Study of the Rectification Process in a Column with a Perforated Spiral-Shaped Contact Element

STUDY OF THE RECTIFICATION PROCESS IN A COLUMN WITH A PERFORATED SPIRAL-SHAPED CONTACT ELEMENT

Communication 1

V. I. KOSINTSEV, V. P. PISHCHULIN, G. M. IZMAILOV

(Presented by the scientific seminar of the Department of Processes, Apparatuses, and Cybernetics of Chemical Production)

The development and study of high-capacity and efficient mass-transfer equipment for gas (vapor)–liquid systems is one of the principal directions of contemporary scientific research in the field of mass transfer. In order to intensify mass-transfer processes, a whole series of designs of vortex contact elements for columns has recently been developed [1].

At the Department of Processes and Apparatuses of Chemical Production of the Chemical-Technological Faculty of TPI, for the intensification of a number of absorption and rectification processes, a contact element has been applied that is made in the form of a continuous single-pass spiral of mesh tape, developed by Yu. I. Sobchuk, N. G. Elukhen, and L. S. Akselrod [2] and partially improved by the authors. The apparatus for carrying out absorption and rectification processes is a column and consists of a shell and an auger-like element made of spiral mesh tape, the inner end of which is fastened to a rod centered in the apparatus shell. Between two adjacent turns of the spiral along its entire length, at certain intervals, partitions made of gas-impermeable material are installed for the movement of the gas phase through the openings in the contact element and to prevent its movement through the inter-turn space. Each partition is attached to the plane of the upper turn in such a way that a gap remains between the lower edge of the partition and the lower turn of the spiral for the movement of the liquid phase along the spiral and for equalizing the thickness of the liquid layer. The liquid (absorbent in absorption or reflux in rectification) enters the column onto the upper turn of the mesh spiral and, under the action of gravity, flows downward in a thin layer along the spiral-shaped contact element. The gas or vapor phase rises from bottom to top through the column, successively bubbling through the liquid layers on each turn of the spiral; in this process, intensive heat and mass transfer occurs. In the laboratory of the Department of Processes, Apparatuses, and Cybernetics of Chemical Production, tests were carried out on a model of the contact element with an outer turn diameter of 56 mm, a screw-spiral pitch of 40 mm, and a free cross section of openings 2.5 mm in diameter of 14%.

This communication presents the results of tests of a laboratory model of an installation with a perforated spiral-shaped contact element.

Investigations of the hydrodynamics of the process in the contact element were carried out for the water—air system in the range of gas velocities in the free cross section of \(2.5 \cdot 10^{-2}\ \text{m/sec}\) to \(14 \cdot 10^{-2}\ \text{m/sec}\), at irrigation densities from 0.26 to \(0.45\ \text{g}/\text{cm}^2 \cdot \text{sec}\). Similar studies for comparison were conducted on sieve trays with an unorganized overflow of the liquid phase, the free area of the openings of diameter 2.5 mm also amounting to 14%.

As the investigations showed, at irrigation densities of \(0.26 \div 0.45\ \text{g}/\text{cm}^2 \cdot \text{sec}\), a stable hydrodynamic operating regime of the column was observed at gas velocities in the free cross section from \(4 \cdot 10^{-2}\) to \(8 \cdot 10^{-2}\ \text{m/sec}\). The pressure drop in this case was, respectively, 15–20 mm of water column.

It was established that, under identical experimental conditions, the hydraulic resistance of the column with the perforated spiral-shaped contact element is 1.5–2 times lower than that of the column with a flat sieve tray.

The choice of the gap between the surface of the turn and the lower edge of the radial partitions had a large effect on the nature of liquid movement through the column. It was established that, in order to equalize the thickness of the liquid layer over the cross section of the turn and to prevent breakthrough of the gas phase into the inter-turn space, the height of the gap should be about \(1/3\) of the thickness of the liquid layer on the turn.

The rectification process was investigated for the ethyl alcohol—water system with an alcohol concentration in the initial mixture of 8–12%.

The results of the study showed that the perforated spiral-shaped contact element, in comparison with sieve trays, provides, all other conditions being equal, a higher alcohol concentration in the distillate (by 2%) and the most stable operation under fluctuations of the vapor- and liquid-phase loads.

Thus, for example, for a column with sieve trays and an unorganized overflow of the liquid phase, the stable regime of the column corresponded to irrigation densities of \(0.07 \div 0.17\ \text{g}/\text{cm}^2 \cdot \text{sec}\), while for a column with a perforated spiral-shaped contact element it corresponded to \(0.26 \div 0.45\ \text{g}/\text{cm}^2 \cdot \text{sec}\), in the range of vapor velocities in the free cross section for both columns of \(4 \cdot 10^{-2} \div 8 \cdot 10^{-2}\ \text{m/sec}\).

Conclusions

1. Tests have been carried out on a column apparatus with a perforated spiral-shaped contact element for the rectification process.

2. It has been established that the perforated spiral-shaped contact element reduces the hydraulic resistance of the column by 1.5–2 times in comparison with sieve trays, all other conditions being equal, increases the rate of the mass-transfer process, and ensures a stable operating regime over a broader range of liquid- and vapor-phase loads.

Literature

1. V. N. Stabnikov. Calculation and Design of Contact Devices of Rectification and Absorption Apparatuses. Kiev, Tekhnika Publishing House, 1970.

2. Yu. I. Sobchuk, N. G. Elukhen, L. S. Akselrod. USSR Author’s Certificate, class 12a, 5, No. 85573, 1950.