Energy savings in preheat trains with preflash

An eastern European refinery combined pinch technology and process simulation to achieve the most cost effective energy savings

KBC Process Technology

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Article Summary

Increasing global competition and more stringent regulation on greenhouse gas emissions have pushed refineries to look for available methods to reduce their carbon footprint and energy bill in a cost effective manner. In this regard, crude distillation units (CDU) are particularly important as this type of unit typically accounts for approximately 25% of an existing refinery’s total energy consumption. This shows the need to improve the economic efficiency of the operation, which is key for overall site energy efficiency.

Pinch technology is one of the ways to improve economic efficiency and reduce carbon footprint. ‘Energy pinch’ is a well established, rigorous, structured, thermodynamic approach for identifying energy efficiency opportunities that can be used to tackle a wide range of such process and utility related problems as, for example, reducing operating costs, debottlenecking processes, and improving energy efficiency. However, traditional pinch techniques alone do not allow the rigorous assessment of the impact on the overall heat and material balance of modifying the inlet conditions of existing preflash drums/towers. Therefore, unless inlet conditions to the preflash remain unchanged, a combination of traditional pinch techniques and process simulators should be used when evaluating energy savings projects on CDU preheat trains with preflash. For these tasks, KBC uses its in-house software SuperTarget for determining energy saving ideas and Petro-SIM (KBC’s process simulator) for evaluating the effect of any changes to product yields, as well as confirming possible fuel savings.

This article gives a brief introduction to energy pinch and illustrates how combining pinch with process know-how helps to overcome the challenge of achieving energy savings on CDUs with preflash.

CDUs separate and recover the relatively lighter fractions from fresh crude oil charge, while the vacuum distillation unit processes the crude distillation bottoms to produce an increased yield of liquid distillates and heavy residual material. The function of a preflash device (either drum or tower) is simply to remove light components of the crude before entering the CDU furnace. While the vapour stream is then sent either to the furnace outlet, to a desired location in the crude column, or to downstream columns (such as debutanisers) for further separation after being mixed with naphtha from the CDU, flashed crude, in its turn, is either sent directly to the CDU furnace or to a preheat train for further heating before being sent to the furnace.

Crude units account for approximately 25% of a refinery’s total liquid fuel consumption. Their operation is, therefore, key in terms of a refinery’s overall energy efficiency. Over the last few years, high fuel prices in addition to an ever more carbon-conscious operation have driven refiners to look for ways to improve the energy efficiency of their units, in particular of CDU preheat trains, as one of the few available avenues for effective cost reduction.

What is pinch technology?
Pinch technology is a systematic approach that analyses all process heating and cooling demands in terms of quantity (duty) and quality (temperature). The study of these factors ensures that all opportunities to recover waste heat within a process are identified and maximised, reducing the demand for hot and cold utilities. The fundamental principle behind this technology is the ability to match individual demands for a commodity (in this case heat) with a suitable supply. One of the principal tools used in pinch analysis is composite curves (see Figure 1).

The basic principle of their generation is as follows: the process streams to be analysed are first divided into sources and sinks, corresponding to hot and cold streams. The hot and cold streams are then plotted in terms of quality (temperature) against quantity (heat duty). The resulting curves enclose a representation of the amount of heat in the process and the temperature range over which it is available.

By combining these curves on one diagram, the minimum amount of hot and cold utility requirements (or targets) can be determined. Process heat recovery is possible where the hot and cold composite curves overlap. The remainder of the heat balance must be made up by external hot and cold utilities. Comparing this target with the actual utility consumption quantifies the scope for savings achievable. The ‘pinch’ that gives its name to the technology is the point of the closest approach between the two composite curves in the plot.

As mentioned previously, in the case of preheat trains with preflash devices, traditional pinch techniques alone do not account for changes to the heat and material balance that result from modifying the crude inlet conditions to the preflash device. Therefore, unless inlet conditions to the preflash remain as for the base case, a combination of traditional pinch techniques and process simulators should be used when evaluating energy saving ideas.
The case study presented in this article corroborated the statement that a combined approach (pinch technology/process simulator) should be followed to avoid misevaluating possible savings.

Energy optimisation of a CDU preheat train with a preflash tower
The authors performed a strategic energy review on a crude distillation unit, in a 7 million t/y Eastern European refinery, aimed at improving the energy efficiency of the unit. The preheat train configuration of this unit is illustrated in Figure 2. At the time of the study, the CDU furnace was bottlenecked and the preflash reboiler (a fired heater) was fired up at times when additional capacity was required. By doing this, the crude coil inlet temperature (CIT) to the CDU furnace increased and the CDU furnace was debottlenecked. However, this represents a major operational inefficiency, as much of the heat introduced to the system by the preflash furnace is lost to the preflash overhead coolers.

 In order to analyse the alternatives to improve the performance of the preheat train, a simulation of the crude unit (preheat train and columns) using Petro-SIM based on DCS data provided by the refiner and KBC’s own crude assay data was developed. The SuperTarget software tool was used to extract the relevant energy data and to determine the best possible options to improve the CDU’s energy performance.

While performing the energy evaluation of the existing design, it was noticed that the vacuum residue (VR) is used to preheat the crude through various heat exchangers before being sent to the air cooler at 180°C, where its heat is lost to the atmosphere. According to the pinch analysis, the simplest and most cost effective way to debottleneck the CDU furnace would be to recover this waste heat against the crude. The most efficient way of doing this is by installing a new heat exchanger downstream of the preflash column (see Figure 3).

Pinch techniques alone would mis-estimate energy savings. Deviations occur as the interactions within the preflash tower cannot be captured by pinch techniques:
• Preflash reboiler duty will change to meet product specifications.
• Preflash reboiler duty is not a direct function of crude inlet temperature, as part of the feed heat is lost to the preflash overhead coolers.

Therefore, having defined this structural change 
to the heat exchanger network using pinch techniques, the full system must then be optimised based on process simulation. This is done to capture the effect of changes to the preflash feed inlet temperature.

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