656
Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 656-661
Scale-up for Chromatographic Separation of p-Xylene and Ethylbenzene Maoml Seko,’ Hiroshi Takeuchl, and Tsutomu Inada Asahi Chemical Industry Co., Lid., 1-2, Yurakucho 1-chome, Chiyodaku, Tokyo, Japan
A new adsorption process for separating p-xylene and ethylbenzene from mixed xylene has been developed. This
“displacement chromatography” process employs a zedie-desorbent system of excellent selectivity and adsorption capacity, thus making it possible to separate p-xylene and ethylbenzene simultaneously from mixed xylene at low cost. The separation efficiency is generally said to decline when the diameter of the adsorbing colllmn is increased for scale-up in the band-chromatography process. The authors have determined the cause of this decline and successfully devised technology for scale-up of the chromatography process. Further, a simulation model has been developed to predict separation efficiency by chromatography in the scale-up. In addition, the adsorption process is not only highly economical for new plants but also makes it possible to increase production capacity and produce p -xylene and ethylbenzene simultaneously at existing plants.
Introduction Mixed xylene is composed of o-xylene (OX), m-xylene (MX),p-xylene (PX), and ethylbenzene (EB), all of which have similar boiling points (Table I), and their separation by distillation is therefore uneconomical. Cryogenic separation (crystallization) utilizing the difference in their melting points has therefore been the conventional method of separation. Commercial production of P X from petroleum based on cryogenic separation was developed in 1950 by the Standard Oil Co. However, with the rising costs of naphtha and energy, the world trend for some years has been toward the replacement of cryogenic separation with adsorption separation. Since its commercialization in 1971, UOP technology based on an adsorption separation method commercialized by URBK has been adopted in a number of new plants. In 1980, approximately 22% of the world PX production capacity was based on adsorption separation, and this share will continue to increase (Table 11). Asahi Chemical Industry Co., Ltd., has developed a new adsorption separation process called “displacement chromatography”, which utilizes a unique method of chromatography and improved zeolite (Seko et al., 1979). This process allows simultaneous isolation of p-xylene and ethylbenzene at low cost. Previous methods of chromatography involved numerous engineering difficulties in scale-up to commercial size and particularly in relation to increasing the diameter of the adsorption column. The authors have succeeded in developing technology for scale-up of band chromatogaphy through research and engineering with a large-scale pilot plant. The technology is broad in scope and includes developments in zeolite characteristics, tower packing methods, tower design, and process design. Development and testing of the Asahi process has been completed, and it is now available for commercial application. Asahi Process In the conventional adsorption process, referred to as elution chromatography, xylene bands are developed with large amounts of eluting solvents. The low selectivity and small adsorption capacity of conventional zeolite necessitate the use of a long migrating distance to achieve sufficient separation. This results in a low concentration of xylene in the eluent solution and large fuel consumption for separation from the solution by distillation. Construction costs are also high, since 0196-4321/82/1221-0656$01.25/0
Table I. Components of Mixed Xylene
ox bp,”C mp,”C content,% use
MX
144.4 139.1 -25.2 -4 7.9 21 41 phthalic isophthalic anhydride acid
PX 138.4 13.3
EB
136.2 -95.0 18
20 ester styrene production
large-diameter, complex adsorption columns of the “simulated moving bed” design incorporating rotary valves are required. In the Asahi process, the selectivity of the zeolite-desorbent system has been greatly increased through the following developments (Seko et al., 1979): (a) achievement of optimum zeolite acidity, through replacement of metal ion in the zeolite with other appropriate metal ion by ion exchange; (b) appropriate modification of the molar ratio of SiOz/A1,O3in zeolite; (c) discovery of a desorbent with an appropriate level of adsorbence to zeolite, making possible symmetrical band formation. With these modifications, the selectivity bas been improved from the range of 2 to 4 found in the conventional processes to a range of 6 to 8 in the Asahi process. The adsorption capacity of the zeolite has also been greatly increased by a new zeolite production process. These improvements make it possible to achieve sharp separation gradients for the xylene isomers with a shortdistance migration. Therefore, there is substantially no desorbent migration ahead of the adsorption band in this displacement chromatography. The energy consumption for separation from the eluent solution is therefore low, and construction costs are reduced substantially. The Asahi process makes it possible not only to separate p-xylene but also to separate ethylbenzene simultaneously because of the extremely large P,‘, (selectivity of p-xylene over m-xylene), the large distance between the p-xylene peak from the o- and m-xylene peaks, and the position of the ethylbenzene peak between these two. The simultaneous recovery of ethylbenzene from the B and C fractions can be effected by the addition of two small subcolumns. This also serves to decrease isomerization loss, since the return of raffinate xylenes to the isomerization process is reduced. These unique features of the Asahi process make it economically highly competitive. Chromatographic Method and Scale-up Tee hnology a. General Technological Requirements for Commercialization. In the development of technology for 0 1982 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982 657
Table 11. World PX Production Capacity (1000 tonslyear)=
U.S.A.
West Europe Japan
Communist Bloc NIC’s: (Korea, Taiwan, Brazil, Mexico, etc.) total a
cryogenic
adsorption
planned expansion
total
2010 ( 9 ) 930 ( 9 ) 520 ( 5 ) 450 ( 1 1 ) 310 ( 6 )
580 ( 3 ) 240 ( 4 ) 350 ( 2 ) 20 ( 1 ) 0 (0)
0 (0) 230 ( 2 ) N.A. ( 1 ) 1260 ( 8 ) 1220 ( 9 )
2590 ( 1 2 ) 1400 ( 1 5 ) 870 ( 8 ) 1730 (20) 1530 ( 1 5 )
4220 ( 4 0 )
1190 (10)
2710 (20)
8120 ( 7 0 )
Note: for 1980; includes estimates where data is unavailable. Figures in parentheses show number of companies. -
nj-1 r---------
---- - -1
+
I
I
P inboard programmer
I
I
I
I
I I I
C
Fraction collector
L
I
Analyzer (Gas chromatograph)
I
Figure 1. Continuous minichromatography apparatus.
selective adsorption separation utilizing a molecular sieve, process design and selection of adsorbents and eluents are generally investigated on a laboratory scale with a minichromatographic column (Figure 1) of small diameter (several mm to several cm) and a pulse column. Further developmental studies of the promising processes are then conducted. For development for a commercial process, the following requirements must be fulfilled. (1) Column Scale-up. The column diameter in a commercial plant ranges from several meters to several hundred millimeters, depending on the production capacity requirements. For columns of this diameter, measures must be devised to prevent disruption of the boundary layers at both ends of the band and achieve a uniform migration through the column. (2) Lengthening of System Life span. To achieve a satisfactory life span in commercial operation, it is necessary to prevent degradation of the separation efficiency by contamination with impurities in the raw materials and the system. For this purpose, it is necessary to develop technology for renewal and reactivation of the system. Sufficient mecanical hardness in the adsorbent is also essential. (3) On-Line Detection and Fractionation of Bands. It is necessary to develop methods for analysis of regularly discharged bands and for the separation and collecton of each fraction. Conventional methods available for this purpose are high-speed gas chromatography, high-speed liquid chromatography, and infrared spectrophotometry (Miyashita et al., 1978). An effective new system utilizing
a differential refractometer and infrared spectrophotometer in combination has been developed (Takeuchi and Saeki, 1979). It is also possible to use a computer or timer valves with these systems. (4) Other Requirements. Investigaiton of equipment materials and methods for increasing column efficiency is also necessary. b. Disruption of Boundary Layers with Increasing Column Diameter. Column efficiency is generally expressed inversely in terms of HETP, which is determined by the following equation HETP = Lc/S(t,/fl)2 where L, = column length, t, = elution time at maximum concentration, and 0 = bandwidth at concentration of Co/e. (See Figure 2.) In theory the HETP should not be influenced by the column diameter as long as it is sufficiently large to ignore wall effect, but often during scale-up it is actually found to increase with an increase in column diameter. This is attributable to the following causes: (1) nonuniform distribution of liqyid at top of column; (2) nonuniform collecton of liquid at bottom of column; (3) disruption of band within packed column; (4) mixing in pipelines and pumps. The resulting decline in performance can be observed with column diameters of as little as several centimeters (Figure 3). Investigations by the authors have shown that the greatest cause of declining performance is band disruption within the packed column. This disruption can be at-
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982
658
1
l o
I tire
e1,tion P
rr
Figure 2. Calculation of HETP.
1_.___I- -_--
01.l--
Figure 5. Dropping segregation (left) and rolling segregation (right).
___L----
:.aTettiI
(--I
I
,
-
-
i i
2
iI
-
0.5
Figure 3. Correlation between column diameter and separation efficiency.
Ranqe of particle diameter
1.0
Average particle diameter
Figure 6. Classification effect of zeolite particle. Table 111. Properties of ASAHI Zeolite function void fraction, mL/g
Figure 4. Scheme of critical velocity theory.
tributed to the following factors. (1) Pressure Differences within Column. According to the critical velocity theory of Hill (1952), the pressure difference is determined by the equation (Figure 4)
P1 - P2 = g(Pl - P Z M X - kV(P1 - PU2)6X
P = hydrostatic pressure, g = acceleration of gravity, p = density of the fluid, 6x = depth of incipient bulge at interface, k = resistance constant of packing, V = superficial velocity of flow, and p = viscosity of fluid. Instability and disruption tend to occur at the solution boundary layer when P1< Pz. Although it is impossible to maintain a stable state (Pz< P1)at all points in the band, in the Asahi process this instability occurs only at the rear boundary of the p-xylene band, where the adverse effects of disruption are small, and the stable state (P2< PI) is maintained at all other points. (2) Nonuniform Adsorbent Packing. During the packing of the column by conventional methods, segregation due to differences in particle size tends to occur (Figure 5). This results in nonuniform distribution of particle size and void fraction, which in turn causes a nonuniform flow of solution through the column. c. Asahi Scale-up Technology. As described below, quantitative analysis has shown that the major causes of band disruption arise with scale-up of the column. Engineering studies by the authors with adsorption columns of various sizes have resulted in the development of new methods for minimization of adsorption band disruption accompanying scale-up. ( 1 ) Improvements of Zeolite. The formed particle zeolite adsorbent is produced from microscopically crys-
water adsorbency, wt %
configuration size
shape
Asahi 0.42-0.45 30-35 100-400 pm spherical zeolite conventional 0.21-0.22 20-25 350 pm t o chip or zeolite several mm spherical
-
talline ( 1pm) zeolite. The product zeolite is formed into chips or spheres of approximately 100 pm to several mm, by various processes. In studies on particle size distribution the authors have found that separation efficiency can be increased greatly by minimization of variation in particle size to prevent particle segregation in the column, as indicated by Figure 6. The separation efficiency was found to increase continually with decreasing particle size deviation. This was found to apply not for a small range of particular particle sizes but for particles of any size, which is of great economic advantage. Additional advantages of the Asahi zeolite over the conventional zeolite are shown in Table 111. (2) Development of Packing Methods. Improved versions of the conventional SOCK,COP, sieve (Takeuchi and Inada, 19801, balanced density slurry packing, and dynamic packing methods were found to be inadequate for elimination of the problems encountered in column scale-up. One important factor is uniformity of particle size along the column radius. Variation of particle size in this direction results in locally nonuniform turbulent flow and serious adsorbent band disruption. The authors were able to eliminate the column scale-up problems by packing the column with an inside diameter more than 20 times the particle diameter in layers, with particle diameter variation distribution uniformly within 5 to 10% of the mean particle diameter (Figure 7 ) .
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982 659 Vibration
n
n
Figure 7. Layer packing with internal vibration.
It is desirable to have the neighboring layers as close as possible in particle diameter, and sequential packing from one end of the column is therefore employed. The other important factor related to scale-up problems is column bed stability. Insufficient packing density will result in nonuniform contraction of the bed during chromatographic operation and a consequent variation in local void fractions. To prevent such contraction, the packing density is increased during or after packing by rapid small-stroke vibrators. The vibration must be uniform across the column section (Figure 7). With the above two measures, it is possible to achieve completely uniform void fraction distribution across the column section and to ensure a stable packing configuration. (3) Process Development. In conventional chromatography, in which p-xylene is recovered by collection of the extract portion at line I in Figure 8, any disruption of the band with even a slight trailing of the rear ethylbenzene boundary will result in its admixture to the p xylene fraction and a consequent decline in the rate of recovery and/or the purity of the product p-xylene. With the conventional methods, it is also impossible to increase the product p-xylene concentration by increasing the p-
xylene concentration in the feed xylene. The authors have found that in the Asahi process it is possible to increase the p-xylene concentration in the band by increasing its concentration in the feed xylene, without increasing the bandwidth at a rate proportional to the square root of the factor of concentration increase in the feed xylene. This has allowed the developmet of a partial recycling process, in which the fraction containing both p-xylene and ethylbenzene is recycled into the feed stream (Figure 9, B). With this partial recycling, the product p-xylene (extract portion) cut (Figure 8,II) is located an appreciable distance from the ethylbenzener e a boundary, and the influence of band disruption is greatly reduced. The distance between cut positions corresponds to the waiting time. The process results in a high concentration of p-xylene in the extract, a corresponding reduction in fuel consumption for distillation of the extracted solution, and the maintenance of a high level of product purity. The authors have named this process the HB process, because of the “humpback” form of the p-xylene rear boundary. By dividing the recycle portion (Figure 9, B) into two or more parts and returning them to the feed stream in sequence, the waiting time is further increased. This greatly facilitates scale-up and results in narrowing of the band and a corresponding increase in the production capacity of the plant. (4) Improvement of Column Design. The fundamental difficulty in scale-up of chromatographic processes lies in the maintenance of uniform conditions across the column section. To maintain this uniformity, it is necessary to achieve liquid distribution and collection which are uniform across the column, as well as a similary uniform adsorbent packing. Numerous devices have been employed at the top and bottom of columns to maintain uniform distribution and collection of the liquid. Various devices have also been employed within the columns, in attempts to reduce structural variation and prevent band disruption. One such attempt is the installation of circular baffle plate “internals” perpendicular to the column axis by Abcor (Baddour, 1966). Investigation by the authors of various internals has shown tournament piping internals (Figure 10) to be highly effective. Although the conventional tournament piping is bulky, since it requires a multi-tiered piping system, the authors have developed single-tier systems for both disI
Cut position in conventional process HB process chromatogram
-
-
-
Y
-
8
m
--- - ---
I
I1
Cut position in
HB process
I
Conventional process chromatogram
C
Figure 8. Chromatogram with p-xylene recycling.
extract
680
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982
Product
Mixed xylene
PX
Figure 9. p-Xylene recycling process. Table IV. Effect of Internals
fTTfi53
Conventional
a
ASAHI ' s
i
, l l ,
1
I l / I / / /
HETP. mm
none bead packing circular baffle Asahi tournament piping
20.6 21.0 20.9 16.7
Pulse test with KCl/H,O solution:
tribution and collection, thus avoiding the need for substantial enlargement of the column. Pulse tests of these intern& with aqueous KC1 solution have shown the HETP with Asahi internals to be approximately 20% lower than that with conventional baffles (Table IV). (5) Series Mixing Model for Quantitative Analysis of Band Disruption. Although HETP is often used as a measure of the decline in separation efficiency at scale-up, its calculation is based on the assumption that the chromatographic curve is a regular distribution curve, thus making its application difficult in cases where the chromatographic form is complex. To achieve a more realistic measure, the authors have employed the multistage series mixing model shown in Figure 11. The chromatographic curve is obtained by integration of the curve obtained for the small-diametercolumn and those obtained for an N-stage series of mixing vessels which are arranged in series and effect various retention times. Curve fitting with those obtained from actual performance of largediameter columns is then conducted to determine the corresponding number of stages (N). One instance of the
Figure 10. Tournament piping.
-
internals
,
I t I l W 8
I
I
I
A
Conventional packing and chromatography
C
ASAHI's packing and conventional chromatography
13
Conventional packing and ASAHI's (HB) chromatography
3
ASAHI's packing and chromatography
I I I I L
column diameter
Figure 12. Relationship between column diameter and number of mixing vessels.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 4, 1982 661 N
Critical ,:e?ocitf theory
2 u i n t l f IC3t13? 3 f ‘ ” l x l r , 3
‘/e:oclty
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distris-tron
S x c h a n g e cepacit, 5 i s t r i b u t i o n
Uersit:
“I
difference
Chromatographic cdzve disruotion
Figure 13. Procedure of simulation model.
Figure 15. Longitudinal variation of hydraulic radius. Table V. Energy Consumption case 1
case 2
~
product plant capacity, tonsly ear PX EB total mixed xylene feed,a tons/year energy consumption,b kcal/kg of product (PX t EB) adsorbent initial charge, tons kg/kg of product (PX t EB)
Figure 14. Definition of two-dimensional hydraulic radius.
column diameter is shown in Figure 12. In this case, the piping above and below the adsorbent column corresponded to 2 or 3 mixing vessels, or Np= 2 or 3 stages. As the figure shows, the value of N (or number of vessels) obtained with utilization of the Asahi process scale-up technology is so small that it is nearly equivalent to that of the piping at the top and bottom of the column. d. Simulation Model Analysis. Various attempts have been made to build simulation models capable of integrating chromatographic performance of scaled-up columns. In many cases, the models based on variables from investigations with small-diameter columns do not agree with the actual performance of the scaled-up column because of their failure to accurately reflect the nonuniform distributions of particle diameter and void fraction which tend to disrupt the band in the scaled-up column. The authors have therefore attempted to build a simulation model composed of the steps shown in Figure 13 which can accurately predict the chromatographic performance in scaled-up columns, in which: (1)nonuniformity of the packed adsorbent in the scaled-up column is expressed quantitatively; (2) these quantitative results are integrated to show the disruption of the flow pattern in the column; (3) the critical velocity effect is entered into the calculations. In the authors’ model, the hydraulic radius measure (Figure 14) of Yamazaki and Jimbo (1979) has been adopted for accurate quantification of the nonuniformity of conditions in the column. To relate the conditions in the column to the degree of disruption, the “parallel pore model” (Figure 15) of Carbonell (1979) and the “agglomeration model” of Moulijn (1976) are employed. The simulation model is of great interest from the standpoint of engineering, and investigations of its validity are now being conducted in a semicommercialscale plant.
PX only
PX and EB
70 000
70 000 83 510
70 000 15 340 85 340 93 450
1.7 x 103
2.2
270 0.00078
170 0.00039
x 103
Feed composition: MX, 41%; OX, 21%; PX, 20%; EB, Based o n total steam, electricity, and fuel consumption other than byproduct fuel, a
18%.
Important Features of the Asahi Process The following are major advantages of the Asahi Process (Table V): (a) zeolite of high selectivity and adsorption capacity, relatively small packing volume, process simplicity, and low construction cost. (b) low heat consumption due in part to high concentration of product obtained by “displacementchromatography”;(c) capability for production of ethylbenzene simultaneously with that of p-xylene, and a consequent reduction in the isomerization load. This process is not only economically superior in application at new plants, but it can also be adopted at existing plants for capacity expansion and for simultaneous production of p-xylene and ethylbenzene. Literature Cited Baddour, R. F. U S . Patent 3 250 058, 1966. Carbonell, R. G. Chem. Eng. Sci. 1979, 3 4 , 1031. Hill, S. Chem. Eng. Sci. 1952, 1 , 247. Mlyashlta, Y.; Oikawa, K.;Hiklchl, M.; Abe, A.; Sasaki, S. Anal. Chem. Jpn. 1978, 27, 741. MoullJn,J. A. Chem. Eng. Scl. 1978, 3 1 , 845. Seko, M.; Mlyake, T.; Inada, K. Ind. Eng. Chem. Prod. Res. D e v . 1979. 18, 263. Takeuchl, H.; Inada, T. unpublished report, 1980. Takeuchl, H.: Saekl, T. unpublished report, 1979. Yamazaki, R.: Jimbo, G. Chem. Eng. Jpn. 1974, 38(4). 299.
Received for review February 9, 1982 Revised manuscript received June 1, 1982 Accepted July 27, 1982 Presented a t the seminar on “Molecular Sieves and Their Applications”, held in New Delhi, Nov 13-14, 1981.
