Rectification to separate benzene-toluene mixture

Contents

Mass exchange processes. Classification and general characteristics

Mass transfer equation

Basics of Mass Transfer in Free Interface Systems

Material balance of mass exchange processes

Molecular diffusion

Mass exchange processes

Mass exchange processes are processes in which the main role is played by the transfer of matter from one phase to another. The driving force of these processes is the difference in chemical potentials. As in any other process, the driving force of mass exchange characterizes the degree of deviation of the system from the state of dynamic equilibrium. Within this phase, the substance is transferred from a point with a higher concentration to a point with a lower concentration. Therefore, usually in engineering calculations, the driving force is expressed approximately through the concentration difference, which greatly simplifies the calculations of mass exchange processes.

Mass exchange processes are widely used in industry:

- to separate liquid and gas homogeneous mixtures,

- for their concentration,

-to protect the natural environment (primarily for wastewater and exhaust gas treatment).

Classification and general characteristics

The following mass exchange processes have become most widespread:

1. Absorption

2. Distillation and rectification

3. Extraction (liquid)

4. Adsorption

5. Ion exchange

6. Drying

7. Dissolution and extraction from solids

8. Crystallization

9. Membrane processes

In all of the above processes, the transition of substance (s) from one phase to another is common.

The process of transferring a substance (or several substances) from one phase to another in the direction of achieving equilibrium is called mass transfer.

The transfer of matter inside the phase - from phase to phase interface or vice versa - from interface to phase - is called mass transfer (by analogy with the process of transfer of heat inside the phase - heat transfer).

Mass transfer processes are usually reversible. Note here that substance transition direction is determined by substance concentrations in phases and equilibrium conditions.

Methods of heat supply to the lower part of the column

Heat must be supplied to the bottom of the column to create a vapor stream. In this case, part of the reflux is evaporated and the vapor flow necessary for rectification is created. The following heat supply methods are most often implemented: in a preheater with a steam space (partial reboiler), in a heat exchanger with the subsequent OI of the heated flow to the bottom of the column (hot jet). Due to the insufficient volume of the lower part of the distillation columns, heat is usually supplied to special remote devices: preheaters with steam space, heat exchangers, tubular furnaces.

Heat supply to heater with steam space. In this case, the liquid entering the preheater is heated to the boiling point of the residue. The resulting D0 vapors are in equilibrium with the reboiler residue W. Thus, this heat supply method is equivalent in separation action to one theoretical plate (partial reboiler).

The mass of the hot jet decreases with increasing temperature.

Hot jet heat supply. This method of supplying heat is used in cases where heating of the residue with conventional heat carriers is not possible or feasible. The heated circulating liquid (hot jet) enters the column. As it enters the column, the circulating stream undergoes an OI process, separating into steam and liquid streams. The reflux from the bottom tray and the circulating flow liquid are mixed and drained to the bottom of the column. From here, part of the stream is withdrawn as residue W and the other part is sent to the preheater. Steam D0 is supplied under the lower tray.

PRESSURE SELECTION IN DISTILLATION COLUMN

The pressure in the distillation column is determined primarily by the heat resistance of the separated products and the possibility of using affordable and cheap cooling agents (water, air) and heat transfer agents (steam). Therefore, the pressure in the column should be higher than atmospheric if the separated substances have low boiling points at atmospheric pressure (for example, hydrocarbon gases), otherwise special refrigerants (ammonia, propane, freons, etc.) would be required for their condensation. The increase in pressure leads to an increase in temperatures in the column, which allows vapor condensation using conventional heat carriers. When rectifying hydrocarbon gases, a pressure of up to 4 MPa is used. In the case of separation of high-boiling products (fuel oil, oil fractions, etc.), it is necessary to reduce the pressure below atmospheric. This makes it possible to separate hydrocarbons having boiling points at atmospheric pressure above 500 ° C at temperatures below 400 ° C without noticeably decomposing them. Typically, a residual pressure of 6.7 kPa or less is used in the separation of high boiling hydrocarbon mixtures.

It should be understood that when the pressure in the column increases, the relative volatility of the components generally decreases, resulting in the need to increase the number of trays in the column or the reflux rate. In general, when selecting the pressure in the column, it is necessary to take into account both the operational and economic indicators of the rectification process. However, if there are no specific process requirements, the operation of the atmospheric distillation column should be preferred.