Packed distillation tower hydraulic design method and mechanical

capacity at which the packing performs with the normally expected distillation efficiency. This differs from the Sherwood method of correlating the hy...
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Ind. Eng. Chem. Res. 1993,32, 2400-2407

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Packed Distillation Tower Hydraulic Design Method and Mechanical Considerations Frank Rukovena’ and T. Daniel Koshy Norton Chemical Process Products Corporation, P.O. Box 350, Akron, Ohio 44309-0350

Capacity and efficiency data on five sizes of metal structured packing in distillation services operating over a pressure range of 6.7 kPa (50 mmHg absolute) to 2200 kPa (319 psia) are correlated with a Sherwood type correlation into a capacity design method. The structured packing capacity correlation is done using the maximum efficient capacity (MEC) of the packing, which is the maximum hydraulic capacity at which the packing performs with the normally expected distillation efficiency. This differs from the Sherwood method of correlating the hydraulic capacity of packing without regard for the packing mass-transfer efficiency. Also presented are performance data on the deep beds of structured packing with height-to-diameter ratios up to 15.5 and with packed depths of 11 532 mm (37 f t 10 in.) showing no deterioration of the mass-transfer efficiency. In addition, there is discussion of the mechanical consideration and liquid distributor design considerations, both of which can greatly affect the packing performance. Information is also presented on commercial operating distillation towers with structured packing. This information includes packed depth to tower diameter ratios from 0.14 to 16.1 at operating pressures from 9 kPa (1.24 psia) to 2200 kPa (319 psia) and single bed depths from 2804 mm (9.2 ft) to 12 645 mm (41.5 ft). Introduction The increased use of structured packing for masstransfer services, particularly in large distillation towers in the past 20 years, has been made possible by improvements in packing design and in design of the associated liquid and gas distributors. In his paper, “Distillation: King of Separation” (199Q), Dr. James R. Fair states, “During the last decade, the biggest news in distillation has been the gradual displacement of the tray-type contactor by the packed contactor for vapor-liquid contacting.” This has been possible because of an increased understanding of equipment design on how to scale up packed towers to world class size. Even so, many questions remain unanswered about the “absolutely correct” equipment design. The first equipment decision a designer faces when selecting the hardware to use for a new tower is the choice between trays and tower packings. The choice between these two devices is decided on the basis of economics and the process condition to be met. Many times in the new tower case, economics favor a trayed tower, if the process can tolerate the higher pressure drop and liquid hold-up of this device over packing. In new towers where the process conditions require low liquid hold-up, and the lowest possible operating temperature (which implies lowest operating pressure) to prevent unwanted side reactions, tower packings are favored, especiallystructured packing. Also, if the tower diameter has a large impact on the overall cost of the tower, as in the case of highpressure separation, packings are the equipment of choice because their use leads to smaller, more efficient towers compared to trays. In order to choose between the alternatives, it is necessary to design the tower using the various devices and compare the resulting economics. The topics covered in this paper will be (1)determination of packed tower capacity and (2) mechanical/hydraulic considerations such as tower aspect ratios and distributor design. Prediction of Capacity Traditionally, determining the diameter of packed towers has been done using one of two methods. One

method uses a percent of the packing flood line of the packing flooding correlation by Sherwood et al. (1938), and the other method uses a percent of a pressure drop line of the generalized pressure drop correlation (Leva, 1953,1954) as a surrogate for the packing flood line of the Sherwood et al. correlation. There is an inherent problem using both of these techniques. First, the flood point (if it can be accurately identified) is a hydraulic limit and does not necessarily equal the point where a packed tower starts to lose mass-transfer efficiency. Second, the use of a single pressure drop criterion, such as the 2 in. of water per foot of packing, does not apply equally to all packings. This is so because the pressure drop points at which different sizes and types of packings start to reach their hydraulic limit or decline in efficiency are not identical. Also, if different designers use a different pressure limit, it is not possible to compare the design on the basis of a percent of flood. Therefore, what is the “ideal” way to determine the maximum capacity of a packed distillation device? It is to relate the maximum hydraulic capacity point to the hydraulic capacity at which the efficiency starts to decline. This was done by Strigle and Rukovena (1979) using the relationship of height equivalent to a theoretical plate (HETP) vs capacity for random packings (Figure 1). Point F of the curve shown on Figure 1 is commonly known as the maximum efficient capacity (MEC, C,). Point F represents the maximum rate at which the packing can be operated in a specificdistillation service while still maintaining the typical HETP as represented by the B to C portion of the curve. Dolan and Strigle [in their paper, “Advances in Distillation Column Design” (November, 1980)and updated in October 19831extended this MEC concept for packing a step further and modeled a MEC correlation after Dr. James Fair’s tray capacity correlation [as presented by Smith (196311. This procedure for packing correlates the flow parameter, X,to the ,) defined in the terms of the Souders MEC (point F, C and Brown (1934) entrainment parameter, C,. A t C,, standardized to CO(see the following discussion and eq 3) Figure 2 shows this (COvs X) correlation for a typical high-performance random packing, where

Q888-5885/93/2632-24OO$Q4.Q0/00 1993 American Chemical Society

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(entrainment parameter) (2) This same concept is now being used for predicting the maximum capacity of structured packing. There is, however, a minor complication. It is that the typical HETP vs capacity curve for structured packing can be slightly different in shape from that of random packing. Figures 3-7 show the typical shapes of structured packing HETP vs C,curves. These data were taken in Norton’s 387-mm(15.25-in.) diameter distillation pilot plant tower with approximately a 3000-mm- (9.8-ft) deep bed. As can be seen, the curves of the smaller packings (1T and 2T) tend to have an increasing slope with throughput which makes the selection of point F slightly more difficult because when the MEC point is picked where the HETP curve starts to have a sudden increase in slope (a very rapid

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CS = CAPACITY FACTOR, M/SEC Figure 5. Intalox Structured Packing 3T. System: isooctane/ toluene, 13.3 kPa abs (100 mmHg abs). Data by Norton.

determines the 100% MEC point as the point where the pressure drop curve starts to have an infinite slope (see Figure €9, and this point is confirmed by heat balance as mentioned above. This method differs from the Strigle/ Rukovena method in that the Strigle/Rukovena method did not use the onset of the infinite slope of the pressure drop as a selection criterion for the MEC point, but it used the intersection of the increasing HETP curve at high throughput with the projection of the BC portion of the HETP curve (see Figure 1). In the case of the larger size structured packing the same infinite slope of the pressure drop curve and heat balance method was used even though the HETP did not deteriorate continuously with increasing capacity. The large structured packing HETP curves go through a minimum like random packing and then show a sudden upturn in HETP due to massive liquid entrainment. In the case of Intalox Structured Packing 5T (Figure 7)the reason for nondeteriorating of the HETP is undetermined. During the experimentation with the larger packings, attempts were made to continually increase the vapor rate until the HETP and the

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pressure drop would increase dramatically, but only large amounts of additional entrainment were observed for very small increases in vapor rate without change in the HETP. In this case the heat balance data was used to get the MEC point. On the basis of this type of typical data (Figures 3-7)the MEC capacity curves (Figures9-13) were developed using the systems indicated. The data base includes separation performed under vacuum to 2200kPa (319 psia) (Hausch et al., 1992) (see Table I). The raw MEC-C, values obtained from each HETP vs C,plot were then brought to a common base by Hausch and Petschauer (1991),using the standardizing equation developed by Dolan (1980).

(3) A similar correlation was presented by Fair and Bravo (1987)based upon the flood point data by Billet (1986)on various structured packings. Whether this was entrain-

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Mechanical Considerations

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Once a process designer, using the above methods or pressure drop criteria (methods not discussed), has established the tower diameter required for the specified throughput and the amount of packed depth required for the separation, consideration must be given to the mechanical design of the tower hardware. The hardware to be considered includes the liquid and gas distributors, packing support plates, packing hold-downs, and the tower configuration itself. The ensuing discussion will cover the following topics in reference to structured packing: (1) general concerns about liquid distributor design; (2) how deep a bed of structured packing can be; (3) if the tower is existing, what should be done about old tray rings; (4) what should be done about existing manways; ( 5 ) what maximum bed height to tower diameter ratio is acceptable without affecting efficiency.

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The proper design of a liquid distributor is critical to obtaining the optimum performance from the structured packing selected. When designinga liquid distributor one must think on two levels, macroscale and microscale. Macroscale thinking means thinking about how to achieve the same liquid to vapor ratio in all areas of the tower cross section that was used in the process calculations to determine the number of stages required. When dealing with tower diameters up to 12m and beyond, it is necessary that the liquid be fed onto the liquid distributor in such a way that it does not cause the primary distributor to feed more liquid to one part of the tower over another, and that the primary distributor itself has liquid cross flow capacity to even out the unevenness of the feed system without feeding significantly more liquid to one part of the tower. All packings will eventually even out the liquid flow given enough packed height, but while this is taking place hydraulically on a macro- and microscale,the desired separation will not be achieved because the areas of the tower starved for liquid will suffer a compositional pinch and the over refluxed portion of the tower cannot make up for it. On a microscale, the designer must consider three things: (1)the number of distribution points per unit area; (2) the liquid variation allowed between distribution pointa;

Figure 8. Structured packing. Height equivalent to a theoretical plate in distillation service.

ment flood or liquid hold-up flood is unknown, so Figure 8 of Fair and Bravo’s work was reproduced without modification as part of Figure 14. The Intalox Structured Packing 2T data was added to the Fair and Bravo figure in a paper by Rukovena and Strigle (1989), as shown on Figure 14, with the Hausch et al. (1992) data on the 2200 kPa (319psig)depropanizer added to the Strigle/Rukovena line for Intalox Structured Packing 2T. As can be seen, the shape of the curve is similar to that of the other structured packings. Thus, the use of the COVS X empirical correlation can be used to design the hydraulic capacity of a distillation tower while maintaining the expected efficiency of the packing. Table I1 lists commercially operating structured packing towers covering a broad range of process conditions, from vacuum to high pressure, including the high-pressure tower described by Hausch et al. (1992), which were designed by this method. A paper by Dolan and Sauter (1992) gives detailed examples of applying the use of the above design methods for designing an ethylbenzene/styrene recycle column and other applications using packed towers.

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(3)the laying out of the liquid distributor points over the tower cross-sectional area. The number of distribution points per unit of tower

area normally does not exceed 150/m2(15/ft2),and it was found by Kunesh et al. (1987) that 100 points/m2 (10 points/ft2) is sufficient. It is the authors’ experience that

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Figure 13. Table I. Intalox Structured Packing: Additional Data on the Packings Used in the Capacity Correlations, Figures 9-13 test results test conditions HETP at 90% MEC, M a t 90% MEC, Intalox pressure, AP at MEC, kPa/m (in. of HzO/ft) mm (ft) kPa/m (in. of HzO/ft) Structured Packing systema kPa (mmHg) 305 (1.0) 0.33 (0.40) 0.57 (0.70) 1T i-octltol 98.7 (740) 0.085 290 (0.95) 0.37 (0.45) 0.57 (0.70) 0.107 1T i-octltol 13.3 (100) 280 (0.92) 0.47 (0.57) 0.114 0.78 (0.95) 1T Eb/Sty 6.7 (50) 293 (0.96) 0.42 (0.52) 0.124 0.65 (0.80) 1T 0,P-XY 2.1 (16) 299 (0.98) 0.45 (0.55) 0.82 (1.00) 0.113 1T 0,P-XY 6.7 (50) 0.51 (0.63) 381 (1.25) 0.86 (1.05) 0.112 2T i-OCt/tol 98.7 (740) 0.41 (0.50) 366 (1.20) 0.75 (0.92) 2T i-&/tol 13.3 (100) 0.131 0.69 (0.85) 372 (1.22) 1.06 (1.30) 0.152 2T Eb/Sty 6.7 (50) 455 (1.46) 0.45 (0.55) 0.82 (1.00) 0.134 2T CdCI 33.3 (250) 0.49 (0.60) 390 (1.28) 0.82 (1.00) 0.116 2T CdCI 107.0 (800) 0.47 (0.58) 366 (1.20) 0.82 (1.00) 0.110 2T CdCI 165.0 (24) 328 (1.07) 0.43 (0.56) 0.46 (0.52) 0.78 (0.95) 0.110 2T CdCI 413.0 (60) 466 (1.53) 0.82 (1.00) 3T i-oct/tol 98.7 (740) 0.119 442 (1.45) 0.41 (0.50) 0.86 (1.05) 3T i-octltol 13.3 (100) 0.140 594(1.95) ' 0.31 (0.38) 0.61 (0.75) 0.122 4T i-&/to1 98.7 (740) 0.36 (0.44) 549 (1.80) 0.78 (0.95) est. 4T i-octltol 13.3 (100) 0.149 0.40 (0.49) 762 (2.50) 0.82 (1.00) 5T i-octltol 98.7 (740) 0.125 0.45 (0.55) 716 (2.35) 5T i-octltol 13.3 (100) 0.165 0

Abbreviations: i-oct, isooctane; tol, toluene; Eb, ethylbenzene; Sty, styrene; o,p-xy, o,p-xylene. 30

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normally 60 points/mZ is sufficient and under certain conditions 40/m2 (4/ft2) is adequate. The reasons for

choosing one distribution point count over another are several. First is the minimum practical size of an orifice.

2406 Ind. Eng. Chem. Res., Vol. 32, No. 10, 1993 Table 11. Bed Height to Tower Diameter Ratios for Typical Commercial Appliations of Intalox Structured Packing height/diameter diameter, mm (ft) deepest bed, mm (ft) service pressure top, kPa (psia) packing ISP 10363 (34) 1494 (4.9) crude vac wash section 3T ISP grid 0.14 0.34 8230 (27) 2804 (9.2) top fractionator 9 (1.2) 2T 1.8 5029 (16.5) 8992 (29.5) Eb/Sty recycle 11 (1.6) 2T 3.8 1981 (6.5) 7437 (24.4) depropanizerrectification 1841 (2.67) 3T 4.6 2743 (9.0) 12497 (41.5) acetone finishing 67 (9.7) 2T 5.3 1524 (5.0) 8001 (26.3) amines 759 (110) 1T 6.0 1219 (4.0) 7315 (24.0) depropanizer (strip.) 2200 (319) 2T 9.9 457 (1.5) 4547 (14.9) depropanizer 1841 (267) 1T 10.4 914 (3.0) 9550 (31.3) pentanol/creosol 170 (25) 2T 13.6 610 (2.0) 8280 (27.2) refrigerant 138 (20) 1T 15.5 743 (2.4) 11532 (37.8) n-butanol/ MIBK/p-xylene 101 (14.7) 2T 16.1 432 (1.4) 6858 (22.5) propane/propylene upgrader 1827 (265) 2T

+

Table 111. Intalox Structured Packing- Performance theoretical dates and HETP Norton: Intalox Hukill: Intalox Structured ’kg at Structured Pkg 2Tbat bed depth, mm (ft) bed depth, mm (ft) 11532 (37.8) liquid 2908 (9.5) 5828 (19.1) distributor, theor HETP, theor HETP, theor HETP, PTS/m2 plates mm plates mm plates mm 8.6 20.6 283 60.3 10.5 277 21.5 271 85.3 30-32 384-360 100.3 10.6 274 22.5 259 163.5 11.2 260 22.5 259 a H/DRatio = 7.5 and 15.1, respectively;tower i.d. = 387 mm; p - , o-xylene, 6.7 kPa (50 mm Hg abs). H/D ratio = 15.5; tower i.d. = 743 mm; n-butanol/MIBK/p-xylene, 101 kPa (760 mm Hg abs) (Matthew, 1990).

Generally,the larger the orifice diameter, the more difficult it is to foul. The smaller the orifice, the greater care should be taken to filter the process stream entering the distributor. A distributor with the largest orifice size possible is the best choice consistent with achieving acceptable tower performance. A plugged high point count distributor does the tower performance little good. A second influence on the number of distribution points is the natural number of rivulets created by the packing being used. It is not, however, essential that the distributor actually match the packing’s natural flow characteristics (see Table 111)since within a certain depth of packing the natural flow characteristics of the packing will be attained. The data in Table I1 shows that for a 5828-mm- (19.1-ft) deep bed 387 mm (1.3ft) in diameter of Intalox Structured Packing lT, the HETP only improves 4.4 % as the liquid distribution point increases from 60.3 to 100.3 per m2,and that no improvement in the HETP is seen as the liquid distribution point count is increased from 100.3 to 163.5 per m2. This strongly suggests that as deeper beds are used, fewer distribution points per unit area are required to achieve the optimal packing performance. Also, the 2908 mm (9.5 ft) of Intalox Structured Packing 1T in the same 387-mm (1.3-ft) tower is shown in Table I11 to have achieved the same efficiency as the deep bed but with the 163.5points/m2. A thesis by Hoek (1983)gives an excellent discussion of the liquid distribution topic. In general, however, Hoek states that small packings should have higher liquid distribution point counts, but that the liquid distribution point count does not have to equal the natural rivulet frequency of the packing to achieve good packing efficiency. The point count used must be consistent with not making the orifices too small or the distributor too costly. As a practical matter, 2-3 mm is normally considered the minimum liquid orifice diameter that can be easily manufactured. An orifice this small would only be used in clean services.

Also affecting the number and size of liquid orifices in a liquid distributor is the height of minimum liquid head in a distributor necessary to maintain a reasonably low flow variation between orifices. The flow through an orifice, when it is full of liquid, is a function of the orifice area and the square root of the liquid head. It is the authors’ recommendation that sufficient liquid head should be maintained to limit the individual orifice flow variation to *5 % of the average flow rate. This normally calls for a minimum head of about 50 mm with a welldesigned feed pipe. Some engineers are specifying individual orifice flow variations within f 2 % of the average. Some researchers, such as Kuneshet al. (1987),have found that a *25% random flow variation between orifices is not extremely detrimental to separation efficiency, but it is very detrimental to separation efficiency if all the variation is located in one area of the tower (macro maldistribution). The layout of the liquid orifice pattern on a macroscale should also be considered. Patterns that cluster distribution points, resulting in overirrigation of some tower areas and underirrigation of other areas, cause a decrease in packing performance due to macro maldistribution. An empirical method by Moore and Rukovena (1986)provides a method for evaluating a liquid distributor orifice layout and relating it to the packing performance. In this method the tower cross-sectional area is divided by the number of distribution points and the resulting area is converted to an equivalent circle. In its simplest form, this method assumes the center of the circle lies directly on the top of the packed bed under the distribution point. The analysis assumes that the liquid flow from each distribution point is equal. The resulting pattern, formed by the circles, is then analyzed for over- and underirrigated areas. This method is a useful way of laying out liquid orifice patterns to assure even liquid distribution on a macroscale.

Packed Depth/Height-to-Diameter Ratio Once the liquid distributor is designed, the next question to be addressed is how deep a bed of structured packing can be used before redistribution is required. Table I11 presents two sets of data which show that it is possible to employ deep beds with high height to diameter ratios with two different sizes of Intalox Structured Packings, 1Tand 2T, than was heretofore thought possible based upon the rule of thumb of using only 20 theoretical plates per bed or limiting the height-to-diameter ratio to 8 or 10. One set of data on Table I11was taken in a 743-mm- (29.25-in.) diameter column operated by the Hukill Chemical Corporation (Matthews, 1990). This tower was packed with a single 11532-mm- (37.8-ft) deep bed of Intalox Structured Packing 2T. The height-to-diameter ratio is 15.5to 1. This bed generated as many as 32 stages. The other set of data in Table I11 is on Intalox Structured Packing

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1Tand was taken in a 387-mm- (15.25-in.)diameter Norton test tower. The bed depth was 5828 mm (19.1 ft), giving a height-to-diameter ratio of 15.1to 1. This bed generated as many as 22.5 stages. Low bed depth data is also shown. An additional example of the use of deep packed beds is given in an article by Fulmer and Graf (1991). This article describes the use of both structured and random packing in deep beds applied to an acetone finishing tower, operating with two liquid phases (see Table 11). Table I1 also lists other operating towers with height-to-diameter ratios ranging from 0.14 to 16.1. The deepest bed shown in Table I1 is the acetone finishing tower [2743-mm (108in.) diameter] which has a 12 645 (41.5-ft) deep bed of Intalox Structured Packing 2T with a height-to-diameter ratio of 4.6. The propane/propylene upgrading tower has the highest height-to-diameter ratio, 16.1. Table I1 also shows that structured packing can be used over a broad range of operating pressures from 9.0 kPa (1.2 psia) to 2200 kPa (319 psia) (Hausch et al., 1992). Tray Rings/Manways When structured packing is used for revamping a trayed column, it is necessary to remove the tray rings and bolting bars from the tower to within 6 mm of the tower wall. The main concern here is that the packing wall wipers must contact the wall in order to prevent vapor and liquid bypassing the packing. This is different than in the case of random packings where the tray rings may be left in the tower if they do not occupy more than 10%-12% of the cross-sectional area. This tray ring percentage removal rule for random packing is just a rule of thumb, and the final decision to remove the tray rings when installing random packing depends on the throughput requirements of the particular design. Manways should be located at every distributor and redistributor location. This facilitates their installation, and should the liquid distributor become fouled, the manway provides access to the distributor without having to remove the packing. Since it is possible to put in deep beds of structured packing, it is usually not necessary to install new manways in an existing column. When an existing tower is revamped, should an existing manway fall within a bed location, it should be covered over with a shroud contoured to the tower wall. It is not necessary that this shroud be attached to the tower shell. It can be supported from the manway cover. Norton’s test data (not published) taken on a 914-mm- (3-ft) diameter and a 387-mm- (1.3-ft) diameter column have shown that the separation efficiency of a tower can be halved if the manway nozzle is left open to the packed bed. It is theorized, by the authors, that this decrease in efficiency is a result of gas back-mixing in the bed caused by eddy currents created by the noncontinuous tower wall. The most pronounced effect of this phenomenon experienced by the author has been under vacuum conditions. Conclusions From the information presented, it can be concluded (1)that structured packing can be used across a broad range of operating conditions from vacuum to pressure applications, (2) that it is possible to use a Sherwood/Fair type capacity correlation for predicting structured packing maximum operational capacity over a broad range of operating conditions, and (3) that structured packing can be used in deep beds with high height-to-diameter ratios and high theoretical stage counts. Nomenclature Co = standardized entrainment parameter per eq 3, m/s

C, = maximum efficient capacity entrainment parameter at

process conditions and/or predicted from eq 3, m/s C, = entrainment parameter, eq 2, Souder and Brown, m/s G = gas rate, kg/(m%) L = liquid rate, kg/(m%) V = vapor velocity, m/s X = flow parameter, eq 1 Greek Letters p~ = gas density, kg/m3 p~ = liquid density, kg/m3 1 = liquid viscosity, mPa.s

or CP

a = surface tension, dyn/cm

Subscripts

G = gas L = liquid 0 = standardized to a = 20 and 1= 0.20 per eq 3 s = Souders and Brown sc = Souders and Brown at process conditions at maximum efficient capacity

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Received for review February 1, 1993 Reuised manuscript received April 20, 1993 Accepted April 28, 1993.

* Abstract published in Advance ACS Abstracts, September 1, 1993.