Thoughts on Reading “Chemical Process Design” III – Separation

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Reflections on “Chemical Process Design” – Part 3: Separation. As a component of chemical processes, separation plays a crucial role; it has a significant impact on reactions, product quality, energy consumption, and investment. In particular, when a product reaches a state of saturation or oversaturation, a company’s ability to survive depends largely on its production costs. These costs, in turn, depend significantly on energy consumption. The quality of the design of separation systems directly determines the level of energy consumption. From the perspective of the sequence of steps in the process, separation generally involves the purification of raw materials as well as the separation of the materials after the reaction, and both are of great importance. Generally, the higher the purity of the raw materials, the more favorable it is for the reaction. The impurities brought in by the raw materials affect the system in the following ways: 1. They may cause catalyst poisoning; for example, in most hydrogenation reactions, strict requirements are placed on the sulfur, chlorine, arsenic, phosphorus, and other elements present in the hydrogen used as a raw material ； 2. It inhibits the main reaction, increases the occurrence of side reactions, and makes it more difficult to separate the products ； 3. If a gas-phase feed is used, it will increase the amount of off-gas; if a liquid-phase feed is used, it will increase the amount of liquid discharged. This leads to higher raw material consumption and increased costs for subsequent treatment. Of course, in some reactions, taking into account the safety of the system, inert components are intentionally added to the system in order to dilute the concentration of the reactants and ensure the safe and reliable progress of the reaction; this is the case, for example, in the reaction that produces methyl nitrite. Some responses also consider investment and process simplicity, and opt for reducing the concentration of the raw materials as a solution. For example, in the catalytic oxidation unit of sulfur recovery processes, if the H2S concentration in the raw materials is high, using an adiabatic bed reactor can lead to overheating of the reactor, rendering the catalyst unusable. On the other hand, while using an isothermal bed reactor results in higher equipment costs and a more complex process, reducing the raw material concentration while still employing an adiabatic bed reactor can be a viable approach. Here too, there is no one-size-fits-all solution; it is necessary to conduct an economic analysis of both options based on factors such as raw material concentration and gas volume, before deciding which method to adopt. Regarding the separation of the material after the reaction, that is, the separation of the crude product, its quality directly determines factors such as product quality, yield, and energy consumption. In particular, when the crude product contains many components, including substances prone to polymerization or oxidation; when the boiling points of these components are very similar or there is azeotropy between them; and when interaction parameters between the components are lacking, requiring numerous experimental measurements and data fitting, the entire separation process becomes extremely complicated. Distillation in the production of ethylene glycol from syngas is one of the most typical examples. The materials to be separated are divided into homogeneous and heterogeneous types. Since the separation of phases is relatively easy, in such processes, the separation of heterogeneous mixtures is generally carried out first, followed by the separation of homogeneous mixtures. Heterogeneous mixtures can be separated based on the density difference between their different phases. For the separation of homogeneous mixtures, a new phase must be added or generated to carry out the separation process. If a solvent is added to the gas mixture, it can dissolve one or several components of the mixture; after further separation, the solvent can be reused. For example, if the reaction produces a gas mixture, a new phase—the liquid phase—can be formed through partial condensation. After condensation, the gas phase becomes rich in volatile components, while the liquid phase is rich in non-volatile components, thereby achieving separation. Separation of heterogeneous mixtures 1. Sedimentation: During the sedimentation process, particles are separated from the fluid due to the effect of gravity. The particles referred to here are solid particles or droplets, while the fluid refers to liquids or gases. For example, in flash vaporization equipment used in gas-liquid mixture separation processes, it is required that the vaporization rate of the gas be lower than the settling rate of the liquid droplets ； A demixer used for liquid-liquid phase separation, which requires a low enough horizontal velocity of the fluids so that low-density droplets can rise from the bottom of the container to the interface, while high-density droplets sink to the interface and merge together ； A gravity sedimentation tank used for gas-solid or liquid-solid separation, whose ratio of vertical height to particle settling velocity must be less than the air residence time. Settling equipment is used in most chemical processing units, such as common gas separation tanks/towers, thickeners in ammonium carbonate or ammonium sulfate production plants, and clarifiers in the water treatment units of coal gasification plants. 2. Flotation: Flotation is a method for separating mixtures based on the differences in the surface properties of particles. Bubbles formed in the liquid interact with the surfaces of these particles or those of immiscible droplets, causing certain particles or droplets to rise to the surface thereby achieving separation. It is commonly used to separate solid-solid and liquid-liquid mixtures, especially in mineral processing to separate different minerals. 3. Centrifugal separation: When the density of the particles is similar to that of the fluid, the particle size is too small, or a stable emulsion has formed, gravity separation often results in a very slow separation rate. Centrifugal force is generated when an object rotates at a constant distance around an axis; this force increases the effect on particles. Many particles that cannot be separated by gravitational sedimentation can thus be separated using centrifugal force. Devices such as cyclone separators and centrifuges all belong to centrifugal separation equipment. 4. Filtration is a process used to separate solid particles suspended in liquids or gases. The suspension passes through a porous medium; this medium allows only fluids to pass through while solids cannot. If solids remain on the surface of the medium, it is known as filter cake filtration ； If the solids remain inside the porous medium, it is called deep filtration. Filter media can be installed in filtration equipment in various forms. Separation of homogeneous mixtures by distillation: As an important separation technique, distillation has a wide range of applications. It can be used with large flow rates and different feed concentrations, and it enables the production of products with high purity. With advances in technology, especially in the petrochemical industry, the diameter of many distillation columns has reached over ten meters, or even larger. However, distillation is not applicable in some cases, such as: 1. the separation of low-molecular-weight materials ； 2. Separation of high-molecular-weight thermosensitive substances ； 3. Separation of low-concentration components ； 4. Classification and separation of components ； 5. Separation of compounds with low relative volatility ； 6. Separating volatile substances from non-volatile substances. Usually, at the initial design stage, a choice needs to be made regarding the separation process and equipment, but the evaluation of the separation unit must be considered in conjunction with the entire system. Separation units often consume a lot of energy; however, when they are thermally coupled with each other and integrated with other units in terms of energy, the energy can be utilized comprehensively. Consequently, areas that previously required external heating or cooling can now be supplied with the necessary thermal energy—either fully or partially, without any need for external input. When designing a separation system, it is necessary to make preliminary selections regarding its parameters. Several important parameters include pressure: as pressure decreases, the following changes occur: 1. The relative volatility increases, which reduces the difficulty of separation; simultaneously, both the number of tray levels and the reflux ratio decrease ； 2. The latent heat of vaporization increases, resulting in an increased thermal load on the top condenser and reboiler ； 3. Gas density decreases, and tower diameter increases ； 4. The temperatures of the reboiler and condenser decrease. As pressure increases, the behavior is the opposite of what was described above. However, vacuum operation should be avoided as much as possible, and condensers should not be cooled by chilled water, as both of these will lead to an **increase in equipment costs and operating expenses. In addition, it has increased the complexity of the process. If conditions permit, the operating pressure should be ≥ atmospheric pressure, and the condenser should be water-cooled or air-cooled. When determining the pressure, it should be considered that the bubble point temperature of the overhead product should be 10°C higher than the temperature of the cooling water in summer. Of course, in special cases where the material to be separated is a high-molecular-weight thermosensitive substance or a substance prone to polymerization, vacuum operation is employed as needed to lower the boiling point of the material and prevent decomposition or polymerization, which could affect the separation efficiency and product quality; for example, most of the columns used in the ethylene glycol separation process operate under vacuum. Under the same separation requirements, as the reflux ratio increases, the number of theoretical plates decreases; in other words, energy consumption rises while equipment costs decrease. Therefore, it is necessary to find the optimal point of efficiency through economic analysis between the two. Typically, in the initial design stage, the reflux ratio is set at 1.1 times the minimum reflux ratio; in general engineering designs, it is taken to be greater than 1.1 times this value, in order to ensure effective separation and to prevent failure to meet the separation requirements due to production fluctuations or errors in design and manufacturing. Optimizing a single tower holds little significance; it is necessary to optimize this value at the later stages of design, after system integration, in order to maximize efficiency. In the design of feed conditions, bubble-point feeding, that is, feeding with saturated liquid, is generally used, so as to make the gas flow rates above and below the feed point equal. With the use of subcooled feed, the number of plates in the distillation section decreases, the number of plates in the stripping section increases, the reboiler heat load rises, and the condenser cooling capacity decreases. As the process design progresses, parameters such as the aforementioned pressure, reflux ratio, and feed conditions will frequently change; therefore, it is not necessary to carry out optimization before integrating the system processes. The separation of light and heavy key components in a material to be separated in near-boiling or azeotropic systems is much more complex compared to simple distillation, if these components can form an azeotrope. First, examine the sensitivity of the azeotrope composition to pressure; when a pressure change can cause the azeotrope composition to vary by more than 5%, pressure swing distillation should be considered as the preferred method for separation. Based on whether the formed azeotrope is a minimum-boiling or maximum-boiling azeotrope, set up the process and determine the recycle stream. The smaller the change in the azeotrope composition with pressure, the greater the flow rate of the circulating stream. At the same time, thermal coupling is considered preliminarily, with a final decision to be made after the heat integration of the entire system. However, based on my own practical experience, when integrating the heat across the entire system, it is still necessary to prioritize the independence of each unit as much as possible, avoiding the use of the lowest consumption values derived from purely theoretical calculations to design the processes, as this could result in the system not being able to operate in practice. Pressure swing distillation is widely used in chemical plants; the separation of methanol and dimethyl carbonate is a typical example. When the azeotropic composition is insensitive to pressure or in cases involving difficult-to-separate mixtures, another substance must be added to alter the relative volatilities of the key components, thereby achieving the separation objectives. This method is generally divided into: azeotropic distillation and extractive distillation. In azeotropic distillation, an azeotrope-forming substance is usually added; this substance is volatile and forms an azeotrope with the key component, thereby enabling the separation of the raw materials ； In extractive distillation, the extractant used is usually a poorly volatile component that is removed from the bottom of the column; it does not form an azeotrope with any other components, thus offering a wider range of selection options. Typically, the extractant has a structure similar to that of the heavy key component and can form an almost ideal mixture with it; however, it forms a non-ideal mixture with the light key component, thereby increasing the relative volatility of the key components. The higher the extractant flow rate, the better the separation effect, but the energy consumption also increases and the distillation temperature rises as well. Although the use of mass separation agents solves separation problems that cannot be addressed by pressure swing distillation, it inevitably introduces new substances. Moreover, losses during the distillation process can lead to pollution in downstream environments. The best approach is to reduce such losses at the source, but completely avoiding the loss of separation agents at the source is costly and difficult to achieve. Therefore, in general engineering projects, substances existing in the original system process should be used as mass separators as much as possible, without the need to introduce new substances. Absorption and desorption are inverse processes; it is a common method for separating low-molecular-weight substances. The absorption process requires the use of an absorbent; substances present in the system process should be used as absorbents whenever possible. When a gas mixture passes through a liquid solvent, the solvent preferentially absorbs one or several components. Several important factors affecting the absorption efficiency are temperature, pressure, and liquid flow rate. In the absorption tower, the solute transfers from the gas phase to the liquid phase, releasing heat and causing the temperature to increase from the top of the tower to its bottom. If the component concentration is low, less heat is absorbed and the temperature rise along the tower height is small; otherwise, the temperature rise is large. Lowering the temperature increases the solubility of the solute, while raising the temperature has the opposite effect; therefore, it is advisable to avoid operating at high temperatures as much as possible. Furthermore, to prevent the adverse effects of temperature rise on absorption, the absorption liquid is sometimes cooled in the middle of the absorption tower, usually with cooling water, and a refrigerant is used if necessary. This approach is used in processes such as low-temperature methanol washing or alcohol scrubbers in ethylene glycol. There are also solutions that involve cooling the absorbed medium or cooling the absorbent. If the absorbent is a volatile component, it can easily be carried away from the top of the tower by the gas phase, resulting in loss of the absorbent or an inability for the downstream system to receive it; therefore, it is necessary to cool or condense the gas phase at the tower top, as in the case of the oxalic acid absorption tower using ethylene glycol. As pressure decreases, the solubility of the solute decreases; as pressure increases, the solubility of the solute increases, but this requires more energy input, raising the issue of an optimal choice. The ease with which a component is absorbed depends on the absorption factor; the greater the absorption factor, the easier it is for the component to be absorbed, and fewer theoretical plates are required to meet certain separation requirements. However, when the absorption factor is relatively large, increasing the liquid phase flow rate has little effect on this value. The optimal absorption factor range is 1.2 to 2.0, with a typical value of around 1.4. After the solute is dissolved in the liquid phase, desorption is required to achieve separation; the absorbent can be reused, and the desorption factor should be sufficiently high to improve the desorption efficiency. For desorbers, the optimal range for the desorption factor is 1.2–2.0; a commonly used value is approximately 1.4. Contrary to absorption, during desorption the temperature decreases from the top of the tower to the bottom. Reducing pressure and increasing temperature are beneficial for the desorption process. The number of evaporation stages is determined based on factors such as production capacity, economic considerations, and the constraints related to raw materials and products. When the required production capacity is low, single-effect evaporation is used. Multiple-effect evaporation can further recover the latent heat of the vaporized substance. There are generally the following processes for multi-effect evaporation: 1. Co-current feeding evaporation process, in which both the fresh feed and steam flow in the same direction, progressing from the first effect to the next one in sequence. The boiling point of the solution decreases gradually, making it suitable for systems involving the high-temperature decomposition of products. The pressure in the evaporation chamber is reduced step by step; transfer between stages is achieved through pressure differences, eliminating the need for a transfer pump ； 2. The counter-current feeding evaporation process, in which the feed liquid enters at the last stage while the concentrated product is obtained from the first stage; this process is suitable when the viscosity of the concentrate is high. In the preceding stage, a higher temperature can reduce viscosity, and the heat transfer coefficient is higher. However, since the flow direction of the solution is opposite to that of the pressure drop, it is necessary to add inter-stage transfer pumps ； 3. Parallel-flow feeding evaporation process, in which the feed solution is added to each stage separately, and the concentrated solution is also withdrawn from the bottom of each stage respectively. The flow of steam still goes from the first stage to the last stage. This process is applicable to solutions whose feed liquid is near saturation, especially those in which crystallization occurs during the evaporation process. As the number of evaporation stages increases, the equipment cost rises, but energy consumption decreases; there is a minimum total cost, hence an optimal number of stages that needs to be determined after thermal integration of the entire system. The maximum allowable temperature of the evaporator is limited by the product decomposition and sludging behavior. Therefore, the pressure at the highest pressure level must keep the operating temperature below this limit temperature ； The minimum pressure is determined by the ability to use cooling water or air for cooling. Drying refers to the process of removing water or other moisture from wet materials. In industrial production, it is generally used to further remove moisture from substances obtained through other separation processes such as distillation, evaporation, centrifugal separation, or plate and frame filtration. Drying is often achieved by heating and evaporating the moisture in solids or liquids, allowing it to enter the gas stream. Commonly used drying equipment includes: tunnel dryers, rotary dryers, drum dryers, spray dryers, fluidized bed dryers, etc. Tunnel dryers can be used when the material cannot flow freely ； A rotary dryer can be used when the material can flow freely, but since the residence time of the material is long, it is not suitable for drying heat-sensitive materials ； For processing small to medium quantities of paste-like mixtures or solid slurries, a drum dryer can be chosen ； One of the greatest advantages of a spray dryer is that the liquid can be sprayed and evaporated at lower temperatures, without being affected by hot air. In a spray dryer, small droplets of a liquid or slurry are exposed to hot air for a short period of time; the feed is atomized under pressure. The resulting product consists mostly of low-density, porous particles, making this process particularly suitable for drying heat-sensitive products. In the technologies for producing sulfuric acid from desulfurization liquid and sulfur paste, due to the high moisture content in the raw materials, if they are fed directly into the incinerator, it will inevitably result in high fuel gas consumption, large-sized equipment, low content of useful gases, adverse effects on subsequent conversion reactions, an imbalance in heat generation, and high electricity costs due to the need to use electric furnaces. In light of this, some processing technologies use spray dryers to dry and remove water from the raw materials before feeding them in. From the perspective of overall process integrity, it has indeed played a positive role. However, the drawbacks are as follows: 1. Environmental issues caused by hot air carrying away the materials ； 2. When the downstream system has problems and is unable to receive the material coming from the spray dryer, storage of this material must also prevent caking from occurring. The selection of drying equipment is usually determined by factors such as the physical properties and characteristics of the material to be dried, its tendency to decompose under heat, and the required drying efficiency. Most of the above views and theories come from the original book; the author’s own ideas and understandings have been added for everyone’s discussion and exchange!