Control of sulphuric acid losses in contact plant stack gases

7/ Central Street Winchester, Mass.,

May 30, 1930

Professor A. L. Merrill Secretary of the Faculty

Massachusetts Institute of Technology Cambridge, Massachusetts

Dear 'Sir:

As partial fulfillment of the requirement for the Degree of Bachelor of Science this thesis entitled "Control of Sulphuric Acid Losses in Contact Plant Stacic Gases" is hereby submitted.

Sincerely yours,

Signature redacted

Signature redacted

94

CONTROL OF SULPHURIC ACID LOSSES

sNST. 7 20 AUG 1930

IN

CONTACT PLANT STACK GASES

by

Elmer W. Harmon Morris N. Young

Submitted in Partial Fulfillment of the Requirement for the

Degree of

Bachelor of Science

from the

Massachusetts Institute of Technology 1930

Signatures of Authors

Signature

redacted

Signature redacted

Certification of the Department of Chemic&J Engineering: Professor in Charge of Research

Head of Department

Signature redacted

na, re dat

AN ACKNOWLEDGMENT

To Dr. F. W. Adams, for his frequent advice and helpful criticism; and to the Merrimac Chemical

Company and Mr. Kraich, Superintendent of the Contact Plant at Everett, for their earnest co5peration in making this investigation possible.

INDEX SUBJECT . . . . OBJECT . . . . ABSTRACT . . . . INTRODUCTION . . . . . . . . . . . . RESULTS:

Table of Mist, Vapor, Acid Concentration and Temperature Variation . . . . Plots of Above Variations . . . . . . DISCUSSION OF RESULTS . . . . . o . . . .

CONCLUSIONS . . . . . . . . . . . . . ..

RECOMMENDATIONS . . . .

APPENDIX:

Sketch of Mist Determination Apparatus Method of Procedure:

Description of Apparatus

Method of Sampling . . . .

Method of Analysis . . .

Temperatures. . . . .

Summarized Data and Calculations: Mist and Vapor in Stacks Temperature and Concentration

Cottrell Washings . . . . Calculations.& Concentration of Acid . . . . . . .

*

Page Calculations (cont'd) Cottrell Washings . . . 24 Volume of Sample. ... .. 25 Acid Loss . . . .

25

Calibration of Flow Meters . . . 27

Standardization of NaOH . . . * 28

Trial Run . . . 29

Test Run . . . 30

Analysis of Samples . . . . . . . 33

Plant Data. . .. ... 36

Calibration Curves of Flowmeters . . . . 37

-1-SUBJECT

Control of sulphuric acid losses in contact

plant stack gases, from number three unit at the Everett plant of the Merrimac Chemical Company, May, 1930.

OBJECT

To determine the stack loss as sulphuric acid or its equivalent and the factors affecting it for number three contact unit.

ABSTRACT

Gas samples from the stacks were passed through a "dry" and then a "wet" electrical precipitator to obtain the mist and 30 vapor content. At the same time samples

of the recirculated acid and its temperature were taken. Acid concentration of both the precipitator and the tower acid samples were determined by titration.

Losses were found to vary with the temperature of the recirculated acid and inversely with its concentra-tion over the range covered by the test. Losses were

found to be at a minimum when the temperature of the recir-culated acid was about 750C., and its concentration

between 98.5 and 98.8 per cent.

The total loss as sulphuric acid was found to be about one hundredth of one per cent of the acid made by the set over a twenty-four hour period.

-2-INTRODUCT ION

A satisfactory determination of the actual effect of different concentrations and temperatures upon the quantity of acid mist and vapor losses .in stack gases of the absorbing towers of a Contact Plant has never been made up to the present. According to plant practice, for good absorption of SO., the temperature of the feed acid

should be kept low and its concentration should be kept at 98.5%. Experimental confirmation or elaboration of

the factors influencing the acid mist and vapor losses by an accurate quantitative test upon the operation of the absorption towers would establish a definite basis for utilizing the properties of some instrument or for the devising of one for the purpose of controlling the acid

losses.

A "titrimeter" installation, for the automatic control of the concentration of the acid fed to the tops of the towers, offers a possible solution if it can be shown that the quantity of mist and vapor in the stack gases varies with the concentration of the feed acid. Unfortunately, the rather erratic operation of the "titrimeter" leaves this method of control open to question.

We have been informed of some work which was recently carried out by Lu and Youngson upon this

-3-ttitrimeter" with a view toward discovering the defects in its operation and rectifying them. The application of their recommendations may lead to the successful operation

of the ttitrimeter" and eventually may even result in a method for the control of the mists in the gases by the automatic regulation of the concentration of the feed acid.,

At present, frequent observations of the gases leaving the stacks are made by the operator of the

Contact Plant before the adjustment of the valves controlling the concentration of the acid fed to the tops of the

towers, or before cleaning the holes through which the acid enters the towers. Plugging up of these holes is often the chief cause of "gassing," so that it often happens that no adjustment of the valves whatsoever will be of any value in reducing the mist to a minimum.

There are nine possible factors which may in-fluence the extent to which mist is present in the stack gases (in addition to the one mentioned above), namely,

(a) the concentration of the feed acid,

(b) the acid temperature,

(c) the rate of feed,

(d) the velocity of the gas, (e) the temperature of tie gas,

-4-(g) the air temperature, (h) the humidity,

(i) the atmospheric pressure.

Of these, the gas velocity, temperature and 803 content may be assumed to be held at a constant optimum value;

the air temperature, atmospheric pressure and humidity are economically beyond control.

With the rate of feed held fairly constant, an accurate determination of the relations existing between the concentration of the feed acid, its temperature and the amount of sulphuric acid mist and vapor in the stack gases might provide some enlightenment on the problem of mist control. This tnhesis was undertaken with this end in view.

Accurate determination of the mistmand vapor in the stack gases offered the principle problem in the

carry-ing out of this thesis. Several means for the analysis of gases for sulphuric acid mist and vapor content have been

investigated. 0. Fitzsimons tried (a) bubbling his gas through water followed by a bottle of caustic solution; (b) bubbling his gas through several bottles of caustic solution without the primary water bubbler; (c) effecting the bubbling through alundum thimbles immersed in solu-tions of caustic. No attempt was made to distinguish between the sulphuric acid mist and S03 vapor. Although

-5-Fitzsimons regards the "alundum thimble method" as the most satisfactory of the three methods which he tried, yet he expresses some doubt as to its accuracy.

R. B. Abrams and T. B. Drew, working with Professor Harold C. Weber, devised an "Electrical Precipitator for

the Quantitative Determination of Mists" which was par-ticularly designed for the removal of sulpuric acid

mists from gases. It is recommended by them as a "portable apparatus for plant use in such determinations".

In its most recent form, the precipitator proper consiste of a 7/8" thin lead glass tube, eighteen inches in length, sealed at one end and having two glass side arms on opposite sides of the tube. The gas is admitted through the lower arm and leaves through the other. The open end of the lead glass tube admits a rubber stopper in which is fixed a glass tube in the shape of a miniature violin bow with a .platinum wire in place of the hair. The wire is sealed to the glass at one end of the bow and

passes through the "frog" (being sealed at its entrance to the "frog") connecting the central electrode with the lead wire. The central electrode can thus be readily removed, together with the rubber stopper, and the inside of the precipitator washed out. A coat of paraffin is applied to the bottom of the rubber stopper and to the surface of the glass bow to facilitate the formation of

-6-droplets of precipitate and to prevent short-circuiting. The outer electrode is made of copper foil, 0.002 inches in thickness, wrapped tightly around the lead glasstube between the two side arms and affixed with shellac. The precipitator operates on the Cottrell principle, A Ford spark coil being used to actuate the apparatus.

This precipitator, with slight variations in its construction, appeared to be well adapted for use in mist determination, the principle of the "Mist and Vapor Determination Apparatus" used in this thesis being

fundamentally dependent upon the qualifications of the "Electrical Precipitator" as determined by Abrams and Drew.

+ 4-4 7 17t ,Aot A-,( +T y _7 + 4 4-4

o O c 'i eL Ilk

-9-DISCUSSION OF RESULTS

As shown by the plot of the results, both the acid mist losses and the So3 vapor losses vary with the

tempera-ture of the recirculated acid and inversely with its con-centration. That a relation exists between the acid mist loss and these two factors is quite well shown by the results. From the appearance of the curves the S03 vapor losses seem to vary similarly to the acid mist losses. The data taken is insufficient to determine definitely whether or not this is true. However, some very

interest-ing bbservations can be made as to the stack losses as a whole.

Losses were found to decrease as the temperature of the tower acid went down throughout the range

covered by the test. A minimum loss was found when the temperature of the tower acid was about 75 0. It was also found that the losses decreased as the concentration of the tower acid went up, reaching a minimum when the strength of the acid was between 98.5 and 98.8 per cent. These check very well wiih the values that have been found to be "good practice" in operation of the towers. Incidentally it was found that if the holes through which the acid in the top of the tower drips down into

the absorption- chamber become plugged the stack nearest the plugged holes begins to "mist" very heavily.

-10-All these factors which affect the stack losses are open to control by tne plant. Plugging of the drip holes is now eliminated by the operator regularly running copper rods down through the holes. Unless an automatic device could be installed to prevent plugging the holes

little more can be done.

Variations in the temperature of the recirculated acid are caused by fluctuations in the amount of S03 gas ooming to the tower; in the quantity of acid recirculated in the tower; and in the concentration of the acid coming to the tower from the storage tank. All these factors

are inter-related.

At present the quantity of 803 gas coming to the tower is as constant as it can be maintained and these

fluctuations will have to accepted. Also acid from the same storage tank goes to both the number two and three towers and must meet fluctuations in S03 from two gas sources.

If the operator finds from observation of the thermometer in the acid at the top of the tower that the temperature is off he may do either of two things. He may change the setting of the valve on the top of the tower which controls the volume of acid recirculated or he may change the valves which control the concentration

-11-which controls the amount of acid recirculated affects only one tower wnile changing the concentration affects both. From this point of view it would be more advisable,

if possible, to control the temperature of the acid by the volume flowing in the tower. An automatic device could be installed on the tower which would regulate this valve by the temperature of the acid.

Variation of tne concentration of the tower acid is also created by the quantity of 803 gas coming to the tower, but as was mentioned all that can be done to

eliminate this is being done at present. However, the greatest cause of variation is the continual resetting of the valves which control the flow of drying acid, the 660 acid, and the water to the storage tank.

Changes in these settings are made by the operator for three reasons. He is guided in trying to maintain a constant concentration by the conductivity of the acid

just before the tower balanced against a standard con-ductivity cell. If the conductivity is off he may

change the valves. He may change the valves in an

effort to control the temperature of the acid. Also the operator regularly observes the mist coming from the stacks and if he finds it to be too high he changes the valves feeding the tank.

If the automatic titrimeter, which is now not in operation, could be made to work it would keep the

-12-concentration of the acid nearer the proper value than the operator is able to by hand. Also as pointed out above, an automatic device could be installed to control the temperature of the acid by regulating the flow of acid in the tower. If the operator were able to make continuous and instantaneous adjustment of these two factors he could keep them at the proper value for a minimum acid stack loss. The automatic devices would be able to do this.

Then there would be no question of the operatorts judgment as to. the "mis-ting" of the s tacks and it would eliminate the necessity of resetting the valves to the

storage tank to overcome this "misting." As a further check an apparatus could be devised to indicate by the electrical conductivity of the stack gas when the misting was too great. This would be used as an indicator that

something was wrong with one of the other two devices or that the holes were plugged.

As is also brought out in the tabulated results it was found that the total acid loss from the stacks is about one hundredth of one per cent of the acid made by the set over a given time . This figure was lower than that which had been anticipated, but as there was no

mist observable inthe glass vacuum receiver through which all the gas sample passed after going through the apparatus

-13-it seems reasonable to assume that all the mist and 303 vapor was caught by the precipitators.

This then would leave the volume of flue gas as calculated in question. For this point, however, if

the calculated volume was a little off the true average it would not greatly affect the value of the per cent

loss. As great a change as fifty per cent in the average

volume of the stack gases would only change the per cent acid loss about three thousandths of one per cent.

In carrying out this test it was found to be

questionable as to whether or not the electrical precipi-tator is satisfactory in the determination of variations of 303 vapor losses with tower conditions. In this test samples were taken from the precipitators every three hours. So small an amount of 303 vapor was found in the precipitator after a three hour run that it was very difficult to determine its amount with any precision.

On the other hand,if the samples were taken over a longer period of time, the averaging of the tower conditions for the period of sampling would cover up the relations that are being sought.

-14-CONCLUSIONS

That the plant is keeping the stack losses of acid mist and SO3 vapor about as low as possible.

That the stack losses could be maintained auto-matically at a minimum by overhauling the automatic

regulator of the tower acid concentration and installing an automatic regulator on the amount of acid recirculated to keep it at a constant temperature.

RECOMMENDAT IONS

That the automatic titrimeter be overhauled nnd that an automatic control actuated by the temperature of the tower acid be installed to control the volume of acid

I x

LI

<

0

E

LL

-16-METHOD OF PROCEDURE

Description of Apparatus

Electrical Precipitators: These precipitators were built on the same principle as the Cottrell Precipitator. A lead glass tube eighteen inches long and seven-eighths of an inch in diameter was equipped with a side arm two inches from either end and in opposite directions. The bottom was sealed to a stopcock so that the samples could be washed out without dismounting the apparatus.

For the outer plate of the precipitator this tube was covered with copper foil about 0.002 inches in thickness. This plate was soldered on and then shellacked. A A;ad was soldered to it for electrical connections.

A glass bow similar to a voilin bow was made but with a platinum wire instead of the hair to be used as the inner electrode. The platinum wire went into the glass at the top of the bow and was carried on up

through the inside of the gube. A seal was made about one half inch from the top of the tube and the top of the tube was filled with mercury so that contact could be made electrically with the platinum wire. A rubber stopper was used to hold the bow in place and this

stopper together with the glass part of the bow was covered with paraffin to prevent Short-dirduiting.

-17-Bubbler: Two precipitators were used on each apparatus being mounted on an upright board with a base attached. Between the two precipitators the gas was bubbled

through water in an eight inch test tube. This was intended to humidify the gas so that the second preci-pitator would be able to precipitate any 30, vapor in the sample-. Gas from the first precipitator entered the

bubbler through a half inch glass ball with holes in

the sides and submerged in water. All the gas connections were made by glass tubing, joints being made by

bring-ing the edges of the glass together and holdbring-ing them with rubber tubing which had collodion on the inside,

and then the whole joint was covered with collodion. Flow Meter: A flow meter of the type shown in the diagram was used. It was calibrated by passing air through it in series with a calibrated gas meter. A uniform flow of gas for the calibration was obtained by runninig two vacuum pumps to a carboy and a line from the carboy to the apparatus. A screw clamp on the suction line controlled the flow. A constant head was maintained on the flow meter for five minutes and

the gas meter read before and after. This gave a

value for calculating the head equal to a certain flow in cubic feet per minute. Several runs at different heads were made on each apparatus and the results

-18-plotted as head versus cubic feet per minute. Potential: Potential for the precipitators was supplied by Ford induction coils. The coil on one apparatus was .actuated by a storage battery and on the other by a six volt generator. Connections on the secondary side were made by insulated wires inside glass tubing. A secondary coil for emergency use was put on each apparatus with suitable switches so that

it could be thrown in if the other coil burned out. Vacuum: Suction for the apparatus during the run was provided by a compressed air injector. This was

donnected to a carboy and a vacuum of about five inches of mercury was maintained in the carboy. A line was run from the carboy. to the apparatus. Two electrically driven vacuum pumps were also connected

to the carboy for use in case of failure of the com-pressed air supply.

Difficulties: Several difficulties attend the use of the electrical precipitator. Careful insulation of the secondary leads from the coil are necessary in order that the potential on the precipitators be high enough. This necessitates careful adjustment of the interrupter. The common primary of the coil must be connected to the outside electrode. The inner

-19-all through the precipitator may be the same. Also the efficiency of the precipitator on small amounts of mist is uncertain for the only other work done in this way was on mist from the heating of 60% oleum by

Abrams and Drew.

Metod of Samplin Gas Sampling

A continuous sample of gas was drawn from the stack through a tube of 1/4" standard iron pipe curved 900 at one end so that the opening faced the flow of gas at the center of the stack. This tube was

directly connected to nine feet of 1/4-3/8" hard lead tubing. A rubber connector, coated on the inside with collodion, fastened the lead pipe to the lower glass arm of the precipitator.

The same method of sampling was used on both stacks of the absorption tower.

Acid Sampling

A sample of the recirculated acid entering the top of the tower was taken in a glass bottle every 15 minutes. 25ml. of this sample were placed in a glass stoppered bottl6. To this was added the next

three samples, the total lUUml. of acid being considered as the average sample of the acid- being circulated

-2u-Precipitated Acid

At the expiration of a test period, as two, three, or four hours, the current to the coils was stopped, the suction was turned off and the inside of the Cottrell was washed with water. This included the washing of the glass bow, the platinum wire, and the

inner surface of the lead glass tube. All washings were run into glass stoppered bottles. To distinguish between the .samples from the four Cottrells, the

arbitrary notation 1A, 1B, 10 and lD, was adopted for the sample bottles, the "Att and "iCi" samples being the sample from the first Cottrell in Apparatus #1 and #2 respectively and samples "B" and "D" being the

corresponding samples for the second Cottrell of each respective apparatus.

(Apparatus #2 was used solely on the outer stack).

Methods of Analysis Recirculated Acid

The method finally used was that employed by the Plant Laboratory for "98" acid. This consists in

drawing off about half a milliliter of the acid in a pipette and adding this to a previously weighed glass

stoppered weighing bottle. This is weighed and then diluted by the rapid addition of 7ml. of distilled

-21-water. Titration of the acid with normal NaOH is performed by adding the alkali directly to the weigh-ing bottle in order to avoid errors in the transferrweigh-ing of the sample. Methyl orange was employed as an

indicator.

Cottrell Washings

Each sample was diluted to 50ml. and titrated in a porcelain dish with 0.1145 normal ,NaOH. Methyl orange was used as the indicator.

Standardization of NaOH

The normal solution of sodium hydroxide was

prepared and standardized according to the method given in Treadwell-Hall'is "Analytical Chemistry", Volume II, no attempt being made to free the alkali from carbonate. This necessitated the use of methyl orange indicator for titrations.

The sodium hydroxide was standardized against

standard hydrochloric acid which in turn was standardized against Na₂CO₃ prepared from C.P. NaHCO3.

The 0.1145 normal NaOH was prepared by diluting lO0ml. of the standard NaOH (1.1454 normal) to 1000ml. in a measuring flask.

Temperatures

Recirculated Acid. The temperature of the recirculated acid was read at 15 minute intervals from a mercury in

glass Centigrade thermometer which was partially immersed in the acid.

-7 '

7,

-A L

71

-- 7

-7

Sda r t+ o n of CIn i n th N-N "N tare 9 6 t Na rC 4 _{,i it} t. reare tar Cl ~ ~ ~ 4 37 4orai ru tie n r- a 'Le rL

T

C&

7~~ 61 ont

3.45 13. 1 :3 .3 t .3 a

To~ ~ iswr ew batt e r

t l. )

1-

I1I.3

C)

A

T 7'T

t "A

3-7 [7

17 914

S 4 'D

A-10

'I.

0~ 17

-14 17. 4l".

133 35 3 3 1 .3 1.10 to] .1

..

.

.

-39-BIBLIOGRAPHY

H. C. Wfber:

0. Fitzsimons.

G.M. Tu, H.P.

"Quantitative Analysis of Mists and EBogs." (In Absorption Symposium: papers presented before the _{Div. of Indus. and} Eng'g Chem. at the 68th meeting of the Am. Chem. Soc. 1924.)

"Stack Losses in the Contact Plant". December 1928. (Test carried out at the Boston Station of the M.I.T.S.C.E.P.) Dayton and R. Simard. "Test on S03

Absorption". October 25, 1928. (Test carried out at the Boston Station of the M.I.T.S.C.E.P.)

R.B. Abrams and T.B. Drew. "An Electrical Precipitator for the Quantitative Determination of Mists". December 1923. (Plant

Investiga-tion at the Boston Station of the M.I.T.S.C.E.P.)

K.I. Lu and R.L. Youngson. "Automatic Control of Sulfuric Acid by Electrical Conductivity"t_{. May,}

1930. (Bachelor's Thesis at the Boston Station of the M.I.T.S.C.E.P.)

F.P. Treadwell and W.T. Hall. "Analytical Chemistry", Volume II. John Wiley and Sons, Inc.