Discussion on the graywater heat exchanger in the Texaco gasification process
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Let’s discuss this: In Texaco’s gasification process, many of the heat exchangers used for treating ash water, such as wash water coolers and ash water heaters, are of the floating-head type. This design is chosen to facilitate the removal of scale formed by the process fluid. However, it has disadvantages such as complex structure, high metal consumption and resulting costs, as well as large floor space. Could U-tube heat exchangers be used instead? Is the scaling in the graywater tanks and after the deaerator that severe? This post was last edited by liuze78 on 2008-3-14 11:01]
U-type heat exchangers cannot be used, as the gray water contains many impurities; although it’s called gray water, it is actually quite dark in color
It is better to use a floating-head heat exchanger for the wash water cooler; the graywater heater and the startup preheater can be considered for removal. A factory in Shanxi removed these two heat exchangers from the start of construction, and they have been functioning well. Furthermore, don’t place excessive trust in Texaco’s technology; generally, design institutes are reluctant to take risks, which results in designs that are all identical to one another, and this hinders the absorption and utilization of such technology.
It seems that scaling in the gray water isn’t that serious… As long as the stabilizers are used properly, scaling hardly occurs. I don’t understand what a floating-head heat exchanger is; the heat exchangers in my plant have a tube side, through which the gray water flows, while flash vapor flows outside the tube side. The heat exchangers are only inspected and maintained when they are shut down
Question: Is it reasonable to disable the graywater heater for energy recovery?
It has been stated that for things like graywater heaters, floating-head heat exchangers are commonly used, and this is to facilitate the removal of scale formed by the process fluid. Due to the bent sections in U-tube heat exchangers, they are difficult to clean ； And after removing the floating head from a floating-head heat exchanger, it consists of straight tubes, making it easy to clean. That’s the reason.
I personally believe that removing the graywater heat exchanger would result in significant heat loss; the sensible heat of the flash vapor would then not be utilized. To bring the temperature of the deaerator to around 108 degrees, a large amount of steam would have to be consumed. The use of a floating-head heat exchanger increases the shell side for heat exchange, enabling more thorough heat transfer and thus maximizing heat recovery. I don’t think the scaling in the deaerator and the graywater tank is very severe; a rinse during maintenance will be sufficient.
Actually, it makes sense to use a graywater heater: 1. Scaling in the system is inevitable; don’t expect dispersants to solve all problems. 2. It helps ensure the system operates stably over a long period of time and reduces downtime. 3. A graywater heater costs only a few hundred thousand dollars, while if the system has to be shut down for scaling removal, it will take at least 3 months, and the scaling may not even be completely removed – the advantages are clear.
It is understood that the reason for removing the graywater heater was as follows: During the early stages of project implementation, the factory investigated the operation of various pressurized gasification units in China. At that time, it was found that the problem of severe contamination of the quenching chamber with ash and water was a common issue, and this condition was one of the main factors affecting the long-term safe operation of the gasification furnace. In response to this situation, it was suggested that one of the causes of this phenomenon was an excessive heat load inside the quench chamber; therefore, the idea of reducing the temperature of the ash water entering the furnace was put forward, and it was ultimately decided to eliminate the ash water heater. As for the issue of increased steam consumption in the deaerator caused by the removal of that heater, it was believed at the time that the steam discharged from the high-pressure flasher was rather dirty and had no suitable use; therefore, using it for deaeration was an acceptable option. After implementing the above modifications, the gasifier operates quite stably, with phenomena of carrying ash and water occurring very rarely. Here, we also hope that everyone will have an in-depth discussion on the advantages and disadvantages of the above modifications, so as to gain a better understanding of the water-coal slurry pressurized gasification process!
A graywater heat exchanger with U-tubes can also be used; as far as I know, some manufacturers do employ U-tubes, but most manufacturers use floating-head or shell-and-tube types, and the reason for this is ease of cleaning
Brother from Building 9#, what you said about removing the graywater heat exchanger makes some sense. I wonder which chemical plant it is?
Our unit uses floating-head heat exchangers, mainly to facilitate cleaning
Has any company done this? Which company?
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