December 2017

Process Optimization

Maximizing energy efficiency in paraxylene production—Part 1

Due to reduced margins, higher energy costs and rising atmospheric carbon dioxide (CO2) content, energy efficiency has never been more important in refining and petrochemicals.

Colling, C., BP AMOCO

Due to reduced margins, higher energy costs and rising atmospheric carbon dioxide (CO2) content, energy efficiency has never been more important in refining and petrochemicals. This is particularly true in the modern aromatics complex, which produces paraxylene (pX), benzene and other aromatics from naphtha. Part 1 will review the energy-efficient process design principles that are widely used in aromatics processing. 

In Part 2, case studies will be presented on pX manufacturing by crystallization and selective adsorption—the two principle pX manufacturing methods—to show how to maximize energy efficiency in pX manufacturing and the aromatics complex. Past literature has shown that crystallization is more energy efficient in the pX unit of the aromatics complex.1 The following will provide data that the inherent energy advantage of crystallization is maintained even when considering the broader scope of the entire aromatics complex.

The aromatics complex

FIG. 1. Schematic of an aromatics complex.<sup>2
FIG. 1. Schematic of an aromatics complex.2

This complex refers to the unit operations involved in the production of pX, benzene and other aromatic compounds from petroleum naphtha. A schematic of an aromatics complex is shown in FIG. 1. Typically, the aromatic complex’s feedstock is total reformate from the bottom of the debutanizer or depentanizer of a naphtha reformer. Part of the production chain between naphtha and aromatics—the design of the naphtha reformer is typically optimized separately—and the naphtha reformer is not heat-integrated significantly with the aromatics complex.

The aromatics complex begins with the reformate splitter, which is usually a single distillation column that produces light reformate (toluene and lighter compounds) and heavy reformate (xylenes and heavier compounds). Light reformate passes to the aromatics extraction unit, where aromatics are separated from nonaromatics using extractive distillation. Raffinate, benzene and toluene are produced from the aromatics extraction unit. Heavy reformate passes to the pX unit from the reformate splitter. Two major manufacturing methods are used in the pX unit: crystallization and selective adsorption. Both methods contain an isomerization section where an equilibrium mixture of xylene isomers is produced and ethylbenzene is converted; a fractionation section where xylenes are separated from byproducts, including heavy aromatics (C9+ aromatics); and a separation section where either crystallization or selective adsorption is used to produce pX with a purity of greater than 99.8 wt%.

Typically, the modern aromatics complex contains a transalkylation (TA) unit, which converts toluene and heavy aromatics into predominantly xylenes. These xylenes pass to the pX unit to further enhance the yield of pX from reformate.

Depending on the particular need for pX, benzene and other aromatics, numerous configurations are possible for the aromatics complex. Several trade names are used, by individual licensors, for sections of the aromatics complex.

Process change analysis for the aromatics complex

The introduction of pinch analysis techniques has had a significant impact on energy efficiency in the refining and petrochemical industry. Pinch analysis techniques have been widely applied to design heat exchanger networks to improve energy efficiency and minimize energy consumption. An important outcome of this work is that process designers now ask how process conditions, such as the pressure of a distillation column, can be adjusted to enhance energy integration. Process change analysis, a form of pinch analysis, allows the process designer to understand how changing process conditions will affect the energy usage of the entire plant.3

Process change analysis is routinely applied in the design of the aromatics complex. Since the aromatics complex uses numerous distillation columns and high temperatures, as well as a refrigeration section in the case of crystallization, the design is well-suited for process change analysis. Pressures can range from ambient pressure to nearly 40 barg. Heat recovered in one section of the plant can often be used to do useful work in a different section of the plant, provided that process conditions are suitable.

Design optimization of the aromatics complex involves using process change analysis to adjust the temperature and/or pressure of one section of the plant so that energy is effectively used in a different section of the plant. While a complete discussion of how process change analysis can minimize the energy usage of the aromatics complex is beyond the scope of a single article, it is important to point out the key concepts of process change analysis that have been applied in the design of the aromatics complex. These concepts include pressure cascade, intermediate condenser and/or reboiler, feed preheating, distillation column sequencing and complex columns.

Pressure cascade

FIG. 2. A simple schematic of a two-column pressure cascade.
FIG. 2. A simple schematic of a two-column pressure cascade.

Pressure cascade refers to a process with multiple distillation columns where the pressure of one column is elevated so that the condenser temperature of that column is sufficient to supply heat to the reboiler of another column, or several other columns (FIG. 2). The heat cascades from the condenser of the high-pressure column to the reboiler of the low-pressure column. Pressure cascades are widely used in the aromatics complex, despite the operational and startup difficulties associated with integrated unit operations.

FIG. 2 shows a schematic where heat is directly transferred from one column to another. In some cases, it may be beneficial to transfer heat from the high-pressure column to a transfer medium, such as steam, and use the produced steam to supply heat to the low-pressure column. This setup is especially useful when the condensing duty of the supply column is adequate to supply heat to multiple users.

A series of simulations was carried out on the aromatic complex’s benzene and toluene columns to show how a pressure cascade can be used to reduce energy usage. The results are shown in TABLE 1. The pressure of the toluene column was increased to raise the toluene column condenser temperature, so it was higher than the reboiler temperature of the benzene column. An adequate difference must exist between the toluene column condenser temperature and the benzene column reboiler temperature. This study used a difference of 15°C. Lower temperature differences are possible, but they require more heat exchanger area, which increases capital costs. High-flux tubes could be used to reduce the required temperature difference, but they  significantly increase capital costs, as well. To provide the desired 15°C difference, the pressure of the toluene column needed to be increased to approximately 2 barg (TABLE 1). As the pressure of the toluene column increased, the duty of the toluene column reboiler also increased. However, until the target driving temperature difference was reached, it was not possible to achieve the pressure cascade. TABLE 1 shows that once the pressure cascade was achieved, the total reboiler duty was about 24% lower than the lowest energy without the pressure cascade.

A 2011 study simulated a benzene-toluene-xylene distillation to show how much energy savings are possible when a pressure cascade is utilized.4 In the study, benzene was first distilled overhead in a low-pressure column, and the bottoms of the first column passed to a second column operating at higher pressure where toluene and xylene were produced. The energy savings, as measured by operating costs, were 10%–30% lower for the pressure cascade, as compared with a conventional system. These results are similar to the results shown in TABLE 1. A higher level of savings was observed when the purity specifications of the distillation products were higher.

The primary pressure cascade in the aromatics complex using selective adsorption is a high-pressure xylene column in the pX unit to supply heat to other users in the aromatics complex. This pressure cascade has not fundamentally changed since it was developed in the 1980s. The xylene column separates xylenes from heavy aromatics in the pX unit. If the pressure of the xylene column is raised so that the condenser temperature is approximately 230°C, then heat can be supplied to users in aromatics extraction, TA and selective adsorption separation. The purity specification of the overhead of the xylene column for selective adsorption is high (less than 500 wppm heavy aromatics), since the adsorbent is sensitive to impurities, and heavy aromatics can build up in the desorbent when using a heavy desorbent. Consequently, the heat of the xylene column’s overhead condenser is sufficient to supply most of the needs to the aromatics complex. Since crystallization does not use sensitive adsorbent and desorbent, the purity specifications in the xylene column are much less stringent, and the heat needed to separate xylenes is significantly lower. Consequently, different process change analysis concepts are utilized for crystallization. Specific examples of the impact that pressure cascades have in lowering the energy used in the aromatics complex will be discussed in Part 2.

Intermediate reboilers and condensers

Pressure cascades increase the pressure and condenser temperature in a distillation column to supply heat that is warm enough to be used elsewhere in the aromatics complex. The condenser temperature must be higher than the temperature of the user (including a reasonable approach temperature). Occasionally, column pressure does not need to be increased because the temperature of the user is between the condenser and reboiler temperatures of the supply column. In this case, an intermediate reboiler or condenser is used for heat recovery. An intermediate condenser condenses vapor from the supply column at a temperature between the condenser and reboiler temperature to supply heat. An intermediate reboiler boils liquid from the supply column at a temperature between the condenser and reboiler temperature to exchange heat. The use of intermediate reboilers and condensers reduces the vapor and/or liquid traffic in the column. This setup often reduces the capital cost of the supply column. However, it is important that the reduction of vapor and/or liquid traffic in the supply column does not reduce the separation efficiency of the supply column and lead to a significant increase of the energy needed in the supply column.

FIG. 3. Schematic of a xylene column with an intermediate condenser.5
FIG. 3. Schematic of a xylene column with an intermediate condenser.5

US Patent Application 20160318831 discloses an intermediate condenser in the xylene column of a pX unit that uses crystallization (FIG. 3).5 The condenser is placed in the xylene column so that the condensing temperature is higher than the temperature of the user (including a reasonable approach temperature). Typically, the energy in the xylene column condenser of a pX crystallization unit is waste heat that is transferred into the environment via air and/or water cooling. The intermediate condenser condenses vapor at a temperature higher than that of the column overhead, thereby increasing the overall energy efficiency of the aromatics complex by using this heat for useful work rather than creating waste heat.

Locating the intermediate condenser higher in the column decreases the condensing temperature where it may not be adequate to transfer heat to the energy user. Placing it lower in the column increases the condensing temperature. However, placing it too low in the column may not allow for adequate contacting of the vapor and liquid in the column for the vapor to strip components out of the liquid to meet the column bottoms specifications. In one example in US Patent Application 20160318831, the amount of heat lost in the xylene column condenser was reduced by more than 46%. The heat was used elsewhere in the aromatics complex, thereby reducing the overall energy usage.

Feed preheating

Feed preheating involves heating a subcooled liquid before it enters the column, making it easier to vaporize the feed in the column and reducing the energy needed in the column reboiler. Feed preheating is especially efficient when the medium used to preheat the feed is a waste heat source (e.g., the column bottoms). TABLE 2 shows an example of where the energy needed in the xylene column of a pX crystallization unit is lowered by preheating the feed using hot reactor effluent. The hot isomerization reactor effluent (350°C–400°C) in a pX unit is usually cooled to near ambient conditions to separate unreacted hydrogen and other light gases, and recycle them within the isomerization section. In this example, some of the reactor effluent heat has been used to preheat the xylene column feed. TABLE 2 shows that it was possible to reduce the energy usage of the xylene column by up to 4% by preheating the feed.

Distillation column sequencing

When separating a multicomponent mixture, where it is desirable to obtain one or more components in a high-purity form, the process designer has the choice of the sequence in which the high-purity components are produced. For many years, designers followed a rule of thumb called the direct sequence. This sequence produces the most volatile component first, followed by successively less volatile components.

Past technical literature points out additional heuristics for multicomponent distillations and several pitfalls to following these heuristics.6 Typically, the use of computer software to simulate distillation of multicomponent mixtures is used to evaluate many sequences and determine the configuration that minimizes energy usage. Additionally, computer software is available for the sequencing of columns containing many short-cut methods, which is helpful in screening multiple sequences. For the aromatics complex, a study investigated the use of an indirect sequence for the separation of benzene, toluene and xylene using shortcut methods.a,7 By using an indirect sequence, researchers were able to reduce energy consumption by more than 28%, and decrease capital cost by more than 8%.

Complex columns

Traditional distillation columns comprise an overhead vapor and/or liquid product, and a bottoms liquid product using a single distillation shell. Conversely, complex columns utilize more than one distillation shell where liquids and/or vapors flow back and forth between the shells. Complex columns can also have walls within a single shell where the number of trays, type of trays, amount of packing, or vapor and liquid loads vary on either side of the wall. Occasionally, complex columns utilize total liquid or vapor removal by separating a single shell into multiple fractionation zones. Complex columns include side streams, side strippers, split-shell side strippers, side rectifiers, a prefractionator and divided wall columns (DWCs).6 The use of complex columns for distillation has grown, as they offer significant energy and/or capital cost savings compared to traditional columns.

FIG. 4. Schematic of the reboiled prefractionator and side-draw column.
FIG. 4. Schematic of the reboiled prefractionator and side-draw column.

Researchers simulated a benzene-toluene-xylene distillation to show how much energy savings are possible when a DWC is used.4,8 The DWC resulted in a reduction in capital costs of approximately 20% compared to traditional distillation. The cost savings were primarily due to the fact that the distillation could be done in a single distillation shell, which also reduced the number of condensers, reboilers and other pieces of equipment. The energy savings of the divided wall system depended on the purity specifications of the products. When the purity specification of the benzene product was higher, the DWC did not save as much energy as the pressure cascade, even though both used less energy than traditional direct sequence distillation.

World Patent Application WO2015123065 shows how a reboiled prefractionation column and side-draw column can be used to reduce the energy used in the TA and pX units. In a typical aromatics complex, the separation of benzene, toluene and xylene in the TA effluent follows the direct sequence previously discussed. The xylene produced is sent to the xylene column in the pX unit. The reboiled prefractionation and side-draw column replaces the direct sequence and xylene column (FIG. 4). In this case, the benzene column bottoms are sent to the reboiled prefractionator, where a vapor and liquid stream are produced and sent to the side-draw column. The side-draw column produces toluene, xylene and xylene column bottoms streams to the same specifications as the basic design. The reboiled prefractionation and side-draw column consumes 10% less energy for selective adsorption and 25% less energy for crystallization than the basic aromatics complex design.

Takeaways

Higher energy costs and rising atmospheric CO2 content make it more important than ever to reduce energy consumption in the aromatics complex. Part 1 described many of the energy-efficient process design principles used in the aromatics complex, including pressure cascades, intermediate reboilers and condensers, feed preheating, distillation column sequencing and complex columns. Part 2 will provide case studies for crystallization, selective adsorption using heavy desorbent and selective adsorption using light desorbent. HP

NOTES

a   Refers to Aspen HYSYS.

REFERENCES

  1. Roberts, S., J. Amelse and C. Colling, “BP’s energy efficient technology for the production of paraxylene,” Petrotech 2010, New Delhi, India, October 2010.
  2. Meyers, R. A., Handbook of Petroleum Refining Processes, McGraw-Hill, New York, New York, 2004.
  3. Kemp, Ian C., Pinch Analysis and Process Integration—A User Guide on Process Integration for the Efficient Use of Energy, 2nd Ed., Elsevier Publishing, Amsterdam, The Netherlands, 2007.
  4. O’Brien, D. and L. Weaver, “BTX Fractionation: Conventional, pressure cascade or dividing wall?” AIChE Spring Meeting, Chicago, Illinois, March 2011.
  5. Colling, C. W., US Patent Application 20160318831, “Enhanced heat recovery in paraxylene plant,” 2016.
  6. Sinnott, R. K. and G. Towler, Chemical Engineering Design—SI Edition, 5th Ed., Elsevier Publishing, Amsterdam, The Netherlands, 2009.
  7. Masoumi, M. E. and S. Kadkhodaie, “Optimization of energy consumption in sequential distillation column,” International Journal of Chemical, Molecular, Nuclear, Materials and Metallurgical Engineering, 2012.
  8. O’Brien, D. and L. Weaver, “Energy and capital efficiency differences in benzene-toluene distillation configurations,” AIChE Spring Meeting, San Antonio, Texas, March 2010.

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