2 FUNDAMENTALS OF MATERIAL BALANCES
2.21. PROBLEMS
2.1. The composition of a gas derived by the gasification of coal is by volume percentage: carbon dioxide 4, carbon monoxide 16, hydrogen 50, methane 15, ethane 3, benzene 2, balance nitrogen. If the gas is burned in a furnace with 20% excess air, calculate
i. The amount of air required per 100 kmol of gas;
ii. The amount of flue gas produced per 100 kmol of gas;
iii. The composition of the flue gases, on a dry basis.
Assume complete combustion.
2.2. Ammonia is removed from a stream of air by absorption in water in a packed column. The air entering the column is at 760 mmHg pressure and 208C. The air contains 5.0% v/v ammonia. Only ammonia is absorbed in the column.
If the flow rate of the ammonia-air mixture to the column is 200 m3=s and the stream leaving the column contains 0.05% v/v ammonia, calculate
i. The flow rate of gas leaving the column;
ii. The mass of ammonia absorbed;
iii. The flow rate of water to the column, if the exit water contains 1% w/w ammonia.
2.3. The off gases from a gasoline stabilizer are fed to a steam reforming plant to produce hydrogen.
The composition of the off gas, molar%, is CH477:5, C2H6 9:5, C3H8
8:5, C4H104:5.
The gases entering the reformer are at a pressure of 2 bara and 358C and the feed rate is 2000 m3=h.
The reactions in the reformer are
1:C2H2nþ2þn(H2O)!n(CO)þ(2nþ1)H2 2:COþH2O!CO2þH2
The molar conversion of C2H2nþ2 in reaction (1) is 96% and of CO in reaction (2) 92%.
Calculate
i. The average molecular mass of the off gas;
ii. The mass of gas fed to the reformer, kg/h;
iii. The mass of hydrogen produced, kg/h.
2.4. Allyl alcohol can be produced by the hydrolysis of allyl chloride. Together with the main product, allyl alcohol, diallyl ether is produced as a byproduct.
The conversion of allyl chloride is typically 97% and the selectivity to alcohol is 90%, both on a molar basis. Assuming that there are no other significant side reactions, calculate masses of alcohol and ether produced, per 1000 kg of allyl chloride fed to the reactor.
2.5. Aniline is produced by the hydrogenation of nitrobenzene. A small amount of cyclo-hexylamine is produced as a byproduct. The reactions are
1:C6H5NO2þ3H2!C6H5NH2þ2H2O 2:C6H5NO2þ6H2!C6H11NH2þ2H2O
Nitrobenzene is fed to the reactor as a vapor, with three times the stoi- chiometric quantity of hydrogen. The conversion of the nitrobenzene, to all products, is 96%, and the selectivity for aniline is 95%.
The unreacted hydrogen is separated from the reactor products and re- cycled to the reactor. A purge is taken from the recycle stream to maintain the inerts in the recycle stream below 5%. The fresh hydrogen feed is 99.5%
pure, the remainder being inerts. All percentages are molar.
For a feed rate of 100 kmol/h of nitrobenzene, calculate i. The fresh hydrogen feed;
ii. The purge rate required;
iii. The composition of the reactor outlet stream.
2.6. In the manufacture of aniline by the hydrogenation of nitrobenzene, the off gases from the reactor are cooled and the products and unreacted nitrobenzene condensed. The hydrogen and inerts, containing only traces of the condensed
materials, are recycled. Using the typical composition of the reactor off gas given below, estimate the stream compositions leaving the condenser.
Composition, kmol/h: aniline 950, cyclo-hexylamine 10, water 1920, hydro- gen 5640, nitrobenzene 40, inerts 300.
2.7. In the manufacture of aniline, the condensed reactor products are separated in a decanter. The decanter separates the feed into an organic phase and an aqueous phase. Most of the aniline in the feed is contained in the organic phase and most of the water in the aqueous phase. Using the data given below, calculate the stream compositions.
Data:
Typical feed composition, including impurities and byproducts, weight%:
water 23.8, aniline 72.2, nitrobenzene 3.2, cyclo-hexylamine 0.8.
Density of aqueous layer 0.995, density of organic layer 1.006. Therefore, the organic layer will be at the bottom.
Solubility of aniline in water 3.2% w/w, and water in aniline 5.15% w/w.
Partition coefficient of nitrobenzene between the aqueous and organic phases: Corganic=Cwater¼300
Solubility of cyclo-hexylamine in the water phase 0.12% w/w and in the organic phase 1.0% w/w.
2.8. In the manufacture of aniline from nitrobenzene, the reactor products are condensed and separated into aqueous and organic phases in a decanter. The organic phase is fed to a striping column to recover the aniline. Aniline and water form an azeotrope, composition 0.96 mole fraction aniline. For the feed composition given below, make a mass balance around the column and determine the stream compositions and flow rates. Take as the basis for the balance 100 kg/h feed and 99.9% recovery of the aniline in the overhead product. Assume that nitrobenzene leaves with the water stream from the base of the column.
Feed composition, weight percentage: water 2.4, aniline 73.0, nitrobenzene 3.2, cyclo-hexylamine trace.
2.9. Guaifenesin (Guaiacol glyceryl ether, 3-(2-Methoxyphenoxy)-1,2-propane- diol, C10H14O4) is an expectorant that is found in cough medicines such as ActifedTM and RobitussinTM. U.S. 4,390,732 (to Degussa) describes a preparation of the active pharmaceutical ingredient (API) from guaiacol (2-methoxyphenol, C7H8O2) and glycidol (3-hydroxy propylene oxide, C3H6O2). When the reaction is catalyzed by NaOH, the reaction yield is 93.8%. The product is purified in a thin-film evaporator giving an overall plant yield of 87%.
i. Estimate the feed flow rates of glycidine and guaiacol that would be needed to produce 100 kg/day of the API.
ii. Estimate how much product is lost in the thin-film evaporator.
iii. How would you recover the product lost in the evaporator?
2.10. 11-[N-ethoxycarbonyl-4-piperidylidene]-8-chloro-6,11-dihydro-5H-benzo- [5,6]-cyclohepta-[1,2-b]-pyridine (C22H23ClN2O2) is a non-sedative anti- histamine, known as Loratadine and marketed as ClaritinTM. The preparation of the active pharmaceutical ingredient (API) is described in U.S. 4,282,233 (to Schering). The patent describes reacting 16.2 g of 11-[N-methyl-4-piperidylidene]-8-chloro-6,11-dihydro-5H-benzo-[5,6]- cyclohepta-[1,2-b]-pyridine (C20H21ClN2) in 200 ml of benzene with 10.9 g of ethylchloroformate (C3H5ClO2) for 18 hours. The mixture is cooled, poured into ice water, and separated into aqueous and organic phases. The organic layer is washed with water and evaporated to dry- ness. The residue is triturated (ground to a fine powder) with petroleum ether and recrystallized from isopropyl ether.
i. What is the reaction byproduct?
ii. The reaction appears to be carried out under conditions that maximize both selectivity and conversion (long time at low temperature), as might be expected given the cost of the raw material. If the conversion is 99.9% and the selectivity for the desired ethoxycarbonyl substituted compound is 100%, how much excess ethylchloroformate remains at the end of the reaction?
iii. What fraction of the ethylchloroformate feed is lost to waste products?
iv. Assuming that the volumes of water and isopropyl ether used in the quenching, washing, and recrystallization steps are the same as the initial solvent volume, and that none of these materials are reused in the process, estimate the total mass of waste material produced per kg of the API.
v. If the recovery (plant yield) of the API from the washing and recrystalliza- tion steps is 92%, estimate the feed flow rates of 11-[N-methyl-4- piperidylidene]-8-chloro-6,11-dihydro-5H-benzo-[5,6]-cyclohepta-[1,2- b]-pyridine and ethylchloroformate required to produce a batch of 10 kg of the API.
vi. How much API could be produced per batch in a 3:8 m3(1000 U.S. gal) reactor?
vii. What would be the advantages and disadvantages of carrying out the other process steps in the same vessel?
Note:Problems 2.5 to 2.8 can be taken together as an exercise in the calculation of a preliminary material balance for the manufacture of aniline by the process described in detail in Appendix F, Problem F.8. Structures for the compounds in problems 2.9 and 2.10 can be found in the Merck Index, but are not required to solve the problems.