Chapter 6 · MCQ Read Mode

Mass Transfer

Fick's first law of diffusion states that the diffusive flux is proportional to

The order of magnitude of the diffusion coefficient for a gas at ordinary temperature and pressure is

The diffusivity of a gas varies with absolute temperature and total pressure approximately as

For diffusion of A through stagnant non-diffusing B, compared with equimolar counter-diffusion under the same conditions, the flux of A is

In equimolar counter-diffusion of A and B through a gas film, the partial pressure profile of A is

The Wilke-Chang correlation is used to estimate the diffusivity of

The effective diffusivity in a porous solid is related to the free-fluid diffusivity by

Knudsen diffusion becomes important when

The Knudsen diffusion coefficient is

Fick's second law of diffusion is mathematically identical in form to

The Stefan tube or Arnold diffusion cell is used to measure

The relation between the total flux Nₐ and the diffusive flux Jₐ is

For a binary gas mixture, the relation between the two diffusivities is

The Schmidt number is the mass transfer analogue of the

The Sherwood number is defined as

The Schmidt number for most gases is of the order of

According to the penetration and surface renewal theories, the mass transfer coefficient is proportional to

The Chilton-Colburn analogy states that

The Reynolds analogy is most nearly valid when

The analogies between momentum, heat and mass transfer fail when

For the air-water system, the wet bulb temperature and the adiabatic saturation temperature are nearly equal because

Interphase mass transfer ceases when

According to two-film theory, the overall gas-phase resistance is given by

The absorption of a highly soluble gas such as ammonia in water is

The absorption of a sparingly soluble gas such as oxygen in water is

An ideal or theoretical stage is defined as one in which

Separation of a binary mixture by ordinary distillation is impossible when the relative volatility is

An azeotrope is a mixture in which

In the McCabe-Thiele method, the assumption of constant molal overflow requires that

The slope of the rectifying section operating line is

For a feed entering a distillation column as a saturated liquid at its bubble point, the q-line is

Operating a distillation column at total reflux gives

At the minimum reflux ratio, the number of theoretical stages required is

Increasing the reflux ratio in a distillation column

The Fenske equation is used to calculate

Steam distillation is particularly useful for separating

In extractive distillation, the added solvent

In liquid-liquid extraction, the solvent-rich product stream is called the

On a ternary phase diagram for liquid-liquid extraction, the point at which the tie line length reduces to zero is the

The solubility of a gas in a liquid

A large value of the Henry's law constant for a gas indicates that the gas is

Flooding in a packed absorption column occurs when

In a packed column, liquid redistributors are installed at intervals in order to

For an absorption operation, the operating line lies

At the minimum liquid-to-gas ratio in an absorber

The number of transfer units (NTU) in a packed absorption column depends on

In packed column design, the mass transfer coefficient and the interfacial area are combined as a single quantity Ka because

The absorption factor A is defined as

The Langmuir adsorption isotherm assumes

A favourable adsorption isotherm is one which is

Breakthrough in a fixed-bed adsorber occurs when

The distinguishing feature of a molecular sieve compared with other adsorbents is its

The driving force for crystallization is

To produce large crystals, a crystallizer should be operated

Crystallization by evaporation rather than by cooling is used when the solute's solubility

Secondary nucleation in an industrial crystallizer is caused principally by

The delta L law of crystal growth states that

In the calculation of crystal yield, a common and serious error is to neglect

The equilibrium moisture content of a solid is

During the constant-rate period of drying, the rate is controlled by

During the constant-rate period, the temperature of the wet surface is approximately

The critical moisture content of a solid

In the second falling-rate period of drying, the rate is governed by

Case hardening during drying occurs when

The time required for the falling-rate period of drying, assuming the rate is proportional to the free moisture content, varies

In a bubble column, the dispersed phase is

A packed tower is preferred to a tray tower when

A tray tower is analysed in terms of theoretical stages, whereas a packed tower is analysed in terms of

Weeping in a tray column occurs when

Which tray type offers the best turndown ratio?

The diameter of a tray column is determined principally by

The overall efficiency of a distillation column is defined as

Spray drying is suitable for heat-sensitive materials despite the high inlet gas temperature because temperature

Freeze drying removes moisture by

The lowest temperature to which water can theoretically be cooled in a cooling tower is

The range of a cooling tower is defined as

In a cooling tower, blowdown is carried out in order to

Most of the cooling achieved in a cooling tower is due to