Distillation column problems
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Our plant has a distillation tower used to distill recycled methanol containing about 2% water. The required level of water content in the distilled methanol is 0.3%. Feed is supplied intermittently. The inner layer of the bottom of this tower is made of carbon steel and stainless steel composite material; its volume is approximately 2000 liters. The jacket surrounding the tower is made of carbon steel and is heated using steam. The tower body itself (with a diameter of 400 mm) is also made of carbon steel, with a height of 8.5 meters. It is equipped with stainless steel corrugated packing inside. The outlet at the top of the tower is a seamless steel pipe with a diameter of 159 mm, which is connected to a horizontal methanol condenser located about 3 meters below it, with an area of 25 square meters. The outlet for the condensed liquid (a steel pipe with a diameter of 57 mm) is connected to the upper part of a buffer tank with a capacity of 1 cubic meter. The outlet at the bottom of this buffer tank is connected to a methanol magnetic pump, and the pump’s outlet is connected to a distribution station. From this distribution station, one pipeline leads back to the top of the tower via a rotameter, while another pipeline leads back to this buffer tank via another rotameter. Once the buffer tank is filled with methanol, it continues to be pumped into the storage tank for qualified methanol. The reflux ratio is kept around 2; other values for the reflux ratio have also been tried, but very little qualified methanol is produced. The steam pressure in the jacket is around 4 kg, the temperature at the top of the tower is 70 degrees Celsius, and the temperature at the bottom of the tower is 80 degrees Celsius. The steam pressure has also been tried at 8 kilograms, but still only 800 liters of methanol are produced per shift. Regardless of whether the methanol is qualified or not, the low output volume is a problem. Even when the methanol magnetic pump is not in use, the flow rate can be seen to be low when observing through the sight glass at the bottom of the horizontal methanol condenser. Is the problem related to the bottom of the distillation tower, the process itself, or the operation? I would appreciate advice from those who are more experienced. Thank you in advance! handshake
It seems that the heat exchange area is too small; for a 2-ton reactor, the external surface area is likely insufficient. It is recommended to add internal coils. Also, is the tower’s insulation sufficient? Since feeding is intermittent, the heat dissipation during the evaporation process also has a significant impact
May I ask the original poster, what is the amount of steam added to the distillation tower? What is the diameter of the steam pipe? Is it the insufficient heat load that results in less output?
Find the design documents for this tower, and then check whether there are any differences between the on-site equipment and the design specifications, paying special attention to comparing the designed temperatures and actual temperatures of the inlet and outlet water for the condenser cooling. Also, it is useless to try to determine the flow rate by eye; once everything is running normally, without adding back methanol, let all the liquid flow directly into the buffer tank. The total amount of fluid exiting the top of the tower can be estimated by using the volume of the buffer tank. Only in this way can one see the tower’s capabilities. Personally, I think the data you provided isn’t complete; 4 kilograms of steam is definitely enough for heating. If possible, measure the pressure drop of the tower.
The main problem is that the heating area at the bottom of the tower is too small; let’s use a reboiler with a surface area of 10 m2
As a supplementary note, there is no flow meter for the steam in the distillation tower; the diameter of the steam pipe is 57. When the pressure in the jacket is 4 kilograms, the pressure at the bottom of the tower is around 0.4 kilograms, while the pressure on the condenser is essentially zero
It’s mainly a problem with the tower low reboiler
Use a larger reboiler, and then turn the steam trap fully open
1. In terms of the heat exchange area of the distillation column bottom jacket, it is around 8 square meters (the steam pressure and pipe diameter are fine). The heat exchange area isn’t the main issue, but I’m not sure about the efficiency of heat exchange in your case! If the heat loss is high and the heat exchange efficiency is poor, it will result in insufficient evaporation in the tower, poor separation performance, and an impact on product quality. 2. The heat exchange area of the condenser at the top of the tower is fine. 3. It is recommended that you first measure the actual evaporation rate when the tower is operating normally before making any judgments. Another suggestion: if the purpose of this distillation is merely dehydration, why not use other methods, such as molecular sieve dehydration?
As a supplementary point: The distillation vessel of 2M3 can use a shell-and-tube reboiler at the bottom of the tower, which yields better results than a jacketed heat exchange system!
Thank you all; I will try each of your suggestions one by one:lol :handshake
There should be no problem with the heat exchange area of the reboiler; the data provided is incomplete. Additionally, the drain valve cannot be opened fully to atmospheric pressure, as this makes it even more difficult to control the temperature and pressure inside the tower. «Methanol (with about 2% water content); the required water content in the methanol after distillation is 0.3%.» The product at the top of the tower still does not meet the specifications, so try adjusting the process parameters inside the tower. Personal opinion, learn from each other*:) :)
It’s a problem with the reboiler; the temperature at the bottom of the tower and the temperature at the top of the tower differ by only about 10 degrees, resulting in poor distillation efficiency – methanol cannot be completely extracted in the distillation section.
The main problem is that the heating area at the bottom of the tower is too small, and there are also issues with the reboiler.
The heat balance needs to be maintained, and appropriate modifications can be made.