Basis for selecting the distillation column feed
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As the title suggests, what are the criteria for selecting the feed inlet of a distillation tower?
The composition of the material in the feed inlet is similar to that of a certain tray inside the tower. It can be obtained using the operating line equation and the q-line equation.
It is usually a liquid feed, selected to have a composition similar to that of the liquid on the tray.
Thank you. Floors 2 and 3: The choice of feed inlet is related to the composition of the feed – is it also related to the feed temperature? What should be done if the feed inlet is selected based on the feed composition but the temperature is not appropriate?
The feed location depends not only on the feed composition but also takes factors such as feed temperature and feed volume into account; a comprehensive balance is achieved by considering the material and heat balances throughout the entire tower
It should be related to temperature; there are five types: feed at the bubble point, feed below the bubble point, feed at the dew point, feed between the bubble and dew points, and feed above the dew point
The choice of feed inlet involves selecting one with a liquid phase composition similar to that at the inlet and outlet trays; if software simulation is used, the feed inlet that results in the lowest loads at the top and bottom of the tower is considered the best.
It is best to feed at a position where the composition and temperature are similar to those of the feeding plate! ! ! !
The optimal feeding point is when the load at the top and bottom of the tower is at its lowest
The main options are feeding at the bubble point, above the bubble point, or below the bubble point. The bubble point is calculated; generally, it is best to feed at the bubble point. In this way, once the material enters the tower, its composition is similar to that of the liquid phase on the tower trays, which prevents any impact on the composition of the material in different sections of the tower. This avoids changes in the height of the distillation section and the stripping section, thereby facilitating operational control. Of course, if it is considered troublesome to add a heat exchanger or similar device at the tower feed point to control the temperature, feeding at the bubble point may not be necessary. However, in such cases, the design of the distillation section and stripping section of the tower must take this effect into account, especially when the required distillation efficiency of the tower is not very high.
Could you be more detailed? If a suitable feed plate is selected based on its composition, but the temperature is not appropriate, what should be done? Is composition-based selection given priority?
There is no perfect option – only relatively reasonable ones. It is recommended to first design one that you consider reasonable, and then have others give it some feedback
The optimal position of the feed plate refers to the position that provides the highest separation capacity under the same number of theoretical plates and identical operating conditions, or the position that requires the fewest theoretical plates under the same operating conditions. In production, most distillation columns are equipped with two or more feed plates, and the position of these feed plates is adjusted based on changes in the composition of the feed. When the proportion of light key components in the feed is lower than under normal operation, the position of the feed tray should be moved downward to increase the number of trays in the distillation section, thereby enhancing its separation capacity. Conversely, moving the feed plate upward increases the number of plates in the stripping section, thereby enhancing its separation capacity. In summary, the content of the light key component in the feed components on the feed plate should be less than that on the lowest tray in the distillation section, and greater than that on the highest tray in the stripping section. This prevents the material composition on each tray within the tower from being disrupted after feeding, thereby maintaining stable operation. There are five types of feed conditions for distillation columns: ① Cold feed ； ②Sponge feeding ； ③Gas-liquid mixed feed ； ④Saturated steam feed ； ⑤Superheated steam feed. For ease of analysis, let δ=/. It can be seen from the above equation that δ>1 for cold feed, δ=1 for feed at the bubble point, 0<δ<1 for gas-liquid mixed feed; δ=0 for feed of saturated steam, and δ<0 for feed of superheated steam. When the feed conditions change, the δ value also changes, which directly affects the amount of reflux in the stripping section. This in turn alters the operating line equation for that section, and the position of the feed plate changes as well. Changes in the position of the δ line lead to changes in the number of theoretical plates, as well as in the distribution of plates between the distillation section and the stripping section. For a column with fixed feeding conditions, any change in the feeding conditions will affect the product quality and the amount of loss. In production, bubble feeding is commonly used; in this case, the flow rates of the vapor rising in the distillation section and the stripping section are equal, so the tower diameter is also the same, which makes the design calculations simpler. Typically, a distillation tower has three feed inlets. Installed at different heights on the tower. In production, the appropriate feed inlet should be selected based on the specific circumstances, and adjustments may be necessary. As defined by the optimal position of the feed plate, for a distillation column fed at or near the bubble point, the selection of the feed inlet is determined based on ensuring that the composition of the feed matches that of the feed plate. Generally speaking, when the proportion of volatile components in the mixture to be separated increases, a feed port located at a higher position is chosen; the feed port is adjusted accordingly when the feeding conditions change. When the feed temperature decreases, a feed port at a higher position is used ； Conversely, use a feed inlet at a lower position.
The compositions should be similar (for example, the liquid feed and the liquid on the plates should be similar). If the temperatures differ from those of the feed plate, use PROII’s CASE STUDIES to estimate the impact of feeding liquids with similar compositions near that plate, in order to determine the appropriate location for the feed plate. If there is a significant temperature difference, it may affect the quality of the product produced by the tower; this can be addressed by using heat exchange to bring the temperature of the feed liquid to match that of the feed plate, thereby enabling thermal balance calculations for the tower and achieving the desired product quality. These considerations are mainly relevant during the design phase, but they also come up when modifying existing installations. Last edited by Albertlu on 2009-4-4 12:09
The feed temperature can be adjusted based on the feed components, thereby changing the feed position