Q: How is the minimum liquid-to-gas ratio (L/G)min determined?

The minimum L/G ratio occurs when the operating line touches the equilibrium line (pinch point), requiring infinite stages/transfer units. For Henry's law equilibrium (y*=mx), the maximum liquid outlet composition at minimum L/G is x*_max = y_in/m, giving (L/G)min = (y_in - y_out) / (x*_max - x_in). In practice, the actual L/G is set to 1.2 to 1.5 times (L/G)min.

Q: When should a packed column vs. a tray column be used?

Packed columns (random or structured packing) offer low pressure drop, large interfacial area, and suitability for corrosive fluids and small-diameter columns. They are designed using NTU-HTU methods. Tray columns (sieve, valve, bubble cap) offer better flexibility for wide liquid flow ranges, easier cleaning, and more predictable performance. They are designed using theoretical stage methods. Tray columns are generally preferred for large-diameter columns (>1.5 m) or systems with fouling risk.

Question 1

What is the difference between NTU and HTU?

Accepted Answer

NTU (Number of Transfer Units) is a dimensionless measure of the separation difficulty, calculated as NTU = ∫dy/(y-y*). It depends on the inlet and outlet compositions and the equilibrium relationship. HTU (Height of a Transfer Unit) reflects the mass transfer performance of the packing or trays, depending on fluid dynamics and packing type. The total column height is Z = NTU × HTU. Typical HTU values range from 0.3 m for high-performance structured packing to 1.5 m for random packing.

Question 2

What is the absorption factor A and what is the optimal value?

Accepted Answer

The absorption factor A = L/(mG) is the ratio of the operating line slope L/G to the equilibrium line slope m. For A > 1, absorption is feasible (operating line steeper than equilibrium line). For A < 1, complete absorption is impossible. Practically, A = 1.3 to 1.7 is common — higher A reduces NTU but increases liquid rate L and energy cost. An economic optimum is typically around A ≈ 1.4 to 1.6.

Question 3

How is the minimum liquid-to-gas ratio (L/G)min determined?

Accepted Answer

The minimum L/G ratio occurs when the operating line touches the equilibrium line (pinch point), requiring infinite stages/transfer units. For Henry's law equilibrium (y*=mx), the maximum liquid outlet composition at minimum L/G is x*_max = y_in/m, giving (L/G)min = (y_in - y_out) / (x*_max - x_in). In practice, the actual L/G is set to 1.2 to 1.5 times (L/G)min.

Question 4

When should a packed column vs. a tray column be used?

Accepted Answer

Packed columns (random or structured packing) offer low pressure drop, large interfacial area, and suitability for corrosive fluids and small-diameter columns. They are designed using NTU-HTU methods. Tray columns (sieve, valve, bubble cap) offer better flexibility for wide liquid flow ranges, easier cleaning, and more predictable performance. They are designed using theoretical stage methods. Tray columns are generally preferred for large-diameter columns (>1.5 m) or systems with fouling risk.

Absorption Column Design – NTU/HTU Method

Absorption Factor A vs NTU

Design Equations