In principle, distillation separation is realized by using different boiling points of substances. There is no doubt that the effective energy obtained by distillation product is separation work, and the effective energy input in distillation process is temperature difference effective energy. The ratio of the separation work obtained in the distillation process to the effective energy of the input temperature difference is the effective energy efficiency of the distillation process, or distillation efficiency for short.
What are the factors affecting the distillation efficiency of a binary system? The first factor is, of course, the number of trays. When the number of trays is greater and the product purity remains unchanged, the smaller the reflux ratio is required, the lower the energy consumption is, and the higher the distillation efficiency is. The second influencing factor is the composition ratio of the binary system. The closer the composition ratio is, the higher the distillation efficiency is. Of course, this is a rough conclusion. There are other factors, such as the different phase heating of high and low boiling point components. The reason why the composition of the binary system affects the distillation efficiency is that when conventional distillation and heat pump distillation processes are adopted, There is often a problem of disharmony between the liquid-gas ratio in the distillation section and the gas-liquid ratio in the stripping section. To make the liquid-gas ratio in the distillation section and the gas-liquid ratio in the stripping section meet the requirements of product purity at the same time, it is necessary to increase the consumption of steam cooling water or increase the circulation of heat pump, and the distillation efficiency will naturally decrease.
The solution to this problem in the conventional distillation process is to add an open heat pump distillation. The heat pump may be set between the distillation section or stripping section or distillation section. For the conventional distillation, it is a single heat pump technology, and for the heat pump distillation process, it is a double heat pump distillation scheme, The heat pump distillation process itself is a heat pump, plus a heat pump. Of course, it is a double heat pump. The disharmony between the liquid-gas ratio in the distillation section and the gas-liquid ratio in the stripping section sometimes has a great impact on the distillation efficiency. The greater the proportion deviation of binary system, the more significant the influence. After the application of heat pump distillation technology, it is normal for the conventional distillation process to adopt the improvement of single heat pump and the addition of a heat pump to form a double heat pump distillation scheme.
So under what circumstances do we need to use multi heat pump technology? A case of preparing high-purity products. The problem of product purity is essentially the problem of the number of trays. Theoretically, if the number of trays is infinite, any high-purity products can be prepared as long as the reflux ratio is greater than the minimum reflux ratio, but in practice, the number of trays available is always limited. In this case, high-purity products can be prepared through the reflux ratio, However, the increase of reflux ratio means the increase of energy consumption and the decrease of distillation efficiency, especially limited by the maximum reflux ratio. If the energy consumption of a given product purity is unacceptable under the limit of the number of available trays, or the energy consumption is acceptable but still can not be realized, it is necessary to adopt multi heat pump technology to reduce the energy consumption for preparing high-purity products and realize the preparation of high-purity products.
There is another case where multi heat pump technology needs to be used, that is to further reduce distillation energy consumption and improve distillation efficiency. Conventional distillation adopts single heat pump technology. The heat pump distillation process adds a heat pump to form a double heat pump scheme, which coordinates the liquid-gas ratio in the distillation section and the gas-liquid ratio in the stripping section. Can a completely reversible distillation process be formed under the infinite number of trays, and the distillation efficiency reaches 100%? No. Its irreversibility is also great. The fully reversible distillation process requires not only an unlimited number of trays, but also an unlimited heat pump setting for the minimum reflux ratio. By adding reasonable heat pump settings, we can always further reduce energy consumption and improve distillation efficiency. Of course, the effect is getting smaller and smaller. This is a question of whether it is worth it.
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