PTQ Q2 2022 Issue

Using future rows capacity to debottleneck fired heaters Opportunistic usage of the future rowprovision can lead to amultitude of benefits, provided the refiner knows what they are looking for during the revamp


F ired heaters are a critical clog in the wheel for a profitable refin - ery 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 prod - ucts at high temperatures, foul - ing 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 dimen- sion of fired heater operation is to operate the equipment with ‘mini- mal Opex’. This effectively means that the efficiency has to be max - imised. 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 21 000 tonnes of fuel in an operating year. Critical parameters in fired heater operation Fired heater design and subsequent operation generally satisfy cer- tain critical parameters that define the integrated operating window (IOW). Some of these parame- ters, 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 tempera- ture (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 pres - sure 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 tem - perature of the tubes in which the process fluid is heated. This metal temperature must be kept within a limit governed by tube or coil met - allurgy. Well-established codes and standards define this temper - ature 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 percent - age ratio of the heat absorbed in a furnace to the total heat input, where the heat absorbed is calcu - lated 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 unu - tilised heat input in fired heaters. A basic rule of thumb is that effi - ciency is enhanced significantly as the flue gas temperature at stack is reduced. However, from a pre - ventive maintenance point of view, this flue gas temperature at stack must remain well above the sul - phur dew point. One or more of the above param - eters may emerge as a bottleneck whenever more throughput is planned to be processed through

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