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Column Flooding

Column Flooding

Introduction:

Understanding and Preventing Flooding in Distillation Columns

Flooding is a phenomenon that can occur in distillation columns, leading to a decrease in separation efficiency and potential equipment damage. It is characterized by an excessive accumulation of liquid inside the column, which disrupts the normal flow of vapors and liquids. This can result in increased pressure drop, poor separation, and even column inoperability.

Defining Flooding

There is some ambiguity in the definition of flooding, as it can refer to different stages or degrees of the phenomenon. Some definitions describe flooding as excessive accumulation of liquid, while others emphasize the inability to maintain net downward liquid flow or the carrying of feed liquid out with the overhead gas.

Variations in Definition

Fortunately, the differences in flooding definitions don't significantly affect the performance of newly designed columns. Well-designed columns typically have a generous safety margin against flooding, and different incipient-flood-point (IFP) definitions don't produce significantly dispersed flood-point data. However, when troubleshooting or debottlenecking existing columns, inconsistencies in the definition of flooding initiation can lead to varying, and sometimes incorrect, revamp designs.

Detecting Flooding with Gamma Scans

Gamma scans have emerged as a reliable tool for detecting and quantifying flooding in distillation columns. These scans directly measure the liquid holdup inside the column by simultaneously lowering a gamma-ray source and a radiation detector down the sides of the column.

Factors Affecting Gamma-Ray Transmission

The gamma-ray transmission through the column is influenced by several factors, including column internals, process fluids, and external influences such as manways or stiffening rings. Any external influences are documented during the scan and are labeled as comments on the data results.

Evaluating Column Internals

The impact of column internals, such as trays, packing, or internal pipes, upon gamma scans can be assessed from mechanical drawings and past experience. This information allows for the exclusion of these factors, enabling the isolation of the liquid holdup signal from the process fluids.

Analyzing Dynamic Flooding Mechanism

In-depth analysis of the dynamic flooding mechanism can provide valuable insights for developing advanced control systems and optimizing column operation. This analysis can help operators better understand the factors leading to flooding and develop strategies for pushing the column to its maximum capacity while maintaining efficient separation.

Identifying Flooding Location and Timing

Incipient-flood-point (IFP) data plays a crucial role in understanding and preventing flooding in distillation columns. It answers two critical questions: when (at what throughput) and where (at what location) the column starts to flood.

Limitations of Traditional Measurements

Traditional methods like monitoring pressure drops, temperature profiles, and liquid levels often fail to pinpoint the exact location of flooding, especially in columns with numerous trays or a tall packing bed. By the time these measurements exhibit a response, the IFP may have already passed.

Interpreting Pressure Drop Changes

Pressure drop (ΔP) increase has been widely suggested as an indicator of flooding. Let's examine how ΔP responds to incipient flooding in a trayed column.

ΔP Change in a Sieve Tray Column

A typical sieve tray has a ΔP of 5 mmHg. For a 30-tray column with 24-inch (610 mm) tray spacing and a 2-inch (50 mm) outlet weir, the total pressure drop from top to bottom would be 150 mmHg. Assuming one tray floods due to fouled sieve holes, liquid backup in the downcomer will increase, causing the pressure drop across the tray to rise from its normal height of 18 inches (460 mm) to its IFP height of 26 inches (660 mm).

ΔP Increase at the IFP

The ΔP of the incipient flood tray will increase by the liquid head equivalent of 26-18 = 8 inches (200 mm) of froth height. Assuming the froth contains 50% vapor and the liquid specific gravity is 1.0, the column's pressure drop increase will be 4 inH2O (7.5 mmHg).

Locating the Flooded Tray

In other words, at the IFP, the column's pressure drop would change only about 5%. Depending on the location of the pressure drop measurement points, the pressure drop data may not be able to identify the flooded tray.

While traditional measurements like pressure drop can provide some indication of flooding, they often lack the precision and location specificity needed for effective troubleshooting and prevention. Advanced methods like gamma scans offer a more accurate and reliable approach to identifying and understanding flooding in distillation columns.

Gamma Scans: A Reliable Tool for Flooding Detection

Gamma scanning emerged as a valuable tool for detecting and quantifying flooding in distillation columns. This technique directly measures the liquid holdup inside the column, providing a precise and location-specific assessment of flooding.

Gamma Scan Principle

Gamma scans utilize a gamma-ray source and a radiation detector. The gamma rays emitted from the source penetrate the column and interact with the process fluids, producing a signal. The detector measures this signal, which is inversely proportional to the liquid holdup.

Benefits of Gamma Scans

Gamma scans offer several advantages over traditional methods for flooding detection:

Gamma Scan Applications

Gamma scans find applications in various scenarios, including:

Jet Flooding and Entrainment Flooding

Jet flooding, also known as entrainment flooding, refers to the upper operating limit of a distillation column where the spray or froth height on trays increases beyond the tray spacing, resulting in massive entrainment. This flooding phenomenon develops over a wide range of vapor-liquid rates, from the incipient flood point (IFP) to a fully flooded column.

The entrainment rate is not uniform throughout the column, varying with tray design and location. In industrial columns, where internals can be compromised and hydraulics are complex, predicting both the entrainment rate and its flood point can be challenging.

A key characteristic of jet flooding is the filling of the normal vapor-liquid disengagement spaces above the tray decks with droplets or frothy liquid. This phenomenon is readily identifiable from gamma-scan plots. However, solely relying on gamma scans may not be sufficient to differentiate between jet flooding and severe entrainment.

Many distillation columns can tolerate a certain level of entrainment without compromising overall performance. In fact, under conditions of severe entrainment, the column can function well if the downcomer area is sufficient to accommodate the entrained liquid.

Case Study: Entrainment Flooding in an 80-Tray Column

Gamma Scan Results

Figure illustrates a gamma scan for an 80-tray column undergoing a pre-turnaround inspection to assess the overall integrity and operating conditions of the trays. The scan revealed that the column was operating with severe entrainment. Gamma-ray counts throughout the tray vapor spaces never reached the clear-vapor bar, except at the top of the column, near manways, and below the bottom tray.

Clear-Vapor Bar

The clear-vapor bar serves as a reference point based on the location of minimum vapor density within the column. This clear vapor is assumed to be devoid of liquid droplets and serves as a benchmark for normal disengagement between trays.

Entrainment Propagation

A closer examination of the bottom trays (1-20) indicated a gradual increase in entrainment intensity from Tray 1 to Tray 20. This suggests that liquid was being entrained upward, tray by tray, leading to an overall liquid rate passing through each downcomer that exceeded the rate of the tray below. However, this entrainment propagation ceased at Tray 20 due to an increased tray spacing caused by a manway.

Prevention of Fully Flooded State

The presence of larger spaces at the manways prevented the column from reaching a fully flooded state. This was possible because the upper trays were still operating below the entrainment flood point.

Impact of Severe Entrainment

While severe entrainment can reduce tray efficiency, requiring more trays to achieve the desired separation specifications, the additional trays in this particular case compensated for the efficiency loss due to entrainment.

Downcomer Backup and Entrainment Flooding

Downcomer backup occurs when the liquid level in the downcomer exceeds the tray spacing, preventing proper vapor-liquid disengagement and leading to entrainment flooding. This phenomenon is often caused by excessive liquid flow rates, tray spacing that is too small, or clogged or damaged downcomers.

Consequences of Downcomer Backup

Downcomer backup can have detrimental consequences for distillation columns. It can result in increased pressure drop, reduced separation efficiency, and even column inoperability. Additionally, it can cause erosion and damage to tray decks and downcomers.

Detecting Downcomer Backup

Traditional methods for detecting downcomer backup, such as visual inspection and liquid level measurement, can be time-consuming and unreliable. Gamma scans offer a more accurate and efficient approach to identifying downcomer backup.

Gamma Scans for Downcomer Backup

Gamma scans can directly measure the liquid level in downcomers, providing a clear indication of backup conditions. The liquid level signal is inversely proportional to the gamma-ray counts transmitted through the downcomer.

Case Study: Downcomer Backup in a 30-Tray Column

A gamma scan of a 30-tray column revealed significant downcomer backup on several trays. The gamma-ray counts transmitted through the downcomers were significantly lower than the clear-vapor bar, indicating a high liquid level in the downcomers.

Remedial Measures

Based on the gamma scan results, the operator implemented several remedial measures to address the downcomer backup. These measures included reducing the liquid flow rate, increasing the tray spacing, and cleaning and repairing clogged downcomers.

Flooding in Packed Columns

Packed columns, like trayed columns, have limits on their capacity to handle vapor and liquid loads. Flooding in packed columns is more complicated than in trayed columns, with a greater divergence of opinions on its definition. Liquid buildup is more difficult to observe in a continuous packed bed than on staged trays, even in laboratory columns.

As liquid backs up the column and fills all of the void space in the packing bed, flooding occurs at the high end of liquid and vapor rates for a given packed column. Poor vapor-liquid disengagement (backmixing) reduces separation efficiency, and the high liquid holdup in the bed increases the pressure drop.

Traditionally, flooding in packed columns is analyzed by measuring pressure drop. At low liquid rates, the open area of the packing is similar to dry packing. In this regime, pressure drop is proportional to the square of the vapor flow rate (range AB). As the vapor rate continuously increases, a point is eventually reached at which the vapor begins to interfere with the downward liquid flow, causing liquid buildup in the packing. The pressure drop increases proportionally to the vapor rate raised to a power greater than 2 (range B-C).

The pressure drop starts to increase rapidly at this point due to the accumulation of liquid in the packing, which reduces the void area available for vapor flow. This region is called the "loading zone." As the liquid accumulation continues, a condition is reached where the liquid phase becomes continuous. The slope of the pressure-drop curve increases further, such that even small increases in vapor flow significantly increase the pressure drop. Point C is typically referred to as the flood point.

A difficulty with this traditional approach is distinguishing the transition points of loading or flooding from the pressure-drop curve. Several suggestions have been made for the definition of when a packed column becomes fully "flooded":

Even with laboratory columns having excellent controls, it is still difficult to consistently predict the flooding point based solely on pressure drop. Fortunately, as with trayed columns, gamma scanning can assist in determining the stages of flooding and the approximate point where flooding begins in packed columns.

Gamma scanning detects areas of liquid buildup and quantifies its amount in packed columns. However, the point at which the buildup constitutes "flooding" depends on the desired level of performance. There are two forms of liquid holdup in packed columns: static holdup and operating or dynamic holdup.

Static holdup is the amount of liquid that remains on the packing after it has been wetted and then drained. It refers to the film of liquid or droplets of liquid that adhere to the packing. The amount of static holdup depends on the physical properties of the liquid and the type and material of the packing.

Operating or dynamic holdup is the amount of liquid that is held in the packing by the interaction of the vapor and liquid flows. It must be measured experimentally. To measure dynamic holdup, the liquid and vapor flows are instantaneously stopped, and the volume of liquid that drains from the packing is collected and measured. The total liquid holdup of the packing is the sum of static and dynamic holdup.

In a plant column, the best way to measure liquid holdup on a macro scale is to perform a baseline gamma scan of the column at "safe" operating rates - rates that are well below any operating limit of the column. A scan at these conditions serves as a benchmark for the "normal" liquid loading of the packing.

To study flooding using scans, focus on how the total liquid holdup changes, particularly how much it changes and where the buildup begins. Since static holdup is constant, operating or dynamic holdup changes proportionally to changes in liquid and vapor rates. The void fractions in a packing bed may vary across the bed due to fouling or damage, and vapor-liquid loads may be different along the bed for different operating conditions. The peak loading could occur anywhere in a packed bed, or a liquid distributor could initiate the flooding. Traditional measurements such as pressure drop and theoretical correlations are not very helpful in identifying the flooding point. Gamma scans simplify the identification of flooding to a measurement of liquid holdup in columns.

Gamma scans provide a non-intrusive and accurate method for identifying flooding in both trayed and packed columns. They can help operators determine the point of flooding, assess the extent of flooding, and identify the location of flooding events. This information can be used to optimize column performance and prevent flooding-related problems.

Optimizing Column Performance with Gamma Scans

Gamma scans provide a valuable tool for optimizing column performance by identifying and addressing flooding issues. By visualizing the liquid distribution within the column, gamma scans can pinpoint the location and extent of flooding, allowing operators to take targeted corrective actions.

Identifying Incipient Flooding Points (IFPs)

Gamma scans can identify the IFPs of a column, which are the points at which the liquid holdup starts to increase significantly, indicating the onset of flooding. This information can be used to establish operating limits and ensure that the column operates within a safe and efficient range.

Detecting Localized Flooding

Gamma scans can detect localized flooding, which may not be apparent from traditional measurements like pressure drop. This can be particularly useful for identifying issues caused by fouling, damaged packing, or uneven liquid distribution.

Monitoring Flooding Progression

Gamma scans can be used to monitor the progression of flooding over time, allowing operators to track changes in liquid holdup and identify potential problems before they lead to column shutdown.

Optimizing Liquid Distribution

Gamma scans can provide insights into liquid distribution within the column, helping to identify and correct maldistribution issues that can affect separation efficiency.

Enhancing Column Stability

By identifying and addressing flooding issues, gamma scans can contribute to enhanced column stability and prevent unexpected shutdowns or performance deterioration.

Applications of Gamma Scans

Gamma scans have a wide range of applications in optimizing column performance, including:

Conclusion

Flooding is a complex phenomenon that can significantly impact the performance and efficiency of distillation columns. Understanding the different stages of flooding and employing reliable detection methods like gamma scans are crucial for preventing and mitigating this issue. Additionally, analyzing the dynamic flooding mechanism can lead to improved control systems and optimized operating procedures.

While traditional measurements like pressure drop can provide some indication of flooding, they often lack the precision and location specificity needed for effective troubleshooting and prevention. Advanced methods like gamma scans offer a more accurate and reliable approach to identifying and understanding flooding in distillation columns.

Gamma scans have established themselves as a valuable tool for detecting and preventing flooding in distillation columns. Their precision, location specificity, and non-intrusive nature make them a preferred choice for troubleshooting, design optimization, and risk mitigation in the distillation industry.

Jet flooding or entrainment flooding represents a critical upper limit for distillation columns. Gamma scans offer a valuable tool for detecting and assessing the severity of entrainment flooding, enabling effective troubleshooting and preventative measures. By understanding the causes and consequences of jet flooding, operators can optimize column performance and ensure efficient separation.

Downcomer backup is a common and problematic issue in distillation columns. It can lead to severe performance degradation and even column failure. Gamma scans offer a reliable and non-intrusive method for detecting and quantifying downcomer backup, enabling timely interventions to prevent these issues. By proactively addressing downcomer backup, operators can maintain optimal performance and ensure the continued safe operation of their distillation columns.

Gamma scans have established themselves as an invaluable tool for optimizing column performance and preventing flooding. Their ability to visualize liquid distribution, identify flooding locations, and monitor flooding progression makes them essential for troubleshooting, design, and operation of distillation and absorption columns. By utilizing gamma scans, operators can enhance column stability, improve separation efficiency, and extend the lifespan of their columns.