Topic guide 13.2: Factors affecting the selection of a chemical process

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Unit 13: Industrial chemistry

The chemical industry produces in excess of 70 000 different product substances. For each product there will one (or more) specific process that has been developed to form the product to a suitable degree of purity and in a way that is cost-effective, safe and minimises the effect on the environment.

In this topic guide you will look at the factors that are taken into account in the design of such processes, using the production of silicone polymers as a detailed case study. You will be introduced step-by-step to the key features of this process and encouraged to apply a similar analysis to your chosen process in order to build up your assessment portfolio.

On successful completion of this topic you will: • understand factors affecting the selection of a chemical process (LO2).

To achieve a Pass in this unit you need to show that you can: • explain the relevance of process factors to a chemical process (2.1) • explain the importance of the quality of the product (2.2) • explain the importance of co- and side-products to the overall

profitability of the process (2.3) • assess safety aspects and discuss measures required for compliance with

the Environmental Protection Act (2.4).

Factors affecting the selection of a chemical process13.2

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13.2: Factors affecting the selection of a chemical process

1 Process factorsProcess factors are factors that directly relate to the chemical reaction that forms the product.

These can relate to: • the starting materials (raw materials) needed in the reaction • the yield and rate of the reaction • the operating conditions • the carbon footprint of the process.

Raw materialsA key factor in the selection of a chemical process is the raw materials from which the product is formed. The term ‘raw material’ is often used rather imprecisely and in different contexts should probably be replaced by the terms ‘feedstock’ and ‘commodity chemicals’. These terms can themselves overlap, but they represent the different levels of processing needed to form the chemical.

The cost of the chemical increases with the amount of processing required, so in general a commodity chemical will be more expensive than a feedstock, which in turn will be more expensive than the raw material. Added to these are the costs of transport.

ActivityThe typical cost of crude oil in 2010 was $76 per tonne; in the same period, naphtha typically cost around $420 per tonne and benzene $1090 per tonne.

• Suggest reasons for the differences in these prices.

Case study: The manufacture of silicone polymersSilicone polymers are based on chains of alternating silicon and oxygen atoms, as shown in Figure 13.2.1.

Si

CH3

CH3

O Si

CH3n

CH3

O Si

CH3

CH3

H3C CH3

Figure 13.2.1: The structure of a silicone polymer.

They are non-volatile liquids with a low surface tension that have a wide range of applications in the production of products such as insulating gels, non-stick coatings, adhesives and waterproof sealants.

They are produced in a multi-step process:

Step 1: Reaction of silicon with chloromethane to form dimethyldichlorosilane:

Si + 2CH3Cl ➝ (CH

3)

2SiCl

2

Step 2: Hydrolysis of chlorosilanes to silanols:

(CH3)

2SiCl

2 + 2H

2O ➝ (CH

3)

2Si(OH)

2 + 2HCl

Continued

Key terms Raw materials: The basic materials from which a product is manufactured; this usually means a naturally occurring substance in an unprocessed (or only minimally processed) state. Examples: crude oil, air, metal ores, and biological materials such as beef tallow. Recycled materials such as glass could also be classified as raw materials.

Feedstock: Raw materials that have been processed (for example, by distillation or cracking) to convert them into a form that can be added to a reactor. Examples: petroleum gases, naphtha, oxygen, iron sulfide, silicon dioxide.

Commodity chemicals: The key building blocks of chemical reactions that have been formed by chemical reactions from basic feedstocks and purified to enable them to be sold to other sectors of the chemical industry. Examples: benzene, ethene, titanium oxide, sulfuric acid, magnesium, ethanol.

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Step 3: These silanols polymerise in the presence of the HCl (which acts as a catalyst).

HO Si

CH3 nn

CH3

O HSi

CH3

CH3

OHHO H2O+ (n – 1)

Figure 13.2.2: The polymerisation of silanols.

Step 4: The HCl is then used in a reaction with methanol to generate the chloromethane used in step 1:

CH3OH + HCl ➝ CH

3Cl + H

2O

Questions: • Identify the starting materials for this process. Which term do you think best describes them:

raw materials, feedstocks or commodity chemicals? • The silicon used in this process is produced by heating silicon dioxide with carbon. What

starting materials provide a source of these two substances?

Take it furtherDow Corning, a US-based company, manufactures and supplies a wide range of silicone-based products. The company website provides an accessible introduction to the use and manufacture of silicones and full details of the silicone manufacturing process. It can be found at http://www.dowcorning.com/content/discover/discoverchem/si-manufacturing.aspx.

The Essential Chemical Industry (Allan Clements et al., 2010) also has a very useful section on the manufacturing process (p209–212).

Yield of reactionThe yield of a process is simply the mass of product formed, but is usually expressed as the percentage yield:

percentage yield =

mass of product formedtheoretical mass of product formed

× 100

Take it furtherThe calculation of theoretical mass of product formed uses the mole concept as the basis of chemical calculations. More information about these calculations can be found in level 2 or level 3 textbooks such as GCSE Chemistry (Saunders and Saunders, 2011, OUP), Chemistry in Context (Hill & Holman, 2000, Nelson Thornes), or Chemical Ideas (ed. C. Otter, 2008, Heinemann), or on the website http://chemistry.about.com/od/workedchemistryproblems/a/How-To-Calculate-Theoretical-Yield-Of-A-Chemical-Reaction.htm.

Reasons for low percentage yield could include: • the formation of unwanted side-products (by-products) • a reaction not going to completion (because, for example, it reaches an

equilibrium state or because it may take too long to go to completion) • a large number of stages in the process, as material may be lost at each stage

and the cumulative effect of this may be significant.

Key termSide-products: Substances formed in a chemical process as a result of unwanted chemical reactions alongside the major process.

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Low percentage yield adds to the cost of a process because: • a greater mass of starting materials will be required to produce the same mass

of product • there will be increased separation or purification costs • greater mass of waste products increases the environmental burden of

the process.

Co-products

Look at the two equations in Figure 13.2.3. They represent two different processes which could be used to manufacture propanone (CH3COCH3); you will be studying these reactions in more detail in Topic guide 13.4.

H3C CH3

CH3 CH3

OH

OH

H2OH+

CH3+ +

H3C

O

H3C H2H3C +

O

The equation for both of these reactions shows you that co-products will be formed alongside the propanone. The formation of co-products may still create issues of separation and purification, but can contribute to the overall cost-effectiveness of the process as they may well be highly marketable. This is certainly true for the co-products of the two processes – phenol and hydrogen – in the reactions shown in Figure 13.2.3.

In some cases, however, the co-products have no market value and may be environmentally damaging. Disposing of these co-products in a safe way without damaging the environment will add significantly to the cost of the process.

Case study: Formation of chlorosilanesThe reaction in stage 2 of the silicone polymers process produces dimethyldichlorosilane in approximately 70–90% yield. This is because of the formation of other chlorosilanes: CH

3SiCl

3 and

(CH3)

3SiCl.

1 Are these alternative chlorosilanes co-products or by-products?2 Explain why they may increase the cost of the process.

Atom economy

The presence of co-products means that even if the yield of a reaction is close to 100%, not all the mass of the starting materials will end up as product molecules. The atom economy of a process can be calculated to give a useful guide to this:

atom economy =

relative molecular mass of product moleculerelative molecular mass of all reactant molecules

× 100

Key termCo-products: Substances formed in a chemical process alongside the product; they appear in the chemical equation and will be formed in a precise and predictable ratio to the main product.

Figure 13.2.3: Both of these processes for the manufacture of propanone

involve the formation of co-products.

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Case study: Atom economyThe overall process for the production of silicone has a very high atom economy, which means that it satisfies one of the criteria for a ’green chemistry‘ process. Some careful thought is needed to explain how this high value for atom economy comes about.1 What is the atom economy of stage 1?2 Calculate the atom economy of stage 2 of the process (A

r values: C = 12, H = 1, O = 16, Si = 28,

Cl = 35.5).3 Stage 2 does not have an atom economy of 100%. Suggest why this does not affect the atom

economy of the complete process (hint: look at stage 4).

Case study: Co-productsThe silicon for the production of silicones is produced by the reduction of silicon dioxide by carbon at high temperature (a process known as smelting):

SiO2 + 2C ➝ Si + 2CO

• Explain why the formation of carbon monoxide as a co-product is likely to add to the environmental burden and cost of this process.

Rate of reactionThe rate of a chemical reaction is clearly important in determining the output rate of a process; for a batch process, the faster the rate, the shorter the time spent in the reaction vessel. For a continuous process, the rate needs to be fast enough so that complete reaction will occur during the time that the reactants spend in the individual reactors of the process.

The different designs of reactors used in batch and continuous processes are shown in Figure 13.2.4.

Batch reactor(a)

Startingmixture

Productmixture

Product mixturecontinuously

removed

Reactants in Reactants added continuouslyAt start Some time later

Stirrer

(b) Continuous (stirred tank) reactor

Key termsSmelting: The extraction of elements from molten ores.

Batch process: Starting materials are put into a reactor and allowed to react. When the reaction is judged to be complete, the products are separated from the reaction mixture.

Continuous process: Starting materials are continuously fed into one side of a reactor and products are continuously withdrawn from the other side.

Link This links to ideas about rate encountered in Unit 5, Topic guide 5.1.

Figure 13.2.4: A comparison of simple reactors used in batch

and continuous processes.

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Case study: Batch and continuous processesThe smelting reaction that produces silicon is a batch process: silicon dioxide and carbon are heated in a furnace to about 1400 °C. Molten silicon collects at the bottom of the furnace and is drained off and cooled.

The multistage process that produces the silicone polymers is then run as a continuous process. This is shown in Figure 13.2.5.

Siliconmetal

Grinder

Ground silicon metal

MethanolMethylchoride

Catalyst

Recycled acid

Hydrochloricacid

Polymerisationand finishing

processes

Silanols

Water

Distilledchlorosilanemonomers

Chlorosilanemix

Figure 13.2.5: The production of silicone polymers uses a sequence of continuous processes.

• Suggest reasons why it might be difficult to operate smelting reactions as continuous processes.

Rate and yield

Although the rate of a chemical reaction is an important factor in determining conditions, the desirability of increasing the rate must be set against the possible effect on the yield of the reaction. Increasing temperature and pressure may have undesirable effects on the yield of a reaction.

High temperatures and pressures will also increase the cost and environmental impact of the process.

Temperature

For conventional chemical processes, an increase in temperature will be accompanied by an increase in rate. Reactions involving biological agents such as enzymes will have an optimum temperature producing a maximum rate, usually in the region 20–60 °C.

However, high temperatures may be undesirable for other reasons: • for exothermic reactions that reach equilibrium, increasing the temperature

will decrease the yield • high temperatures increase cost of fuel • high temperatures increase the carbon footprint of the process • it may be difficult to control the reaction safely (particularly for exothermic

reactions).

Key termCarbon footprint: A measure of the total amount of greenhouse gas emissions caused by a particular activity. Greenhouse gases can include methane and nitrous oxide as well as carbon dioxide, but carbon footprint is usually measured in tonnes of CO

2 equivalent.

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Take it furtherCalculations of the carbon footprints of a range of products of the chemical industry have been made, and are available on websites such as http://www.ihs.com/products/chemical/ technology/pep/reviews/estimating-carbon-footprint.aspx (subscription required).

Pressure and concentration

For gas phase reactions, increasing pressure will increase rate.

However, an increase in pressure may be undesirable: • for reactions that result in a net increase in the volume of gaseous products,

increasing the pressure will decrease the yield • high pressures require energy to create and sustain them, thus increasing

fuel costs • high pressure may increase the hazards of the process, for example, by

increasing the risks of explosions or leaks • high pressure increases the carbon footprint of the process.

For reactions in solution, increasing the concentration of some or all reagents will increase the rate.

Case study: Conditions for the reactions in the formation of siliconesThe production of chlorosilanes (step 1) is carried out at 280 °C and a slightly raised pressure (1–5 bar).

Steps 2, 3 and 4 take place at a sufficiently high rate at room temperature. Increasing pressure has no effect on the rate of reaction, so 1 bar pressure is used.

Questions:1 What can you conclude about the reactivity of the chlorosilanes and silanols in steps 2 and 3 if

the reactions occur rapidly at room temperature?2 Suggest why increasing pressure has no effect on the rate of reaction. (Hint: if the reaction is

done at room temperature, what can you say about the likely states of the reactants?)

Catalysts

Catalysts are necessary in most chemical processes for two reasons: • they decrease the temperature required to carry out a reaction at an

acceptable rate • they provide selectivity in the process, often reducing or preventing the

formation of by-products.

You will read more about catalysts in Topic guide 13.3.

http://www.ihs.com/products/chemical/technology/pep/reviews/estimating-carbon-footprint.aspx

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2 Quality of productProduct quality can include features such as:

• the purity of the product • the presence of any co- or side-products formed • the ease by which the product can be separated from these co- and side-

products • the ease of purification of the product • the method used in quality control of the product.

Difficulties of purification and separationFor many liquid products, repeated fractional distillation will produce a pure sample of the required product.

Fractionating towers (also known as fractionating or distillation columns) are a familiar sight in petrochemical plants, as shown in Figure 13.2.6. The design of a fractionating tower, in which repeated cycles of evaporation and condensation enable separation of components with similar boiling points, is shown in Figure 13.2.7.

Figure 13.2.7: The process of fractional distillation. Hot vapour rising up a column undergoes a series of condensation and evaporation stages, resulting in separation

of the system into a series of fractions with different boiling ranges.

Gases

Petrol

Kerosene

Diesel

Fuel oil

Asphalt

Heated crude oil

Vapour

However, some mixtures (for example, water/ethanol or propanone/hexane) form azeotropic mixtures. This means that repeated distillation of the mixture produces only a mixture of the components. In these cases, other components can be added to disrupt the interaction between the molecules in the mixture.

Alternatively, other forms of separation can be used, for example, molecular sieves that contain tiny pores that can be penetrated by water molecules, which are then adsorbed on the surface of the sieve.

LinkDetails of a wider range of separation and purification processes will be found in Topic guide 13.3, along with a discussion of the theoretical basis of these techniques.

Figure 13.2.6: Fractional distillation towers.

LinkAzeotropic mixtures and the process of distillation are discussed in more detail in Unit 6, Topic guide 6.2.

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Take it furtherA list of some common azeotropic mixtures and the problems that these can cause for separation of products from solvents can be found at http://www.solvent--recycling.com/azeotrope_1.html.

Other methods of separation include solvent extraction, crystallisation and chromatography.

Case study: Separation of products in the formation of chlorosilanesIn a multi-step process, such as the one used in the formation of silicones, separation of the required product will need to occur at the end of each stage.

In stage 1, several chlorosilanes are formed. These have slightly different boiling points:

CH3SiCl

3: b.p. = 66 °C

(CH3)

2SiCl

2: b.p. = 70 °C

(CH3)

3SiCl: b.p. = 57 °C

The dimethyldichlorosilane is the main product, and makes up 70–80% of the product mix. The other two products have uses in the processing of the silicone polymer to produce cross-linked gels, elastomers and resins.

Question: • Comment on the separation method and any problems that need to be overcome.

3 Environmental and safety aspectsEnvironmental Protection ActThe main aspects of this act that are relevant to the chemical industry are:

• some substances and processes are described as prescribed – limits are set for the emissions of prescribed substances into the environment

• the disposal of any waste classified as controlled waste (which includes industrial waste) must be regulated and licensed

• remediation of any contaminated land is made compulsory • statutory nuisances are defined. Several of these are relevant to the chemical

industry including noise, smoke, dust, fumes and smells which may affect health or cause a public nuisance in the immediate environment of the plant. Local authorities can serve an abatement order on any premises which cause these nuisances

• there are also extra regulations relating to radioactive substances and genetically modified organisms.

Take it furtherThe Environment Agency website (http://www.environment-agency.gov.uk/business/default.aspx) contains up-to-date information about environmental permitting regulations.

Environmental permits are required for many industrial activities. Local councils maintain records of applications made to operate prescribed processes and the permits issued. By contacting the relevant local council it is possible to get copies of the permits issued and the applications made for a particular prescribed process.

LinkYou will read more about these methods of separation, and others, in Topic guide 13.3.

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COSHH assessmentCOSHH stands for Control of Substances Hazardous to Health. The definition of such substances is quite wide-reaching and includes chemicals and biological agents (although not lead, asbestos or radioactive materials, which have separate regulations).

COSHH assessments apply to any workplace in which hazardous substances are used, so not just the chemical industry, but obviously they apply to every aspect of a chemical process from operating the plant to cleaning the building in which it may be housed.

Carrying out a COSHH assessment

For every activity that involves chemicals, the following assessment should be made: • what hazardous substances are involved? • how do these cause harm (the hazard)? • how can the risk of harm occurring be minimised (the control measures)?

Take it furtherThe Health and Safety Executive website has more details of COSHH regulations and how to carry out a COSHH assessment: http://www.hse.gov.uk/coshh/.

Risk analysis COSHH assessments restrict themselves to the impact of chemicals on workers. A wider risk assessment will look at other kinds of risk and attempt to quantify the impact of the risks.

Take it furtherThe Management of Health and Safety at Work Regulations (1999) set out the legal obligations of an employer in respect of the health and safety of employees and any temporary workers. It is available at http://www.legislation.gov.uk/uksi/1999/3242/contents/made.

If a new process is being introduced, a company will carry out a risk assessment on the complete process.

In addition to the risk of workers being exposed to chemicals covered in the COSHH assessment, there are other aspects that may be covered by such a risk assessment. These could include:

• the impact of heat, noise or operating heavy machinery on workers • the possible effect of fire or explosion on the immediate environment • the risks to the wider environment of emissions and disposal of waste

(including, for example, heated coolant water).

COMAH regulations

The risk of fire or explosion due to the operation of a process is regulated by the measures outlined in the Control of Major Accident Hazards (COMAH) regulations.

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This ensures that businesses: • ’Take all necessary measures to prevent major accidents involving dangerous

substances’ (for example, by ensuring that there is sufficient segregation between large-scale stocks of flammable solvents and potentially explosive reactors)

• ’Limit the consequences to people and the environment of any major accidents which do occur‘ (for example, by ensuring that there are appropriate firefighting systems in place and that there is an appropriate evacuation plan).

Emissions and waste disposal

As noted in Topic guide 13.1, the principles of ’green chemistry‘ have led to reductions in emissions at source for many manufacturing processes. However, some emissions and production of waste are unavoidable. Strategies for these include:

• ’scrubbing‘ the gaseous emissions to remove harmful gases such as sulfur dioxide

• treating the aqueous emissions to remove contaminants such as toxic metal ions, oils or complex organic molecules.

Take it furtherThe COMAH regulations can be found at www.hse.gov.uk/comah/background/index.htm.

ActivityLook at the details of the silicone manufacturing process provided in the case studies above and from the Dow Corning website (http://www.dowcorning.com/content/discover/discoverchem/si-manufacturing.aspx).

• Identify the possible hazards that may be associated with the process. Include the hazards of all the chemicals used. Consider also the possibility of additional hazards, such as heat, fire or explosions and the emissions from the process, as discussed above.

• Is there any indication in the information provided about the control measures that are used to minimise the risk of harm due to these hazards?

Portfolio activity (2.4)Assess the hazards in your chosen process and find out what control measures are used to minimise the risk of them causing harm. In your answer:

• include details of COSHH assessments that have been carried out • discuss what aspects of the process would be covered in a wider risk assessment of the

complete process • find out the details of any measures which are required for compliance with the Environmental

Protection Act.

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Further readingA helpful overview of much of the material in this topic guide can be found in Chemical Ideas (ed. C. Otter, 2008, Heinemann), which is a textbook aimed at A-level Chemistry students. Chapter 15 covers aspects of process factors, yield calculation and waste disposal along with safety and environmental issues.

Details of the silicone manufacturing process, which forms the basis for the case studies in this topic guide, can be found in The Essential Chemical Industry (Allan Clements et al., 2010) on pages 209–212, or alternatively via the Dow Corning website at http://www.dowcorning.com/ content/discover/discoverchem/si-manufacturing.aspx (Dow Corning is a US-based company which manufactures a wide range of silicone-based products).

The legislative background to the environmental issues and health and safety aspects can be explored in more depth on the websites of the relevant bodies: the Environment Agency http://www.environment-agency.gov.uk and the Health and Safety Executive http://www.hse.gov.uk.

AcknowledgementsThe publisher would like to thank the following for their kind permission to reproduce their photographs:

Imagestate Media: John Foxx Collection; Veer/Corbis: jwolf 8

All other images © Pearson Education

We are grateful to the following for permission to reproduce copyright material:

Diagrams of simple reactors used in batch and continuous processes, from Salters’ Advanced Chemistry (G.Burton et al) Heinemann, 2000 p355. Reproduced by permission of Pearson Education Ltd; Diagram based on one shown on Dow Corning website, dowcorning.com ©2000 - 2013 Dow Corning Corporation. All rights reserved. Used by permission of Dow Corning.

Every effort has been made to trace the copyright holders and we apologise in advance for any unintentional omissions. We would be pleased to insert the appropriate acknowledgement in any subsequent edition of this publication.

http://www.dowcorning.com/content/discover/discoverchem/si-manufacturing.aspx

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Topic guide 13.2: Factors affecting the selection of a chemical process