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What Is Number of Transfer Units (NTU)? | Process Engineering Glossary
What Is Number of Transfer Units (NTU)?
In piping engineering and process engineering, the number of transfer units (NTU) is a dimensionless measure of the difficulty of a mass transfer separation. It quantifies how far a separation departs from the starting composition relative to the available driving force throughout the contacting device. A higher NTU value means the separation is more difficult, requiring more equilibrium stages, more packing height, or more contact time to achieve the required outlet specification. NTU is always a property of the separation duty, not of the equipment. The equipment is characterised separately by the height of a transfer unit (HTU), and their product gives the required packed height or equivalent contact length.
Applications of NTU
Amine Gas Treating Unit Design
Amine absorption units remove hydrogen sulphide and carbon dioxide from sour natural gas. The NTU for each acid gas component depends on the inlet partial pressure, the required outlet specification, the lean amine loading, and the temperature-dependent equilibrium relationship between the acid gas and the amine solution. The NTU for hydrogen sulphide removal is typically higher than for carbon dioxide at the same column temperature because hydrogen sulphide has a higher affinity for the amine and the equilibrium line is steeper, producing a larger driving force at a given amine rate. The total column height is the sum of the packed heights for each component’s NTU.
Air Stripping for Groundwater Remediation
Air stripping columns remove volatile organic contaminants from groundwater by transferring them into an upward-flowing air stream. The NTU depends on the inlet groundwater concentration, the required outlet standard, and the Henry’s Law constant for the contaminant at the water temperature. Contaminants with high Henry’s Law constants have favourable equilibrium that produces a large driving force and a low NTU, requiring shorter columns. Contaminants with low Henry’s Law constants require higher air-to-water ratios or taller columns to achieve the required treatment standard.
Wet Scrubber Sizing
Wet scrubbers for acid gas removal or odour control use the NTU framework to determine the required packing height for the specified outlet gas quality. The NTU for the scrubber depends on the inlet contaminant concentration, the permitted outlet concentration, the scrubbing liquid flow rate, and the solubility of the contaminant in the scrubbing liquid. For highly soluble gases such as hydrogen chloride and ammonia, the NTU is typically low because the equilibrium strongly favours dissolution into the liquid and the driving force is large throughout the column. For sparingly soluble gases, the NTU is much higher and a reactive scrubbing liquid that chemically converts the dissolved gas may be required to achieve a practical column height.
Benefits of the NTU Framework
Separation of Duty from Equipment
The NTU-HTU framework separates the separation duty calculation from the equipment design calculation. The NTU is fixed by the process requirement and is independent of the equipment type or packing selected. The HTU is determined by the equipment design and the operating conditions. This separation allows the process engineer to calculate the separation difficulty from the process specification before the equipment vendor is engaged, providing a clear and unambiguous basis for specifying the required column performance.
Sensitivity Analysis
Calculating NTU as a function of the operating conditions, solvent rate, and equilibrium relationship allows the engineer to identify which variables have the greatest influence on the separation difficulty. A sensitivity analysis that shows how NTU changes with a five percent change in solvent flow rate, a two degree Celsius change in column temperature, or a ten percent change in the equilibrium constant gives the process operator a clear understanding of how the column performance will change when operating conditions vary from the design point.
Troubleshooting Column Performance
The NTU framework provides a structured approach to troubleshooting an underperforming packed column. The required NTU is calculated from the actual measured inlet and outlet compositions and compared against the available NTU from the installed packing height divided by the current HTU. If the available NTU has decreased relative to the design value, the cause is either increased HTU from fouled or damaged packing, or the packing height has been reduced by settling or mechanical damage. If the required NTU has increased, the cause is a change in the inlet composition, the equilibrium relationship, or the operating temperature.
Limitations to Consider
Applicability to Dilute Systems
The analytical NTU formula using the logarithmic mean driving force applies only when both the equilibrium curve and the operating line are straight, which requires dilute concentrations and linear equilibrium. For concentrated systems, curved equilibrium lines, or non-ideal phase behaviour, graphical or numerical integration is essential. Applying the dilute-system formula to a concentrated system can produce significant errors in the calculated NTU and therefore in the required column height.
Equilibrium Data Accuracy
The NTU calculation depends critically on accurate phase equilibrium data, particularly the shape and position of the equilibrium curve relative to the operating line. Small errors in the equilibrium data, especially near the pinch region where the two lines are close together, produce large errors in the NTU because the driving force in this region is small and the NTU integral is sensitive to the exact spacing between the lines. Reliable experimental equilibrium data for the specific fluid system at the operating conditions is more important than sophisticated numerical integration methods for accurate NTU calculation.
Mass Transfer Resistance
The overall NTU based on gas-phase driving forces assumes that the gas-film mass transfer resistance controls the rate of transfer. When the liquid film resistance is significant, the overall driving force must be expressed using overall liquid-phase units and the NTU_OL must be calculated instead. Mixing the gas-phase and liquid-phase NTU frameworks without correctly accounting for the controlling resistance produces incorrect column sizing. The two-film theory analysis of the mass transfer resistances on each side of the interface determines which phase controls and which NTU framework to apply.
NTU FAQ
What is the number of transfer units in process engineering? The number of transfer units is a dimensionless measure of the difficulty of a mass transfer separation in a packed column or other continuous contacting device. It equals the integral of the reciprocal driving force over the composition change from inlet to outlet. Process engineering uses it together with the height of a transfer unit, HTU, to calculate the packed height required for a given separation: packed height equals HTU multiplied by NTU. The NTU is determined by the separation duty, and the HTU is determined by the equipment design and operating conditions.
How does NTU relate to absorption column design and solvent selection? In an absorption column the NTU depends on the inlet and outlet gas compositions, the liquid-to-gas flow ratio, and the equilibrium relationship between the gas and liquid phases, which is described by Henry’s Law for dilute systems and by more complex models for concentrated or reactive systems. Increasing the solvent flow rate moves the operating line away from the equilibrium curve, increases the driving force, and reduces the required NTU. Selecting a solvent with a lower equilibrium constant, meaning greater affinity for the target component, produces a more favourable equilibrium relationship and further reduces the required NTU. The lean/rich loading of the solvent in amine gas treating units directly determines the position of the operating line and therefore the NTU for hydrogen sulphide and carbon dioxide removal.
How is NTU used in distillation and gas scrubber design? In packed distillation columns, the NTU of the rectifying and stripping sections is calculated from the vapour-liquid equilibrium curve and the respective operating lines. The sum of the two section heights gives the total column height. In gas scrubbers for emission control, the NTU is calculated from the inlet contaminant concentration, the permitted outlet concentration, and the concentration gradient driving force throughout the packed bed. Highly soluble gases such as hydrogen chloride produce low NTU requirements, allowing short scrubbers. Sparingly soluble gases require either very large air-to-liquid ratios to maintain driving force or reactive scrubbing liquids that convert the dissolved gas to a non-volatile species, effectively driving the equilibrium constant to near zero and eliminating the concentration gradient limitation.
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