762
Ind. Eng. Chem. Process Des. Dev. 1986, 25, 762-767
to that of fracton 3 of diesel fuel. However, nearly equal relative intensities of resonances near 152, 157, and 161 ppm lead us to propose the presence of substituted quinolines. This is further supported by the aromaticity of this fraction (34% vs. 28% for the equivalent diesel fraction). This fraction contains about 3% nitrogen and 2.4% oxygen, and the NMR spectra again indicate the predominance of methyl- and ethyl-substituted pyridine carbons with small quantities of phenolic OH and carbonyl groups. Fraction 4, containing 3.9% N and 2.1% 0, has spectra similar to those of fraction 3; it appears that alkylquinolines are concentrated in this fraction. Both the IR and NMR spectra of the SESC fractions of the hydrogenated gas oil cut are essentially identical with those of the raw gas oil with the exception that the IR signal due to carbonyl groups in fraction 4 is reduced by a factor of 2. As observed in the elemental analysis of the various nitrogen concentrates, high levels of oxygen were also observed. These oxygen-containing compounds may represent components carried along as a result of hydrogen loading or as a result of compounds having both oxygen and nitrogen functionality. It is unfortunate that additional time was not available to isolate and characterize these compounds to a greater extent. Conclusions
Previous papers in this series indicated that essentially all the nitrogen-containing compounds in shale-derived jet fuel can be removed by mild hydrodenitrogenation followed by ion-exchange treatment. This type of treatment is less effective in the case of diesel fuels due to molecular size considerations. As part of this DOE project, samples of raw and hydrogenated distillate cuts along with ionexchange isolated nitrogen concentrates were characterized by column chromatography, FTIR, NMR, and GC/MS. Hydrodenitrogenation, of course, reduces the nitrogen content of the various fractions but does not greatly change the remaining type of nitrogen-containing compounds. These compounds primarily consist of alkylpyridines with low levels of alkyl-substituted indoles and quinolines. No pyrroles were observed. The spectral characteristics of the nitrogen concentrates isolated from both the raw and hy-
drogenated distillates are essentially the same, with differences primarily being related to the degree of substitution and chain length. The nitrogen-containing compounds of the jet fuel cuts have an aromaticity of 50-60%, while those of the diesel and gas oil cuts are about 28% and 34% , respectively, the latter containing more quinolines. The low level of observed indoles and pyrroles in the nitrogen concentrate was less than that given in the results of Holmes (19831, who used a different extraction technique for sample isolation. Acknowledgment
We acknowledge the financial support of the US Department of Energy (Contract No. DE-AC17-80BC10313). Also acknowledged is the support of Dexter Sutterfield (presently with the National Institute for Petroleum and Energy Research), A. B. Crawley (DOE/Bartlesville, OK, Project Office), W. H. Waitz of Rohm and Haas Co., A. B. King, D. L. Fuller, and M. J. Kurtz of GR&DC for suggestions and experimental support. L i t e r a t u r e Cited “Annual Book of ASTM Standards”; ASTM. Philadelphia, 1983; Vol. 05.01 ASTM-D1319-82, pp 710-7 16. Clutter, D. R.; Petrakis, L.; Stenger, R. L.; Jansen, R. K. Anal. Chem. 1972, 4 4 , 1395-1405. Cronauer, D. C. DOE Contract No. DE-AC17-80BC10313, Oct 1984. Dcddreli, D. M.; Pegg. D. T.; Bendall, M. R. J . Magn. Reson. 1982, 4 8 , 323. Farcaku, M. Fuel 1977 56, 9-14. Holmes, S. A., Thompson, L. F. Proceedings of the Eastern Oil Shale Symposlum, 1982, Lexington, KY. Holmes, S. A. “Shale Oil Upgrading and Refining”; Newman, S. A,, Ed.; Butterworths: Boston, 1983; pp 159-182. Marcelin, G.; Cronauer, D. C.; Vogei, R. F.; Prudich, M. E.; Solash, J. I d . Eng. Chem. Process Des. Dev., preceeding paper in this issue. Nowack, C. J.; Delfasse, R. J.; Speck, G.; Solash, J.; Hazlette R. N. In “Oil Shale, Tar Sands, and Related Materlak”; Stauffer, H. C., Ed.; American Chemical Society: Washington, DC. 1981; ACS Symp. Ser. No. 163. Patt, S. L.; Shooiery, J. N. J . Magn. Reso. 1982, 4 6 , 535-539. Prudich, M. E., Cronauer. D. C.; Vogel, R. F.; Solash, J. Ind. Eng. Chem. Process Des. Dev., first paper of three in series in this issue. Seshadrl, K. S.; Cronauer. D. C. Fuel 1983, 62, 1436-1444. Solash, J.; Hazlett. R. N.; Burnett, J. D.; Beai, E.; Hall, J. M. “Oil Shale, Tar Sands, and Related Materiels”; Stauffer, H. C., Ed.; American Chemical Society: Washington, DC, 1981; ACS Sym. Ser. No. 163. Young, D. C., Galya, L. G. Liq. Fuels Techno/. 1984, 2(3), 307-326.
Received for review December 26, 1984 Reuised manuscript received November 21, 1985 Accepted January 9, 1986
Control of a Complex Sidestream Column/Stripper Distillation Configuration Imad Y. Alatlql and WlHlam L. Luyben’ Process Modeling and Control Center, Department of Chemical Engineerlng, Lehigh Universiv, Bethlehem, Pennsylvania 180 15
The dynamics and control of a complex, multivariable, interacting sidestream cdumn/stripper distillation configuration (SSS) were explored via digital simulation. The dynamic response of this complex system was compared quantitatively with the conventional sequential “lightsut-first” two-column configuration (LOF). The complex SSS configyration was found to be controllable by using four Conventional SISO controllers. The load response of the complex SSS configuration was better than the response of the more simple, conventional LOF system.
There have been very few dynamic studies of complex distillation configurations in the literature. These systems feature columns with multiple feeds and multiple products. They often use significantly less energy than sequences of 0196-4305/86/1125-0762801 SO10
simple two-product, single-feed columns. Therefore, the dynamic operability of these more interacting and more multivariable processes has yet to be firmly established. Single sidestream columns have been 0 1986 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 3, 1986 763
PI
@
= 14.7 p d g
PCI =6.8xIO6 BTUlhr P2: 14.7 psiq
@
g
l
J
OC2. 4 . 0 ~ 1 0 ' BTUlhr
SI?,=261 I D
R ~ 258 =
DET2
1, =
Tg.236.9 P'14.7
XD( Ib0.W XO(21a0.05 0.00
VS'70.96
x 0 (31
I IO'
TBa 346
BOTl
.
I b molrr hr
'1
XB(21-0.05 XB(31- 0.95
1
X2
F
600
0.45
0.10
DlSTl
284.2
0.95
0.048
SOT,
315.8
0.0002
0.1465
DIST2
35.8
0.001
0.90
280
0.00
0.05
SOT2
BOT 1284.05 XI)[ I 1. 0.00
I SOT2
I
55'35.29 XS(I 1'0.0954 xs~21*0.9000 XS (31*0.0048
0.002
HOLOUPS, FLOWS, TEMPERATURE, PRESSURE, ENERGY,
Ib.molee I b . moles I hr F pslg BTUlhr
]
Figure 2. Basic design for SSS system. 0.099
studied by Tyreus and Luyben (1975) and Ogunnaike et al. (1983). Prefractionator schemes have been studied by Doukas and Luyben (1978), Lenhoff and Morari (1982), and Elaahi and Luyben (1985). None of these papers give a direct comparison of conventional and complex configurations. The purpose of this paper is to present the results of a quantitative study of the dynamics and control of two alternative distillation systems for separating ternary mixtures that contain small amounts (less than 20%) of the intermediate component in the feed. Alatiqi and Luyben (1985) showed that for these feed concentrations, the complex sidestream column/stripper configuration (SSS) was more energy efficient than the conventional two-column sequential "lighboubfirst" (LOF) configuration. This paper demonstratesthe controllability of the SSS system and compares it with the controllability of the LOF system. The SSS system presents a challenging 4 X 4 multivariable control problem. Significant questions must be addressed concerning control system structure, tuning, stability, and robustness. Steady-State Designs The ternary system benzene/ toluene/o-xylene was chosen as a typical industrially important separation. Benzene and xylene product purities of 95 mol % and toluene product purity of 90 mol % were used. The steady-state designs of the conventional LOF and the complex SSS configurations are given in Figures 1 and 2. The LOF columns were both designed by using conventional short-cut methods to determine the number of trays with 1.1 times the minimum reflux ratios. The SSS configuration was designed by using an evolutionary pro-
cedure. Detailed design information is given by Alatiqi and Luyben (1985) for both systems. The SSS configuration uses 15% less energy under steady-state conditions than the LOF confiiation for the 10 mol % toluene feed composition. A major aspect in the steady-state design of the SSS system is the amount of liquid sidedraw rate (LS) fed to the stripper. The higher this rate, the lower the total energy consumption (main reboiler heat duty QB plus stripper reboiler heat duty QBS). However, there is a limiting value of LS beyond which the purity of the toluene product from the stripper base can no longer be attained. This is due to the increase in the heaviest component (xylene) around the sidedraw tray as LS is increased. Any xylene that enters the stripper leaves in the toluene product. Therefore, LS cannot be increased beyond the limiting rate and still attain toluene product purity (90mol %),
In order to provide some room for the SSS system to handle changes in feed concentrations, the design value of LS was set at 90% of the maximum value. This resulted in a toluene product with 1 mol 70 xylene and 9 mol % benzene impurities. For the LOF system, any benzene that leaves in the bottoms from the fust column shows up as impurity in the toluene product from the top of the second column. Therefore, to provide some room for the LOF system to handle changes in the feed compositions, the system was designed for 1 mol % benzene and 9 mol % xylene in the toluene product. Alatiqi (1985) showed that the total energy consumption was fairly insensitive to the changes in toluene product impurities. These considerations explain why the amount of the light and heavy impurities in the toluene product are different in the LOF and SSS systems. This also produces slight differences in the flow rates of the benzene and xylene products. Dimensions of the Control Systems Both the SSS and the LOF configurations have three
764
Ind. Eng. Chem. Process Des. Dev., Vol. 25,
No. 3, 1986
~
/
16.0 14.0
LS CONSTANT
10
0
*
10.0
a z
w
8.0 0.06
2
t
--I i 8.0
, 0.06
, 0.08
0.10
0.18
0.14
X B I( I 1 = 0.003 XD2( I ) = 0.050
,
,
,
0.10
0.12
0.14
AT CONSTANT
0 MINIMUM
ZF(2)
Figure 3. Effect of intermediate feed concentration on total energy consumption (LOF).
z a W
compositions to be controlled and four manipulated variables (after subtracting the manipulated variables needed for inventory control). LOF
sss
controlled variables
XDl(1) XD2(2) XB2(3)
manipulated variables
R1
XD(1) XS(2) XB(3) R
QBl RR 2 QB2
QBS QB
0.40
0.45 ZF ( 1 )
0.50
Figure 4. Effect of controlled AT on energy relation with feed composition.
LS
A. LOF System. Heat inputs and reflux flow rates to each column can be manipulated. The reflux ratio was used in the second column because of the high reflux ratio. Since there is one more manipulated variable than is needed for composition control of the three product purities, parametric studies were made to determine what manipulated variables should be used. It was found that keeping the heat input to feed ratio (QBlIF) constant in the first column resulted in energy consumptions that were only slightly higher than the cases where another composition or temperature was controlled (Figure 3) as the feed composition was changed. Hence, the control system chosen for the first column was a single SISO control loop, controlling the benzene product purity XDl(1) by manipulating the reflux flow rate R1. The second column was a 2 X 2 multivariable system. An alternative control policy of also controlling a tray temperature in the stripping section of the first column was also tested. This increased the dimension of the control problem to 2 x 2 in the first column. The load responses of the two alternatives were essentially the same. B. SSS System. A major question in the SSS control problem was the manipulation of the sidedraw rate LS. In theory, the LS rate could be held constant, and the other manipulated variables could be used to control the three product purities. However, when the intermediate feed concentration was changed, it was found that LS manipulation was necessary to maintain toluene product purity and to minimize energy consumption. Parametric steady-state studies showed that maintaining a constant temperature difference (AT) between trays above and below the sidedraw tray by manipulating LS kept energy consumption near its minimum. See Figure 4. The sen-
I
230
270 310 TEMPERATURE (F)
J 350
Figure 5. Temperature profile for different sidedraw rates.
sitivity of AT (tray 16 - tray 28) to changes in LS is shown in Figure 5. Control Systems Analysis Multivariable analysis and control methods were used for the development of the final control schemes for the LOF and SSS systems. The general procedure proposed by Yu and Luyben (1985) was used for tackling the problems of variable pairing, controller tuning, disturbance rejection, and robustness characteristics. Table I gives the transfer functions of the LOF system, obtained from pulse testing the nonlinear mathematical model, with the 3 x 3 constant Q B l / F control structure. Table I1 gives the transfer functions for the SSS system. Table I11 gives values for both systems of RGA, MIC, and MRI. As discussed by Grosdidier et al. (1985), since the MIC values are all positive, the systems are “integral controllable”. Thus, both the RGA and MIC values indicate that both systems have variables paired in a reasonable way. The MRI values (Morari resiliency index) for the LOF and SSS systems are both about 0.3. This indicates that both of these systems are about equally controllable. The
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 3, 1986 785 Table 111. Steady-State Parameter Values I. RGA LOF Ri RR2 QB2 1.0 0 0 0 1.04 -0.04 0 -0.04 1.04 h
SSS:
I
L
L
QBS
R 3.11 -5.03 -0.08 3.01
QB -0.90 4.67 0.05 -2.83
-0.48 -0.04 1.55 -0.03
11. MIC LOF: SSS:
2.7 13.8
2.1 4.8
0.316 0.75 0.5751'
111. MRI LOF: SSS:
0.29 0.33
3 3
+
XDl(1) XD2(2) XB2(3)
LS -0.73 1.40 -0.52 0.85
XD(1) XB(3) XS(2) AT
+
1 %
-
*
g-118
h
3
+
RATIO
--LULg-
A
Figure 6. Basic control scheme for LOF system.
X h
3
+
i, w
---
Figure 7. Energy balance control of SSS system. X
MRI is the minimum singular value of the plant transfer function gain matrix. Figures 6 and 7 give the control systems for the two systems. Figures 8 and 9 compare the frequency-dependent MRI (singular values of G(iw))and TLC (Tyreus Load-Rejection Criterion), which is a measure of the feed composition disturbance attenuation capacity of the closed-loop system. The SSS configuration has a MRI (a, minimum singular value) that is higher than the LOF configuration over most of the frequency range, indicating a more easily controllable process. The TLC curve indicates that the SSS and LOF systems should attenuate disturbances in the intermediate feed composition ZF(2) about the same, except for the benzene product purity where LOF looks better. The BLT method was used to get initial estimates of the
766
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 3, 1986
I
0.880
0.860 0.960 -4.01 -4.0
\ I
-3.0
I
I .o
-2.0 -1.0 0 LOG FREOUENCY (RADSIMIN)
Figure 8. Dynamic resilience of SSS and LOF system.
0.940
BENZENE
t c t
0.930I 0
XYLENE
-sss
--- LOF 1.0
2.0 3.0 TIME ( H R S )
I
4.0
5.0
Figure 10. Controlled variables responses to a +20% change in ZF(2) for SSS and LOF systems, Dt= 3.
sss
LOF
-2.0 -1.0 LOG FREOUENCY ( R A D I M I N )
0
-3.0
Figure 9. Load frequency responses of SSS and LOF systems tuned for 3-min analyzer dead times.
controller parameters. The final controller settings were found by empirical tuning. For the SSS system, they were loop XD1-R XB3-QB XS2-QBS AT-LS
KC
0.55 0.95 0.36 0.35
'I
30 20 23 39
For the LOF system, they were loop
XDl(l)-R, XD2( 2)-RR, XB2( 3)-QB,
KC
3.8 1.38 3.1
71
13 129 27
Simulation Results The closed-loop load responsm of the SSS and the LOF systems to intermediate feed composition disturbances are
a? 0
-LOF sss ---
- 13.0
-------------------r 4.0 /
- 11.0
6.0
-
5.0
-
i--
3 .O
m 0
- 9.0 7.0
TIME ( H R S )
Figure 11. Manipulative variables movements of SSS and LOF systems in response to a +20% change in ZF(2), Dt= 3.
shown in Figures 10 and 11. A composition analyzer dead time of 3 min was assumed for the composition loops. The response of the SSS system is certainly as good as the LOF, if not somewhat better, particularly for the toluene product purity. This somewhat unexpected result may be due to the more reversible nature of the SSS system. There is recycle between the main column and the stripper which tends to dampen out disturbances. The LOF structure looks
Ind. Eng. Chem. Process Des. Dev. 1986, 25, 767-771
more simple but in fact it is more sensitive to disturbances in the intermediate component feed composition. Any light component that drops out of the bottom of the first column of the LOF system must appear as impurity in the toluene product. In the SSS structure, the light component that enters the stripper tends to be stripped out. Notice that the simulation results are consistent with the theoretical predictions. The MRI values of the two configurations indicated essentially equal controllability, which was seen in the simulations. The MIC and Niederlinski index predicted the system to be integral controllable, which it was. The TLC curves predicted that the toluene and xylene product purities would be slightly better controlled in the SSS configuration, and this was observed in the simulations. The TLC curves predicted that the benzene product purity would be better controlled over the low-frequency range in the LOF configuration but would be better controlled over the high-frequency range in the SSS configuration. These results can be seen in the simulation results. Thus, the simulations confirmed the reliability of the various indexes and measures of performance. Conclusion The complex, interacting, multivariable SSS configuration was successfully controlled by using four conventional P I controllers. The sidedraw rate had to be manipulated to maintain energy efficiency and rangeability. The load response of the SSS system was as good as, if not better than, the response of the conventional LOF system. The recycle and coupling nature of the SSS system contributed positively to disturbance attenuation. Nomenclature d = disturbance (intermediate feed composition ZF(2)) G = open-loop plant transfer function matrix F = feed flow rate, lb mol/h LOF = light-out-first configuration LS = liquid draw rate from main column to stripper, lb mol/h
MIC = Morari index of integral controllability MRI = Morari resiliency index QB = main column reboiler heat-transfer rate, Btu/h
787
QBl = first LOF column reboiler heat-transfer rate, Btu/h QB2 = second LOF column reboiler heat-transfer rate, Btu/h QBS = stripper reboiler heat-transfer rate, Btu/h R = reflux flow rate, lb mol/h R1 = first LOF column reflux flow rate, lb mol/h RGA = relative gain array RR2 = reflux ratio in second LOF column S S S = sidestream stripper configuration AT = temperature difference, O F XB(J) = bottoms composition (mole fraction of component J)
XBP(J) = second LOF column bottoms composition (mole fraction of component J) XD(J) = distillate composition (mole fraction of component J)
XDl(J) = first LOF column distillate composition (mole fraction of component J) XD2(J) = second LOF column distillate composition (mole fraction of component J) XS(J) = sidestream product composition (mole fraction of component J) y, = jth output variable Greek Symbols u = minimum singular value o = frequency, rad/min Registry No. Benzene, 71-43-2;toluene, 108-88-3;o-xylene, 95-47-6. Literature Cited Alatiqi, I. M., Ph.D. Thesis Lehigh University, 1985. Alatlqi, I. M., Luyben, W. L. Ind. Eng. Chem. ProcessDes. D e v . 1985, 2 4 ,
500. Doukas, N., Luyben, W. L. Inst. Techno/. 1978, 2 5 , 43. Elaahi, A., Luyben, W. L. Ind. Eng. Chem. Process Des. Dev. 1985, 24, 368. GrosdMier, P., Morari, M., HoR, 8. R. Ind. Eng. Chem. Fundam. 1985, 2 4 , 221. Lenhoff, A. M., Morari, M. Chem. Eng. Sci. 1982. 37, 245. Ogunnaike, B. A. et ai. AIChE J. 1983, 29, 4. 632. Tyreus, B. D., Luyben, W. L. Ind. Eng. Chem. Process Des. L b v . 1975, 14, 391. Yu, C. C . , Luyben, W. L. Ind. Eng. Chem. Process Des. D e v . 1986, 25, 498.
Received for review July 18, 1985 Revised manuscript received December 23, 1985 Accepted January 22,1986
Thermolytic Reactions of Isomeric Ethylphenols with Solvent Dodecane Pelrheng Zhou and Bllly L. Qynes" School of Chemical Engineering, Oklahoma State Universiv, Stillwater, Oklahoma 74078
Low-conversion thermolyses of p - and m-ethylphenols (PEP and MEP) in dodecane under N, or H, atmosphere were conducted, and the results are compared with those of o-ethylphenol (OEP). Overall reactivities, as well as those for dehydroxylation and dealkylation, fall in the order of ortho > para > meta. This is the reverse of their reactivity order in catalytic dehydroxygenation. OEP, PEP, and MEP also inhibit dodecane cracking, while the latter accelerates their conversion. This promotion effect is suppressed in the presence of molecular hydrogen. The same mechanism is believed to apply to these three isomers: molecular dissociatian followed by radical reactions but with insignificant chain lengths. The variations in their reactivities and product patterns are attributed to different isomeric configurations.
Phenols are the most abundant organic oxygen-containing compounds in coal-derived liquids. For a better
* To whom correspondence should be sent. 0196-4305/86/1125-0767$01.50/0
understanding of their thermal behavior during coal oil processing, ethylphenol was chosen as representative of the alkyl-substituted, single-ring phenols with a single OH group, existing in coal oils. The reaction characteristics 0 1986 American Chemical Society
