C₃ splitter revamp case study emphasizes importance of balancing in four-pass trays to achieve separation efficiency and capacity

S. Krishnamoorthy, Sulzer Chemtech USA Inc., Tulsa, Oklahoma; S. LATURKAR, Sulzer India Pvt. Ltd., Pune, India; V. KALE, Sulzer Chemtech Middle East W.L.L., Manama, Bahrain; D. R. SUMMERS, Sulzer Chemtech USA Inc. (ret.), Tulsa, Oklahoma; D. SHAHARE and M. A. KHAWDAH, Saudi Kayan, SABIC, Al-Jubail, Saudi Arabia; M. A. AL-SEKHAN, M. KUMAR S. and M. AL GHAMDI, SABIC, Al-Jubail, Saudi Arabia; and A. MERENOV, SABIC, Sugar Land, Texas

The C₃ splitter in an olefins plant is one of the most important and critical pieces of equipment: it separates polymer-grade propylene from propane. The relative volatility between propylene and propane is so low that it requires a significantly large number of trays to achieve the desired specification and product recovery. Saudi Kayan had been operating a C₃ splitter with two physical columns, C-16040A and C-16040B (FIGS. 1 and 2), each equipped with 100 four-pass trays. The column overhead is coupled with a heat pump system to generate the necessary reflux and vapor load from the reboiler. The trays operated satisfactorily as long as the tower was not pushed near its original design capacity. When Saudi Kayan tried to increase the feed to the C₃ splitter tower’s capacity, propylene losses from the bottom of the tower were so high that the loss of production went well beyond acceptable limits. Subsequently, this was followed by tower flooding that affected stable column operation. Saudi Kayan wanted to address the problem of the poor tray efficiency as well as increase the capacity of the column to handle higher throughput, considering future expansion.

Operating condition before revamp. The authors’ companies worked together to evaluate the performance of the column. Sulzer performed several analyses based on various sets of operating data collected during 2018–2019. The operating data collected in June 2019 will be presented in this paper. This data was used as a basis for the revamp of this column. The feed flowrate to this column was not measured. The top and bottom product flowrates were used to back-calculate the feed flowrate and feed composition. The overall mass balance before the revamp is presented in TABLE 1. Note: All flowrates in TABLES 1–5 have been normalized to match 1,000 kg/hr as the feed flow to the plant before revamp. This normalization was enacted to protect confidential operating data.

The column was simulated based on Sulzer’s proprietary vapor-liquid equilibrium (VLE) and enthalpy model to generate the internal vapor and liquid loads. Sulzer then performed the tray’s hydraulics rating based on the existing geometry of the trays. It was noticed that the four-pass trays were never balanced to regulate the flow of liquid from the off-center downcomer (OCDC). A typical arrangement of a four-pass tray is presented in FIG. 3. The OCDC downcomer provides the liquid to both the side downcomer (SDC) and center downcomer (CDC) on the tray below. Based on the geometry using an equal flow path length type design, the active area of Panel B is larger than Panel A. However, the weir length of the CDC is proportionally much larger when compared to the SDC. Therefore, Panel B was getting more liquid than Panel A. Subsequently, the froth height and pressure drop of Panel B is larger than Panel A. In this situation, more vapor will tend to go to Panel A, where there is the least resistance and pressure drop. Ultimately the vapor/liquid ratio on these two tray panels will become unequal.¹

Based on Sulzer’s calculations, it was found that the vapor/liquid (V/L) ratio in the tray panels were 1.35 and 0.8, respectively. The V/L ratio is a measure of the relative amount of vapor and liquid on each tray panel. This means that Panel B of the four-pass tray panels was getting very high liquid compared to Panel A. Generally, the vapor should distribute proportionately to each tray panel as per the active area; however, the liquid presents several variables (weir length, weir height, downcomer clearance area) that must be satisfied to allow it to match the vapor distribution to each tray panel (and enable a V/L ratio of 1.0 on each tray panel).

The unbalanced design of these trays contributed to a very high degree of mal-distribution inside this column. Subsequently, this contributed to lower tray efficiency and a higher than anticipated reflux ratio to achieve the desired product purities. Ultimately, this efficiency reduction negatively affected the tower’s overall capacity. Overall tower tray efficiencies were estimated to be at about 60% of design operating conditions. The polymer-grade propylene purity cannot be compromised. The only option is to lose propylene in the bottom product and keep the column in operation.

To reduce the propylene losses in the bottom product, Saudi Kayan tried to further increase the reflux to the column. The column overhead vapor was compressed in the heat pump system to heat the reboiler. The maximum reflux was limited by the capacity of the compressor. The reflux flow to column was increased until the compressor was operating at its maximum limit. This can be seen in the compressor curve (FIG. 4), where the operating point before revamp is indicated.

Based on this curve, it is clearly understood that the operating point before the revamp was outside the standard operating curve. Therefore, no further reflux was available in the system without revamping or getting a new compressor, which is a major capital expense. At this point, Saudi Kayan decided to focus on the column internals rather than increasing the compressor capacity.

Several revamp ideas were considered, including performing a partial revamp and fixing the most critical section of the tower to improve tray efficiency. A partial revamp, however, would leave unbalanced trays in some sections of the tower, which would continue to suffer poor tray efficiency. Considering future expansion requirements, it was decided to conduct a complete revamp of the full C₃ splitter tower.

Revamp proposal. In 2019, Saudi Kayan was interested in not only achieving the original design capacity, but also wanted to de-bottleneck the entire tower to obtain additional capacity over the original design.

Before the revamp, the column was operating with full compressor capacity in the integrated heat pump system. However, the heat pump system was unable to provide any additional reflux. As a result of this, the plant was unable to achieve design propylene purity specification. Considering overall economics, it was decided that revamping the compressor was an infeasible option to enhance capacity. Therefore, revamping the tower with high-capacity and high-efficiency trays was pursued with Sulzer, which decided to focus on all aspects of the new tray’s design to achieve both the column capacity and higher efficiency. The proposed new trays should be able to work at much higher tray efficiency to reduce propylene slippage, and should have additional spare capacity to cater to future requirements. Sulzer observed that it would be difficult to use any conventional tray technology to achieve higher efficiency with higher capacity. Based on detailed assessment of Saudi Kayan’s requirements and current tower operations, it was decided to use high-capacity the company’s proprietary downcomer trays^a trays with its proprietary valves^b and enhanced downcomers technology to achieve higher efficiency and capacity (FIGS. 5 and 6).

The proprietary valve^b releases the vapor laterally onto the tray deck, which allows the liquid to flow without obstruction. This helps to reduce the liquid gradient along the flow path length, reduces the vertical jetting and minimizes the entrainment to increase the deck capacity, which is critical to meet future capacity requirements. The company’s valves are punched out of the tray deck and have no moving parts, providing longer life without any wear or tear. The downcomers for the proposed new tray design need to handle the increased liquid load. The downcomer velocity was very high at 0.102 m/sec, as shown in TABLE 3.²

In a four-pass tray, the side downcomer is typically limited by high weir loading. This is a geometrical limitation when compared to the weir length of center and off-center downcomers. It was proposed to use proprietary type side downcomers^c, which increases outlet weir length and reduces the weir loading. To improve the tray efficiency, push valves were employed to minimize the liquid gradient on the relatively long flow path lengths. These devices also improve liquid plug flow, enhance liquid aeration, eliminate any vapor cross flow channeling, and improve the mass transfer efficiency.^3,4 Sulzer put together an aggressive design that maximized both capacity and tray efficiency with a fully balanced four-pass tray design.

Turnaround installation and operating data after revamp. Sulzer executed all installation activities on a turnkey basis, including labor, consumables, crane, scaffolding, power generators, ventilation, air compressors, forklift, trailers, etc. The company also complied with all necessary labor safety training and certification to meet SABIC Safety, Security, Health and Environmental Management Standards (SHEMS) procedures. The installation was carried out during the peak of the COVID-19 pandemic. There was a resource crunch for both labor and material due to travel restrictions. Additionally, when installation work was in progress, onsite personnel were faced with very high winds for several days. This high wind reduced the installer’s ability of crane movement and work at elevated locations inside the tower.

The project team was also impacted significantly due to the COVID-19 pandemic. Constant video surveillance monitored all activities inside the column and resources were expedited whenever necessary. Shutdown was scheduled for 25 d and the Sulzer installation crew managed to complete the installation safely and successfully within the schedule.

The plant restarted after the revamp in April 2021. Following plant stabilization, Sulzer analyzed the operating data over a duration of 2 mos and examined the detailed process trends such as flowrate, pressure and temperature. A 24-hr operating window was chosen in May 2021. This set of data showed stable operating conditions and was suitable for the plant’s performance evaluation. The laboratory analysis for the top and bottom product purity was also available during this period. As mentioned earlier, the feed flowrate was not measured, but was calculated based on the top and bottom product flow streams. The composition of the feed was also calculated based on the mass balance of the product streams.

The operating data presented in TABLE 4 was used as a basis for the simulation to analyze the operating performance of the column. The reflux ratio is the reflux flowrate to the column before flash over the distillate flowrate. The number of theoretical stages in the simulation was adjusted to match the measured reflux flowrate. The C₃ splitter is designed with two physical columns and the plant instrumentation can measure the reflux fed into both the upper and lower columns. To validate the calculated tray efficiency, the simulated results were verified based on reflux flow measurements from both towers. Based on a rigorous plant simulation model and validation of the operating data, it was confirmed that a tray efficiency of > 95% was achieved for the new trays^a in this C₃ splitter using the authors’ company’s proprietary VLE and enthalpy model. The vapor and liquid internal loads from the simulation were used to perform hydraulic ratings for the trays. The results of the evaluation are presented in TABLE 5. FIGS. 7, 8 and 9 represent the impact of the tray revamp on key process trends like steam consumption in the heat pump turbines, and propylene losses in the bottom of the C₃ splitter. As shown in these figures, due to the improved efficiency of the revamped trays, the overall steam consumption, propylene losses and reflux flow have decreased significantly, reiterating the increased tray efficiency.

Takeaways. This paper illustrates a successful revamp of a C₃ splitter that was installed safely within a plant’s narrow turnaround window during the global COVID-19 pandemic. The revamp of the Saudi Kayan C₃ splitter, with proprietary trays^a, achieved > 95% efficiency, which was higher than the target requirement for this revamp. Subsequently, it also helped to reduce propylene losses in the bottom product. One of the main reasons for the initial low tray efficiency of the original trays was the employment of an unbalanced four-pass tray design that was expected to operate very near to its maximum capacity.

Following the tower revamp, the reflux ratio required to minimize product losses and achieve the desired product quality was reduced by ~25%. Because of this, the heat pump compressor’s duty has been significantly reduced, which translated to lower steam consumption in the turbines. The plant is presently showing stable operation at design capacity without any challenges. Based on a tower hydraulics assessment using the operating data after the revamp, it was confirmed that the C₃ splitter equipped with the authors’ company’s trays^a has a margin to accommodate more than 25% additional feed capacity. HP

ACKNOWLEDGEMENTS

The authors would like to thank Saudi Kayan and Sulzer for supporting the publication of this article.

NOTES

^a Sulzer Chemtech Ltd.’s VGPlus™ trays

^b Sulzer Chemtech Ltd.’s MMVG™ valves

^c Sulzer Chemtech Ltd.’s ModArc™ downcomers

LITERATURE CITED

1. Summers, D. R., “Designing four-pass trays,” Chemical Engineering Progress, April 2010.
2. Jenkins, S., “Facts at your fingertips: Distillation trays and packing,” Chemical Engineering, June 2010.
3. Summers, D. R., "Push valve experience on distillation trays," AIChE 2005 Spring Meeting, Atlanta, Georgia, April 12, 2005.
4. Summers, D. R. and D. Ehmann, "Enhanced V-grid trays increase column performance," AIChE Annual Meeting, Indianapolis, Indiana, November 4, 2002.

SENTHIL KRISHNAMOORTHY is a Key Application Specialist for Sulzer Chemtech USA. With more than 20 yr of experience, he is responsible for column design in major olefins and styrene projects around the globe. He has published several technical articles on distillation column design and troubleshooting and holds one European patent for column internals. He has served as a member of the FRI Design Practices Committee since 2020. Krishnamoorthy received an MS degree in chemical engineering from the Regional Engineering College in India.

SHASHANK LATURKAR is an Application Manager for Sulzer India.

VINIT KALE is Head of the Sales downstream business at Sulzer Chemtech and has 18 yr of experience in refinery, petrochemical and specialty chemical processing. He holds a BS degree in chemical engineering from India and has published five technical papers.

Until January 2021, DANIEL R. SUMMERS served as the Tray Technology Manager for Sulzer Chemtech. He was a 1977 graduate in chemical engineering from the State University of New York (SUNY) at Buffalo, New York, and also worked for Union Carbide, UOP, Stone & Webster, and Nutter Engineering. His entire career has been focused on distillation. Summers is the author of more than 70 papers and is a listed inventor on three U.S. patents. He now works as a consultant for Fractionation Research Inc.’s (FRI’s) Design Practices Committee and was the Chair of that committee for 12 yr. Additionally, he serves as a Director of AIChE’s Separations Division, is a Fellow of AIChE and is a registered Professional Engineer in New York and Oklahoma. Summers is also the recipient of the 2016 AIChE Gerhold Award for outstanding work in chemical separations technology.

DIPAK SHAHARE is a highly experienced process engineer with more than 18 yr of experience in the petrochemical industry. Employed by Saudi Kayan, a SABIC affiliate, for the past 8 yr, Shahare also spent a decade working for Reliance Industries Ltd. in India. He has significant expertise in managing olefins, benzene, utilities processes and energy management. He holds a degree in chemical engineering from Sardar Vallabhbhai National Institute of Technology, Surat, India, and is also a certified Energy Auditor by the Bureau Of Energy Efficiency (BEE), India.

MOHAMMAD A. KHAWDAH is the Process Engineering Department Senior Manager at Saudi Kayan Petrochemical Complex, a SABIC affiliate company. He earned a BS degree in chemical engineering from King Fahd University of Petroleum and Minerals and has more than 13 yr of international technical experience working with a world-leading technology licensor in the oil and gas industry specializing in process technology licensing, catalysts and adsorbents manufacturing. Khawdah has served in several technical positions as Technical Advisor, engineering technology Superintendent, and process engineering and operation Manager for pre-commissioning and commissioning activities of mega-project startup and operational technical support for a number of refineries, petrochemicals and gas processing plants worldwide.

MOHAMMAD ALI AL-SEKHAN is responsible for global cracker technology at SABIC.

MAHESH KUMAR S is the Chief Scientist with SABIC and has 19 yr of experience in the petrochemical industry with a focus on identifying areas for improvement resulting in feedstock maximization, asset utilization, yield Improvement and loss minimization. Key aspects of his work include working with an interdisciplinary team to address process issues in crackers and to identify new opportunities and implement cost-effective solutions. He has published more than 15 papers in peer reviewed journals and has filed and been granted five international patents.

MISFER AL GHAMDI is responsible for process safety and process risk management at SABIC.

DR. ANDREI MERENOV is the Chief Scientist with SABIC.