CHEE 2404Unit Operations and Unit Process

EquipmentDr. A. Ghanem

• Lets look at the overall process flow of a modern oil refinery:

The overall process is broken down into individual subsets:

PhysicalChemical

Thermal Catalytic

Distillation

Solvent extraction

Propane deasphalting

Solvent dewaxing

Blending

Visbreaking

Delayed coking

Flexicoking

Hydrotreating

Catalytic reforming

Catalytic cracking

Hydrocracking

Catalytic dewaxing

Alkylation

With their own flow arrangement:

Distillation

Fluid Catalytic Cracking

We notice…..

• The number of individual processes is large, each one can be broken down into a series of steps that appear in process after process

• The individual “steps” have common techniques and are based upon the same scientific principles

Fluid Dynamics

Heat Transfer

Evaporation

Humidification

Gas absorption

Solvent Extraction

Adsorption

Distillation

Drying

Mixing

Classification

Fluidization

Filtration

Screening

Crystallization

Centrifugation

Comminution

Materials handling

• Fluid dynamics:– a study of the behaviour of fluids– In Chemical processes fluid streams flow from

one process to another through pipes and ducts.

– Process fluids are moved by pumps and compressors

– Fluid flowrates must be monitored by meters and are controlled by valves.

• Heat Transfer: – process fluids may need to be heated up to a certain

temperature – heat from a process stream may be recovered– This can be done by contacting two streams in a heat

exchanger.

• Evaporation:– A special case of heat transfer, where a phase change takes

place. – Concentrate a solution consisting of a nonvolatile solute and a

volatile solvent– The volatile solute evaporates leaving a more concentrated

solution.

• Humidification:– Transfer of material between a pure liquid phase and

a fixed gas phase that is nearly insoluble in the liquid– Example: Water vapour is added to a air, cooling

tower to decrease water temperature

• Gas absorption– Also known as stripping or desorption– a mass transfer operation– A soluble vapour is absorbed from its mixture with an

inert gas by means of a liquid in which the solute gas is more soluble.

– Example: the removal of CO2 and H2S from natural gas or syngas by adsorption into amines or alkaline salts

• Leaching or Liquid extraction– Solid extraction involves the dissolving of soluble

matter from its mixture with an insoluble solid– Liquid extraction is the separation of two miscible

liquids by the use of a solvent that preferentially dissolves one of them.

– LE an alternative to distillation for difficult separations– Example: penicillin is separated from fermentation

broth by extraction with butyl acetate

• Distillation– Production of a vapour by boiling the liquid mixture to

be separated and then condensing the vapours without returning any to the still. (flash distillation)

– Return part of the condensate to the still under conditions where it can be in contact with the vapours on their way to the condenser. (rectification)

– Either operation may be done in batch or continuous mode.

• Adsorption – a separation process where the fluid is contacted with

small particles of a porous solid which selectively adsorbs or complexes with certain components of the feed.

– The solid adsorbent is usually held in a fixed bed.

• Drying– Removal of water (usually small amounts) or other

liquid from a solid material to reduce the content of residual liquid to an acceptable low value

– Water may be removed by presses or centrifuge (mechanical) or thermally by vaporization

• Agitation and Mixing– Induced motion of a material in a container– Random distribution into and through one another, of

two or more initially separate phases.

• Fluidization– Particle fluid interaction– When a gas or liquid is passed upward through a bed

of particles, at a certain velocity the particles will become suspended in the fluid.

• Filtration– the removal of solid particles from a fluid by passing

the fluid through a filtering medium on which the solids are deposited.

• Crystallization– The formation of solid particles within a homogenous

phase– Formation of solid particles in a vapour, solidification

from a liquid melt, or crystallization from liquid solution– A variety of materials are marketed in crystallized

form.– Many are vacuum units where adiabatic evaporative

cooling induced supersaturation.

• Centrifugation– a given particle settles under gravitational force at a

fixed maximum rate– to increase the settling rate we replace the force of

gravity by a much stronger centrifugal force.– More effective than gravity separators because they

will separate fine drops and particles and are much smaller in size for a given capacity.

– Solids removal from gas: cyclones

• Materials handling – Classification:

• characterization of solids by size and shape• Done in a series of standard screens or woven wire test

sieves arranged serially on a stack, with the smallest mesh on the bottom and the largest on top.

– Comminution• Size reduction of solid particles• For example, chunks of coke must be reduced to workable

size• Compression, impact, attrition (rubbing) or cutting• Crushers and grinders are good examples.

Unit Process: Reactors

• While not a unit operation, reactors are essential process operations

• They are identified on flowsheets by the type of vessel and their flow:– Batch reactor– Continuous stirred tank reactor (CSTR)– plug flow or tubular reactor

• Batch reactor– A tank or vessel where reactants have

been placed and products are removed– There is no inflow or outflow of reactants

or products.

• Continuous Stirred Tank Reactor (CSTR)– Run at steady state (ie no accumulation)– Very well mixed– Modelled as having no spatial variations in concentration,

temperature, pH or reaction rate in the vessel– T an C are identical everywhere in the vessel, they are the same

at the exit as in the tank.

• Plug Flow or Tubular Reactor– Consists of a cylindrical pipe– Assume the flow is highly turbulent and there

is no radial variation in concentration.