Equipment

Batch distillation equipment can be custom-made to meet particular design specifications or be directly purchased by catalogue selection if no stringent construction features or materials are required. Ordering can be greatly facilitated by a previous search of the manufacturers or suppliers in the worldwide web. Equipment intended to be used for research or educational purposes should be made of glass whenever possible, given the easy observation of the internal flow regimes and their change with the internal flow rates. An existing batch glass column is described here as an example.

Figure 3 shows a distillation column which has a simple conceptual design but is versatile enough to be used for research or teaching applications. The column is atmospheric and functions as the rectifying section of a regular distillation column. The feed drum doubles as a kettle vessel where the feed is vaporized by a coil heater having steam as the heating medium. Instrumentation is reduced to the essentials: the distillate and reflux flow rates are controlled by varying the rotation speed of the distillate and reflex pumps, and the feed flow rate can be controlled by varying the steam pressure in the coil. The safety system consists of a relay actuated by the occurrence of any of the failure conditions in the column, which are pressurization within the equipment, zero flow of condenser cooling water or loss of power to the ventilation system. When any of these conditions occurs, steam admission to the feed drum is switched off.

The pumps are actuated in the remote mode by a driver board that receives signals in the range between 4 and 20 mA from an analog output board installed in a PC. The connection between the pump driver board and the analog output board consists of a screw terminal connector. The variation of the output signal to the pumps is accomplished via

Figure 3 Rectifying batch distillation column. The abbreviations are as follows: A/D board = analog to digital interface board, CW = cooling water, D. Pump = distillate pump, F. Pump = feed pump, FIC = flow indicator control, P. chiller = product chiller, P. Tank = product tank, PI = pressure indicator, PIC = pressure indicator control, R. Disk = rupture disk, S. Valve = safety valve and TI = temperature indicator.

Figure 3 Rectifying batch distillation column. The abbreviations are as follows: A/D board = analog to digital interface board, CW = cooling water, D. Pump = distillate pump, F. Pump = feed pump, FIC = flow indicator control, P. chiller = product chiller, P. Tank = product tank, PI = pressure indicator, PIC = pressure indicator control, R. Disk = rupture disk, S. Valve = safety valve and TI = temperature indicator.

a software utility provided with the board that emulates a control panel, where each of the instruments hooked to the board is assigned a channel number displayed in the panel screen. The user varies the pump flow rate by changing the output current at the computer screen. Manual local control of each pump is also provided in case of failure of the computer-interfaced control. The stepper motor of the steam valve for the feed drum steam coil is also interfaced in a comparable manner.

The column is also provided with thermocouples for each stage, including condenser and reboiler. The thermocouples are wired to a screw terminal connector, that provides the interface to an analog/digital I/O board installed in the PC. When there is a significant difference between the boiling points of the two components of a binary system, the stage temperature is an efficient and straightforward way of evaluating compositions. Under these circumstances, the realtime composition profile within the column can be updated to the computer screen. Sample ports for the liquid phase are installed in every stage, including the condenser and reboiler to corroborate thermal measurements under circumstances where thermal gradients are not sufficiently steep to provide accurate composition correlation. The composition analysis can be performed by a variety of methods. If there is a significant density difference between components being separated, composition can be deduced from a density-concentration curve. Density can be deter mined gravimetrically, or equipment is available for online density measurement. A particularly versatile online implementation of density measurement is in the condensate stream of an off-set condenser as described in more detail below. If one of the components is an organic acid, sample analysis can be carried out either by titration or by the measurement of any other property related to the dissociation state (such as pH), provided the metering apparatus is sufficiently accurate to discriminate stage-to-stage differences. For organic mixtures, other properties such as refractive index may also be used as analytical method.

The feed drum in the described pilot-scale column is a large glass bulb equipped with a steam heating coil to vaporize the feed. A pressure relief rupture disk provides a mechanical fail-safe against reboiler pressurization. Another pressure gauge is installed at the steam inlet to the coil. The steam outlet is provided with a trap to ensure the total condensation of the steam, and therefore the use of its latent heat. A useful energy balance is achieved by cooling the condensate as it is discharged to the drain. The heat load to the column can be crudely calculated by measuring the discharge flow rate and multiplying this valve by the heat of vaporization of the steam at the inlet pressure.

The distillate is collected in a separate vessel whose volume equals approximately half that of the feed drum. The product collection vessel is fitted to permit charging of material to the feed drum. The remaining material in the drum after a batch processing can be discharged by a valve in the bottom. The subsequent batch charge can also be combined with the remaining heavy ends of the previous operation.

The off-set condenser depicted in Figure 3 provides for direct measurement of condensate flow rate. This eliminates the need to calculate condensate from the condenser energy balance. This is particularly important for pilot-scale units where complete condensation may not be achieved at high boil-up rates. The condensed top vapours drip down and accumulate in the bottom part of the vessel, from which they are removed either to the distillate tank or back to the column as reflux. A match between the condensation rate and sum of product and liquid flow rate returned to the column can be assured by visual monitoring of the condenser liquid level or computer monitoring of the liquid head in the bottom of the condenser with a pressure transducer. The gas entrance to the condenser doubles as a liquid overflow in the event of excessive condensate accumulation.

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Solar Panel Basics

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