4.1 Industrial chemical processes

Question 1

The principles of iron and steel-making have not changed much since Roman times. The process is simple and uses four major raw materials:

Answer 1

iron ore, coke (from coal), limestone and air.

Question 2

Increasing the collision rate between particles will increase the rate of reaction. The key variables are: 5

Answer 2

 Temperature: Increasing temperature speeds up a reaction (except for those involving enzymes).  Pressure: Pressure increases will increase the rate of reactions involving gases.  Concentration: Increasing concentration normally increases the rate of reaction.  Surface area of reactant: Increasing the surface area of reactants increases the rate of reaction.  Catalyst: The presence of a catalyst is intended to increase the rate of a reaction.

Question 3

Exothermic reaction means…

Answer 3

In many chemical reactions, energy is given out by reactants as they form products, causing the temperature of the surroundings to rise.

Question 4

Endothermic reaction means…

Answer 4

the opposite of exothermic reactions. In an endothermic reaction energy is taken in by the reactants to form products. The energy comes from the surroundings which lose energy and cool down, resulting in a drop in temperature.

Question 5

Thermal runaway begins when

Answer 5

the heat produced by an exothermic reaction exceeds the heat removed. The surplus heat raises the temperature of the reaction mass, which causes the rate of reaction to increase, which in turn accelerates the rate of heat production.

Question 6

At best, a runaway causes loss and disruption of production, at worst it may result in a major accident. The harmful effects could include: 6

Answer 6

 the boiling over of the reaction mass  large increases in temperature and pressure resulting in an explosion  blast and missile damage  secondary fires or explosions (if the materials are flammable)  contamination of the workplace by hot liquors and toxic materials  toxic clouds that may spread off-site.

Question 7

An analysis of thermal runaways in the UK has indicated that incidents occur because of: 4

Answer 7

 inadequate understanding of the process chemistry and thermochemistry  inadequate design for heat removal  inadequate control systems and safety systems  inadequate operational procedures, including training.

Question 8

Inherently safer methods Wherever possible hazards should be eliminated or reduced by inherently safer design. Examples include: 4

Answer 8

 Replacing hazardous materials with safer ones.  Having less unreacted material in the reactor. For example: using a continuous process instead of a batch reactor.  Using a semi-batch method (in which one of the raw materials is added over time) instead of a batch process.  Using a heating medium which has a maximum temperature that is too low for the reaction mixture to decompose.

Question 9

Process control includes the use of

Answer 9

sensors, alarms, trips and other control systems that either take automatic action, or allow for manual intervention to prevent the conditions for uncontrolled reaction occurring.

Question 10

Protective measures do not prevent a runaway but are intended to reduce the consequences should one occur. They are rarely used on their own as some preventive measures are normally required to reduce the demand upon them. Examples include: 5

Answer 10

 Designing the plant to contain the maximum pressure.  Fitting emergency relief vents and ensuring that the vented material goes to a safe place.  Crash cooling the reaction mixture if it moves outside set limits by providing additional cooling by: - bringing a reflux condenser on-line - using a refrigerant in the reactor cooling coils or jacket - pumping the reactor contents through an external heat exchanger.  Adding a reaction inhibitor to kill the reaction and prevent runaway. For example: by free-radical scavenger injection or by poisoning a catalyst.  Dumping the reaction into a quenching fluid.

Question 11

Exothermic chemical processes rely on efficient control of temperature and pressure to prevent the phenomenon of ‘runaway reaction’ (thermal runaway) when mixing chemicals in a reactor. Outline the methods used to control temperature and pressure in such circumstances. 10 marks

Answer 11

An important method to control temperature and pressure would be the installation of high integrity temperature detection linked to the cooling and reactant addition systems with pressure rise detection linked to cooling, venting or auto shut-down. Consideration could also be given to the external cooling of the reactor by chilled water jacketing, an air cooled heat exchanger or the use of material with high heat conductivity in the construction of the reactor or internal cooling by the use of water or fluid refrigerant cooled coils. A further method would involve the control of the reactant by pre-chilling it, limiting the rate of its addition to the reactor, controlling the subsequent reaction by high efficiency temperature detection loops and the depth and level of reaction by pocketed thermal probes. The prevention of reactant bulk insulation can be prevented by efficient mixing with a mechanical stirrer while it would be advisable to locate the reactor away from external sources of heat and to introduce a system of regular internal cleaning to prevent the build-up of insulating residues.