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Oct-2013

Improving the distillation energy network

Energy-efficient design applied to the refit of a distillation unit was achieved through optimisation between the distillation column and heat network system.

SOUN HO LEE, GTC Technology
KWANG GIL MIN, GS Caltex Corporation
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Article Summary
Energy costs are the largest percentage of a hydrocarbon plant’s operating expenditures. This is especially true of the distillation process, which requires substantial energy consumption. Concerns over recent high costs and economic pressures continually emphasise the need for efficient distillation design and operation without a loss of performance.

This article illustrates how 
energy-efficient design can be applied in a distillation unit through optimisation between the distillation column and heat network system. Through a case study, a successful retrofit of an aromatics distillation unit is discussed. Detailed retrofit activities, including complex heat network evaluation, process simulation modelling and energy-friendly, high-performance distillation equipment implementation, are described.

Strategies for improving the distillation energy network
As continuous distillation requires simultaneous heat input and removal (thus requiring significant energy consumption), complex heat integration becomes more common for modern distillation units to improve unit energy efficiency. Since a distillation column’s degree of separation and enthalpy balance influence each other, it is critical to evaluate and optimise the distillation column and heat exchanger networks together in order to maximise plant economics.

There are numerous strategies to improve the energy efficiency of distillation processes, with the amount of improvement through each strategy varied according to process conditions. The following are common strategies that can be applied to practical energy improvement projects.

Feed temperature
Feed temperature is a major factor influencing the overall heat balance of a distillation column system. Increments in the feed enthalpy can help reduce the required energy input from the reboiler at the same degree of separation. Installing a feed preheater is a very common process option to minimise reboiler heat duty. If the feed preheater can be integrated with other valuable process streams (as a heating medium), overall energy efficiency of the distillation system can be improved further. However, increasing the feed temperature does not always improve the overall energy efficiency of a distillation unit. Excessive feed temperature increments can cause a significant amount of flash of heavy key and non-key components at the distillation column feed zone. In this case, a higher amount of reflux stream is necessary to maintain required overhead distillate purities. This augmented reflux ratio thus requires a higher boil-up ratio. Overall energy efficiency is eventually aggravated.1 Therefore, careful review of the feed temperature and phase is critical to minimise the overall energy consumption of the distillation unit.

Feed location
Improper feed location of a distillation column can also increase the reflux/boil-up ratio and energy consumption. An ideal feed location is a section of the distillation column where the composition of column internal liquid traffic is similar to feed stream composition. In this case, the composition gradient between feed stream and distillation internal fluids is minimised. In actual operation of the distillation column, feed compositions are often changed from the original design conditions. In cases of significant deviation, discrepancy between column internal liquid composition and feed stream composition can increase, which results in a non-optimum feed location. Therefore, evaluating feed location is an essential step for successful distillation unit energy improvement.
 
Inter-condensers and inter-reboilers
Adding inter-condensers and/or inter-reboilers can help improve overall energy efficiency. Pumparound, one of the inter-condenser concepts, has been widely applied to numerous petroleum multi-product fractionators. On the other hand, implementing an intermediate reboiler can reduce the main reboiler duty. As the required temperature of an intermediate reboiler is lower than that of the main reboiler, this strategy may allow heat integration with other valuable heat sources that are not as costly or not fully utilised in the plant.

Column operating pressure
Relaxation of the column top operating pressure decreases the distillation column’s temperature profile and results in a lower reboiler duty. It has been observed that numerous commercial distillation columns have been operated with lower operating pressures than their original design values. However, this strategy is not applicable to columns operated under an atmospheric pressure range. Column overhead circuit pressure drop and condenser temperature approaches both heavily influence feasibility. In addition, column pressure reduction expands vapour traffic and pushes the limits of existing distillation equipment.

Column pressure drop
Reducing column pressure drop can lower reboiler duty at the same degree of separation. The amount of reboiler duty saving relies on operating pressure and enthalpy balance. This strategy is generally feasible when the distillation column is operated under vacuum pressure range. Meanwhile, pressure drop improvement does not often provide noticeable energy savings in high-pressure range distillation service.

Column efficiency improvement
Column efficiency improvement can reduce the reflux/boil-up ratio at a given degree of separation. This strategy can be delivered by increasing the number of theoretical stages and/or enhancing the efficiency of distillation equipment. The feasibility can be gauged by a dedicated sensitivity analysis. Constructing a column efficiency curve with a reflux ratio is one of the core tools for sensitivity analysis. A typical curve is shown in Figure 1. This curve visualises column efficiency sensitivity and energy-saving gain. The curve can be categorised by three district zones: steep, moderate and flat sensitivity.2
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