7. Chemical Economics - A variety of costs involved! Sociological, environmental issues etc.

• The greater the amount of starting materials (reactants) the greater amount of new substances (products) formed.
• 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 section 6a. and

• 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 depends on:
  • Price of energy (e.g. gas, electricity).
  • Starting materials (reactants).
  • Labour (wages).
  • Equipment (chemical plant e.g. machines, reactors, heat transfer systems).
  • Speed of manufacture (time efficiency).
• These cost factors can be analysed in more detail e.g.
  • 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 temperature.
  • 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 catalysts etc.
    • 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 iron,
    • the Haber synthesis of ammonia,
    • and the manufacturing sulphuric acid by the Contact process.
• Locating a chemical works: Many factors need to be considered.
  1. 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 steel works.
  2. 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.
     • See Issues related to limestone quarrying
  3. Availability of suitable workforce:
     • Are there enough people locally to operate the works AND with the requisite skills?
  4. 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 costs
  • 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 further use.
    • 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 developing country.
  • Various ways of dealing with the problem of waste plastics is encouraging novel ideas to recycle plastic/polymer materials.
  • For specific metal recycling examples See
    • iron extraction/recycling
    • aluminium extraction/recycling
    • General ideas on chemical economics involving metal extraction/recycling.
• Case studies:
  • Synthesis of ammonia page illustrates some of these ideas,
  • Some issues and ideas related to mining and the extraction of metals
  • Issues related to limestone quarrying

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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