Distillation column operation

With no operational changes and constant draw rates from the top and bottom of the tower, changes in the temperatures at these locations can be used to reflect changes in the composition of the feed. The temperatures at the top and bottom of the tower increase, and the amount of light components in the feed decreases ； The temperatures at the top and bottom of the tower decrease, and the proportion of light components in the feed increases.

Thank you for your answer. I’ve asked many people now, and I’ve received answers of both increasing and decreasing – it’s really confusing

An increase in the top temperature of the tower and a decrease in the bottom temperature indicate that the feed composition has become lighter; conversely, it means the feed has become heavier.

The raw material becomes lighter, with an increase in light components in the feed and a decrease in heavy components. The rising vapor phase inside the tower increases, while the descending liquid phase decreases. This is reflected in terms of temperature as both the top temperature and the bottom temperature increase, while the heat at the bottom of the tower remains unchanged

A normal distillation column, when the column pressure remains relatively constant, can be regarded as a single function of temperature. When a distillation column is operating normally, the liquid levels in the reboiler and the top reflux drum are under automatic control. In principle, the removal of light components from the top of the column and heavy components from the bottom does not affect the thermal balance within the column; the temperature of the sensitive trays is also usually controlled automatically. The heat source at the bottom of the column is dynamic. Assuming that the feed rate and feed temperature remain constant while only the composition becomes lighter, theoretically, in order to reach a new equilibrium the temperature gradient within the distillation column decreases. In simpler terms, due to the increase in light components, it is no longer necessary for such high temperatures at the bottom of the column to maintain the purity of the heavier components there; likewise, a higher temperature at the top of the column is no longer required to vaporize all the light components. To ensure the purity of the light and heavy components at the top and bottom of the tower, normal operation requires lowering the temperature of the sensitive tray, removing the heat source, and simultaneously reducing the reflux flow rate or increasing the reflux temperature. However, this tower is automatically controlled; when the composition becomes lighter, the automatic control system continues to maintain the original temperature of the sensitive tray. This results in the heavier components being overheated. The temperature at the top of the tower remains unchanged, while the amount of light components increases. As a result, the gas load at the tower top rises, and so does the pressure there. The temperature at the top of the tower increases as the heat source is automatically adjusted up. Since there is no corresponding adjustment to the reflux rate, the concentration of heavy components at the top of the tower exceeds the desired level ； Even in manual control mode, the conclusion remains the same: the heat source should be removed without reducing the flow rate; as a result, the bottom temperature will rise, and the heavier components will still make their way to the top of the tower, leading to a decrease in the purity of the lighter components. By the time the high gas-phase load causes the pressure at the top of the tower to increase, thereby suppressing the rising top temperature, it will already be too late. In summary, when the feed composition becomes lighter, with all other conditions remaining unchanged, the temperature at the top of the tower will increase, and the temperature at the bottom of the tower may also rise. By the same logic, and by extension, as the concentration of the feed components increases, the temperature at the top of the tower will decrease, and the temperature at the bottom of the tower may also drop. In order to re-establish a new balance, the temperature gradient within the tower will increase; therefore, it is necessary to raise the temperature of the sensitive tray, increase the heat source, and also increase the reflux ratio.

It can be understood in this way: the temperature of each tray corresponds to the saturation temperature of the material on that tray, which represents the gas-liquid equilibrium of the material containing certain components at the current pressure. Premise: It is assumed that the feed rate remains constant, the temperature remains constant, the column pressure remains constant (of the type controlled via stage control, thus very stable), the temperature of the sensitive plate remains constant, and the steam flow at the bottom of the column is controlled in a cascade manner based on the temperature of the sensitive plate. If the light components are reduced, then the heavy components increase. More heat is required to reach the saturation temperature in order to maintain it; the temperature of the sensitive plate is increased by using a cascade control system to regulate the steam flow, thereby maintaining the set temperature, and as a result the temperature of the upper trays in the tower increases compared to before. Conversely, if there is an excess of light components, the temperature above the sensitive plate will drop. Whether it is the light component or the heavy component, if it exceeds the design processing concentration range of the tower, the distillation tower will fail. If the light components are present in excessive amounts, then no amount of steam added to the distillation tower will be able to establish equilibrium. A typical symptom of this is that only the bottom of the tower boils, with the temperature remaining normal, while all other temperatures on the trays are lower than usual, as if the trays were empty; as a result, the vapor can easily reach the condenser at the top of the tower.

Without adjusting the heat source at the tower bottom or the reflux at the tower top, an excess of light components results in no change in the temperature of the sensitive plate, while the temperature at the tower bottom decreases. In a properly operating distillation column, the temperature of each tray is unrelated to the heat source at the bottom of the column; as long as the composition of that tray does not change, its temperature remains unchanged. From the perspective of thermal balance, the heat sources within the tower are the heat carried by the feed and the heat supplied at the bottom of the tower. The heat losses consist of the heat taken away by the products at the top of the tower, the products at the bottom of the tower, the heat removed by the cooler at the top of the tower, and other forms of heat loss. When other operating conditions remain unchanged, that is, when the heat input to the tower stays constant, the amount of heat carried away by the components at the top of the tower remains unchanged, and thus the quantity of products at the top of the tower does not change. As a result, neither the sensitivity plate temperature nor the temperature at the top of the tower changes. However, since there are more light components at the bottom of the tower, its temperature decreases

Under normal conditions, with all other factors remaining unchanged, a decrease in the heavy components in the feed leads to a drop in the temperature at the top of the tower (the sensitive plate), while the pressure increases； The Reconstituted components increases, pressure decreases, and temperature rises.

It can be determined by the levels in the tower bottom and the reflux tank, or by the temperatures in the stripping section and the distillation section: if more light components are fed in while operating conditions remain unchanged, the level in the reflux tank will rise; the temperature of the circulating water returning to the top condenser will increase slightly (as the condenser is responsible for this effect), the level at the tower bottom will drop, and the overall temperature of the tower will decrease. The pressure difference across the tower will increase. The opposite occurs when more heavy components are fed in. . .

The simplest way is to observe the changes in the liquid level at the top of the tower and at the bottom of the tower