The main factors that affect the operating cost of distillation plant is the volume of the steam in the tower. Having a certain production capacity which means that the amount of D is certain, the size V depends on the reflux ratio. The actual reflux ratio is always between the minimum reflux ratio and the full reflux, because the size of reflux ratio not only affects the required number of theoretical plates but also has an effect on the consumption of the heating and cooling, and it even affects the choice about the structural size of tower plate, distillation tower and condenser.
Thus, choosing a suitable reflux ratio is an important problem.
The appropriate reflux ratio is decided by economic accounting, so if the ratio, generated under the circumstance that the cost of operating and plant depreciation is the lowest, is the appropriate reflux ratio. However, as curriculum design, it is difficult to make this kind of accounting. So we usually decide the reflux ratio depending on these three methods:
1. According to the specific condition of the waste oil distillation equipment design, we can make a reference on the empirical data of the rational ratio during the production process;
2. First is that we need to account the smallest ratio Rmin. According to previous experiences, we should take the operating ratio which is the 1.1 to twice as much as the smallest ratio, so it means the R is equal to (1.1 to2) Rmin;
3. In the certain range, choose 5 different reflux ratios and account the related theoretical plate number, and then using these numbers, we can draw the curve of reflux ratio and theoretical plate number. When the R is equal to Rmin, the plate number is limitless; when the R>Rmin, the plate number is limited from limitless; if the R is continuous to increase, even though the plate number will reduce, the speed of it will be slower. So we can choose a appropriate ratio from the part of slash area.
The purity or the recycling rate of the product
The purity of the product is decided by the demand of clients. If clients have the requirement on the distillation tower and the purity of products, the recycling rate of product is certain; but if clients just have requirement on the purity of one kind of product, the designer can decide the recycling rate based on the economic analysis. Increasing the product’s purity means the improvement of product’s recycling rate to get more economic benefit. But the increasing of purity needs to increase the plate number and reflux ratio, which means the increasing of plant cost and operating fees. So we decide the purity and recovery rate of products only through the economic analysis.
Utilization of heating energy
The heating efficiency is low in the oil refinery process, and the 95% energy of re-boiler is taken away by the cooling material of condenser, which is installed in the top of tower. So only about 5% of the heating energy can be efficiently utilized. The introduction of heating pipe technology can help increasing the steam temperature of tower, and then using the heating steam to heat the liquid in reactor. At the same time, the steam in the top of tower will be condenser. By this method, it can not only save a lot of heating steam, but also save large cooling medium. Certainly, steam of tower can be used as the heating source in low temperature system, and can produce the low pressure steam for use in other places through the waste heat boiler. When we fully consider how to utilize the heat energy, we should also pay attention to the investment of additional plants and the effects that the crude oil refinery plant can bring.
