Desorption hydrolysis modification
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For the hydrolysis process, we use a sieve tray tower designed by the Fourth Research Institute. Currently, the wastewater contains high levels of ammonia nitrogen and urea (50 for ammonia nitrogen and 100 for COD); we plan to reduce the ammonia nitrogen level to below 10 and the COD level to 25. Those who are capable of analyzing hydrolytic modification please reply with a brief overview of the modification approach, investment required, and expected results.
Has this indicator been poor recently, or has it always been like this?
It’s always been like this; I’ve wanted to make changes for a long time. Waste liquid recovery is difficult, and operational flexibility is low.
The distillation tower in our company was designed by Donghua; its trays are vertical sieve trays. After it was put into operation, the performance regarding waste liquid treatment was poor. Various methods were tried, but the ammonia nitrogen level remained at least at 50 ppm. Later, when visiting the Nanyang Fertilizer Plant, it was found that plate corrugated packing towers were used there, with good results – the ammonia nitrogen level in the waste liquid was only 1–2 ppm. We followed the example of that company by replacing the reactive towers with packed towers (as the analysis of the liquid output from the hydrolyzer showed that one of the towers was functioning well). After this modification, the ammonia nitrogen level in the waste liquid dropped to 20 ppm, with values as low as 16 ppm at times. Various methods are currently being employed to further adjust this level; if they prove successful, your organization can consider adopting this approach as well.
Also, please tell me about the status of your hydrolysis process. I’m puzzled by the high urea content in your waste liquid – how high is it? COD stands for Chemical Oxygen Demand; is it meaningful to measure this value in wastewater analysis?
Urea was determined by microtitration at 0.7 ppm, which I consider to be inaccurate. The outlet temperature of the hydrolysis tower is 185°C, and the pressure is 1.27 Mpa. I believe the current reason for the high COD level in the waste liquid is the urea content in it.
Measure the urea content in the waste liquid and give me a figure
Hi, those of you on the 4th floor – which company’s technology was used for the vertical sieve plates you previously had? Patent holders that supply internals for towers must first understand the urea process; at least a thorough understanding of the process and operational characteristics of the deep hydrolysis system is required in order to design internals suitable for desorption towers. It remains to be verified through practice whether structured packing is suitable for the operation of a desorption tower. If regular packing is used in the original design of the desorption tower, the desired results can be achieved, but the consumption of low-pressure steam will be higher than that in a plate tower. If several domestic design institutes use domestic process packages and design the desorption towers with structured packing, the consumption of low-pressure steam is relatively high. The most critical issue is the frequent replacement of the structured packing. A large fertilizer manufacturer in Jiangsu originally used a packed tower; it would be interesting to find out how many times they have replaced the packing. After replacing the structured packing in the domestic Wushihua, Lanzhou, Jiujiang, and Inner Mongolia (CNOOC Tianye) plants, the requirements specified in the design were not met; the replacement only reduced the liquid carryover in the first desorption tower. Therefore, when modifying the desorption tower, it is essential to consult the supplier of the tower internals for clear information.
It was the company’s own proprietary technology that led to the decision to use a packed tower; this decision was made after an inspection of Nanyang Fertilizer Factory, where the use of packed towers had yielded good results. The actual amount of steam used is not a problem – when the load is high, there is an excess of low-pressure steam. We have already reached the limit when using low-pressure steam as a blowing gas.
The destination of low-pressure steam is determined by the overall plant steam balance. Fertilizer plants have excess low-temperature heat, but low-pressure steam can still be supplied to the pipeline network; for example, some companies carry out overall plant balancing by sending low-pressure steam to the boiler deaerator. Regarding the packing used in the desorption tower, it needs to be evaluated through long-term operation. Replacing the packing in every major overhaul cycle increases costs. If an appropriate separation technique is chosen, it can essentially solve the problem once and for all. There are several NH3 stripping plants in the southwest; the desorption towers have been in operation for 13 years now, and during each major maintenance session, the manholes are opened to check whether the bolts of the trays at those locations are loose.
It is indeed necessary to open the manhole during each major repair to check the condition of the bolts. The packing tower currently uses a detachable tray-type gas-liquid distributor, and during this major repair it will be checked whether the distributor has been displaced. Currently, the ammonia nitrogen level in the waste liquid has risen from 20 ppm to 50 ppm, and the cause is being investigated.