Chemical Economics - A variety of costs
environmental issues etc.
- The greater the amount of starting
materials (reactants) the greater amount of new substances
- However in the real world chemical
processes are not 100% perfectly efficient!
- The amount that you actually make
is called the yield.
- The percentage % yield = actual
yield x 100 / predicted yield
- The predicted yield assumes there
is no loss of product, i.e. no waste, and the reaction goes 100%
in the desired direction.
- If no product is obtained then
the yield is 0%!
- In reality, yields can typically
range from 5% to 95% for a variety of chemical processes.
- The atom economy is
another important consideration.
- % atom economy = mass of useful
product x 100 / total mass of products
- See Chemical Calculations
- Why aren't processes 100% efficient?
Typical reasons are:
- Loss in filtration of a solid
product, i.e. some may get through as very fine particles or more
likely dissolved in the liquid residue.
- Loss in evaporation if the
product is a volatile liquid.
- Loss in transferring liquids,
i.e. traces left on the sides of containers.
- The reaction may be an
equilibrium, so its impossible to get 100% yield anyway and this
means that the yield of an equilibrium reaction depends on
the conditions used.
- The costs of making new substances
- Price of energy (e.g. gas,
- Starting materials (reactants).
- Labour (wages).
- Equipment (chemical plant e.g.
machines, reactors, heat transfer systems).
- Speed of manufacture
- These cost factors can be analysed in more
- The higher the operating pressure
of the reactor, the higher the cost. The engineering is more
costly due to e.g. thicker steel reaction vessel, higher health
and safety standards require.
- The higher the temperature the
higher the energy cost. Fortunately this cost is reduced if the
reaction is exothermic and the reaction does go faster at higher
- Time is money! so catalysts save time and
money by speeding up the reaction.
- The rate of reaction must be high
enough to give a reasonable yield in reasonable time e.g. at least
within 24 hours for a continuously working plant.
- Often with equilibrium reactions,
it is possible to recycle unreacted starting materials back
through the reactor. The % yield must be high enough at least
per day, but an initial low yield is quite acceptable if the
unreacted starting materials can be recycled many times on a
continuous basis through the reactor.
- Optimum reaction conditions are
geared to the lowest cost situation. This often means
'balancing' the rate of reaction versus the highest % yield. It
is often best to get a low yield fast and recycle!
- Automating the chemical plants
with sensors, controls, computer software etc. significantly
reduces the wages bill.
- Batch and continuous processes
- A batch process in
chemical manufacturing is where the reactant chemicals (raw
materials/feedstock) have to me mixed
in a reactor vessel or furnace etc. When the reaction is completed as far as it
will go, the product is then extracted.
- The reactor must then
be cleaned out before it can be re-used to make the next 'batch'
by re-filling the reaction vessel with more reactants.
- It is
generally less economic than continuous processes (see below).
Typically salts, drugs, alcohol from fermentation, making
specialised steel alloys etc. are
examples of chemicals made by batch processes.
- In a continuous process the
reactants are continuously fed into the reactor vessel or
reaction chamber and the products are continuously
extracted and removed.
- This is usually more economic than
batch processing because the is no stopping and starting
situation and the chemical plant may run for 6-12 months before
shutting down for essential maintenance or replenishing damaged
- Another advantage of a
continuous processes is that unreacted chemicals can usually be
separated from the product and recycled through the reactor, so
ALL the chemical feedstock (the reactants) are eventually used
up to form the desired product.
- Examples are: the
blast furnace extraction of
Haber synthesis of ammonia,
- and the
acid by the Contact process.
- Locating a chemical works:
Many factors need to be considered.
- Good transport links to
bring raw materials in and products out.
- e.g. you need at least
good road links and possibly rail or even water links e.g. if
factory was located on an estuary for importing iron ore to a
- Environmental, and health
and safety issues:
- e.g. how does the
factory impact on the local population from the point of
increase in road traffic, dangers from chemicals and pollution
from the chemical processes involved?
- How might it affect the
surrounding natural environment e.g. the flora (plants) and fauna (animals) of the locality if adjacent
or close to 'green land'?
- Is the land suitable and
planning permission granted? e.g. the land well drained, stable,
maybe a brown site of previously used land so as not to use
protected 'green belt' land.
Issues related to limestone quarrying
- Availability of suitable
- Are there enough people
locally to operate the works AND with the requisite skills?
- The availability of raw
materials and energy requirements:
- Are the raw materials
available locally or are they readily imported in?
- Can the energy demands
of the factory and offices be met by the e.g. the electricity grid?
- Is the supply of water
sufficient for the chemical processes involved?
- Recycling - way of saving on
- Recycling metals like aluminium and
iron/steel saves on costs AND allows a mineral resource like iron
ore to last a lot longer.
- Recycling metals may use as little
as 5% of the energy used to transport ore, extract the metal and
process into a useful product either as the pure metal or alloy.
- Therefore savings include, transport
costs may be less, but more importantly
- mining costs are omitted -
mining, crushing all use energy and machinery, and the
- cost of actually extracting the
metal from its finite ore resource - eg the chemical and
processing plants costs etc.
- So, scrap metal merchants are doing
a roaring trade at the moment.
- The savings are partly reduced by
the cost off collecting waste/scrap metal and purifying them for
- Quoted figures from the 1990s (and
some for 2008) for the UK (Britain), all are probably increasing at
the moment, but the data I have found at the moment - % of metal
recycled in metal products was
- Al aluminium 28% (39% in 2008), Cu
copper 18% (32% in 2008), Fe iron 40% (42% in 2008), Pb lead 60%,
tin 30%, zinc 30%
- As you can see, for a country with
little economic metal mineral ore deposits, the percentages are
quite (and should be) high.
- It should be pointed out in all
fairness, the extraction of metal ores and their overseas sales is
very important source of employment and revenue for an often poor
- Various ways of dealing with the
problem of waste plastics
is encouraging novel
ideas to recycle plastic/polymer materials.
- For specific metal recycling
- Case studies:
Notes information to help revise KS4 Science
Additional Science Triple Award Separate Sciences GCSE/IGCSE/O level
Chemistry Revision-Information Study Notes for revising for AQA GCSE Science, Edexcel
GCSE Science Edexcel IGCSE Chemistry & OCR 21st Century Science, OCR Gateway Science WJEC/CBAC
GCSE science-chemistry CCEA/CEA GCSE science-chemistry
(and courses equal to US grades 8, 9, 10) also useful revising and introduction to
metal extraction for A level AS/A2/IB chemistry students
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