Revamps 2024 Issue

Vacuum heater operational cycle improvement study

Coke formation creates short vacuum heater runs. Controlling film temperature and oil residence time can help reduce coke formation rate inside radiant tubes

Haytham Al-Barrak, Abdulaziz Mubarak and Mahendran Sella Saudi Aramco

P lant vacuum heaters have been experiencing short run lengths due to coke formation, which requires shutdown. A comprehensive approach must be fol - lowed to evaluate the heaters' performance, condition, and deficiency. This article focuses on vacuum heaters experi - encing a short run length of 24 months. The surveyed two vacuum heaters have four passes each, and the flow rate is distributed equally for each pass. There are two radiant cells for each heater, with both cells sharing one common convection section. Each cell is side-fired at the bottom with 18 burners, divided equally by a wall with nine burners on each side. The process tube inlet is located at top of the radiant section, while the tube outlet is located at the bottom of the radiant section close to the burners. Coke formation in vacuum heaters is caused by differ - ent mechanical and process factors, where radiant tubes absorb the released heat by burners through the tube’s out - side surface. Once the film temperature and residence time exceed the oil’s thermal stability, coke formation will occur. Crude oil stability varies and cannot be controlled; however, the oil film temperature and residence time can be controlled through heater design and operation. The main factors of coke formation in vacuum heaters include: • Crude residence time • Crude film temperature and thermal stability • Heater design. Crude oil residence time depends on feed rate, radiant section tube size and length, and velocity of steam injec - tion. Radiant sections contain between two and five tube sizes from the inlet to the outlet due to oil vapourisation. Oil film temperature increases when the tube size expands because the oil mass velocity decreases; thus, oil residence time increases. Radiant tubes with high residence time and high film tem - perature are more prone to coke formation. Injecting velocity steam into radiant section tubes lowers oil residence time, which reduces the coke formation rate. However, injecting steam may increase the pressure drop and have other side effects on the downstream tower. Despite the changes in the heater's feed composition and mass flow rate, the velocity steam was not adjusted to accommodate those changes. Crude film temperature determines the susceptibility of a

process fluid towards coking, where film temperature varies with composition change. If the film temperature exceeds the limit, the fluid film on the inside tube's surfaces is subject to thermal decomposition, which results in coke deposition at the location. Oil film temperature mainly depends on the critical balance between incoming tube external heat flux and internal oil mass velocity. The heat flux is described as the quantity of heat absorbed by the radiant tube per unit external tube area. Mass velocity is the mass of oil flowing through the heater tube per unit of internal cross-section area and unit time. Oil thermal stability varies depending on crude type. Some crude oils are less stable than others. The original crude’s maximum allowable film temperature is 510ºC based on crude composition specified at the design stage. However, it is worth mentioning that the current crude feed may have a lower film temperature than the design feed, which means it is susceptible to coking at a lower temperature than the design limit. The current crude blend should be thoroughly analysed in order to identify the new film temperature limit at which coking will start. This new film temperature will deter - mine the new tube metal temperatures (TMT) and heat flux. Heater design has a major role in influencing coke for - mation key factors. One of the important criteria in vacuum heater design is radiant heat flux. It is defined as heat trans - ferred per unit of tube (external surface) area. Designing the heater above the standard heat flux limitation may accelerate the coking rate formation. A second major factor of coke formation is the location of the outlet tubes where the feed is at the highest temperature. Vacuum heater out - let tubes have a larger diameter, resulting in lower mass flux combined with higher oil film temperature and, hence, increasing oil residence time. Therefore, this leads to a high rate of coke formation inside the tubes located near the highest heat flux zone. Methodology Several simulations of vacuum heaters have been con - ducted utilising simulation software. This software is well proven to be reliable and trusted industry-wide. The pro - gram calculates operating parameters such as firing rate, heater efficiency, and tube skin temperatures. In addition to its calculations, the software can simulate most types of

Revamps 2024