The main process operations in chemical engineering systems consists of:

1. Reaction
2. Separation
3. Mixing
4. Physical Transformation

Reaction

Reaction is the chemical transformation of raw materials to desired products. There are two common reactor configurations. The first is known as the Continuously Stirred Tank Reactor (CSTR). Here, the reactor is a tank usually jacketed by steam or cooling water to attain specified temperature settings. The reactants, usually both liquid, are fed into the tank and a stirrer ensures that reactants are mixed properly. The other configuration is the Plug Flow Reactor (PFR). Here, the reactor is a pipe containing baffles inside. A jacket of steam or cooling water also cover the pipe. The reactants, gaseous or liquid, flow into the reactor and are mixed turbulently as they pass through the baffles.

For a reaction to proceed to the desired degree and distribution, several variables need to be set up correctly.

1. The right amount of reactants have to be fed. This does not necessarily imply stoichiometric proportions. For instance, if one of the reactants is inexpensive and easily removed, excess of this reactant may be desirable.
2. Temperature and pressure need to be set carefully. Some reactions do not procede until a certain temperature is met, i.e.\ the energy of activation has to be overcome.

If insufficient heat is supplied to endothermic reactions, the reaction may slow down and be extinguished. The other case is more serious, if heat is not removed properly from an exothermic reaction, a thermal runaway will occur. The temperature can rise indefinitely and can cause disaster.
 
 

Separation

Separation processes require the extraction of a desired product at specified purity. The other exit streams contain by-products, solvents or unreacted chemicals. The feed to separation units could come from direct raw materials or from reactor effluents.

Most separation schemes take advantage of phase differences, gravity or size distribution. A phase is simply a region which appear to exhibit homogenous physical features. Based on gravity, the system usually consists of a heavy phase and a light phase. The convention is to use x_i todenote the mole or mass fraction of component i in the heavy phase, while y_i is used to denote the mole or mass fraction of component i in the light phase.

Again, temperature and pressure will be very important variables that determine the distribution of a desired component between the phases:

1. Distillation. The heavy phase is liquid while the light phase is vapor. For the binary case, say a mixture of compounds A and B, separation depends strongly on the relative volatilities of each component. If both have the same volatility then separation via distillation is not effective. However, as will be shown later, the volatilities depend on the vapor pressures at the operating temperature and the column pressure used.
2. Absorption. For gas absorption, the gas contains a desired component, say A. A liquid solvent which has specific affinity only to A is then contacted with the gas stream, and a fraction of A coming into the absorber is "absorbed'' by the entering liquid. Ideally, the chosen solvent should have low volatility such that almost none of the solvent escapes with the gas. The fraction absorbed is again strongly dependent on the operating temperature and pressure, where lower temperature and higher pressure is more desirable. The solvent containing absorbed A is then passed to another separation unit such as a distillation column to finally separate the two.
3. Stripping. Considered the reverse of absorption, a liquid stream contains the desired A. A gas such as air or nitrogen which is devoid of A (or at least is very dilute in A) is mixed or bubbled through the liquid feed and thus "strips'' a fraction of A in the liquid. Again, both temperature and pressure is crucial for determining how much stripping is possible. The contaminated gas is then usually passed through a condenser where most of A is liquified.
4. Crytallization. A solution is heated to evaporate the solvent and is then cooled through careful temperature schedules to obtain predetermined crystal size and composition. Then the slurry is either passed through a series of settling tanks or filtration units.
5. Liquid-Liquid Extraction. A solution containing the desired component A and solvent C is mixed with another liquid B. A fraction of A is then ``extracted'' from C into B. However, B can not be completely miscible with C. The greater the immiscibility between B and C, the better. Also, the B-rich phase containing A has to be easier to separate than the C-rich phase, otherwise the whole extraction scheme becomes unnecessary.

From the brief discussion of typical chemical engineering operations, the following questions need to be answered:

1. How do we determine the temperature, pressure and other intensive variables to achieve desired extents of reaction and/or separation?
2. Knowing what the operating temperature and pressure are, how could we transfer heat or mechanical energy effectively?
3. How are material balances affected by energy balances, and vice versa?

Chapter 7 introduces the different forms of energy and how total energy has to be balanced. Thus it discusses how energy can be transferred across system boundaries either through heat removal or heat supply or mechanical energy (stirrers, pump, turbines). A short discussion of the Bernoulli equation is also included to deal with energy balance during transport of fluids.

Chapter 8 continues to answer question 2. This chapter introduces the idea of process paths, i.e. a strategy in which one state can be brought into a desired state. The heat transfer along each segment of the path can be calculated and then the total energy balance results from adding the energy balance of each segment along the path. We will limit these path segments to changes only in one state variable, e.g. change in temperature (sensible heat) or change is phase aggregation (latent heat).

Chapter 9 focuses specifically on reacting systems. Although it could be considered as just another segment of the process path introduced in Chapter 8, several estimation techniques are involved in obtaining heats of reaction. It also attempts to answer question 2.

For question 3, all four chapters are needed depending on the complexity of the problem. For instance, reaction will determine the composition. Reaction will sometimes also determine the phases . For separation system, evaporation and condensation require heat transfer between hot and cold streams, i.e. depending on how much heat is transferred, the composition of the streams can be very different.

Tomas B. Co
Associate Professor
Department of Chemical Engineering
Michigan Technological University
1400 Townsend Avenue
Houghton, MI 49931-1295