Process Engineering Fundamentals – Distillation Process Essay Example

Process engineering fundamentals

Introduction

Distillation is a process where components are separated from a liquid mixture through selective condensation and evaporation. It is a physical process which exploits the difference in boiling point of the substances in the mixture to separate them.

Distillation was perfected in the middle ages by Muslim alchemists and were later introduced in Europe. The distillates were used almost exclusively for medicinal use. Wine was heated in a clay with long tube that extend outwards from the top and inkling downwards like in a retort flask. Alcohol, has low boiling point compared to water and it evaporates first and condense in the cold tube (Helstosky, 2015).

Major industrial application

  • Distillations is used to separate air into its components such as argon, nitrogen and oxygen to be used in the industries.

  • Distillation is used in refining oils from the crude oil in the industry. Hydrocarbons are separated through fractional distillation because they boil at different temperatures.

  • It is also used to produce alcohol or the fermentation products for commercial purposes.

Different types of distillation

Simple distillation

This is a distillation process where the vapour channeled immediately into the condenser. This method produce impure distillate. It is normally used to separate liquid with boiling points that differ greatly (Speight, 2014).

Fractional distillation

This type of distillation is used two separate a mixture of two or more liquids with different boiling points. The mixture is separated into fractions through condensation and boiling, and therefore the liquid with lower boiling point vaporize and collected first. This method is normally used to separate crude oil into its constituents (Speight, 2014).

Vacuum distillation

This technique takes advantage of volatility of liquids by decreasing the pressure. Some liquids are sensitive to heat and they decompose at a temperature beyond their boiling points. Vacuum distillation is carried out at lower temperatures because the boiling points of liquids decrease with decrease in pressure (Ledgard et al., 2006).

Steam distillation

This technique takes advantage of volatility force of water on some liquids or solids. Some solids and liquids are volatize partially in the presence of steam without changing to gas. The steam then carry them through conventional distillation manner to the receiving flask. The liquid volatize to from micro sized liquid droplets and the solids volatize to from micro particle (Ledgard et al., 2006).

Heat exchanger is used to transfer heat between two or more fluids which may be in direct contact or may be separated by a solid wall. The feed stream is heat up by hot bottoms stream before they enter the still. On the other hand, the bottom products is cooled before they enter into the storage. The steam is used to heat and in turn condense (Considine & Considine, 1995).

Reboilers are used to heat up the bottom of the distillation column. The boiler boils the liquid at the bottom of the distillation column in order to produce vapors that enters the column to energize the distillation.

The role and importance of reflux in distillation process

Reflux is used to control the temperature at the top therefore controlling the purity of the product at the overhead. Liquid is heated and the vapor condense and flow back to the flask. This allows separation of components of the mixture, by dissolving hard to dissolve compounds through reflux. This helps the reaction to reach completion (Ledgard et al., 2006).

The role and importance of material and energy balance for distillation process

Material and energy balances are necessary for control of processing, specifically in controlling the product yields. Materials balance are first determined at exploratory stages and improved through pilot plant experiments as the process is planned and tested. It is also checked out during the commissioning of the plant and maintained and refined as the production continues. The material balance needs to be determined once more if the process is changed. Energy balance is used to examine differnt5 stages of the process, on the whole process and over total production from the initial to the final process.

Process Engineering Fundamentals – Distillation Process

Solution

  1. The expression used for enthalpy is

H = mCP∆T

M is mass, CP is the heat capacity and ∆T is change in temperature.

For one pound of feed, H = 1lb x 0.65 Btu/ (lb) (0F) x 200F = 13J

Heat change at vaporization = 1.0 x 140 = 140J

Total heat change = 140 + 11 = 153 J

There is heat exchange at the reboiler where the heat given out by the steam is equal to the heat received by the fluid at the bottom.

Change of heat for steam, H = mCV

Latent heat of vaporization for water, CV = 970.4 Btu/lb

Mass of steam required, m = H/CV = 153/ 970.4 = 0.158 lb

In 30 psig, m = (30+14.7) x 0.158/250 = 0.0283 lb

  1. Process Engineering Fundamentals – Distillation Process 1

From the data provided, 1 kg of the feed has 0.35kg of ethylene (35% of ethylene in the feed).

Process Engineering Fundamentals – Distillation Process 2Mass of ethylene in the distillate per hour =

Mass of the distillate = 509.25 x 100/97 = 525 lb/hr

Enthalpy change at vaporization,

H = mCv = 509.5 x 135 = 68,782.5J/hr

Where m is mass, Cv is the heat of vaporization of ethylene

Enthalpy change in 250F rise of refrigerant is given by

H = mCP∆T

Process Engineering Fundamentals – Distillation Process 3Mass of refrigerant =

lb = 0.1198 gal

2,751.1 lb = 2,751.1 lb/hr x 0.1198 gal = 329.47 gal/hr

  1. Mass of the distillate = 509.25 x 100/97 = 525 lb/hr entering the condenser

The mass of bottom product can be obtain by assuming a balance stsyem

Thus, mass at the bottom = 1500lb/hr — 525lb/hr = 975 lb/hr

Enthalpy at the preheater, H = mCP∆T

H = 1500klb/hr x 0.65 Btu/ (lb) (0F) x 200F = 19500J = 19.5 kJ

Change in temperature at the bottoms at the preheater

∆T = H /m CP = 19.5 kJ/(975 x 0.65) = 30.760F

The temperature at the bottom as it enters the preheater = -80 + 30.76 = -49.240F

References

Helstosky, C. (2015). The Routledge history of food. London: Routledge.

Ledgard, J. B., & Mazal Holocaust Collection. (2006). A laboratory history of chemical warfare agents: A book. Place of publication not identified: Jared Ledgard

Speight, J. G. (2014). The chemistry and technology of petroleum. Boca Raton: CRC Press.