Distillation problem

Let’s see how the difference between these two substances’ boiling points changes with pressure; if the pressure increases, the greater the difference in their boiling points, and thus an increase in pressure facilitates separation.

When the boiling point difference is small, a larger number of theoretical plates are required; atmospheric distillation is a better option, as vacuum distillation involves larger volumes of gas, resulting in larger equipment and higher costs.

Where the conditions of utility systems permit it, and normal-pressure distillation can be used, vacuum distillation should be avoided as much as possible. 1. Increase equipment investment; the tower has a large diameter, requiring a vacuum system and even chilled water. 2. Under reduced pressure, it is of little significance if the required number of theoretical plates cannot be significantly reduced.

As pressure decreases, the relative volatility may increase or decrease; it’s uncertain.

1. Atmospheric distillation: The pre-treated crude oil is heated and fed into the initial distillation column of the atmospheric distillation unit (Figure 2), where most of the light gasoline is distilled off. The crude oil from the bottom of the initial distillation tower is heated to 360–370°C and then fed into an atmospheric distillation tower (with 36–48 trays). The product at the top of this tower is the gasoline fraction (also known as naphtha), which, together with the light gasoline from the top of the initial distillation tower, can be used as feedstock for catalytic reforming, as raw material for the petrochemical industry, or as a component in gasoline blending. The side stream from the atmospheric pressure column enters the stripping tower, where it is heated with steam or a reboiler to evaporate the light components, thereby controlling their content (as indicated by the product’s flash point). Typically, the side stream 1 consists of jet fuel (i.e., aviation kerosene) or kerosene fractions, side stream 2 comprises light diesel fractions, and side stream 3 contains heavy diesel or transformer oil fractions (which belong to the lubricant fraction category); the bottom product of the tower is atmospheric residue (i.e., heavy oil). 　　2. Vacuum distillation, also known as reduced-pressure distillation. The boiling points of the heavy fractions in crude oil range from about 370 to 535°C. To distill these fractions at atmospheric pressure, it is necessary to heat the mixture to above 420°C; at such temperatures, the heavy fractions undergo a certain degree of cracking. Therefore, vacuum distillation is usually carried out after atmospheric distillation. At an absolute pressure of about 2–8 kPa, the heavy components are distilled at a temperature at which no significant cracking reaction occurs. Atmospheric residue is heated to about 380–400°C in a vacuum heating furnace and then fed into the vacuum distillation tower. Vacuum distillation can be divided into two categories: lubricating oil type (Figure 3) and fuel oil type. The former requires a high level of precision in the separation of its various fractions, with 24–26 plates ； The latter has low requirements, with 15–17 trays.

It depends on what’s in the material; if reducing pressure can increase their relative volatility, then vacuum distillation is used； If pressure is required to increase the relative volatility, distillation under pressure is used.

An analysis is conducted based on the relationship between the boiling point of the materials and pressure, as well as the properties of their mixture.

Generally, atmospheric distillation can be used for mixtures at normal pressure whose boiling point ranges from room temperature to around 150°C. For mixtures that have a high boiling point at atmospheric pressure or are prone to degradation, polymerization, and other deterioration reactions at higher temperatures—i.e., thermosensitive substances—vacuum distillation is often used to reduce the operating temperature. Reducing the operating temperature can increase the relative volatility to a certain extent, making separation easier.

Whether to use pressurized or vacuum distillation cannot be determined solely based on the boiling point difference of the materials. In my opinion, whether to use pressure or reduced pressure should be determined by the boiling point of the material, rather than the boiling point difference. For example, regardless of the difference in boiling points among different C4 compounds, pressure must be applied; otherwise, condensation at the top of the tower is not possible. If the material at the bottom of the tower has a relatively high boiling point or is heat-sensitive, then vacuum distillation is used for separation in the end.

It mainly depends on the materials involved; if vacuum distillation can **increase the relative volatility of the two substances, then it is better to use vacuum distillation. Regarding the investment issue, it can be considered comprehensively to find the best point.