Avoiding unscheduled shutdowns
Case studies have demonstrated that correct heater design and consequent performance will determine crude unit run length
Gary R Martin and Tony Barletta, Process Consulting Services
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Several refiners are targeting crude unit run lengths of six to eight years to maximise profits through increased onstream factor and lower maintenance costs. Moderate to rapid fired-heater coking causes unscheduled shutdowns. Coke forms because the oil in the tubes is no longer thermally stable at the operating conditions. Oil temperature and oil residence time control the rate of coking for a given feedstock.
Oil temperature alone is not a good predictor of coke formation rate. Crude oil feedstock stability varies, but it cannot be controlled. However, temperature and oil residence time can be controlled through heater design and operation. Fundamental design principles influence coke formation rates and correct design parameters must be used to build a more reliable heater or understand why an existing heater is coking so that it can be fixed.
For many refiners, the vacuum heater performance will determine crude unit run-length. Two case histories will highlight some of the problems that cause rapid coking.
Meeting six- to eight-year run-length and product yield targets requires very low coking rates. Coke forms an insulating layer inside the tube, which increases the outside tube metal temperature (TMT). Once the maximum TMTs are reached, the coke must be removed. This requires a shutdown, otherwise, tube life is reduced or tube failure can occur. Minimising oil residence time and oil film temperature are the keys to meeting run-length.
Minimising oil residence time at high film temperature is essential to limit coke formation. Very high oil film and bulk temperatures can be maintained at an acceptable coke formation rate if the oil residence time is kept low. Conversely, if residence time is high, then the oil temperature must be kept low.
High oil residence time heaters must operate at low outlet temperature and low heavy vacuum gasoil (HVGO) product cutpoints. Radiant section residence time varies from less than 10 to more than 90 seconds, depending on heater design and operation.
Oil film temperature, not the bulk oil temperature, should be maintained as low as possible. The bulk oil temperature measurement may be 790°F, while the peak film temperature is 950°F (Figure 1).
Peak, or maximum, oil film temperature occurs on the inside wall of the tube. This temperature is dependent on peak heat flux and oil mass velocity. Peak heat flux occurs on the 15–20 per cent of the tube outside surface area facing the burner flame. Burner fuel combustion rate depends on flame length and flame stability; therefore, measured peak heat fluxes vary from the heater floor to the radiant section outlet (Figure 2).
The lower the peak heat flux, the lower the peak oil film temperature for the same bulk oil temperature. Oil mass velocity depends on feed rate and radiant tube size. Increasing the oil mass velocity will lower the peak film temperature for a fixed heat flux and bulk oil temperature.
Radiant section heat flux rate is defined as the quantity of heat absorbed by a given outside surface area of the tube:
Heat flux =
Quantity of heat absorbed/ Outside tube area = Btu/hr-ft2
Mass velocity (flux rate) is the mass of oil flowing through the heater tube cross-sectional area:
Mass flux rate =
Mass rate of oil/Inside Cross-sectional area of heater tube = lb/sec-ft2
Dry and wet vacuum units
Vacuum units are designed either dry or with steam (wet or damp). Wet or damp systems use steam in the heater coils, while dry units do not inject steam. Dry versus wet design is a controversial issue, with the arguments for dry design focusing on lower capital and operating costs versus wet designs that can achieve higher product yield and better heater reliability.
Dry vacuum units are less costly to build and they have lower operating costs. Generally, dry vacuum units operate at relatively low HVGO product yields (lower TBP cutpoints) or they have short heater run-lengths. A dry vacuum heater can be designed for high HVGO cutpoints on light crude oil. However, it is not possible to achieve long run-length and a high cutpoint when processing heavy crude oil.
Dry heaters must be designed carefully, otherwise they will coke or they must be operated at low HVGO cutpoint. Product yield and heater reliability are important factors because of their impact on profitability.
Process side and/or fired side problems can cause high rates of coking. Average radiant section heat flux, total firing rate, and oil outlet temperature are often used to characterise heater severity. While these parameters are useful and can help, they may not be accurate predictors of coking rate.
Monitoring coking rate
The rate of coke formation cannot be measured directly; however, it can be inferred. A common method uses infrared scans to determine TMTs, which help identify “hot spots” and areas prone to coking. Hot spots indicate high heat flux and/or coke. Once enough coke is deposited to form hot spots, it is often too late to take corrective action and heater firing must be reduced. Oil cracking produces coke and gas. Vacuum ejector system offgas flow rate is a good measure of the rate of cracking and it should be used to infer coking.
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