Operation -- Rapid establishment of distillation column equilibrium

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How can adjustments be made to enable the distillation column to reach equilibrium quickly? By doing so, the startup process can be accelerated, and energy and material losses can be reduced. It is well known that the operation of a distillation column is governed by three types of equilibrium: mass balance, heat balance, and gas-liquid equilibrium. Only by achieving these three balances can the distillation tower be said to be in a normal operating condition, and qualified products can be produced as required. Therefore, the basic approach we take to adjusting the distillation column is to find the equilibrium points of these three balances as quickly as possible. However, these three equilibrium points are not intuitive; we need to find them. Mass balance is relatively easy to determine; the sum of all the materials entering the tower equals the sum of all the materials exiting the tower. All one needs to do is add up the relevant values. However, the gas-liquid equilibrium cannot be seen; the amount of rising gas and the amount of falling liquid on each tray, as well as the ratio between them, and the height of the liquid layer on each tray, are all invisible. However, there are some phenomena that can reflect this, such as the online analysis values of the product at the top of the tower, the pressure difference across the tower, and the liquid levels in the reflux tank and the bottom of the tower. The same is true for heat balance. In other words, these three types of balance are distinct from one another, and it is not possible to achieve them all at the same time. But they are connected. Among them, mass balance is a prerequisite; if the feed rate and the extraction rate do not match, nothing else can be considered. Gas-liquid equilibrium is the goal; it reflects the product composition, yet there is no direct means to control it. Heat balance is the key factor we adjust; it is based on mass balance. By controlling the amount of cooling energy applied, we can influence the amount of liquid that condenses and returns, while the amount of reboiling heat used allows us to control the volume of steam produced, thereby affecting the gas-liquid equilibrium. In practical operations, under certain pressures, a specific temperature corresponds to a particular composition of the material. Therefore, by determining the pressure and controlling the temperature, we can achieve the desired material composition. This is the relationship between temperature, pressure, and composition. Due to the need for material balance as well as the requirement to obtain the desired product, it is essential to strictly control the reflux ratio. The reflux ratio is the ratio of the amount of material returning to the reactor to the amount taken out from the top of the tower. A high reflux ratio helps to concentrate the components at the top, but it increases the load on the reflux system and the bottom of the tower; in severe cases, this can disrupt the operational balance of the tower. Hence, it is necessary to strictly control the reflux ratio. The focus of mass balance lies at the top of the tower. The important parameter is the reflux ratio. First, ensure that the liquid level in the reflux tank remains within the normal range. When there is an excess of light components, they are discharged to the front system through a pressure-controlled valve; at this time, less liquid condenses, and accordingly, both the amount of product extracted and the amount of liquid returned also decrease ； When there is a high amount of heavy components, the reflux flow rate must be increased to increase the amount of cold liquid on each tray, thereby reducing the evaporation of heavy components. For heat balance, the tower pressure is considered first, followed by temperature (mainly the temperature of the sensitive plate). When the tower pressure is high, reduce the reboiling heat input and increase the condensation volume at the tower top ； The opposite is true when the tower pressure is low. When the pressure difference is high and the temperature of the sensitive plate is low, it indicates that the amount of reflux liquid is too large; it is necessary to reduce the reflux volume and the amount of cooling agent at the tower top, while increasing the amount taken from the tower top ； A high pressure difference and a high temperature of the sensitive plate indicate that too much steam is being generated; therefore, the heating amount for reboiling needs to be reduced. When the pressure difference is high, adjustments should be made as soon as possible to prevent flooding. When driving the process, since it is a full reflux operation, the reflux rate is reduced by one distillation column after feeding begins. Once the composition and feed conditions are determined, the reflux ratio is also fixed. A simple approach is to control the yield and reflux rates at the bottom and top of the tower based on material balance calculations, until the full reflux rate is brought within the desired range. This is the fastest and most stable way of operating. When these three balances are achieved, all parameters reach their normal values, allowing for the proper production of qualified products.