Chemical Engineering Design 2
Design Project
Objective
The design project within the module SIC2019 (Chemical Engineering Design 2) aims to provide students the opportunity to apply the knowledge and skills that they have learnt to a real chemical engineering environment for a specific design. It is by doing so, they can integrate their knowledge into a real design process and further develop their understanding of how chemical engineering design is put into practice.
Arrangement
Students are divided into groups of around six members and are normally given 6 weeks (including the 3 weeks in Easter break if the Easter break is within the six week period) to complete ONE of the design statements detailed from the next page. Each group will then need to produce ONE written report to address their specific design including but not limited to a proper flow diagram, relevant calculations for stream composition (material balance), energy balance and size of the equipment if specified. If the submission date hits a holiday period, then the due date will be postponed to 5 pm Friday of the immediate week after the holiday week provided that the whole length of time satisfies at least the 6 weeks required.
Assessment
This design project forms a part of the assessment. It is 15% in combination with the 20% assessed tutorials plus the 5% process simulation towards in total their 40% coursework for the whole module they are attending.
As the report is produced by the whole group, in general, each member of the group will receive the same mark for their report. So it is expected that each member of the group should contribute approximately equally to the design and the writing of the report.
The assessment criteria to the report are: Abstract (10%), Introduction (10%), Process and Equipment Design (60%), which includes: process design, chemical engineering design of specific processes, flowsheet development, specific equipment design if applicable and safety analysis (MSDS for one particular chemical, HAZOP, FMEA and instrument control for one particular unit operation) if applicable, Discussion (10%), Conclusion (5%) and Reference (5%).
The structure of the report should follow the elements specified in the assessment criteria and the number of pages is not limited.
In the event that dispute arises due to the unequal contribution from each member of the group, e. g. someone may not or may not have sufficiently carried out their work towards their assigned task for the report writing, such dispute will need to be discussed and best resolved within the group and finally a consensus should be reached. In such circumstances, the contribution from each student should be declared on the front page of the report. If a consensus cannot be reached, then a peer assessment will have to be undertaken, in which case, a student will need to give a grade for others and others do the same, a calculation based on the average score and individual scores will be made to determine the contribution of each student of the group. The students will be informed of their final contributions.
Note
1. Each group should choose and appoint a member to lead the design and make efficient communication with the rest of the group; also to combine individual works into a proper report for submission.
2. The design project requires efficient communication among the members of the group, i.e., meetings should be held to discuss allocations of specific tasks to individual members of the group and to report their progresses towards their assigned tasks.
3. Each member of the group should focus on what they are initially allocated and set the target (timeline) in order for their work to be combined into the group report.
Design Statement 1
The Design
Design a process to produce 1 × 108 kg per year of acrylonitrile (CH2:CH.CN) from propylene and ammonia by the ammoxidation.
Feedstock: Ammonia: 100% NH3; Propylene: Commercial grade containing 90% C3H6 and 10% paraffins.
Services available: Dry saturated steam at 140 C. Cooling water at 24 C.
The Process
Propylene, ammonia, steam, and air are fed to a vapor-phase catalytic reactor (item A). The feed stream composition (mole percent) is propylene 7; ammonia 8; steam 20; air 65. A fixed-bed reactor is employed using a molybdenum-based catalyst at a temperature of 450 C, a pressure of 3 bar absolute, and a residence time of 4 seconds. Based upon a pure propylene feed, the carbon distribution by weight in the product from the reactor is:
Acrylonitrile: 58%, Acetonitrile: 2%, Carbon dioxide: 16%, Hydrogen cyanide: 6%, Acrolein: 2%, unreacted propylene: 15%, Other byproducts: 1%.
The reactor exit gas is air-cooled to 200 C and then passes to a quench scrubber (B) through which an aqueous solution containing ammonium sulfate 30 wt% and sulfuric acid 1.0 wt% is circulated. The exit gas temperature is thereby reduced to 90 C. From the quench scrubber (B) the gas passes to an absorption column (C) in which the acrylonitrile is absorbed in water to produce a 3 wt% solution. The carbon dioxide, unreacted propylene, oxygen, nitrogen, and unreacted hydrocarbons are not absorbed and are vented to atmosphere from the top of column (C). The solution from the absorber (C) passes to a stripping column (D) where acrylonitrile and lower boiling impurities are separated from water. Most of the aqueous bottom product from the stripping column (D), which is essentially free of organics, is returned to the absorber (C), the excess being bled off. The overhead product is condensed, and the aqueous lower layer returned to the stripping column (D) as reflux.
The upper layer which contains, in addition to acrylonitrile, hydrogen cyanide, acrolein, acetonitrile, and small quantities of other impurities, passes to a second reactor (E) where, at a suitable pH, all the acrolein is converted to its cyanohydrin. (Cyanohydrins are sometimes known as cyanhydrins.) The product from the reactor (E) is fed to a cyanohydrin separation column (F), operating at reduced temperature and pressure, in which acrolein cyanohydrin is separated as the bottom product and returned to the ammoxidation reactor (A) where it is quantitatively converted to acrylonitrile and hydrogen cyanide.
The top product from column (F) is fed to a stripping column (G) from which hydrogen cyanide is removed overhead. The bottom product from column (G) passes to the hydroextractive distillation column (H). The water feed rate to column (H) is five times that of the bottom product flow from column (G). It may be assumed that the acetonitrile and other byproducts are discharged as bottom product from column (H) and discarded. The overhead product from column (H), consisting of the acrylonitrile water azeotrope, is condensed and passed to a separator. The lower aqueous layer is returned to column (H).
The upper layer from the separator is rectified in a column (I) to give 99.95 wt% pure acrylonitrile.
Aspects of the design work required
Process Design
a. Sketch the block flow diagram showing the major processes.
b. Present a material balance for the process.
c. Work out a heat balance for the reactor (A) and quench column (B).
d. Prepare a process flow diagram showing the items of major equipment, pipelines and control instruments.
e. Indicate the instrumentation and safety procedure required for this process bearing in mind the toxic and flammable materials being handled.
Chemical Engineering Design
Present a chemical engineering design of reactor (A) and either column (B) OR column (D).
References
1. Towler G, Sinnott R (2013) Chemical Engineering Design, 2nd ed. (Elsevier)
2. Perona JJ and Thodos G (1957) AIChE J, 3, 230
3. Kolb, HJ and Burwell RL (1945) J Am Chem Soc, 67, 1084
4. Rudd, DF and Watson, CC (1968) Strategy of Process Engineering (New York: John Wiley & Sons
Inc.)
Data
The Reactions:1. C6H6 + Cl2 = C6H5Cl + HCl, 2. C6H5Cl + Cl2 = C6H4Cl2 + HCl
The dichlorobenzene may be assumed to consist entirely of the para-isomer and the formation of trichlorobenzenes may be neglected. The rate equations can be written in first-order form when the concentration of dissolved chlorine remains essentially constant. Thus:
rB = k1xB, rM = k1xB – k2xM, rD = k2xM
where r is the reaction rate, k1 is the rate constant for reaction (1) at 328 k1 = 1.0 × 10–4 s–1, k2 is the rate constant for reaction (2) at 328 k2 = 0.15 × 10–4 s–1. Note, x denotes mole fraction. The subscripts B, M, and D denote benzene, monochlorobenzene, and dichlorobenzene respectively.
Yields for the reactor system should be calculated on the basis of equal liquid residence times in the two reactors, with a negligible amount of unreacted chlorine in the vapour product streams. It may be assumed that the liquid product stream contains 1.5 wt% of hydrogen chloride.
Solubility of the water/benzene system.
Temperature (K) 293 303 313 323
g H2O in 100 g C6H6 0.05 0.072 0.102 0.147
g C6H6 in 100 g H2O 0.175 0.190 0.206 0.225
Thermodynamic and Physical Properties.
C6H6 (L) C6H6 (G) C6H5Cl (L) C6H5Cl(G) C6H4Cl2 (L) C6H4Cl2 (G)
Heat of formation (kJ kmol-1) 298 K 49.0 82.9 7.5 46.1 -42.0 5.0
Heat capacity (kJ kmol-1 K–1) 298 K
350 K
400 K
450 K
500 K 136
148
163
179
200 82
99
113
126
137 152
161
170
181
192 92
108
121
134
145
193
238
296
366 103
118
131
143
155
Density (kg m–3) 298 K
350 K
400 K
450 K
500 K 872
815
761
693
612
1100
1040
989
932
875
1230
1170
1100
1020
Viscosity (Pa s) 298 K
350 K
400 K
450 K
500 K 0.598 10–3
0.326 10–3
0.207 10–3 0.134 10–3
0.095 10–3 0.75 10–3
0.435 10–3
0.305 10–3
0.228 10–3
0.158 10–3
0.697 10–3
0.476 10–3
0.335 10–3
0.236 10–3
Surface tension (N m–1) 298 K
350 K
400 K
450 K
500 K 0.028
0.022
0.0162 0.0104
0.0047 0.0314 0.0276 0.0232 0.0177
0.0115
0.0304 0.0259 0.0205
0.0142
Note, L – Liquid, G – Gas.
References
1. Bodman, SW (1968) The Industrial Practice of Chemical Process Engineering (The MIT Press)
2. Seidell, AS (1941) Solubilities of Organic Compounds, 3rd ed, Vol. II (Van Nostrand)
3. Perry, RH, Chilton CH (1973) Chemical Engineers’ Handbook, 5th ed. (McGraw-Hill)
4. Kirk-Othmer (1964) Encyclopaedia of Chemical Technology, 2nd ed. (JohnWiley&Sons)
5. Towler G, Sinnott R (2013) Chemical Engineering Design, 2nd ed. (Elsevier)
