August 2022

Special Focus: Refining Technology

Column revamp using packings and internal modeling and revamping technology

The challenge of frequent plugging in vent gas scrubbers is resolved with the highest degree of maleic anhydride (MAN) product recovery.

Shirpurkar, V., Sahara International Petrochemical Co.; Arora, A., Kale, V., Sulzer Chemtech Middle East WLL

The challenge of frequent plugging in vent gas scrubbers is resolved with the highest degree of maleic anhydride (MAN) product recovery. This article describes the details of a problem in a packed bed column, the troubleshooting approach, an overall revamp study, features of the packing products supplied by the author’s company and the benefits achieved by the authors’ company.

The vent gas scrubber is a packed bed column equipped with a conventional structure packing bed. The column operates at a moderately elevated temperature and a pressure of 25 mbara. The lean oil solvent is fed to the top of the vent scrubber to absorb the MAN and the non-condensable vapor from the vent scrubber passes to the vent header. A rich oil stream from the bottom of the vent scrubber is sent to another column to recover the MAN.

A generalized schematic simulation scheme showing the packed bed arrangement is illustrated in FIG. 1.

FIG. 1. Process flow diagram. Source: AspenTech.
FIG. 1. Process flow diagram. Source: AspenTech.

The vent gas scrubber was experiencing frequent plugging of the column internals. At the start of operation, the column pressure drop was 5 mbar, but increased to as high as 20 mbar within 3 mos–4 mos. Due to the process nature of the application, deposits were detected on the packing during plant operation. This resulted in the plugging of the packed bed and eventually required column packing washing once every 3 mos–4 mos.

A simulation was performed for the column, and vapor liquid traffic hydraulics results were shared with the author’s company, which validated the hydraulics calculation and proposed a next-generation proprietary structured packinga and a high-performance proprietary distributorb to resolve the problem.

Historic plant data

The authors’ companies studied the actual column pressure drop and plant data to analyze and understand the problem. From plant operation data, it was observed that the increase in pressure drop across the column was gradual; within 3 mos of operation, the column required solvent washing to remove the plugging material from the bed. The packing and internals were inspected during a scheduled shutdown to determine the reason for this high pressure drop. The internals were found to be intact, but severe fouling was seen by strong disposition of acidic materials due to the nature of the process.

The hydrolysis of MAN by water forms maleic acid as part of the process, and maleic acid is thermally converted to fumaric acid. The formation of fumaric acid accounts for most difficulties in the column high pressure drop due to blockage of the equipment and surface fouling.

The use of computational fluid dynamics (CFD) revealed that the shape of the existing packing type had supported the accumulation of liquid/solids at the junction of two subsequent structure packing layers due to the abrupt change of direction, which is detrimental for lower pressure drop.

FIG. 2 shows the existing packing pattern causing material accumulation at the packing junction. The effect of pressure drop on MAN recovery is directly proportional. The column operational trendline clearly specified that a low column pressure drop provides improved MAN recovery.

FIG. 2. CFD revealed that the existing packing shape supported the accumulation of liquid/solids at the junction of two subsequent structure packing layers.
FIG. 2. CFD revealed that the existing packing shape supported the accumulation of liquid/solids at the junction of two subsequent structure packing layers.


In any unit operation, future modifications without a basic engineering study are a risky adventure. Simulation provided by an industry leader was used due to the company’s widespread use of 40-yr databases of tuned models with specific vapor-liquid equilibrium (VLE) packages. These databases are important when designing distillation columns and are one of the key reasons that many distillation experts use the process simulation softwarec. In this case, a rigorous distillation calculation was performed on the existing column using the actual temperature, pressures, compositions, etc., of the column to accurately match the original heat and material balance and tune the steady-state model.

Rating the existing column

With the two tuned models, the column was rated for the new conditions. The rating determined that the existing equipment could be used in the new hydraulics operation. The hydraulic calculations were then evaluated, and the column capacity was reviewed using an analysis of simulated internal vapor and liquid flows. With the original heat and material balances, the VLE data and packed column efficiencies were checked. The existing vent gas column was rated for the correct number of transit units (NTS), reflux-to-stages ratio and packing hydraulics.

A plant match case simulation was performed with the objective of determining whether lower surface area packing should be used to improve column run length. However, the simulation showed significant loss of MAN from the top due to reduced NTS. Therefore, it was decided to not use the lower surface area packing.

Furthermore, from the author’s company’s hydraulics calculation of equivalent existing packing, it was observed that the existing packing bottom section was operating near to flood (79%) in a clean condition. This means that as soon as fouling starts in the packing, the surface area and overall cross-sectional area of the packing are reduced to the extent of the fouling, resulting in the packed bed operating under flood condition. As soon as the packing flooded, the pressure drop across the packing was increased exponentially.

Application of structured packinga

To resolve the structure packing capacity limitation and to reduce the overall pressure drop across the bed, the author’s company checked the hydraulics of its latest generation structured packinga. The pressure drop across the packing is much lower and the maximum capacity can be extended up to 20%–30%. The new style of packing incorporates a patented modification to the lower and upper end of each packing element. The new packing’s internals were verified with various operational variables/changes to meet the desired targets of the highest degree of MAN recovery by 6.5 wt%. During simulation, various options (pressure changes, temperature reflux rate changes, optimizing the feed, etc.) were studied.

For the revamp, different options were also studied, including random packings. The pros and cons of each option were identified and compared; in this service, however, random packing seems inapplicable due to the increased settlement of fouling materials and eventual high pressure drop caused by liquid hold-up in the random packings.

Most column revamps necessitate the evaluation and modification of ancillary equipment. However, associated equipment such as condensers, receivers, pumps, valves and instrumentation may remain as they are. With the latest generation structured packinga with enhanced geometric structure, the pressure drop across the packing is much lower and the maximum capacity can be extended up to 20% compared to conventional structure packing in a simulation study. The new packing has a smooth curved shape at the junction of two packing layers, lowering the accumulation of liquid/solid at this point and resulting in high packing capacity. The corrugation angle in relation to the vertical is gradually reduced to zero at both ends of each sheet, as seen in FIG. 3.

FIG. 3. The flow pattern of the proposed proprietary structured packing<sup>a</sup>.
FIG. 3. The flow pattern of the proposed proprietary structured packinga.

This design modification of the corrugation angle causes smooth and steady changes of flow direction, as opposed to abrupt direction changes found in conventional structure packing. The result is a reduction in pressure drop and reduced shear force between the gas and liquid phase, and a reduction of gas velocity.

Hydraulics calculations of a packed bed based on plant match conditions showed that the proprietary structured packinga operated at 62.7% of its capacity compared to 79.4% of the existing packing (providing an 16.7% additional capacity). This indicates better column performance with the new packing if the “exponential increase in pressure drop of the existing packing” could be resolved. TABLE 1 shows capacity and pressure drop values for the existing packing and the new structured packinga from hydraulic calculations.

Replacement of existing liquid distributor

Precise liquid distribution is crucial for the performance of packed beds, and distributor selection is vital to achieve maximum packing performance—even the best packing will never provide its full performance if the related internals are not designed appropriately. In a simulation study, the authors’ company noticed that the existing liquid distributor drip point density was limited for distribution. The existing liquid distributor was replaced with a high-performance liquid distributorb (with a drip point density of 116 holes/m2) to improve liquid distribution. In turn, this provided better performance in product recovery and delayed plugging problems.

Revamp results

The replacement of the entire packed bed internals and distributor was conducted without any hot work. On startup, the pressure drop across the packing was reduced. The column has been in operation for 8 mos without any operational problems compared to the cleaning/washing cycle required every 3 mos before the revamp.

The project shows how proper cooperation between the authors’ companies resulted in successful troubleshooting of a long-prevailing problem. The revamp of the column provided energy savings and increased product yield. MAN recovery has been improved compared to the old packing (FIG. 4).

FIG. 4. Pressure drop and MAN recovery post revamp.
FIG. 4. Pressure drop and MAN recovery post revamp.

This successful revamp illustrates how unexpected downtime for packing washing—a costly operation for any plant operation in the oil and gas industries—can be avoided. The project achieved numerous benefits with a lucrative payback period of 0.5 mos. HP


   a  Sulzer’s MellapakPlus™

   b  Sulzer’s MellaTech™

   c  AspenTech’s Aspen Plus


The case study presented here was performed by Sahara International Petrochemical Company (Sipchem) Saudi Arabia. Sulzer extends its thanks to Ali S. Al-Wadie (Process Engineer and Control System Manager) for his encouragement and engineering approval for this project, and Mohammad M. Al-Zahrani (Operation Manager) for his professionalism, approval and coordination provided within the team during project execution.


   1  Gmehling, J., et al., “Chemical thermodynamics for process simulation,” 2nd Ed., Wiley-VCH, Germany, June 2019.

The Authors

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