In-depth article ‖ How to properly renovate old equipment to achieve energy savings and improved efficiency!

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In-depth article ‖ How to properly renovate old equipment to achieve energy savings and improved efficiency! 2016-08-13 Petrochemical Bonds: An Attitude Towards Life through Petrochemical Bonds. When carrying out renovations to increase the processing capacity of a tower, it is necessary to address certain issues, such as what potential the existing equipment has, what are the key factors limiting its processing capacity, and what methods can be used for renovation that are both cost-effective and capable of meeting the requirements for increased production. The key issue here is essentially the prediction of the tower’s maximum processing capacity. However, this is different from the design phase; during potential exploitation and renovation, more accurate predictions are required. A 10%–20% error margin is generally acceptable during the design phase, but such an error level is far too large in the context of potential exploitation and renovation. At this point, the advantage is that there are existing older devices, along with extensive operational experience and relevant documentation. These favorable conditions should be fully utilized to make predictions more accurate and realistic. The prediction of the maximum capacity in the bottleneck elimination phase is based on reliable measurements, rather than general correlation formulas. It should be said that this is a fundamental principle for retrofitting old equipment; the same applies to improving product quality and energy conservation. To achieve the maximum processing capacity of the old device, that is, the maximum raw material processing capacity or the maximum output volume, two tests should be conducted. First, change the feed position to determine the optimal feed position ； Then gradually reduce the reflux ratio R while checking the quality of the product, in order to determine the minimum operating reflux ratio required to produce qualified products. Since this value is smaller than that of normal production operations, it indicates that the tower has some capacity left. Finally, a flooding test of the column is conducted at this minimum operating reflux ratio to determine the maximum flux of the column. Through the aforementioned tests, it is possible to estimate the maximum raw material processing capacity or maximum output of the old device; if this processing capacity is sufficient to meet the requirements for increased production, then the issue is fully resolved. If there is still a certain gap, the tower should be modified. At this point, it is necessary to identify the factors limiting the tower’s processing capacity before deciding on a renovation plan. Simulation calculations of towers play an important role in equipment modification, but the various equations and basic data used in these simulations must be verified using actual measurement data from the distillation tower, to ensure that the results of the simulations reflect the true operating conditions of the tower. The most fundamental correlations and data in the simulation calculations of distillation columns include the following aspects. 1. Phase equilibrium correlations: The correlations should preferably be derived from experimentally measured phase equilibrium data, and such data must be of high quality. That is, to pass the thermodynamic consistency test, the error between the phase equilibrium data predicted using the established correlations and the measured values must be small, and the concentration range of the measured data should cover the entire concentration range in the column, thereby avoiding extrapolation. For example, if the concentration of a product is 99.5%, but the measured value is only 95% or less, then the range of such measured values is too narrow. Although the extrapolation from 95% to 99.5% seems minimal, purifying the product within this range requires a large number of theoretical plates, and even slight errors can significantly affect the calculated values. 2. Mass transfer efficiency data: For plate towers, tower efficiency is often used; different values can be applied to the distillation section and the stripping section. For packed towers, HETP is used. With the correct phase equilibrium relationships, the actual number of theoretical plates in the entire column (and in each section) can be determined through simulation calculations, based on the measured product composition, feed composition and temperature conditions, feed and product flow rates, reflux ratio and operating pressure, as well as the location of side streams, the amount and composition of those streams (if any). This allows for the determination of the column efficiency (or the efficiency of each section) or HETP. In this test, a higher reflux ratio should be used to avoid pinch points that could lead to the separation of key components and result in incorrect findings. Special attention must be paid to the quality of liquid distribution by the distributor in packed tower tests. By checking the structural drawings, it is possible to determine the distribution quality of the distributor. Generally, for a tower that operates properly and meets the design specifications, its liquid distributor will have a distribution quality that satisfies the design requirements. If there are suspicions regarding the distribution quality of the liquid distributor during operation, this can be verified through on-site testing. To strictly verify whether the equilibrium data and efficiency values correspond to the actual production operations, on-site tests should be conducted to determine the temperature distribution along the height of the tower; of course, it would be even better to be able to measure the concentration distribution as well. By using these data to compare with the results of simulation calculations, and by analyzing and adjusting the relevant relationships repeatedly, more realistic phase equilibrium relationships and efficiency values can be obtained. 3. Enthalpy correlation for the gas-liquid two-phase system: It would be ideal to have experimental data for verification, but such data is limited; therefore, it is often necessary to rely on those correlations that are consistently recommended in the literature. The accuracy of enthalpy primarily affects the vapor-liquid flow rates in the tower, as well as the heat loads on the condenser and reboiler; if the error in these heat loads is small, the results obtained from simulations using this value are acceptable. 4. Relationships between some hydrodynamic properties of tray or packed columns – The most important one is the relationship between the flooding vapor velocity and the pressure drop. It is first necessary to determine the cause of flooding, and then select and verify the appropriate correlation formula. Kist provided an example in which the flooding correlation given by the packed tower supplier was overly optimistic, leading to misjudgments regarding the scale-up potential that could be achieved after the modification. Understanding the amount of experimental data on which the correlations in the literature are based, as well as the range of variable variations and the experimental systems, is crucial for correctly selecting reliable correlations. By properly selecting the appropriate various correlations and using factory equipment as well as necessary test data to verify these correlations, it is possible to ensure that the results of the simulation calculations accurately reflect the actual conditions within the tower; only then can such simulations serve as a useful tool for identifying the main factors that limit the processing capacity of the equipment and for developing improvement plans. Ways to increase the tower’s processing capacity ► Change the internal components of the tower equipment. ►Adjust the process according to local conditions. ►Increasing the operating pressure of the tower, as well as reducing the reflux ratio by adding more trays, can also help to increase production. 1. Modification of internal components in tower equipment: (1) Using new, high-efficiency packing in place of older types of packing or trays. Third-generation packing and structured packing offer lower pressure drops, higher efficiency (lower HETP), and higher flux (lower FP) compared to second-generation packing; by replacing the old packing, the liquid flooding velocity in the tower – that is, the flux – increases significantly ； Additionally, the number of theoretical plates for packing layers of the same height has increased; consequently, the operating reflux ratio can be reduced, indirectly increasing the feed treatment capacity and output. It should be emphasized that the liquid distributors and redistributors also need to be modified to meet the requirements of the new type of high-efficiency packing. When operated under optimal conditions, both packed towers and plate towers can function at reduced pressure, atmospheric pressure, or low pressure; structured packing offers advantages in terms of efficiency and throughput ； Under high-pressure conditions (>2 MPa), the aforementioned advantages of structured packing disappear, while highly efficient bulk packing holds certain advantages. In the modification of the tower, using packing in place of trays yields a more significant increase in production, as the pressure drop per theoretical tray can be reduced by almost 90%. Generally, the pressure drop per tray ranges from 0.4 to 1.05 kPa; it is lower in vacuum distillation. The pressure drop for bulk packing materials such as metal saddle rings can be as low as 55 Pa per theoretical tray, while that for structured packing can reach 13 Pa per theoretical tray. This significant advantage gives it a clear edge in the renovation of high-vacuum distillation columns, as the column diameter in such cases is determined by the specified pressure drop, allowing fillers with a lower pressure drop to utilize a higher vapor velocity. According to Strieger’s estimates, at the same pressure drop, metal rectangular saddle ring packing can achieve a processing capacity about 70% higher than that of plate towers, while the use of structured packing can yield an even greater processing capacity. Modifying styrene distillation columns using high-efficiency packing is a typical example of achieving significant economic benefits; Tianjin University has successfully modified more than a dozen such columns by using plate corrugated structured packing. This modification not only increases production volume, but also virtually eliminates losses due to styrene polymerization by reducing the bottom temperature and the average temperature throughout the distillation section, as well as decreasing the liquid holdup in the column (thereby shortening the residence time), thereby improving the styrene yield. For atmospheric and high-pressure distillation, the advantages of packed towers, such as low pressure drop and low liquid holdup, are no longer significant; however, using packing instead of tray towers can still achieve a considerable increase in production. Although Brierley believed that plate towers are more reliable at high pressures (>2 MPa) due to their long history of successful application, he acknowledged that bulk packing can also achieve significant results under high pressure. Compared to plate towers, structured packing exhibits greater processing capacity and efficiency in most applications in the low-pressure range. Striegel believes that replacing trayed sections with metal Intalox saddles in high-pressure towers can result in an approximately 25% increase in productivity. The main reason is that under high pressure, a plate tower must allocate a considerable area for the overflow zone, whereas in a packed tower, the entire area can be used for two-phase contact mass transfer. In the high-pressure demethanizer of a 300,000-ton ethylene plant, replacing the floating valves in the section that restricted production with metal rectangular saddle rings resulted in an increase in output of about 20%. The use of structured packing in high-pressure areas is not yet widespread, and care must be taken when applying it. The literature contains examples of successful modifications using a dry methane removal column (at 3 MPa), a propane removal column (at 1.8 MPa), an ethane removal column (at 2 MPa), and a C3/C4 column (at 3.2 MPa), achieving yield increases of 20% to 25%. However, the separation efficiency is not as good as that at lower pressures, and further research is needed. Literature reports indicate that the American Distillation Research Company conducted industrial-scale tests on the Intalox 2T structured packing at a pressure of 3.2 MPa, using isobutane-n-butane as the feed mixture; however, consistent and satisfactory results were not achieved regarding separation efficiency and throughput. When packing is used in place of tray plates, the original tray support rings in the tower must be removed, and the remaining width should be less than 12 mm; otherwise, it will lead to poor distribution of the liquid on a large scale, reducing separation efficiency as well as the tower’s throughput. In addition, the raw material interfaces, steam interfaces, as well as the interfaces for instruments used for pressure and temperature measurement must also be carefully located and inspected; sometimes they need to be repositioned. It should be noted that what is being discussed here is the capacity enhancement retrofit of the tower. By modifying its internal components, the requirement for increased production can be met; this is far more economical than constructing a new unit. Even if the cost of the replacement components is somewhat higher, it remains a worthwhile investment. This is different from the design for building new plants; under no circumstances is a packed tower always better than a tray tower. (2) Modifying the structural dimensions of the existing tray design or adopting new trays with higher efficiency and greater processing capacity. Generally, due to various associated errors in the original design, a safety factor is used to make the design more reliable, which also facilitates further increases in production capacity. However, often due to the constraints of a certain factor, it is difficult to achieve significant increases in production without appropriate modifications. After verifying the correlations for tray fluid dynamics using experimental data, it is possible to make the redesign calculations more practical, identify limiting factors, and potentially achieve the optimal structure. Kapus suggests using the tray load performance diagram for determination (Figure 7-1). Figure 7-1 shows the performance diagram for determining whether the structural dimensions of the tray are optimized. As shown in Figure 7-1b, premature flooding of the downcomer can occur; this issue can be resolved by adjusting the tray opening ratio, the height of the overflow weir, the gap at the bottom of the downcomer, or the tray spacing ； Figure 7-1c shows that the drop tube has an excessively small cross-section, and its blockage by flooding limits the tower’s processing capacity ； Figure 7-ld shows that excessive mist entrainment or liquid flooding limits the tower’s processing capacity; optimization can be achieved by adjusting parameters such as plate spacing, weir height, and orifice diameter. By making appropriate modifications to the tray, it is possible to achieve a significant increase in production to meet the requirements. Selecting a newly developed tray that is particularly suitable, taking into account the specific system being separated, the operating conditions, and the vapor-liquid phase loads, is also a way to increase the production capacity of practical distillation columns. Guided floating valves seem to be able to replace sieve trays and conventional floating valve trays on a wide scale, thereby achieving increased production levels. 2. Modification process: When the capacity for separation and processing of an idle tower does not meet the requirements, combining it with the existing production tower results in a significant improvement in its processing capacity. Figure 7-2 shows three possible combination schemes, and the most suitable one can be determined through process simulation. Figure 7-2 Combination of idle tower and production tower 1 – Production tower ； 2-Idle tower: When the separation of a certain product is carried out using two towers built sequentially in series, the tower used for the distillation section clearly has a higher processing capacity than that used for the stripping section; meanwhile, flooding in the stripping section severely limits the overall processing capacity of the entire tower. To improve the handling capacity of the entire tower, an intermediate reboiler can be added to the lower part of the distillation section, thereby transferring a portion of the vapor-liquid load from the stripping section to the distillation section and achieving a balanced load distribution between the two sections, thus enabling an increase in production. From the perspective of energy utilization, the heat input to the intermediate reboiler enables separation in the distillation section, but its efficiency is low. If the factory has surplus low-temperature heat sources available, rational comprehensive utilization of energy can be achieved. Under the condition that an increase in tower height is permitted, appropriately increasing the number of trays (which can reduce R), adjusting the location of the feed inlet to the optimal position, and modifying the thermal state of the feed (since the stripping section is key to limiting the processing capacity, switching to a mixed vapor-liquid feed or a vapor feed) can all help to increase production to an appropriate level. 3. Increase the operating pressure of the tower. The reason for operating under pressure is to raise the temperature of the top condenser, so that air cooling, water cooling, or a refrigerant with a higher temperature can be utilized ； The reason for using reduced-pressure operation is to lower the temperature at the bottom of the tower, in order to prevent the material from overheating and deteriorating, or from suffering from coking, polymerization, etc., or to enable the use of steam or a heating agent with a slightly lower temperature. It can be seen that as long as the temperature at the bottom of the tower is not determined by issues such as material degradation, polymerization, or coking, whether the tower pressure can be increased is a matter of economic consideration. If increasing the pressure can help achieve higher production levels and thereby save on the costs associated with building new facilities, it is often economically attractive. The reason why increasing pressure leads to higher production is that as pressure rises, the density of steam increases proportionally. Although the vapor velocity for liquid flooding in the column will decrease slightly with an increase in column pressure, the allowable gas mass flow rate of the column will increase significantly. If the reflux ratio remains unchanged, the output of the tower or the amount of raw material processed will increase by the same amount. However, as the pressure increases exponentially, the relative volatility decreases slightly, resulting in an increase in the minimum reflux ratio; consequently, the operating reflux ratio also rises, which partially offsets the effect of pG and the increased yield improvement. Generally speaking, vacuum distillation and distillation at near-atmospheric pressures are easier to modify for increased production under pressure, as a modest increase in absolute pressure is sufficient to achieve a significant increase in pG, without any issues related to equipment strength. In high-pressure distillation, doubling the pG value presents challenges related to equipment strength; moreover, the normal operation of high-pressure packed columns is more difficult, and plate columns may also lead to the problem of overly small downcomers. This article is from Chemical Engineering 707; it is reproduced solely for information sharing, and the copyright belongs to the original author