Revamps 2023 Issue

Detecting and monitoring temperature runaway in the catalytic bed is crucial to identifying this phenomenon and taking appropriate mitigation measures in a timely manner. A key temperature runaway indicator is an abrupt and abnormal increase in reactor temperature. It is important to regularly monitor temperature distribution throughout the catalytic bed to identify any potential issues. Temperature runaway can have various adverse effects, including a sig- nificant increase in coke laydown rate and sintering of the catalyst’s active phase, leading to an elevation in pressure drop within the reaction section. Therefore, a rising pres- sure drop can serve as an indication of temperature run- away in the hydroprocessing unit. Finally, it is essential to ensure that the processing unit exhibits a uniform tempera- ture distribution across the catalytic bed and to monitor for any abnormal increase in pressure drop. High-pressure separator vs cold low-pressure separator Hydrotreating units that process lighter feeds, such as naphtha, exhibit different physical properties in the hydro- treated stream compared to water. In these cases, a single separation vessel is sufficient. However, when dealing with heavier feeds like LCO and gasoils, two separation vessels are employed due to the closer physical properties between hydrotreated feed and water, making the separation pro- cess more challenging. A high-pressure separation vessel, typically a vertical vessel, is used to separate the recycled gas (H₂, H₂S, and NH₃) from the liquid phase containing water, hydrocarbons, and dissolved H₂S and NH₃. The liquid phase then under - goes separation in a low-pressure separation vessel, com- monly a horizontal vessel. This configuration is commonly found in high-severity hydrotreating units, particularly in modern diesel hydrotreaters handling unstable feeds like LCO and coker gasoil (CGO). The choice between various separator configurations – hot high-pressure separator (HHPS), cold high-pressure sep - arator (CHPS), and cold low-pressure separator (CLPS) – is influenced by energy consumption analysis. Implementing an HHPS configuration offers the advantage of potential energy savings since the hydrotreated stream can be fed to the stripping section without the need for reheating. Furthermore, the HHPS configuration allows for smaller air cooler systems and enables water injection after the hot separator, utilising high-quality metallurgy materials in a smaller section of the processing unit. On the other hand, a CHPS configuration helps maintain low contaminant con - centrations in the recycle gas, which is crucial for maintaining the performance of hydrotreating reactions by preserving hydrogen partial pressure. However, this configuration may require higher consumption of make-up hydrogen. When selecting the configuration, performance con - siderations often lead to the choice of HHPS and CLPS for high-severity processing units handling heavier feeds. Nonetheless, factors such as unit dimensions and the com- position of make-up hydrogen should also be considered. Additionally, historical considerations of the refining asset may influence the preference for low-pressure separators

to ensure the safety of operators by avoiding exposure to high-pressure systems. Steam turbine replacement The choice of a backpressure steam turbine instead of a motor can prove beneficial as boiler steam generation efficiency is typically 80+% while the thermal efficiency of electricity generation is typically about 33%. Therefore, the energy requirement of the backpressure steam tur - bine is typically less than 50% of that of the electric motor. An important consideration, however, is that the exhaust steam from the turbine must be of use within the refinery. A case study was presented wherein a hydrotreater was designed with a compressor driven by a backpressure steam turbine. The exhaust steam from the turbine, which operates between the high-pressure (600 psig/4.14 MPa) and medium-pressure (150 psig/1.03 MPa) headers, is sent to hydrotreater reboilers with excess medium-pressure steam utilised by other consumers in the refinery. However, at certain time periods throughout the year, the exhaust flow of medium-pressure steam is more than demand due to seasonal factors and changing refinery-unit-oper - ating modes. This leads to the letdown of the excess to the low-pressure (25 psig/0.17 MPa) header for use in the refinery and the venting of a large amount of steam. To remedy this problem, the backpressure steam turbine can be replaced with an extraction turbine, which will pro- vide both medium- and low-pressure exhaust streams. This replacement is beneficial as the specific steam consumption for the expansion of the steam from 600 psig (4.14 MPa) to 25 psig (0.17 MPa) is about 45% lower than for the expan - sion from 600 psig (4.14 MPa) to 150 psig (1.03 MPa). When the demand for medium-pressure steam drops, the amount of steam exhausted to the low-pressure header is significantly lower than with the backpressure steam tur - bine and no venting takes place. The $1MM higher invest - ment cost for this turbine and the $400 K/y in operating savings translates into a payback period of 2.5 years. Hydrogen consumption The most significant energy savings will be indirectly real - ised through technological developments that reduce over- all H₂ demand. Hydrotreating units require a large amount of hydrogen to operate successfully. Many advances are being made with the introduction of new processes and catalysts that will require less hydro - gen during processing. Hydrogen production can be very energy-intensive and, therefore, a reduction in hydrogen demand will indirectly result in significant energy savings for the overall plant. Optimised consumption Hydrogen consumption associated with the hydrotreatment process is inevitable, as it directly involves hydrogenation reactions targeted at achieving precise product specifica - tions, such as hydrodesulphurisation or aromatics hydro- genation. Hydrogen consumption in a hydrotreater arises from both hydrotreatment reactions and losses occurring within the process.

Revamps 2023