Make a preliminary design for a vertical thermosyphon for a column distilling crude aniline. The column will operate at atmospheric pressure and a vaporisation rate of 6000 kg/h is required. Steam is available at 22 bar (300 psig). Take the column bottom pressure as 1.2 bar.

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Physical properties, taken as those of aniline:

Boiling point at 1.2 bar [latex]190^{\circ} C[/latex]

Molecular weight 93.13

[latex]T_{c}[/latex] 699 K

Latent heat 42,000 kJ/kmol

Steam saturation temperature [latex]217^{\circ} C[/latex].

[latex] ext { Mean overall } \Delta T=(217-190)=27^{\circ} C ext {. }[/latex]

[latex] ext { Reduced temperature, } T_{r}=\frac{(190+273)}{699}=0.66[/latex]

[latex] ext { From Figure } 12.59 ext {, design heat flux }=25,000 W / m ^{2}[/latex]

[latex] ext { Heat load }=\frac{6000}{3600} imes \frac{42,000}{93.13}=751 kW[/latex]

[latex] ext { Area required }=\frac{751 imes 10^{3}}{25,000}=30 m ^{2}[/latex]

Use 25 mm i.d., 30 mm o.d., 2.44 m long tubes.

[latex] ext { Area of one tube }=\pi 25 imes 10^{-3} imes 2.44=0.192 m ^{2}[/latex]

[latex] ext { Number of tubes }=\frac{30}{0.192}=157[/latex]

Approximate diameter of bundle, for 1.25 square pitch

[latex]D_{b}=30\left[\frac{157}{0.215} ight]^{1 / 2.207}=595 mm[/latex] (12.3b)

A fixed tube sheet will be used for a vertical thermosyphon reboiler. From Figure 12.10, shell diametrical clearance = 14 mm,

Shell inside dia. = 595 + 14 = 609 mm

Outlet pipe diameter; take area as equal to total tube cross-sectional area

[latex]=157\left(25 imes 10^{-3} ight)^{2} \frac{\pi}{4}=0.077 m ^{2}[/latex]

[latex] ext { Pipe diameter }=\sqrt{\frac{0.077 imes 4}{\pi}}=0.31 m[/latex]

Question 12.10: Make a preliminary design for a vertical thermosyphon for a ...

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