Alarm interlock control requirements for hydrogen chloride and acetylene mixers

Our factory sets the temperature based on that of the mixer; ours is 55°C.

The commonly used method in China at present is the intuitive temperature monitoring, alarm, and control approach. By monitoring the temperature and setting an upper temperature limit along with an alarm, on-site operators can handle the situation. But it often doesn’t work well! Because chloroacetylene is extremely reactive, it almost causes an explosion as soon as we become aware of the temperature rise; therefore, effectively solving this problem is something that can be achieved through simple monitoring! First, the issue of free chlorine monitoring at the source needs to be addressed; secondly, accidental factors such as free chlorine can be reduced by adsorbing HCL! Safety is the lifeblood of a chemical plant! Establishing a strong safety mindset is more important than anything else! !

HCL, containing free chlorine, reacts with acetylene to produce chloroacetylene; the occurrence of an explosion is likely related to the amount of free chlorine present in hydrogen chloride, as well as temperature and flow rate. Because sometimes the temperature of the mixer does not change, but the temperature of the preheater does respond; at other times, the presence of free chlorine leads to an increase in temperature as well, yet no explosion occurs. The speed at which the temperature rises, as well as whether timely measures are taken, play a role here. Additionally, the temperature of the mixer changes with fluctuations in outdoor temperatures throughout different seasons. Based on our years of operational experience, having good temperature monitors along with experienced and responsible operators is sufficient to minimize the risk of explosions. This post was last edited by shizhiyong001 on 2009-2-19 10:57.]

There are two temperature transmitters at the mixer outlet; tantalum material is preferred as it is resistant to corrosion and allows for rapid heat transfer. The acetylene cut-off valve shuts off quickly when the temperature reaches 40°C. There is an interlock system in place for hydrogen chloride, but it has not been put into use yet, out of concern that overpressure in the main pipes could lead to more serious safety accidents. The acetylene cut-off valve responds quickly; we have conducted tests and found that it can be fully closed within 2.5 seconds once a command is given.

The temperature on the mixer is almost useless for control. During HCL hyperchlorination, since the temperature detection lags far behind the explosion of chloroacetylene, by the time the temperature rises, the explosion has already occurred; it may only be useful in cases of a slight excess.

At present, although a few manufacturers in China use online free chlorine monitoring instruments, the results are not ideal; otherwise, this would indeed be a good preventive measure

Our factory is equipped with online free chlorine monitoring instruments in its design, and a interlock system to shut off acetylene and hydrogen chloride has been installed on the mixer; however, it has not been put into operation yet, so we don’t know what the results will be

Its alarm and interlock systems ensure rapid response of the acetylene shut-off valve based on the mixture temperature

Since the reaction is slow, just set the interlock temperature lower! 55 degrees is too slow, 40 is also delayed – so should we set it at 35 degrees? This post was last edited by clq20040107 on 2009-2-18 23:19.]

If the temperature setpoint is too low, it becomes problematic in summer, as alarms and interlocks are often triggered due to high temperatures (without the generation of chloroacetylene).

What was said upstairs is correct; in order to ensure the safety of the mixer, it is most important that the operators have experience and be conscientious and responsible. It is recommended to conduct emergency drill* in the HCL synthesis section (multiple times). Appropriate rewards and penalties should be implemented for safe production – for example, a bonus of XXXX yuan if no accidents occur over XX months, or a bonus of XXX yuan if emergencies are handled promptly. However, a deduction of XX yuan would be applied in case of accidents. I’m not sure how effective this approach will be.

Our factory sets the temperature based on that of the mixer; ours is 35°C. For the mixer’s two temperature monitoring points, emergency shutdown occurs as soon as one of them reaches 35°C, with an alarm at 30°C.

Your company is probably in the north; otherwise, during summer, the temperature of the air-fuel mixture can easily reach 35°C.

In winter, the set value is 35 for interlocking operations and 40 otherwise; in summer, it is 45 for interlocking operations and 50 otherwise. When the free chlorine content is low, a temperature rise occurs, and measures can be taken promptly. However, when the free chlorine content is too high, an explosion occurs immediately, as there isn’t enough time for the temperature to rise. The key to controlling free chlorine lies still in the hydrogen chloride synthesis process!

The rise in temperature leading to an explosion can be roughly divided into two types: one is a gradual increase in temperature until an explosion occurs; in this case, an alarm is triggered at a temperature of 50 degrees (55 degrees in summer), and the acetylene valve is shut off. To enhance effectiveness, an additional criterion of a temperature change rate exceeding 10 degrees per second can be used as a supplementary alarm to trigger the shutdown of the acetylene valve； Second, the temperature rises instantly to the point of explosion; in such cases, alarms are ineffective as there’s no time to react. This is only possible by adding a free chlorine detection instrument at the HCL location, and only if the HCL pipeline is relatively long. Of course, the operator’s sense of responsibility and skills are the most important, as they can prevent and reduce losses in many cases.

Is the gas phase temperature in the mixer affected so much by season? Are there any issues with process management? Is the amount of chilled water and cooling capacity sufficient? ； Is equipment management in place? Has cold storage for the equipment been arranged? Have the employees received training? Will they adjust the amount of water used for freezing based on the mixer’s temperature? This post was last edited by clq20040107 on 2009-2-23 12:35.]

Does the gas phase of your company’s mixer often reach 35 degrees Celsius in summer? Ah! The purpose of mixed freeze-drying is to remove the moisture content from the mixed gas, and low temperature is required for this purpose, which is what controls the process. Which other PVC manufacturer can achieve a gas phase temperature of 35 degrees Celsius during normal production? This post was last edited by clq20040107 on 2009-2-23 12:53.]

Before entering the mixer, hydrogen chloride is cooled and dehydrated using a hydrogen chloride cooler, so the temperature of the mixture in summer does not exceed 35°C.

Is it really that difficult? Looking at the temperature lag, and the free chlorine meter is inaccurate, resulting in poor performance – is there no mature experience available in China?

The mixer temperature is above 45 degrees, so the acetylene quick-shut valve closes； If the temperature of the mixture rises by more than 8 degrees within 60 seconds, the acetylene quick-shut valve closes.