8/10/2019 Industrial & Engineering Chemistry Volume 39 Issue 1 1947 Carlson, Harrison C. -- Absorption and Humidification

1/2

R P T l O N

I F I C A T

Harrison

C

Carlson,

1413 VAN

BUREN

STREET,

WILMINGTON

DEL.

HE theory and practice of absorption and humidification

have advanced slowly during the past year. Marshall 12)

T iscussed the selectionof finned coils cooled by a boiling refrig-

erant for dehumidifying and cooling air. His presentation of the

equivalent by-pass theory considers tha t part of t he air is cooled

to the surface temperature and part unchanged in temperature.

The same conclusions could be reached

if

the ratio of the heat

transfer coefficient to t he mass transfer coefficient were taken

equal to the humid heat and if th e transfer units for heat and

mam

transfer are equal in number.

When cooling water is limited, the refrigeration industry usee

evaporative condensers for condensing vapors inside tubes with

water recirculated over the outside, but cooled only by the air

blown countercurrent to the descending water films. Goodman

(7)

had analyzed the rate of heat transfer

to

the air by sensible

and latent heat effects, and showed that the enthalpy driving

force between the water and the inlet air could be used to calcu-

late the rate of heat transfer. Thomsen

(19)

substituted an ap-

proximate expression for the enthalpy as a function of the wet-

bulb temperature, obtaining an over-all coefficient to be used

with the temperature difference between the condensing vapor

and the average wet-bulb temperature of the air. He analyzes

the operation on a graph of temperature plotted against resistance

to heat flow between the vapor and cooling air.

Ferencz

5 )

reformulated the hea t and material balance equa-

tions and rates

of

heat and material transfer in the countercurrent

contact of a liquid and gas in a packed tower for dehumidification

and water cooling. He suggested a method

of

solving the differ-

ential equations by successive approximations. The concept

of

the enthalpy driving force, however, gives quicker solutions to

some cases than does the general equations based oh over-all

transfer coefficients presented by Ferencz.

Estignard-Bluard 4 ) applied the Ponchon diagram to calcu-

lations of the separation of two gases by a solvent (extractive dis-

tillation). He shows how the minimum reflus and the number

of theoretical plates a t infinite or finite reflux ratios can be calcu-

lated for an isothermal and isopiestic column, using a nonvolatile

solvent in which the gases are only slightly soluble. An example

calculated

for

separating a 20y0 mixture of propylene in propane

using water at atmospheric temperature and pressure indicated

twenty-five theoretical plates and a circulation of 7 cubic meters

of water per cubic meter of propylene recovered, when the top and

bottom products contained less than 0.5% impurity.

Geddes (6) developed a method of predicting local hfutyhree

plate efikiencies in bubble-cap absorption and distillation col-

umns. Although Geddes points out tha t some

of

the assumptions

are inexact, the predicted plate efficiencies agree with the meas-

ured values over a wide range of conditions. The gas film re-

sistance is based on the theoretical equation for unsteady-state

diffusion from a spherical gas bubble to a constant interface coni-

position. The time of contact is calculated from an empirical

correlation of the velocities of rise of single gas bubbles and the

head of unaerated liquid over the slot, The paper is character-

istic

of

the theoretical attack on absorption aqd distillation prob-

lems which designers

of

commercial equipment are forced

t o

make because

of

the lack

of

experimental data.

O'Connell gave an empirical correlation of the over-all plate

efficiency of bubble-cap absorbers ( Id ) , using the ratio of the gas

solubility to the liquid viscosity. O'Connell terminated his cor-

relation a t a plate efficiency of 70%; this probably indicates

that , for higher plate efficiencies, he residance in the liquid film

becomes less important and other variables would have to be in-

cluded to correlate the gas film resistance.

The design of absorbers for natural gasoline to operate

at 1000

t o

2000 lb./sq. in., when the dry ga8 is to be reinjected into the

petroleum reservoir, was discuesed by Wade 10). A t

1800

lb./sq. in. methane is almost immediately dissolved

in

the lean

oil at the top of the absorber,

so

that the amount

of

liquid is

nearly doubled and the oil tempera ture raised. With cool enter-

ing gas the oil temperature may actually decreme as it flows

through the high pressure absorber. At low pressure the major

absorption is of butanes and higher hydrocarbons; this occurs

at

the bottom of the absorber, so that the oil temperature risesm t

flows through the absorber. As the solubility of methane in-

creases and that of butane decreases with increasing pressure,

the lean oil rate is nearly constant for pressures between 1000and

2000 lb./sq. in. Wade discussed the choice of absorption pressure

and the method of separating the large amount of methane from

the desired components in the rich oil.

HUMIDITY EAYEREVENT.he familiar Carrier psychrometric

chart of humidity os. dry-bulb temperature was redecorated by

Palmatier and Wile

15)

with an auxiliary scale for the enthalpy

of saturated air in the range of 20 t o

110" F.

As unsaturated

air has nearly the same enthalpy as saturated air

of

the same

thermodynamic wet-bulb (adiabatic saturation) temperature,

deviations in the two enthalpies are shown by contour lines on the

chart.

A

table of corrections to the saturated humidities and

enthalpies in the range of 20

o

84

F.

s given for total pressures

between 24 and

31

inches of mercury.

For calculating the humidity from wet- and dry-bulb tem-

perature measurements with air in the range of

50-200

F.

and

5-100 lb./sq, in., Rohsenow

(f7)

resented two graphs based on

the hope tha t the adiabatic saturation temperature is the same as

the wet-bulb temperature. He made the minor correction for

gas law deviations, which are less than the uncertainty in reading

from the small charts. The amount

of

labor expended in calcu-

lating charts of adiabatic saturation compared to the actual

measurement

of

humidity as a function

of

reliable wet- and dry-

bulb temperatures is really amazing in view

of

the lack of defini-

tive data .

Williams and Schmitt 91 ) solved t ie problem of humidity

measurement by wet-bulb psychrometry in rotary dryers of

water-soluble salts, where the wet-bulb thermometer can be con-

taminated with sal t. The. wet-bulb Ieading can be used to cal-

culate the humidity by using a saturated solution of the salt

being dried on the wick of the wet-bulb thermometer.

Williams

and Schmitt developed the psycliromctric equation for this cme,

which required tha t the vapor pressure of the saturated salt s o h -

tion and the heat of crystallization be kiiown. Humidity meas-

urements in an uncontaminated air stream with the wet bulbs

moistened with water

as

well as with saturated solutions

of

sodium nitrate, ammonium nitrate, or magnesium chloride hexa-

14

8/10/2019 Industrial & Engineering Chemistry Volume 39 Issue 1 1947 Carlson, Harrison C. -- Absorption and Humidification

2/2

January

1947

I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y 15

hydrate gave the same calculated humidity ; this indicated that

their extension

of

the psychrometric equation

was

correct.

Humidity-temperaturo charts showing wet-bulb lines for these

three salt solutions were published.

EQUIPMENT Since 1940 glass and ceramic equipment for

hydrogen chloride absorption has been largely replaced by

Karbate. Hatfield and Ford (9) recounted the improvements

in the resin used to make the carbon or graphite base impervious,

which extended its temperature and corrosion resistance. They

also reviewed the mechanical improvements in Karbate pumps,

valves, coolers, towers, and heat exchangers.

The ease of fabri-

cating this material of high thermal conductivity and corrosion

resistance into shell and tube exchangers means tha t the acid and

hydrogen chloride gas can be contacted directly on the water-

cooled surface. These cooler absorbers, which may be con-

structed with either horizontal or vertical tube bundles, are pre-

ferred with concentrated hydrogen chloride gas to the old system

of a number of towers in series, each with its external cooler for

the acid circulated over the tower. Cooler-absorbers are char-

acterized by compactness and low pressure drop.

EQTJIPXENT

ERFORMANCE

ATA The flooding velocities of

various sizes and types of tower packing have previously been

correlated with a packing constant, the ratio

of

the surface area

per unit volume to the cube of the percentage

of

voids. For

small packings this ratio is a function of the method

of

packing

the tower as well as the diameter of the tower and the source of

th e packing. Lobo, Friend, Hashmall, and Zena 1 2 ) found that

if l/)-inch Raschig rings were poured into a water-filled tower,

the packing constant was 360, but if the packing were then

shaken, the ratio increased to 725. The ratio for l/a-inch rings

may

\ary

from 593 to 1664, depending on the manufacturer.

These tiuthors recorrelated the existing data on flooding velocities

of dumped packings, using the most probable values

for

the pack-

ing constant, but t he points are still scattered in a band

of 100%

around the mean curve.

Boelter, Gordon, and Griffin 8)measured the rate of evapo-

ration of water from a 1-foot diameter horizontal surface into

quiet air a t 65" to 80"

F.

and

54

to

98

relative humidity when

the water temperature was varied from 63" to

200 F.

Evapo-

ration coefficients

for

free convection were correlated with the

Grashof

and Schmidt groups and compared with similar data on

heat transfer.

During the war about 250,000 tons a year of butadiene were

purified from the associated C4 hydrocarbons by absorption or

extraction with an aqueous cuprous ammonium acetate solution.

Morrell, Paltz, Packie, Asbury, and Brown I S ) give the physico-

chemical data underlying the process, which depends on the for-

mation of a loose chemical compound. This chemical absorbent

has the advantage

of

higher selectivity over the usual extractive

distillation agents. The diolefins are much more soluble than

the olefins or saturated paraffins, particularly at

low

tempera-

tures. For absorption on 1-inch Raschig rings, a height equiva-

lent to a theoretical plate was between

10

and 12 feet. The

greater selectivity at

low

temperatures and the boiling points

of

the hydrocarbons indicated the liquid-liquid extraction would be

better than absorption.

PigulevskiI and Ilyina

(16)

measured the rate

of

absorption of

ethylene in sulfuric acid in a flask rotated to keep the walls re t .

In fresk

95.5%

sulfuric acid at

7 0

C.,

K g

was 0.9 X

Ib.

moie/(hr. ) (sq. ft.) (atm. ). The absorption coefficient increased

twenty-five fold as the acid concentration was increased from

86.5 to

99.4 .

The absorption coefficient increased 1.33 times

for

a

10 C.

rise in temperature. The absorption rate decreased

the initial value to 0.1 when a mole of sulfuric acid had absorbed

a mole

o

ethylene.

Bosworth (3) bsorbed carbon dioxide in a

56%

sugar solution

made alkaline with

0.078

N calcium oxide, using a I-foot spray

tower. The effective tower height was varied from 5 to 96

inches, Ind the amount of carbon dioxide absorbed was found

proportional

t o

the

1/,

power of the tower height. The resistance

is mainly in the liquid film, as Johnstone and Williams IO) ound

1 N sodium hydroxide necessary t o eliminate the liquid film re-

sistancc. Equations for the unsteady-state diffusion through a

stagnant drop indicate tha t the amount absorbed should be pro-

portional to th e square root of the time. As the liquid velocity

from the spray was less than ft./sec., the time of fall was pro-

portional to the square root of the tower height; this explains the

observed variation of the amount absorbed with the

I/4

power

of

the tower height.

It

is interesting to note that Guyer, Tobler,

and Farmer

(8),

esorbing carbon dioxide from single water

drops, found no appreciable change in the desorption coefficient

on changing the tower height.

Using

a

10-mm. tube packed with 5-mm. balls, Amelin

1 )

found

that the percentage

of

sulfur trioxide absorbed in

75-95%

sulfuric

acid passed through a minimum at about

120'

C. as the acid tem-

perature was varied from 20-200 C.

He gave a quantitative

explanation of this in terms of the relative rates of absorption of

water and sulfur trioxide and the equilibrium partial pressures

from the solution.

Abstracts of the following articles from Russia are all th at are

available to the reviewer

at

present.

Zhavoronkov and Furmer

28)measured heat transfer coefficients for cooling air with water

in towers packed with Raschig rings, coke, and wooden grids.

Air velocities up to

2

ft./sec. and liquid rates

up

to 5000 lb./(hr.)

(sq. ft.) were employed. Shabalin and Blyakher

18)

measured

the plate efficiency of a bubble-cap column absorbing sulfur

tri-

sxide in sulfuric acid.

Varying the gas rate and liquid depth

from

1.2

to

2.5

inches gave Murphree efficiencied between

81

and 97% at pressure drops of

4

to 7.5 inches of water. Cooling

coils on the plates had over-all heat transfer coefficients of 200

B.t.u./(hr.)(sq. ft.)('

F.).

L I T E R A T U R E C I T E D

Amelin, A. G., J . App l i e d Chem.

17, 319-25 1944)

;

18, 509-17

Boelter, L. M . K. Gordon, H.

S.,

and Griffin,

J.

R.

IND.

ENQ.

Bosworth,

R.

C.

L., Australian Chem. I d . J .

&

Proc. 13,

Estignard-Bluard,

J.,

Chinie

&

industrie

52, 25-9 1944).

Ferencz, P., Can.

Chem. Process Inds. 30 47-9 1946).

Geddes,

R.

L.,

Trans. Am. Inst. Chem. Engrs. 42, 79-105

G o o d m a n , W.

Heating Piping Air Conditioning 10, 165-8,

Guyer A Tobler, B., and Farmer, H., Chem. Fabrik 9, 5-7

Hatfield, M

R.,

and Ford,

C. E.,

Trans. Am. Inst.

Chern.

Johnstone, H. F. and Williams, G. C.,

IND.ENQ

CHEM.,

1 ,

Lobo,

W.

E. ,

Friend,

L.

Hashmall, F., and Zenz,

F.,

Trans. Am .

Marshall, G. K., Heating

&

Ventilating 42, 82-7 1945).

Morrell, C. E., Paltz, 1 '. J., Packie, J. W., Asbury, W. C

and

Brown,

C. L., Trans. Am. Inst . Chem.

Engrs .

42, 473-94

1946).

1945).

CEEX. ,

38, 596-600 1946).

53-9 1946).

1946).

255-8, 327-8 1938).

1936).

Engrs. 42, 121-47 1946).

993-1001 1939).

Inst. Chem. Engrs. 41, 693-710 1946).

O'Connell,

H. E.,

Ibid.

42, 741-55 1946).

Palmatier,

E.

P.,

and

Wile,

E. E., Refrig. Eng.

52, 31-6,

74

PigulevskiI,

V.

V., and Ilyina, S.

I., Compt. rend. m a d . s c i .

Rohsenow, W .

M.,

Refyig. Eng. 51, 423-4, 464, 466 1946).

Shabalin, K. N., and Blyakher, I. G., Khimicheskaya Prom.

Thomsen,

E. G. Refrig. Eng. 51, 425-31 1946).

Wade,

H.

N.,

Oil

Gas J .

44 50), 120, 122-4, 126 1946);

Williams, G. C.,

and Sohmitt, R. O., ISD ENG.CHEM.,

8,

Zhavoronkov, N . M . , and

Furmer,

I E., Rhimicheskaya Prom.

1946).

U.R.S .S . 45, 331-3 1944).

1945,

NO.

11, 14-16.

Petroleum Refiner 25, 206-10 1946).

967-74 1946).

1944, NO. 12, 7-9.