COMPILATION OF INTERVIEW QUESTIONS ON

CHEMICAL REACTION ENGINEERING,

PROCESS CONTROL, FLUID MECHANICS,

PETROLEUM REFINING & PETROCHEMICALS

Prepared ByG. SANJAY KUMAR

Assistant Professor (Selection)College of Engineering Studies,

University of Petroleum and Energy StudiesP. O. Bidholi, Via Prem Nagar

Dehradun 248 007, Uttarakhand, INDIAEmail: gsanjay@ddn.upes.ac.in, gudasan@gmail.com

Interview Questions in ChemicalReaction Engineering

This is based on Levenspiels classic book. [1]

Kinetics

1. What is meant by chemical reaction?

2. List three common chemical reactions. Solution:

(a) DecompositionCH3CH3 H2 + CH2 = CH2

(b) CombinationN2 + O2 2NO

(c) IsomerizationC2H5CH = CH2 CH2 = C(CH3)2

3. Define/Explain the following

(a) Rate of a reaction

Change in number of moles of a species per unit volume per unit time

(b) Elementary reaction

When the rate law corresponds to the stoichiometry of the equation as written

(c) Molecularity

Actual number of molecules taking part in a reaction or reaction step.

(d) Order

The powers to which the concentrations of reactants are raised to in the ratelaw when the rate law is of the form rA = kCaACbB...

(e) Units of rate constant

4. Give examples for

5. The pyrolysis of ethane proceeds with an activation energy of 300 kJ/mol. How muchfaster is the decomposition of ethane at 650oC than at 500oC?

6. Explain the Differential method of Analysis of Data.

Solution: This method deals directly with the rate equation to be tested. All wehave to do is assume the rate equation, evaluate all the terms of the rate equationincluding dC/dt and test the fit of the data to the rate equation.

The procedure is given in detail below.

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(a) Hypothesize a mechanism and obtain a rate equation from it OR try out stan-dard rate forms. The rate equation will be of the form

rA = dCAdt

= kf(C)

(b) Plot a graph of concentration versus time from the data collected by performingthe reaction experiment.

(c) Draw a smooth curve through these data

(d) Determine the slope of this curve at suitably selected concentration values (bydrawing tangents). These slopes, dCA/dt are the rates of the reaction at theselected concentrations.

(e) Evaluate f(C) at the selected concentrations.

(f) Plot a graph of dCA/dt i.e. rA v/s f(C). If we get a straight line passingthrough the origin then the rate equation assumed is consistent with the data.If we do not get a straight line then another equation should be tested.

7. Explain the Integral method of Analysis of Data

Ideal Reactors

8. Draw a sketch of a batch reactor with accessories. Solution:

Figure 1: Typical batch reactor

9. What assumptions are made when modeling an ideal tubular reactor?

10. What assumptions are made when modeling an ideal CSTR?

11. What is the performance equation of an ideal batch reactor?

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12. What is the performance equation of an ideal CSTR?

13. What is the performance equation of an ideal PFR?

14. What does the performance equation for a CSTR become if rA = kCA(at constantvolume)?

15. Calculate the time to reduce the number of moles by a factor of 10 in a batch reactorfor the reaction with rA = kCA, when k = 0.046 min1.

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Interview Questions inInstrumentation and ProcessControl

Basis is books on control by Coughanowr [2] and Stephanopoulos.

Basics/Introduction

1. What is process control?

Process Control is the study of automatic control principles applied to chemical pro-cesses. It applies principles of mathematics and engineering science to the regulationof the dynamic operation of process systems

2. Why is control necessary?

Process plants do not operate at steady state - they wander from there because ofdisturbances, hence it is essential to control them.

3. What is a control system?

A Control System is a device, or a collection of devices that manage the behavior ofother devices.

Some devices are not controllable.

A control system is an interconnection of components connected or related in such amanner as to command, direct, or regulate itself or another system

4. What is a controller?

A controller is a control system that manages the behavior of another device or system

5. What are the main jobs of a control system?

The main jobs of a control system are

(a) Set Point tracking the ability to shift from one desired operating point toanother (like you driving your car)

(b) Disturbance rejection the ability to maintain an operating point despite fluc-tuating conditions and external forces (like your thermostat)

6. What are the hierarchical jobs of a control system?

Control systems

(a) allow basic operation by managing throughput and inventory

(b) ensure safety by preventing dangerous conditions

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(c) guide transitions between operating states (startup, shutdown, setpoint track-ing)

(d) maintain product quality by rejecting outside forces which act on the process

(e) implement optimizing strategies devised by management or supervisory comput-ers

7. What special features of chemical process systems differentiates them from (say) robotcontrol?

Chemical process systems are distinguished by:

(a) longer time constants (minutes for a HX, hours for many columns)

(b) long transportation lags or dead time (minutes)

(c) nonlinearities (reaction kinetics)

(d) distributed parameters (coupled material and energy balances)

8. Describe the basic components of a control system.

The basic components of a control system include

(a) Measurement devices for variables affecting the system

(b) a specified desired value or range of values for the controlled variable (the set-point).

(c) a control calculation or algorithm

(d) a way of adjusting the system to reflect the results of the control calculation(the control element).

9. What are the components in a computer controlled system?

Additional components over a usual computer system include a filter, A/D convertorand D/A convertor

10. Describe/draw the block diagram of a basic feedback control system

Figure 2: Block Diagram of a Control System

11. List the steps involved in control system design.

The steps are:

(a) determine control objectives

(b) identify measurable variables, available manipulators

(c) pair variables (choose controller structure)

(d) select controller algorithms

(e) tune controller (adjust sensitivity)

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12. Define / Explain what is meant by steady state.

Steady State: A steady state system does not change with time. Mathematically,this means the time derivatives in the balance equations (the accumulation terms)are zero. Often, systems will reach steady state if given a long time to settle usually,real systems dont get the time. This leads to another mathematical approximation steady state is the behavior of the system as time approaches infinity. Some peopleuse the words static or stationary as synonyms for steady state.

13. Define / Explain what is meant by dynamic/transient systems.

Dynamic (or transient) systems are time dependent. All real systems are dynamic;this makes process control necessary. Dynamic systems must be modeled using dif-ferential equations, unlike steady state systems where algebraic systems will suffice.

14. Define / Explain what is meant by inputs and outputs.

Inputs and Outputs are not necessarily material flows. An input is a variable thatcauses an output to change. Both inputs and outputs may be measurable or theymay not. Disturbances are inputs that cannot be adjusted, and often they are notmeasurable.

15. Define / Explain what is meant by error.

Error is the difference between the measured behavior of a process output and itsdesired behavior or setpoint. Never forget that the measured values of the outputsare only representations of the real values, and may be limited in accuracy.

16. Define / Explain what is meant by feedback and feedforward control

Feedback Control: information from an output of a system is used to adjust a ma-nipulator to change an input to the system to try and compensate for disturbancesafter they have changed the system.

Feedforward Control: information from measured disturbances is used to adjust amanipulator to try and compensate for disturbances as they occur. Feedforwardallows for the possibility of perfect control, but only if all disturbances are measuredand the adjustments are fully understood. This means you must have a complete andvery accurate model of the process not an easy achievement. Feedback controladjusts for all disturbances and does not require an exact process model.

17. Define / Explain what is meant by negative feedback

Negative feedback reduces the difference between the actual and desired values, so itis beneficial. Positive feedback increases the difference, so it is undesired.

18. Define / Explain what is meant by open loop and closed loop

When a system is operating without control, we say it is operating Open Loop. AClosed Loop system has controllers on-line.

19. Define / Explain what is meant by stability

One of the most important things we will be watching is the stability of the system.The error of an unstable system becomes larger and larger (unbounded) with time,often leading to undesirable consequences.

20. What are the hardware components of a control loop?

A control loop is built from mechanical and electrical devices. These usually include

(a) a sensor

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(b) a transmitter

(c) a controller

(d) an actuator, and

(e) a final control element

21. What are two common forms of control signals used?

4 to 20 mA current3 to 15 psig compressed air

22. What is meant by a First Order system?

(a) Any process system which follows first order dynamics

(b) One with a transfer functionKps+1

(c) One which follows a I order ODE i.e. output w.r.t input is related by a I orderODE

23. Give me some examples of First Order Systems Mercury thermometer

Liquid level system - single tankRC Circuit

24. What is a PID controller?

Proportional - Integral - Derivative controller

25. Why do you need integral action?

To eliminate offset

26. How can you reduce offset in proportional control?

By increasing K (Proportional gain)By decreasing the porportional band

27. When will you use derivative control? For which variables is it usually used? Why?

(a) When process has a large capacity or dead time

(b) Temperature is a typical example of a variable which requires derivative ac-tion. Another is composition control. (since there is a large lag in measuringinstrument)

(c) Using derivative action is a crude method of predicting the behaviour of theprocess and taking action early.

(d) Derivative action is used for slowly changing control variables

28. What is the problem with derivative action?

It introduces more oscillations into the controlled response

29. Which type of controller will you use for given variable?

See Table 1

30. Is PID controller a feedback or feed forward controller?

Feedback

31. What are the components of a feedback control loop?

The components of a feedback control loop are

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Control Controller

Level control (loose) P with low to moderate gain

Level control (rigid) P with high gain

Pressure control PI

Flow control PI

Temperature control PID

Table 1: Controller vs Variable

(a) Process

(b) Sensor (measuring element)

(c) Comparator (for comparing Setpoint and Process value)

(d) Controller

(e) Final Control Element

(f) (Process)

32. Describe some standard inputs to a control system with diagram. Step, Pulse, Ramp,Sinusoidal

33. What is the Laplace Transform of sin t? cos t?

s2 + 2s

s2 + 2

34. What is a Bode plot?

A Bode Plot is a useful tool that shows the gain and phase response of a given LinearTime Invariant (LTI) system for different frequencies. Bode Plots are generally usedwith the Fourier Transform of a given system.

An example of a Bode magnitude and phase plot set. The Magnitude plot is typicallyon the top, and the Phase plot is typically on the bottom of the set.

35. How can you draw a Bode plot? (approximate)

36. What is the Bode Stability criterion?

37. What is a Nichols plot and Nichols stability criterion

38. What is internal model control?

39. How is a servo mechanism different from regulator mechanism in a tank heater /reactor control system?

40. Measurements conducted on a servo controller gave the system response c(t) = 1 +0.2e60t1.2e10t when a unit step input in set point was given. Obtain an expressionfor the closed loop transfer function. Determine also the undamped natural frequency(n) and the damping ratio ().

41. Explain the working principle of (i) bubbler level measurement system (ii) level mea-surement in pressure vessels (

42. Determine the offset for a proportional controller towards a step change in loadvariable.

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Figure 3: Bode Diagram

43. A proportional controller is used to control temperature within the range of 60 to100oC. The controller is adjusted so that the output pressure goes from 3 psi (valvefully open) to 15 psi (valve fully closed) as measured temperature goes from 71 to75oC with the set point held constant. Find gain and proportional band.

44. Show that the effective transfer function of a control valve in a process may beapproximated by a constant.

45. A control system has the following elements: A second order process with characteris-tic time 10 sec, damping coefficient 0.6 and gain 2; A first order measuring instrumentwith time constant 2 sec and gain 1.5; A P-controller. Draw a neat block diagram ofthe system. Find the closed loop transfer function for regulatory control (

46. A unity feedback system is characterized by the open loop transfer function G(s) =1/s(0.5s+1)(0.2s+1). Determine the steady state errors for unit step and unit rampinputs.

47. Discuss briefly about valve actuators and positioners.

48. Discuss valve characteristics in detail.

49. Is the PID transfer function suitable for feedforward control?

50. Explain with a diagram (i) feedforward control and (ii) cascade control of a stirredtank heater.

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Figure 4: Valve Characteristics

Dynamic Models

Response of Standard Systems

Controllers

Stability

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Interview Questions on FluidMechanics

Fluids and fluid Properties

Hydrostatics

Kinematics

Basic equations of Fluid Mechanics - Macroscopic

Basic equations of Fluid Mechanics - Differential Form

Pumps and Compressors

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Interview Questions on HeatTransfer

1. What is the difference between diffusion and radiation heat transfer ?

Solution:

Diffusion heat transfer is due to random molecular motion. Neighboring moleculesmove randomly and transfer energy between one another - however there is no bulkmotion. Radiation heat transfer, on the other hand, is the transport of heat energyby electromagnetic waves. All bodies emit thermal radiation. In particular, noticethat unlike diffusion, radiation heat transfer does not require a medium and is thusthe only mode of heat transfer in space. The time scale for radiative heat transfer ismuch smaller than diffusive heat transfer.

2. How is natural convection different from forced convection ?

Solution:

In natural convection, the movement of the fluid is due entirely to density gradientswithin the fluid (e.g. hot air rises over cold air). There is no external device orphenomenon which causes fluid motion. In forced convection, the fluid is forced toflow by an external factor - e.g. wind in the atmosphere, a fan blowing air, water beingpumped through a pipe. Typically heat transfer under forced convection conditionsis higher than natural convection for the same fluid.

3. Define a black surface

Solution:

A black surface is defined by three criteria:

(a) it absorbs all radiation that is incident on it

(b) it emits the maximum energy possible for a given temperature and wavelengthof radiation (according to Plancks law). the radiation emitted by a blackbodyis not directional (it is a diffuse emitter)

(c) A black surface is the perfect emitter and absorber of radiation. It is an idealizedconcept (no surface is exactly a black surface), and the characteristics of realsurfaces are compared to that of an ideal black surface.

4. What is the range of values for the emissivity of a surface ?

Solution: The emissivity e ranges between 0 and 1.

5. What are the conditions to be satisfied for the application of a thermal circuit ?

Solution:

The problem must be a steady state, one-dimensional heat transfer problem.

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6. Will the thermal resistance of a rectangular slab be increased or decreased if: thethermal conductivity is increased ?the cross sectional area is increased ?the thickness of the slab is increased ?

Solution:

resistance will decrease resistance will decrease resistance will increase

7. State the condition which must be satisfied to treat the temperature distribution ina fin as one-dimensional.

Solution:

When ht/k 1 where h is the convective heat transfer coefficient, t is the thicknessof the fin and k is the thermal conductivity of the fin, one can consider that thetemperature gradient in the thickness direction is very small and the analysis can beconsidered as one-dimensional.

8. Define and state the physical interpretation of the Biot number.

Solution:

The Biot number is given by:

Bi = hL/k

where

h = convective heat transfer coefficient,

k = thermal conductivity

L = characteristic length.

It is a ratio of the temperature drop in the solid material and the temperature dropthe solid and the fluid. So when the Bi 1 , most of the temperature drop is in thefluid and the solid may be considered isothermal

9. What is a lumped system ?

Solution:

A lumped system is one in which the dependence of temperature on position (spatialdependence) is disregarded. That is, temperature is modeled as a function of timeonly .

10. When can the unsteady temperature in a spatial body be considered uniform ?

Solution:

When the Biot number is small (Bi

12. What is internal energy generation ? Give examples where internal energy generationoccurs.

Solution:

Internal energy generation is the generation of heat within a body by a chemical,electrical or nuclear process. Examples are the heating of a nuclear fuel rod (dueto fission within the rod), the heating of electrical wires (due to the conversion ofelectrical to heat energy), microwave heating and the generation of heat within theEarth. The heat generated in each case is being converted from some other form ofenergy.

13. What do you understand by stability criterion for the solution of transient problems?

Solution:

When solving transient problems using finite-difference methods, it is possible thatthe solution undergoes numerically induced oscillations and becomes unstable i.e. thetemperature values diverge. The stability criterion is a restriction on the values of Dtand Dx which ensures that the solution remains stable and converges. The criterionis usually expressed as a function of Fouriers number. For example, for an interiornode in a two dimensional system the stability criterion is :

Fo < 1/4 or

Dt/(Dx)2 < 1/4

14. Both the Nusselt number and the Biot number have the same form. What are thedifferences between them in terms of the variables employed and their physical sig-nificance ?

Solution:

Both the Biot number and the Nusselt number are of the form (hL/k). However, forthe Biot number, the thermal conductivity k used is that for the solid; for calculatingNusselt number the k value as that of the fluid. The Biot number is a measure ofthe ratio of the temnperature drop in the solid material and the temperature dropbetween the solid and the fluid. The Nusselt number is a dimensionless version ofthe temperature gradient at the surface between the fluid and the solid, and it thusprovides a measure of the convection occurring from the surface.

15. What is the effect of the Prandtl number of a fluid on the relative thicknesses ofvelocity and temperature boundary layers when the fluid flow is parallel to a flatplate ?

Solution:

For laminar flow, the ratio of the boundary layer thickness d to that of the thermalboundary layer, dt, is given by:

d/dtPrn

The higher the Prandtl number, the larger is the ratio.

16. Two fluids, with different properties, flow with equal free stream velocities parallelto a flat plate. What property of the fluid determines whether the velocity boundarylayer of one is thicker than the other ?

Solution:

The thickness of the boundary layer depends on the Reynolds number:

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17. What do you understand by the terms fully developed velocity and temperatureprofile regions in internal flow ?

Solution:

In the fully developed region, the cross-sectional velocity/temperature profile is of aconstant shape at any axial location. Thus the profile has ceased to change. Alsothere is no radial component of velocity i.e. every particle of fluid is flowing purelyin the axial direction.

18. Do you expect the convective heat transfer coefficient in the thermally developingregion to be higher or lower than the convective heat transfer coefficient in the fullydeveloped temperature profile region ? Support your answer with qualitative logic.

Solution:

We should expect that the convective heat transfer coefficient is higher in the ther-mally developing region. Near the tube entrance, the thickness of the boundary layeris very small, and the temperature gradients at the surface will be high, implying highrates of convective heat transfer. As the flow develops, the thickness of the boundarylayer increases and the temperature gradients decreases, decreasing h. In the fullydeveloped region, the temperature gradients are constant and h is also a constant.

19. Explain why the temperature boundary layer grows much more rapidly than thevelocity boundary layer in liquid metals.

Solution:

Liquid metals are characterised by very low Prandtl numbers since their thermalconductivity is high, hence the heat diffusion is much faster than momentum diffusion.

20. You are told that in a particular case of fluid flow over a flat plate the temperatureboundary layer thickness is much smaller than the velocity boundary layer thickness.What can you conclude about the nature of the fluid ?

Solution:

You can conclude that the fluid is a high Prandtl number fluid e.g.oil.

21. What is a gray surface ?

A gray surface is defined as one for which the emissivity (e) and the absorptivity (a)are independent of wavelength (l).

22. What is a diffuse surface ?

A diffuse surface is defined as one for which the emissivity (e) and the absorptivity(a) are independent of direction (q).

23. Define a view factor.

A view factor is defined in the context of two surfaces A and B. It is defined as thefraction of radiation leaving A which is incident directly on surface B. A view factormust be defined in terms of surface A to surface B (FAB).

24. If a surface emits 200 W at a temperature of T, how much energy will it emit at atemperature of 2T ?

Since E T4, a 2-fold increase of temperature brings a (24) = 16-fold increase inenergy. Thus the surface will emit (16)(200) = 3200 W.

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25. You might have observed early morning frost on a clear day even when the minimumair temperature during the night was above 0 C. On a clear day, the effective skytemperature can be as low as -45 C. Explain how such frost formulation takes place.

The frost is created because of radiative losses to the sky

26. A greenhouse has an enclosure that has a high transmissivity at short wavelengthsand a very low transmissivity (almost opaque) for high wavelengths. Why does agreenhouse get warmer than the surrounding air during clear days ? Will it have asimilar effect during clear nights ?

Solar radiation is skewed towards shorter wavelengths. On a clear day the glassof the greenhouse admits a large proportion of the incident radiation. Inside thegreenhouse, the various surfaces (plants etc.) reflect the radiation; but the reflectedradiation is spectrally different, having more of a high wavelength contribution. Thusthe reflected radiation is not transmitted well by the glass, and is reflected back intothe greenhouse. The interior heats up due to this trapped radiation. The sameeffect will not be seen on a clear night, since there is no solar radiation.

27. Define overall heat transfer coefficient.

Solution:

The overall heat transfer coefficient is defined in terms of the total thermal resistancebetween two fluids. If there are a number of thermal resistances between the twofluids, the overall heat transfer coefficient is given by:

U = 1/SR

28. Your friend asserts that, in a heat exchanger, it is impossible for the exit temperatureof the cold fluid to be greater than the exit temperature of the hot fluid when bothfluids are single phase fluids. What is your response ?

Solution:

The statement is true for a parallel flow heat exchanger. However, in a counterflowheat exchanger the outlet temperature of the cold fluid can in fact exceed the outlettemperature of the hot fluid.

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Interview Questions on PetroleumRefining and Petrochemicals

Refining

Crude Oil and its Properties

1. How can you characterize crude oil?

Crude oil can be characterized based on

(a)

(b) Watsons K Factor K = 3Tb/Sp.Gr

(c) API Gravity

2. What is API Gravity? Why is it defined and used?

API Gravity is a modified scale for stating the specific gravity (density) of a crude.oAPI is given by

oAPI =141.5

Sp.Gr 131.5

It can be used for crudes lighter than water.

3. List some crudes and their properties

Crude Name Source K Factor PONA Type oAPI Suphur (%)

Bombay High Bombay High 11 Paraffinic/Waxy 35 1

Table 2: Important Crudes with Properties

Products and their Specifications

4. List the major products from a petroleum refinery.

The major product include

(a) Gases (Fuel Gas, Ethylene, Propylene)

(b) Naphtha

(c) Gasoline

(d) Superior Kerosene

(e) Diesel

(f) Fuel Oils

(g) Lubes

(h) Solvents

(i) Asphalt

(j)

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Primary Processing

5. Explain how a electrostatic desalter works with a neat picture.

6. What is meant by preheat train?

7. To what temperature is crude typically heated before (a) crude preheat furnace (b)main Crude distillation column

8. What is overflash?

9. What is the typical yield of each product in the CDU?

10. What are the cut points/ draw temperatures for each product in a CDU?

11. What are the different type of trays are used in a CDU?

12. Why is steam used in CDU?

13. What is the operating pressure of a CDU? How is it maintained?

14. Why is a VDU used?

15. What are the products from a VDU? Yields?

16. What are the cut points/ draw temperatures for each product in a VDU?

17. What kind of trays are used in a VDU? Why?

Secondary Processing

18. List the main secondary processing operations in a refinery.

19. What is reforming? Why is it done?

20. What is Platforming? Draw its PFD.

21. What is the temperature of operation for platforming?

22. Draw PFD for Continuous Catalytic Reforming.

23. What is the catalyst used for CCR?

24. What is the temperature of operation for platforming?

25. Draw PFD for Hydrocracker/Hydrotreater? How are they different?

26. What is FCCU?

27. What are the modes of operation of an FCCU?

28. What is the temperature of operation for FCC?

29.

Petrochemicals

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Bibliography

[1] Octave Levenspiel. Chemical Reaction Engineering. John Wiley and Sons, 3rd edition,1999.

[2] Coughanowr. Process Systems Analysis And Control. McGraw Hill, 2nd edition, 2003.

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