Chemical Engineering Interview Questions

Question: What does the catalystic converter on an automobile really do?
Ans: A catalytic converter is a device that uses a catalyst to convert three harmful compounds in automobile exhaust gas into harmless compounds. The three harmful compounds are:
1. Hydrocarbons (in the form of unburned gasoline)
2. Carbon monoxide (formed by the combustion of gasoline)
3. Nitrogen oxides (created when the heat in the engine forces nitrogen in the air to combine with oxygen).
Carbon monoxide is a poison for any air-breathing animal. Nitrogen oxides lead to smog and acid rain, and hydrocarbons produce smog. In a catalytic converter, the catalyst (in the form of platinum and palladium) is coated onto a ceramic honeycomb or ceramic beads that are housed in a muffler-like package attached to the exhaust pipe. The catalyst helps to convert carbon monoxide into carbon dioxide. It converts the hydrocarbons into carbon dioxide and water. It also converts the nitrogen oxides back into nitrogen and oxygen. TOP

Question: What is an effective means of removing silicon from aluminum?
Ans: Silicon is well known for its chemical inertness, (i.e. it tends not to react with many other chemicals). Depending on what type of silicon you are dealing with, this may or may not be easy to solve. If the silicon is from a lubricant, it is probably the graphitic form, which is soluble in a strong combination of nitric, and hydrofluoric acids, neither of which I would recommend for you to use...nor hydrofluoric acid is not easy to come by. If it is silicon from an acidic form (probably any other form other than a lubricant), you should try ammonia. In either case, leave your acetone at home...it will NEVER work! UPDATE: An ammonia solution worked very well in this case TOP

Question: We wanted to know how to impart various colors to copper wire by simply dipping them into various chemicals, formulations, etc. This copper wire is to be used by us for our hobby of making various art objects from copper wire.
Ans: Changing the color of copper by means of chemical reactions is a dangerous Endeavour that I really do not recommend. However, there is something you can do to get a green color, if fact if you are familiar with the Statue of Liberty here in America, this would explain why it is green. You see, the outside of the statue is coated with copper and being in New York City, it is subjected to acid rain. This causes the formation of another chemical that coats the copper and gives the statue its green color. The two acids that you can use are nitric acid (which works best) or sulfuric acid (which will probably require some gentle heating along with the acid). I am not sure if there were a good way to get nitric acid out of something you may have around the house, you would probably have to buy it.
Sulfuric acid can be obtained from car batteries (the liquid inside). You will want to boil the mixture (to concentrate it by evaporating the water), until you see white fumes (which are very dangerous). Then put your copper is while the acid is hot and leave it there until you get the color you would like. If you are going to do this, please do it outside or in a well ventilated area and make sure you have some baking soda handy is case you get some of the acid on your skin. If you are looking for a different color or more colors...I suggest contacting someone who manufactures copper or someone who specializes in it. Please be very careful if you do any of the experiments that we have suggested. The green color that you get will not be an artistic bright green, but an unattractive dark, milky green. You may have to consider making your art out of something that is a bit easier to color. Good luck with your project! TOP

Question: After conducting an internet search for ways to reduce energy costs, I found a recipe for whitewashing that is said to reflect sunlight. The recipe calls for 20 pounds hydrated lime to 5 gallons water to 1-quart polyvinyl acetate. What is a good source of polyvinyl acetate that I can buy at a local store?
Ans: Well your recipe sounds exciting. Finding your polyvinyl acetate should be easy. Go to your nearest department store and pick up a large container of plain white glue! The chief active ingredient in this glue is polyvinyl acetate. Good luck with your project! TOP

Question: What are some characteristics of bulk solids that can affect their ability to flow properly?
Ans: Four (4) main factors to consider include moisture content, temperature, particle size (and shape), and time at rest.
1) An increase in moisture content will generally make solids more "sticky". Some solids will absorb moisture from the air, which is why nitrogen is often used as a carrier gas (among other reasons).
2) For some solids, their ability to flow can be adversely impacted by temperature or even the length of time that the particles are exposed to a specific temperature. For example, soybean meal flows nicely at 90 °F but start to form large bridges at 100 °F.
3) Generally, the finer a bulk solid becomes, the more cohesive the particles. Round particles are generally easier to handle than "stringy" or oddly shaped particles.
As particles rest in a bin, they can compact together from their own weight. This can create strong bonds between the particles.
4) Often times, re-initiating flow can break these bonds and the solids will flow as normal, but this can depend on the load at given locations in the bin. TOP

Question: What is a "saltation velocity" and how is it used in designing pneumatic conveying systems?
Ans: The saltation velocity is defined as the actual gas velocity (in a horizontal pipe run) at which the particles of a homogeneous solid flow will start to fall out of the gas stream.
In designing, the saltation velocity is used as a basis for choosing the design gas velocity in a pneumatic conveying system. Usually, the saltation gas velocity is multiplied by a factor, which is dependent on the nature of the solids, to arrive at a design gas velocity.
For example, the saltation velocity factor for fine particles may be about 2.5 while the factor could be as high as five for course particles such as soybeans could. TOP

Question: How can one determine the particle size distribution for a given bulk solid?
Ans: While there are high-tech methods of performing such an analysis (laser-diffraction and video imaging system are available), the simplest way is to use a sieve stack. For example, to analyze a particular solid, one would stack several different mesh sizes into a cylinder with the largest mesh opening on the top and progress down through the cylinder to finer mesh. The cylinder would contain a pan on the bottom. Before beginning, weight the test sample, each piece of mesh, and the pan. Then, the sample is loaded into the top of the test cylinder and the cylinder is exposed to a combination of movements (shaken) to allow the solids to pass through the appropriate mesh sizes. TOP

Question: What is the most common cause of solid size segregation in bulk solid systems?
Ans: Many engineers usually point directly to the pneumatic conveying system as a source of such a problem. The truth is that in most cases, segregation occurs because of the differences in sizes of the articles. As a rule-of-thumb, if the size ratio extends outside of around 1:1.3, then there will most likely be segregation. This being said, one should inspect the equipment responsible for determining the particle size rather than the pneumatic conveying system if this problem is occurring. Reference: Richard Farnish, the Wolfson Centre for Bulk Solids Handling Technology TOP

Question: What can cause bulk solids to stop flowing from a bin?
Ans: Causes of such problems can fall into one of two categories: Material strength or Bin Geometry: Factors that can affect material strength include. Moisture is especially with particles, which fuse together with moisture. TOP

Question: What compounds are responsible for the odors that come from wastewater treatment plants?
Ans: Compounds such as hydrogen sulfide, methyl mercaptan, dimethyl sulfide, and dimethyl disulfide can all contribute to this foul odor. TOP

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