Z. Jegla

Plant energy saving through efficient retrofit of furnaces

Abstract Tubular process furnaces belong to energy demanding equipment in the process industry, especially in the chemical and petrochemical process plants and refineries. Several ways of energy saving in such plants usually exist. Retrofit of furnaces can be considered as one of the straightforward and efficient ways. However, operational and geometrical constraints of an existing furnace are the reasons due to which the retrofit of a furnace is a very difficult task. Therefore, the process retrofit is usually focused on heat exchanger network (HEN) retrofit considering maximum furnace duty. Nevertheless, the furnace retrofit should be considered wherever possible. In some older plants, the placement of new shells or topology changes in HENs can be expensive due to various reasons and only minimum topology modifications are usually allowed. The furnace retrofit procedure described in this paper is based on an advanced furnace integration approach using some principles of Pinch Analysis and considering furnace limitations. It can bring surprising results. This method combines principles of an effective design of both processes and equipment. An efficient methodology for furnaces retrofit, using optimization of both stack temperature and air preheating system, is applied. An advantage of this approach is demonstrated through a case study — retrofit of furnace in petrol hydrogenation refining plant for energy efficiency improvement.

The Conceptual Design of a Radiant Chamber and Preliminary Optimization of a Process Tubular Furnace

The design procedure of a process tubular furnace (or fired heater) can generally be divided into three design stages: the preliminary design of furnace, a detailed thermal and hydraulic simulation of the furnace, and final design solution, and the mechanical solution of the furnace (stress analysis, drawings preparation, etc.). The first design stage (the preliminary design of the furnace and cost prediction) is usually connected with a proposal for the customer when only the basic process and furnace design data are usually known. In this design stage, it is appropriate for the furnace designer to not have much detail for a reliable design method. The procedure for the preliminary design of a radiant chamber is usually a main part of such a design method because the radiant chamber represents a basic and dominant part of the modern process tubular furnace. The conceptual (or preliminary) design of the radiant chamber makes up the main part of this process. The presented method is based on standard, time-tested design methods (e.g., the Lobo-Evans method and Belokons method). It is shown how these standard global design methods can be (for common operating conditions) suitable, generalized, and simplified. It allows for the purpose of the conceptual radiant chamber design the arrangement of the basic heat transfer equation for the radiant chamber. The derived form of the heat transfer equation then allows one to obtain basic process and geometrical radiant chamber characteristics of the given furnace type (cylindrical, box, etc.) iteratively. A developed radiant chamber calculation connected with standard procedures for the design of the furnace convection parts and stack (together with cost predictions) is used, and the method for a quick preliminary evaluation of the influence of the main design and process furnace parameters (dimensions of radiant chamber and convection parts, average heat flux to radiant tubes, absorbed heat in radiant chamber and convection part of furnace, stack size, and fuel consumption) for the total costs was developed. It allows the optimization of the furnace from an investment, operating, or total cost point-of-view in the preliminary design stage of the furnace. The developed method can also be used for the effective solution of the furnace integration into the process. The application of the developed method is demonstrated through a case study—the optimum design of a furnace for a crude atmospheric distillation unit.

Global algorithm for systematic retrofit of tubular process furnaces

Abstract Tubular process furnaces are widely used especially in the chemical and petrochemical process plants and refineries. Retrofit of a furnace is usually recommended for increasing plant capacity or improvement of the furnace (and/or plant) energy efficiency. Many different constraints of existing furnace design and operation are the reasons for which the retrofit of a furnace is a very difficult task. Main operational and geometrical constraints are taken into account in furnace retrofit integration approach created earlier. This approach was developed primarily for improvement of the furnace energy efficiency. When furnace retrofit for increasing the plant capacity is performed additional criteria have to be considered. Then fluid hydrodynamic behaviour plays an important role and influences the retrofit strategy. This problem is discussed in the paper and results in suggestion of the global conceptual algorithm for furnace retrofit strategy. Practical case studies demonstrate applicability of the proposed global furnace retrofit algorithm.

Optimum Arrangement of Tube Coil in Radiation Type of Tubular Furnace

Tube coil arrangement dominantly influences the thermal-hydraulics characteristics of furnace and its economy. The presented method is based on standard long-used design methods (Lobo-Evans method, Belokon method). It is shown how these standard global design methods can be (for common-operating conditions) suitably generalized and simplified. It allows (for the purpose of optimum design of coil arrangement) to arrange the basic heat transfer equation for radiant chamber. In connection with investment cost relations for individual furnace subsystems (tube coil, burners, lining, etc.) and operating cost relations (fuel consumption, fluid pumping cost), the final objective function of total cost can be obtained. This objective function then allows finding the optimum coil arrangement from minimum total cost point of view. Moreover, the obtained results allow one to formulate some general recommendations suitable for furnace designers. The developed method can be used for the individual solution of a radiation-type of tubular furnaces and also for the first (preliminary) design stage usually connected with a proposal for the customer when only basic process and furnace design data are known. The developed method can be also used for the effective solution of the integration of radiation furnaces into processes. The application of the developed method is demonstrated through a case study—the optimum arrangement of coil in a gas plant regeneration furnace.

Numerical Analysis of Radiant Section of Fired Heater Focused on the Effect of Wall-Tube Distance

Abstract Vertical cylindrical fired heater as used in crude oil atmospheric distillation unit is simulated numerically to analyse the distribution of thermal loading on the tubes. The model describes flow, gas combustion and radiative heat transfer inside the radiation section of the fired heater. Attention is focused on the tube-wall distance and the effect it has on circumferential distribution of heat flux on tube walls. It is shown that assumptions invoked in 1D design calculations of fired heaters are significantly underestimating heat flux variability.

Combined Approach Supporting Integrated Furnace Design and Retrofit

Existing complicated and complex calculation procedures for the design of process tubular furnaces create the main reason why the connection of targeting and detail design stage of furnaces (i.e., the continuous design and/or retrofit approach) is a serious problem in design practice. A complex, newly developed approach of furnace design and/or retrofit is presented in this paper. This comprehensive approach involves the application of heat transfer (radiation and convection) in equipment design, thermodynamic analysis for process design from the point of view of furnace integration, and the latest approach consisting in involving process fluid flow and hydrodynamic analysis to avoid potential problems in operation. This new methodology bridges existing gaps between modern targeting methods based on pinch analysis (providing optimized parameters such as the temperature of preheated air, stack temperature, and excess air) and the detailed design of furnaces. It is partly interactive and based on three stages of design: targeting, synthesis, and detailed design. It can be applied for both grassroots and retrofits problems.