Jul-2010

Process and energy analysis for cost savings and CO2 reduction

Applying heat pinch technology and process know-how to maximise energy savings. In an increasingly carbon-conscious market, improving energy efficiency is one of the few available avenues for cost- effective reduction of a facility’s carbon footprint.

Mehmet Alkan and Osman Karan, Tüpras̨
Maryro Méndez and Juan Gómez-Prado, KBC Advanced Technologies

Article Summary

Pinch technology is a well-established approach for identifying economic and practical opportunities for reducing energy costs in the oil refining and chemical industries. This systematic approach analyses demand for process heating and cooling in terms of quantity (duty) and quality (temperature). The study of these factors ensures that all opportunities to recover waste heat within a process, by reducing the demand for hot and cold utilities, are identified and maximised. Including process analysis in a traditional pinch study enables non-intuitive modifications to the process itself to be identified (for instance, a new pumparound), to enhance heat recovery without affecting process yields. 

This article provides a brief introduction to pinch technology and includes a case study for such an approach, combining energy-focused pinch with process know-how to obtain practical and economical opportunities for saving energy.

The crude unit operation at any refinery is key in terms of the overall energy efficiency of the refinery. Typically, the crude and vacuum unit furnaces combined account for approximately 25% of a refinery’s total energy consumption. Heating such a large amount of crude requires significant energy and, therefore, the preheat train and furnace need to be designed and operated to the highest standards of energy efficiency. KBC and TüpraÅŸ  carried out an energy review of the crude distillation units (CDU) and vacuum distillation units (VDU) at the TüpraÅŸ refinery site in Kirikkale, Turkey. A combined pinch/process/utility analysis was used to determine the scope for improvements in waste heat recovery and any other process changes that could produce energy savings (see Figure 1).

KBC developed a base case using test run data provided by TüpraÅŸ and a rigorous simulation of two new heat exchangers that had been commissioned and were to be brought on-line following the study (see Figure 2). 

The tools used for the study included PetroSim process simulation software, SuperTarget pinch technology targeting and design software, and ProSteam utility modelling software. In addition, HTRI Xchanger Suite was used when rigorous modelling of heat exchangers was required.  
 
Heat and power model
A complete utilities model of heat and power for the Kirikkale site was developed. Aimed at evaluating the effect of steam-related projects on the site’s overall steam system, this utilities model was developed within the framework of a ProSteam Model Builder, which incorporates the major components of the utility system. The ProSteam Model Builder has built-in models for boilers, deaerators, turbogenerators, gas turbines and other plant equipment used to create a steam model. It is also able to construct automatically the complex heat and mass balances for the steam system. Figure 3 provides an overview of the utilities model corresponding to the base case scenario. This model was used to calculate the marginal cost of steam.

Steam costs are based on the actual cost of the marginal fuel and power and on the current configuration of the utilities system. Marginal costs of steam at the low-pressure level (LPS) and medium-pressure level (MPS) are high and essentially the same as the marginal mechanism for LPS. MPS is steam letdown from high- pressure steam to fulfill the steam requirements at the different steam level conditions.  

Within the context of the CDU-VDU unit pinch analysis, interactions with the steam system can happen via changes in the waste heat recovery section of the main furnaces as duties are reduced, as well as changes in steam generation from hot process streams. This can be either increased opportunities for steam generation or reduced steam generation to increase crude preheat. It is important to understand how these changes in steam balance affect the site’s overall steam balance, particularly as opportunities are identified and developed for the site as a whole.

Pinch analysis and targeting
Pinch technology is a rigorous, structured thermodynamic approach to identifying opportunities for energy efficiency that can be used to tackle a wide range of process and utility-related problems such as reducing operating costs, debottlenecking processes and improving energy efficiency. Pinch analysis is the process by which systematic application of pinch technology is applied to industrial processes. It can also be applied to identify opportunities for capital cost reduction, and it can be used in capital investment planning.

The fundamental aim of this technology is 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 the composite curve (see Figure 4). The process streams are divided into sources and sinks, corresponding to hot and cold streams, then plotted in terms of quality (temperature) against quantity (heat duty). The resulting curves represent the amount of heat in the process and the temperature range over which it is available.

By combining these curves in a single diagram, the minimum requirements for hot and cold utilities (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 actual consumption of utilities quantifies the scope for saving. The “pinch” that gives its name to the technology is the point of closest approach between the two composite curves in the plot. It is a rigorous thermodynamic formulation of minimum possible utility consumptions for the system. The composite curves are annotated to show the main hot and cold streams.

Construction of the composite curves, and analysis of the heat transfer inefficiencies derived from these curves, was carried out using KBC’s SuperTarget pinch software. Petro-Sim was used for the development of the reconciled test run data and all project ideas were evaluated in this process simulator.

It is important to note that when implementing projects aimed at saving hot utilities, the same savings are also achieved in the cold utilities. While not typically given much attention from the point of view of savings in utilities costs, this can be a significant consideration if process cooling limitations are a bottleneck in certain areas of the plant, or if water is an expensive or limited resource.