Methanol-Water Distillation Column Design

Introduction

Rectification of binary systems is the process of separating solutions into one or two practically pure components by multiple two-way mass exchange between counter-current moving vapors and liquid carried out in a rectification column.

Although in industrial practice there are relatively rare cases of separation of two-component systems, the theory of rectification of binary mixtures is of great cognitive importance, because it allows us to clearly identify the techniques and methods of studying the process taking place in the rectification apparatus and present the results of such a study in very clear calculation diagrams. Using graphical techniques, it is possible to most simply present the principles of calculating the various operating modes of the column, which are common and equally applied to more difficult cases when the separation is no longer a binary, but a much more complex multi-component system.

The reaction of phases during rectification is the diffusion of a low boiling component from liquid to steam and a high boiling component from steam to liquid, due to the difference in concentrations of components in mass-exchanging flows of vapors and liquid. The main condition for the effective exchange of substance between phases is their most intense contact.

The distillation column trays, which may be of a wide variety of design, interact intensively between the ascending steam and descending liquid streams. In the extreme case of the tray operation, the energy exchange between the contacting vapors and the liquid leads to the equalization of their temperatures, as a result of the exchange of substance, the equilibrium values ​ ​ of the phase compositions are established, and the process of their interaction ceases, since the vapor-liquid system enters an equilibrium state. The vapors and liquid are separated from each other and the process continues due to the new contact of these phases already at the next stage with other liquid and steam streams.

In the thermodynamic theory of mass exchange separation processes during the transition from phase compositions in one interstage compartment to phase compositions in the adjacent quantitative basis, the hypothesis of the theoretical plate (stage) is taken as a quantitative basis. The peculiarity of this theory is that it does not deal with the issue of the mechanism of the process and does not investigate the diffusion nature and kinetic picture of the mass transfer phenomenon at the contact stage.

The theory of mass exchange separation processes, based on the concept of a theoretical plate (stage), studies the limit conditions for the process and establishes standards with which you can get a correct judgment about the operation of the practical apparatus and the degree of its deviation from the sample most perfect in these conditions.

Currently, distillation and rectification are widespread in chemical technology and are used to obtain a variety of products in pure form, as well as to separate gas mixtures after their liquefaction (separation of air into oxygen and nitrogen, separation of hydrocarbon gases, etc.).

Substantiation of design selection of the main units of the apparatus

Mainly poppet columns are common, but the designs and types of plates in them are diverse and often quite complex. Advantageously, the use of poppet columns in distillation processes is due to their significantly higher productivity compared to packing columns. Thus, the experimental data show that the capacity of the packing column with a diameter of 1.5 m is equal to the capacity of the poppet column having a diameter of 1 m, and this diameter difference does not compensate even that the height of the poppet column increases.

When choosing the type of distillation column for the designed separation, it should be borne in mind that the plate columns of very small diameter are much more expensive than the corresponding packing columns, but as the diameter increases, the cost of packing columns increases much faster; for an approximate rough estimate, it can be considered that the cost of the packing column increases in proportion to the square of the diameter, and the cap - to the diameter in the first degree. Therefore, beyond some boundary diameter value, the use of poppet columns should be more economical.

The long experience of industrial operation of packed columns has shown the expediency of their use at diameters of not more than 0.8 m. With a further increase in the diameter of the packed column, the uniform distribution of reflux over the packed column deteriorates, channels are formed along which reflux predominantly rushes, and the efficiency of the column sharply decreases. Packing columns have insufficient flexibility in operation, expressed in the need for relatively large reflux numbers; in addition, it is difficult to maintain a stable operating mode in them.

Bubbling phenomenon in column consists in contact of vapors and reflux by passing ascending steam flow through layer of liquid moving along tray.

Cap poppet columns are most common in practice, although recently they have gained the advantage of sieve, valve, flake and other more effective types of bubbling devices, the main purpose of which is the maximum development of the surface of interphase contact, which contributes to the intensification of mass exchange between vapors and reflux. In addition, the choice of the type of contact device is also determined by factors such as material savings, cost, ease of manufacture, cleaning and repair, corrosion resistance, low pressure drop during the passage of vapors, and the breadth of the range of stable operation of the tray.

In our case, a column with screened trays is most suitable. It is a vertical cylindrical body with horizontal trays in which a significant number of holes with a diameter of 1-5 mm are drilled uniformly over the entire surface. To drain liquid and adjust its level on the tray, overflow tubes are used, the lower ends of which are immersed in cups.

The gas passes through the holes of the tray and is distributed in the liquid in the form of fine jets and bubbles. At too low a gas velocity, the liquid can leak (or "fall") through the holes of the tray to the downstream, which should lead to a significant decrease in the intensity of mass transfer. Therefore, the gas must move at a certain speed and have a pressure sufficient to overcome the pressure of the liquid layer on the tray and prevent the liquid from flowing through the holes of the tray. Sieve trays are characterized by the simplicity of the device, ease of installation, inspection and repair. The hydraulic resistance of these trays is small. The sieve trays operate stably in a fairly wide range of gas velocities, and in a certain range of gas and liquid loads these trays have high efficiency. At the same time, sieve trays are sensitive to contaminants and precipitates that clog the holes of the trays. In case of sudden cessation of gas supply or significant reduction of its pressure, all liquid is drained from the sieve trays, and in order to resume the process it is necessary to start the column again.

Despite these disadvantages, the sieve trays are more suitable for separating a given mixture.

Conclusion

This course project describes the basics of theory and calculation of vapor-liquid equilibrium in systems of real solutions, elements of the doctrine of inter-phase mass transfer, namely thermodynamic theory and methods for calculating rectification of binary systems.

A continuous poppet distillation column with a capacity of 4000 kg/h was designed to separate the liquid mixture of methanol and water. Content of light-volatile component for this mixture: in the initial mixture 30% (wt.), in distillate 96% (wt.), in the bottom residue 3.5% (wt.). During the calculations, the following main dimensions of the rectification column were obtained:

diameter - 1000 mm;

height of the poppet part of the column - 5.7 m

full height of the column - 8.7 m

According to this data, the rectification column drawing is made.

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