Managing Chemical Processes: Optimizing Production

Managing Chemical Processes

Learning Intentions

  • Industrial processes are designed to maximise profit and to minimise impact on the environment.

Success Criteria

  • Explain how certain reaction conditions represent a compromise that will give maximum yield in a short time.

  • Explain the impact of increases in temperature and pressure on manufacturing conditions and costs, and on the environment.

  • Explain how use of a catalyst may benefit both the manufacturer and the environment.

Chemical Industry: Factors Influencing Production

  • Rate: Industry aims for the fastest possible production.

  • Yield: Industry seeks the maximum possible product amount from the reaction.

  • Cost: Industry desires the highest possible profit, meaning the lowest production costs.

  • A balance between these three factors is crucial, as the fastest rate may not yield the best outcome, and the best yield may not be the cheapest.

Chemical Industry: Considerations for Setting Up a Plant

  • Availability of raw materials.

  • Ease of handling and transporting the chemicals.

  • Value and demand of any by-products.

  • Environmental concerns and impact.

  • Availability of the workforce.

Chemical Industry: Overview

  • The chemical industry applies chemistry knowledge to produce useful materials on a large scale.

  • An industrial chemical process usually consists of an integrated series of chemical reactions and associated steps.

  • In the process, raw materials are converted into desired products.

  • The process is usually carried out in an assembly of equipment through which materials travel continuously.

  • A flow chart is often used to represent the movement of materials through the various components of the plant.

Raw Materials, Waste Products, and By-products

  • Raw Materials

    • An unprocessed material used to produce more useful materials or energy in an industrial process.

    • Examples:

      • Coal for electricity production.

      • Crude oil for the production of fuels.

      • Water as a solvent or coolant.

      • Air for heat production (combustion).

  • Waste Products

    • Any unusable product of a reaction.

    • These products are unwanted or undesirable and are often difficult to dispose of.

  • By-products

    • Any usable product of a reaction in an industrial process.

    • By-products could have a secondary use on site at the industrial plant or can be sold to generate profit.

Flow Charts

  • A diagram that identifies different stages of a chemical process, progressing from the initial processing of raw materials to the extraction, isolation and purification of the final products.

  • Raw materials, waste products and by-products are all identified on a flow chart.

  • A flow chart is usually presented in a line and block diagram. Some quantities of materials used and produced can be shown.

  • The convention is to represent each piece of equipment as a labelled rectangle or circle.

  • Annotated arrows are used to show the movement of materials through the process.

Example Flow Chart: Production of Zinc from Sphalerite (ZnS)

  • Stage 1: Froth flotation

    • Raw materials: Sphalerite (ZnS), water, air (contains O_2)

    • Waste: Waste rock

    • Products: ZnS

  • Stage 2: Roasting

    • Reactant: ZnS, Oxygen (O_2)

    • Products: Zinc Oxide (ZnO), Sulfur Dioxide (SO_2)

  • Stage 3: Leaching

    • Reactants: Zinc Oxide (ZnO), Sulfuric Acid (H2SO4)

    • Product: Zinc Sulfate (ZnSO_4)

  • Stage 4: Purification

    • Reactant: Zinc Sulfate (ZnSO_4)

    • Waste: Metal residues

    • Product: Pure Zinc Sulfate (ZnSO_4).

  • Stage 5: Electrolysis

    • Reactant: Zinc Sulfate (ZnSO_4)

    • Product: Zinc (Zn)

Yield

  • Many of the wealthiest companies in the world are manufacturers of chemicals, and their profits are made by optimising the chemical processes that take place.

  • The term yield refers to the quantity of the product that was obtained in a chemical process. The more efficient a process is, the higher the yield will be.

  • The percentage yield for a chemical process can be used to measure the efficiency of a process.

  • Percentage Yield = (Actual Yield / Theoretical Yield) * 100

    • Actual yield – the quantity of product recovered

    • Theoretical yield – the quantity of product predicted by the stoichiometric ratio.

Maximising Yield

  • For a particular production process, the conditions of the chemical reactions are controlled to produce the best yield of product possible at an economic rate of formation.

  • Conditions that give the highest yield may not give a fast rate of reaction and there may be a need for a compromise between these factors.

  • The balance between yield and rate of formation must be considered when determining the reaction conditions.

  • Some of the yield may be sacrificed to permit an economic rate of formation of product.

High Pressure

  • Some chemical processes may involve a reversible reaction between gases. To obtain a high yield of product using Le Chatelier’s Principle, high pressure is sometimes required.

  • This is not always possible in industry, because:

    • Maintaining high pressure is expensive and often dangerous.

    • High pressure pumping equipment needs constant maintenance and repair.

    • The increased pressure is only cost effective if the profit generated from the higher yield of product exceeds the amount of money required to maintain high pressure.

High Temperature

  • High temperatures are used to increase the rate of formation of a product in a chemical process.

  • Temperature also affects the equilibrium position in a reversible reaction.

  • If a reaction is exothermic, higher temperatures cause the equilibrium position to shift towards the formation of reactants, reducing yield.

  • Low temperatures are not favourable however, as the rate of reaction will decrease and this leads to an increase in costs associated with power generation and staff wages.

  • A compromise is made between the rate of reaction and yield of product.

Catalysts

  • Catalysts are used in industrial processes to increase the rate of formation of products.

  • This minimises the reaction time and therefore reduces operating costs.

  • Catalysts are not used up in a chemical reaction and they can therefore be reused throughout the process.

  • A catalyst can also benefit the environment, as less energy is required to heat the process, reducing the emissions of fossil fuels.

Learning Intentions (Flow Charts)

  • Designing chemical-synthesis processes involves constructing reaction pathways that may include more than one chemical reaction. The steps in industrial chemical processes can be conveniently displayed in flow charts.

Success Criteria (Flow Charts)

  • Interpret flow charts and use them for such purposes as identifying raw materials, chemicals present at different steps in the process, waste products, and by- products.

The Haber Process – Production of Ammonia (NH_3)

  • The Haber process is the industrial process used throughout the world to produce ammonia.

  • N2(g) + 3H2(g)
    ightharpoonup 2NH_3(g)
    ightharpoonup ∆H = −46 kJmol^{-1}

  • Using Le Chatelier’s principle:

    • Should the reaction occur at a high or low pressure to maximise yield? High P

    • Should the reaction occur at high or low temperatures to maximise yield? Low T

Haber Process: Industrial Conditions

  • In reality, nitrogen and hydrogen gases are reacted at approximately 400°C and 250 atmospheres pressure in the presence of an iron catalyst.

  • N2(g) + 3H2(g)
    ightharpoonup 2NH_3(g)
    ightharpoonup ∆H = −46 kJmol^{-1}

  • Under these conditions the yield of ammonia is approximately 45% of the theoretical yield.

  • The actual pressure is high as predicted, however the temperature is also high which contradicts Le Chatelier’s principle.

  • The higher temperature means the rate of formation of product is increased.

Haber Process: Economical Yield

  • Although the yield of ammonia produced is lower under these conditions, the conditions used in industry are those which provide the most economical yield of ammonia (in terms of time and cost).

Haber Process: Flow Chart Considerations

  • Raw Materials: Natural gas, water, air

  • Waste: Carbon dioxide

  • Environment effect:

    1. Increase emission of CO_2, increase amount of greenhouse gases, cause global warming

    2. Use of energy to provide high temperature, burning of fuel, increase green house gas emission

Haber Process - Benefits

  • Manufacture of cleaning agents

  • Manufacture of N-based fertilisers e.g. ammonium nitrate, ammonium phosphate

  • Manufacture of dyes

  • Manufacture of explosives

  • Manufacture urea and UAN*

  • Refrigeration using ammonia systems

  • Food production e.g. ammonium bicarbonate

  • Manufacture of nitric acid

  • Photography as fixers in processing

  • UAN is a solution of urea and ammonium nitrate.

The Contact Process – Production of sulfuric acid (H2SO4)

  • Most sulfuric acid is produced by the chemical industry via the Contact Process

  • The first step involves burning of elemental sulfur or metallic sulfides (e.g. zinc sulfide) in air at high temperature to produce sulfur dioxide.

  • S(s) + O2(g) → SO2(g)

  • A mixture of sulfur dioxide and air is then passed through beds containing vanadium pentoxide catalyst. The sulfur dioxide reacts with oxygen to form sulfur trioxide.

  • 2SO2(g) + O2(g)
    ightharpoonup 2SO_3(g) ∆H = -99 kJmol^{-1}

Contact Process: Key Step

  • The key step in the Contact Process is the production of sulfur trioxide.

  • 2SO2(g) + O2(g)
    ightharpoonup 2SO_3(g) ∆H = -99 kJmol^{-1}

  • Using Le Chatelier’s principle:

    • Should the reaction occur at high or low pressure? High P

    • Should the reaction occur at high or low temperature? Low T

Contact Process: Industrial Conditions

  • The actual pressure used for the contact process is atmospheric pressure.

  • The actual temperature is moderately high.

  • A higher temperature is used to increase the rate of reaction.

  • The conditions used in industry are those which provide the most economical yield of sulfur trioxide (in terms of time and cost) Industry Conditions 450°C, 1atm

Contact Process: Flow Chart Considerations

  • Raw Materials: sulfur, air, water

  • Bi-product: H2SO4 produced in step 4, then it was re-introduced to step 3 as a reactant.

  • Catalyst: V2O5

Uses of Sulfuric Acid

  • Manufacture of metals e.g. leaching processes

  • Manufacture of fertilisers e.g. superphosphate

  • Manufacture of paints

  • Manufacture of plastics e.g. rayon & nylon

  • Manufacture of electrolytes e.g. car batteries

  • Manufacture of explosives

  • Manufacture of pesticides

  • Manufacture of soaps & detergents

TERM 1 HOLIDAY TASKS

  • Read SASTA 2nd workbook, page 192-202

  • Complete SASTA workbook 2nd Ed

  • Page 203 Q66-69

  • Page 210 Review TEST 2 Q1-4

  • Advanced notice: SAT 2 – Tuesday week 2 term 2

  • Term 2 week 2 SAT 2 Revision