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Thermal Separation Process

◈ Distillation

Distillation

Distillation is a unit operation separating physically homogenuous liquid mixture consisted of more than two components into a single component or highly purified target component by intensive mixing of 2 phases inducing mass and heat transfer with subsequent evaporating and condensing enhanced by different volatilities in each stage of the multi-stage column.

Distillation in general provides the most competitive solution to the industries for separating the liquids to it's consisted components except some restrictions stipulated below.

The difference of volatility between the components is small.
A small quantity of low volatile component is to be recovered from the feed that distillation requires the whole feed should be vaporized to recover this small quantity.
A component is termally unstable even under vacuum atmosphere.
The mixture is extremely corrosive or highly fouling.

At the basic design stage, the type of internals should be decided whether tray or packings. We are using slit tray or channel tray for aqueous mixture and structured packing for hydrocarbon mixtures normally because of it's surface tension, however, this may not be a absolute rule that is to be dependant on the ration of vapor to liquid load and pressure drop availabe in case heat regeneration of overhead vapor is planned by mechnical vapor recompression.

Fixed valve tray column

We use fixed valve tray which is a further development of the standard sieve tray whose simple holes have been replaced by slit valves to allow horizontal vapor flow through the tray.

Liquid droplets brought by the vapor flow to flat trajectory that reduces entrainment remarkably and shorten stage distance which lowers the total column height consequently. Intensive mixing of vapor and liquid phase as well as long contact times contributes to higher stage efficiency. Right photo shows the slit tray column rectifying ethanol up to 94.5 wt% installed in the year 2005.

Structured packing column

The table below shows ranges of typical data at atmospheric pressure test with different packing densities.

Descriptions	150 ㎡/㎥	230 ㎡/㎥	320 ㎡/㎥	450 ㎡/㎥
Theoretical stages/m	2	3.5	5	7
F-factor	0.7∼2.5	1∼3	0.9∼2.3	0.8∼1.8
Presure drop, mbar/m	0.13∼6	0.35∼5	0.6∼3..5	0.7∼4.5

The left photo shows acetic acid and acetic anhydride separtion column using 320 ㎡/㎥ density structured packing.

Feed : 55% acetic acid + 45% acetic anhydride
Rectified distillate : acetic acid 〉 99%
Stripped residue : acetic anhydride 〉 99%
Column diameter : 500 mm
Rectification section : 4,160 mm stage height
Stripping section : 3,120 mm stage height

Packed columns are extremely flexible as far as gas load is concerned, while tray column requires a minimum gas load to prevent weeping, low liquid load of 0.2∼50 ㎥/㎡h is required for packed column.

Another advantage to use packed column is low pressure drop typically 0.5 mbar per an equlibrium satge which ranges one order of magnitude than in tray column. This will hint us to apply overhead vapor compression to reuse for reboiler heat source.

Up to diameter of 800 mm is usually installed as a circular pad. For larger diameters up to 5 m, the packing elements are devided into segments for introduction through manholes and subsequent packing elements are rotated by 90°.

Distillation column overhead vapor compression to regenerate heat for reboiler

The overhead vapor leaving column top contains enthalpy close to the heat duty of reboiler that is being condensed by cooling water in the condenser.

In order to utilize the latent heat of overhead vapor, it should be compressed to the elevate temperature high enough to reboil the bottom product. After releasing the latent, the condensed distillate shall be cooled down to the column top temperature to reflux and discharge.

For temperature elevation, multi-stage turbo fan as MVR compress the overhead vapor adiabatically for the temperature rise of 30∼50℃ depending on Kappa value (Cp/Cv) of the vapor.

It is generally understood that the multi-compoents overhead vapor may not be suitable for this application, because their different vapor pressure requires a wide span of condensing temperature.