CONSERVATION: MASS, MOMENTUM, AND ENERGY BALANCES - Biomass as a Sustainable Energy Source for the Future - page 93

First law of thermodynamics
Internal energy
Kinetic energy
Potential energy
Work

SHORT-ANSWER QUESTIONS

3.1 When can a system be considered in “ steady state ” ?

3.2 What is the difference between an open system and a closed system?

3.3 What is the difference between microscopic and macroscopic balances? For
which situations in general do you use each of both approaches?

3.4 What is the difference between the “ degree of conversion ” and the “ relative

degree of conversion ” for a chemical reaction 2A ! Β ? What changes when

the degree of conversion is expressed on a molar basis?

3.5 In the energy balance, the change in potential energy is usually based on the
gravity field; which other fields might be relevant as well and which terms would
then appear?

3.6 Indicate whether the following statements are in general true or false :

￿ Enthalpy is always conserved.

￿ Momentum is always conserved.

￿ The number of moles of a chemical component is always conserved in a chem-

ical reaction.

￿ The mass of each element is conserved during combustion of biomass.

PROBLEMS

3.1 Using Equation (3.6), derive a material balance for a reactant in a closed

system.

3.2 For a reaction mixture with a density that is linearly dependent on the conver-

sion, with ε ζ Α ), derive a relation for the concentration of species

A(c A ) as a function of the relative degree of conversion

ρ = ρ 0 (1 +

ζ Α .

p − p vap

3.3 The cavitation number is defined as Ca =

v 2 .
Now, for ethanol produced by sugar fermentation, a company has installed a
pump for the transport of the liquid product, which is assumed to be pure. The
ambient pressure is 1020 hPa and the pump is situated 1 m below a vessel from

ð ρ 1 = 2