Chimica industriale - struttura dell'industria chimica
The concept of flowsheet
Unit operations in chem. processes
pretreatment of the feed (purification, heating, etc.)
▸ chemical transformation (reactor)
▸ continuous or batch-type
◦ mono- or multi-phase
◦ fixed bed, stirred tank, multiphase
▸ mechanical (not usual)
◦ different composition between gas/liq. phase (distillation)
◦ different solubilities (extraction, absorption-stripping,
◦ crystallization, ...)
preferential adsorption (active carbones)
UniME and many more
The flowsheet is the “road-map’’ of a process, and serves to
▸ identify and focus the scope of the process for all interested and
associated functions of the project. As a project progresses, the
various engineering disciplines read their portions of responsibility
from the flowsheet, although they may not understand the process
or other details relative to some of the other phases of engineering.
The flowsheet also describes the process to management as well as
▸ those concerned with preparing economic studies for process
A good process flowsheet pictorially and graphically identifies the
▸ chemical process steps in proper sequence. It is done in such a
manner and with sufficient detail to present to others a proper
mechanical interpretation of the chemical requirements.
vent transalkylation vent
heat out toluen
- input-output, mass flow
sequence of operations
- main equipments (with possible omissions)
UniME - 10
Type of flowsheets
▸ Usually used to set forth a preliminary or basic processing concept
◦ without details. The blocks do not describe how a given step will be
achieved, but rather what is to be done. These are often used in survey
studies to management, research summaries, process proposals for
“packaged” steps, and to “talk-out’’ a processing idea. For management
presentations the diagrams are pictorial and help illustrate the basic
Ammonia is an important primary inorganic material. 85% of
▸ the worldwide production is utilized in the manufacture of
N2 + 3 H2 2 NH3, H = -91.6 kJ/mol
▸ favored by: (i) high pressure, (ii) low temperature,
◦ (iii) active catalyst, (iv) pure gas (low inert gas )
catalyst: (from magnetite), promoted by
◦ (a) K2C03 (increases activity),
(b) Al203, SiO2, (protect against sintering),
(c) CaO (reduces susceptibility to sulfur and
Ammonia and Nitric Acid
ammonia synthesis ammonia (NH3) oxidation
to nitric acid (HNO3)
Nitric acid (HNO3) production by ammonia
UniME 14 14
It is shown the sequence of unit operations and the
▸ flows between them.
◦ only main operations
◦ also T,P and composition (in, out of main equipments)
◦ also the main instruments to control operations
Main symbols (in flowsheets)
1 vertical empty reactor
2 filling (catalyst., resins,
1 2 3 4 3 fixed bed reactor (or column)
4 multitubular reactor
5 continuous mixed reactor
6 gas-lig (or liq/liq) separat.
7 plate distillation column
5 6 7
UniME 8 9 10 heat cyclone valves
pumps (gas, liq.)
agitated with immersion jacketed
plate column packed column spray column reactor heat exchanger
pneumatic electric flow manual pneumatic
depuration air from benzene 18
Gas absorption is a unit operation in which soluble components of a
▸ gas mixture are dissolved in a liquid. The inverse operation, called
stripping or desorption, is employed when it is desired to transfer
volatile components from a liquid mixture into a gas. Both
absorption and stripping, in common with distillation, make use of
special equipment for bringing gas and liquid phases into intimate
Absorption, stripping, and distillation operations are usually carried
▸ out in vertical, cylindrical columns or towers in which devices such
as plates or packing elements are placed. The gas and liquid
normally flow countercurrently, and the devices serve to provide
the contacting and development of interfacial surface through
which mass transfer takes place.
Absorption is favoured at low T and high P, the contrary for
UniME stripping 19
Block diagram depuration air from benzene
Reading flowsheets - Reactors 1/...
high and low conv.
Reading flowsheets - Reactors 2/...
gas-gas, fixed bed gas-liq, slurry (suspended cat.)
Reading flowsheets - Reactors 3/...
liq-liq, moderate exothermicity gas-liq, exothermic reaction
Reading flowsheets - Reactors 4/...
gas-liq, exothermic reaction gas-gas, high exothermic
Mass and heat balances
Quantification of mass/heat transfer 1/..
Chemical processes may be classified as batch, continuous or semi-batch
▸ and as either steady-state or transient. Although the procedure required
for performing mass, or material, balances depends on the type of
process, most of the concepts translate directly to all types.
The general rule for mass balance in a system box (a box drawn around
▸ the complete process or the part of the process of interest) is:
input + generation - output - consumption = accumulation
▸ input is the material entering through the system box. This will include feed and makeup
generation is the material produced within the system, such as the reaction products in a
output is the material that leaves through the system boundaries. These will typically be
◦ the product streams of the process;
consumption is the material consumed within the system, such as the reactants in a
accumulation is the amount of material that builds up within the system.
UniME ◦ 29
Quantification of mass/heat transfer 2/..
In a steady-state continuous process, the accumulation should always be
▸ zero, which leads to a more simple mass balance equation:
input + generation = output + consumption
In the case of systems with no reaction, where mass is neither generated
▸ nor consumed, the result is even simpler:
input = output
Example 1: Mass Balance on a Continuous Distillation Process
▸ Suppose that a 1000 kmol h-1 feed stream, consisting of 30.0% by mole n-
◦ pentane and the remainder n-hexane, is to be separated into 95.0% pure
pentane and 95.0% pure hexane streams using a distillation column.
Determine the flow rates of the output streams through the use of mass
◦ balances, assuming steady-state operation.
pentane has a lower boiling point than hexane.
first step is to draw and label a flowsheet
• diagram indicating the process steps and all
F is the feed stream, D the distillate or tops
• product stream (which will be primarily
pentane, the lighter of the two species), B
the bottoms product stream (primarily
hexane), p refers to pentane and h to
mole fractions hexane
input = output
• we have two unknowns. Therefore, we
• need to generate two independent
total and equations that will allow us to solve for
partial mass these unknowns (degrees of freedom)
Mass Balance on a Process with Reaction
Suppose an initially empty tank is filled with 1000 mol of ethane and the
▸ remainder with air. A spark is used to ignite this mixture and the following
combustion reaction takes place:
2C2H6 + 7O2 4CO2 + 6H2O
Assume that the amount of air provides twice the stoichiometric
▸ requirement of oxygen for this reaction, and that air is composed of 79%
nitrogen and the remainder oxygen. Suppose that the reaction reaches a
90% conversion. What is the composition of the mixture in the tank at the
end of the reaction?
E for ethanol, O for oxygen,
N for nitrogen, C for carbon
dioxide and W for water
UniME input + generation = output + consumption 32
g is used to indicate the amount generated and the subscript c indicates the amount consumed
As there are five species involved, five mass balance equations can be defined.
• We have 19 unknown variables. As we have just defined 5 equations, we have
• 14 degrees of freedom remaining. To solve this problem, therefore, we need to
define at least 14 more equations.
We can write down new equations relating the unknown and known variables by
• making use of the stoichiometric coefficients This set of nine equations
reduces the degrees of
freedom to 5
Further equations can be defined on the basis of the specifications
of the feed and the conversion of the reaction:
The result is that we have
19 equations and 19
unknowns giving zero
degrees of freedom.
Given nl,E = 1000 mol, the initial amount of ethane
Energy balances can be treated in much the same way as
▸ material balances
Energy may be transferred between a system and its
▸ surroundings in two ways:
As heat, or energy that flows as a result of a temperature difference between a
◦ system and its surroundings. The direction of flow is always from the higher
temperature to the lower. Heat is defined as positive when it is transferred to
the system from its surroundings.
As work, or energy that flows in response to any driving force other than a
◦ temperature difference. For example, if a gas in a cylinder expands and moves a
piston against a restraining force, the gas does work on the piston. Energy is
transferred as work from the gas to its surroundings, including the piston.
Positive work means work done by the system on its surroundings, although
this convention is sometimes not followed and one should be careful to note
the convention used by other people.
the full energy balance equation is
As internal energy, U, is typically difficult to measure or estimate, enthalpy is instead used.
Specific enthalpy (enthalpy per unit mass), denoted by H, is defined as
In working with changes of energy, it is often useful to choose a reference 3/..
state, a state in which one of the quantities is assumed to be zero.
UniME (A) 37
Energy Balance on a Distillation Column 1/..
We consider the distillation unit introduced before, updated
▸ with temperature information for each of the streams. The
temperatures have been estimated using a physical property
estimation system. More streams are included. Specifically,
the vapour stream, V, from the top of the column to the
condenser and the liquid reflux stream, L, from the condenser
back into the column.
The relationship between the liquid reflux stream back into
▸ the column and the actual distillate product stream (D) is
given by the reflux ratio (typical value between 1.3 and 1.6;
1.6 in this case):
UniME Neglecting the effect of pressure on enthalpy, estimate the
▸ rate at which heat must be supplied 38
+1 anno fa
I contenuti di questa pagina costituiscono rielaborazioni personali del Publisher Giuseppe^^ di informazioni apprese con la frequenza delle lezioni di Chimica industriale e studio autonomo di eventuali libri di riferimento in preparazione dell'esame finale o della tesi. Non devono intendersi come materiale ufficiale dell'università Messina - Unime o del prof Perathoner Siglinda.
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