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Jun-2018

Boosting energy efficiency in aromatics processing

Simulation of a revamped process scheme with improved heat integration indicates big gains through energy savings and emissions abatement.

SUNIL KUMAR, PRASENJIT GHOSH and SHRIKANT NANOTI
CSIR-Indian Institute of Petroleum
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Article Summary
A new energy efficient method for processing the aromatics mixture to produce pure benzene, toluene, and xylene (BTX) in an aromatics production plant was conceptualised for a significant increase in process energy efficiency and financial savings. A techno-
economic study was carried out to estimate the value of energy savings, capital investment and additional capital requirement for revamping the existing design of an aromatics production plant. The results of the study show large scale savings in hot utility, cold utility, electricity and overall operating costs as a result of implementing the new method. The payback period for additional expenditure required to install heat exchange area piping modifications would be less than a year, while process emissions of CO2 would fall significantly.

BTX is produced on a large scale by processing the reformate and pyrolysis gasoline streams in an aromatics production plant. The aromatics rich stream is first processed in the extraction section to remove non-aromatics. Subsequently, the aromatics stream is further treated in a clay tower to remove olefinic impurities, followed by two distillation columns to produce pure benzene and toluene. Nowadays, divided wall column (DWC) technology, which claims 30% energy and capital savings over the conventional two column arrangement, seems to be the upcoming approach for fractionation of a BTX mixture in the aromatics plant.1,2 A DWC application in a grassroots unit for BTX production seems very promising. However, there is a need for 110 trays for BTX fractionation. Therefore, the application of DWC for revamping an existing two column distillation process does not seem feasible.

A new method for BTX processing was conceptualised by optimising the operating conditions and adopting new heat integration for the revamp of a conventional two column process to deliver substantial energy savings and reduce the process’s carbon footprint. A 
techno-economic analysis was carried out to estimate the quantitative benefits of the proposed scheme over a conventional scheme.

Conventional process
A schematic of the conventional process used for processing the aromatics mixture (BTX mixture) obtained from a solvent recovery column is shown in Figure 1. A hydrocarbon stream is heated in the heat exchanger with the clay treating tower bottoms, and then with steam to the desired temperature, before entering the clay tower for removal of olefinic impurities.

The clay treated BTX mixture is processed in a benzene separation column (DCI) and toluene-xylene separation column (DCII) to obtain pure BTX (see Figure 1).  The xylene stream, which is generally processed downstream, is either routed hot or water-cooled for storage.

Energy requirement in a conventional process
Simulation of a conventional process as per the schematic shown in Figure 1 was carried out using Aspen Hysys. A rigorous distillation column model and Peng Robinson thermodynamic model were used in the simulation. The proportions of benzene, toluene, o-xylene, p-xylene and m-xylene in the hydrocarbon stream were taken as 12.55 wt%, 39.30 wt%, 16.05 wt%, 16.05 wt%, and 16.05 wt%, respectively. A flow rate of 20000 kg/h and a temperature of 40°C were used as conditions to define the feed. A pressure drop of 0.5 bar was used across the process heat exchangers. A benzene purity of ≥99.95 wt%, a toluene purity of ≥99.5 wt%, along with a recovery level of ≥99.5%, were targeted in the study. The storage temperature of BTX was taken as 45°C. The clay treating column was not simulated as its operation is beyond the scope of an energy optimisation study. The columns DCI and DCII were simulated using 36 and 32 theoretical trays, respectively. Both distillation column condenser pressures were kept close to normal atmospheric pressure (1.1 bar). A reasonable pressure drop across the various equipment, such as 0.2 bar across the condenser of DCI, 0.4 bar across the air cooler of DCII, and 0.2 bar across the column, were used in the simulation. The reboiler pressures of the columns DCI and DCII used in the simulation were 1.5 bar and 1.7 bar, respectively.

Enhancing process energy efficiency
A new, energy efficient distillation configuration based on DWC technology reduces the reboiling and condensing duty to minimise the net process energy requirement. The reboiler and condenser duty of a given distillation column depends on operating conditions such as feed temperature, column pressure, reflux ratio to meet product purity, number of trays used, and so on. However, it is essential to note that the net process energy requirement is governed not only by the distillation column configuration but also by the process-to-process heat recovery potential. The extent of process-to-process heat recovery is governed by the operating conditions in the different types of process and process heat integration equipment in the scheme adopted. Physical insight and pinch analysis of the conventional process reveal that a significant amount of energy is wasted in air or cooling water used for condensing the huge amount of toluene vapour produced in DCII. It would be a great achievement if this energy could be integrated into the process to reduce the net process energy requirement.

The boiling points of BTX are significantly different. Thus, it seems quite realistic to suggest that the condensation energy of toluene vapour can be used for preheating the feed to the clay treating tower and replacing part of DCI’s reboiler duty, provided there is sufficient temperature gradient for heat transfer. This temperature gradient will depend upon the condensation temperature of toluene vapour which increases along with a rise in pressure in DCII. However, a rise in DCII’s pressure will also increase the reboiler and condenser duty for specified toluene purity and recovery values due to reduced relative volatility. On the other hand, higher pressure will lead to a lower volumetric flow rate of vapour in DCII. Considering this, a sensitivity analysis of the effect of DCII’s pressure on these parameters was carried out to understand their interaction quantitatively.

The results of this sensitivity analysis indicated that reboiler and condenser duty increases with a rise in pressure, whereas maximum actual vapour flow decreased, as expected. Thus, an increase in pressure does not seem to be beneficial with regard to energy savings until and unless the introduction of more energy does not facilitate the utilisation of toluene vapour energy in the process itself for a significant reduction in the net process energy requirement. To find out the optimum value of DCII’s pressure, the net process energy requirement was estimated using the pinch analysis tool in Aspen Hysys for various pressure values of DCII. The results are shown in Table 1.

The results clearly indicate that a DCII pressure of ~3.0 bar is the optimum value to minimise the net process energy requirement. Subsequently, a new processing scheme with modified heat integration was conceptualised (see 
Figure 2).
 
Comparative analysis
Conventional and proposed processes with an optimum pressure for column DCII as shown in Figures 1 and 2 were simulated in a closed loop for evaluation of their actual energy requirement. In the present study, the annual operating cost was estimated for an operating rate of 8000 h/y and for utility prices of steam at Rs. 1500 per ton, cooling water at Rs. 1.8/ton and electricity at Rs. 4.2/kWH ($ = Rs60).3 The capital cost of the heat exchanger network, reboiler and condenser were estimated using correlations reported in established and reliable literature.4,5,6,7 The payback period for additional expenditure required for new heat exchangers and pipeline rerouting was estimated using the ratio of capital cost to annual profit through energy savings. The cost of piping modification was assumed to be 30% of the installed cost of new heat exchangers.
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