【Haichuan Chemical Salary Updates】Maoming Petrochemical: The RTO unit for exhaust gas purification at Silo No. 1 has been successfully put into operation

The last edit to this post was made by Desert Fish on 2018-11-13 at 14:40; it was originally posted by shangnenglsg on 2018-11-11 at 10:08. I’ve learned that thermosensitive materials have a significant impact on the operation of pumps. Everyone is welcome to participate in the following series of discussions – just click on the link to access them. -------------------------------------------------------------------------- This is a compilation post for the 【Haichuan Equipment Guessing Game】 series; all equipment enthusiasts are encouraged to take part! https://bbs.hcbbs.com/thread-2079686-1-1.html (Source: Haichuan Chemical Industry Forum) Summary post of the 【Haichuan. Pioneers in the Chemical Industry】 series. https://bbs.hcbbs.com/thread-1842627-1-1.html (Source: Haichuan Chemical Industry Forum) Summary post for the 【In-depth Discussion on Modular Installation in Haichuan】 – everyone is welcome to participate in the discussion! https://bbs.hcbbs.com/thread-1773168-1-1.html (Source: Haichuan Chemical Industry Forum) The compilation posts of the 【Guess the Haichuan Food Items】 series are being updated continuously – with thousands of different foods, can you recognize them all? https://bbs.hcbbs.com/thread-2142205-1-1.html (Source: Haichuan Chemical Industry Forum Website)

A Regenerative Thermal Oxider (RTO) is an energy-efficient and environmentally friendly device used to treat volatile organic waste gases with moderate to low concentrations. I. Introduction to the principle: Its basic principle is to oxidize organic waste gases at high temperatures (≥760°C) to produce CO2 and H2O, thereby removing them. The typical three-bed RTO main structure consists of a combustion chamber, three ceramic packing beds, and six switching valves (Figure 1). The regenerative ceramic packed-bed heat exchanger in this device enables maximum recovery of thermal energy (with a heat recovery rate of over 95%), allowing little or no fuel to be used in the treatment of VOCs. RTO has three states: startup warm-up state, operating state, and fault state. The procedures for each process are described as follows. When the RTO is started in the preheating mode, the main fan comes online, and fresh air is blown into the RTO combustion chamber through periodic switching of 6 valves. Purging is done for safety reasons, to prevent high concentrations of exhaust gases from remaining and causing an explosion during ignition. After the purging is complete, the RTO burner system starts operating. First, it checks whether the natural gas pressure is within normal ranges; if not, the conditions for ignition are not met, and the RTO will not ignite automatically. At this point, an alarm is displayed on the screen, and the relevant alarm issue needs to be investigated. Once the ignition conditions are met, the RTO initiates automatic ignition. First, the pilot flame is lit; once this pilot flame stabilizes, the gas solenoid valve activates to inject natural gas into the combustion chamber for burning. The UV flame detector checks for the presence of a flame every second. After successful ignition, the process continues until the combustion chamber reaches the temperature required for the reaction to take place. During this process, the main fan will keep operating to supply clean air through the purification pipeline; through the cyclic switching of the 6 valves, the preheating of the 3 packing beds is accomplished. In normal operation, the exhaust gas enters packing bed A through an automatic switching valve, where it is preheated to over 850°C. An oxidation reaction occurs in the combustion chamber, after which the gas exits via packing bed C. Once its temperature has dropped, it passes through the automatic switching valve and is discharged at the bottom of the chimney. Due to the high VOC concentrations and excess heat, some of the gas in the combustion chamber is discharged into the chimney through the high-temperature bypass valve. During this process, additional clean air enters the combustion chamber from packing bed B to purge the exhaust gases that remained in packing bed B from the previous process. After reaching the cycle time (T), the switching is controlled by a PLC program via 9 automatically switching valves at the bottom of the 3 packing beds; the exhaust gas enters through packing bed C and exits from packing bed B, while packing bed A undergoes a purging procedure. Continuous purification of exhaust gases can be achieved by using a PLC program to control the switching of the automatic valve. Fault condition: When a fault occurs or the system needs to be shut down for maintenance, the exhaust valve is opened, the waste gas flow to the RTO is stopped, the RTO goes offline, and the waste gas is discharged directly. At this point, the RTO combustion chamber is cooled and purged; the main fan continues to operate, and fresh air is circulated through 6 switching valves on a cyclic basis to cool the combustion chamber and the ceramics. Once the desired temperature is reached, the system stops. The tin-coating industry involves printing and coating inks on the surface of tinplate, followed by drying in an oven. During this drying process, solvents on the tinplate surface evaporate, generating VOCs, which are collected by exhaust fans and sent to the RTO exhaust manifold. Typically, metal stamping factories have several production lines, and VOCs are collected centrally for centralized treatment.

II. Advantages of RTO units The RTO unit has the following advantages: First, it has low operating costs and extremely low fuel expenses; when the concentration of organic waste gas is above 450PPM, no additional fuel is required for the RTO unit； Second, it has a high purification rate; the purification rate of the three-bed RTO is over 99% ； Third, it does not generate secondary pollutants such as NOX ； Fourth, fully automatic control with simple operation ； Fifth, waste solvents can be used as fuel ； Sixth, completely eliminate the odor ； Seventh, there are no side effects on the products produced in the drying room ； Eighth, low maintenance costs ； Ninth, it offers high security.

Over the past few years, as requirements regarding the emission of volatile organic compounds (VOCs) in environmental protection have become stricter, RTO technology has become increasingly widely used for the recovery and treatment of such organic waste gases. It is currently employed extensively in the petrochemical, pharmaceutical chemical, painting booth, paint and coating production, and chemical manufacturing industries. RTO technology provides an effective solution for the treatment of organic waste gases, opening a new chapter in the recovery and treatment of such gases in industries with intermittent production processes such as the chemical and pharmaceutical sectors. It has been almost 40 years since the first RTO was put into operation. Since the 1990s, RTOs have seen significant development; they have almost replaced traditional thermal incineration units and hold a dominant position in most areas of organic waste gas purification technology. Due to the significant differences in the basic conditions of various types of enterprises, the current limitations in the application of RTO systems, and the lack of safety-focused design on the part of RTO manufacturers and enterprises, production safety accidents have occurred after these systems were put into use for various reasons. For example, on December 10, 2017, an explosion occurred in the RTO environmental protection facility of a certain chemical (**) company; the cause of the explosion was a gas explosion, resulting from an ignition source within the gas pipes, although no casualties were reported. But it has also cast a shadow over the use of RTO by some companies. How to use and select RTOs safely and effectively has become an issue for enterprises.

Possible causes of the explosion at an enterprise’s RTO exhaust outlet: The concentration of organic waste gas emitted was extremely high in a short period of time (exceeding the design limit), which led to a sudden rise in temperature inside the combustion chamber as well as extremely high temperatures in the exhaust gases. After the intake of organic waste gas was shut off via interlocks, the gases were discharged directly through a bypass. Since this direct discharge pipeline was the same as the exhaust gas release pipeline, the high-temperature exhaust gases mixed directly with the high-concentration organic waste gas, resulting in an explosion in the exhaust pipe. Additionally, because no flame arrester was installed on the waste gas intake pipeline, the backflow from the explosion caused a fire to break out in that pipeline as well. Possible cause of the fire in a certain enterprise: The RTO was operating under positive pressure; when exhaust gas was introduced, the high-temperature gases in the combustion chamber flowed back, causing the PVC pipes (which are flame-retardant with a ignition temperature of around 256°C) to catch fire. Since no flame arrestor was installed on the exhaust gas inlet line, the fire spread upstream. Possible causes of a fire in a heavy oil storage tank at a certain enterprise: The exhaust gases from the production units combine with those from the tank’s exhaust lines and are sent to the RTO. In the event of a failure in the exhaust fan of the RTO, high-concentration gases from the production units flow back into the heavy oil storage tank; the high-speed movement of these gases generates static electricity, which causes the gases inside the tank to catch fire.

1 Overview of the incident: The RTO purification system at a chemical manufacturing company in Jiangsu experienced two explosions on March 8, 2015, at 9:43 a.m., and on March 27, 2015, at 3:20 a.m. The accident did not result in any casualties; however, the induced draft fan in the polymer polyol workshop was damaged, the instruments on site were destroyed, and parts of the RTO unit were severely damaged. The direct economic loss amounted to over 1 million yuan. 2. Production conditions in the workshop at the time of the accident: The company operates on an intermittent production basis. According to the information provided by the company, at the time of the accident, only the process exhaust gases from the POP, PL1/PL2 products were collected through metal pipes with diameters ranging from DN50 to DN350 (the main pollutants included ethylene oxide, propylene oxide, trimethylamine, isopropanol, styrene, acrylonitrile, etc.). After being collected, these exhaust gases were sent to an RTO for combustion using exhaust fans; this RTO was of the R-RTO type (rotary regenerative thermal oxidizer). The detailed process for exhaust gas collection and treatment is shown below.

3 Analysis of the accident causes 3.1 Direct causes of the accident Investigations revealed that the temperature at the exhaust outlet of the enterprise’s vacuum pump was above 73°C. Based on calculations regarding the condensation efficiency of organic waste gases, the formulas are given in (1)–(3). At 75°C, the maximum saturated mass concentration of organic compounds in the exhaust gas, as well as related data on the explosion range, are shown in the table below. According to theoretical calculations, when the temperature of the exhaust gas at the vacuum pump outlet is 75°C, the saturated concentrations of various organic compounds are extremely high; thus, safety accidents can easily occur if the dilution of the exhaust gas is insufficient. On-site measurements showed that the flow rate of ethylene oxide waste gas from a single PL vacuum pump was approximately 120 m3/h; with 3 such pumps in use, the total flow rate was 360 m3/h. At the time of the incident, the actual air handling capacity of the incinerator did not exceed 5000 m3/h. The overall volume percentage of organic compounds in the mixed gas was about 7.2%. Even when only one pump producing ethylene oxide waste gas was in use, the average concentration of 2.4% fell within the explosive range. It is evident that the exhaust gas leaving the vacuum pumps had an excessively high temperature, and since the boiling points of organic compounds are low, this led to high concentrations of pollutants. Additionally, the dilution factor was insufficient. Moreover, propylene oxide and ethylene oxide are chemically reactive substances, which ultimately resulted in the exhaust gas fed into the incinerator reaching its explosive limits, thereby causing the explosion. 3.2 Indirect causes of the accident 1) Improper design of the collection system. The investigation revealed that for high-concentration organic waste gases from vacuum pumps, enterprises did not carry out any pre-treatment such as condensation and recovery. Moreover, the collection methods currently used by these enterprises for the exhaust gases at the outlet of the PL system’s vacuum pumps are highly unreasonable; the umbrella-shaped collectors installed at the pump outlets have a limited capacity to capture gas, and the main exhaust gas collection pipe is only DN50 in diameter, which makes it difficult to ensure an adequate amount of dilution air during normal operation of the system. 2) The pre-treatment measures are inadequate. The activated carbon adsorption systems used in the POP, PL1, and PL2 workshops of this enterprise to handle organic waste gases are not equipped with desorption and regeneration systems, making them essentially ineffective. The stainless steel high-pressure fans installed at the end of these systems lack variable frequency control, which results in excessive negative pressure in the waste gas collection pipelines. This leads to higher energy consumption and hinders the condensation and recovery of organic substances. The water scrubbers made of metal are quite strong, but they cannot provide effective explosion relief in the event of accidents such as explosions; as a result, the shock waves generated by the explosions propagate along the pipelines into the production workshops, exacerbating the secondary damages caused by the explosions. 3) There are issues with the RTO furnace itself. Some of the products in this project contain chlorine. The main body of the equipment used is made of SUS304, while the rotating valves are made of SUS316L. Numerous cases have shown that due to their large mass, the support components are subject to significant stress corrosion. When the system contains chlorine (such as epichlorohydrin), pollutants like HCl are generated during high-temperature combustion, which can cause severe corrosion to the equipment’s main body and the rotating valves of the RTO furnace, making it difficult for the system to operate stably and effectively. Secondly, the waste gases from the project contain organic substances such as acrylonitrile and styrene. These substances tend to undergo self-polymerization at higher temperatures, which leads to an increase in the resistance of the regenerative thermal oxidizer’s heat storage ceramics after they have been in use for some time. Additionally, high-boiling-point organic substances adhere to the bottom of these ceramics, increasing the risk of safety incidents such as fires.

4 Accident prevention measures: RTOs are not prone to fire or explosion accidents under normal operating conditions. However, due to the complex and variable composition of waste gases in the fine chemical industry, as well as the large fluctuations in their concentration, this can lead to poor operational stability of incinerators, posing certain safety risks. It is recommended that RTO manufacturers and users adopt the following safety measures to prevent fires and explosions involving RTOs: 1) Optimize the collection system. The selection of exhaust hoods and fans should be carried out through standardized design; meanwhile, the exhaust gas collection pipelines need to be planned comprehensively, forming a collection and treatment system that consists of branch pipes, main pipes, treatment units, and a main exhaust outlet, to ensure effective exhaust gas collection. When designing the collection and pretreatment systems for flammable and explosive waste gases, it is better not to pursue excessive strength, as this contributes to the safety of the system. However, even when using equipment and materials with lower strength, blast relief diaphragms still need to be installed at components such as the RTO furnace itself and the waste gas collection pipes. 2) Strengthen pretreatment measures. Due to the significant fluctuations in the emission concentrations of waste gases in the fine chemical industry, it is necessary to mix, buffer, and pre-treat organic waste gases of various concentrations. It is recommended that companies use PP packed towers for the pre-treatment of such waste gases; however, since PP packed towers do not have high strength, they are prone to explosion in the event of an accident, so it is important to take measures to ensure the safety of the system as much as possible. 3) Gradual scientific debugging. When commissioning an RTO furnace, it is appropriate to start with no-load testing first. Once the no-load operation is stable, low-concentration organic waste gases can be introduced gradually, such as those generated from covered wastewater tanks in enterprises or from ventilation in workshops. Eventually, high-concentration waste gases can be introduced as well. At the same time, the emission flow rate and concentration of these high-concentration waste gases must be monitored, with particular attention paid to the peak concentrations. The organic concentration at any single exhaust point should be kept below 1000 ppm, with a maximum limit of 5000 ppm. 4) Install an online monitoring system and set up an operation room for the electrical control system. The RTO furnace purification system is a device that combines human operation with automated functions to a high degree. Although it is highly automated, it still requires dedicated personnel for maintenance and management. For example, before an explosion occurs in the RTO furnace, the concentration of organic substances often rises rapidly in a short period of time; if the system is monitored by humans, early warnings can be issued and necessary actions taken to prevent accidents from happening ； At the same time, an online TVOC concentration monitoring system is installed for the exhaust gases from various RTO systems, providing the necessary data support for enterprise management.

【HaiChuan Process Discussion】Summary and discussion on the fire separation distances between RTO incinerators and various enterprise facilities https://bbs.hcbbs.com/thread-2142307-1-1.html (Source: HaiChuan Chemical Industry Forum)