ChemSOF

Tips & Tricks for Chemical Engineers

McCabe Thiele Diagram

This article shows how to draw McCabe Thiele diagram for doing binary distillation analysis in a spreadsheet. A detailed writeup on mccabe thiele method is provided at Distillation Column Design.

Determine total number of stages and feed stage for separation of a binary mixture with relative volatility (α) 2.5, Feed composition (zF) 0.36, Distillate Composition (xD) 0.915, Bottom composition (xB) 0.05 and feed quality (q) 1.5. Reflux ratio to be 1.5 times the minimum reflux ratio (Rm).


Minimum Reflux Ratio (Rm)

Calculate the intersection of q-Line and equilibrium curve.

 y = qx /(q-1) - zF /(q-1)
y = αx / (1+ x (α - 1))

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Gravity Settling Separation

This article illustrates determination of terminal velocity required for gravity separation. Performing a force balance on liquid droplet settling out of gas stream results in following relationship.

force balance on liquid droplet

where FB is force due to Buoyancy, FD is force due to Drag and FG is force due to gravity.

 FG = m.g
FB = m.ρV.g/ρL
FD = CD.(ρV.Vt²/2).AP

Assuming spherical liquid droplet with diameter DP, above equation gets reduced to

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Specific Heat Ratio of Real Gas

Equation of state is used to derive variety of thermodynamic properties. This article illustrate calculation of specific heat ratio from Peng Robinson Equation of state.

Calculate specific heat ratio ( γ = Cp/Cv ) for methane gas at 11 Bar & 300 °K. Critical constants for Methane are as following

  • Critical temperature, Tc : 190.6°K
  • Critical Pressure, Pc : 46.002 bar
  • Accentric Factor, ω : 0.008
Ideal gas specific heat constants CpIG = A + B.T + C.T² + D.T³ are as following
  • A = 4.5980
  • B = 0.0125
  • C = 2.86 x 10-6
  • D = -2.7 x 10-9
where Cp is in cal/mol-K

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Centrifugal Pump Curve Fitting

Pump manufacturers provide Head versus Flow curve for various pump models. This article shows how to convert it into a 2nd order polynomial using Excel's LINEST function. This curve can then be used to predict pump head curve at diffent impellor diameter and pump speed based on Affinity Laws.

Pump head versus flow curve is available for impellor diameter 210 mm. Estimate modified head vs flow curve at impellor diameter 250 mm.

Flow (m³/h) Head (m)
415.00
614.50
813.75
1012.70
1211.25
149.50

LINEST function in Excel is used to do 2nd order polynomial curve fitting to get constants a0,a1 and a2.

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