Molecular diffusion and convective diffusion efficiency

 

Molecular diffusion and convective diffusion efficiency

DIFFUSION

Fig: Diffusion

                It is the movement of a personal part through a combination from a district of upper concentration to a district of lower concentration at fastened temperature associate degreed pressure with or while not the assistance of an external force. A mixture that is non-uniform at the start are ultimately delivered to uniformity (a mixture is everyplace uniform within the concentration of its constituents) by diffusion since the concentration gradient that acts as a propulsion for diffusion tends to maneuver the part in such a direction on equalize its concentration and destroy the gradient. If we have a tendency to maintain the concentration gradient by perpetually providing/supplying the disseminative part to the high concentration finish and removing it at the low concentration finish, then the flow of disseminative part is continuous. This movement is used in mass transfer operations. 

                     When diffusion results from the random movement/motion of the molecules, it's known as molecular diffusion. because the molecular diffusion needs actual migration of molecules, the speed of mass transfer is higher in liquids and veritably slow in solids. When the movement of the motes occurs with the help of an external force (e.g. mechanical shifting and convective movement of the fluid), also it's called circle or turbulent prolixity. The molecular prolixity is a slow process, whereas the circle prolixity is a fast process. The molecular prolixity is the medium of a stationary fluid, i.e., a fluid at rest and fluids in laminar inflow. In case of fluids in turbulent inflow, the medium of mass transfer is by circle prolixity. The transfer of material in the presence of a attention grade is classified as molecular transfer and circle transfer. Molecular transfer of mass occurs in stationary fluids or fluids in laminar inflow, whereas eddy transfer occurs in fluids under turbulent conditions.

ROLE OF DIFFUSION IN MASS TRANSFER

                                        Prolixity may do in one phase or in both phases in all the mass transfer operations. In case of distillation, the more unpredictable element diffuses through the liquid phase to the interface between the phases (liquid and vapour phases) and down from the interface into the vapors phase. The less unpredictable compendia diffuse in the contrary direction and passes from the vapour phase into the liquid phase. In case of gas immersion, the solute gas diffuses through the gas phase to the interface and also through the liquid phase (liquid detergent phase) from the interface between the phases. 

                     In case of crystallization, the solid solute diffuses through the mama liquor (liquid phase) to the solid phase (chargers) and deposit on the solid shells. In case of drying operation, liquid water (humidity) diffuses through the solid towards the face of the solid, evaporates and also diffuses as a vapour into the gas phase (drying medium). In case of liquid-liquid birth, the liquid solute diffuses through the raffinate phase and also into the excerpt phase (detergent phase) from the interface between the phases.

MASS TRANSFER OCCURS BY TWO INTRODUCTORY MECHANISMS

(1) molecular prolixity by arbitrary and robotic bits’ movement of individual motes in a gas, liquid, or solid as a result of thermal stir. (2) circle (turbulent) prolixity by arbitrary, macroscopic fluid stir. Both molecular and/ or circle prolixity constantly involve the movement of different species in opposing directions. When a net inflow occurs in one of these directions, the total rate of mass transfer of individual species is increased or dropped by this bulk inflow or convection effect, which may be considered a third medium of mass transfer. Molecular prolixity is extremely slow, whereas circle prolixity is orders of magnitude more rapid-fire. Thus, if artificial separation processes are to be conducted in outfit of reasonable size, fluids must be agitated and interfacial are as maximized .However, using small patches to drop the distance in the direction of prolixity will increase the rate , If mass transfer in solids is involved.

Molecular diffusion

Fig: molecular diffusion 

Frequently simply called prolixity, is the thermal stir of all (liquid or gas) patches at temperatures above absolute zero. The rate of this movement is a function of temperature, density of the fluid and the size (mass) of the patches. Diffusion explains the net flux of motes from a region of advanced attention to one of lower attention. Once the attention are equal the motes continue to move, but since there's no attention grade the process of molecular prolixity has desisted and is rather governed by the process of tone- prolixity, forming from the arbitrary stir of the motes. The result of prolixity is a gradational mixing of material similar that the distribution of motes is invariant. Since the motes are still in stir, but an equilibrium has been established, the result of molecular prolixity is called a" dynamic equilibrium". In a phase with invariant temperature, absent external net forces acting on the patches, the prolixity process will ultimately affect in complete mixing. 

STEADY-STATE, ORDINARY MOLECULAR DIFFUSION

 Suppose a spherical glass vessel is incompletely filled with water containing a answerable red colour. Clear water is precisely added on top so that the dyed result on the bottom is unperturbed. At first, a sharp boundary exists between the two layers, but after a time the upper sub caste becomes multi-coloured, while the sub caste below becomes less multi-colored. The upper sub caste is more colored near the original interface between the two layers and lower colored in the region near the top of the upper sub caste. During this color change, the stir of each color patch is arbitrary, witnessing collisions substantially with water motes and occasionally with other color motes, moving first in one direction and also in another, with no bone direction preferred. This type of stir is occasionally appertained to as a arbitrary- walk process, which yields a mean-square distance of trip for a given interval of time, but not a direction of trip. Therefore, at a given vertical aeroplane through the result in the cylinder, it isn't possible to determine whether, in a given time interval, a given patch will cross the aeroplane or not. Still, on the normal, a bit of all motes in the result below the aeroplane will cross over into the region over and the same bit will cross over in the contrary direction. Thus, if the attention of color motes in the lower region is lesser than in the upper region, a net rate of mass transfer of color motes will take place from the lower to the upper region. After a long time, a dynamic equilibrium will be achieved and the attention of color will be invariant throughout the result.

 1. Mass transfer by ordinary molecular diffusion occurs because of a concentration, difference or gradient; that is, a species diffuses in the direction of decreasing concentration.

 2. The mass-transfer rate is proportional to the area normal to the direction of mass transfer and not to the volume of the mixture. Thus, the rate can be expressed as a flux.

 3. Net mass transfer stops when concentrations are uniform. 

Fick’s Law of Diffusion

The above observations were quantified by Fick in 1855, who proposed an extension of Fourier’s 1822 heat-conduction theory. Fourier’s first law of heat conduction is

qz = −k Dt/ dz

where qz is the heat flux by conduction in the positive z direction, k is the thermal conductivity of the medium, and dT/dz is the temperature gradient, which is negative in the direction of heat conduction.

What is Convection?

Convection is the process of heat transfer through the bulk movement of motes within fluids. The fluid can be either a gas or a liquid. Originally, the heat transfer between the fluid and an object occurs through conduction; still, the bulk heat transfer occurs due to the stir of the fluid. We can simply say convection is the heat transfer process in fluids through the factual stir of matter. This process can do as a natural or a forced process.

                                                             Fig: Convection

Concerning the process of convection, hitting a fluid causes thermal expansion, and the layers which are closest to the heating source gets hotter and come less thick. Therefore, followed by this is the rising up of the hotter part of the fluid according to buoyancy where the colder fluid layers tend to replace the rising hot fluid layers. This process reprises, and it's the process of convection where heat transfer occurs. 

• Convection is the process of heat transfer in fluids by the actual motion of matter.
• It happens in liquids and gases.
• It may be natural or forced.
• It involves a bulk transfer of portions of the fluid.

 There are two forms of convection as natural convection and forced convection. Natural convection occurs due to the buoyant force, and forced convection occurs due to an external source similar as wind from a addict or a pump.  

 Natural convection 

  Fig: Natural convection

When convection takes place due to buoyant force as there is a difference in densities caused by the difference in temperatures it is known as natural convection.

Examples of natural convection are oceanic winds.

Forced convection 

 Fig: Forced convection

When external sources such as fans and pumps are used for creating induced convection, it is known as forced convection.

Examples of forced convection are using water heaters or geysers for instant heating of water and using a fan on a hot summer day.

SIGNIFICANT PARAMETERS IN CONVECTIVE MASS TRANSFER

Dimensionless parameters are often used to correlate convective transfer data. In momentum transfer Reynolds number and friction factor play a major role. In the correlation of convective heat transfer data, Prandtl and Nusselt numbers are important. Some of the same parameters, along with some newly defined dimensionless numbers, will be useful in the correlation of convective mass-transfer data.

  What is Diffusion?

Prolixity is the movement of motes from a region of high attention to a low attention via a attention grade. These movements do in the same result. The factors that affect the attention grade affects prolixity as well.

 In prolixity, this stir is terminated when the attention of the two regions come equal at every point. This means this stir occurs until the attention grade disappears. Also the motes spread everyplace inside the result.

Fig: Diffusion

The rate of the movement of the motes through prolixity is a function of temperature, the density of the gas (or fluid) and flyspeck size. Generally, molecular prolixity describes the net flux of motes from a region of high attention to a low attention. When considering the two systems, A1 and A2, which are at the same temperature and are able of swapping motes between them, a change in the implicit energy in either of these systems can produce an energy inflow from one system to the other (from A1 to A2 or vice versa). This is because any system naturally prefers low energy and high entropy countries. This creates a state of molecular prolixity.

What is the Difference Between Convection and Diffusion?

Convection and prolixity are physical processes we can describe chemically through the movement of patches. The crucial difference between convection and prolixity is that convection is the large movement of a large mass of patches in the same direction through the fluid, whereas prolixity is the movement of single patches and transfer of flyspeck’s instigation and energy to other patches in the fluid.

Author by : Rajat bhakare, afsar ansari