Controlling film temperature in fired heaters
Film temperature control is critical to the successful design of fired heaters, especially for heaters employed in upgrading heavy feedstocks
JINYU JIAO, YURIY MORAYKO, MORTEN THEILGAARD and MICHAEL HO
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Direct-fired heaters have been widely used in the oil refining and chemical process industries to heat the crude oil contained in tubular coils by the combustion of fuel within an internally insulated enclosure. A successful fired heater design relies on many factors. Film temperature control is one of the key factors that play a crucial role in fired heater design, particularly for units processing heavy feedstocks that are thermally unstable, such
as Canadian oil sands-based feedstocks.
Film temperature determines the susceptibility of a process fluid towards coking. Bulk oil temperature plus a temperature rise across the oil film sets the film temperature. In most applications, it is the oil film temperature, not the bulk oil temperature, that limits the heater duty and the oil life.1 Film temperature is an important factor in fired heater design for many reasons. Firstly, oil degradation starts in the fluid film, since this is the hottest place for the bulk oil. Fluid life is shortened because of degradation, which can lead to a costly result. Secondly, if the film temperature exceeds the limitation, the stationary fluid film on the inside tube surfaces is subject to thermal decomposition, which results in coke deposition at that location. Coke deposits increase resistance to heat transfer and raise the tube metal’s temperature. Once the tube wall temperature reaches the design temperature, the heater must be shut down for decoking to avoid coil damage. Thirdly, overheating of the fluid film accelerates the fouling rate. Fouling requires more heat input and a hotter tube metal temperature to maintain the same heater outlet temperature. These factors cause heaters to shut down much more frequently and eventually reduce the whole plant’s profitability.
Due to the importance of the film temperature, its control has become a hot topic for fired heater designs, especially for crude heaters, vacuum heaters and coker heaters. In this article, some feasible methods of controlling the film temperature for fired heater design are presented. Methods including steam injection, reducing tube size, using double firing and lowering the average heat flux are discussed, and some methods are presented with accompanying examples.
Steam injection is one of the best options for lowering the film temperature, as long as there are no unintended consequences downstream of the units. Steam reduces the oil residence time by increasing the fluid velocity. High fluid velocity improves heat transfer in the film layer, which lowers the differential temperature between the tube wall and the bulk fluid. Figure 1 shows the effect of steam injection on film temperature for one crude heater. The horizontal axis represents the radiant coil growth from inlet to outlet. It can be seen that the film temperature drops around 20°C after injecting 1 wt% of steam into the fluid. The results shown in Figure 1 arise from the simulation of a crude heater at a design duty of 63 MW with diluted bitumen as the process feed. The simulation was performed with a commercial fired heater rating program, FRNC-5PC, which has been widely used and has been proven to be reliable in predicting heater performance.
Selecting the correct location and amount of steam injection is critical. It must be injected upstream of the heater tubes with the highest film temperature, yet far enough downstream in the radiant section to minimise incremental pressure drop to ensure charge pump capacity is not exceeded.2
Steam injection can also change the flow regime for two-phase flow. The problem associated with slug flow can be mitigated by steam injection. However, it is not the intention of this article to discuss the effect of steam injection on the flow regime because it is not related to the topic. Caution is advised in selecting a suitable steam condition to match the process fluid condition, to make sure that no condensation occurs after steam injection into the process fluid.
Reducing tube size
Depending on the allowable pressure drop, fired heater coils are usually divided into multiple passes to accommodate the total flow and to meet pressure drop requirements. For a given flow rate and the number of flow passes, oil mass velocity increases with reducing tube size. A higher oil mass velocity reduces the oil residence time and increases the film heat transfer coefficient, therefore lowering the film temperature. Figure 2 shows the effect of tube size on film temperatures. The horizontal axis represents the heater coil growth from radiant inlet to radiant outlet. The results come from the simulation of a small heater with a design duty of 15 MW. The heater is a vertical cylindrical type with four-pass flow. To demonstrate the effect of tube size on the film temperature, the radiant tube size is changed from 6.625in to 4.5in, with no change in the size of the convection tubes. More radiant tubes have been added to the radiant section to compensate for surface loss caused by the reduction in tube size, to keep the same heat flux. Case 1 is for the heater with all tubes sized at 6.625in, while case 2 is for the same heater with tube sizes changed from 6.625in 4.5in, for the radiant coils only. It can be seen that the film temperature in the radiant section has been decreased because of the reduction in tube size.
Reducing the tube size increases the heater pressure drop, which requires a much higher pump head upstream of the heater. Since film temperature control is basically intended to control the peak film temperature, it is advisable to reduce tube sizes for tubes with peak film temperature only, to minimise the increase in pressure drop caused by reducing tube sizes.
Double fired vs single fired
A fired heater can be single or double fired. The heat flux on the tube’s circumferential surface is not uniform because of the shading of radiant heat. The single-fired heater receives radiant heat on one side of the process tubes (directly from the burner flame), while the other side of the tubes, facing the heater wall, gains radiant heat from the refractory. The portion of the tube facing the burners has a higher local heat flux, while the side facing the refractory is much lower. For a given fired heater with nominal two diameter tube spacing and a very uniform longitudinal heat flux distribution, the local peak heat flux (qm) is approximately 1.8 times the average heat flux (qa) for single-fired heating. In contrast, the double-fired heater has radiant heat on both sides of the tubes, which greatly reduces the peak flux to about 1.2 times the average heat flux.3 The correlations mentioned above for single and double fired can be simply represented in the following equation:
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