Ind. Eng. Chem. Fundam. 1984, 23, 252-256
252
Acknowledgment This work received financial support from the Petroleum Research Fund (Grant No. 11309-AC7).
Nomenclature a = interfacial area per unit volume of liquid phase, A / V, cm-'
A = interfacial area, cm2 Ce = concentration of liquid reactant, g-mol/L
C* = equilibrium concentration of dissolved gas in liquid, g-mol/L D = diffusion coefficient in the liquid, cm2/s E,, Eb = activation energies in eq 4 E,, Ed = activation energies in eq 7 k = reaction rate constant, s-l k,, kb = kinetic constants in eq 3 and 4 k,, kd = kinetic constants in eq 5, 6, and 7 k L = mass transfer coefficient in the liquid phase, (DS)1/2, cm/s Q, = rate of gas absorption, g-mol/s S = surface renewal rate, s-l V = volume of the liquid phase in the reactor, cm3 u = liquid flow rate, cm3/s Registry No. Propionaldehyde, 123-38-6. Literature Cited Astarita, G. "Mass Transfer with Chemical Reaction"; Elsevier: New York, 1967. Baeyer, A,; Villiger, V. Ber. 1899, 3 2 8 , 3625.
Bawn, C. E. H.; Wllliamson, J. B. Trans. Faraday SOC. 1951, 4 7 , 721. Bawn, C. E. H.; Jolley, J. B. R o c . R . SOC.London, Ser. A 1956, 327, 296. Beenacker, A. A.; Van Swaaij, W. P. M. R o c . Eur. Chem. Eng. Symp. Hiedelberg, 1976. Brostrom, A. Trans. Inst. Chem. Eng. 1975, 53, 26. Carpenter, B. Ind. Eng. Chem. Process Des. Dev. 1985, 4 , 105. Charpentier, J. C.; Laurent, A. AIChE J. 1974, 2 0 , 1029. Charpentier, J. C. Chem. Reactlon Eng. Rev. Houston 1978, 223. Danckwerts, P. V. "Gas-Liquid Reactions"; McGraw-Hill: New York, 1970. Denisov, E. T.; Mitskevlch, N. F.; Agabekov, V. F. "Liquid Phase Oxidation of Oxygen Containing Compounds"; Consult. Bureau: New York, 1977. DeWall, K. J. A.; Okeson, J. C. Chem. Eng. Sci. 1966, 21, 559. Ganguli, K. L.; Van Den Berg, H. Chem. Eng. Sci. 1978, 3 3 , 27. Gurumurthy, C. V. J. Appi. 8iotechnol. 1973, 2 3 , 769. Gurumurthy, C. V.; Govindarao, V. M. H. Ind. Eng. Chem. Fundam. 1974, 73,9. Hendriks, C. F.; Hendrik, C. A.; Heertjes. P. M. Ind. Eng. Chem. Prod. Res. Dev. 1978, 1 7 , 261. Hobbs, C. C.; Drew, E. H.; Van7 Hof, H. A,; Mesich, F. G.; Onore. M. J. Ind. Eng. Chem. Prod. Res. Dev. 1972, 1 7 , 220. Kagan, M. I.; Lyubarskii, G. D. Zh. Fir. Khim. 1935, 6 , 536. Kulov, N. N.; Malyusov, V. A. Doki. Akad. Nauk SSSR 1966, 177(6), 1288. Ladhabhoy, M. E.; Sharma, M. M. J. Appl. Chem. 1989, 19, 267. Laurent, A.; Charpentier, J. C. J. Chim. Phys. 1974, 77, 613. Llnek, V.; Mayrhoferova, J. Chem. Eng. SCi. 1970, 25. 787. Marta, F.; Boga. E.; Matok, M. Discuss. Faraday SOC. 1988, 46, 173. Mutzenburg, A. 6.; Parker, N.; Fischer, R. Chem. Eng. Sept 13, 1965, 175. Penney, W. R.; Bell, K. J. Ind. Eng. Chem. 1987, 59(4), 40. Prasher, B. D. AIChE J. 1975, 27,407. Robinson, C. W.; Wilke, C. R . AIChE J . 1874, 2 0 , 285. Spidharam, K.; Sharma, M. M. Chem. Eng. Sci. 1976, 31, 767. Venugopal, E.; Kumar, R.; Kuloor, N. R. Ind. Eng. Chem. Process Des. Dev, 1967, 6 , 139.
Received for review January 10, 1983 Accepted November 22, 1983
On-Line Analysis of Fischer-Tropsch Synthesis Products Ronald A. Dlctor and Alexis T. Bell' Materials and Molecular Research Division, Lawrence Berkeley Laboratory, and Department of Chemical Engineering, University of California, Berkeley, California 94720
A gas chromatographic system has been developed for the rapid, on-line analysis of products produced during Fischer-Tropsch synthesis. The system utilizes a single chromatograph fitted with two columns. A packed column containing Chromosorb 106 is used to separate C,-C5 hydrocarbons, low molecular weight oxygenated compounds, C02 and H20. All C5+ products are separated using a capillary column coated with OV-101. Complete analysis of products containing up to 32 carbon atoms can be achieved in 2.5 h.
Introduction Interest in Fischer-Tropsch synthesis for the production of liquid fuels and chemicals has motivated the development of improved methods for product analysis. Primary attention has focused on gas chromatography, which in some instances has been combined with other analytical methods such as mass spectrometry. In most investigations, the unconsumed reactants and noncondensable products (e.g., C1-CB hydrocarbon, low molecular weight oxygenated compounds, and COP) are analyzed on-line, whereas the condensable products (e.g., C6+hydrocarbons and oxygenated compounds, and HzO) are collected, separated into an organic and aqueous phase, and then analyzed off-line (Bauer and Dyer, 1982; Deckwer et al., 1982; Huff et al., 1982; Feimer et al., 1981; Atwood and Bennett, 1979). This approach, while capable of providing good product identification, suffers from a number of disadvantages. The principal ones are the following: (1) Quantitation: Accurate volumetric measurements of each phase must be made. Components usually appear in more 0 196-431318411023-0252$01.50f 0
than one fraction (or phase), and fractions must be added to yield the full product spectrum. (2) Handling: Problems of vaporization and oxidation of products during product storage and handling can be severe (Huff et al., 1982). It is also possible that reactions may occur in the condensate (e.g., hydrolysis of esters to acids and alcohols). (3) Conversion: Reactors must be run at high conversion in order to accumulate sufficient amounts of condensate in short periods of time (typically, 5 to 10 h). Low conversion studies, ideal for determining kinetics, are impractical. (4) Analysis turnaround: One complete analysis necessarily involves many hours of collection plus analysis time. This makes it difficult to follow the dynamics of catalyst deactivation or changes in reaction conditions. Many of the difficulties cited can be overcome by complete on-line analysis of all products, and several groups have reported on efforts to develop chromatographic systems for this purpose. Everson et al. (1978) used a column packed with Chromosorb 102 to analyze the complete product spectrum. The mole fractions of CH,, CO, G 1984 American Chemical Society
Ind. Eng. Chem. Fundam., Vol. 23, No. 2, 1984
C2H4,and C2H6were determined with a second column, but the packing in this column was not specified. By combining the information from both columns, the proportions of C,-C12 products could be determined. A dual-column system has also been described by Nijs and Jacobs (1981). The analysis of CO, COP, and CHI was accomplished on a 1.5 m long, 2.5 mm i.d. column packed with Carbosieve B. A 150 m long, 0.5 mm i.d. WCOT capillary column coated with OV-101 was used to identify C1-C12 hydrocarbons. The total time for analysis of products containing up to 12 carbon atoms was 200 min. A chromatographic system suitable for analysis of Fischer-Tropsch products has also been described by Schulz et al. (1978). Gas samples were collected in evacuated glass ampules and then analyzed on a 100 m long, 0.25 mm i.d. glass capillary column coated with squalane. By use of this system, C1-Clo products could be resolved in about 100 min. The chromatographicsystem described in this paper was designed to permit rapid on-line analysis of FischerTropsch products. The central component is a dual-column, temperature-programmable gas chromatograph, equipped with both thermal conductivity and flame ionization detectors. The chromatograph is interfaced with a small computer which is used for both data acquisition and reduction. Total turnaround time for an analysis of products containing up to 32 carbon atoms is 2.5 h.
Experimental Section Product analysis was carried out with a Varian Model 3700 gas chromatograph adapted for subambient operation. CO, COP, C1 through C5 hydrocarbons, and low molecular weight oxygenated compounds were separated on a 2.7 m long, 3.2 mm 0.d. stainless steel column packed with Chromosorb 106 (80/100 mesh). The products eluting from this column were detected by a thermal conductivity detector. To maintain a stable baseline, the reference side of the detector was connected to a packed column identical with that used for product separation. The analysis of C1 through C32 products was accomplished on a 50 m long, 0.25 mm i.d. low load OV-101 WCOT column. The inlet end of this column was connected to a standard injectorsplitter system. The gases eluting from the capillary column were detected by a flame ionization detector. The injector temperatures were set at 310 and 170 "C for the capillary and packed columns, respectively. The flame ionization detector temperature was maintained at 300 "C, and the thermal conductivity detector was maintained at 220 OC, with the filament kept at 300 "C. The head pressure of the capillary column was kept at 25-30 psig to maintain a helium carrier gas flow rate of 1 to 2 cm3/min. The helium flow rate through the two packed columns (reference and sample columns) was balanced at approximately 25 cm3/min. A make-up flow rate of 30 cm3/min of helium was used for the capillary column. The voltage outputs from the thermal conductivity and flame ionization detectors were amplified and then interfaced to a Commodore PET microcomputer via a voltage-to-frequency converter. The counter connected to the converter was controlled by an external time base. A data collection rate of 5 counts s-l per channel proved to be more than sufficient. The raw data were stored in sequential files on floppy disks using a machine language program. Peak identifications and integrations were carried out on the stored chromatograms using a program written in Commodore BASIC. The connection of the chromatograph to the reactor effluent is illustrated in Figure 1. A high-temperature valve located immediately downstream from the reactor
CO+H2
253
Heated L i n e Vent
48
Water-Cooled Condenser
To Drain Valve Oven
Injector
Vent
Amplifiers V o l t a g e - t o Frequency Converter
Computet
Figure 1. Schematic of the gas chromatography system.
is used to bleed a small fraction of the product gases through a heat-traced line to a heated sampling valve (Valco). To avoid the formation of hydrocarbons in the transfer line, its temperature is maintained below 250 "C. The sample loop connected to the sampling valve has a volume of approximately 2 cm3. The effluent leaving the first sampling valve passes through a low dead volume, water-cooled condenser. The noncondensable gases leave the condenser and pass through a second sampling valve (Valco) which is connected to the packed column. The volume of the sample loop connected to this valve is approximately 1 cm3. To begin an analysis, the chromatograph oven is cooled to -80 "C, and a sample is injected onto the capillary column. A split ratio of 501 is maintained during, and for at least 5 min following, injection. The initial temperature is maintained for 3 min following injection, after which the oven temperature is increased at the rate of 45 "C/min. At -5 "C the rate is decreased to 5 OC/min, and the injection onto the packed column is made one minute later at 0 "C. The rate of 5 "C/min was chosen to obtain good separations on both the packed and capillary columns, concurrently. A final oven temperature of 235 OC is set by the Chromosorb 106 packing used in the packed column. Once the oven reaches 235 OC, this temperature is maintained until all compounds of interest have eluted. A complete analysis can be carried out every 2.5 h. Manufacturers of OV-101 glass capillary columns do not recommend using the columns below 0 "C; yet with the exception of a slight initial loss of resolution the columns appear to age very slowly. Losses in resolution can largely be recovered by removing one loop of the column nearest the injector where deterioration is most significant. By this approach, a given capillary column can be cycled in excess of 500 times. The products eluting from the packed column were identified by comparison of the elution times with those of pure substances injected onto the column. Bottled calibration gases containing C1 through C6 hydrocarbons CO, and C 0 2 were used for this purpose. The thermal conductivity detector response was assumed to be linear
254
Ind. Eng. Chem. Fundam., Vol. 23, No. 2, 1984
for hydrocarbon concentrations between 0 and 500 ppm. The calibration factor for C 0 2 increased by 22% for concentrations between 5000 and 71000 ppm, but was assumed to be constant for concentrations between 0 and 1000 ppm. Many of the products containing 6 to 12 carbon atoms were identified by gas chromatography/mass spectrometry with a Finnigan Model 4023B gas chromatograph/mass spectrometer. For this purpose, reaction products were collected at 0 "C in liquid form. Separate analyses were made for the organic and aqueous condensates. A capillary column coated with either DB-5 or OV-101 was used to separate the compounds present in both phases. The flame ionization detector responses for hydrocarbons were assumed to be proportional to the number of carbon atoms in the product. For straight-chain olefins and paraffins this presents little error (Dietz, 19671, even for short chains (Stockinger, 1977). Response factors for other compounds are given by Dietz (1967). The molar flow rates of the lght gases (e.g., C1 through C,) in the gas stream leaving the condenser are the same as those of the same gases in the analysis stream taken from the reactor. As a consequence, the light gases can be used to link the chromatograms obtained from the packed and capillary columns. This link was usually made at Cq.To do so, the broad peak comprised of butene and butane appearing in the chromatograph from the capillary column was assigned the same molar concentration as that corresponding to the sum of the butene and butane peaks observed in the chromatogram from the packed column. The concentrations of the C5+products appearing in the capillary column chromatogram were obtained by dividing the peak areas by the area of the C4 peak and then correcting for the number of carbon atoms in the product. The C5peaks were not used to link the two chromatograms because of the interference from oxygenated products. A further reason is that C5+ products are lost in the condenser, and consequently, their molar flow rates in the light gas stream are not representative of their true rates of production. The data-reduction program was designed to pass through the files of collected data and identify peak starting and ending times, the corresponding baseline values, and peak areas. Peak areas were calculated by summing all the data points collected between a peak beginning and end, after correcting for the baseline height. This approach is equivalent to the use of the trapezoidal method for quadrature and is highly accurate, considering that a typical peak from the packed column is divided into approximately 200 intervals, and a peak from the capillary column is divided into approximately 40 intervals. In the case of overlapping peaks, integrations were made by dropping perpendiculars to the calculated baseline. Linear extrapolations were made for the baselines of peaks eluting on the tails of other products-such as normal hydrocarbons eluting on the tails of aldehyde peaks. Any questionable integrations could be recalculated at any time, as long as the data file was retained. The approach outlined here has considerable advantages over the use of commercial integrators which generally do not store data. Results and Discussion Figures 2 and 3 are typical chromatograms of the products produced during Fischer-Tropsch synthesis over a fused iron catalyst contained in a well-stirred slurry reactor. The reactor operating conditions are indicated in the caption for each figure. Each of the peaks appearing in Figure 2 is identified in Table I. Very good resolution can be achieved for all
I
0
6
I
12
,
1
1
1
18
24
30
36
2
t (min)
Figure 2. Packed column chromatogram. Reactor operating conditions: T = 574 K;p = 10 atm; H 2 / C 0 = 2; Qo = 300 cm3(STP)/ min; X C O= 0.52.
I
0
5
IO
15
20
40
-1hr-
t (min)
25
30
35 t iminl
Figure 3. Typical capillary column chromatogram. Reactor operating conditions: T = 518 K; p = 10 atm; H2/C0 = 2; Qo = 480 cm3(STP)/min;X C o = 0.03.
products and the quantities of all species except H2 and H20 can readily be determined. Hydrogen produces a negative response in a helium carrier gas, which is highly nonlinear in H2 concentration. The H 2 0 peak is always broad and exhibits a long tail. Because of the latter feature, it is difficult to obtain an accurate determination of the peak area. This problem is particularly severe when the H20concentration in the sample is large. Nevertheless, an estimate of the H 2 0 content in the products can be obtained by integrating the water peak and using the calibration factor given by Dietz (1967). The peaks observed in the chromatographobtained from the capillary column, seen in Figure 3, are identified in Table 11. The first five peaks are attributed to C1through C4 hydrocarbons. While ethylene and ethane can be fully
Ind. Eng. Chem. Fundam., Vol. 23, No. 2, 1984 255
Table I. Assignment of the Peaks Shown in Figure 2 peak no.
compounda
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
hydrogen carbon monoxide methane carbon dioxide ethylene ethane water propylene propane acetaldehyde 1-butene n-butane propanal 3-methyl-1-butene 1-pentene n-pentane
a All peaks were identified by comparison of the retention times with those for pure compounds. Peaks 11-16 were also identified by GCIMS.
resolved, it is evident that the C3 and C4 olefins and paraffins cannot be separated. For products containing five or more carbon atoms, good resolution of the a-olefin and normal paraffin is observed, and these species are seen as the principal features in each cluster of products. The Cn-z aldehyde is observed as a peak immediately preceding the C, a-olefin peak. The Cn-3 alcohol appears as a peak
preceding the Cn-Paldehyde. Identification of aldehydes containing 12 or more carbon numbers becomes impossible, inasmuch as the aldehyde peaks merge into the olefin peaks for higher carbon numbers. The cluster of peaks preceding the Cn-3alcohol, and following the Cn4 paraffin, is largely comprised of isomers of hydrocarbons containing n carbon atoms. The predominant branched products are the methyl-substituted olefins, the ( n - 2l-methyl-lC+,,-ene generally being present in largest quantity. Doubly substituted chains are less abundant but are generally the first products to elute for a given carbon number. Also found in the cluster of peaks are unsaturated compounds of carbon number n - 1, such as cyclic olefins, cyclic diolefins, and aromatics. Toluene, xylene, and ethylbenzene have all been identified, but very little benzene has been observed. While methanol and ethanol do not appear in the chromatograms presented in Figures 2 and 3, the concentration of these products can be determined with the system described here. Methanol, when present, appears as a peak located between the peaks for propane and aldehyde eluting from the packed column. Since the tailing of the methanol peak is much less pronounced than that for water, the concentration of methanol can be determined accurately. Ethanol appears as a leading shoulder on the 1-butene peak of the chromatogram obtained from the packed column. The area of the ethanol peak can be calculated indirectly. To do so, the packed-column and
Table 11. Assignment of Peaks Shown in Figure 3 peak no.
compound
method of ident peak no.
1
carbon monoxide, methane
a
2 3 4
ethylene ethane propylene, propane butene, butane methylbutene acetone 1-pentene prop anal n-pentane 4-methyl-1-pentene 2-methylpentane 1-butanal 2-butanone 1-hexene n-hexane C,H,, benzene, dimethylpentene
a a a
5 6 7d 8 9d 10 11 12 13 14 15 16 17 18
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
3-methyl-1-hexene 5-methyl-1-hexene 4-methyl-1-hexene 2-methyl-1-hexene 1-pentanal 1-heptene n-heptane toluene, C,H,, 6-methyl-1-heptene 1-heptanol 1-hexanal 1-octene n-octane C,H,, (likely 2-octene) xylene, ethylbenzene 7-methyl-1-octene 1-hexanol 1-heptanal 1-nonene
a, b b a,b a, b arb a,b b b a,b b a, b a, b b
b b b b a,b b,c b, c b b a, b a, c b,c b, c b b b a,b
compound
method of ident
37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55
n-nonane 1-heptanol 1-octanal 1-decene n-decane 1-octanol 1-nonanal 1-undecene n-undecane 1-nonanol 1-decanal 1-dodecene n-dodecane 1-decanol 1-undecanal 1-tridecene n-tridecane 1-undecanol 1-dodecanal. 1-tetradecene
b, c b, c b, c b, c b, c b,c b, c b,c b,c b,c b, c b, c a, c
56 57 58 59 60 61 62 63 64 65 66 67 68 69 IO 71 72 73
n-tetradecane 1-pentadecene n-pentadecane 1-hexadecene n-hexadecane 1-heptadecene n-heptadecane n - c ,,H,, n-CJL n-CzoH,, n-Cz1H44 n-CzzH, n-Cz,H,, n-Cz4H5, n-Cz5H52
C
n-C26
54
-
C C C
C C C
C C C
a, c C C C
C C C C
C C C C
C n-C 2 7 H 56 C b, c n-Cz,H58 Identified by GCIMS. Identified by a Identified by comparison of retention time with that of pure compounds. Propanal, acetone, and ethanol all analogy t o the relative retention times of smaller members of the homologous series. elute beneath C,. Elution -times are concentration dependent and there may be some uncertainty in these assignments. C
256
Ind. Eng. Chem. Fundam. 1984, 2 3 , 256-260
capillary-column chromatograms are normalized at Cs, and then the integrated area for C4 hydrocarbons determined from the capillary-column chromatogram is used to identify the fraction of the integrated peak area for ethanol plus C4 hydrocarbons observed in packed-column chromatogram that can be attributed to ethanol. The system described in this paper has been used successfully to analyze the products formed in both fixed bed and slurry reactors. Both types of reactor have been operated over a broad range of conditions, with CO conversions between 1 and 70%. The rapid turnaround of the system and the good resolution of products has made it possible to acquire rate data in an efficient manner. Since all of the data are stored in digital form, calculations of CO conversion and plots of product distribution can be produced immediately after the completion of an analysis. This capability greatly facilitates data evaluation. Acknowledgment
This work was joint supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Chemical Science Division, and the Assistant Secretary for
Fossil Energy, Office of Coal Research, Liquefaction Division, of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098through the Pittsburgh Energy Technology Center, Pittsburgh, PA. Registry No. Carbon monoxide, 630-08-0;carbon dioxide, 124-38-9; water, 7732-18-5. L i t e r a t u r e Cited Atwood, H. E.; Bennett, C. 0. Ind. Eng. Chem. Process Des. Dev.
1979, 18, 163. Bauer, J. V.; Dyer, P. N. Chem. Eng. Prog. 1982, 51. Deckwer, W. D.; Serpemen, Y.; Ralek, M.; Schmidt, 8. Ind. Eng. Chem. Process Des, Dev. 1982, 2 1 , 222. Dietz, W. A. J . Gas Chromatog. 1987, 5 , 68. Everson, R. C.; Woodburn, E. T.; Kirk, R. M. J . Catal. 1978, 53, 186. Feimer, J. L.; Silveston, P. L.; Hudgins, R . R. Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 609. Huff, G. A.; Satterfield, C. N.;Wolf, M. H. Ind. Eng. Chem. Fundam. 1983, 22, 258. Nijs, H. H.; Jacobs, P. A. J . Chromatog. Sci. 1981, 79, 40. Schultz, H.;Gregor. B.; Lochmiller. R.; San Min, S.Erdol Kohie, DGMK Compendium 78/79 (1978). Stockinger, J. H. J . Chromafog. Sci. 1977, 15, 198.
Received for review February 10, 1983 Revised manuscript received December 27, 1983
COMMUNICATIONS Improved Preparative Chromatography: Moving Port Chromatography
Two new methods for improving the feed throughput of preparative chromatography are introduced. Example calculations using the local equilibrium theory show a 69.5% increase in throughput for moving product withdrawal and a 246 % increase for moving port chromatography when compared to traditional preparative chromatography at the same resolution. For center cuts where only a single solute is desired, moving port chromatography may be able to input feed throughout the entire cycle. Example calculations are done for the system naphthalene, anthracene, pyrene in 2-propanol on a polyvinylpyrollidone resin.
There has recently been renewed interest in large-scale applications of chromatography. The most obvious approach is to scale up an analytical chromatograph. This approach has been applied to gas chromatography (e.g., Bonmati et al., 1980),liquid chromatography (e.g., Heikkila, 1983), and size exclusion chromatography (e.g., Yau et al., 1979). The performance of these preparative systems can be improved by recycling part of the product (Bailly and Tondeur, 1982; Snyder and Kirkland, 1979), by backflushing (Bailly and Tondeur, 1981; Snyder and Kirkland, 1979), and by using column switching (Snyder and Kirkland, 1979). An alternate method which has been commercially successful is the simulated countercurrent or simulated moving bed (SMB) approach. In this method countercurrent motion of the solid and fluid phase is simulated with packed beds by switching the locations of feed and product ports every few minutes. Commercial applications of this method by UOP (Sorbex process) for liquids are reviewed by Neuzil et al. (1980). SMB separations have been developed for gas chromatography (see Barker and Liodakis, 1978) and for size exclusion chromatography (see Barker et al., 1978). Wankat (1982) presented a local equilibrium analysis of SMB systems. The SMB is a
continuous process which separates the feed into two products. If multicomponent separations are required several columns will be required. The SMB is quite a bit different than the usual preparative chromatograph, which is a batch process capable of separating multicomponent mixtures. A hybrid process, moving feed point chromatography, which combines the idea of moving the location of the feed port and normal preparative chromatography, was suggested by Wankat (1977). Wankat and Ortiz (1982) studied moving feed point size exclusion chromatography and found a 40% increase in throughput compared to traditional preparative chromatography a t the same resolution. McGary and Wankat (1983) studied moving feed point liquid chromatography and observed up to a 300% increase in throughput. Although backflush and recycle were not studied, they could both be used with moving feed point chromatography. The moving feed point system is now commercially available (Waters, 1983). In this paper we develop the idea of moving product withdrawal and illustrate theoretically how this idea can be used to increase throughput in preparative chromatography. Moving product withdrawal can be considered an extension of either SMB ideas or column switching
0196-4313/84/1023-0256$OlSO/O (C 1984 American Chemical Society
