Chemical Engineering Interview Questions And Answers

Check / consider the following:

► The upper pressure range and /or the smaller pipe diameters prompts me to investigate the possibility that the gas is reaching critical flow somewhere downstream within the pipe. When a gas gets to critical flow, sonic booms (producing vibration) are expected. In fact, one of the main means by which the additional pressure in the pipe is lost.

► If the source is a compressor, look for surging.

► If the source is a tower, look for pressure cycling in the tower

► Look at critical flow through any control valve that may be in the line.

► Are there any vapors in the line, which can condense and produce two-phase flow? Two-phase flow can cause vibration.

In chemical plant design, if we suspect two-phase flow, we instruct the piping designers to provide special anchoring.

The RheoVac air in-leak monitor by Intek, Inc. in Westerville, OH is a viable meter that gives the actual air in-leakage flow rate. It also gives you exhauster capacity and a vacuum quality reading. If you want to find more information, you can view their web site below.

A 600 psig, 3" steam line is experiencing "passing" or internal leakage. If you order to replace the valve, the process would have to be taken offline. A temporary solution to the problem is sought to get the plant to their next scheduled shut down ANSWER Research on-stream leak sealing services. This problem is quite common. What they would do in this case is drill a hole into the bypass valve on the upstream side but not completely into the line. They would then tap the hole and install one of your injection fittings, which is like a small plug valve. They would then take a long 1/8" drill bit and drill through the open injection fitting and into the pressurized line. The drill bit is then removed and our injection equipment is then attached. Sealant (heavy fibers and grease) is pumped into the line and caught in the flow, which will bind up against the leaking seat on the bypass valve. If done properly, this technique can be both effective and safe.

A rule of thumb is to turn the tank over three (3) times and then sample the tank for mixture properties. By "turn the tank over”, we mean to force the entire volume of the tank through the pump at least three times. More turnovers may be necessary, but three times is a good starting point.

There are various traditional methods to employ waste steam in an operating plant:

1. You can generate electricity through a steam turbine-generator set. The electricity is usually put back in the line; this is the idea behind the "Co-Gen" concept used today in many USA plants. Steam turbines can effectively use saturated steam supply down to 75 - 100 psig. In special conditions, they have used down to 50 psig as a turbine steam supply. I have used steam as low as 100 psig.

2. You can pre-heat process streams that require pre-heating; this is done by applying heat exchangers.

3. You can employ the waste steam as a refrigeration source by employing it in vacuum jet ejectors and producing 50 of cooling water.

You have to consider these as viable options if you can identify the heating, cooling and energy conservation requirements. An economic analysis is required to identify the most attractive option. You usually utilize a Discounted Cash Flow analysis to base your decision and that means you must study each case as to savings generated. A fourth method might be that you can use the steam for environmental heating (if you live in a cold climate).

It is unclear as to whether or not you know the total steam consumption. If you do not, one way to get it is to take the nominal capacity of the boiler in terms of heat, i.e. the total rated Btu/hr. This is usually available either through the documentation you have for the boiler or even on the nameplate. You also must know the steam pressure you are producing. Using the steam tables, get the enthalpy of the steam and divide it into the nominal boiler capacity to get the total rate. I hope that you also know how much of the capacity you are using, 50%, 75% etc. Multiply this by the total lb/hr to get your rate. Another way to get the capacity is by using the amount of boiler feed water you are sending to the boiler and the known level of steam you are producing. Do not forget to include the blow down in your heat & mass balance. Getting the rate to each plant is more difficult if you are lacking in instrumentation. Use as much plant instrumentation as possible; flow meters, pressure and temperature indicators. If you do not have a meter in each header to each plant, then see if you have them in sections or to pieces of equipment using the steam. Another way is to measure the amount of condensate you are returning to the boiler. If you are dumping the condensate, you may be able to collect and measure the amount in a pail from each source. Another way is to use the process instrumentation and do some mass and energy balances around the steam users.

Slurry lines should be flushed with a minimum fluid velocity of 10 ft/s and the total flushing liquid volume should equal 3-6 times the total piping volume.

Design of slurry piping systems should follow ANSI/ASME B31.1 and B31.11 Codes. A simple equation for this calculation is as follows: t = (PD) / (2S) + C where: t = pipe wall thickness, in. P = maximum design pressure of the pipe, psig S = maximum allowable design stress, psig C = corrosion or erosion allowance, in.

Even long radius elbows should be avoided in slurry pipes and lines. They are often the site of severe erosion or solid/liquid separation. Only gentle pipe bends or sweeps should be used to turn a slurry line. Industrial experience has shown that a bend-radius-to-pipe-diameter ratio of 3-5 is recommended.

Manufacturers of these systems recommend bin agitation or blowing air into the top of the feeding bin. These methods can prevent fine particle from bridging near the rotators valve. Two types of particles that are especially prone to bridging include titanium dioxide and calcined- kaolin clay.

When selecting a piping material to transport slurries, corrosion and erosion considerations must be accounted for. Some of the most popular piping materials include:

► Carbon Steel

► Stainless Steel

► High Density Polyethylene (HDPE)

► Acrylonitrile butadiene stryene (ABS)

► Unplasticized polyvinyl chloride (uPVC)

► Fiberglass reinforced plastic (FRP)

► Elastomer-lined carbon steel

Probably the most common problem encountered in dilute phase pneumatic conveying is the wearing of the rotary valve that serves as an air lock where the product is introduced into the system. If excess air is allowed to pass by the rotary valve, this can cause bridging of the material the flow can be slowed or stopped.

Dense phase pneumatic conveying, typically experiences one common problem from system to system: plugging in the line due to a malfunctioning booster valve. Dense phase systems require these booster systems to introduce new, pressurized air. These boosters are nearly always accompanied by a check valve. If the check valve becomes stuck, the product is allowed to plug the line.

While many applications utilize air as a carrier gas, others are not suited for using air. For example, if the substance being conveyed reactions with moisture in the air or if there is a threat of dust explosions, nitrogen is likely choice.

Essentially, there are two types of pneumatic conveying systems. In dilute phase systems, the solids are suspended in the carrier gas and transported to their destination. In dense phase systems, the solids-to-gas ratio is much higher. The gas in these systems acts more like a piston to push the product to its final destination. Dilute phase systems are more typical than dense phase systems because they can employ positive pressure displacement or a vaccum system. Dense phase conveying is useful if the product degrades easily (works at lower velocities) or is particularly abrasive.

Pneumatic conveying is a method of moving bulk solids from one place to another with the help of a carrier gas. A differential pressure is applied inside a conveying line. The flow always moves from a region of higher to lower pressure.

As a rule, pneumatic conveying will work for particles up to 2 inches in diameter with a typical density. By "typical density”, we mean that a 2 inch particle of a polymer resin can be moved via pneumatic conveying, but a 2 inch lead ball would not.

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.

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

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.

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.

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.

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!

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...

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