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Separation Process Principles

J. D. Seader, Ernest J. Henley

Chapter 17

Crystallization, Desublimation, and Evaporation - all with Video Answers

Educators


Chapter Questions

09:19

Problem 1

Estimate the sphericities of the following simple particle shapes:
(a) a cylindrical needle with a height, $H$, equal to 5 times the diameter, $D$
(b) a rectangular prism of sides $a, 2 a$, and $3 a$

Subash Charan
Subash Charan
Numerade Educator
01:57

Problem 2

A certain circular plate of diameter, $D$, and thickness, $t$, has a sphericity of $0.594$. What is the ratio of $t$ to $D ?$

Heather Zimmers
Heather Zimmers
Numerade Educator
10:36

Problem 3

A laboratory screen analysis for a batch of crystals of hypo (sodium thiosulfate) is as follows. Prepare both differential and cumulative-undersize plots of the data, using a spreadsheet.
$$
\begin{gathered}
\text { U.S. Screen } \\
6 \\
8 \\
12 \\
16 \\
20 \\
30 \\
40 \\
50 \\
70 \\
100 \\
140 \\
170 \\
230
\end{gathered}
$$
$$
\begin{gathered}
\text { Mass Retained, gm } \\
0.0 \\
8.8 \\
21.3 \\
138.2 \\
211.6 \\
161.7 \\
81.6 \\
44.1 \\
28.7 \\
13.2 \\
9.6 \\
8.8 \\
7.4 \\
\hline 735.0
\end{gathered}
$$
In preparing your plots, determine whether arithmetic, semilog, or log-log plots are preferred.

Susan Hallstrom
Susan Hallstrom
Numerade Educator
05:31

Problem 4

Derive expressions for the surface-mean and mass-mean diameter from a particle-size analysis based on counting, rather than weighing, particles in given size ranges, letting $N_{l}$ be the number of particles in a given size range of average diameter, $\bar{D}_{p_{v}}$ -

Khoobchandra Agrawal
Khoobchandra Agrawal
Numerade Educator
04:51

Problem 5

Using the screen analysis of Exercise 17.3, calculate, with a spreadsheet, the surface-mean, mass-mean, arithmetic-mean, and volume-mean crystal diameters, assuming that all particles have the same sphericity and volume shape factor.

Keshav Singh
Keshav Singh
Numerade Educator
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Problem 6

A precipitation process for producing perfect spheres of silica has been developed. The individual particles are so small that most cannot be discerned by the naked eye. Using optical microscopy, the particle size distribution has been measured, with results given in the table below. Using these data on a spreadsheet program:
(a) Produce plots of the differential and cumulative particle-size analyses
(b) Determine:
(1) surface-mean diameter
(2) arithmetic-mean diameter
(3) mass-mean diameter
(4) volume-mean diameter
(table can't copy)
(table can't copy)

Rashmi Sinha
Rashmi Sinha
Numerade Educator
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Problem 7

A screen analysis for a sample of glauber's salt from a commercial crystallizer is as follows, where the crystals can be assumed to have a uniform sphericity and volume shape factor.
$$
\begin{gathered}
\text { U.S. Screen } \\
14 \\
16 \\
18 \\
20 \\
25 \\
30 \\
35 \\
40 \\
45 \\
50 \\
60 \\
70 \\
80
\end{gathered}
$$
$$
\begin{gathered}
\text { Mass Retained, gm } \\
0.0 \\
0.9 \\
25.4 \\
111.2 \\
113.9 \\
225.9 \\
171.7 \\
116.5 \\
55.1 \\
31.5 \\
8.7 \\
10.5 \\
4.4 \\
875.7
\end{gathered}
$$
Use a spreadsheet to determine in microns:
(a) a plot of the differential analysis
(b) a plot of the cumulative oversize analysis
(c) a plot of the cumulative undersize analysis
(d) the surface-mean diameter
(e) the mass-mean diameter
(f) the arithmetic-mean diameter
(g) the volume-mean diameter

Rashmi Sinha
Rashmi Sinha
Numerade Educator
06:55

Problem 8

$1,000$ grams of water is mixed with 50 grams of $\mathrm{Ag}_{2} \mathrm{CO}_{3}$ and 100 grams of $\mathrm{AgCl}$. At equilibrium at $25^{\circ} \mathrm{C}$, calculate the concentrations in moles/liter of $\mathrm{Ag}^{+}, \mathrm{Cl}^{-}$, and $\mathrm{CO}_{3}^{-}$ions and the grams of $\mathrm{Ag}_{2} \mathrm{CO}_{3}$ and $\mathrm{AgCl}$ in the solid phases.

Jorge Villanueva
Jorge Villanueva
Numerade Educator
03:13

Problem 9

$5,000 \mathrm{lb} / \mathrm{h}$ of a saturated aqueous solution of $\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{4}$ at $80^{\circ} \mathrm{C}$ is cooled to $30^{\circ} \mathrm{C}$. At equilibrium, what is the amount of crystals formed in $\mathrm{lb} / \mathrm{h}$. If during the cooling process, $50 \%$ of the water is evaporated, what is the amount of crystals formed in $\mathrm{Ib} / \mathrm{h}$ ?

Lottie Adams
Lottie Adams
Numerade Educator
03:13

Problem 10

$7,500 \mathrm{lb} / \mathrm{h}$ of a $50 \mathrm{wt} \%$ aqueous solution of $\mathrm{FeCl}_{3}$ at $100^{\circ} \mathrm{C}$ is cooled to $20^{\circ} \mathrm{C}$. At $100^{\circ} \mathrm{C}$, the solubility of the $\mathrm{FeCl}_{3}$ is $540 \mathrm{~g} / 100 \mathrm{~g}$ of water. At $20^{\circ} \mathrm{C}$, the solubility is $91.8 \mathrm{~g} / 100 \mathrm{~g}$ water and crystais of $\mathrm{FeCl}_{3}$ are the hexahydrate. At equilibrium at $20^{\circ} \mathrm{C}$, determine the lb/h of crystals formed.

Lottie Adams
Lottie Adams
Numerade Educator
03:13

Problem 11

The concentrate from an evaporation system is $5,870 \mathrm{lb} / \mathrm{h}$ of $35 \mathrm{wt} \% \mathrm{MgSO}_4$ at $180^{\circ} \mathrm{F}$ and 25 psia. It is mixed with $10,500 \mathrm{lb} / \mathrm{h}$ of saturated aqueous recycle filtrate of $\mathrm{MgSO}_4$ at $80^{\circ} \mathrm{F}$ and 25 psia. The mixture is sent to a vacuum crystallizer, operating at $85^{\circ} \mathrm{F}$ and 0.58 psia in the vapor space, to produce steam and a magma of $25 \mathrm{wt} \%$ crystals and $75 \mathrm{wt} \%$ saturated solution. Determine the $\mathrm{lb} / \mathrm{h}$ of water evaporated and the maximum production rate of crystals in tons/day (dry basis for $2000 \mathrm{lb} /$ ton).

Lottie Adams
Lottie Adams
Numerade Educator
03:22

Problem 12

Urea is to be crystallized from an aqueous solution that is $90 \%$ saturated at $100^{\circ} \mathrm{C}$. If $90 \%$ of the urea is to be crystallized in the anhydrous form and the final solution temperature is to be $30^{\circ} \mathrm{C}$, what fraction of the water must be evaporated?

Sima Sarker
Sima Sarker
Numerade Educator
09:52

Problem 13

In Examples $17.3$ and $17.5$, heat addition to the crystallizer is by an external heat exchanger through which magma is circulated, as shown in Figure 17.16. If instead the heat is added to the feed, determine the new feed temperature. Which is the preferable way to add the heat?

Mohammad Mehran
Mohammad Mehran
Numerade Educator
02:01

Problem 14

For the conditions of Exercise 17.11, determine the rate at which heat must be added to the system.

Nathan Silvano
Nathan Silvano
Numerade Educator
02:12

Problem 15

For the conditions of Example 17.4, calculate the amount of heat in calories/100 grams of water that must be removed to cool the solution from 100 to $10.6^{\circ} \mathrm{C}$.

Zulfiqar Ali
Zulfiqar Ali
Numerade Educator
01:27

Problem 16

Based on the following data, compare the effect of crystal size on solubility in water at $25^{\circ} \mathrm{C}$ for (1) $\mathrm{KCl}$ (see Example $17.7$ ), a soluble inorganic salt, with that for (2) $\mathrm{BaSO}_{4}$, an almost insoluble inorganic salt, and (3) sucrose, a very soluble organic compound.
$$
\begin{aligned}
&\sigma_{\text {2.L }} \text { for barium sulfate }=0.13 \mathrm{~J} / \mathrm{m}^{2} \\
&\sigma_{\text {s, L }} \text { for sucrose }=0.01 \mathrm{~J} / \mathrm{m}^{2}
\end{aligned}
$$
What conclusions can you draw from the results?

Lottie Adams
Lottie Adams
Numerade Educator
00:52

Problem 17

Determine the supersaturation ratio, $S$, required to permit $0.5-\mu \mathrm{m}$-diameter crystals of sucrose $\left(\mathrm{MW}=342\right.$ and $\rho_{c}=1,590$ $\mathrm{kg} / \mathrm{m}^{3}$ ) to grow if $\sigma_{\mathrm{s}, \mathrm{L}}=0.01 \mathrm{~J} / \mathrm{m}^{2}$.

Nicole Smina
Nicole Smina
Numerade Educator
24:43

Problem 18

The Kelvin equation, $(17-16)$, predicts that solubility increases to infinity as the crystal diameter decreases to zero. However, measurements by L. Harbury [J. Phys. Chem., $50,190-199$ (1946)] for several inorganic salts in water show a maximum in the solubility curve and a solubility that approaches zero as crystal size is reduced to zero. Harbury's explanation is that the surface energy of the crystals depends not only on interfacial tension, but also on surface electrical charge, given by
$$
2 q^{2} v_{s} / \pi \kappa R T D_{p}^{4}
$$
where
$q=$ electrical charge on the crystal
$\kappa_{\kappa}=$ dielectric constant
Modify (17-16) to take into account clectrical charge. Make sure your equation predicts a maximum.

Dr. Rajveer Singh
Dr. Rajveer Singh
Numerade Educator
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Problem 19

Using the following data, compare the effect of supersaturation ratio over the range of $1.005$ to $1.02$ on the primary homogeneous nucleation of $\mathrm{AgNO}_{3}$. $\mathrm{NaNO}_{3}$, and $\mathrm{KNO}_{3}$ from aqueous solutions at $25^{\circ} \mathrm{C}$ :
$\begin{array}{llll} & \mathrm{AgNO}_{3} & \mathrm{NaNO}_{3} & \mathrm{KNO}_{3} \\ \text { Crystal density, } \mathrm{g} / \mathrm{cm}^{3} & 4.35 & 2.26 & 2.11 \\ \text { Interfacial tension, } \mathrm{J} / \mathrm{m}^{2} & 0.0025 & 0.0015 & 0.0030\end{array}$

Ronald Prasad
Ronald Prasad
Numerade Educator
01:21

Problem 20

Estimate the effect of relative supersaturation on the primary, homogeneous nucleation of $\mathrm{BaSO}_{4}$ from an aqueous solution at $25^{\circ} \mathrm{C}$, if
$$
\begin{aligned}
\text { Crystal density } &=4.50 \mathrm{~g} / \mathrm{cm}^{3} \\
\text { Interfacial tension } &=0.12 \mathrm{~J} / \mathrm{m}^{2}
\end{aligned}
$$

Narayan Hari
Narayan Hari
Numerade Educator
01:41

Problem 21

Repeat parts (g) and (i) of Example $17.9$ if the solution velocity past the crystal face is reduced from $5 \mathrm{~cm} / \mathrm{s}$ to $1 \mathrm{~cm} / \mathrm{s}$.
Section 17.4

Chai Santi
Chai Santi
Numerade Educator
04:02

Problem 22

The feed to a cooling crystallizer is $2,000 \mathrm{~kg} / \mathrm{h}$ of $30 \mathrm{wt} \%$ $\mathrm{Na}_{2} \mathrm{SO}_{4}$ in water at $40^{\circ} \mathrm{C}$. This solution is to be cooled to a temperature at which $50 \%$ of the solute will be crystallized as the decahydrate. Estimate the required heat-transfer area in $\mathrm{m}^{2}$ if an overall heat-transfer coefficient of 15 Btu/h $-\mathrm{ft}^{2}{ }^{-}{ }^{\circ} \mathrm{F}$ can be achieved. Assume a constant specific heat for the aqueous solution of $0.80 \mathrm{cal} / \mathrm{g}-{ }^{\circ} \mathrm{C}$. Chilled cooling water will flow countercurrently to the crystallizing solution, entering the crystallizer at $10^{\circ} \mathrm{C}$, and exiting at a temperature sufficient to give a log-mean driving force of at least $10^{\circ} \mathrm{C}$.

Sri Datta Vikas Buchemmavari
Sri Datta Vikas Buchemmavari
Numerade Educator
01:05

Problem 23

Two tons per hour of the dodecahydrate of sodium phosphate $\left(\mathrm{Na}_{3} \mathrm{PO}_{4} \cdot 12 \mathrm{H}_{2} \mathrm{O}\right)$ is to be crystallized by cooling, in a cooling crystallizer, an aqueous solution that enters saturated at $40^{\circ} \mathrm{C}$ and leaves at $20^{\circ} \mathrm{C}$. Chilled cooling water flows countercurrently, entering at $10^{\circ} \mathrm{C}$ and exiting at $25^{\circ} \mathrm{C}$. The expected overall heattransfer coefficient is $20 \mathrm{Bt} / \mathrm{h}-\mathrm{ft}^{2}{ }^{-}{ }^{\circ} \mathrm{F}$. The average specific heat of the solution is $0.80 \mathrm{cal} / \mathrm{g}$. ${ }^{\circ} \mathrm{C}$. Estimate:
(a) The tons $(2,000 \mathrm{lb})$ per hour of feed solution.
(b) The heat-transfer area in $\mathrm{ft}^{2}$.
(c) The number of crystallizer units required if each 10-ft-long unit contains $30 \mathrm{ft}^{2}$ of heat-transfer surface.
Section I7.5

Manik Pulyani
Manik Pulyani
Numerade Educator
03:13

Problem 24

An aqueous feed of $10,000 \mathrm{~kg} / \mathrm{h}$, saturated with $\mathrm{BaCl}_{2}$ at $100^{\circ} \mathrm{C}$, enters a crystallizer that can be simulated with the MSMPR model. However, crystallization is achieved with negligible evaporation. The magma leaves the crystallizer at $20^{\circ} \mathrm{C}$ with crystals of the dihydrate. The crystallizer has a volume (vapor-space free basis) of $2.0 \mathrm{~m}^{3}$. From laboratory experiments, the crystal growth rate is essentially constant at $4.0 \times 10^{-7} \mathrm{~m} / \mathrm{s}$. Using the data below, determine:
(a) The $\mathrm{kg} / \mathrm{h}$ of crystals in the magma product.
(b) The predominant crystal size in mm.
(c) The mass fraction of crystals in the size range from U.S.
Standard 20 mesh to 25 mesh.
Data:
Density of the dihydrate crystals $=3.097 \mathrm{~g} / \mathrm{cm}^{3}$
Density of an aqueous, saturated solution of barium chloride at $20^{\circ} \mathrm{C}=1.29 \mathrm{~g} / \mathrm{cm}^{3}$

Lottie Adams
Lottie Adams
Numerade Educator
07:12

Problem 25

The feed to a continuous crystallizer that can be simulated with the MSMPR model is $5,000 \mathrm{~kg} / \mathrm{h}$ of 40 wt\% sodium acetate in water. Monoclinic crystals of the trihydrate will be formed. The pressure in the crystallizer and the heat-transfer rate in the associated heat exchanger are such that $20 \%$ of the water in the feed will be evaporated at a crystallizer temperature of $40^{\circ} \mathrm{C}$. The crystal growth rate, $G$, is $0.0002 \mathrm{~m} / \mathrm{h}$ and a predominant crystal size, $L_{\mathrm{dd}}$, of 20 mesh is desired.
Determine:
(a) The $\mathrm{kg} / \mathrm{h}$ of crystals in the exiting magma.
(b) The $\mathrm{kg} / \mathrm{h}$ of mother liquor in the exiting magma.
(c) The volume in $\mathrm{m}^{3}$ of magma in the crystallizer if density of the crystals $=1.45 \mathrm{~g} / \mathrm{cm}^{3}$
density of the mother liquor $=1.20 \mathrm{~g} / \mathrm{cm}^{3}$
Solubility data:
$\begin{array}{cc}T,{ }^{\circ} \mathrm{C} & \text { Solubility, g sodium acetate/ } 100 \mathrm{~g} \mathrm{H}_{2} \mathrm{O} \\ 30 & 54.5 \\ 40 & 65.5 \\ 60 & 139\end{array}$

Niamat Khuda
Niamat Khuda
Numerade Educator
04:47

Problem 26

An MSMPR-type crystallizer is to be designed to produce $2,000 \mathrm{lb} / \mathrm{h}$ of crystals of the heptahydrate of magnesium sulfate with a predominant crystal size of 35 mesh. The magma will be 15 volo crystals. The temperature in the crystallizer will be $50^{\circ} \mathrm{C}$ and the residence time will be $2 \mathrm{~h}$. The densities of the crystals and mother liquor are $1.68$ and $1.32 \mathrm{~g} / \mathrm{cm}^{3}$, respectively. Determine:
(a) The exiting flow rates in cubic feet per hour of Crystals
Mother liquor
Magma
(b) The crystallizer volume in gallons, if the vapor space equals the magma space.
(c) The approximate dimensions in feet of the crystallizer, if the body is cylindrical with a height equal to twice the diameter.
(d) The required crystal growth rate in feet per hour.
(e) The necessary nucleation rate in nuclei per hour per cubic feet of mother liquor in the crystallizer.
(f) The number of crystals produced per hour.
(g) A screen analysis table covering a U.S. mesh range of $3-1 / 2$ to 200 , giving the predicted $\$_{c u m u l a t i v e ~ a n d ~}$ ? differential screen analyses of the product crystals.
(h) Plots of the screen analyses predicted in part (g).
Section 17,6

Prachi Joshi
Prachi Joshi
Numerade Educator
03:00

Problem 27

Refer to Example 17.12. In Run 15, Fitchett and Tarbell also made measurements of number density of crystals at $200 \mathrm{rpm}$, for which the data can be fitted well by the equation
$$
\text { In } n=26.3-0.407 L
$$
where
$n=$ number density of crystals
$L=$ crystal size, $\mu \mathrm{m}$
Using the MSMPR model, determine in the same units as for Example 17.12:
(a) $n^{\circ}$
(b) $G$
(c) $B^{\circ}$
(d) mean crystal length
(e) $n_{c}$
(f) Are your results consistent with the trends found in Example 17.12?
(g) Using your results and those in Example 17.12, predict the growth rate and mean-crystal length if no agitation is used.
Figure 17.37 Population density of $\mathrm{CaCO}_{3}$ for Exercise 17.28.
$17.28$ Tai and Chen $[A I C h E J .41,68-77(1995)]$ studied the precipitation of calcium carbonate by mixing aqueous solutions of sodium carbonate and calcium chloride in an MSMPR crystallizer with $\mathrm{pH}$ control, such that the form of $\mathrm{CaCO}_{3}$ was calcite rather than aragonite or vaterite. In Run S-2, which was conducted at $30^{\circ} \mathrm{C}$, a pH of $8.65$, and $800 \mathrm{rpm}$, with a residence time of $100 \mathrm{~min}$, the crystal population density data were as shown in Figure 17.37. Because the data do not plot as a straight line, they do not fit (17-38).
(a) Develop an empirical equation that will fit the data and determine, by regression, the constants.
(b) Can nucleation rate and growth rate be determined from the data? If so, how?
17.29 Tsuge and Matsuo ["Crystallization as a Separation Process," ACS Symposium Series 438 , edited by Myerson and Toyokura, ACS, Washington, DC (1990), pp. 344-354] studied the precipitation of $\mathrm{Mg}(\mathrm{OH})_{2}$ by reacting aqueous solutions of $\mathrm{MgCl}_{2}$

Khoobchandra Agrawal
Khoobchandra Agrawal
Numerade Educator
01:46

Problem 28

Tai and Chen $\left[\right.$ AlChE $\left.J_{.}, 41,68-77(1995)\right]$ studied the precipitation of calcium carbonate by mixing aqueous solutions of sodium carbonate and calcium chloride in an MSMPR crystallizer with pH control, such that the form of $\mathrm{CaCO}_{3}$ was calcite rather than aragonite or vaterite. In Run S-2, which was conducted at $30^{\circ} \mathrm{C}$, a $\mathrm{pH}$ of $8.65$, and $800 \mathrm{rpm}$, with a residence time of $100 \mathrm{~min}$, the crystal population density data were as shown in Figure 17.37.
Because the data do not plot as a straight line, they do not fit (17-38).
(a) Develop an empirical equation that will fit the data and determine, by regression, the constants.
(b) Can nucleation rate and growth rate be determined from the data? If so, how?

Sheryl Ezze
Sheryl Ezze
Numerade Educator
06:56

Problem 29

Tsuge and Matsuo ["Crystallization as a Separation Process," ACS Symposium Series 438 , edited by Myerson and Toyokura, ACS, Washington, DC (1990), pp. 344-354] studied the precipitation of $\mathrm{Mg}(\mathrm{OH})_{2}$ by reacting aqueous solutions of $\mathrm{MgCl}_{2}$
Figure 17.38 Crystal-size distribution of $\mathrm{Mg}(\mathrm{OH})_{2}$ for Exercise 17.29.
and $\mathrm{Ca}(\mathrm{OH})_{2}$ in a 1-liter MSMPR crystallizer operating at $450 \mathrm{rpm}$ and $25^{\circ} \mathrm{C}$. Crystal sizes were measured by a scanning electron microscope (SEM) and analyzed by a digitizer. Crystal size was taken to be the maximum length. A typical plot of the crystal-size distribution is given in Figure $17.38$ for an assumed residence time of $5 \mathrm{~min}$. Assuming that the number of crystals is proportional to $\exp (-L / G \tau)$, as in (17-38), determine:
(a) Growth rate
(b) Nucleation rate
(c) Predominant crystal size

Adriano Chikande
Adriano Chikande
Numerade Educator
06:22

Problem 30

The feed to the top of a falling-film crystallizer is a melt of 60 wt\% naphthalene and 40 wt\% benzene at saturation conditions. If the coolant enters at the top at $10^{\circ} \mathrm{C}$, determine the crystal-layer thickness for up to $2 \mathrm{~cm}$, as a function of time. Necessary physicalproperty data are given in Example 17.13.

Satpal Satpal
Satpal Satpal
Numerade Educator
16:58

Problem 31

Paradichlorobenzene melts at $53^{\circ} \mathrm{C}$, while orthodichlorobenzene melts at $-17.6^{\circ} \mathrm{C}$. They form a eutectic of $87.5 \mathrm{wt}$ ? of the ortho isomer at $-23^{\circ} \mathrm{C}$. The normal boiling points of these two isomers differ by about $5^{\circ} \mathrm{C}$. A mixture of 80 wt $\%$ of the para isomer at the saturation temperature of $43^{\circ} \mathrm{C}$ is fed to the top of a fallingfilm crystallizer, where coolant enters at $15^{\circ} \mathrm{C}$. If $8-\mathrm{cm}$ i.d, tubes are used, determine the time for the crystal-layer thickness at the top of the tube to reach $2 \mathrm{~cm}$. Which isomer will crystallize? Necessary physical properties are given in Perry's Chemical Engineers' Handbook, except for crystal thermal conductivity, for which we assume a value of $0.15 \mathrm{Btu} / \mathrm{h}-\mathrm{ft}-{ }^{\circ} \mathrm{F}$ for either isomer.

Susan Hallstrom
Susan Hallstrom
Numerade Educator
01:35

Problem 32

Derive (17-67).
Section 17.8

Aman Gupta
Aman Gupta
Numerade Educator
01:17

Problem 33

Derive the following expression for the average impurity concentration over a particular length of crystal layer, $z_{2}-z_{1}$, after one pass or partial pass of zone melting.
$$
w_{a v_{g}}=w_{0}\left\{\frac{\ell(1-K)}{K\left(z_{2}-z_{1}\right)}\left[\exp \left(-z_{2} K / \ell\right)-\exp \left(-z_{1} K / \ell\right)\right]+1\right\}
$$
Using the results of Example 17.14, calculate $w_{\text {avg }}$ for $z_{1}=0$ and $z_{2} / \ell=9$.

Narayan Hari
Narayan Hari
Numerade Educator
02:25

Problem 34

In Example 17.14, let the last $20 \%$ of the crystal layer be removed, following the first pass, to $z / \ell=9$. Calculate from (1), in Exercise $17.33$, the average impurity concentration in the remaining crystal layer.

Adriano Chikande
Adriano Chikande
Numerade Educator
00:54

Problem 35

A bar of 98 wt $\%$ Al with 2 wt $\%$ of Fe impurity is to be subjected to one pass of zone refining. The solid-liquid equilibrium distribution coefficient for the impurity is $0.29$. If $z / \ell=10$ and the resulting bar is cut off at $z_{2}=0.75 z$, calculate the concentration profile for Fe and the average concentration from (1) in Exercise 17.33.
Section $17.9$

Lottie Adams
Lottie Adams
Numerade Educator
10:21

Problem 36

A desublimation unit of the heat-exchanger type is to be sized for the recovery of $200 \mathrm{~kg} / \mathrm{h}$ of benzoic acid (BA) from a gas stream containing $0.8 \mathrm{~mol} \%$ BA and $99.2 \mathrm{~mol} \% \mathrm{~N}_{2}$. The gas enters the unit at 780 torr at $130^{\circ} \mathrm{C}$ and leaves without pressure drop at $80^{\circ} \mathrm{C}$. The coolant is pressurized cooling water that enters at $40^{\circ} \mathrm{C}$ and leaves at $90^{\circ} \mathrm{C}$, in countercurrent flow to the gas. The heatexchanger tubes are of the type in Example 17.15.
Some properties of benzoic acid are given in Exercise 17.37. In addition,
$$
\begin{aligned}
&k_{c} \text { of solid benzoic acid }=1.4 \mathrm{cal} / \mathrm{h}-\mathrm{cm}-{ }^{\circ} \mathrm{C} \\
&\rho_{c} \text { of solid benzoic acid }=1.316 \mathrm{~g} / \mathrm{cm}^{3}
\end{aligned}
$$
Determine the number of tubes needed and the time required to reach the maximum thickness of benzoic acid of $1.25 \mathrm{~cm}$.

Chareen Guzman
Chareen Guzman
Numerade Educator
01:18

Problem 37

Benzoic acid is to be crystallized by bulk-phase desublimation from $\mathrm{N}_{2}$ using a novel method described by Vitovec, Smolik, and Kugler [Coll. Czech. Chem. Commun, 42, 1108-1117 (1977)]. The gas, containing $6.4 \mathrm{~mol} \%$ benzoic acid and the balance $\mathrm{N}_{2}$, flows at $3 \mathrm{~m}^{3} / \mathrm{h}$ at $1 \mathrm{~atm}$ and a temperature of $10^{\circ} \mathrm{C}$ above the dew point. The gas is directly cooled by the vaporization of $150 \mathrm{~cm}^{3} / \mathrm{h}$ of a water spray at $25^{\circ} \mathrm{C}$. The gas is further cooled in two steps by nitrogen quench gas at l atm as follows:
$\begin{array}{ccc}\text { Step } & \text { Quench Gas Flow Rate, } \mathrm{m}^{3} / \mathrm{h} & \text { Quench Gas Temp., }^{\circ} \mathrm{C} \\ 1 & 1.5 & 105 \\ 2 & 2.0 & 25\end{array}$
The quench gases enter through porous walls of the vessel so as to prevent crystallization on the vessel wall. Based on the following data for benzoic acid, determine the final gas temperature and the fractional yield of benzoic-acid crystals, assuming equilibrium in the exiting gas.
Melting point $=122.4^{\circ} \mathrm{C}$
Specific heat of solid and vapor $=0.32 \mathrm{cal} / \mathrm{g}-{ }^{\circ} \mathrm{C}$
Heat of sublimation $=134 \mathrm{cal} / \mathrm{g}$
Vapor pressure:
$T,{ }^{\circ} \mathrm{C} \quad$ Vapor Pressure, torr
The vapor pressure data can be extrapolated to lower temperatures by the Antoine equation.

Aadit Sharma
Aadit Sharma
Numerade Educator
02:36

Problem 38

Derive (17-75),
Section 17.10

Mikayla Stephens
Mikayla Stephens
Numerade Educator
06:11

Problem 39

Fifty-thousand pounds per hour of a $20 \mathrm{wt} \%$ aqueous solution of $\mathrm{NaOH}$ at $120^{\circ} \mathrm{F}$ is to be fed to an evaporator operating at $3.7$ psia, where the solution is concentrated to 40 wt $\% \mathrm{NaOH}$. The heating medium is saturated steam at a temperature $40^{\circ} \mathrm{F}$ higher than the exiting temperature of the caustic solution. Determine:
(a) Boiling-point elevation of the solution
(b) Saturated-heating-steam temperature and pressure
(c) Evaporation rate
(d) Heat-transfer rate
(e) Heating-steam flow rate
(f) Economy
(g) Heat-transfer area if $U=300 \mathrm{Btu} / \mathrm{h}-\mathrm{ft}^{2}-{ }^{\circ} \mathrm{F}$

Qiao Ruan
Qiao Ruan
Numerade Educator
01:05

Problem 40

A 10 wt $\%$ aqueous solution of $\mathrm{NaOH}$ at $100^{\circ} \mathrm{F}$ and a flow rate of $30,000 \mathrm{lb} / \mathrm{h}$ is to be concentrated to 50 wt\% by evaporation using saturated steam at 115 psia.
(a) If a single-effect evaporator is used with $U=400 \mathrm{Btu} / \mathrm{h}-\mathrm{ft}^{2}-{ }^{\circ} \mathrm{F}$ and a vapor-space pressure of $4 \mathrm{in}$. Hg, determine the heat-transfer area and the economy.
(b) If a double-effect evaporator system is used with forward feed and $U_{1}=450 \mathrm{Btw} / \mathrm{h}-\mathrm{ft}^{2}-{ }^{\circ} \mathrm{F}$ and $U_{2}=350 \mathrm{Btu} / \mathrm{h}-\mathrm{ft}^{2}{ }^{\circ}{ }^{\circ} \mathrm{F}$, and a vaporspace pressure of $4 \mathrm{in}$. $\mathrm{Hg}$ in the second effect, determine the heattransfer area of each effect, assuming equal areas, and the overall economy.
17.41 A 10 wt\% aqueous solution of $\mathrm{MgSO}_{4}$ at $14.7$ psia and $70^{\circ} \mathrm{F}$ is sent to a double-effect evaporator system with forward feed
17.40 A 10 wt $\%$ aqueous solution of $\mathrm{NaOH}$ at $100^{\circ} \mathrm{F}$ and a flow rate of $30,000 \mathrm{lb} / \mathrm{h}$ is to be concentrated to 50 wt\% by evaporation using saturated steam at 115 psia.
(a) If a single-effect evaporator is used with $U=400 \mathrm{Btu} / \mathrm{h}-\mathrm{ft}^{2}-{ }^{\circ} \mathrm{F}$ and a vapor-space pressure of $4 \mathrm{in}$. Hg, determine the heat-transfer area and the economy.
(b) If a double-effect evaporator system is used with forward feed and $U_{1}=450 \mathrm{Btw} / \mathrm{h}-\mathrm{ft}^{2}-{ }^{\circ} \mathrm{F}$ and $U_{2}=350 \mathrm{Btu} / \mathrm{h}-\mathrm{ft}^{2}{ }^{\circ}{ }^{\circ} \mathrm{F}$, and a vaporspace pressure of $4 \mathrm{in}$. $\mathrm{Hg}$ in the second effect, determine the heattransfer area of each effect, assuming equal areas, and the overall economy.

Aadit Sharma
Aadit Sharma
Numerade Educator
01:23

Problem 41

A 10 wt\% aqueous solution of $\mathrm{MgSO}_{4}$ at $14.7$ psia and $70^{\circ} \mathrm{F}$ is sent to a double-effect evaporator system with forward feed

Hast Aggarwal
Hast Aggarwal
Numerade Educator