Analysis and improvement approaches for ammonium blockage in the ceramic monolith of Regenerative Thermal Oxidizer in the Chemical industry
1. Introduction to Ceramic Monolith
Ceramic monolith holds a crucial position in RTO. With the advancement of RTO, the types and geometric structure of heat storage monolith have witnessed significant alterations. Initially, the researchers employed elliptical pebbles as the thermal storage materials, considering that they could effectively store heat energy and were relatively easy to obtain. Nevertheless, the pebbles had issues such as poor ventilation and the tendency to compress the gaps, leading to suboptimal ventilation and influencing the efficiency. Additionally, pebbles were not resistant to high temperatures and temperature fluctuations, and were prone to breakage, which could have adverse effects on equipment efficiency. Subsequently, ceramic saddle rings became the option for a new type of heat storage media, addressing the problem of pebbles. This type of heat storage material ensures the consistency of the gaps, enhances the uniformity of the material, and significantly boosts the efficiency of the heat storage process. Currently, modern RTO systems typically utilize honeycomb ceramic monolith fillers with high recovery efficiency and low airflow resistance, including regularly arranged fillers and dispersed particle fillers, as well as other shapes such as spherical, tubular, corrugated plate, and saddle-shaped ceramic monolith. As depicted in table 2-1, honeycomb ceramic monolith is usually favored because it has a short commutation cycle, smaller resistance loss, larger specific surface area, and higher heat transfer efficiency. Besides, it has a smaller coefficient of thermal expansion, a more compact structure, and a longer service life.
Table 2-1 Comparison of commonly used ceramic regenerator in RTO
| Performance | Spherical ceramic monolith | Honeycomb ceramic monolith |
| Specific surface area | Small | Large |
| Density | Honeycomb ceramic is 1/10 of ceramic ball | |
| Heat storage and release capacity | Low | High |
| Commutation cycle | 180-300s | 30-60s |
| Resistance of airflow | Large | Small |
| Uniformity of medium temperature | Nonuniform | Uniform |
| Service life | No difference | No difference |
| Material requirements | Low | High |
| Accumulate dust | Easy | Difficult |
"The material of the heat storage monolith is of crucial significance. In RTO equipment, metal media are not appropriate as the high-temperature working condition may cause damage to metal materials. The ceramic monolith emerged as the prime choice, encompassing materials like mullite and cordierite. Ceramic monoliths possess multiple advantages, like oxidation resistance, high-temperature endurance, chemical corrosion resistance, strong thermal conductivity, good thermal shock resistance, and a relatively low cost, thus they are widely utilized."
2. Analysis of the Ammonium Salt Blockage Issue in Ceramic Monolith
Due to the periodic heat storage and release in RTO, ceramic monoliths are periodically subjected to both high and low temperature environments and are inclined to phenomena such as pore collapse, rupture, and agglomeration. In practical applications, it has been discovered that apart from the blockage phenomenon caused by the ceramic monolith itself, the composition and treatment process can also lead to the blockage of the ceramic monolith. Below, we will take actual engineering projects as examples to analyze in detail the composition and formation mechanisms that result in the ammonium salt blockage of the ceramic monolith.
A pharmaceutical company in Hubei Province mainly manufactures Vitamins and other drugs through fermentation technology. The main components of the exhaust gas are ethanol, benzene, toluene, ethyl acetate, triethylamine, oxazole, HCl, heptacycline, n-butanal, methane, hydrogen sulfide, ammonia, etc. The total flow rate is 50,000 m³/h. During the production process, the fermentation workshop will generate a large quantity of exhaust gas and wastewater, among which the wastewater will produce a peculiar odor when discharged into the sewage treatment station for treatment. The "spray + RTO" process is utilized to treat these two parts of the exhaust gas and discharge them after meeting the standard. After a month of stable operation of the process, the RTO inlet and outlet pressure difference gauge indicated a pressure difference greater than 4500Pa, so the equipment was shut down for maintenance. During the maintenance, it was found that there was obvious blockage in the ceramic monolith, as shown in Figure 3.1.
Figure3.1 Comparison of ceramic monolith before and after being blocked by ammonium salt
2.2 Analysis of Blockage Induced by Ammonium Salt
Ammonium salt blockage is one of the crucial problems confronted by ceramic monolith. The main cause for the formation of ammonium salts is that the exhaust gas encompasses compositions which are prone to generating ammonium salt after RTO incineration, and they form in the low-temperature area beneath the ceramic monolith. As shown in Figure 3.2:
Figure 3.2 Schematic Diagram of Ammonium Salt Formation in Ceramic Monolith
The ammonium salts that cause the blockage of thermal storage systems typically include ammonium chloride, ammonium sulfate, ammonium carbonate, ammonium nitrate, and triethylamine hydrochloride.
Specifically, the following are the properties and effects of different ammonium salts:
(1)Ammonium Chloride: Ammonium chloride is a white crystal that is readily soluble in water but insoluble in ethanol and ether. Decomposing into ammonia and hydrogen chloride at high temperatures can cause corrosion to RTO, especially in the low-temperature region of RTO.
(2)Triethylamine Hydrochloride: Triethylamine is an organic compound that forms triethylamine hydrochloride with HCl. This substance is irritating and prone to moisture absorption.
(3)Ammonium Sulfate: Ammonium sulfate is an acidic salt that easily decomposes into ammonia and ammonium bisulfate at high temperatures and is also hygroscopic. This salt can cause acidic conditions in the RTO device and damage the equipment.
(4)Ammonium Nitrate: Ammonium nitrate decomposes into ammonia, nitrogen dioxide, and water at high temperatures, which can lead to unstable gas composition, and its hygroscopicity can aggravate the blockage problem of ceramic monolith.
(5)Ammonium Carbonate: Ammonium carbonate is easily decomposed into ammonia, carbon dioxide, and water at high temperatures and has high hygroscopicity, which has adverse effects on RTO devices.
Based on the exhaust gas parameter table and on-site production process of this project in Hubei, as well as the analysis of the RTO site situation, the main reason for the blockage at the bottom of the ceramic monolith is caused by salt substances such as triethylamine hydrochloride.
3. Control Measures for Ammonium Salt Crystallization
Ammonium salts have the following significant characteristics:
(1)Ammonium salts are crystalline and ionic compounds.
(2)Ammonium salts are inorganic salts that are readily soluble in water.
(3)Ammonium salts are unstable and prone to decomposition when heated.
(4)Ammonium salts are mainly generated in the bottom layer of ceramics.
Based on its characteristics and practical engineering experience, the following measures are adopted:
3.1 Prevention of Ammonium Salts Formation
A. Classified Collection and Governance
a.Collect and treat ammonia-containing exhaust gas separately and do not mix it with chlorine and sulfur-containing exhaust gas.
b.Collect and treat chlorine-containing exhaust gas separately and do not mix it with ammonia-containing exhaust gas.
c.Collect and treat sulfur-containing exhaust gas separately and do not mix it with ammonia-containing exhaust gas.
B. Take Pretreatment Measures to Reduce at the Source
a.For exhaust gas containing both small amounts of ammonia and organic compounds such as chlorine, sulfur, and nitrogen, use acid washing, alkali washing, and demist processes to remove ammonia-containing components from the exhaust gas at the front-end to reduce the generation of ammonium salts.
b.For exhaust gas containing both ammonia and a small amount of HCl and SO2, use alkaline washing + demist process to remove acidic components from the exhaust gas at the front end to reduce the generation of ammonium salts.
3.2 Slow Down the Generation of Ammonium Salts
Based on the decomposition temperature, adopt measures such as preheating, heat tracing, hot air blowing, and insulation at the front pipeline to increase the temperature and reduce the generation of ammonium salts.
3.3 Slow Down the Blockage of Ammonium Salts
Employ ceramic monoliths that are not prone to blockage, such as honeycomb ceramics with large pore size and plate-type ceramic, which can effectively reduce the risk of ceramic monolith blockage.
3.4 Special Structural Design for RTO
A. Quick Disassembly Design of Access Door
Install access doors around the ceramic monoliths at the bottom of RTO. When ammonium salts occur, the 12 access doors can be quickly opened and then water can be used to rinse the ceramics to dissolve the ammonium salts inside the ceramics. And the water can flow out from the bottom drainage outlet.
Figure 4.1 Quick disassembly design
B. Comprehensive Drainage Structure of RTO Body
RTO is furnished with a thorough structure at the bottom. When rinsing the ceramics, the water can rapidly discharge from the bottom drainage outlet. Each regenerative chamber is provided with a drainage outlet, with a total of 12 drainage outlets for discharge. The outlet chamber is also outfitted with a drainage outlet. All the water from the drainage outlets is converged to the main sewage pipeline for discharging.
Figure 4.2 Thorough drainage structure
Figure 4.3 Drainage design
Transformation Measures and Effects
Based on the on-site circumstances, there are two approaches to reduce and eliminate triethylamine salts within the ceramics.
Measure one: Incorporating mixed high-temperature steam into the purging air pipeline
According to current testing and verification, high-temperature steam can effectively dissolve and thermally decompose the triethylamine hydrochloride substances accumulated in the bottom ceramics. When high-temperature steam is introduced into the purging pipe, the frequency of the rotary valve motor can be reduced to 30Hz, and the frequency of the purging fan can be reduced to 30 - 35Hz, raising the temperature of the purging air and increasing the residence time of the steam in the purging area. This measure can better remove the triethylamine hydrochloride substances in the bottom ceramics. The system can also be modified for automatic flushing.
Measure two: Preventing the generation of salts.
Before the exhaust gas enters RTO, add a pre-treatment system to lower the concentration of triethylamine and prolong the blockage time of the bottom ceramics. According to the in-site process, there are 7 - 12 feeding reactions per day (considering an average of 10), and for each feeding cycle, approximately 67kg of triethylamine will be lost. Considering the slightly soluble nature of triethylamine in water, the vast majority of triethylamine will evaporate into the exhaust gas. According to the main fan frequency of around 20Hz, the volume of exhaust gas is about 20000m³/h, and the concentration of triethylamine is 700mg/m³ - 1000mg/m³.
Transformation Measure:
1. Collect the triethylamine-containing exhaust gas of the front-end process separately and add an acid wash pre-treatment system. Triethylamine is weakly alkaline, and after being sprayed and washed by dilute sulfuric acid, the concentration of triethylamine is significantly reduced. After the acid washing pre-treatment system, the exhaust gas enters the existing alkali washing, water washing, and demist system. After acid washing, there are triethylamine sulfate substances in the aqueous solution, which can enter the triethylamine recovery kettle to recycle and reuse the triethylamine in the aqueous solution.
2. Collect the triethylamine-containing exhaust gas in the front-end process separately, change the existing pre-treatment system to acid washing, alkali washing, and demist (with a backwash structure for demist). After acid washing, there are triethylamine sulfate substances in the aqueous solution, which can enter the triethylamine recovery kettle to recycle and reuse the triethylamine in the aqueous solution.
The pretreatment renovation has been accomplished and is currently in normal use. By retrieving data from the on-site pressure sensors, there has been no notable increase in system pressure loss since the pre-treatment system was put into operation (about one year ago).
4 Summary
For the chemical industry, our recommendation is that the design units should meticulously develop the reasonable treatment plan and fully understand the enterprise's exhaust gas volume, composition, concentration and emission patterns when conducting process deepening design. Especially for the cases where the exhaust gas contains multiple components such as halogenated hydrocarbons, ammonia and organic amines, priority should be given to the classification, collection and treatment of organic and inorganic exhaust gas. If complete separation is not achievable, the exhaust gas entering RTO should be purified in stages by pre-treatment and post-treatment devices in accordance with the specific exhaust gas working condition to avoid problems such as ammonium salt blockage, corrosion, excessive emission of secondary pollutants.
The successful operation of this project offers valuable experience and reference for the RTO system design of other fine chemical enterprises. We believe that this project will have a profound influence in the fields of environmental protection and exhaust gas treatment, contributing to a cleaner and sustainable industrial development.