Month: June 2020

Jun 12

Boiling Point Diagram for Ideal Plate

Boiling point diagram for ideal plate is shown in the figure below. The concentration of fluid entering and leaving the nth plate is shown in in the figure. By definition of ideal plate, vapor and liquid leaving the plate n are in equilibrium. So Xn and Yn represents equilibrium concentration.

Since concentration of both phase increase with height of column, Xn-1 is greater than Xn and Y1 is greater than Yn+1. Although the streams leaving the plates are in equilibrium, those entering are not.

When the vapor from plate n+1 and liquid from plate n-1 are brought into intimate contact their concentration tends to move towards an equilibrium state as shown by arrows in above figure.

Here, liquid streams are at their dew point. Heat necessary to vaporize component A is sullied from heat released in condensation of component B.  Thus each plate act as an interchange apparatus in which component A is transferred to vapor stream and component B to liquid stream.

Since concentration of A in both liquid and vapor increases with column height, temperature decreases. Temperature of plate n is greater than that of plate n-1 and less than that of plate n+1.

Equilibrium concentration doesn’t necessarily means that reactants and products are present in equal amount. It means that reaction has reached a point where concentration of reactant and product are unchanging with time because forward and backward reaction have same rate.

 

 

About Author

Name : Pratiksha Shrestha

pratiksha.shrestha2001@gmail.com

Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.

 

Jun 11

Food Hygiene and Food Spoilage

More than cleanliness, food hygiene is

  1. Protecting food from risk of contamination including harmful bacteria, poison and other foreign bodies.
  2. Prevent proliferation of microorganism present in food, which would otherwise cause food spoilage or related illness.
  3. Destruction of pathogenic microorganism.
  4. Discarding unfit or contaminated food

Read more

Jun 10

Continuous distillation with reflux (Rectification)

It is the process of separation of components of comparable volatility. Production of vapor by boiling liquid mixture and condensing the vapor and allowing part of condensed liquid to still is the working principle of rectification.

In every distillation process, vapor formed in kettle is richer in more volatile component than in remaining liquid. However, the vapor contains certain amount of both components and distillate is rarely a pure substance. In order to increase concentration of volatile component in vapor and to separate the less volatile one, vapor is brought into contact with the stream of boiling liquid that runs down the column.

A stream of liquid with high concentration of volatile component is introduced at the top of the column. The most volatile components are concentrated in vapor phase and least volatile in liquid phase. This way vapor becomes richer in volatile component as it goes up the column, while liquid phase becomes richer in heavier component as it descends the column and separation is achieved.

Separation of mixtures depends upon relative volatilities of components, number of contact stages and reflux ratio. Rectification unit consist of

  1. Steam or reboiler in which vapor is generated
  2. Rectifying or fractionating column, which is divided into large number of perforated trays that permit upward flow of vapor.
  3. Condenser, condense all vapor, leaving the top of column, sending a part of condensed liquid (reflux) back to column to descend counter to raising vapor and delivery the part of condensed liquid as product.

Typical diagram for rectifying column is shown as below.

In the above figure, feed is introduced into the column ‘A’ at the point that is nearly its center. The plate on which feed enters is called feed plate. All plates above the feed constitute rectifying section. The plates below the feed, including the plate itself constitute stripping section.

The feed flows down the stripping section to the bottom of the column in which definite level of liquid is maintained. Liquid flows by gravity to reboiler ‘B’. Reboiler generates steam (vapor) and returns it to the bottom of the column, which then passes up to entire column.

Fig: Schematic diagram of reboiler

At one end of reboiler is a weir (dam) and downstream side of this weir collects bottom product which is withdrawn and passed through cooler ‘G’. This cooler also preheats the feed by heat exchange with the hot bottoms.  The vapors rising through rectifying section are completely condensed in condenser ‘C’ and the condensate is collected in accumulator ‘D’.

Reflux pump ‘F’ takes liquid from accumulator and delivers it to the top plate of tower. This liquid stream is called ‘external reflux’.

Condensate not picked up by reflux pump is cooled in heat exchanger ‘E’ called product cooler and withdrawn as overhead product.

 

 

About Author

Name : Pratiksha Shrestha

pratiksha.shrestha2001@gmail.com

Ms. Shrestha holds masters degree in food engineering and bioprocess technology from Asian Institute of Technology (AIT) Thailand. She is currently working for Government of Nepal at Department of Food Technology and Quality Control (DFTQC), Kathmandu. She is also a teaching faculty in College of Applied food and Dairy Technology (CAFODAT) affiliated to Purbanchal university, Nepal.

 

Jun 09

Food safety management system (FSMS)

It is group of practices and procedures intended to control risk and hazard throughout the handling of food. Hazard can be

  1. Biological: microorganisms like bacteria, yeast, mold and insects and pests
  2. Physical: stone, metals and wood chips
  3. Chemical: cleaning detergents and corrosives and poisons

Foundation of food safety management system: Read more

Jun 08

How to access online courses for free: Food Technology and Food Science

Many of us might not know that prestigious universities and different learning platforms have their free online courses! We might not have that opportunity to join high ranking colleges and universities. Yet, we can learn virtually from online courses by ingenious professors and lecturers. Below is the link to find what are the online courses related to food technology and food science and  how to access them for free.

https://www.youtube.com/watch?v=_tdF2q709pA

Jun 07

Azeotropes

Azeotropes or constant boiling mixtures is a mixture of two or more liquid which cannot be separated by simple distillation. This happens because when azeotrope is boiled, vapor has same proportion of constituents as unboiled mixture.

Maximum boiling azeotropes:

These are obtained from solution that shows negative deviation from ideal solution. We know, vapor pressure is inversely proportional to boiling point. Since first type of mixture showing negative deviation have lowest vapor pressure, they form maximum boiling azeotropes. Here the mixtures will boil at the temperature above boiling temperature of components from which they are made and the mixture will boil undistilled. Read more

Jun 07

Diet and Diseases

Person may suffer from different diseases and therefore need to adopt different diet plan to recuperate form the ailment. Different food have different functions and thus the same diet cannot benefit all diseased condition. Special care should be given in diet when suffering from body illness like fever, hypertension, diabetes etc. There are diet that should be strictly prohibited in certain diseases while some food are highly recommended for recuperating the disease or fighting the disease. Also care should be taken about recommended dietary allowances of nutrients for specific age group and gender and particular diseases. Read more