Liquid carryover occurs in gas-liquid separation processes across the oil, gas and petrochemical industries. If not prevented or investigated, this can cause damage to downstream equipment, reduce product quality and affect processing efficiency.
Learning how to prevent this problem can save you from extended downtime and added costs from replacing equipment.
What is Liquid Carryover?
Liquid carryover refers to droplets become entrained in a gas or vapor stream downstream of the mist eliminator. Droplets can vary in size and can occur in many different processes that involve gas and liquid separation such as turbines, distillation columns, compressor systems and scrubber systems.
Consequences of Liquid Carryover
Liquid carryover can impact several different factors depending on the application:
- Downstream equipment can become damaged or operate with a reduced performance or efficiency.
- Valuable gases can be carried out of the vessel rather than being recovered, increasing production waste.
- Quality or composition of the refined product can be affected, resulting in potential financial losses.
- Maintenance requirements may increase due to equipment fouling and damage.
- Health and safety concerns could arise as liquid degrades and corrodes other equipment and safety measures.
These consequences can have a huge impact on production efficiency and finances, making prevention a high priority for the affected industries.
What Causes Liquid Carryover?
To prevent these problems, it’s important to understand what can actually cause liquid carryover. This depends on the process application, and can be a multitude of different factors.
- Excessive gas velocity: if the velocity of the gas or vapor stream is too high, liquid droplets can be forced through the mist eliminator before they have chance to drain properly.
- High liquid loading: if the level of liquid droplets within a gas or vapor stream suddenly increases, this can exceed the capabilities of what the mist eliminator was originally designed to handle, causing the droplets to build up on the material.
- Incorrect mist eliminator specifications: mist eliminators are designed and manufactured to meet certain specifications regarding chemical erosion, gas velocity and pressure drop. If these specifications are incorrect, then the mist eliminator will not perform as intended, allowing droplets to pass through.
- Poor liquid drainage: once the droplets have been captured within the mist eliminator, they then need to be able to effectively drain away otherwise they can be carried downstream. Vessel internal designs and drainage equipment can affect the droplets’ ability to drain correctly, but fouled mist eliminators can also contribute to poor drainage.
Preventing Liquid Carryover
Ensure the correct specifications for mist eliminators
Mist eliminators should be made to meet the operational needs, rather than relying on a standard specification. You should consider:
- Gas flow rate
- Gas density
- Liquid density
- Liquid loading
- Operating pressure and temperature
- Droplet size distribution
- Required separation efficiency
- Chemical compatibility
- Vessel geometry
Even slight changes to any of these parameters can alter the performance of the mist eliminator.
Control Gas Velocity
Gas velocity should remain unchanged from what was specified during the design process of the mist eliminator. If changes need to be made, then evaluation of the current mist eliminator may be required, and a new unit issued if the velocity will be permanently altered.
Evaluate Drainage Method
Ensure that the method of draining is adequate for the volume of droplets in the gas stream. If liquid carryover does occur, then your drainage methods may need addressing.
Effective Monitoring Practices
Regular monitoring through the process can help to identify issues such as liquid carryover before they begin to cause further problems. Measuring your pressure drop can indicate if your mist eliminator is working effectively. Any drop in pressure could be an indication of a spoiled mist eliminator, which can also result in liquid carryover.
Liquid levels can also be monitored throughout the process using monitoring equipment that provides real time data. Once liquid is detected downstream from the separator, then you can begin investigating the root cause before any real damage is done.
Is your mist eliminator causing liquid carryover?
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