Determination of Thiophene, Carbon Oxysulfide, and Carbon Disulfide in Producer Gas A Spectrophotornet ric Method L. J. BRADY' Mellon Institute, Pittsburgh, Pa.
4 simple, rapid method is presented by which the individual concentrations of carbon disulfide, carbon oxysulfide, and thiophene in a gas may be determined. Thiophene is quantitatively removed from a gas sample by scrubbing it with a piperidine-ethanol solution, and carbon oxysulfide and carbon disulfide are removed by solution as piperidine oxythiocarbamate and piperidine dithiocarbamate. Concentrations of thiophene and the piperidine carbamates are determined spectrophotometrically.
T
H E need for a rapid, simple method foi the determination of where T is the transmission of the component, e is its extinction organic sulfur in producer gas has long been recognized. Recoefficient,c is its concentration, z is the length of the optical path cently this need has expanded rapidly, paralleling closely the use through the sample, and d is its optical density. I t follows that of catalysts susceptible to sulfur poisoning, such as those emthe transmittance of all components of a solution can be expressed ployed in the Fischer-Tropsch process for the production of synas thetic fuels, and the attendant need of developing scrubbers for removing sulfur compounds. D = d i + d z + d a + . . . . . dn (2) The organic sulfur compounds present in the gas are believed to where D is the optical density of the solution and d, is the optical consist almost entirely of mercaptans (thiols), thiophene, carbon density of the nth component. I n general, n such equations, one disulfide, and carbon oxysulfide. Methods for the determination for each of n selected spectral positions, are required to deterof some or all of these constituents have been described by Hutchmine the concentration of each component present. ison ( I ) , Kemper and Guernsey (b), Oldach and Field (6),hIacHattie and McNiven (3),Martin, Rueck, Knaggs, and Hakewill ( d ) , Riesz and Wohlberg ( 6 ) , and Shaw ( 7 ) . Of all the compounds present only the mercaptans 100 can be determined directly by absorption and analysis of the absorbing solution. The others have heretofore 80 been determined indirectly by scrubbing the gas with suitable selective absorbents and estimating the re2 sidual sulfur content of the gas by burning it and absorbing the sulfur oxides formed. Obviously such a ? = procedure is time-consuming and has other serious 2 7 limitations that restrict its usefulness. 5 10 This paper presents a simple, rapid method of analysis which makes it possible to determine the indiT H I O P H E N E 3 O L U T l O N CONTAINS 01 IS U L f U R l L SOLVENT CARBAMATE S O L U T I O N CONTAINS I O I S S U L f U R I L SOLVENT vidual concentrations of carbon disulfide, carbon oxySOLVENT C D N T A I N D 0 5 I P l P E R l D l N E I L ETHANOL *O sulfide, and thiophene in a gas. The minimal amount of sulfur that is conveniently evaluated by this method 0 is 0.013 grain per 100 cubic feet of gas, which is equal 120 e30 wo LX) 260 em zno 4 9 0 300 310 WAVELENSTH IN M I L L I M I C R O N S to 0,0002 mg. of sulfur per liter. Figure 1. Ultraviolet Spectrograms of Thiophene, Piperidine The determination is based on the observations that Oxythiocarhamate, and Piperidine Dithiocarbamate thiophene is quantitatively removed from a gas sample by scrubbing i t with a piperidine-ethanol solution and that carbon oxysulfide and carbon The wave lengths chosen for the analysis of organic sulfui 111 disulfide are effectively removed from the gas by solution as producer gas are 230, 240, and 290 mp. At 290 mp piperidine piperidine oxythiocarbamate and piperidine dithiocarbamate, respectively. The thiophene and the piperidine carbamates abdithiocarbamate absorbs strongly, whereas both thiophene and sorb strongly in the ultraviolet portion of the spectrum, and it is piperidine oxythiocarbamate are relatively transparent, as shown therefore possible to determine their concentrations spectroin Figure 1. photometrically. Spectrophotometric measurements are con4 n inspection of Figure 2, which reveals the ultraviolet spectrograms of benzene, toluene, ethylbenzene, and a mixture of the veniently made with a Beckman model DU quartz spectrophotometer equipped with ultraviolet accessories. isomeric xylenes, the aromatic hydrocarbons most likely to occur The transmittance of any component of a solution is expressed in producer gas in small quantities, shows that these materials, mathematically as n hile transparent a t 290 mp, absorb a t 230 and 240 mp. Therefore, a correction for their absorption is applied where necessary. TTo = ecz = d log,, (l) That ethanol and the piperidine-ethanol solution scrub benzene from the gas with equal facility is shown in Table I. 1 Present address, Air Reduction Sales Company, Stamford, Conn.
s
8 g
512
513
V O L U M E 20, N O . 6, J U N E 1 9 4 8
using fresh 95% ethanol as the comparison standard. From the relationship, 100
52
(3)
80
i? % * 2
2c
5
60
40
e
2
e0
0
0 WbVELENGTM
Figure 2.
I N MlLLlMlCRONS
Ultraviolet Spectrogramsof Benzene, Toluene, Ethylbenzene, and a Mixture of Isomeric Xylenes
the optical density a t wave length X is calculated. Here DX is the optical density of the piper idine-ethanol scrubbing liquor, a t wave length X (230 or 240 m p ) , Dzjj is the optical density of the ethanol scrubblng liquor a t 255 nip, esjj is the extinction coefficient of benzene a t the same wave length, and eX is the extinction coefficient of benzene atwavelengthX. Because benzene is the principal aromatic present inproducer gas, the error caused by ascribing all absorption from aromatics t o benzene is insignificant. For most work the difference between the transmittances of ethanol and the pipeiidineethanol solution need not be considered. PROCEDURE
Thioohene. Eastman grade. is Durified to remove carbon disulfide i s follow. One Gilliliter of-piperidine, Eastmari grade, is added to 50 ml. of thiophene. The product is then distilled through a fractionating column, and the
e -.
Table I. Extraction
Removing Benzene from Producer Gas Benzene, Dilution of Transmittance of Diluent M g , per Liter Scrubbing Solution a t 255 m r of Gas
Benzene Removed from Gas. %
Scrubbing Efficiency of Ethanol First Second Third
100-1 Sone None
66.1 41.1 97.1
r-7'
118 2.5 0.085
/i
97.9 96.8
...
Scrubbing Efficiency of Piperidine-Ethanol Solution First Second Third
'
100-1 Kone iione
70.0 63.1 100.0
101.2 1.3 0.000
9S6 100.0
...
Table 11. Scrubbing Efficiency of Piperidine-Ethanol Solution in Removing Thiophene from Gases Extraction
Dilution Solution
of
Transmittance of Diluent a t 230 mg
Sulfur, blg. per Liter of Gas
Sulfur Removed from Gas, %
First Second Third Fourth
1000-1 10-1 Sone None
55.0 40.6 86.1 100.0
30.20 0.47 0.008 0.000
98.4 98.4 100.0
...
_________ .. I n making these tests a Tutweiler buret (Figure 3) was evacuated and a known amount of benzene vapor vias added, after which the gas pressure was brought, to atmospheric by adding carefully scrubbed producer gas. The scrubbing liquor was brought into int,imate contact with the gas by shaking the buret for 3 minutes. At, the end of t,hat time the liquor was removed and the buret' washed 100 carefully with small successive portions of fresh liquor; the contact between gas and wash liquor was kept to a minimum. Washings and the scrubbing liquor were combined and the benzene concentra80 tion was det,ermined spectrophotometrically. The w U operation was repeated until the spectrophotometer 2 showed no benzene in the gas.
Figure 3. Modified Tutweiler Buret for Determining Thiophene, Carbon Oxysulfide, and Carbon Disulfide in Producer Gas
2 eo
I n Table I1 is portrayed the scrubbing efficiency of the piperidine-ethanol solution in removing thiophene from gases. Figure 4 demonstrates that alcoholic solutions of thiophene, carbon oxysulfide, and carbon disulfide are relatively transparent a t 255 nip; accordingly, no appreciable error is caused by ascribing all absorption a t 255 mp to aromatic hydrocarbons. I n order to correct for t,he absorption of aromatic hydrocarbons a t 230 and 240 mM, resPcctively~the gas is scrubbed with g5% ethanol and its transmittance measured a t 265 m,u,
3
z
rO W -0.U)G.
---
u
2
G CO3lL. ERLNOL C S I I L . ETMANOL
0.050. T H I O P I I I N E I L ITMANOL
=O
0 WAVELENGTH IN MILLIMICRONS
Figure 4.
Ultraviolet Spectrograms of Carbon Disulfide, Carbon Oxysulfide, and Thiophene
514
ANALYTICAL CHEMISTRY
forerunnings and tails are discarded. Piperidine dithiocarbamate is prepared by the reaction of piperidine with carbon disulfide in petroleum ether. The ether is removed by filtration through a sintered-glass funnel. The solid piperidine dithiocarbamate is washed several times n-ith small amounts of petroleum ether, dried, and kept in a glass-stoppered bottle. Piperidine oxythiocarbamate is made by reacting a saturated solution of potassium thiocyanate with 5070 sulfuric acid, keeping the temperature of the reactants below 45' C. The gases evolved are scrubbed successively with a 30% potassium hydroxide solution and a solution containing 25% aniline and 7570 ethanol. The carbon osysulfide is then passed through ice and absorbed in a solution of piperidine and petroleum ether. The ether is removed by filtration. The precipitate, piperidine ouythiocarbamate, is washed several times with small portions of petroleum ether, dried, and kept in a glass-stoppered bottle. (Carbon oxysulfide is toxic.) A piperidine-ethanol solution (0.5 gram of piperidine per litrr of 95% ethanol) is prepared. I t is stirred thoroughly in contact with air before it is used. Solutions are made containing accurately weighed amounts of thiophene within the concentration range of 4 to 10 mg. of thiophene per liter of piperidine-ethanol solvent. Convenient concentrations are 4, 6, 8, and 10 mg., respectively. Similar solutions of piperidine oxythiocarbamate and piperidine dithiocarbamate are prepared. All solutions are kept in glassstoppered bottles. Because the carbamates decompose on standing fresh reagents are prepared as required. dolutions containing accurately weighed aiiiounts of benzene in 95% ethanol are prepared and stored in glass-stoppered bottles. The solutions should contain approximately 0.1, 0.2, 0.3, and 0.5 gram of benzene per liter of ethanol. The optical densities of the "standard" solutions of thiophene, piperidine oxythiocarbamate, piperidine dithiocarbamate, and benzene are determined a t 230,240,255, and 290 mp. From these measurements the extinction coefficients are ascertained. Two gas samples, 1 liter each, are required for each analysis. Rubber connections are kept to a minimum between the sampling point and gas collection bottle, The rubber connections are boiled in 30Y0 caustic brfore they are used. The Tutweiler buret (Figure 3), made from a 1-liter round-bottomed flask, a 60-cc. dropping funnel, and t\vo stopcocks, is employed for scrubbing the gas sample, It is also recommended that a pair of such burets be used as gas-collecting bottles, thereby avoiding transfer of samples. The gas sample is introduced into two clean Tutweiler burets. Twenty-five milliliters of 95% ethanol are added to the first buret and the gas sample is scrubbed with the alcohol by shaking the buret for 2 to 3 minutes. The scrubbing ethanol is then removed from the buret and its optical density is measured a t 255 mp, using ethanol as the reference standard, Twenty-five milliliters of the piperidine-ethanol solution are added to the second buret. hfter the gas has been scrubbed with this liquor bv shaking the buret 2 or 3 minutes, the scrubbing liquor is removed and its transmittance is determined a t 230, 240, and 290 mp, piperidine-ethanol solution being employed as the reference standard. The optical density of the piperidine-ethanol scrubbing liquor on a benzene-free basis is found, using the relationship
e$, and e3 are the extinction coefficients of piperidine dithiocarbamate a t 230, 240, and 290 mp; e : , e:, and e: are the extinction coefficients of piperidine oxythiocarbamate a t the same wave lengths, respectively; and e; and e," are the extinction coefficients of thiophene a t 230 and 240 mp, respectively. The length of the optical path, in this case 1 em., is designated by 5 .
Repeated testa have indicated that both carbon ouysulhde and carbon disulfide are completely removed from gases by scrubbing in the manner described with the piperidine-ethanol solution. Hydrogen sulfide is also removed as dipiperidine hydrosulfide, but this material is transparent when i t is present in low to moderate concentrations. Mercaptans and other sulfur compounds such as dimethyl sulfide do not effect this determination; either they do not react with the piperidine or the reaction products are relatively transparent. I n Table I11 the analytical results for a number of synthetic samples are given. The synthetic gas samples were prepared by successive dilutions of gas samples using a precision manometer to measure all gas pressures. Because the errors involved in preparing the samples in this manner are large, it is apparent that the values given in Table I11 are only indicative of the results that ran be obtained. I t is believed that the information contained in Tables I and 11, inclusive, in addition to the fact that carbon disulfide and carbon osysulfide are completely removed from the gas by the piperidine-ethanol solution, is more pertinent in evaluating this procedure than are the results shown in Table
111.
Table 111. Determination of Thiophene, Carbon Oxysulfide, and Carbon Disulfide in Synthetic Producer Gas Concentration of Thiophene Present Found c-
15 15 15 15
13
18 16 14
Concentration of Carbon Oxysulfide Present Found
Concentration of Carbon Disulfide Present Found'
M g . p e r liter of gas 0 0 0 15 13 25 0 0 25 17 0 15
C~~;CBee"n~~n
0
28 23
2
Present 0 50 50
50
The speed of analysis and the very low organic sulfur concenti ations that can be determined are the distinguishing character-
istics of this method. The siniplicity of the techniques involved makes it particularly valuable as a control procedure. ACKNOWLEDGMENTS
The author wishes to thank J. A. Shaw of Nellon Institute for his many helpful suggestions during the course of this study. He is also grateful to Ruth Cardell and Rachel Comer of Mellon Institutp, x h o carried out most of the esperimental work.
and the equation d,
=
D
- DX
(4)
where D is the observed optical density of the piperidine-ethanol scrubbing liquor and d, is its optical density on a benzene-free basis. The concentrations of thiophene, carbon oxysulfide, and carbon disulfide are found by solving the following simultaneous equation
in which d, is the optical density a t 230 mp corrected for benzene, d: is the corrected optical density a t 240 mp, D3 is the optical density a t 290 mp; c1, CZ, and c3 are the concentrations of carbon disulfide as piperidine dithiocarbamate, carbon oxysulfide as piperidine oxythiocarbamate, and thiophene, respectively; e,,
LITERATURE CITED
(1) Hutchison, W. K., Institution of Gas Engineers, Pubn. 175/64, p. 37 (1937). (2) Kemper, W. A,, and Guernsey, E. W., Am. Gas. Assoc. Proe., 24, 364 (1942). (3) MacHattie, I. J. W., and McNiven, N. L., Can. Chem. Proceae Ind.,30,87 (1946), No. 7, July. (4) Martin, S.W., Rueck, E. M., Knaggs, E. A,, and Hakewill, H., Organic Sulfur Subcommittee, Am. Gas. Assoo., Report 28 (1945). (5) Oldach, C. S.,and Field, E., IND.ENG.CHEM., ANAL.ED., 18, 669 (1946). (6) Riesz, C. H.,and Wohlberg, C., Am. Gas. Assoc. Proc., 25, 4 (1943). (7) Shaw, J. A., IND.ENQ.CHEM.,ANAL.ED., 12, 668 (1940). RECEIVED July 14, 1945. Contribution from Koppers Cnmpany'a Fellowship on Coal Product Analysis, Mellon Institute, Pittsburgh, Pa.
