When designing distillation columns, engineers focus on selecting the required number of trays or packing height and column diameter to achieve target performance. As a result, distillation column feed condition is often overlooked. Improper feed design can hinder performance and influences unit economics.
This article will illustrate useful distillation column feed design optimization methodologies. A case study is demonstrated in detail and highlights how optimum feed design enhances distillation column performance.
Technical considerations for feed location.
Selecting optimum feed location is critical to maximizing distillation column performance. Improper feed location of a distillation column can downgrade column performance; the degree of separation is decreased at the same reflux/boil up ratio or the higher reflux/boil-up ratio is required to maintain the degree of separation.
An ideal feed location is feeding to a section of the distillation column where column internal liquid traffic composition is similar to feed stream composition. In this case, it can minimize the composition gradient between feed stream and distillation internal fluids. In distillation column operations, it is often seen that feed compositions are changed from original design conditions. In the case of significant deviation, discrepancy between column internal liquid composition and feed stream composition can increase, and results in non-optimum feed location. Therefore, evaluating the feed location is an essential step for a successful distillation unit revamp or optimization.
However, it is very difficult to sample and analyze the column internal liquid traffic composition in most of the distillation columns that are commercially operated. Instead, process simulation modeling has been widely utilized to predict internal liquid composition and determine the optimum feed location in the actual industry design. As feed optimization through simulation modeling is convenient and does not require additional costs for field measurements and laboratory analysis, it seems to be a very convenient procedure on the surface. Nevertheless, reputable simulation software itself does not promise the reliability of simulation modeling. Inherent gaps between actual conditions and theoretical simulation modeling should not be ignored.1 The appropriate simulation flow sheeting methodology is necessary to bridge between actual conditions and the simulation model. It is common for improper simulation modeling to give misleading results regarding the optimum feed-point location and cause poorer column performance than expected.
Key ratio plotting is a useful tool to evaluate optimum feed location in simulation modeling. This key ratio is usually expressed as the mole fraction ratio of light keys to heavy keys in a semi-logarithmic scale chart. A benefit in using the chart is that retrograde distillation due to non-optimum feed location can be visually identified. Optimum feed location can be graphically selected through the chart. Meanwhile, it should not be overlooked that key ratio plotting only shows binary key component behaviors. Light non-key behaviors are not recognized in this plotting.2 Relying on key ratio plotting is dangerous in determining optimum feed location. Reviewing non-key component composition profiles through the column and various sensitivity analyses is also required to ensure optimum feed location.
Arranging multiple feed locations to a single distillation column is one of the solutions for a column in which feed compositions are frequently varied. Actual feed location can be switched to suit varying feed composition. Switching methods can be arranged either by manual block valves or automatic control valves. This design is usually feasible in a trayed distillation column. It is difficult to arrange multiple feed locations in most packed column cases unless an extra bed is inserted to allow an alternate bed.
Feed temperature is one of the major factors in influencing the overall heat balance of a distillation column system. Increased feed enthalpy can help reduce the required energy input from the reboiler at the same degree of separation. Installing a feed pre-heater is a very common process option to minimize reboiler heat duty. If the feed preheater can be integrated with other valuable process streams (as a heating medium), overall energy efficiency of the distillation system can be improved further. A simple heat integration in the distillation block is heating feed streams using the bottom product.
However, increasing feed temperature does not always improve the overall energy efficiency of a distillation unit. Excessive feed temperature increments can cause a significant amount of flash of heavy key and heavy non-key components at the distillation column feed zone. In this case, a higher amount of reflux stream is necessary to maintain required overhead distillate purities. This augmented reflux ratio thus requires a higher boil-up ratio. Overall energy efficiency is eventually aggravated.3 Therefore, carefully reviewing feed temperature and phase is critical to minimize the distillation units overall energy consumption. Identifying feed condition at a reliably measured feed temperature should be conducted to evaluate optimum feed conditions. Sensitivity analysis is a required step as well.
Internal feed-distributor design.
The importance of the internal feed distributor cannot be ignored in distillation column feeding optimization. The equilibrium stage basis simulator predicts distillation column performance based on ideal mixing between feed and column internal traffic. This means that the feed distribution quality is not reflected in the equilibrium stage calculation result. Therefore, feed distribution quality should be evaluated separately to ensure that simulated distillation column performance predictions are met.
In real distillation column operation, feed distribution quality influences overall performance. Non-optimized feed fluid distribution can cause non-uniform concentration across a distillation column cross-sectional area and result in downgrading performance. The importance of feed distribution is emphasized when a distillation column has multi-pass trays and/or packing. In multi-pass trays, it is essential that uniform internal liquid-to-vapor (L/V) ratio in each section shall be maintained.4 Also, the feed distribution ratio shall be matched to the tray L/V ratio as closely as possible for best performance. Otherwise, poor feed distribution can cause an imbalanced L/V ratio in each pass. In an imbalanced L/V ratio, the reduction of tray efficiency can be observed and non-uniform froth height generation can reduce overall column capacity.
Non-optimum tray layout selection can increase the difficulty of feed distribution. A typical example is when the distillation columns top tray is configured with two off-center positioned downcomers in four-pass-tray geometry. In this geometry, an internal feed/reflux distributor has to be fed to one center and two side inlets. With a conventional distributor design, this almost guarantees that the split of liquid flow was not proportional to the vapor from each pass. Poor feed/reflux distribution to the tray inlet panel causes an uneven L/V ratio in each active area.
If the top tray is converted to the tray with two sides and one center downcomers, feed/reflux distribution problems can be resolved. However, this conversion requires complete layout changes of all trays in the column, which may not be feasible in a revamp solution. Since it requires high modification costs, it is not a feasible solution in most cases.
Alternatively, this feed maldistribution can be fixed by an enhanced feed distributor design. Installing controlling orifices can split liquid flow to match the tray pass liquid ratio. Fig. 1 displays the feed distributor for four-pass trays with two off-center downcomers. Since controlling orifices are positioned inside distributor arms, these features are not revealed in Fig. 1. A drawback of this design is creating an additional pressure drop through the feed/reflux distributor, thus influencing pipeline hydraulics. Therefore, it is necessary to check whether a sufficient driving force is still maintained at a higher feed distributor pressure-drop scenario.
| Fig. 1. Feed distributor and four-pass top tray |
with two off-center downcomers.
A packed column is more vulnerable than a trayed column with regard to feed distribution. Like multi-pass trays, a trough-style liquid distributor with multiple primary troughs (parting boxes) requires uniform incoming liquid distributions to multiple primary troughs. Otherwise, uniform liquid distribution cannot be maintained at the liquid distributor component irrigating the next packed bed.
Proper internal feed distributor sizing is also necessary to achieve uniform feed distribution. High feed fluid velocity can allow more fluid flow at the end of the distributor. Pressure drop among the entry header, lateral arm and discharge hole needs to be optimized. It is usually recommended that progressive pressure drop increments through the feed distributor provide adequate feed distribution. In case of a packed tower trough-style distributor, high inlet liquid velocity can cause liquid splashing, allowing liquid flows outside the primary trough(s) of the gravity distributor. Slightly submerged guide-tube installation with properly sized discharge holes can help prevent liquid splashing.
External feed-line configuration.
Inappropriate external feed configurations also influence distillation column performance. If feed flow is split and introduced to the distillation column through multiple locations, all multiple-branch pipes shall be symmetrical. Otherwise, feed flows are not introduced in uniform manners and column performance is influenced. Unless external feed-line balancing is strongly specified in process design materials, such as piping and instrumentation diagrams (P&IDs) and process data sheets, this line balancing may be overlooked during the final piping design.
External feed-line geometry is also a critical issue when two-phase fluid is formed in the feed stream line. Certain distillation columns, such as refinery multi-product fractionators, are inherently designed with two-phase feed conditions. Quite a few energy saving projects specify two-phase feed conditions.
As is well-known in the industry, a two-phase feed condition necessitates more complex design steps than a single-phase feed condition. Undesirable two-phase flow patterns are prone to causing unit troubles including entrainment, flow instability, temperature and/or pressure fluctuation, hammering and pipe or equipment erosion.5 It is generally known that a slug flow regime should be avoided at two-phase feed inlets. A highly aerated slug can act like frothy surge and can cause not only column instability but also severe hydraulic pounding and distillation equipment damage.6 For a particular vacuum tower inlet, a mist flow regime can create liquid entrainment in the flash zone.7 At high two-phase flow velocities, most of the liquid components are turned to mist droplets and distributed into the vapor phase. This liquid mist is prone to be entrained in the vacuum tower flash zone and impacts distillate product yield and/or qualities.
Understanding two-phase flow patterns and proper piping geometry is required to avoid unit troubles. However, it is very difficult to identify two-phase flow patterns in an accurate manner. There is no reliable calculation method to predict void fractions for pressure drop and fluid residence time yet. Due to a lack of a universal model, two-phase flow pattern prediction is varied upon the model chosen. Graphical and empirical methods are widely used as well. Design practices regarding piping geometry and fitting methods are available in the industry.
The case study demonstrates how column performance is improved by distillation column feeding optimization. The solvent-recovery column function is to separate water and impurities from spent hydrocarbon solvent and to supply regenerated solvent back to the main process unit. Using contaminated solvent as the feed stream, a reflux stream combines with the feed line and is pre-heated prior to entering the column at one single point located at the columns top. Water and other impurities are stripped from the hydrocarbon solvent, and purified solvent is produced at the bottom of the column and recycled back to the main process unit. The original solvent-recovery column configuration is described in Fig. 2. As capacity of the main process unit is expanded, the capacity of the solvent-recovery column should also be increased to supply enough solvent to the main process unit. However, the solvent columns maximum capacity was lower than the required capacity.
| Fig. 2. Original process configuration. |
An engineering firm that conducted the main process unit expansion work originally evaluated the solvent recovery column capacity expansion. The expansion study concluded that the existing column diameter was not large enough to handle the required column internal traffic and implementing a larger diameter column with other periphery equipment modifications was suggested. Since this modification plan required a high capital expenditure and a long shutdown period, this modification scenario was not accepted in the given overall project schedule and budget. A more feasible modification scenario was desired to meet the project schedule as well as the performance targets.
An alternative set of unit evaluation and optimization studies was conducted by another party. The evaluation started with the identification of the root cause, which is an essential step for unit optimization. Operating condition data as well as design documents and materials, were gathered. A preliminary study was done based on operating conditions. The study revealed that the maximum design and simulated duties of the feed pre-heaters were not matched in a reasonable manner. Simulated pre-heater duties at measured feed temperatures were much higher than the maximum calculated heat exchangers duties. Meanwhile, all design material data were consistent: P&ID and equipment data sheets showed the same heat-exchanger duties. To confirm the feed pre-heating system design, a field survey was conducted and compared with the design materials. Surprisingly, the field survey disclosed that all design materials were not updated to the real equipment configurations. There were five heat exchangers positioned in the actual feed pre-heating circuit, while the P&ID only showed three exchangers. Through clarifications with the technical/operating staffs, it was found that two more heat exchangers were added to increase feed temperature, but design document materials were not updated. The belief was that higher feed temperature always improves the energy consumption in a distillation unit.
To evaluate the column performance in detail, rigorous simulation modeling was conducted based on operating conditions. The purpose of modeling was to identify the bottleneck point and to construct a base model for revamping. Obtaining pertinent operating data is necessary for reliable simulation modeling. A review of operating data showed that daily operating conditions were not suitable for simulation modeling. As measured stream volumetric flowrates were not standardized, mass-balance closure could not be reviewed. Also, gathered temperature and pressure data were not consistent. A dedicated test run was required to gather reliable operating data. A set of operating data was obtained at the rate just before the maximum operating point in a snap-shot basis. Overall mass and component balance closure data were compiled. Each instrument position was checked and confirmed through a field survey.8
Before performing rigorous simulation modeling, the thermodynamic package was reviewed in the selected commercial process simulator. It was found that the binary interaction parameter and alpha functions were not available between key components in the selected thermodynamic packages data base. In this case, most of the commercial simulators automatically choose the ideal gas law between components and report results. It is difficult to recognize this assumption unless detailed thermodynamic parameters are reviewed among components. For reliable simulation modeling, binary interaction parameters and alpha functions that were regressed through experiment data were applied in the selected liquid activity coefficient model. Through various sensitivity analyses, tray efficiency of the column was quantified.
Rigorous simulation results indicated that two-phase feed was formed at the operating feed temperature. This feed temperature was much higher than the original design feed temperature. The increment of feed temperature was intended to maximize feed preheater duty and reduce overall energy consumption. However, undesirable two-phase feed caused an excessive hydrocarbon solvent amount in the column overhead system. To maintain product purity specifications of the bottom product, the reflux rate needed to be increased. A higher reflux rate generated more vapor/liquid traffic inside the column and limited column capacity. To destroy this vicious cycle, the feed temperature should be decreased to maintain the liquid feed.
A case study was conducted to check whether the existing feed point was optimum. Required reflux rates were simulated with various feed points at the same bottom solvent purity. These results are displayed in Fig. 3. This figure shows that adding a rectification section helps to minimize the reflux rate at the same product purity and the feeding at Stage 3 shows the minimum reflux rate. Partial key ratio plots are highlighted among stages 1 and 5. Fig. 4 shows key ratio plot changes as the feed point is elevated down. Monitoring key ratio behavior with various feed points shows that retrograde distillation was observed at the Stage 5 feeding.
| Fig. 3. Reflux rate vs. feed stage at same |
solvent product purity.
| Fig. 4. Key ratio plot. |
The solvent recovery unit was modified according to the feed-optimization study. The feed tray was relocated from the top tray to the tray matching the third theoretical stage. The reflux stream was no longer combined with the feed stream and was introduced to the top tray independently. The feed temperature was reduced to maintain liquid-phase feed condition. New feed temperature was set at a temperature slightly lower than the simulated bubble-point temperature. In spite of the slightly sub-cooled feed, this temperature maintains a stable liquid-phase feeding against a minor feed composition variation. The internal feed distributor was designed and installed at the new feed location. Since the feed stream contains some fouled materials, the discharge hole size was optimized to prevent potential plugging. It was also found that the control valve flow was reaching the critical flow zone. A new control valve was installed to prevent the choked flow. Modified process schemes are depicted in Fig. 5.
| Fig. 5. Modified process configuration. |
Table 1 summarizes the solvent columns pre- and post-modification operating data. The maximum feed rate of the column is increased by 35%. The metered reflux rate is significantly improved without sacrificing bottom product purity. The new reflux ratio is only 52% of the previous amount. The reboiler heating medium (steam) consumption per feed is reduced by 12%. HP
1 Lee, S. H., et al, Optimizing crude unit designs, Petroleum Technology Quarterly, 2Q, 2009.
2 Kister, H. Z., Distillation design, McGraw-Hill Company, 1992.
3 Lee, S. H., et al., Minimizing energy consumption in distillation units AIChE Spring national meeting, April 2009.
4 Bolles, W. L., Multipass flow distribution and Mass Transfer Efficiency for Distillation Plates, AIChE Journal, Vol. 22, No. 1, January 1976.
5 Daniels, L., Dealing with two-phase flows, Chemical Engineering, June 1995.
6 Kister, H. Z., Distillation operation, McGraw-Hill Company, 1990.
7 DeGance, A. E., et al, Chemical Engineering Aspects of Two-Phase Flow, Chemical Engineering, March 1970.
8 Kister, H., et al., Sensitivity analysis is key to successful DC5 simulation, Hydrocarbon Processing, October 1998.
|The author |
||Soun Ho Lee is the manager of refining application for GTC Technology US LLC in Irving, Texas, and specializes in process design, simulation modeling, distillation equipment design and field troubleshooting for refining and aromatic applications. Mr. Lee holds a BS degree in chemical engineering from Sogang University, Korea. |
||Michael J. Binkley is manager of product development for the GTC Process Equipment Technology (PET) group in Irving, Texas. He is a registered professional engineer in Texas with 42 years of experience in mass transfer and separations equipment development and applications. Mr. Binkley has invented several separations equipment advancement-related patents, as well as numerous product trademarks. He earned a BS degree in chemical engineering from Texas Tech University. |