Commissioning and production of the crude benzene hydrogenation unit

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Commissioning and Production of the Crude Benzene Hydrogenation Unit Abstract: This paper describes the water flushing, gas purging, drying, airtightness checks, catalyst loading, and tube furnace heating processes carried out prior to the commissioning of Wuhan Iron and Steel Group’s 100,000 t/a crude benzene hydrogenation and refining unit, as well as the resolution of the abnormalities and issues that arose during the trial production phase. Keywords: 100,000 t/a benzene hydrogenation plant; commissioning. 1. Introduction: In order to comply with the industry policies and environmental regulations, address the issues related to the production capacity of Wuhan Iron and Steel Group’s crude benzene refining products, eliminate environmental and safety risks, and produce high-quality products to improve economic efficiency, Wuhan Iron and Steel Group built and put into operation a 100,000 t/a crude benzene hydrogenation refining unit in December 2009 (hereinafter referred to as the benzene hydrogenation plant). The device achieved successful commissioning in one go with qualified products, and then gradually reached full production capacity. During the construction, commissioning, and full-scale operation of a project, various challenges arise that need to be addressed. This section provides a review and overview of these issues. 2. Introduction to the facility: The Wuhan Iron and Steel Company’s 100,000 t/a crude benzene hydrogenation plant has a total investment of nearly 270 million yuan. Its main components include an hydrogen production unit, a hydrogenation unit, a pre-distillation unit, an extractive distillation unit, a xylene unit, oil storage facilities and loading/unloading systems, as well as production and living auxiliary facilities. The main processing technology is the imported German Uhde low-temperature catalytic hydrogenation and extractive distillation process, while the 550 Nm3/h hydrogen production unit utilizes the domestically developed pressure swing adsorption technology for hydrogen production from coke oven gas. The main equipment within this project area was subject to EPC general contracting by MCC Coking & Refractory, while the facilities outside the area, as well as some imported equipment and chemical reagents, were handled by Wugang. 3. Pre-commissioning inspections and trial operations of the unit 3.1 Process compliance checks as well as inspections of equipment, electrical systems, and instruments Given the high temperature, high pressure, flammability, and explosiveness associated with hydrogenation reactions, airtightness checks and hydrostatic tests are crucial for the success of the project; they constitute the prerequisite for its smooth commissioning and form the basis for ensuring safety. Furthermore, prior to the introduction of the process media and the commissioning of the system, process compliance checks and equipment inspections can promptly identify design flaws in the pipes, valves, instruments, flanges, and equipment, thereby saving on construction time and rework costs. During the construction process, strict quality control is implemented in accordance with the relevant standards for pressure pipelines, pressure vessels, as well as design and construction regulations. For key pipelines, valves, flanges, equipment, and welds, in addition to requiring factory inspection reports, multiple samples are taken on-site for testing of chemical composition and mechanical strength. For the nearly 30,000 meters of pressure pipelines and over 3,000 valves in the project, full participation was ensured in pipeline flaw detection, hydrostatic testing, sectional purging of the pipelines, and explosive purging. During the production of oil tank storage tanks, strict controls are applied to aspects such as the quality of steel plates, weld inspection, vacuum chamber testing, and hydrostatic testing, with monitoring and random inspections carried out in accordance with regulatory requirements. Pressure pipeline accessories such as safety valves and pressure gauges are required to be inspected and calibrated in accordance with regulations, and the accessories of equipment and instruments are also subject to relevant standards. During the inspection, the following issues were also identified and resolved: 1) The original design of the project provided only one steam line at 3.2 MPa coming from the coke dry quenching plant; to ensure smooth project progress and production stability, another 3.2 MPa steam pipeline was introduced from a nearby location. During the project commissioning and operation phase, this newly added steam pipeline played a crucial role. 2) A circulation line for the XS fraction at the bottom of the pre-distillation tower was added, ensuring the flow path of the XS fraction during commissioning. 3) It was found that the elevation of the cooler at the inlet of the high-pressure separator was lower than that of the high-pressure separator itself, which could lead to liquid accumulation in a U-shaped bend. After consulting Uhde Company and conducting repeated calculations, and taking the project timeline into consideration, no changes were made in this area. 4) The inverted U-shaped bend in the hydrogen circulation pipe, running from the outlet of the cyclohydrogen compressor to the pre-evaporator, was raised to better prevent the backflow of gas-liquid mixtures in the event of an emergency shutdown. 5) The rust on the inner wall of the main reactor was removed through acid washing and passivation, and the gas distributor at the inlet of the main reactor was modified. 6) Based on the damage to the trays in the pre-distillation tower observed in other projects, measures were taken to reinforce those trays and install check valves. 7) Defects caused by low temperatures that led to damage to certain refractory materials in the heating furnace were repaired; furthermore, since the DN150 valves used in the gas pipelines were difficult to adjust properly, DN50 control valves were added to ensure precise control of the temperature at the outlet of the heating furnace. 8) Given the presence of flammable and explosive hydrogen gas in the hydrogenation and hydrogen production areas, the contractor was instructed to upgrade more than a hundred explosion-proof switches in these areas from IIB grade to IIC grade. 9) In the hydrogen production area, drawing on the experience gained from the hydrogen production units in the cold rolling plant, additional purging pipelines and liquid discharge tanks were installed. Most importantly, a third return pipeline for the gas compressor, which had been overlooked during construction, was added, thereby preventing problems such as high pressure at the third-stage outlet or low pressure at the first-stage inlet of the compressor, which could otherwise make it impossible to make timely adjustments. At the same time, following the advice from the equipment department, diagonal supports were added to the compressor outlet pipeline, which eliminated severe vibration in the pipeline. 10) Cleaning and inspection inside the tower, including the detection of debris and iron pieces inside the tower, as well as serious issues such as misaligned flange gaskets at the reboiler inlet. 11) For the ESD system used for the first time by our company, multiple tests and training sessions were arranged, and the reports from Uhde Corporation were clarified. 12) To address the display defects in the DCS system, it was proposed to add data related to the recycle hydrogen compressor to the hydrogen production PLC display, to add cut-off valves to the DCS display, and to correct the issues in the process and display aspects handled by the design team. 13) In the design, the vent pipeline of the benzene hydrogenation unit was integrated with the coal intake pipeline of the coke oven, with no interface provided. It was necessary to create an opening for a negative-pressure pipeline at our company; after thorough discussions and adjustments to the coke oven operation, a DN500 interface was safely created using an opening machine. Introducing vent gas into the coal gas suction pipeline can reduce the costs associated with building a flare system. 3.2 Water operation, gas purging, drying, and airtightness testing: The main purpose of the water operation process is to remove debris such as rust and welding slag remaining in the equipment and pipelines. It also allows for testing the performance of instruments, control valves, and pumps, as well as providing an opportunity for staff to gain technical skills through training and accident drills. For water circulation, it is important to first flush out debris, install a temporary filter at the pump inlet, and set up a bypass line with a flow meter. During gas purging, drying, and airtightness testing, care should be taken to prevent overpressure in pipelines and equipment as well as to avoid dead zones; further details are not necessary here. 3.3 Catalyst loading and tube furnace heating: Before loading the catalyst, it is necessary to inspect the relevant grills, screens, ceramic balls, and thermocouples. Careful consideration should be given to the loading hoppers, canvas covers, and lifting plans to prevent the catalyst from becoming damp or exposed to rain. The catalyst loading was carried out successfully under the supervision of BASF and Uhde companies. To save time, the pre-reactor, heating furnace, and main reactor are isolated from the rest of Unit 100 using blind flanges, allowing catalyst loading and furnace drying to be carried out simultaneously with the trial operation of the cycle hydrogen compressor and nitrogen pressure testing in the remaining parts of Unit 100. 3.4 Hot-state airtightness, vulcanization, and material feeding It is worth noting that before introducing hydrogen into the entire hydrogenation unit, it is necessary to ensure that all checks and adjustments, as well as the nitrogen airtightness tests, have been completed. Since the foreign party responsible for the cyclic hydrogen compressor prohibits pressure levels exceeding 0.5 MPa when operating with nitrogen, our company adopted a process in which nitrogen was used to flow from the gas compressor to the make-up hydrogen compressor for the nitrogen airtightness tests, thereby enabling the conduct of tests at pressure levels of 0.6 MPa and 1.6 MPa. Care must be taken to prevent the outlet temperature of the hydrogen make-up compressor from becoming too high, and external water spraying should be applied. During the airtightness test, the construction party wrapped polytetrafluoroethylene tape around the exterior of the octagonal gasket to eliminate surface defects on the sealed surfaces of local recesses. The foreign party questioned whether polytetrafluoroethylene could withstand temperatures above 220°C; however, after consulting similar projects and through practical testing, it was confirmed that this would not have any adverse effect on the hot-state airtightness test. After the hydrogen gas hot-seal test was successful, we began the gradual temperature increase and introduced the BT fraction; after operating for some time, it was suddenly discovered that the flange of the short pipe at the inlet of the pre-evaporator was leaking benzene due to internal sand holes. Quick measures should be taken to reduce temperature and pressure, and arrangements should be made gradually to transfer the BT fraction from the high-pressure separator to the raw material buffer tank and then to the vent tank, in order to prevent loss of pure BT fraction. After the system has been cooled and depressurized to the required level, nitrogen purging is carried out, along with venting, while maintaining a slight positive pressure; the short pipe is disconnected for inspection and replacement. After the replacement, once the airtightness test was passed, preparations for vulcanization were carried out. After 32 hours of vulcanization, and once it was confirmed that the hydrogen sulfide concentration stopped decreasing and remained above 5000 ppm, light benzene was added. It should be added that the ratio of pure benzene to pure toluene purchased by our side is 1:1, while Uhde Company requires a ratio of 3:1 for the BT fraction to be used. Through simulations using Pro II software, we determined that this ratio meets the requirements, and practice has also shown that the BT fraction does not cause catalyst damage due to condensation in the pre-reactor under sulfidation conditions. 3.5 Abnormalities encountered during pilot production and their handling: Qualified products were produced within 4 days after the feeding of light benzene, and continuous production lasted for one and a half months, during which more than 7,000 tons of light benzene were processed. The main abnormalities that occurred during this period due to certain defects and insufficient operational experience, along with the corresponding corrective actions, are as follows: 1) Large fluctuations in external gas pressure led to liquid accumulation in the pipelines, which affected the tubular furnace and the hydrogen production unit; as a solution, additional drainage points were added and water was drained regularly. 2) Excessive liquid discharge from the collection tank caused the pressure in the raw material buffer tank to exceed 200 kPa, resulting in the sequential shutdown of UV-1027, UV-1028, UV-1029, and UV-1036. Solution: When draining the liquid from the collector, the manual valve should be opened slowly to prevent excessive pressure fluctuations in the raw material buffer tank. 3) The high-speed pump oil filter became clogged, resulting in low oil pressure that caused the high-speed pump to start operating, thereby leading to a chain shutdown. Solution: Pay attention to adjusting the oil pressure setting; when the oil pressure is low, reverse the pump operation or disable the interlock mechanism in a timely manner. 4) Due to the failure of the flow meter for circulating hydrogen FRCA_1012, the flow rate was set to 10,000 m3/h automatically, which caused the circulating hydrogen bypass valve FV_1012 to remain open. As a result, the H2 flow rate started to decrease, leading to an increase in the liquid level in the multi-stage evaporator and a decrease in the liquid level inside the high-pressure separator. Solution: Shut off the outlet of the high-pressure separator to control the pressure; at the same time, stop the feed to the buffer tank. Set the outlet of the high-speed pump to full reflux, and cease the small reflux at the top of the multi-stage evaporator. Manually control the temperature of the tubular furnace and increase the amount of H2 supplied, in order to vaporize the liquid in the pre-evaporator as well as that in the multi-stage evaporator. The liquid at the bottom of the multi-stage evaporator should be continuously discharged. Only after the system temperature stabilizes by 24:00 should feeding resume until normal conditions are achieved. 5) The vacuum pump tripped multiple times, causing fluctuations in the pressure of the stripping tower. Reason: The solvent vent pipe of the vacuum tank is blocked, resulting in high pressure that prevents gas from being evacuated. Blowing the vent pipe with steam can reduce its pressure. 6) The level gauges in the stabilizer reflux tank showed incorrect readings both at the site and in the control room, resulting in the tank being emptied. Solution: The level gauge has frozen; use steam to purge it to restore normal operation. 7) Solution to the sand in the drain valve at the bottom of the high-pressure separator: After contacting the manufacturer to confirm that there are no issues with the strength of the component, use a plug to seal the leak and arrange for nitrogen purging. 8) The total sulfur content in the pure benzene sample is less than 0.5 ppm, while the sample taken before shipment exceeds 2 ppm, reaching up to 15 ppm in some cases. Solution: The tank truck had previously transported light benzene, and cleaning was not thorough; a professional team was assigned to clean it using high-pressure water guns. Special tank trucks for transporting hydrogenated benzene should be used. 9) Reasons for the rupture of the bellows at the outlet of the pure benzene tank and measures taken: When loading the tank, the operator habitually closes the inlet valve of the loading pump, and the valve at the bottom of the tank is also closed; as a result, the benzene in the intermediate pipe section remains unvented. Under the continuous heating effect of the low-pressure steam heating pipes, the benzene vaporizes, causing the bellows to rupture. Switch to saturated vapor pressure steam and add a vent pipe. 10) Severe leakage in the reboiler E-3108 of the stripper, with steam entering the stripper and resulting in a forced shutdown: The reboiler of the stripper was not designed with expansion bends, and due to manufacturing defects, excessive temperature differential stress caused leakage at the tube sheet. Multiple repair welds proved ineffective, so it was redesigned and manufactured with expansion bends; after being put into operation for over 8 months, no abnormalities were observed. During the trial production phase, the ESD system was configured in strict accordance with Uhde’s design, which led to frequent emergency shutdowns. Additionally, most of the instruments and equipment could not reach their actual operating conditions during the water operation and joint commissioning phases; therefore, it took some time to adjust them. Furthermore, there are significant deviations in the design operating parameters, and it also takes time to determine the appropriate empirical operating parameters. 4. Issues resolved to ensure the normal operation of the unit at full capacity: During the trial production phase, the unit was shut down to replace the reboiler in the stripping tower, replace the adsorbent, add solvent, and address certain leakage issues; after these repairs, it resumed operation and entered normal production mode. Three months later, the processing capacity for light benzene reached 14.5 m3/h, meeting the design specifications. During this period, a deeper understanding was gained of the operations and coordination among various units, and the following issues were resolved: 1) Fluctuations in external steam pressure caused instability in the extractive distillation unit. Reason and handling: Low steam pressure causes fluctuations in the temperatures at the tops of the extractive distillation column and the stripping column, and the temperature at the bottoms of these columns does not reach the required level. Arrange for the power plant to ensure that the steam pressure remains at least 20 kgf; if the pressure is low, have other units reduce their feed accordingly. 2) Excessive drainage from the hydrogen production coal compressor, loss of adsorbents in the preprocessor; reasons for excessive drainage in the preprocessor and solutions: Inspection revealed a leak in the desorption gas heater, which caused steam to mix with the desorption gas and reach the 200# and 100# units ahead. 3) After replacing the adsorbent and the desorption gas heater, as well as fixing the valve with internal leakage; 4) In the event of a power outage outside the facility that caused both high and low voltage supplies to be interrupted, with only the UPS providing power to the DCS and ESD systems – it was necessary to promptly inspect Unit 100, activate the emergency relief valves to release pressure, prevent overheating of the catalyst bed in the main reactor, and switch to manual operation immediately. Stop the steam to the reboilers of each distillation column and arrange the circulation of each product. The UPS system should be checked regularly to ensure it can provide power for at least 30 minutes. 5) Causes of unstable pressure in the hydrogenation unit and solutions: 1) Analyze the raw materials; identify operational defects in the degumming tower in the recycling workshop, as the dry point of some light benzene substances is above 210°C. Strict controls should be implemented to ensure that the dry point of the light benzene used as feed for benzene hydrogenation remains below 150°C ； 2) The addition of demineralized water has a certain impact on the pressure in the hydrogenation unit; it is advisable to switch from continuous water addition to intermittent addition while adjusting the flow rate, ensuring that the pressure difference across the shell side of the pre-evaporator remains below 200 kPa. 3) The valve elements in the coal press of the hydrogen production unit have a short lifespan and are prone to internal leakage, which leads to frequent maintenance issues, insufficient hydrogen production, and low outlet pressure. Replacing these valve elements with plastic ones can extend their service life and allow for a slight reduction in the pressure of the gas being released. 6) Causes of reduced vacuum level in the stripping tower and corrective measures: 1) Inspect the vacuum pump system, replace worn bearings in a timely manner, and replace the filters. 2) Check the amount of cooling water used in the stripping tower’s condenser as well as the temperature after condensation; high temperatures can lead to a decrease in vacuum level, so it is necessary to increase the amount of cooling water in the condenser and reduce the heating in the vacuum pump’s pipelines. 3) Inspect the outlet pipe of the vacuum pump. 4) Check the vacuum pump cooler and the cooling water used. 5) Calibration results: The calibration results after reaching full production capacity for benzene hydrogenation are shown in the table below. | Serial No. | Item | Design Value | Current Operating Level | Remarks |
| --- | --- | --- | --- | --- |
| 1 | Plant capacity | 12.5 t/h | 12.76 t/h | 14.5 m³/h |
| | Operating parameters | Pre-reactor temperature rise: ℃ | 209 | Main reactor temperature rise: ℃ | 4519 |
| | | Circulating hydrogen volume/hydrogen-to-oil ratio | 11874/873 | 10714/740 | Supplementary hydrogen volume | 550/490 |
| 2 | Product quality | Benzene: 99.95%, neutrality level: 99.95% | Toluene: 99.80%, neutrality level: 99.50% | The clay tower is not in use |
| | | Xylene: 96% | 96.5% | |
| 3 | Utility consumption | Cooling water: 1600 m³/h | 1450 m³/h | Medium-pressure steam: 32 t/h | 29 t/h |
| | | Electricity: 840 kWh | 790 kWh | Gas: 450 m³/h | 228 m³/h |
| 4 | Conclusion | 1) Thorough, effective, and meticulous preparation before commissioning is key to ensuring successful operation from the first attempt. <br>2) The installation of the ESD system ensures system safety; interlocks between the tubular furnace, circulating hydrogen compressor, and high-speed pump must be properly implemented, with some settings adjusted according to actual production conditions. <br>3) Inspections of equipment prior to delivery and on-site quality checks play a significant role in achieving continuous and stable production and thus economic benefits. <br>4) Domestic benzene hydrogenation technologies and equipment are now mature, enabling effective reduction of project costs. | |