Heating: A large amount of thermal energy is consumed to heat the mixture to the vaporisation temperature.
Cooling: condensing the vapours requires a certain amount of cooling energy.
In addition, if high purity chemicals are to be obtained, high distillation reflux ratios are required, which in turn increases the energy consumption exponentially.
Melt Crystallisation
Melt crystallisation is a new low-carbon technology in industrial crystallisation, which has an irreplaceable role in obtaining high-purity product purification and breaking through the bottleneck of distillation separation. According to statistics, more than 80% of the organic mixed system is a low eutectic system, theoretically a melt crystallisation can be obtained 100% purity of the product and its impurities of the eutectic mixture, and distillation technology is theoretically impossible to get the pure substance compared to melt crystallisation in the separation of high-purity chemicals have a natural advantage. In recent years, melt crystallisation technology has been widely used and developed in many fields, and the common melt crystallisation process mainly includes the purification of organic substances and the preparation of APIs. Melt crystallisation is mainly through the use of substances in the crystallisation process can be obtained pure substances and low eutectic mixture characteristics, through the crystallisation of a high-purity products.
According to the precipitation method of melt crystallisation and the type of crystallisation device, melt crystallisation is mainly divided into layer crystallisation and suspension crystallisation. Layer crystallisation is the crystallisation process on the cooling surface, usually an intermittent process, divided into different stages, a certain load of material into the crystalliser, in turn, through the crystallisation, mother liquor discharge, sweating (partially melted), melting (fully melted), purified product discharge. Suspension crystallisation, on the other hand, generally involves rapid crystallisation of crystal particles from the melt in a vessel with agitation, with the particles suspended in the melt, usually with additional solid-liquid separation equipment.
Compared with other separation methods, melt crystallisation has the following advantages:
1. High product purity: conventional material system due to the boiling point difference is small, distillation is difficult to get high purity products, usually melt crystallisation can get high purity products (99.9%), and even can get ultra-high purity products (99.99%, 99.999%);
2. Mild operating conditions: Melt crystallisation is operated at the melting point of the product, which is usually much lower than the boiling point, so the operating temperature is low. At the same time, due to the mild operating conditions, no special requirements for equipment, but also reduces the manufacturing cost of equipment, especially for certain heat-sensitive chemicals, melt crystallisation has an inherent advantage.
3. Wide range of products: in addition to the ordinary material system, for many difficult to separate systems such as isomeric systems, heat-sensitive systems can achieve good separation results;
4. Energy saving and environmental protection: usually the heat of melting of substances is much smaller than the heat of vaporisation, and the distillation process has a high reflux ratio and heat loss, so the operation is much smaller than the conventional distillation energy consumption, and at the same time due to the lack of additional introduction of other solvents, not only to save energy but also to reduce environmental pollution.
With the continuous progress of science and technology, melt crystallisation technology has also been continuously improved and developed. Modern melt crystallisation technology combines advanced control and sensing technologies to achieve more precise temperature control and crystallisation process monitoring. In addition, researchers are exploring ways to improve crystallisation equipment, optimise process parameters and develop new types of crystallisers to increase the efficiency and economy of melt crystallisation.
Distillation - crystallisation thermally coupled process technology improvement
With the economic and social development, the purity of chemicals in various fields put forward higher requirements, the demand for high-purity, ultra-pure chemicals is increasing day by day. The use of high-purity chemicals distillation technology there is a high reflux ratio, high energy consumption, ultimately leading to high costs, in the distillation process of trace impurities removal is more difficult, the purity of each level of distillation reflux ratio needs to be doubled or even multiples, which leads to a sharp increase in energy consumption. Distillation operation is generally operated in the material boiling point annex operation, even if the use of vacuum technology to reduce the operating temperature, its condensation must also be higher than the melting point of the product, otherwise there will be due to crystallisation problems lead to condenser blockage, of course, this also provides a theoretical basis for the distillation condensation heat for the heat source of the melt crystallisation.
Melt crystallisation using substances in the crystallisation process can be obtained pure substances and low eutectic mixtures of the characteristics of the crystallisation to obtain high-purity products. When the purity of the raw material for melt crystallisation is low, due to the proximity of the low eutectic point of the mixture, there will be the disadvantage of low yield or even not being able to crystallise the pure substance, however, the purification of the lower purity substance is exactly the advantage of distillation. In addition, since melt crystallisation operates near its melting point, the operating temperature is inevitably lower than the condensation temperature at the top of the distillation column, making it possible to use the condensation heat at the top of the distillation column for melt crystallisation.
The process utilises distillation technology for the initial separation of raw materials and then uses melt crystallisation technology to obtain high purity chemicals, with the melt crystallised mother liquor returning to the distillation for further distillation. The heat from the condenser at the top of the distillation tower is used as a heat source for the melt crystallisation, and the heat from the melt crystallisation is used to cool the condenser at the top of the distillation tower.
Distillation and crystallisation technology coupled, using their respective advantages for high-purity separation, the process is mainly raw materials first distillation purification to a certain purity, distillation process products and then into the crystallisation process to further crystallisation and purification to a higher purity, crystallisation of the mother liquor into the distillation process, so that the yield and purity have reached the optimum. At present, the distillation-crystallisation coupling process has been widely used in industrial production, which can further broaden the separation system, greatly increase the separation effect, and greatly improve the purity of the product. At present, the research and development of distillation-crystallisation coupling technology is still mainly focused on the application of the specific material system, for the continuity of the coupling separation process and the process of energy consumption of the network optimization is relatively weak.
The distillation-crystallisation thermal coupling technology achieves efficient separation and ultra-purification of components in mixtures through the transfer and use of thermal energy in the thermal coupling process. Because for the general material system, the heat of vaporisation is 2~3 times of the heat of melting, while the boiling point is much higher than the melting point, therefore, in the process of thermal coupling, the heat generated by condensation of vapour in the condenser of the distillation process can be used as the energy supply of melting and sweating stages of the crystallisation process, and in the process of distillation-crystallisation thermally coupled ultrapure separation process, the two steps of distillation and crystallisation are coupled with each other to form a cyclic process. This technology not only provides highly efficient separation, but also saves energy and reduces carbon emissions, which meets the requirements of low-carbon energy saving and greening.
Take the distillation and crystallisation coupling of ethylene carbonate (EC) as an example, EC industrial grade above 99% can meet the requirements, but as a lithium solvent must reach 99.99~99.999% of the electronic grade requirements, using only distillation technology to get electronic grade purity is not only high energy consumption, but also difficult to achieve. EC's distillation coupled with melt crystallisation technology, not only can easily obtain high-purity electronic-grade products, and distillation of the top of the tower condensation heat for melt crystallisation heat source to further reduce energy consumption, the same electronic-grade products, distillation - crystallisation coupling technology relative to the traditional distillation method, energy saving of more than 60%.
Distillation - crystallisation heat coupling technology to achieve the following main points:
1, the design and development of high-efficiency condenser can further reduce energy consumption;
2, prompting the distillation tower tower top area of the low-temperature steam heat energy effectively converted to the crystallizer melting stage and sweating stage of the heat source;
3, the distillation process is generally a continuous process, while the crystallisation process is generally an intermittent process, in the establishment of the heat utilization network, the need to consider the coordination and matching of energy;
4, the crystallisation process of the heat conduction medium circulation system and distillation condenser heat control for efficient regulation.
Although thermal coupling technology has many advantages, there are some challenges and limitations:
1, engineering design complexity: the implementation of thermal coupling requires appropriate heat exchange equipment and system design, increasing the complexity of the equipment and investment costs. The control scheme to ensure the effective transfer of heat is a complex engineering problem.
2, process optimisation: thermal coupling requires precise temperature control and heat transfer, with high requirements for process parameters and operating conditions. Optimisation of the parameters and operation of the thermal coupling process requires comprehensive consideration of a number of factors such as heat transfer, material transport and reaction kinetics.
3, material compatibility: in the thermal coupling process, the compatibility and thermal stability of substances need to be considered to avoid adverse chemical reactions or thermal decomposition. Especially for operations at high temperatures and pressures, the choice and durability of materials need to be carefully evaluated.
Conclusion
Distillation-crystallisation thermally coupled ultrapure separation technology is a technology that couples distillation and crystallisation processes to achieve efficient separation and ultrapurification of components in a mixture through energy recovery and reuse, and is a key technology for comprehensive energy saving and heat integration optimisation of the distillation operation process, which has significant advantages in terms of low-carbon energy saving and greening of the production, and can save energy, reduce carbon emissions, and is applicable to a number of industrial fields It can save energy and reduce carbon emissions, and is applicable to the separation and purification needs of a wide range of industrial sectors. With the increasing requirements of greening and sustainable development, the distillation-crystallisation thermally coupled ultrapure separation technology is expected to be more widely applied and promoted in the industrial world, thus reducing the energy consumption of chemical separation processes, contributing to the realisation of the national ‘dual-carbon’ goal, and creating a low-carbon, high-end industrial future.
