DEPARTMENT OF CHEMICAL ENGINEERING

CL 152/2015S/3 Tutorial sheet 3 Revised on: February 5, 2015Tutorial date: February 5, 2014

Question 6 [marked Computational] has to be solved and submitted for evaluation

1. Reaction extent, conversion, etc.; simple reacting systems

(a) In the following cases, the liquid phase reaction: 2A1 + A2 3A3 + 2A4takes place in a batch system. An initial and a final set of concentrations[c1, c2, c3, c4] as measured are given. All concentrations are in the same units.Check if the measurements are consistent, and, if so, determine the limitingreactant, conversion and the reaction extent .

i. [1, 2, 3, 0] and [0.3, 1.65, 4.05, 0.7]

ii. [3, 5, 7, 2] and [1, 4, 10, 3]

(b) The gas phase reaction A1 + 2A2 3A3 + A4 takes place in the followingcases, and the feed is a mixture of 10 moles of A1 and 20 moles of A2. For thefollowing cases, calculate the mole fractions of the components in the productmixture, and any additional information as asked:

i. Batch reactor, with 50% conversion of A1. Use molecular species balancesand relate species consumption/productions through the conversion vari-able X.

ii. As in (a), but 8 moles of A1 are added into the reactor during the reaction;the product contains 15 mol% of A1. What is the conversion of A1 inthis case, and how many moles of reaction have taken place? Commenton how the balance works for this semibatch system.

iii. A continuous flow reactor operating at steady state, with exit mole frac-tion of A1 being 15%. Calculate conversion of A1 and the moles/s ofreaction taking place in the reactor. Use any method of your choice.

2. In the production of integrated circuits (ICs), silicon rods are sliced into wafers,which are lapped and polished to form uniform flat surfaces. Microscopic defectsare then removed by chemical etching. The etching solution typically consists of4:1:3 volumetric mixture of 49% HF, 70% HNO3 and 100% acetic acid respectively.In order to ensure complete removal of defects, etching to a depth of 20 m perside is common. The etching reaction is:

3Si+ 4HNO3 + 18HF 3H2SiF6 + 4NO + 8H2OIf 6000 wafers per hour, each of 150 mm dia, are to be etched, calculate theflowrate of the etching solution required, the limiting reagent, and the exit solutioncomposition. (Data: Si density= 2.33 g/cm3, mol wt. = 28.09; specific gravitiesof HF , HNO3 and acetic acid solutions used are respectively 1.198, 1.413 and1.049; mol. wts of the acids are respectively 20.01, 63.01 and 60.05).

3. Multiple reactions: Methane and oxygen react in the presence of a catalyst toform formaldehyde. An undesired side reaction is that of CH4 oxidizing to carbondioxide:

CH4 +O2 HCHO +H2O (1)CH4 + 2O2 CO2 + 2H2O (2)

The feed at steady state to the flow reactor consists of an equimolar mixture ofmethane and oxygen. Assume a feed rate of 100 mol/s.

(a) For an approach involving molecular species balances and extents of reaction,perform a degrees-of-freedom analysis to determine how many process variablevalues need to be specified in order to calculate the product composition.

(b) Derive the flow rates of the product species out of the reactor in terms of thereaction extents.

(c) If the conversion of methane is 90% and the yield of formaldehyde, definedon the basis of the maximum that can form, is 85.5%, calculate the molarcompositions of the reactor output stream and the selectivity of formaldehyderelative to carbon dioxide.

4. Flowsheets with reaction, purge and recycle: The following is an importantstep in manufacturing methanol using gases from coal conversion:

CO + 2H2 CH3OHThe feed to such a process contains 67.3% H2, 32.5% CO and the rest methane.The gases from the converter are passed through a separator to separate and recyclethe unconverted gases. A purge stream is bled from the recycle stream in order tocontrol methane accumulation. Assume the product to be 100% pure methanol.CO conversion per pass is 18%.

(a) The methane concentration in the gaseous stream leaving the separator isto be less than 3.2%.Compute the recycle rate, product rate and purge rateper mole of fresh feed. Also calculate the composition of the purge stream.

(b) The methane concentration in the gaseous stream entering the separatoris to be less than 0.4%. Perform a DoF analysis and suggest an approachto solving the problem to calculate the same quantities as before. Formulatethe final equations in the minimum number of unknowns (which have to besolved simultaneously). Suggest an approach to completing the solution.

5. Combustion reactions:

(a) Distinguish between partial combustion and incomplete combustion.

(b) A gas contains 80.0 wt% propane, 15.0 wt% n-butane and the balance water.Calculate the molar composition of the gas on both wet and dry bases, andmoles of water vapour per mole of dry gas. If 100 kg/h of this gas has to beburned with 30% excess air, what is the required air feed rate in kmol/h?

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(c) Repeat the air requirement calculation in (3.5.2) for the following cases - (a)the combustion is 70% complete, and (b) 40% of the carbon undergoes partialcombustion to CO and the rest, to CO2. In both of the above cases, calculatethe flue gas composition.

(d) A mixture of 75 mol% propane and 25 mol% hydrogen is burned with 25%excess air. A fractional conversion of propane of 0.95 and, of hydrogen of 0.85are achieved. Of the propane that reacts, 95% reacts to form CO2 and thebalance reacts to form CO. The combustion products flow through a boilerin which the heat is utilized to convert boiler feedwater to steam.

Calculate the concentration of CO in parts per million in the stack gas, andthe complete composition of the stack gas on dry basis.

6. Reaction equilibria (Computational): The following refer to the water-gasshift reaction, CO(g) + H2O(g) = CO2(g) + H2(g), which has an equilibriumconstant at 1105 K of 1.00.

(a) At low to moderate pressures, the variation of the equilibrium constant of thisreaction with temperature is given by K(T ) = 0.0263 exp (4020/T ), where Thas to be in K (is this an exothermic or an endothermic reaction?).

The feed to a batch reactor contains 20.0 mol% CO, 10.0 mol% CO2, 40mol% water, and the balance, an inert gas. On a basis of 1 mol of feed, see ifthere is enough information to calculate the composition of the equilibriummixture for a given reactor temperature.

(b) Use a computer program or spreadsheet to calculate the composition of theequiibrium mixture given the temperature and the mole ratio of the tworeactants in the feed. (Assume the feed does not contain either of the productspecies).

Plot your results in some convenient form to show the effect of temperatureand feed composition on the composition of the equilibrium mixture.

7. Multiple reaction stoichiometry: For the gas phase reaction system beingcarried out in a constant pressure, isothermal batch reactor:

A1 + 2A2 = A3

A3 + A4 = 2A5 + A1

A1 + A4 = A6

A3 + 2A4 = 2A5 + A6

Find the number of independent reactions R, and express all concentrations interms of R independent variables of your choice.

8. Combustion processes

(a) n-Pentane is burned with excess air in a continuous combustion chamber. Atechnician reports the following composition of the flue gas on a dry basis:

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0.270 mol% pentane, 5.3% oxygen, 9.1% carbon dioxide, and the balancenitrogen.

i. Take 100 mol dry gas as a basis, perform a DOF analysis to show thatthe system has -1 DOF. How do you interpret this?

ii. Use (elemental?) balances to show that the reported analysis could notpossibly be correct.

iii. On being asked to redo the analysis the technician returns the followingfigures this time: 0.304 mol% pentane, 5.9% oxygen, 10.2% carbon diox-ide and the rest nitrogen. Show that this result could be correct. If it is,what is the percent excess air fed and what is the conversion of pentane?

(b) Fuel oils contain primarily organic compounds and sulfur. The molar compo-sition of a certain fuel oil is established by elemental analysis as C0.71H1.1O0.003,and it is found to contain 0.02 kg sulfur per kg fuel. Calculate the composi-tion of the stack gas on a dry basis if the fuel is burned with 18% excess air,assuming the oxygen in the fuel molecule to be available for combustion, andaccounting for the air requirement of sulfur (to sulfur dioxide). What is thedifference in the theoretical air requirement if you do not, take into accountthe conversion of sulfur to sulfur dioxide?

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