Page 114 - Chemical engineering design

FUNDAMENTALS OF ENERGY BALANCES
365.12821 365 kJ/kmol
Actual work D
Total work required first step D 718 kJ/kmol 93
The spreadsheet used for this example was Microsoft Works. A copy of the solution using
Microsoft Excel can be found on the Butterworth-Heinemann web site: bh.com/compa-
nions/0750641428.

3.13.4. Electrical drives
The electrical power required to drive a compressor (or pump) can be calculated from a
knowledge of the motor efficiency:
W ð mass flow-rate
Power D 3.40
E e
where W D work of compression per unit mass (equation 3.31),
E e D electric motor efficiency.
The efficiency of the drive motor will depend on the type, speed and size. The values
given in Table 3.1 can be used to make a rough estimate of the power required.

Table 3.1. Approximate efficiencies
of electric motors
Size(kW) Efficiency (%)
5 80
15 85
75 90
200 92
750 95
>4000 97

3.14. ENERGY BALANCE CALCULATIONS
Energy balance calculations are best solved using spreadsheets or by writing a short
computer program. A suitable program is listed in Table 3.2 and its use described below.
The use of a spreadsheet is illustrated in Example 3.14b.

Energy 1, a simple computer program
This program can be used to calculate the heat input or cooling required for a process
unit, where the stream enthalpies relative to the datum temperature can be calculated from
the specific heat capacities of the components (equation 3.11).
Ž
The datum temperature in the program is 25 C (298 K), which is standard for most
heat of reaction data. Specific heats are represented by a cubic equation in temperature:
2
C p D A C BT C CT C DT 3
Any unspecified constants are typed in as zero.
If the process involves a reaction, the heat generated or consumed is computed from
Ž
the heat of reaction per kmol of product (at 25 C) and the kmols of product produced.