Chemical Process Technology
Chapter 1: Introduction
1. Introduction
Chemical processes already exist for centuries and were manly discovered by accident. Examples of
such processes are fermentation in beer, cheese and yoghurt, pottery and porcelain, metals such as
steel which is Iron with Carbon (makes is stronger).
In the 20th century a strong development of the chemical industry was driven by industrial and political
developments. The coil industry was used to produce colorants (e.g. aniline) and steel. The Haber-
Bosch process was found and was/is the main process for the synthesis of ammoniac (NH3). The
drawback of this process is the formation of other nitrogen impurities, nitrates, which are known to
be explosive hence were used during WO I.
During the interbellum, many German companies joined to form IG-Farben, which produced synthetic
gasoline, rubber, ZyklonB (hydrogen cyanide, used in the concentration camps). IG-Farben received a
bad connotation due to their implications in WO II and decided to split afterwards into Agfa, BASF,
Bayer and Hoechst.
Originally, the main drivers in the chemical industry where Germany and America. Nowadays, a shift
towards the ‘developing countries’ such as China, japan and the Middle-East. Furthermore, also more
specialization occurs today: the formation of higher end products with an added value, such as
pharmaceuticals.
The ranking of the biggest chemical industries is solely based on their sales of chemicals and not on
their petrochemical sales. For that reason, companies such as Total and Sinopec are not 1st on the list.
Expanding this idea to what chemicals are mostly produced: sulfuric acid (mostly because it is a
byproduct form the purification of fuels), followed by N 2 (used as fertilizer) and H2, ethylene for the
PE synthesis.
The relative added values in the chemical industry give insights in the magnitude of the chemical
process that is involved for making the product. While crude oil has a value of 1, fuel (refined version)
has a relative value of 2. Extrapolating this scale gives rise to fertilizers with values of 4 and
pharmaceuticals with relative added values of 32. On the other hand, pharmaceuticals are sold in
lower quantities.
Belgium is fourth on the list of turnovers (~ omzet) in the chemical industry in Europe, but on the other
hand for turnovers per capita (per person in a land) Belgium is 1st! The chemical industry in Belgium is
thus of major importance, especially visible in Antwerp.
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,Chapter 2: Flow schemes and balances
2. Flow schemes and balances
In chemical industry, raw materials (minerals, crude oils …) are converted into other chemicals using
both:
- Chemical processes ~ reactions => unit processes
- Physical processes ~purifications and separations => unit operations
2.1. Flow schemes
A flow scheme structures complex systems and represents the underlying structure of the elements
and how they interact.
Example) Methanol
Methanol is formed from: CO + 2 H2 → CH3OH (exothermic)
Both CO (g) and H2 (g) are obtained via the reaction of methane with water. CO and H2 are called
‘syngasses’, which are two gasses that react together.
Thermodynamically: the exothermic reaction should be performed at low T in order for the
equilibrium to be towards the product. Since heat is released during the reaction, the entropy factor
will become more important, and since the latter is negative (less particles), the total term -TS is
positive, hence the reaction becomes less favorable at higher T. Drawback of working at low
temperatures: the reaction rate decreases (Arrhenius). We will work at 350°C.
Furthermore: pV = nRT (ideal gas law) => the pressure is inversely proportionate to the number of
moles in the system. By working at high pressures the equilibrium can be shifted towards the side with
the less moles, hence to the product in this case. For this reason, the reaction will be performed at
300 bar (very high), in order to avoid too low T, hence in order to affect the reaction kinetics.
Kinetically: use a catalyst in order to decrease the activation barrier, hence increase the reaction rate.
Two factors are considered for the choice of the catalyst: how good is the catalyst + how robust (after
how much time do you need to replace it). Here only heterogenous catalysts are considered: catalyst
is in a different phase (solid) as the reagents (gas).
- Cu-Zn on alumina (solid support): large surface (100 m2/g catalyst) + robust + good activity (T
can be lowered to 250°C). Drawback: requires a difficult purification, but faster reaction and
lower T
- Zn-Cr: less reactive catalyst, hence requires T of 350°C + higher resistance against impurities
(sulfates) which poison the catalyst (= deactivate it). Advantage: easier purification, but higher
T + slower reaction
Besides the main reaction, side reactions can also occur:
- CO + 3 H2 → CH4 + H2O (exothermic) => inverted process
- 2 CO + 4 H2 → CH3-O-CH3 + H2O (exothermic) => dimethyl ethyl is volatile and explosive
- The side reactions to heavier alcohols
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,All side reactions are exothermic and since the exothermicity is additive, a lot of heat can be released
if besides the main reaction, the side reactions would also occur. This could result in a runaway
reaction: the released heat results in increased reactions rates, which results in more heat release,
hence higher reaction rates … viscous circle which results in uncontrollable reactions.
In order to avoid such runaway reactions the Seveso restrictions were established (safety
measurements each chemical industry has to obey).
The solution to runaway reactions is: very short residence time in the reactor itself, hence the reagents
are also used as buffers for the released heat. However, this results in low conversions and therefore,
the unreacted reagents are recycled and used back.
The flow scheme of methanol:
The syngasses are added horizontally via a compressor at 10 bar and are partially dissolved into the
alkaline solution (liquid), in fact only the nitrogen and sulfur impurities are dissolved because they
possess acidic properties, hence they are neutralized. During this purification absorption occurs: the
absorption of gas in the bulk of the liquid phase. The remaining gas is transferred further via a
compressor and the pressure is set at 300 bar.
During the cleaning step adsorption is performed: the adsorption of particles to the surface of a solid
material. Due to the adsorption, this material becomes saturated and can no longer adsorb, for that
reason the adsorption towers are switched. However, this step is discontinue, within a whole
continuous process, and is not liked for the reason that discontinuous steps change the process
parameters.
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, Afterwards, the purified syngasses can enter the heat exchanger, which is a way to exchange heat
between two fluids (or gasses) without mixing them. This heat exchanger is used to preheat the
reagents, but is avoided in some cases.
Afterwards the reagents are introduced into the reactor (unit process) and react. The reagents are
added via two different pipes to the reactor which allows a better T control and increased conversion.
The use of different catalyst layers results in a better T spread, hence less heat release. Recall that is
some cases the heat exchanger is skipped, well this is done in order to use cold reagents => cold shot
cooling. Both T control and cold shot cooling are used to avoid runaway reactions.
Afterwards, the pipe containing the liquid is cooled down with water in the condenser, which results
in a separation of the liquid (MeOH, water and side products) from the unreacted gasses which are
recycled and pressurized again with the condensor. The gasses are then transferred back to reactor,
except for a small fraction (about 10%) which is unselectively removed with a T-piece at the purge.
The reason being that inert compounds have to be removed in order for the process the mostly involve
reagentia. The flux of incoming inerts has to be different than the flux of outgoing inerts (= Fp . Xp).
The liquid phase is purified twice with distillation. The first distillation removes all compounds with a
lower boiling point than water, such as dissolved gasses and dimethyl ether. The remaining liquids are
then passed through a boiler into a second distillation in which methanol is separator from water (has
a higher boiling point than MeOH).
2.2. Balances
Different fundamental laws have to be satisfied in order for a chemical process to the established
correctly:
- Conservation laws: impulse, mass, energy
- Equilibrium laws: thermal, liquid-gas
- Kinetic laws: heat- and mass transfer, reaction speed
Those laws allow to determine mass flows and energy requirements for the plants. However, those
laws are not sufficient and other fields of engineering are required:
- Optimization using non-linear programming
- Process controlling: Keep T optimal, no runaway
• Proportional (P) controller: slow convergence to the optimal
T
• Proportional integrate (PI) controller: integration of the
surface that has to be covered, but results in T oscillations around
the optimal T
• PI differential (PID) controller: faster convergence to the
optimal T
- Planning, cost, layout, construction so that safety and efficiency are maximal
- Choice of materials and sustainability
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