7/27/2019 Chemical Process Principles tests

1/2

CHE 205 Chemical Process Principles

Test #2. Open book, closed notes. Use one side of each page, begin each numbered problem on a new

page, show all your work, and box your answers. Number and put your initials at the top of each page.

When done, fold the pages vertically and put your name, section, and Test #2 on the outside.

1. (25) A gaseous stream of n-pentane enters a 20-cm diameter pipe at a flow rate of 100 kmol/h at200C and 1 atm and is cooled at constant pressure to 100oC. In answering the following questions,

use data in Table B.1 when appropriate.

a. (10) State whether the following variables increase (), decrease (), or stay the same () as the

gas moves downstream, or whether you cant tell without more information (?): (i) molar flow

rate; (ii) mass flow rate; (iii) standard volumetric flow rate (SCMH); (iv) actual volumetric flow

rate (m3/h); (v) density (kg/m3); (vi) gas velocity (m/s); (vii) critical temperature; (viii) vapor

pressure. Your answers should just be the numerals followed by one of the given symbols [e.g.,

(ix), (x) ]

b. (3) Would you characterize the pentane as a gas, vapor, or supercritical fluid at (i) the pipe inlet,

(ii) the pipe outlet? Briefly explain your answers.

c. (5) To what temperature would you have to cool the stream to begin condensation?

d. (7) Suppose the stream is brought to 470 K and 11.1 atm and you wish to calculate its volumetric

flow rate using the compressibility factor equation of state. Determinez.

2. (20) The fourth tutorial inInteractive Chemical Process Principles dealt with an absorption tower.

The tower had two feed streams: (i) a gas containing 33.0 mole% O2, 66% N2, and 1% SO2 entering

the tower at 250 L/s, 35oC, and 1.2 atm; and(ii) liquid water flowing at a rate of 15.0 mol/s. The two

product streams were (iii) a liquid containing most of the entering water and a small amount of

dissolved SO2; and(iv) a gas containing all of the entering O2 and N2, the balance of the entering SO2,

and some water vapor at 35oC and 1.2 atm. The distribution of SO2 and H2O between those streams

was described by two relations:

ySO2 P = 4.75xSO2

yH2OP =(1xSO2) pH2O*(T)

wherexSO2is the mole fraction of SO2 in the liquid stream,ySO2 is the mole fraction of SO2 in the gas

stream, andyH2Ois the mole fraction of H2O in the gas stream. P andp* have their usual meanings.

a. (10) Draw andcompletely label a flow chart of this process, including component mole fractions

in your labeling of all but the liquid water feed stream.

b. (10) Prove that the absorber has zero degrees of freedom. (Note: You dont have to have worked

through the tutorial to be able to solve this problem.)

OVER

7/27/2019 Chemical Process Principles tests

2/2

3. (55) Polyethylene (PE), the plastic used to make milk bottles, is produced from ethylene in the

process sketched below for a basis of 60.0 kmol C2H4 fed to the reactor. The fresh feed joins a

recycle stream and goes to the polymerization reactor, where a 60.0% reactant conversion is obtained.

A multiple-unit separator (shown as one unit in the flow chart for simplicity) separates the ethylene

(gas) from the polyethylene (solid), recycles some of the ethylene (P,V, and T given on flow chart),

and sends the rest of the ethylene to a vent flare along with a stream of methane. The methane andethylene are burned completely with 25% excess air based on the methane alone (i.e., 25% more air

than is needed to burn just the methane). The stack gases leave the flare at 1 atm with a dew point of

57.5oC.

FLARE

n5 (kmol CH4) Vstack (m3)@700

oC,760 torr

nstack (kmol)yC (kmol CO2/kmol)

25% excess air based on CH4 only yO (kmol O2/kmol)n6 (kmol air) yW (kmol H2O/kmol)

0.21 kmol O2/kmol air (1yCyOyW) (kmol N2/kmol)0.79 kmol N2/kmol air Tdp =57.5

oC

n4 (kmol C2H4)

REACTOR SEPARATIONPROCESS

n1 (kmol C2H4) 60.0 kmol C2H4 n2 (kmol C2H4) PEPE PE

60.0% conversion

448 SCM C2H4 (standard cubic meters)110

oC, 137 atm

n3 (kmol C2H4)

a. (10) Calculate n1, n2, n3, andn4 in the units shown on the chart.

b. (5) Prove that the assumption of ideal gas behavior is probably a poor one for the recycle stream.

[Note:R = 0.08206 L-atm/(mol-K)]. A calculation is required.

c. (10) Assuming that you know the value ofn4 from Part (a),perform a degree-of-freedom analysis

on the flare based on atomic species balances and prove that there are zero degrees of freedom.

Clearly identify your unknowns and equations. (Note: If you couldnt calculate n4, assume 4.0.)

d. (25) Write a complete set of equations that can be solved using E-Z Solve for the 7 remaining

unknown variables on the flow chart, again considering n4 to be known. (Write them in any order

and normally, not in E-Z Solve syntax.) Identify the source of each equation (for example,

Raoults law or Atomic carbon balance). If you use a tabulated physical property (e.g., a vapor

pressure), state where in the text you would find it. Do not do any arithmetic or algebraic

calculations and dont worry about unit conversions.

e. (5) It has recently been discovered that the fresh feed contains trace amounts of a toxic

hydrocarbon impurity and that the impurity is completely retained in the recycle stream from the

separation process. What will eventually happen if nothing is done about it? What should be

done? (Your answer should involve a device or procedure introduced in the text. For full credit,

also account for the fact that the impurity is toxic.)