Troubleshooting and Rectificationof a Giant C3 Splitter Tower Problem Part 1: Troubleshooting Henry Z. Kister, Fluor, Aliso Viejo, CA Brian Clancy-Jundt and Randy Miller, PetroLogistics Corp., Houston, Texas and Daniel R. Summers, Sulzer Chemtech USA, Inc., Tulsa, OK Presented at the Distillation Topical Conference, AIChE Spring Meeting, New Orleans, Louisiana, March 30 - April 3, 2014 UNPUBLISHED Copyright 2014 Henry Z. Kister, Fluor, Brian Clancy-Jundt, Randy Miller, and Daniel R. Summers. The AIChE shall not be responsible for statements or opinions contained in its publications. Page 1 Troubleshooting and Rectification of a Giant C3 Splitter Tower Problem Part 1: Troubleshooting By Henry Z. Kister, Fluor, Aliso Viejo, CA, Brian Clancy-Jundt and Randy Miller, PetroLogistics Corp., Houston, Texas, and Daniel R. Summers, Sulzer Chemtech USA, Tulsa, OK Summary The PetroLogistics Propylene Dehydrogenation Unit (PDH) 28 feet ID C3 Splitter started up in October 2010 and experienced low tray efficiencies and premature flooding in its first year of operation. Due to the low tray efficiency it could not produce on-spec polymer grade propylene. PetroLogistics, Fluor (who was not involved in the tower design), and Sulzer formed a taskforce to conduct a troubleshooting investigation to determine the root cause of the poor performance and to propose a fix. Our troubleshooting investigation combined hydraulic analysis and detailed multipass distribution calculations with the specialized technique of multichordal gamma scanning with quantitative analysis7. The hydraulic analysis and multipass calculations did not identify a reason for the low tray efficiencies, but confirmed that the trays are prone to channeling and maldistribution due to their large open areas. The gamma scans showed a maldistributed pattern on the trays, with high L/V ratios on the inside panels and low L/V ratios on the outside panels. The scans showed vapor cross flow channeling (VCFC) on the outside panels. Flooding was observed on the inside panels well below the calculated flood point. The scans pointed at a combination of VCFC and multipass maldistribution as the root cause. Our investigation identified the open slot area (15% of the active area) of the fixed valves to be the prime factor inducing the channeling and maldistribution. A likely initiator of the multipass maldistribution was liquid preferentially flowing to the inside panels from the false downcomers distributing the flashing reflux to the top tray’s panels. This preferential flow is believed to have occurred through the gap at which the reflux pipes entered the false downcomers. The high ratios of flow path length to tray spacing (2.4 to 3.7), high weir loads, and integral trusses projecting a significant depth (4”) into the vapor space were other conditions that promoted the channeling. To the best of our knowledge, this is the very first time that field measurements demonstrated interaction between VCFC with the inside-to-outside-pass maldistribution. A lesson learnt is that this interaction must be considered when designing and operating large-diameter towers. Finally, our investigation highlights that excessive open areas render trays prone to channeling and maldistribution, especially in large diameter towers containing multipass trays. Page 2 Open area on the trays was 15% of the active area. 1 and 2) is a heat-pumped 28 ft ID tower operating at 105 psig at the top. Figure 1. PetroLogistics C3 Splitter Tower. Overall view Page 3 . The tower contains 4-pass equal-bubbling-area Sulzer MVG™ fixed valve trays with mod-arc downcomers (MOAD’s) on the outside panels.Background The PetroLogistics giant C3 Splitter (Fig. Figure 2. PetroLogistics C3 Splitter Tower. Close-up Page 4 . 4). Together. A typical pressure drop for good operation is normally about 0. Part 1 of our paper describes the troubleshooting investigation. about 20-30% below the design. The propylene product contained 3. There was a question whether the trays in the tower were flooding or not. The internal vapor traffic is approximately the sum of the reflux and product meters. PART 1 TROUBLESHOOTING Hydraulic Evaluation at Initial Operating Conditions Fig. A point of inflection in such a curve indicates the vapor load at which liquid begins to accumulate in the tower. evaluation. The separation did not improve (if anything. The propylene content of the bottom stream was a little higher than the design. was requested to assist with investigating the root cause of the poor performance and with engineering the fix. and modification was crucial to the modifications and improvement. Part 2 will focus on the rectification of the tower problem based on the troubleshooting diagnosis. who was not involved in the tower design. while pressure drops of 0. the gamma scans concluded that many of the trays were flooded.1 psi per tray.09 psi per tray. The tower had difficulty making on-spec polymer grade propylene.2 psi per tray indicates flooding. The PetroLogistics expertise in operating the C3 Splitter was combined with other specialty resources. it had gotten worse) upon turndown. For the C3 Splitter the pressure drop per tray at the operating conditions was about 0.4% propane (by mole) compared to 0. a troubleshooting task force was formed comprising of representatives from PetroLogistics. Data for typical initial operation were collected at the highest rates at which operation was stable. To determine whether the tower was flooded. and Sulzer. There is uncertainty about the reflux flow rate due to a metering error that plagued the reflux meter.90% tray efficiency experienced with conventional trays in a C3 Splitter. Tracerco was later brought in to provide diagnostic expertise in anticipation of extensive use of gamma scanning in identifying the root cause. The strategy was to conduct a field investigation aimed at identifying possible root causes to the operation difficulties. and is a good indicator of flood(1). Fluor. In contrast.5% design. which argues against flood. a plot of the measured tower pressure drop against the tower internal vapor traffic was prepared (Fig. 3 is a simplified process sketch of the C3 Splitter tower and its auxiliaries. Fluor. about 40-50%.The tower started up in October 2010 and had experienced operational difficulties during its initial 8-months run. To investigate and solve the problem. The tower temperatures and pressures were similar to the design. the task force identified the root cause and came up with an effective fix that solved the problem. and proposing a fix. Sulzer’s expertise in tray design. Initial gamma scans through the center panels indicated flooding. Page 5 . compared to a typical 80 . There was a need to reconcile the two conflicting observations. Tray efficiency appeared to be very low. but has extensive experience in distillation design and troubleshooting. Simplified process diagram of PetroLogistics heat-pumped C3 Splitter and its auxiliaries Page 6 .FIGURE 3. 4. the lower curve is for the trays between the propylene side draw and the feed. Table 1 shows the results of these hydraulic calculations. the upper curve is for the entire tower.5 -14 psi. A simulation was prepared based on the operating conditions on 12/31/2010. but even Fluor’s calculations do not indicate proximity to any flood limits. internal vapor load from plant operating data In Fig. This suggests that at the initial operating loads in the tower were right at incipient flooding. Both curves show a point of inflection at vapor traffic just above 2000 MPH. the significant accumulation of liquid started above these loads. While some flooding could have started earlier. With the loads at which flooding initiated in the tower shown to be well below the design loads.Figure 4 C3 Splitter pressure drop vs. Page 7 . Vapor and liquid loadings from that simulation provided the basis for hydraulic calculations. or a total tower pressure drop of 13. This analysis verified the conclusion that the flood observed in the tower was premature. Fluor’s values are more conservative than Sulzer’s. it was concluded that the flooding was premature. 5 psi per tray of dynamic pressure drop. psi/tray. TABLE 1 HYDRAULIC EVALUATION AT THE 12/31/2010 OPERATING CONDITIONS Top Above Feed Bottom Trays 0.068 to 0.92 Pressure Drop.21 0.01 1.074 0.015 psi per tray.09 CACTIVE.The pressure drops values in Table 1 do not include the vapor static heads.236 0. Subtracting the static pressure drop from the total tower pressure drop of 13.26 % Jet Flood. Fluor 1.09 1.059 0.22 0.4 8.075 0.02 1.5 psi.03 Pass Distribution Ratio.227 GPM / in of outlet weir length 6.074 Pass Distribution Ratio. psi/tray. Sulzer 57 58 56 % Jet Flood. or 0.5 – 14 psi gives 11 11.237 0.060 0. Sulzer 0. ft/s 0. The static vapor head is about 2. This is in good agreement with the Fluor calculation and about 10-20% higher than the Sulzer calculation.09 1.6 DC Entrance Velocity.01 0.02 1. Sulzer 1. FRI (by Fluor) 73 74 73 % Froth in DC (by Sulzer) 49 51 54 % Froth in DC (by Fluor) 73 75 78 % Downcomer Choke (Sulzer) 38 43 51 % Downcomer Choke (Fluor) 67 68 69 Dry Pressure Drop.072 psi per tray. Fluor 0. while the tower pressure measurement does. ft/s Page 8 .9 7. or about 0.060 Pressure Drop. in liq/tray 1. To address this theory. Maldistribution among the panels of multi-pass trays is a common source of tray efficiency loss(3. For good performance. These models are among the most advanced and most reliable in the industry. while other regions where the loads are reduced have surplus capacity. All showed that the distribution ratios for trays in this tower were robust to changes in these parameters and remained small. Variations in the L/V ratio from pass to pass also adversely affect tray capacity. 11-14). as well as the proximity of the pinch. al. Pros: Large maldistribution would explain the efficiency loss and premature flooding.(3) and the Summers(11) multi-pass maldistribution models.Overall. flow path length. Bolles(12) defined the distribution ratio Φ as the ratio of the maximum pass L/V to the minimum pass L/V. Summers’ stricter criterion also keeps the loss of capacity due to maldistribution minimal(3).2 to ascertain good tray efficiency. it would be difficult to adequately model the outlet weir length.1 maximum. and recommended keeping this distribution ratio below 1. and hydraulic gradient. The calculations verified that by itself this theory cannot explain the observations. There was nothing in the calculations that would explain the observed large drop in tray efficiency and severe premature flooding. We therefore reran the models with several variations. al. the inside panels are non-symmetrical to the outside panels. In four-pass trays. and both gave distribution ratios below the stringent value of 1. The following alternative theories were proposed: 1. Page 9 . the main issues identified in the tower were low tray efficiencies and premature flooding. The task force performed a preliminary review of the theories to guide the field tests required to narrow in on the root cause. the L/V ratio needs to be the same for the inside and outside panels. Multi-pass trays maldistribution. Lockett and Billingham(14) show that the efficiency loss depends on the degree of L/V unevenness. Cons: Hydraulic calculations showed low distribution ratios. There were some concerns that with MOAD’s.1. The non-symmetry of multi-pass trays makes them prone to maldistribution. Regions where maldistribution increases the vapor or liquid loads are pushed closer to flooding. method) and the Sulzer calculation (using the Summers method) were in good agreement. Summers(11) tightened Bolles’ criterion to 1. Table 1 shows that both the Fluor calculation (using the Kister et. we applied the Kister et. Possible Theories Several theories were advanced for the low efficiency and premature flood. There are reports(7) of other forms of channeling. but it may be combined with other theories. even though at this point the details were not understood. such as due to excessive forward push (reverse vapor cross flow channeling. RVCFC) or due to vapor maldistribution. Cons: No experiences have been previously reported of interaction between tray channeling and multi-pass maldistribution. Page 10 . Channeling combined with multi-pass maldistribution. This theory also agrees with the observation of efficiency loss without apparent flooding below 13. high weir loads (7. The large open slot area is also conducive to maldistribution between passes in multi-pass trays(3. 15).5 to 14 psi pressure drop. provided as part of the original design to keep pressure drop low for the heat pump system. This theory can combine with channeling on the trays. and these needed to be investigated in the test program. In multi-pass trays. Cons: Hydraulic calculations showed that to lose the downcomer seal it would take a gap about 1 square foot in area. This became by far the leading theory. The trays were thoroughly inspected. This theory leaves unanswered questions regarding the nature of the channeling and its propagation. This theory was therefore regarded highly unlikely. are conditions that when they come together with high open areas lead to VCFC(2). Downcomer unsealing Downcomer unsealing was argued to be caused by vapor entering the off-center and center downcomers via large gaps where the supports go through the downcomers.7).The theory of pass maldistribution was regarded unlikely. VCFC is not the only form of channeling previously experienced on distillation trays. channeling is likely to interact with the split of vapor and liquid between the passes. high ratios of flow path lengths to tray spacing. so if the gaps were properly welded this is unlikely. 2. conditions that apply also for the current trays. One thing they have in common is that they were only experienced at large tray open areas. This theory explains the inability to operate at lower loads. High ratios of flow path length to tray spacing (2. let alone gaps of this magnitude. and no gaps were seen.4 to 3. and high liquid loads.9 gpm/inch of outlet weir) and integral trusses projecting a significant depth (4 inches) into the vapor space. The large open slot areas (15% of the active areas). generating or aggravating inside-to-outside-pass maldistribution. 3. Pros: This theory explains the premature flooding. can render trays prone to various forms of channeling such as vapor cross flow channeling (VCFC) at the high liquid loadings. Pros: This theory explains the premature flooding and low efficiencies. 5. At startup. Also. There is a meter on the heat pump compressed gas. Excessive hydraulic loads and poor metering Excessive reflux and boilup rates due to incorrect metering can overload and flood the trays. Foaming Foaming is known to induce premature flood. The heat pump starts up at near full rates.4. base liquid level is sometimes raised above the reboiler return inlet in anticipation of rapid boiling upon heat pump startup. Pros: This theory explains the premature flood and low efficiencies. Pros: It explains the premature flood. Damage Damage may possibly induce premature flood. This theory was therefore regarded highly unlikely. which renders the tower startup bumpy. This theory does not explain the inability to operate at lower rates and the poor operation below flood. Cons: We are not aware of any foaming cases in C3 Splitters. tray damage incidents may occur in heat-pumped C3 Splitters. a check found the annubar measurement to be within 1% of the value inferred from the compressed gas meter. The tower feed does not appear to contain foaming components in significant concentration. Pros: Metering problems have caused poor operation in many towers. Pros: It explains the premature flood. but their manufacturers claim higher reliability for recent models. Our survey of tower failures reported in the literature(4) does not include a single case of foaming in a C3 Splitter. During a crash shutdown the gas meter was fixed. and again verified the reflux measurement. giving low efficiencies. and there is such a case reported for a C3 Splitter(4). This theory does not explain the poor efficiencies at low rates and below flood. Also. the reflux flow was measured by an annubarTM with no independent check. some of the gamma scan reports mentioned the possibility of foaming. Although uncommon. base liquid level rise above the reboiler return inlet is a common cause Page 11 . Generally. 6. While the compressed gas meter was working. Cons: The annubar was checked and rechecked several times. Reliability problems had been experienced with annubars in the past. In the current tower. but that meter was bad. the annubar needed to be out by quite a factor to explain the observed poor performance. This theory was therefore regarded highly unlikely. Unbolted Manways Unbolted manways is likely to induce channeling and premature flood. the manways were properly installed. However. reducing the reboiler temperature difference and the boiling rate. 7. Also. Another argument against tray damage is that the top to bottom gamma scans did not show any severe local variations. if high liquid level damage. Other sources of damage may be rapid pressuring or depressuring. initiating poor efficiency and possible flooding. as the liquid head above the reboiler rises. We often see people leaving manways unbolted in a 20-tray single pass tower! If all. Page 12 . PetroLogistics personnel were well aware of the risk and very closely inspected the manways installation. Cons: In C3 Splitters. Pros: It agrees with the premature flood. This kind of damage tends to occur near weeping. The number of manways in this tower is well above 1000. The huge volume in this tower is likely to cushion the tower from this mechanism. in the C3 Splitter. Just a few unbolted manways are unlikely to produce the scanning pattern observed. as the reboiler vapor tends to travel through the liquid as slugs that can uplift trays(1). vibrations can be felt. so does the boiling point. or even some. Another source of damage is flow-induced vibrations(5). In contrast. almost clear vapor was reached near each manhole. The damage theory was therefore regarded unlikely. or at least almost all. near the bottom). Also. Usually damage shows local flooding or disturbance (e. closely examined this possibility and determined that it is unlikely in this tower. and a huge number to be bolted up prior to startup. Cons: The installers swore that all manways were bolted adequately. This pattern may be consistent with unbolted manways throughout the tower. Manways not installed.. the high open area and low dry pressure permit easy spread of vapor. heard and measured. Sulzer who have expertise in this type of damage and wrote the paper cited here. This is the largest number of manways we have seen in a single tower. vapor would be channeled into the unbolted region. the problem appears to initiate in every column section. but the section above showed much the same pattern as the section below. They too were sure that all.of tray damage. We have not seen this kind of damage in C3 Splitters. of the manways at the center panels were left unbolted. the potential for slug formation is relatively low due to the small reboiler temperature difference. which may be the case here due to the high open area. are common issues in all towers.g. The gamma scans show a consistent phenomenon throughout the tower. or poorly bolted (and therefore lifted). In the top-to-bottom scans. The scans showed a similar and quite uniform channeling pattern throughout the tower. the early qualitative gamma scans established that the inside active areas were flooded. The early gamma scans of the C3 splitter used this technique. not flooded in others. and hydraulic gradients can be calculated as described in references 7 and 8. and many others. and later developed by Kister(7). Center and off-center downcomers were flooded in some scans. or at most one chord per panel. However. but is unable to detect subtle abnormalities such as channeling. and use this section to represent most of the tower. The gamma scanning technique normally practiced for distillation trays shoots a single chord. To identify more subtle abnormalities. multi-chordal gamma scans with quantitative analysis is invaluable. and to implement an effective fix. This “mapping” provided a cost effective way of gaining a concise definition of the nature of the channeling. froth densities. fouling. map it in detail. the channeling/maldistribution theory towered high above the others. This technique requires top-quality multi-chordal scans of each tray panel. seldom applied by gamma scan vendors due to its high costs. This technique.The unbolted manways theory was therefore regarded unlikely. Likely Theory In summary. blow-by in unsealed downcomers. Often downcomer chords are also shot. Froth heights. and blow-by in unsealed downcomers. foaming. the nature of the channeling and/or maldistribution remained poorly defined. Initial multi-chordal scans confirmed presence of channeling both on the outside and inside active areas of the trays. and high base levels. Page 13 . The costs of multi-chordal scanning with quantitative analysis rapidly escalate with the number of trays scanned and the number of chords per tray. 4 that the trays were at incipient flood at the operating rates. The small section “mapping” permitted shooting a large number of chords per tray to give a good definition of the channeling while keeping the costs down by limiting the number of trays scanned. missing trays. clear liquid heights. There were no signs of unbolted manways. with qualitative interpretation. The uniformity of the channeling pattern throughout the tower made it possible to focus on a relatively small section. Gamma Scans Investigation To diagnose the nature of the suspected channeling or maldistribution. For the C3 splitter. was first proposed by Harrison(6). This simple technique is excellent for detecting gross abnormalities such as flooding. This tied up well with the conclusion based on Fig. abnormal froth structure. we performed extensive multi-chordal gamma scans together with quantitative analysis of the gamma scans. abnormal froth structure. The lead author has used it with great success to diagnose a multitude of subtle abnormalities on trays including various modes of channeling. 5). which are tray sketches drawn to scale with the various measurements also shown to-scale on these diagrams. the inferred trends are quite independent of the estimates and are therefore real and valid. and their locations were chosen to minimize interference from the support trusses (also shown on Fig. The results from the eastern and western inside panels were very similar to each other. the inside active areas. Two of the three outside chords passed through the mod arc downcomers (MOAD’s).In this “mapping” study. the off-center downcomers. As described in Reference 7. Due to scan quality issues. 6 are not accurate. froth densities. These MOAD’s are marked as dashed lines on Fig. Quantitative Analysis of Gamma Scans: Results Fig. Page 14 . The quantitative interpretation required several estimates. As such. 6 shows the results only for the western half of the tower. for the liquid in the mod arc downcomer above the outside even numbered panels. 6 shows the results derived from the multi-chordal gamma scans of the active areas. For each of these chords. and clear liquid heights were calculated. and three more chords on the outside western panels (Fig. 5). and finally the center downcomers. froth heights. where relevant. Spacing between successive outside chords was 23 to 29 inches.5 (extending to 63” from the tower end). Also. Even though the numbers on Fig. another five chords on the inside western panels. the chords were chosen in a manner that minimizes interference of the beams with the measurements. but these trusses are shown on Fig. allowance was made for variation in chord length and. In the interpretation. these results are shown on “Kistergrams”. Fig. they give a visualization of the key hydraulic parameters. five chords were shot on the inside eastern panels. the outside panels. Each sketch terminates just to the right (just east of) the center downcomer. From left to right on each diagram (west to east in the tower) are the side downcomers. entrainment indexes. some chords needed re-shooting to verify repeatability. The spacing between any two successive inside panel chords was about 6 inches. 6 and where relevant were considered in the interpretation. No allowance was made for the radiation absorption by the support trusses above the odd numbered trays. 50% liquid and 50% vapor by volume. The values plotted were obtained by numerical integration of the gamma scan transmission vs. higher L/V ratios on the inside panels.. i. indicative of lower L/V ratios on the outside panels.Figure 5 Chords used for the mapping gamma scan study Figure 6a shows the froth densities (using 1.0 for pure liquid propane/propylene. with a low froth density indicating a low L/V ratio. so that a point on the tray above means a froth density of 0. blue shading indicates high froth density.5. A high froth density generally indicates a high L/V ratio. height above the tray for each scan chord(7) and are drawn on a scale of 0 to 0. 0 for pure vapor). On this diagram. Figure 6a shows higher froth densities in the inside than the outside panels. Page 15 .e. red shading low froth density.5. Results from Multi-chordal gamma scans of active areas (a) Froth densities (b) Froth heights (c) Clear liquids heights (d) Entrainment Page 16 .Figure 6. Again. Most clear liquid heights for the chord closest to the off-center downcomer ranged from 3 inches to 5 inches. Fig. near the off-center downcomers. These are very high and coincide with flood or near-flood operation. No flood was observed in the chord closest to the off-center downcomers and only in two trays in the chord closest to the center downcomers. right there.6b shows flood also at the outlets of the off-center to side outside panels. Fig. A froth height less than tray spacing. Page 17 . The froths at the tray outlets and inlets are less dense than near the tray middle. with blue being high froth density and red low froth density. For the three middle chords of the inside panels. The clear liquid heights on the inside panels.. For the chord closest to the center downcomer. using a scale of 0” to tray spacing.e. the clear liquid heights ranged from 4 to 7 inches. 6b shows that most of the flood occurred near the middle of the inside panels. no flood was observed in the middle. this flood is characteristic of VCFC. and in all but one of the inlet side to off-center panels. The scale is 0” to 10”. On the inside panels. The regions where the flood was observed were those of high froth density. the highest L/V ratios are at the middle of the inside panels. The high L/V ratios in the panel middle suggest liquid accumulation. possibly flood initiation. Fig. The difference between the high densities (at the inside panels and at the inlets to the outside panels) and those in the middle and outlets of the outside panels is typically of the order of 50%. the clear liquid heights were taller in the regions where the froth density was higher and shorter where the froth density was lower. This behavior is typical of vapor cross flow channeling (VCFC). froth heights were comparatively low. with lower L/V ratios at the panels inlets and outlets. In the outside panels. 6b shows the froth heights. There appeared to be a maximum in clear liquid height in the middle of the inside panels. The flood observed here was different from that observed in the inside panels in that it always took place in the low density regions. 6c shows the clear liquid heights on the trays. a point well below the tray above indicates non-flooded operation. especially in the middle. with blue being high and red low.The only places on the outside panels where the froth densities are similar to those in the inside panels are at the outside panel inlets. so that a point at the floor of the tray above coincides with 10” clear liquid height. 6a and represents froth densities. so many points show proximity to flood. The scans are showing higher L/V ratios at the inlets of the outside panels compared to the middle and outside of these panels. Each clear liquid height is simply the froth height multiplied by the froth density. Therefore. The shading shown on the diagram is the same as that shown in Fig. The shading shown on the diagram is the same as that shown in Fig. In the chord closest to the off-center downcomers. A froth height of tray spacing or more indicates flooding and shows up as a point right on the tray above. The accuracy of the froth height determination is plus or minus 20%. most clear liquid heights ranged between 6 and 8 inches. the froth densities are highest in the middle. i. 6a and represents froth densities. As expected. were high. Fig. where most indices were 0. the clear liquid heights ranged from 2 to 5 inches. same as the froth density.12 to 0. For the outside panels.07 to 0. showing lessening entrainment from panel inlet to outlet. Fig. we deviated from the index we usually use(7) in favor of an alternative index that we believe is more meaningful here. except on tray 108. much lower than the inside panels.27. 6c. These were also the peaks that had high clear liquid heights in Fig. This pattern is typical of VCFC. the entrainment index (froth densities at the peaks) ranged from 0. there is not much entrainment (compare Figures 6b and 6d). Figure 6d shows entrainment from the trays.19 and 0. 7. froth densities at the peaks) ranged between 0. or the outside panels contained more vapor. with blue being high and red low. The only exceptions were the inlets to the side to off-center panels. There appears to be VCFC on the outside panels. i. Note that the inlet weep from this panel comes from the inlet region that is much wider than the panel width near the outlet. 6b). This ties in with the low clear liquid heights observed in the middle and outlet of the off-center to side panels. most of the entrainment index values (i.For all the outside panel scans. The flood observed on the outside panels is on the outlet of the off-center to side panels (Fig. These outside side to off-center panels had “entrainment gradients” which tracked the clear liquid heights. or both. For the entrainment index.e. The shading shows that the entraining regions closely track the high froth density regions. 6a and represents froth densities. For the chord closest to the off-center downcomers. we took the froth density at the maximum transmission point. at the vapor peak. The shading shown on Fig..14. The only exceptions were the clear liquid heights at the inlets to the side to off-center panels.0. On these trays (the even trays) there appeared to be a hydraulic gradient of around 3 inches. For the chord closest to the center downcomers the index ranged from 0.22.5. This means that either the inside panels contained more liquid.28 . with hydraulic gradients of the order of 3 inches on the side to off-center panels. Patterns Distinguished from the Analysis of Gamma Scans: Results The side panels appear to operate at lower L/V ratios than the center panels. For the entrainment index. The reason for the low entrainment there could be drying up in this region.e.. 7 illustrates this mechanism. ranging from 0. For the three middle chords of the inside panels.06 to 0.36.3). the index was much less. The scale is 0 to 0. Even though the flood appears there. The clear liquid heights there ranged from 6 to 7 inches. Page 18 . 6d is the same as in Fig. The entrainment is mostly from the inlet areas of the side to off-center panels. The low-density regions nicely correspond to the path of the vapor in Fig. Figure 7. Clear liquid heights diagram with VCFC on outside panels Page 19 . It was realized that there was still a missing link. The horizontal vapor velocity component through the outlet regions of the even outside panels may blow liquid across the off-center downcomers from the outside into the inside panels. the VCFC on the outside panels generated a strong horizontal vapor velocity component in the direction of the inside panels. and the 4” deep integral trusses perpendicular to the liquid flow. Reflux Inlet While closely reviewing the tower internals. Each false downcomer is 43” wide at the top. which would cause more hydraulic resistance on the side panels.With the outside panels operating at VCFC. These include the effective length of the modified arc downcomer (MOAD) being less than that used in the design calculations. The flashed reflux enters the tower via an H-distributor (Fig. vapor disengages upwards. Each lateral has more than 40 2. we found our missing link: reflux entry maldistribution. squirting the feed horizontally onto the walls of two off-center false downcomers that serve as flash boxes. making it shorter than the center weir length. The reflux entering the tower is a flashing liquid. This velocity component would supply the horizontal push in a direction opposite to the liquid flow on the inside center to off-center panels. This too may induce multi-pass maldistribution. while liquid descends and flows onto the top tray through 2” clearances in the bottom of each false downcomer.8). O’Bara(9) who consulted our team on maldistribution identified additional sources conducive to diversion of vapor and liquid into the inside panels. In the false downcomers. Upon flashing to the tower top pressure. but 3% by weight is 48% vapor by volume due to the density difference. The lower efficiency is then both due to the VCFC and the multi-pass maldistribution. there will be an easy path for the vapor to travel without encountering a high liquid head. Although possible. The false downcomers begin tapering 6 inches below the bottom of the laterals. tapering towards the bottom. That may not seem much. One hypothesis was that above the off-center downcomers. The inlet nozzle and the pipe feeding the H are 24”. This theory of VCFC alone explained the resulting flood initiation on the inside panels. This will induce preferential vapor flow through the outside panels and induce multi-pass maldistribution. it generated about 3% vapor by weight at the operating conditions. this hypothesis was not considered likely. Page 20 . The channeling pattern on the inside chords was more difficult to explain. branching into two short 18” pipes which later split into four 18” laterals. the formation of regions of retrograde flow on the outer panels caused by the impact of the liquid on the outlet weir near the tower wall on both sides of the MOAD.5” diameter holes. Figure 8 Reflux inlet arrangement Page 21 . So while the assumption may not be precise. which is quite typical of downcomer entrance velocities. Fig. and it is possible that some of the feed liquid lifted up would overflow the top of the false downcomers. 40% of the reflux would pour through the gap above the pipe. The result is high froth densities on the inside panels and low froth densities on the outside panels as seen in the gamma scans. 9 shows the entry of one of the 18” branch pipes into the false downcomer. Where each pipe enters there is an 18. If it only built up to 6” above the gap. a region of considerable turbulence. A calculation using the Francis weir formula(10.2 ft/s. mostly above the pipe. There were several other less prominent modifications proposed and discussed. With the high open area of the trays.The feed mixture is discharged horizontally towards the wall of the false downcomer at a velocity of about 10 ft/s. raising the height of the reflux inlet false downcomers can help prevent overflow. The presence of vapor at this region will aggravate the turbulence. This is likely to produce some upward lift on the feed liquid when it hits the false downcomer walls. Either way there would be a large scale maldistribution with excess liquid pouring through the gaps into the inside panels. In addition. this fraction would decline to 15%. The region of the gap did not have any pipe holes on either side of the pipe. The downward liquid velocity in the false downcomer at the holes elevation is about 0. the dry tray pressure drop is too low to counter this maldistribution. 16) showed that if liquid built to the top of the false downcomer. The reflux downpour through the gaps would be totally directed to the inside panels. The validity of the assumption of pure liquid may be questioned. it would be beneficial to add anti-jump baffles to stop any horizontal velocity component from the outside panels that may blow vapor or carry liquid into the inside panels. so the maldistribution would persist throughout the tower. Mysteries Explained The VCFC observed on the outside panels can now be combined with the reflux downpour through the gaps in Fig.9 to explain all the observations. The high liquid heads generated on the inside panels would induce vapor to preferentially flow into the outside panels. making the inside panels liquid-rich. Modifications for Overcoming the Problem Based on the above diagnosis.5” x 12” gap. so it is likely to be liquid-rich. Reducing the open area will eliminate the VCFC and will counter sources of multi-pass maldistribution. and the distance from the holes to the false downcomer walls is only about a foot. Page 22 . the key to solving the problem is to reduce the excessive open area (from 15% to about 11%) on the trays and to close the gaps at the reflux pipe entry into the false downcomers. it should be quite a reasonable first approximation. The top of the false downcomer is only 21” above the centerline of the holes. and generate turbulence in this region. Finally. Such overflow is likely to be maldistributed. through which liquid would pour out if froth or liquid built up above the reflux pipe or if there was turbulence in the region. only the most important modifications and those that were easy to do in a short time frame were implemented. Figure 9 Reflux pipe entering the false downcomer.While engineering a detailed solution. for the excellent gamma scans. showing gap at the entrance to the downcomer Acknowledgement The authors express their gratefulness to Tracerco. in particular to Lowell Pless. With timing being short. Page 23 . This provided an opportunity to perform key modifications from this list. the plant experienced the need for a crash shutdown. These will be discussed in Part 2 of our paper. and M. Z. 12. Mathias. 8. p. 2008. in D. Progr. Section 14. E. Phase Dispersion and Phase Separation”. 7. D. B. 2010. Summers. 2006.. Stupin. March 2006.611. TX. 373. 9. Eng. M. 131. “Optimize Separation Efficiency for Multipass Tray”. D. K. Summers.. H.. Kister. 2003. John Wiley & Sons.. D. R. D. p.. AIChEJ. M. Z. "Gamma Scan Evaluation for Distillation Column Debottlenecking". NJ. J. the AIChE Meeting. Penney. NY. Prog. September 1215. Lockett. Kister. April 2011. Chemical Engineering Progress. 81. March 1990. May 2002.. Perry “Perry’s Chemical Engineers’ Handbook” 8th Ed. “Tray Stability at Low Vapor Load”. “Preventing Maldistribution in Four-Pass Trays” (cover story). Kelkar. Z. Z. Summers. 14. R.. p. Gas Absorption. Eng. 80. The Netherlands. R. 153.. and J. S. Eng..References 1. Dionne. November 18. W. Harrison. H. Fair. 22 (1). W. J. W.26. "Harmonic Vibrations Cause Tray Damage". Chem.. W. Larson and P. 2. 10. “Distillation Operation”. Crocker. 85. Kister. Bolles... p. F. Z. “Designing Four Pass Trays” Chem. Olsson. April 2010. Kister.. H. February 2013. 4. San Francisco. Chem. IChemE. Proc. Steinmeyer. Consultant Report. Green and R. H.. Conference Proceedings of “Distillation and Absorption 2010”.. Eng. O’Bara.37-44. M. 11. April. Prog. Kister. November 1992. presented at the AIChE Annual Meeting. 15. and Billingham. P. p. “Multipass Flow Distribution and Mass Transfer Efficiency for Distillation Plates”. E. 3. H. R. Progr. E. Part A.. “Use Quantitative Gamma Scans to Troubleshoot Maldistribution on Trays”. CA. H.. H. Houston. 2010. Z. 1990. and J. J... 5. H. Kister. Jaguste. Hydroc. “Distillation Troubleshooting”. IChemE. Paper 307g. “Is the Hydraulic Gradient on Sieve and Valve Trays Negligible?” Paper presented at the Topical Conference on Distillation.. Eindhoven. Trans. B. D. p. Madsen “Vapor Cross Flow Channeling on Sieve Trays: Fact or Myth?” Chem. April 2012. New York. 6. R.86. “Equipment for Distillation.. January 2003. Trans. R. 1976 13. W. 86 (3). Z. Carmagen. Page 24 . Kister. McGraw-Hill. p. McGraw-Hill. V. Part A. J. Page 25 . 1855. “Flow of Water Over Weirs.16. and in Canals of Uniform Rectangular Section and of Short Length”. Little. Lowell Hydraulic Experiments. Brown and Company. Boston. Francis. B..
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