February 2022

Process Optimization

Debottlenecking of an extractive distillation column for aromatics recovery: Effect of feed pre-vaporization

Extractive distillation is one of the most efficient techniques to separate aromatic and non-aromatic hydrocarbons by increasing the relative volatility of the mixture in the presence of a solvent.

Extractive distillation is one of the most efficient techniques to separate aromatic and non-aromatic hydrocarbons by increasing the relative volatility of the mixture in the presence of a solvent. Extractive distillation is attractive because of its low capital and operating costs, operational ease and flexibility. It has many applications within the refining and petrochemical industries for the recovery of high-purity products, including benzene, toluene and xylene (BTX), butadiene, isoprene and styrene. In the hydrocarbon processing industry, it is commonly used for treating light reformate in refineries. The performance of the extractive distillation column can be restricted for several reasons, ranging from operational to mechanical factors. This article details a case study of high pressure drop observed across the extractive distillation column, the operational reasons for it and a cost-effective solution to this challenge. A significant increase in column pressure drop can lead to flooding. The loss of stages due to flooding adversely affects separation and product quality.¹

FIG. 1 shows a simplified schematic flowsheet for an extractive distillation unit separating aromatics from the hydrocarbon stream. The non-aromatics exit as raffinate from the top of the extractive distillation column. The solvent and aromatics phases are routed to the solvent recovery column, where solvent is separated from aromatics by conventional distillation. The lean solvent reenters the extractive distillation column by exchanging heat with the feed in the preheater.

The operation of the extractive distillation column was constrained due to high pressure drop, especially in the bottom section below the feed tray. To gain insights into the problem, the unit was first simulated using a commercial process simulation software, and the preheat train was simulated using a commercial heat exchanger simulation software.

The issue of high pressure drop was addressed by increasing feed pre-vaporization. Many studies have discussed the impact of feed vaporization on column reboiler duty. However, this study focuses on the impact of extractive distillation feed vaporization on various column parameters, including pressure drop, reboiler duty and aromatics separation efficacy. The process simulation results for this case study showed that increasing the feed pre-vaporization can help to reduce pressure drop across the extractive distillation column, thus saving the reboiler duty without significantly affecting aromatics recovery.

This simple philosophy of increasing feed pre-vaporization to tackle the challenge of high pressure drop in columns can be tested in similar plant problems in other operating plants, considering all the other constraints present in this respective operating plant, some of which are conveyed in this article.

Ideology

The adequacy of the aromatics extractive distillation column is limited by high pressure drop. If altering process parameters does not help, then this issue can be addressed by changing the column internals, which may not always be an economically viable solution. One of the solutions could be to increase the feed pre-vaporization. This would help to reduce the reboiler duty, along with the vapor-liquid traffic in the bottom section of the extractive distillation column, thereby decreasing pressure drop across the column. The degree of pre-vaporization is dependent on the percentage of non-aromatics in the feed. This possibility was checked by analysis of the extractive distillation unit preheat circuit, using a simulation. The extent of feed pre-vaporization is primarily governed by the:

• Maximum feed temperature attainable across the feed preheater in the extractive distillation heat exchanger circuit (the higher the feed temperature, the greater the extent of pre-vaporization)
• Ability of the feed line to sustain the flow regime with increased vapor flow
• Acceptable slippage of aromatics and non-aromatics in the product streams, as per the operating plant’s specifications.

Vaporizing the feed ensures that light boiling components go up the feed tray in the vapor phase. This avoids the light non-aromatic components entering the bottom section of the column, which saves reboiler duty. The reboiler duty goes down to maintain heat balance inside the extractive distillation column, and eventually decreases the vapor flow through the trays between the feed tray and the bottom tray. This helps to reduce the pressure drop in the stripping section of the column. Therefore, feed pre-vaporization might be a practical solution in case the column is limited by flooding.

However, increasing the feed vaporization does not always improve the overall separation efficiency of a distillation unit. Excessive feed temperatures can cause a significant amount of flash of heavy key components at the distillation column feed zone. The extent of pre-vaporization will depend on the allowable specifications of aromatics slippage in the raffinate stream of the extractive distillation, as well as non-aromatics slippage in the extract stream of the solvent recovery column. In addition, the capability of the feed line to endure the increased vapor fraction must be evaluated. Personnel must look at the effect of feed pre-vaporization on the operation of downstream units like the solvent recovery unit (SRU).

Approach

The following methodology was used for the case study:

• Perform a simulation of extractive distillation and solvent recovery columns using commercial simulation software and an appropriate thermodynamic property model
• Check the possibility of feed pre-vaporization, with the help of simulation
• Perform the extractive distillation column hydraulic study (pressure drop studies) by increasing feed pre-vaporization, and study its impacts on reboiler duty and on aromatics and non-aromatics separation.

Analysis of the extractive distillation unit preheat circuit

To check the possibility of feed pre-vaporization and the maximum reasonable feed temperature across the feed preheater, it was important to analyze the extractive distillation column’s preheat circuit to check the adequacy of all heat exchangers in the circuit. Depending on the preheater duty margin and flashing conditions, the authors used simulation models to determine the feed pre-vaporization percent by increasing feed temperature. The authors compared the results for cases with 0%, 0.6%, 3.6%, 5.6%, 7.6% and 18.6% of feed pre-vaporization. Reboiler duty and specifications of the end products were checked for each scenario. In addition, the transition in flow regimes with vaporization was studied to inspect the feed line’s effectiveness.

Impact of feed pre-vaporization on reboiler duty

The impact of feed vaporization to offload reboiler duty in distillation columns is discussed in literature.² Likewise, the authors’ simulation for the case study indicated a reduction in reboiler duty with increasing feed pre-vaporization (FIG. 2). A further reduction in reboiler duty from the existing level would depend on the prospects of pre-vaporization due to the reboiler margin and the preheating of feed to the extractive distillation unit.

Impact of feed pre-vaporization on pressure drop

The increase in feed pre-vaporization decreases vapor-liquid (V-L) traffic in the extractive distillation column, especially in the stripping section (below the feed tray). Using simulation software, stagewise V-L profiles were generated. The decreasing trends for vapor and liquid load below the feed tray to the reboiler for each percent vaporization are shown in FIG. 3. The V-L load is consistent above the feed tray. The impact of pre-vaporization on pressure drops in the stripping and rectification sections of the column, and on pressure drops of the entire column, is depicted in FIG. 4. It is evident that, with increased feed vaporization, pressure drops in the column could be reduced further.

Impact of feed pre-vaporization on aromatics separation

The increase in feed pre-vaporization might affect separation efficiency of the extractive distillation column. When it comes to extractive distillation separation, for the raffinate phase, the loss of aromatic hydrocarbon components must be monitored. Conversely, in the extract phase, non-aromatics content is an important criterion, since it affects final product quality. The impact of feed preheating on both parameters is depicted in FIG. 5. Aromatics slippage in raffinate increases due to increased pre-vaporization of the feed. Similarly, the slippage of non-aromatics also increases. However, it should be confirmed that the loss is well within desired product specifications.

Scope to increase feed throughput

In the studies and feed conditions, the authors found that increasing feed pre-vaporization can help to reduce the pressure drop across the extractive distillation column. This provided the authors with a scope to increase feed throughput by the same fraction as pressure drop decreases, thus maintaining base pressure drop without significantly affecting separation efficiency. For example, at 7.6% feed vaporization, the pressure drop decreases by 2.2% (FIG. 4). Therefore, the feed throughput can be increased by 2.2%, thus debottlenecking the column. The same principle can be applied to other pre-vaporization cases by retaining base pressure drop. The process constraints and product specifications must be monitored for any such alteration.

TABLE 1 considers a base case depicting some column parameters using alphabetical variables. The effect of increasing feed throughput for each pre-vaporization case on column parameters is also represented. It should be noted that the operation of the SRU was undisturbed, and it did not hit any operation or quality constraints due to increased pre-vaporization of the feed.

Takeaway

For this case study, with an increase in feed pre-vaporization, the pressure drop across the extractive distillation column and reboiler duty was found to decrease. The degree of pre-vaporization will depend on the competence of the extractive distillation heat exchanger circuit and desired product specifications.

This case study provides a cost-effective and accessible solution that can be further extended for similar plant challenges where extractive distillation columns are bounded by high pressure drops. It should be noted that the operation of the downstream unit (i.e., the solvent recovery column) was undisturbed by this modification. HP

LITERATURE CITED

1. Buffalo Brewing Blog, “Columns,” March 2021, online: https://www.buffalobrewingstl.com/practical-distillation/columns.html
2. Deshmukh, B., R. K. Malik and S. Bandyopadhyay, “Feed preheat targeting to minimize energy consumption in distillation,” International Symposium on Process Systems Engineering and Control, 2003.

The Authors

Related Articles

From the Archive

Comments