March, 1924
I N D UXTRIAL A N D ENGINEERING CHEMISTRY
253
Removal of Naphthalene from Gas by Oil Scrubbing'J' By Robert D. Pike and George H. West 74
NEW MONTGOHERY ST.,
SAN
FRANCISCO, CALIF.
I n one case the naphthalene content of the gas was reduced by more than 50 per cent by merely transforming the arrangement of the existing scrubbers from a batch system to a Estage countercurrent system. Interpretation of the data with the mathematics of the scrubbing of a gas with a liquid indicates that the best system to employ is Q $ve-stage countercurrent system, which is substantially equivalent to a true countercurrent flow of oil with reference to the gas. I n each stage the oil should have a very great rate of circulation in comparison wilh the net flow of oil through the system in order to supply the necessary surface of contact of gas and oil. I n the case described the rate of circulation of the oil in each stage is one hundred and twenty times as great as the net advance of oil through the system. I n being recirculafed. the oil should not pass into open sumps, but should remain enclosed within the scrubbing system. If this precaution is observed and onIy suficienf net oil is used to remove the naphthalene, there will be practically no loss of heating value from the gas, no matter how active the recirculation within the scrubber. The scrubbing temperature should not exceed 70" F.. but if it is not possible to keep the temperature as low as this by the usual means of wafer scrubbing, it will still be possible, within reasonable
limits, to reduce the naphthalene content of the clean gas to the desired point by proper choice of the amount of net oil and the scrubber volume. No definite limit can be set f o r the safe naphthalene content of the clean gas. This will vary with prevailing temperatures and with the rapidity of temperature changes. Aside f r o m price there are two principal criteria for choke of scrubbing oils: ( a ) The naphthalene vapor pressure characteristic of the oil at constant temperature. Other things being equal, that oil is the best which, at the temperature prevailing in the scrubber, gives the lowest vapor pressure of naphthalene from a given amount of naph. thalene in solution. (b) Viscosity. Other things being equal, that oil is the best which, at the scrubbing temperature, has the lowest viscosity. I f , in examining petroleum scrubbing oils, the viscosity is made the sole criterion, the naphthalene vapor pressure characteristic will, to a considerable extent, fake care of itself; at any rate, a considerable variation in this Characteristic can be allowed for by simply adjusting the amount of net oil so as to keep p. the vapor pressure of naphthalene from the foul oil, within required limits.
EVERAL years ago the Pacific Gas & Electric Company installed new oil gas generators a t its Potrero, San Francisco, plant, and since that time, owing probably to higher temperatures employed in the gas-making process in these new generators, stoppages in service pipes resulting from naphthalene have been on the increase. Accordingly, an investigation was undertaken by the writers with a view to designing or selecting the best available means for removal of naphthalene from the gas before leaving the works. About SO per cent of the city gas for San Francisco is made a t the Potrero plant, the balance being made a t the Metropolitan plant. As naphthalene conditions a t the latter were not corrected during the period under observation, the reduction of naphthalene complaints for the entire city noted below is to be credited entirely to the improved conditions a t Potrero. The gas manufactured is oil gas made from California residuum fuel oil of about 16.5" BB. in the improved Jones generators. The temperature carried in the checkers of these generators is relatively high, leading to a rather prolific production of naphthalene,s which has led to many complaints by customers on account of stopped-up service connections during summer months when sudden lowering of temperatures are frequently brought about by ocean fogs. The nominal capacity of the Potrero plant is 20,000,000 cubic feet 550 B. t. u. gas daily.
pine, spaced 2.5 inches on centers, and the separate trays are piied with the grids a t right angles to each other. The oil is delivered into a spray chamber a t the top of each scrubber and thence trickles down through the grids. The sprays are pointed vertically downwards. I n the batch system of scrubbing the fresh oil was put into service in lots of 5000 to 6000 gallons, and recirculated through the sprays and scrubbers for a period of 10 days to 2 weeks until saturated with naphthalene, when it was removed and a fresh batch substituted. The oil used for scrubbing is a special California crude oil purchased for the purpose, known as Shamrock crude. There follows a distillation test of this oil as received and after using for 10 days.
S
FORMER SCRUBBING PRACTICE Before the changes noted below, scrubbing was conducted under the so-called "batch system." Two pairs of oil scrubbers are used, the gas being split into two streams and each stream being run through three water scrubbers, two oil scrubbers, and one water scrubber. The scrubbers are steel towers 12 feet 7 inches diameter by 39 feet high. They are partly filled with wooden trays of grids made of 1 x 6 inches surfaced 1
Received June 25, 1923.
on an investigation of the removal of naphthalene from gas at the Potrero, San Francisco, works of the Pacific Gas & Electric Company. Egloff and Twomey, J . Phys. Chcm., 20, 121 (1916). a Abstracted from a report
*
-
TABLEI-DISTILLATIONTESTS 08 SHAMROCK CRUDE OIL USED FOR BATCH SCRUBBING AS RECEIVEDAND AFTER USE FOR TEN DAYS RECEIVED-ARTER 'USEGravity at 60' F. Gravity at 60' F: 18.8O BC. = 0.9409 specific gravity 14.2' BC. = 0.9708 specific -gravity -
---As
Distillalion
Light oils Medium oils
i{
l50'F. 200' F. 250°F. 300'F. 350°F.
-Per -.cent - _-3.0 5.2 9.5 11.6 16.9
Distillation
Light oils 250° F. Heavy oil's, 370° F. Water Naphthalene Residue
Loss
-Per -_
rent
27.0 45.5 13.0 4.5 9.0 1.0
IMPROVED SCRUBBING PRACTICE The improvements which were inaugurated in this practice were principally to convert it over to a 2-stage countercurrent system. The layout of the new plant is shown in Fig. 1. Practically no change was made in the net amount of oil used per thousand cubic feet of gas compared with batch scrubbing. The same kind of oil and the same method of passing the gas through the two lines of scrubbers were continued in use. The No. 4 and No. 5 scrubbers shown in Fig. 1 are oil scrubbers, and the No. 6's are water scrubbers. The gas enters the system through the No. 4 scrubbers and leaves from the N0:G's. No change was made in the internal arrangement of the scrubbers except to place one layer of 14-mesh wire screen over the top wooden tray. This serves
INDUSTRIAL A N D ENGINEERING CHEMISTRY
25 4
Vol. 16, No. 3
COUNTER CURRENT *STEM REMOVAL FROM OIL GAS POTRERO GAS PLANT
NAPUTHAL€tdE
FIG.1
to make a better distribution of the oil and a more uniform contact of gas with oil at this level. The oil is supplied in a small, continuous and accurately regulated stream by a “flotation feeder” mounted in the oil storage tank (Fig. 2). This oil is delivered to the sump of the No. 5 scrubbers and is circulated through the scrubber a t the rate of about 6 gallons per thousand cubic feet of gas. The overflow from sump No. 5 passes to sump No. 4,whence it is circulated through scrubbers No. 4 a t about the same rate, and the overflow from sump No. 4 is mixed with the oil t o the gas generators. This establishes in effect a 2-stage countercurrent system for scrubbing with the oil. The system is by no - the gas TABLE 11-TWO-STAGECOUNTERCURRENT SCRUBBING lis. BATCH SCRUBBING %STAGE COUNTERCURRENT SCRUBBING JULY-DEC., DURINO 1922O Naphthalene Naphthalene Content of Gas Content of Gas Leaving Works Leaving Works Grains per Naphthalene Grains per Naphthalene MONTH 100 Cu. F t . Complaints 100 Cu. Ft. Complaints January 6.5 4500 5.0 4800 February 5.0 4000 7.0 3800 March 5.0 4300 5.5 4400 April 6.0 7000 5.0 4000 9.5 7400 5.5 5800 10.0 9.5 11200 14300 45 . 5 9700 6500 E;ust 6.5 15800 3.0 10000 6200 September 6.0 11500 1.7 October 5.0 7000 2.3 5100 November December 5 6.0 5 5200 6000 2 . 70 6200 3500 0 Because plant arrangements make i t necessary to bring in extra generators during the winter months of peak load, it is not possible to use countersurrent scrubbing during this period. ‘BATCH
SCRUBBING-
DURING
Ex
1921
means so good as it might be, either theoretically or practically, but it had a remarkable effect on the naphthalene content of the gas leaving the works and upon the naphthalene complaints as shown in Table 11. The countercurrent system began to be effective in July, 1922, and during the balance of the year effected a great reduction in naphthalene content, with the same kind and net quantity of scrubbing oil. Naphthalene complaints took a decided drop, and it is expected that after a year or so of turning out a gas with a maximum content of 2 grains of naphthalene per 100 cubic feet (see later), thus giving the system a chance to be entirely cleared of naphthalene, the complaints arising from naphhhalene deposits will have been entirely eliminated. Table I11 covers some other important operating and derived data during the period July to November, 1922, when countercurrent oil scrubbing was in effect. During the period covered by these observations this average d d l y output was about 13,000,000 cubic feet.
VAPOR PRESSURE OF NAPHTHALENE FROM OIL I n order to calculate the “driving force” defined above under the conditions existing in the scrubber, it is necessary to determine the naphthalene content of gas which will be in equilibrium with the oil for various naphthalene contents of the latter. It is very necessary that thia determination be made a t the temperature that will prevail in the scrubber, because the vapor pressure of naphthalene from the oil increases very rapidly with the temperature.
INDUSTRIAL A N D ENGl .NEERING CHEMISTRY
March, 1924
This relation for Shamrock crude oil at 65' F. is shown in the curve of Fig. 3. It will be noted that the cume is not a straight line, which would he necessary to meet the requirements of Henry's law. The method employed in getting the data for curve of Fig. 3, which we may term the "naphthalene vapor pressure characteristic of the oil at constant temperature," was as follows: The naphthalene content of some of the used scrubber oil was determined. Naphthalene-free gas was then passed through it and the naphthalene picked up by the gas determined. The train consisted of a pressure regulator on the high-pressure gas line, two picric acid wash bottles to remove any traces of naphthalene in this gas, two wash bottles fdled with the oil to be tested. two picric acid wash bottler to pickup the naphthalene carried over by the gas, and a laboratory wet-type gas meter to measure the gas passed. The high-pressure gas was found to be practically naphthalene-free. The oil frothed a great deal, increasing the surface of contact and permitting a more rapid current oi gas than would otherwise be the case. Thomas&found that in saturating gas with solid naphthalene he could use a volume of 0.1 cubic foot per hour. The writers have been able to obtain close checks at twice that rate from oil. This method depends on the fact that the quantity of naphthalene removed from the oil is such that it changes the concentration in the oil by only an infinitesimal amount. The experiments were carried on in a room having a temperature varying not over two degrees from 65' F., and no attempts were made to regulate the temperature more closely. A water bath would he advisable, if it were desired to maintain a t.emperature differing from room temperature.
255
REMOVAL OF HEATINQ VALUEFRO% GASIN SCRUBBINQ Because of difficulty of sampling, i t has not been possible to determine accurately the loss of B. t. u. of the gas through the oil scrubbers, but the following points seem to he well established from the miters' observations. 1-If the amount of net snubbing oil used is that given in Table 111-namely, 0.05 gallon per thoitsand cubic feet of gas-the amount of benzene or other of the heavier hydrocarbons prcsent in the gas which is required to saturate this oil under the conditions existing in the scrubber is negligible compared with the total amount prcsent, and B. t. u. losses from this loss are thnefore negligihlc. 2-The rate of oil in circulation in each scrubber is more than one hundred times as great as the amount of net oil advanced through the scrubbers, and if this circulating oil passes through an open sump an excellent opportunity is offeredfor evaporation of the heavier hydrocarbons and d m of the methane in solution
active the brcuiation of oil within the sy&&
VAPORI'EESSURE OF NAPHTBALENE The determinations of the vapor pressure of naphthalene are quite numerous. The results of Schlumberger,5 Thomas,e Gensee,' and Barker,a have been compared in the curve of Fiv. 4.
TABLE I I I - O P ~ R A ~ ~ NA N OD DKRIYBD DATAO N T W O - S T ACou~riin. ~E CURRENT ScKvaarnc Demees F. 1 Avcisge temperature gzu 63 a >%*rim"mtemperntuze sns 68 3 MLnimum tempeiaturc sa' 56 4 Average temperature scrubbing oil 04.5 5 Maximum temperature scrubbing oii 68 B Minimum temoeralure rcriibbinr oil 58 Saybolt SecunCIs 7 ViseOs$y of sciubb/ng oil at 65O F. iifin iy Viscosity after w e m scrubber No. 5 582 7b Yiscosiui alter use in scrubber No. 4353 Giains per 100 Cu. Ft. 8 Total nnphthalene content of gas entering No. 4 scrubbers (foul gsr) 28.4 9 Same ulnthined in tar mist in g=$ 12.7 10 same cootained ill gas as vapor 15.7 11 Total niphthaiene content 01 gsn leaving last oil scrubber No. 5 (clean sas) 3.81 12 Same contained i n tar miaf in gas 2.14 13 Same contained in vspoz in 1.67 14 Told naphthalene content of gas leaving water scrubber No. G 4.60 15 Same contained in tar mist in garr 2.96 16 Same contained a8 vapor in bras 1.64 17 Total naphthalene coutent of gas le~uingwork$ ail contained in c i s a8 v a ~ o r
22 23
84
1.53
P C diiy ~
Scrubber constant K b for No. 4 scrubbers 0.0534 Scrubber constant X far No. 5 scrubbers 0.0619 N a ~ h t h d e n ebalance: Per rent ~~~. by No. 4 scrubbers 59.1 Removed from gas{ by No. 5 scrubheis 27.4 m tar from centrif~galexhauster 7.9 Sent. out SF. . of ranrlts ~... The viscosity of the oil is iedueed by iaking lighter hydrocarbon3 ~
'
~
F l O . 2-orL-PEEDINo
~
XnscnANrsr
in'a.4..*;"-
In this curve the ordinates are the reciprocals of temperatures in degrees centigrade absolute and the abscissas are log P, P being t.he vapor pressure of napht,halene in millimeters of mercury. The curve is a straight line, as predicted by the Clausius-Clapeyron equation, and shows a good agreement among the different points. For everyday use it is more scrviceable to d o t the curve with direct J . Gasbd., 56, 1251 (19121. J . SOC.Chms. Ind., 56, 500 (i9lG). ,So
