H82CPE: Chemical and Phase Equilibria – Coursework Assignment
Heats of Mixing and Reaction
Nitrobenzene is made by the direct nitration of benzene by nitric acid in the presence of strong sulphuric acid solution:
C6H6 + HNO3 C6H5NO2 + H2O
The actual nitrating agent is the nitronium ion, NO2+, and the sulphuric acid is needed to generate this from the nitric acid feed.
Sulphuric acid, of strength w1% by weight and T1 ˚C is mixed with pure nitric acid at 25˚C to give a nitric acid strength in the mixture of w2% by weight and temperature of X˚C. The mixed acid is then passed through a heat exchanger to bring its final temperature to Y˚C and fed to the reactor. Liquid benzene at 25˚C is also fed to the reactor at a rate to give ap% stoichiometric excess based on the nitric acid flow.
The reactor operates adiabatically and gives 100% conversion of the nitric acid. All products leave the reactor as liquids at T2 ˚C. The sulphuric acid is re-concentrated to w1% and recycled, while the nitrobenzene is sent for purification.
Using your personalised values of w1, w2, T1, T2 and p, with the attached thermodynamic data, you need to calculate:
(i) The temperature, X˚C of the mixed acids after mixing;
(ii) The temperature, Y˚C, at which the mixed acids enter the reactor.
You may assume that the organics benzene and nitrobenzene form. an ideal liquid mixture, and there is complete immiscibility between organics and inorganics.
Thermodynamic data
This section provides you with all the data needed to solve the problem. However, as data gathering is such an important part of design work, and one of the main reasons why you are doing this course, considerable detail is given on data sources, and why one set rather than another has been selected.
a Heats of Formation
The standard general sources (Perry, CRC, etc) are satisfactory for benzene, nitric acid and water, but nitrobenzene is surprisingly hard to find. “Thermodynamic Data of Organic Compounds” (1986) by J B Pedley, R D Naylor and S P Kirby is the best source for heats of formation of organics, and is used here for benzene, nitrobenzene and water; “JANAF Thermochemical Tables” (3rd ed., 1984) or the “NBS circular 500” are best for inorganics, and JANAF is used here for nitric acid, with values converted from kcal units.
Compound ΔHf˚/MJ kmol−1
C6H6(1) 49.0
C6H5NO2 (1) 12.5
HNO3 (1) −173.35
H2O (1) −285.83
b Specific heat capacities of the organics
There are some data in Perry for both liquids, but they are old and their accuracy is very uncertain.
For benzene, “Thermophysical properties of matter” vol. 6, by Y S Touloukian and T Makita (IFI/Plenum,
1970) gives an equation that can be simplified as follows (cp is in kcal kg-1K-1 and T in K): cp = 0.2837 + 2.7579 x 10−4T
For nitrobenzene, the most complete data set is in “Tables on the thermophysical properties of liquids and gases” by N B Vargoftik (2nd ed., 1975, Hemisphere), but it only covers temperatures up to 70˚C, and it seems to suggest a much larger variation with temperature than the fragmentary data in Touloukian & Makita. For this reason, it has been decided to take values from the PPDS data package, which are given below (cp inkJ kg-1K-1, T in ˚C.
|
T
|
20
|
30
|
40
|
50
|
60
|
70
|
80
|
90
|
100
|
110
|
120
|
130
|
140
|
|
cp
|
1.468
|
1.479
|
1.495
|
1.514
|
1.535
|
1.559
|
1.584
|
1.610
|
1.637
|
1.664
|
1.691
|
1.718
|
1.745
|
c Thermodynamic data for the inorganics
The ideal mixture assumption cannot be made here, so real heat of mixing data are needed. Data for the binary H2 SO4/H2O are readily available in the form. of an enthalpy-composition diagram, and CRC has heats of dilution at 25˚C for both acid/water binaries, together with some cp data at the same temperature. Perry has a graphical presentation of some data for the ternary H2 SO4/HNO3/H2O based on the experiments of Morgan, Bender and Capell (Chem. & Met. Eng.(1943), 50(6), p 122).
This coursework is based on an MEng design project with what was then ICI. Since graphs are hard to read accurately, ICI provided tables for the acid/water binaries and the ternary system, from which relevant selections are reproduced below.
Table I is for the binary system H2O -H2 SO4. The data consist of the enthalpies of mixtures for different compositions (% by weight H2 SO4) at a number of different temperatures (˚C) in units of kcal kg-1, and based on the enthalpies of the pure liquids being zero at 25˚C. The complete table runs to several pages. (These data can be used to construct the H-x diagrams).
Table II is for the ternary system H2O -HNO3 -H2 SO4. The data here are from an internal ICI memo, but are based on the same published data as Perry’s chart. They are given in 3 tables in units of kcal per kg of mixture. Each table is triangular, with the composition of HNO3 reading horizontally and the composition of H2 SO4 reading vertically (both in % weight at 5% intervals). The composition of water can be found by difference.
Table II(a) gives the enthalpy of various mixtures at 25˚C, relative to the pure components at the same temperature. Tables II(b) and II(c) give the specific heat capacity ofthe mixture in the form.
cp = a + b.t
where tis in ˚C. Table II(b) gives values of a, and Table (c) the values of 104 xb.