Author: Jason Swoboda
Suggested Courses: Mass and Energy Balances, Thermodynamics
Level: Sophomore & Junior
Joe has been working with the Polymers-Are-Us Chemicals (PAU Chemicals) for almost 15
years. For most of this time, he's been working as an environmental engineer for the
company, reporting to state and national regulators during inspections and maintaining
records of the company's pollution and wastewater treatment procedures.
Because he has worked so long in the department, Joe has become acquainted with many of
the state regulators and board members personally, and they trust Joe to provide them with
accurate information concerning the company's waste production. As a rule, PAU Chemicals
has always met the state regulations for water quality control, and Joe has always been
very cooperative with the inspection teams from the state.
In recent years, Joe has noticed that even when the company is operating just below the
environmental standards set by the state, there have been significant environmental
problems downstream of the plant outflow. Problems include declining plant life, and
periodic "fish kills." The river isn't used commercially except by PAU Chemicals
as a waste stream release. Few people use this segment of the river, except a rare water
skier or an even rarer fisherman.
After studying the problem, Joe believes that the state regulations are not restrictive
enough and that the company should install another distillation column, which would lower
the plant waste output to a safer level. Joe plans to approach management with a proposal
at the next engineering meeting.
1. A mixture containing 65 mole % A and the balance B is separated in a continuous
distillation column at 1 atm. A flowchart for the operation follows.
The overhead stream from the column is a vapor that passes through a condenser. The
condensed liquid is divided into two equal streams: one is taken off as the overhead
product and the other (the reflux) is returned to the column. The bottom stream from the
column is a liquid that is partially vaporized in a reboiler. The liquid stream emerging
from the reboiler is taken off as the bottoms product, and the vapor is returned to the
column as boilup. Negligible heat is lost from the column, so that the only places in the
system where external heat transfer takes place are the condenser and the reboiler.
Stream Composition Data
Feed - 1: Liquid, 65.0C; 65 mole%A, 35%B
Overhead - 2: Vapor, 63.0C; 98 mole%A, 2%B
Distillate - 3: Liquid, 56.8C; 98 mole%A, 2%B
Reflux - 4: Liquid, 56.8C; 98 mole%A, 2%B
Bottoms - 5: Liquid, 98.7C; 15.5 mole%A, 84.5%B
Boilup - 6: Vapor, 98.7C; 54.4 mole%A, 45.6%B
Thermodynamic Data H(cal/mol)
(here, H is specific enthalpy)
A (1 atm)
| B (1atm)
(a) Taking 100 mol feed as a basis, calculate the net heating requirement (cal) for the
process. You may neglect heats of mixing.
(b) For the same basis, calculate the required heat input to the reboiler and the
required heat removal from the condenser.
(c) Scale up the process assuming a process feed to the distillation column of 35,000lbm/week,
and an average molecular weight of 146 g/mol for both compounds. What is the mass of
dissolved waste (A) is being dumped into the river?
Note: The state limit is 40,000 lbm/week A (found in the output of the
Bottoms product, not counting the Overhead product. Assumed that the A rich compound is
recycled back into the process before the distillation columns).
1. The management team agrees with Joe's analysis of the distillation column, but is
adamant that they do not need to add another column as long as they are already below the
state regulations. Joe feels strongly that PAU Chemicals has a greater responsibility to
the environment and that the state regulations are not low enough. His immediate
supervisor, Tom Terry, asks to speak with Joe in private after the meeting about his
concerns. What should Joe say to convince Tom of the need to install a new distillation
column? How could he justify going beyond the bounds of what is required by the state
regulations? Is there a creative-middle-way approach between doing nothing and adding
another column? Are there several such solutions? Should Joe promote them? Explain.
During routine maintenance of a reactor vessel at your sister plant, technicians find
an irreparable crack in the vessel wall. The plant cannot continue to operate until the
vessel is replaced and must be shut down for an indefinite amount of time (at least one
PAU Chemicals has just signed a new contract for their product with a very large
corporation. The corporation has promised larger contracts in the future if they are
satisfied with PAU's work on this contract. If the company wants to stay on schedule to
keep up with the contract, Joe's plant will have to almost double its production rate. An
increase of this size will have a corresponding increase in pollution into the river.
Joe receives a call from the plant manager, Vince Welling, asking to meet with him that
afternoon. During the meeting, Vince tells Joe that the plant will be running at an
increased rate and will probably run over the state limits for pollution. Joe knows that
an increase like Vince is suggesting means the possibility of very heavy fines, and that
the plant is already putting out far too much pollution for the river environment to
handle. Vince tells Joe that he has to let the management know when the state plans to
visit so that they can hold-up the waste for longer than normal in their temporary storage
ponds before decanting it into the river. That way they can maintain constant production
even while the inspection is going on, and just dump the waste later after the inspection.
Joe reminds him of his earlier presentation which indicates that the river can't handle
these levels of pollution and that more distillation columns are going to have to be added
to handle the higher load. But Vince tells Joe, "It's only temporary, and we'll put
another column or two in after we get this contract. You just let us know when we need to
slow down and don't leave a paper trail for them to follow. Let your managers take care of
the production decisions."
1. Assuming an increase of 200% in the waste stream being fed to the distillation
columns, design a computer program that will calculate how many more columns must be added
in order to cut the waste down to less than 1lbm/week.
2. Assume that the pollutant concentration can be measured as a single variable CA
(moles/volume). Making other assumptions as appropriate, derive the differential equation
dynamic model for pollutant in the river as a function of position and time. You may
consider that the pollutant decays naturally in a first order chemical reaction.
3. The cost of adding a distillation column is $10,000 per column, and the cost of
energy is $1/106 Btu. The mass of waste removed by each column decreases as
another column is added. Plot the total cost of the columns added versus the total amount
of waste removed (in lbm).
- There are several models of professional responsibility. The Malpractice Model requires
engineers to avoid charges of malpractice or breaking the law, and to avoid actions that
will directly harm. The Due-Care Model requires engineers to take a proactive stance that
actively seeks to prevent foreseen or anticipated harm. The Good Works Model requires
engineers to go beyond the Due-Care Model in preventing harm and promoting the public
good, even at considerable personal inconvenience to the engineer. Which model best
describes Joe's recommendation to install another distillation column? Do you think
professional obligation requires Joe to make this recommendation? What tactic should Joe
use in making his recommendations to management, whatever they are?
- Suppose Joe follows Vince's instructions and reports plant emissions at the time the
inspectors are there and not at the time the plant is emptying the waste held in the
storage facilities. (1) What is lying? (2) Is this a case of lying? (3) If it is lying, is
Solutions to Numerical Problems
1. Distillation Column Computations
a) Overall Balances:
Total Moles = 100 = 0.5 n2 + n5
A = 0.65 (100) =0.98(0.5 n2) + 0.155 n5
n2=120 mol n5= 40 mol
Product Flow Rates: Overhead: 0.5(120)(0.98) = 58.8 mol A || 0.5(120)(0.02) = 1.2 mol B
Bottoms: 0.155(40) = 6.2 mol A || 0.845(40) = 33.8 mol B
Overall Energy Balance: Q = H = (out) niHi - (in) niHi
Q = 58.8(0) + 1.2 (0) + 6.2(1385) + 33.8(1812) - 65(271) - 35(257) =
b) Flow Through Condenser: 2(58.8) = 117.6 mols A || 2(1.2) = 2.4 mols B
Energy Balance on Condenser: Qc = H
Qc = 117.6(0-7322) + 2.4(0-6807) = -8.77x105 cal
heat removed from condenser
Assume negligible heat transfer between system and surroundings other than Qc
Qr = Q - Qc = 2.63x104 - (-8.77x105)
= 9.03x105 cal heat added to the reboiler.
2. Calculate scale-up for feed stream (factor of increase) and multiply by the bottom
tails stream. Take the percentage of the bottom tails stream that is A to find your amount
deposited into the stream. The conversion from grams to lbm is 2.20462 lbm/1000g.
3. Using Fortran or any spreadsheet program, repeat iterations of the distillation
process until the bottoms content of A is less than the desired amount. If not, then use
the bottoms tails composition as your feed into a second distilling compound. Add another
$10,000 for each column added, and sum the lbm A in the overhead stream for
each process iteration. Use these two summations for your plot in number 5.
4. Proof of differential equation of dynamic stream model.
- River has a constant cross-section Ax
- River has a constant velocity V
- No axial dispersion (concentration varies only up and down stream)
- Single Concentration Matters - CA
Pollutant Decays as first order chemical reaction
[Rate of Change of Pollution Element] = Bulk Flow In - Bulk Flow Out +
Diffusion In - Diffusion Out - Decay
Pollutant in Element = CAAxX
Diffusive Flux = - D(C/X) (moles/(area*time))
d(CAAxX)/dt = (VAxCA - DAx(CA/X))x
- (VAxCA - DAx(CA/X))X+X - k(AxX)CA
(AxX)d(CA)/dt = (VAxCA -
DAx(CA/X))x - (VAxCA - DAx(CA/X))X+X
(1) d(CA)/dt = ((VCAx - VCAx+x)/x) -
((D(CA/X)x - D(CA/X)x+x)/ x) -kCA
By Definition df/dx = lim(x0) (f(x+x) - f(x))/ x therefore, take the limit of
(CA)/t = -V(CA/x) + D(2CA/x2)
Ethical Solutions (Or Recommended Paths of Thinking)
1. Joe's recommendation does not fit the Malpractice Model, because the plant is not
breaking the law. His action is best described by either the Due-Care Model or the Good
Works Model, depending on such factors as how severe the environmental problems downstream
are, whether the plant can easily afford the new distillation column, how much Joe is
risking his job by making the recommendation, how likely it is that the state regulations
will be changed and how likely it is that human life could be endangered. As with many
ethical problems, we cannot answer the question adequately without additional knowledge of
In the absence of factual knowledge, however, we can make assumptions. Suppose we
assume that the environmental damage is considerable but that human life or health is not
threatened, that the plant can afford the new distillation column and will probably
eventually be required to install it and that Joe's job will not be threatened by any
recommendation he makes. In this case, Joe's recommendation to install another
distillation column falls under the Due-Care Model.
Whether Joe should make this recommendation depends on what model of professional
responsibility you believe he should follow. In general, the Due-Care Model seems to be
the most appropriate. Most professional codes, such as the code of the National Society of
Professional Engineers (NSPE), require engineers to "hold paramount the safety,
health and welfare of the public in the performance of their professional duties."
This seems to involve more than simply avoiding action that breaks the law or provokes
charges of malpractice. This seems to imply the Due-Care Model.
The code, however, refers to human health, and we have assumed that there is no threat
to human health. So the question is what holding paramount public "welfare"
means in this situation. The term is not defined in the code but has been interpreted by
the NSPE's Board of Ethical Review in case 85-2 as requiring engineers to try to get
designs in a defense project modified in order to save tax dollars. Might it also require
an engineer to at least recommend measures that would prevent substantial environmental
harm, assuming that this environmental harm would adversely affect public welfare? If it
does, Joe has an obligation to recommend that another distillation tower be installed.
If we conclude that Joe should recommend that another distillation tower be installed,
there is still a question how the recommendation should be made. When dealing with
business management, the best approach may not be trying to prove that cutting back to
pollution is good for public welfare, but that it is good for business. Joe should try to
convince his manager that cleaning up the river would give the firm good publicity and
that it probably would eventually be required anyhow.
2. (1) We can define a lie as a statement believed to be false or seriously misleading,
made with the intention to deceive.
(2) Whether Joe lies to government officials depends, of course, on what he says.
Suppose he says to officials while they are making their inspection of the plant,
"The present emissions from our plant are never exceeded." Then he would be
lying, because the emissions are exceeded when the excess from the storage tanks is
dumped. Joe might avoid outright lying by refusing to make such statements.
There are however, other types of misrepresentation of the facts besides outright
lying. He might, for example engage in deliberate deception, withhold information, or
simply fail to inform regulators of all of the facts about the plant. If he follows
Vince's instructions, it would probably be impossible for Joe to avoid misrepresentation
in one of these senses, even if he were not to tell a lie. We could certainly consider all
of these types of misrepresentation as types of dishonesty.
(3) Professional codes have a great deal to say about dishonesty. For example, section
II, 3, a of the code of the National Society of Professional Engineers (NSPE) reads:
"Engineers shall be objective and truthful in professional reports, statements or
testimony. They shall include all relevant and pertinent information in such reports,
statements or testimony." This provision speaks directly to Joe's situation and
suggests that withholding information from government officials would be wrong.
There is, furthermore, good reason for the insistence on honesty in the engineering
codes. What engineers (or other professionals) have to offer the public is expert
knowledge and judgment. If professional knowledge and judgment are corrupted by
dishonesty, the value of professional services is undermined. Therefore, the strong
emphasis on honesty in the codes is fully justified. So complying with Vince's instruction
is a violation of professional standards and unjustifiable.