Tips & Tricks for Chemical Engineers

Linear Programming in Excel

Linear Programming is a widely used mathematical technique designed to help managers and engineers in planning and decision making relative to resource allocation. Linear programming (LP) model essentially consists of 3 components.

  • Decision Variables
  • Objective function
  • Constraints

This article shows how to develop LP model in Excel.

Multipurpose plant is used to manufacture three products A, B and C from raw materials Y and Z. Amount of raw materials required, batch times and profits per batch are shown as below. Determine number of batches to be processed every week for each product to maximize the profit. Plant is operating for 150 hours per week. Raw material available per week is Y : 216 units and Z : 200 units.

Linear Programming example multipurpose plant

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Sizing Expansion Tank for Hot Oil System

Expansion tank is provided in hot oil system to accomodate for the increased thermic fluid volume due to thermal expansion. It also helps in removal of moisture and non-condensables during start-up. This article will show how to do sizing of an expansion tank.

Sizing Expansion tank for hot oil system

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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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