Using future rows capacity to debottleneck fired heaters
Opportunistic usage of the future row provision can lead to a multitude of benefits, provided the refiner knows what they are looking for during the revamp.
Akhil Gobind, Ankur Saini, Rupam Mukherjee and Shilpa Singh
Engineers India Limited
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Fired heaters are a critical clog in the wheel for a profitable refinery operation. Fired heaters consume almost 60% of the energy input in a typical refinery, which speaks volumes on the importance of dependability and reliability of this equipment. Interestingly enough, fired heaters have long been an important consideration in any refinery capacity expansion plan, with significant effort and Capex diverted to debottleneck this piece of equipment.
In fact, in the case of existing unit operations, fired heaters are often the reason for operating the plant at less than full capacity. Since fired heaters deal with combustion products at high temperatures, fouling and deterioration of the heat transfer surface area are inevitable occurrences. As a result, regular operation or optimum heat transfer are hampered as the performance parameters hit their allowable limits over time.
Another very important dimension of fired heater operation is to operate the equipment with ‘minimal Opex’. This effectively means that the efficiency has to be maximised. This study shows that even minor efficiency improvements can lead to significant fuel savings with associated reductions in carbon emissions.
Revamps to existing fired heaters may be considered for a number of reasons: capacity expansion of the unit; operational improvement through efficiency increase; or to remove bottlenecks primarily due to ageing and deterioration of the equipment. This article describes a case study evaluating options to debottleneck a mid-size furnace in a hydrotreating unit that consumes around 21000 tonnes of fuel in an operating year.
Critical parameters in fired heater operation
Fired heater design and subsequent operation generally satisfy certain critical parameters that define the integrated operating window (IOW). Some of these parameters, which impact and may limit revamp objectives, are:
Firebox temperature: Operating the fired heater within or near to the design average radiant flux is an important safety consideration. The more tangible form of radiant flux is interpreted as Bridgewall temperature (BWT) or firebox temperature. Design specifications for radiant tube supports and hangers must be adequate for BWT in the radiant section. Over the years, these pressure part supports bear significant loading and will suffer wear and tear. Thus, any increase in BWT may have adverse impacts through increased wear and tear of these mechanical components.
Tube metal temperature: Radiant flux also affects the metal temperature of the tubes in which the process fluid is heated. This metal temperature must be kept within a limit governed by tube or coil metallurgy. Well-established codes and standards define this temperature limit. Standard operating procedures (SOPs) of individual operating companies define this temperature limit, keeping a certain margin below the standards.
Fired heater efficiency: Efficiency is usually defined as the percentage ratio of the heat absorbed in a furnace to the total heat input, where the heat absorbed is calculated from the difference between the heat input and heat losses. The total heat input is the sum of the heat of combustion of the fuel and the sensible heat of all incoming streams:
Thermal Efficiency=(Total Heat input-Stack heat loss-Radiation heat loss)/(Total heat input)
A more common way to express the efficiency of fired heaters is to use ‘fuel efficiency’. The following equation defines the fuel efficiency of fired heaters:
Fuel Efficiency=(Total Process absorbed duty)/(Heat input from fuel only)
In fact, fuel efficiency is a useful indicator of the cost of fuel being fired to operate the heater, the main component in terms of the Opex or cost of energy incurred in attaining a specific process duty.
Stack losses form a major part of efficiency loss among the unutilised heat input in fired heaters. A basic rule of thumb is that efficiency is enhanced significantly as the flue gas temperature at stack is reduced. However, from a preventive maintenance point of view, this flue gas temperature at stack must remain well above the sulphur dew point.
One or more of the above parameters may emerge as a bottleneck whenever more throughput is planned to be processed through a fired heater or when the heater is required to be operated with more severity. Standards for the design of fired heaters (API 560) require space to be provided in the convection section for additional tubes, the so-called ‘future rows’. This article will focus on the possibility of utilising the future rows provision included in the design of the fired heater convection section (see Figure 1) to keep these critical parameters within their intended operating limits and enhance the operating efficiency with minimal complexity.
The American Petroleum Institute’s Standard 5601 rigorously defines best design and sizing practices for fired heaters. The API standard recommends including provision for two future rows in the convection section in the design of new fired heaters. The basic idea is to ensure space is available to install additional rows in the event of the need for further surface area augmentation without any major capital or labour-intensive revamps. See Figure 2 for a visualisation of convection future rows.
If these future rows are located at the breeching or top section of the convection section, utilising them will require changes to the heater terminals and the transfer line, which may not be an easy solution to implement.
Utilising future rows to increase the capacity of fired heaters
Refineries worldwide are pushing their operating units to extract more from their existing hardware. The fired heater often jolts this ambition. The firebox temperature increases near the allowable limit, or the metal temperature, read by tube skin thermocouples, reaches close to its metallurgical limit. In such events, adding rows into the space provided can offer minor relief and an effective solution to increase the unit’s throughput appreciably. Refer to Table 1 for a comparative case study.
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