Process Classification

1
Process Classification

• Before writing a material balance you must first
  identify the type of process in question.

• Batch no material is transferred into or out of
  the system over the time period of interest
  (e.g., heating a beaker of water)
• Continuous material is transferred into and out
  of the system continuously (e.g., pump liquid at
  a constant rate into a distillation column and
  remove the product streams from top and bottom of
  column)
• Semibatch any process that is neither batch nor
  continuous (e.g., slowly blend two liquids in a
  tank)
• Steady-State process variables (i.e., T, P, V,
  flow rates) do not change with time
• Transient process variables change with time

2
Example

• Classify the following processes as batch,
  continuous, or semibatch, and transient or
  steady-state.
• A balloon is filled with air at a steady rate
  of 2 g/min.
• A bottle of milk is taken from the refrigerator
  and left on the kitchen table.
• Water is boiled in an open flask.
• Carbon monoxide and steam are fed into a
  tubular reactor at a steady-rate and react to
  form carbon dioxide and hydrogen. Products and
  unused reactants are withdrawn at the other end.
  The reactor contains air when the process is
  started up. The temperature of the reactor is
  also constant, and the composition and flow rate
  of the entering reactant stream are also
  independent of time. Classify the process (a)
  initially and (b) after a long period of time
  has elapsed.

3
Material (Mass) Balances

• A material balance is simply an accounting of
  material. For a given system, a material balance
  can be written in terms of the following
  conserved quantities
• System a region of space defined by a real or
  imaginary closed envelop (envelopsystem
  boundary)
• may be a single process unit, collection of
  process units or an entire process

• Total mass (or moles)
• Mass (or moles) of a chemical compound
• Mass (or moles) of an atomic species

To apply a material balance, you need to define
the system and the quantities of interest.
4
General Balance Equation
Accumulation In Out Generation
Consumption
system boundary
Input streams to system
output streams from system

-
5
Integral Versus Differential Balances

• Integral Balances written in terms of the
  amounts of a specified quantity over a period of
  time
• E.g. At the start of the year my bank balance
  was 4328. Now it is 2555.
• Accumulation (final output initial input)
• Typically applied to batch processes
• Differential Balances written in terms of rates
  of change of the specified quantity with respect
  to time
• E.g. I get paid 10.50/hr
• Accumulation is a differential term
• Typically applied to continuous steady-state
  processes
• Used extensively in this course

6
Example

• A country has a population of 10 million people
  in 1900 AD. Over the period from 1900 to 2000, 6
  million people immigrated into the country, 2
  million people emigrated from the country, 5
  million people were born in the country and 3
  million people died in the country. What is the
  population of the country in the year 2000 AD?

7
Material Balance Simplifications

• The following rules may be used to simplify the
  material balance equation

Accumulation In Out Generation
Consumption

• If the system is at steady-state, set
  accumulation 0
• If the balanced quantity is total mass, set
  generation 0 and consumption 0 (law of
  conservation of mass)
• If the balanced substance is a nonreactive
  species, (neither a reactant nor a product), set
  generation 0 and consumption 0

? In Out Generation Consumption 0
? Accumulation In Out
? Accumulation In Out
8
Process Unit Basic Functions

• Splitter divides a single input into two or
  more outputs of the same composition (no
  reaction)
• Mixer combines two or more inputs (usually of
  different compositions) into a single output) (no
  reaction)
• Separator separates a single input into two or
  more outputs of different composition (no
  reaction)

splitter
mixer
separator
9
Process Unit Basic Functions

• Reactor carries out a chemical reaction that
  converts atomic or molecular species in the input
  to different atomic or molecular species in the
  output
• Heat exchanger transfers heat from one input to
  a second input (no reaction)
• Pump changes the pressure of an input to that
  of the corresponding output (no reaction)

Actual process units usually combine these
different functions into a single piece of
hardware, and are given different names.
10
Steam Boiler
Steam Boiler
Heat Exchanger Reactor
11
Distillation
Column
Separator
Splitter Heat Exchanger
Reflux Condenser
Separator Heat Exchanger
Bottoms Reboiler
12
Distillation Inside the Column

• Internal trays (or packing) are used to
  enhance component separations
• Each tray accomplishes a fraction of the
  separation task by transferring the more
  volatile species to the gas phase and the less
  volatile species to the liquid phase
  (Thermodynamics!!)
• Can perform material and energy balances on
• an individual tray
• the column, bottoms reboiler, or top condenser
• the entire system

white vapour black liquid
13
Problems Involving Material Balances

• Procedures will be outlined for solving single
  unit processes
• No reaction (consumption generation 0)
• Continuous steady-state (accumulation 0)
• These procedures will form the foundation for
  more complex problems involving multiple units
  and processes with reaction
• Develop good problem solving habits now!!