Heat and mass transfer are central to many engineering processes, including distillation, drying, absorption, crystallization, reactors, and heat exchangers. In these systems, energy moves because of temperature differences, while chemical species move because of concentration or partial pressure differences. Engineers control these transfers to improve yield, safety, efficiency, and product purity.
A process vessel often has heat flow, diffusion, convection, and reaction happening at the same time.
Understanding Engineering: Heat and Mass Transfer in Process
At the particle level, heat transfer means molecules pass energy through collisions or bulk motion. Faster moving molecules usually belong to warmer material. When they contact slower molecules, energy spreads toward a more even temperature.
Mass transfer is different because particular molecules change location. A perfume spreading through air is a simple example.
In a process plant, the moving substance may be water vapour, oxygen, salt, a solvent, or a valuable product. The transfer rate depends on the available driving difference, the area for transfer, and the difficulty of crossing the path.
The main difficulty often sits close to a surface. Fluid next to a pipe wall or a solid particle moves more slowly than fluid in the main stream. This thin region is called a boundary layer or film.
Heat and molecules must pass through it before reaching the well mixed fluid. Faster stirring makes the film thinner, which usually increases transfer. This is why pumps, agitators, fans, and turbulence are important engineering tools.
They improve transfer rates, but they require energy. Engineers must balance the benefit of better mixing against the cost of moving the fluid.
Phase change makes many processes more interesting. When a liquid evaporates, it needs a large amount of energy to separate its molecules. That energy can cool the remaining liquid.
When vapour condenses, the same energy is released. In distillation equipment, heat causes some components to enter vapour more readily than others. The vapour and liquid then contact each other on trays or packing.
Molecules repeatedly transfer between phases, gradually changing the composition of each stream. In drying, heat supplies evaporation energy while water vapour must be carried away. If humid air stays near the wet surface, drying slows even when the air is hot.
Students should learn to identify the direction of transfer before choosing an equation. Sketch the hot and cold regions, or the high and low concentration regions. Then mark every resistance, such as a metal wall, a stagnant film, fouling deposits, or a membrane.
Fouling is a real problem because scale or dirt can form an extra barrier inside equipment. A heat exchanger may have a large metal area yet perform poorly if its surfaces are coated. Pay attention to units and physical meaning.
Thermal conductivity describes how readily energy passes through a material. Diffusivity describes how quickly species spread.
Transfer coefficients summarize complicated flow near surfaces, so their values change when velocity, geometry, or fluid properties change. These ideas appear in radiators, insulated bottles, cooking, air conditioning, medical oxygen delivery, water treatment, and battery cooling.
Key Facts
- Heat conduction through a flat wall: q = kA(Delta T)/L
- Convective heat transfer: q = hA(T_s - T_fluid)
- Fick's law of diffusion: J_A = -D_AB dC_A/dx
- Mass transfer across a film: N_A = k_cA(C_A,s - C_A,b)
- Overall heat transfer rate: q = UA Delta T_lm
- The heat and mass transfer analogy compares Nu = hL/k with Sh = k_cL/D_AB
Vocabulary
- Driving force
- A driving force is the difference in temperature, concentration, pressure, or chemical potential that causes heat or mass to move.
- Transfer coefficient
- A transfer coefficient measures how easily heat or mass crosses a boundary layer between a surface and a moving fluid.
- Diffusion
- Diffusion is the net movement of molecules from higher concentration to lower concentration due to random molecular motion.
- Convection
- Convection is transfer caused by the bulk motion of a fluid carrying heat, mass, or both.
- Boundary layer
- A boundary layer is the thin region near a surface where velocity, temperature, or concentration changes rapidly.
Common Mistakes to Avoid
- Using the wrong sign for flux is incorrect because heat and diffusion flux point from high potential to low potential, while gradients may be written in the opposite direction.
- Mixing area units is incorrect because transfer rates depend directly on area, so using cm2 in one term and m2 in another changes the answer by large factors.
- Treating transfer coefficients as constants in every situation is wrong because h and k_c depend on flow speed, fluid properties, geometry, and turbulence.
- Confusing total rate with flux is wrong because flux is per unit area, such as W/m2 or mol/(m2 s), while total rate includes the transfer area.
Practice Questions
- 1 A heat exchanger wall has k = 16 W/(m K), area A = 2.0 m2, thickness L = 0.010 m, and a temperature difference of 40 K. Calculate the conductive heat transfer rate q.
- 2 A gas species diffuses across a stagnant film with D_AB = 2.0 x 10^-5 m2/s and concentration changes from 0.80 mol/m3 to 0.20 mol/m3 across 0.0030 m. Estimate the molar diffusive flux magnitude using J_A = D_AB(Delta C)/L.
- 3 In a reactor with a hot wall and a membrane that removes product, explain how increasing fluid velocity can affect both heat transfer and mass transfer, and why the two effects are often similar.