Molecular Distillation Mass Transfer Operations

Molecular distillation is vacuum distillation below the pressure of 0.01 torr. 0.01 torr is one order of magnitude above high vacuum, where fluids are in the free molecular flow regime, i.e. the mean free path of molecules is comparable to the size of the equipment. The gaseous phase no longer exerts significant pressure on the substance to be evaporated, and consequently, rate of evaporation no longer depends on pressure. That is, because the continuum assumptions of fluid dynamics no longer apply, mass transport is governed by molecular dynamics rather than fluid dynamics. Thus, a short path between the hot surface and the cold surface is necessary, typically by suspending a hot plate covered with a film of feed next to a cold plate with a clear line of sight in between. Molecular distillation is used industrially for purification of oils.

Vacuum Distillation Mass Transfer Operations

4.Vacuum Distillation

Some compounds have very high boiling points. To boil such compounds, it is often better to lower the pressure at which such compounds are boiled instead of increasing the temperature. Once the pressure is lowered to the vapor pressure of the compound (at the given temperature), boiling and the rest of the distillation process can commence. This technique is referred to as vacuum distillation and it is commonly found in the laboratory in the form of the rotary evaporator.

This technique is also very useful for compounds which boil beyond their decomposition temperature at atmospheric pressure and which would therefore be decomposed by any attempt to boil them under atmospheric pressure.

Steam Distillation Mass Transfer Operations,

3.Steam distillation

steam distillation is a method for distilling compounds which are heat-sensitive. This process involves using bubbling steam through a heated mixture of the raw material. By Raoult's law, some of the target compound will vaporize (in accordance with its partial pressure). The vapor mixture is cooled and condensed, usually yielding a layer of oil and a layer of water.


Steam distillation of various aromatic herbs and flowers can result in two products; an essential oil as well as a watery herbal distillate. The essential oils are often used in perfumery and aromatherapy while the watery distillates have many applications in aromatherapy, food processing and skin ca

Fractional distillation Mass Transfer Operations

2.Fractional Distillation

 Therefore, fractional distillation must be used in order to separate the components well by repeated vaporization-condensation cycles within a packed fractionating column. This separation, by successive distillations, is also referred to as rectification

As the solution to be purified is heated, its vapors rise to the fractionating column. As it rises, it cools, condensing on the condenser walls and the surfaces of the packing material. Here, the condensate continues to be heated by the rising hot vapors; it vaporizes once more. However, the composition of the fresh vapors are determined once again by Raoult's law. Each vaporization-condensation cycle (called a theoretical plate) will yield a purer solution of the more volatile component. In reality, each cycle at a given temperature does not occur at exactly the same position in the fractionating column; theoretical plate is thus a concept rather than an accurate description.

More theoretical plates lead to better separations. A spinning band distillation system uses a spinning band of Teflon or metal to force the rising vapors into close contact with the descending condensate, increasing the number of theoretical plates.

Simple distillation Basic Mass Transfer Operations

1. Simple distillation

In simple distillation, all the hot vapors produced are immediately channeled into a condenser that cools and condenses the vapors. Therefore, the distillate will not be pure - its composition will be identical to the composition of the vapors at the given temperature and pressure, and can be computed from Raoult's law.

USES:(Limitations)

1.  Used only to separate liquids whose boiling points differ greatly (rule of thumb is 25 °C)
2. To separate liquids from involatile solids or oils.

Driving force for fluid flow

Driving force for fluid flow

Driving force for fluid flow is Energy per unit mass or total head available at the point of location.


Justification:


If you observe the flow in venturi meter, it will become more clear. consider 3 points as shown in the below figure.

Assume P1, P2, P3 are pressures at 1, 2 and 3 points. Here according to cross section area variation we can say P1 > P2 < P3. It shows that P3 > P2 but fluid is moving from 2 point to 3 point, so we can say pressure is not the driving force. Similarly we can say velocity is also not the driving force.

So here the driving force is combination of pressure head and velocity head that is total head.

Angular Momentum

Angular momentum in classical mechanics

Definition

The angular momentum L of a particle about a given origin is defined as:

 

where r is the position vector of the particle relative to the origin, p is the linear momentum of the particle, and × denotes the cross product.

As seen from the definition, the derived SI units of angular momentum are newton metre seconds (N·m·s or kg·m²s⁻¹) or joule seconds. Because of the cross product, L is a pseudovector perpendicular to both the radial vector r and the momentum vector p and it is assigned a sign by the right-hand rule.

For an object with a fixed mass that is rotating about a fixed symmetry axis, the angular momentum is expressed as the product of the moment of inertia of the object and its angular velocity vector:

where I is the moment of inertia of the object (in general, a tensor quantity), and ω is the angular velocity.

Angular momentum is also known as moment of momentum.