Top Chemical Engineering Interview Questions & Answers

1. What is entropy?

Entropy is a measure of disorder within a system. It describes the behaviour of substances within an equilibrium environment. Gases inherently have greater entropies than liquids. 

2. What is an isochoric process?

This is a thermodynamic process in which the system has a constant volume. Since the volume is constant the system will have no work. You can then determine the relationship between pressure and temperature within the system.

3. List the advantages and disadvantages of a PFR.

PFR have the highest conversion rates of all the reactor types. They have more control over the system’s reaction and operating conditions such as temperature and have similar working principles as FBCR’s. Despite this, however, they are costly and have high operating and maintenance costs. 

4. What is global warming?

Generically global warming the process by which the global temperature increases due to an increase in the volume of heat-insulating gases are released into the atmosphere, creating the greenhouse effect. 

5. What is carbon sequestration?

Carbon sequestration or carbon dioxide removal is the long-term removal, capture or sequestration of carbon dioxide from the atmosphere to slow or reverse atmospheric CO2 pollution and to mitigate or reverse global warming

6. How does a Spray Condenser work?

The Spray Condenser. The coolant is sprayed, using nozzles, into a vessel to which the vapour is supplied. It is important that the spray nozzles and vessel are designed to produce a fine spray of liquid (to give a large interfacial area for heat transfer), and a long enough residence time of liquid droplets in the vessel.

7. What is Viscosity?

Your answer should include these keywords: Fluid, Resistance, Stress

8. Describe pneumatic conveying?

The principle of pneumatic conveying is based on the fact that bulk goods can be moved by means of air through pipelines. The flowing conveying air transmits a propulsion force on the bulk material and thus conveys it through the conveying line. The pneumatic conveying always requires a pressure difference between the beginning and the end of the pipeline. To overcome this pressure difference, conveying blowers or compressors are used.

9. How does a catalytic converter work?

Catalytic converters change harmful substances in a car’s exhaust gasses, such as carbon monoxide, nitric oxide, nitrogen dioxide and hydrocarbons, into less harmful substances like carbon dioxide and water vapour by means of chemical reactions.

10. How does paint dry?

In the case of paints, the pigment is the tiny particle that is suspended in the solvent-binding medium solution.  When paint dries, the solvent portion of the mixture evaporates away, leaving the pigment suspended in the binding medium in a hard layer.  These evaporating solvents are also largely responsible for the “smell” of the paint you are using.

11. What is Quicklime?

Calcium oxide (CaO), commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline, crystalline solid at room temperature.

12. How should you extract hydrogen from water?

Electrolysis is the process of using electricity to split water into hydrogen and oxygen. This reaction takes place in a unit called an electrolyzer.

13. Describe food additives and how they are used?

Food additives are substances added to foods to perform a range of specific function. Additives may be natural, nature-identical or artificial.

• Natural additives are substances found naturally in a foodstuff and are extracted from one food to be used in another. For example, beetroot juice with its bright purple colour can be used to colour other foods such as sweets.
• Nature identical additives are man-made copies of substances that occur naturally. For example, benzoic acid is a substance that is found in nature but is also made synthetically and used as a preservative.
• Artificial additives are not naturally present in foods and are made synthetically. An example is an azodicarbonamide, a flour improver that is used to help bread dough hold together.

14. Based on two pressure measurements how can you estimate gas flow?

To find the velocity of the fluid flow, you must first determine the pressure at the two reference points. Then multiply the differential pressure by two and divide this number by the density of the gas.

15. What are the implications of the minimum flow required by a pump?

Every pump has a minimum flow requirement. Operating pumps at below minimum flow can result in the following consequences:

• Pitted/worn impeller vanes
• Overheated casing/bearings
• Excessive noise or vibration
• Broken shafts
• Mechanical seal failures
• Poor performance efficiency

Adding a bypass line will allow the pump to sustain the minimum flow requirement, even when the process requires less flow.

16. What problems are associated with large temperature differences in boilers?

When a boiler overheats there are several issues that may arise, most noticeably leaking. The conduction of heat through the material causes expansion and as a result changes in the solid structure can occur causing major structural issues.

17. Describe a Condensate lift.

The term condensate lift is usually associated with a vertical rise of a condensate line AFTER a steam trap. The motive force to lift the condensate is the steam pressure. There are potential problems when there is a modulating valve on the steam inlet because this also modulates the pressure, often causing a ‘stall’ situation in the heat exchanger when there is no longer enough pressure to lift the condensate. The solution here is to use a pump.

18. Describe the characteristics of a good quality water cooling tower.

A good quality cooling tower operates on a pH level around neutral since the carrier fluid is water. Any impurities such as sulphur and chlorine will hinder the performance of heat transfer. They should be situated in locations that have low air humidity and high temperatures to allow the moisture to evaporate effectively. This process is characterised by the wet-bulb temperature of materials.

19. What characteristics of bulk solids can affect their ability to flow properly?

When handling bulk solids, there are several factors that can affect your material flow. Most of these are environmental; all of them need to be taken into consideration. Moisture content, temperature, particle size, and time of storage at rest can have a tremendous impact on your material’s flow properties. How your material flows is a function of its cohesive and friction properties. As such, conditions that affect the cohesive strength and friction properties will have an impact on its handling capabilities.

20. What compounds are responsible for the foul smells from wastewater plants?

Odours in the wastewater treatment plant are caused by a variety of compounds, mainly: 1) reduced sulphur compounds, e.g. hydrogen sulphide, mercaptans, organic sulphides; 2) nitrogen compounds, e.g. ammonia, amines; 3) volatile fatty acids, e.g. formic acid, butyric acid; 4) aldehydes and ketones.