Discussion on the pressure control methods for distillation towers.

Let me reply first: 1) Our company has several towers, with pressures ranging from 300 kpag to 1200 kpag. 2) The gas phase is completely condensed; only trace amounts of non-condensable gas are emitted from the gaps between individual towers. 3) More than a dozen towers employ the \"pressurized tower control – gas-phase choking method\". 4) Advantages: rapid adjustment, ease of startup and shutdown, and fast recovery in case of abnormalities ； The condensate temperature can be adjusted conveniently ； Disadvantages: If the device has a high production capacity, the gas-phase control valve must be large in size, which makes it difficult to select the appropriate model; it is also expensive and hard to maintain.

1. Pressurization tower 2. Complete condensation, subcooling 3. The first method involves controlling the gas phase extraction, but this is achieved through staged control, with pressure relief at high levels and nitrogen injection at low levels.

The summary is good; I’ve also seen another type that uses an inert gas, regulated through a breathing valve. For towers used in the methanol-water separation process via deep cryogenic methanol washing, N2 is used to maintain a pressure of around 0.13 Mpa; a pressure relief valve is set to adjust automatically – it releases gas when the pressure is too high and adds N2 when it is too low. Discussing the advantages and disadvantages of various control methods seems to be putting the cart before the horse; in distillation control, what’s most important isn’t pressure, but rather the balance between feedstock, product extraction, heat sources, and cooling agents. The control system should also be designed by taking into account the most relevant indicators based on the specific characteristics of the process

Hello, isn’t this method not very convenient for fine-tuning the temperature of the condensate, which results in a low temperature of the reflux liquid and increased energy consumption?

The condensate temperature is automatically controlled by the cooling water flow rate.

Previously, my responsibility in the workshop was related to pressurized total condensation and the control of refrigerants; occasionally, non-condensable gases had to be removed, and sometimes nitrogen was added as a substitute for such gases in order to control pressure (on rare occasions). I believe the most important aspect of a distillation tower is pressure control, because only when the pressure is stable can the relative volatility of various components remain constant, and then the \"eye\" of the distillation tower – temperature – can function properly. Without stable pressure, it’s impossible to achieve control, which makes it difficult to establish appropriate control parameters!

Hello, thank you for the reply. You said this falls under the operations of atmospheric pressure towers, and there are also summaries available; they aren’t included in this post. Pressure control in towers is divided into pressurized towers, atmospheric towers, and reduced-pressure towers (vacuum operation). From a design perspective, the pressure control method determines the subsequent operation methods. It is still necessary to determine the optimal pressure control method in consideration of the process design; for example, if the gas-phase load of the designed unit is particularly high, using the gas-phase choke method requires a regulator valve of larger size. And if the gas-phase load is low, the gas-phase choking method is more useful. Furthermore, different pressure control methods may affect subsequent operational costs, as well as the start-up and shutdown speeds. From an operational perspective, different control methods result in varying approaches to the start-up and shutdown processes of the distillation tower, as well as to handling abnormal situations. Adopting a direct and efficient control method contributes to the safety of the device. I have only been exposed to the vapor-phase choking method; I am unaware of the advantages and disadvantages of other control methods, as well as the layout of the equipment and the precautions that need to be taken during operation. Therefore, I ask fellow sailors to share their insights generously.

When comparing the advantages and disadvantages of various options, it is also necessary to consider the range of fluctuations in processing capacity. For example, a production line has three gasification furnaces at its source; all three are unstable and operate intermittently, with the gas supply volume fluctuating between 40,000 and 120,000 standard cubic feet per hour. The subsequent equipment must adapt to these fluctuations, striving to save energy when the load is low, and adjusting itself again when the load is high. Under such circumstances, controlling the extraction, thermal bypass, and bottlenecks is less effective than controlling the refrigerant

In my previous operations, I used pressure control via a pressure tower combined with vapor-phase extraction. The advantage is that this is the method recommended by the manufacturer. As for other methods, here are some suggestions: The refrigerant adjustment method involves controlling the flow rate of cooling water/frozen water; generally, the amount of water used remains relatively constant, but over time erosion problems may arise. There can also be fluctuations in the operating pressure of the cooling water/frozen water. The heat bypass method refers to operating under full condensation conditions, so the pressure is controlled based on the pressure after condensation. In this case, the variable becomes the efficiency of cooling, and the bypass valve is opened or closed to maintain a certain overall pressure; however, adjusting this valve leads to fluctuations in the heat exchange efficiency. The “choke point” method (regardless of whether it’s the liquid or gas phase) isn’t something I really approve of; it might be useful under certain special conditions, but for distillation towers, the amount of vapor produced depends on the quantity and quality of the feedstock entering from upstream. If the choke point method is used, the overall system performance will still be problematic. ------------------------------------------------------------------------------------------------------------ Even when full condensation occurs, there should still be some gases containing impurities, and the system needs to remove these impurities in order to achieve high quality

Another variant of the thermal bypass method involves gas-phase extraction from a reflux tank, with thermal compensation being achieved through segmented control. In general, the choke-type design is used in devices of smaller scale; the condenser must be placed above the reflux tank, which results in higher investment costs and the need to add an additional layer of platform. The advantage is that adjustments can be made more quickly. Thermal bypass is used in larger-scale installations, where the condenser can be placed below the reflux tank; this results in relatively lower investment costs, but the downside is regulatory lag.