Rheology

Rheology is the study of deformation and flow behavior of materials. Rheology is generally associated with the behavior of the fluids, but the semi-solids materials (under specific conditions) some times also possess the rheological properties.

Rheology normally accounts for the flow of unusual materials, generally non-Newtonian fluids, as rheological properties are the parameters in any quantitative functional relation between the stress and strain.And as we have already discussed that the Newtonian fluids have the linear relationship between the stress and strain.

All non Newtonian fluids do not possess same properties. For example paints, blood, custard, ketchup etc do not fall in the same category of the non Newtonian fluids. So these fluids are divided according to their rheological properties. i.e.

1) Non Newtonian fluids with Time independent viscosity:

1.1)          Shear thinning (Pseudo plastic) fluids

In pseudo plastic fluids, the viscosity decreases with the increase in stress or disturbance. i.e. these fluids are also named as shear thinning fluids. The most common examples of these pseudo plastic fluids are paint, blood, syrups, molasses etc.

           

1.2)          Shear thickening (dilatant) fluids

Like the name, the dilatant fluids are those, whose viscosity increases with the increase in the stress. The most common example is the corn syrup, or the sand in water.

1.3)          Bingham plastic fluids

The bingham plastic fluids are also named as the generalized Newtonian fluids. These fluids have the constant viscosity, but unlike non Newtonian fluids, the stress is dependent on the strain rate and the pressure applied on the fluid. Blood plasma and custard are the common examples of these fluids.

2) Non Newtonian fluids with Time dependent viscosity:

2.1)      Rheopectic

The viscosity of the fluid increases with the increase in the duration of stress,

2.2)          Thixotropic

The viscosity of the fluid decreases with the increase in the duration of stress,

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Rheology

Rheology is the study of deformation and flow behavior of materials. Rheology is generally associated with the behavior of the fluids, but the semi-solids materials (under specific conditions) some times also possess the rheological properties.

Rheology normally accounts for the flow of unusual materials, generally non-Newtonian fluids, as rheological properties are the parameters in any quantitative functional relation between the stress and strain.And as we have already discussed that the Newtonian fluids have the linear relationship between the stress and strain.

All non Newtonian fluids do not possess same properties. For example paints, blood, custard, ketchup etc do not fall in the same category of the non Newtonian fluids. So these fluids are divided according to their rheological properties. i.e.

1) Non Newtonian fluids with Time independent viscosity:

1.1)          Shear thinning (Pseudo plastic) fluids

In pseudo plastic fluids, the viscosity decreases with the increase in stress or disturbance. i.e. these fluids are also named as shear thinning fluids. The most common examples of these pseudo plastic fluids are paint, blood, syrups, molasses etc.

           

1.2)          Shear thickening (dilatant) fluids

Like the name, the dilatant fluids are those, whose viscosity increases with the increase in the stress. The most common example is the corn syrup, or the sand in water.

1.3)          Bingham plastic fluids

The bingham plastic fluids are also named as the generalized Newtonian fluids. These fluids have the constant viscosity, but unlike non Newtonian fluids, the stress is dependent on the strain rate and the pressure applied on the fluid. Blood plasma and custard are the common examples of these fluids.

2) Non Newtonian fluids with Time dependent viscosity:

2.1)      Rheopectic

The viscosity of the fluid increases with the increase in the duration of stress,

2.2)          Thixotropic

The viscosity of the fluid decreases with the increase in the duration of stress,

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types of fluid

Fluid mechanics is one of the major sciences involved in chemical engineering, so a chemical engineer is mostly interested in many aspects of the problems involved in the fluid flow. To understand the fluid mechanics, a chemical engineer must have to understand that what is fluid, and the types of the fluid.

The simplest definition of the fluid is that “Any substance which can flow under pressure is fluid”. Fluid can also be defined as “any substance that has no fixed structure, shape or size, and yields easily to the external pressure”.

There are generally two ways to classify the fluids, i.e.

> Compressible and Incompressible Fluids

> According to viscosity change

Compressible And Incompressible Fluids

The nature of the fluid is said to be compressible or incompressible according to its behavior under applied pressure, i.e.

The incompressible fluid is the one, whose volume is independent of its temperature and pressure, i.e. its volume will not be affected by the change of its temperature and pressure. There is no real fluid, which is completely incompressible, however liquids are assumed to be the incompressible fluids, as they sustain the change of temperature and pressure, more then gases.

The compressible fluids change their volume according to the change in their temperature or pressure. The gases are real example of such type of fluid. However, if the percent change is small, then for practical purposes, a gas may be treated as the compressible fluid.

Classification Of Fluids According To Viscosity Change

The fluids can also be classified according to the effects produced on the fluid by the action of the shear stress. This classification is important, as it determines the way in which the fluid will flow. This classification is based on a most important physical property “viscosity”.

Then main two types under this classification are:

> Newtonian fluid

> Non Newtonian fluid

A Newtonian fluid is the fluid, whose viscosity remains constant regardless of any applied stress. That’s why these fluids are also named as “linear viscous fluids”. The most common example of Newtonian fluid is water. The flow of water remains same, whether it flows alone, or in vigorously agitation condition. Its simplest meaning is that, the fluid will continue to flow, regardless of the forces acting on it. The Newtonian fluids behave according to following equation:

                                                            τ = µ (du/dy)

τ is the shear stress exerted by fluid

µ is fluid viscosity, constant for Newtonian fluids

(du/dy) is the velocity gradient, or the strain.

So for Newtonian fluid, according to the equation, the ratio of stress to strain is constant, there fore, the viscosity is constant.

The viscosity of the non Newtonian fluid is variable, and is dependent upon the applied stress on the fluid. These type of fluids also exhibits the rheological properties .The common examples of non Newtonian fluids are solution of corn starch and water, paints, ink, tooth paste, etc.

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types of fluid

Fluid mechanics is one of the major sciences involved in chemical engineering, so a chemical engineer is mostly interested in many aspects of the problems involved in the fluid flow. To understand the fluid mechanics, a chemical engineer must have to understand that what is fluid, and the types of the fluid.

The simplest definition of the fluid is that “Any substance which can flow under pressure is fluid”. Fluid can also be defined as “any substance that has no fixed structure, shape or size, and yields easily to the external pressure”.

There are generally two ways to classify the fluids, i.e.

> Compressible and Incompressible Fluids

> According to viscosity change

Compressible And Incompressible Fluids

The nature of the fluid is said to be compressible or incompressible according to its behavior under applied pressure, i.e.

The incompressible fluid is the one, whose volume is independent of its temperature and pressure, i.e. its volume will not be affected by the change of its temperature and pressure. There is no real fluid, which is completely incompressible, however liquids are assumed to be the incompressible fluids, as they sustain the change of temperature and pressure, more then gases.

The compressible fluids change their volume according to the change in their temperature or pressure. The gases are real example of such type of fluid. However, if the percent change is small, then for practical purposes, a gas may be treated as the compressible fluid.

Classification Of Fluids According To Viscosity Change

The fluids can also be classified according to the effects produced on the fluid by the action of the shear stress. This classification is important, as it determines the way in which the fluid will flow. This classification is based on a most important physical property “viscosity”.

Then main two types under this classification are:

> Newtonian fluid

> Non Newtonian fluid

A Newtonian fluid is the fluid, whose viscosity remains constant regardless of any applied stress. That’s why these fluids are also named as “linear viscous fluids”. The most common example of Newtonian fluid is water. The flow of water remains same, whether it flows alone, or in vigorously agitation condition. Its simplest meaning is that, the fluid will continue to flow, regardless of the forces acting on it. The Newtonian fluids behave according to following equation:

                                                            τ = µ (du/dy)

τ is the shear stress exerted by fluid

µ is fluid viscosity, constant for Newtonian fluids

(du/dy) is the velocity gradient, or the strain.

So for Newtonian fluid, according to the equation, the ratio of stress to strain is constant, there fore, the viscosity is constant.

The viscosity of the non Newtonian fluid is variable, and is dependent upon the applied stress on the fluid. These type of fluids also exhibits the rheological properties .The common examples of non Newtonian fluids are solution of corn starch and water, paints, ink, tooth paste, etc.

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Bleaching agents

Bleaching agents are the materials, which are used for the lightening, decolorizing or whitening of a substrate by the chemical reactions.  The color producing materials are generally organic in nature, containing“chromophores” (Chromophore is that portion of molecule that absorbs light photons).

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The bleaching action is normally based on the oxidation or reduction processes that degrade the color systems. Chemical bleaches works in one of two ways:

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Oxidizing bleach breaks the chemical bonds that make up the chromophores. Thus the molecule either does not contain the chromophore, or the chromophore does not absorb visible light.

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Reducing bleach converts double bonds of the chromophoric carbonyl groups into textiles or pulp.

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The bleaching agents are mostly used for the following purposes:

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Textile bleaching: In textile process industries, these agents are used to remove the remaining unwanted materials (e.g. soil, colored compounds,etc) before dying and finishing. In textile process industries, the bleaching of the fabrics is generally named as “Scouring”, in which the washing of the fabric is carried out in hot alkali solutions.

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Pulp bleaching: bleaching agents are used to decrease the color of the pulp. The main use of this pulp bleaching is to make white paper from the pulp.

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Laundering purposes: normally alkaline aqueous solution of sodium hypochlorite is used for this purpose. This bleach is highly effective for fabrics whitening and germicidal activity, however not suitable for all fabrics.

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Cleaning purpose: Bleaching agents have extensive uses as hard surface cleaners, to remove strains, and also for the disinfection of the surfaces(industrial use). Sodium hypochlorite, hydrogen peroxides or many other alkaline solutions along with surfactants and auxiliaries are used for the purpose.

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Bleaching agents are also used for the bleaching of pulp and paper, hair, fur, foodstuff and oils.

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Bleaching agents are not considered as the environment damaging substances, as most of them are organic substances, however bleaching of the chemical substances or conventional bleaching using elemental chlorine produces large amount of chlorinated organic compounds such as chlorinated dioxins etc. Chlorinated dioxins are highly toxic, and have bad influence on human health.

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Bleaching agents

Bleaching agents are the materials, which are used for the lightening, decolorizing or whitening of a substrate by the chemical reactions.  The color producing materials are generally organic in nature, containing “chromophores” (Chromophore is that portion of molecule that absorbs light photons).

The bleaching action is normally based on the oxidation or reduction processes that degrade the color systems. Chemical bleaches works in one of two ways:

Oxidizing bleach breaks the chemical bonds that make up the chromophores. Thus the molecule either does not contain the chromophore, or the chromophore does not absorb visible light.

Reducing bleach converts double bonds of the chromophoric carbonyl groups into textiles or pulp.

The bleaching agents are mostly used for the following purposes:

Textile bleaching: In textile process industries, these agents are used to remove the remaining unwanted materials (e.g. soil, colored compounds, etc) before dying and finishing. In textile process industries, the bleaching of the fabrics is generally named as “Scouring”, in which the washing of the fabric is carried out in hot alkali solutions. 

Pulp bleaching: bleaching agents are used to decrease the color of the pulp. The main use of this pulp bleaching is to make white paper from the pulp.

Laundering purposes: normally alkaline aqueous solution of sodium hypochlorite is used for this purpose. This bleach is highly effective for fabrics whitening and germicidal activity, however not suitable for all fabrics.

Cleaning purpose: Bleaching agents have extensive uses as hard surface cleaners, to remove strains, and also for the disinfection of the surfaces (industrial use). Sodium hypochlorite, hydrogen peroxides or many other alkaline solutions along with surfactants and auxiliaries are used for the purpose.

Bleaching agents are also used for the bleaching of pulp and paper, hair, fur, foodstuff and oils.

Bleaching agents are not considered as the environment damaging substances, as most of them are organic substances, however bleaching of the chemical substances or conventional bleaching using elemental chlorine produces large amount of chlorinated organic compounds such as chlorinated dioxins etc. Chlorinated dioxins are highly toxic, and have bad influence on human health.

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Air Pollution because of process industries

[caption id="attachment_175" align="alignright" width="300" caption="Air pollution because of process industries"][/caption]

Being a chemical engineer, you must be aware of the hazards and outcomes of the un-managed and poorly handled processes in process industries. One of the major disadvantages of these poorly handled chemical processes is the Air Pollution. However energy generation is also considered as the major source of the air pollution.

In air pollution, the air is contaminated with several unwanted substances, which produces direct measurable effect on receptors [receptor may be humans, animals, plants or environment]. This situation becomes more dangerous when these substances react with other contents in the atmosphere and form other hazardous compounds, causing harmful phenomena like depletion of ozone layer, petrochemical smog, acid rains, or green house gases [generally known as global warming]. We will discuss these phenomena later, in detail.

The original six criteria pollutants, documented by EPA [environmental protection agency] are SO2 [sulfur dioxide], NO2 [Nitrogen dioxide], CO [carbon monoxide], O3 [ozone], suspended particles and VOC [volatile organic compounds]. As this is a serious issue against the global environmental protection, so EPA is trying to implement and make several changes in the clean air act.

The clean air act was first passed in 1956 by the parliament of UK,in response of the London smog 1952. This clean air act 1952 was used and effective till 1964, then there were several modifications in this act.

The 1970 clean air act required that EPA provide a safety margin to protect against hazardous air pollutants by establishing national emissions standards for certain sources.

This act was last amended in 1990,which contains national ambient air quality standards for pollutants which are harmful to receptors. This act identifies two types of national ambient air quality standards:

• Primary standards      : for public health protection.


• Secondary standards  : for public welfare protection.

However, it is also the duty of the process industries / organizations, to take serious steps to control air pollution. They must focus on the reduction of contaminant discharge

• by installing control equipments [scrubbers, gravity settling chambers, etc]


• by changing raw materials, operations, or modes of operations


• by diluting the discharge


• by dispersion of sources locations.

Out of all above, the installation of the pollution control equipments is the best option to go for.This is quite an expensive technique, but is very efficient and most of the industries are using these equipments to regenerate the chemicals from the waste stream, so that their process is highly optimized causing very less pollution.

Read More

Air Pollution because of process industries

Being a chemical engineer, you must be aware of the hazards and outcomes of the unmanaged and poorly handled processes in process industries. One of the major disadvantages of these poorly handled chemical processes is the Air Pollution. However energy generation is also considered as the major source of the air pollution.

In air pollution, the air is contaminated with several unwanted substances, which produces direct measurable effect on receptors (receptor may be humans, animals, plants or environment). This situation becomes more dangerous when these substances react with other contents in the atmosphere and form other hazardous compounds, causing harmful phenomena like depletion of ozone layer, petrochemical smog, acid rains, or green house gases (generally known as global warming). We will discuss these phenomena later, in detail.

The original six criteria pollutants, documented by EPA (environmental protection agency) are SO2 (sulfur dioxide), NO2 (Nitrogen dioxide), CO (carbon monoxide), O3 (ozone), suspended particles and VOC (volatile organic compounds). As this is a serious issue against the global environmental protection, so EPA is trying to implement and make several changes in the clean air act.

The clean air act was first passed in 1956 by the parliament of UK, in response of the London smog 1952. This clean air act 1952 was used and effective till 1964, then there were several modifications in this act.

The 1970 clean air act required that EPA provide a safety margin to protect against hazardous air pollutants by establishing national emissions standards for certain sources.

This act was last amended in 1990, which contains national ambient air quality standards for pollutants which are harmful to receptors. This act identifies two types of national ambient air quality standards:

• Primary standards      : for public health protection.
• Secondary standards  : for public welfare protection.

However, it is also the duty of the process industries / organizations, to take serious steps to control air pollution. They must focus on the reduction of contaminant discharge

• by installing control equipments (scrubbers, gravity settling chambers, etc)
• by changing raw materials, operations, or modes of operations
• by diluting the discharge
• by dispersion of sources locations.

Out of all above, the installation of the pollution control equipments is the best option to go for. This is quite an expensive technique, but is very efficient and most of the industries are using these equipments to regenerate the chemicals from the waste stream, so that their process is highly optimized causing very less pollution.

Read More