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Overcoming pressure-drop constraints
Processes that operate under vacuum or very low pressure immediately upstream of the compressor can be very tricky for suction drum design, because the pressure drop across the mist eliminator must be kept low. However, generally speaking, the lower the pressure-drop characteristics of a mist eliminator type, the lower its efficiency in removing mist. Liquid carryover from the suction drum may be a result of selecting a low-efficiency mist eliminator for the sake of low pressure drop. When high efficiency is not required, a vane unit or low-density mesh pad (Amistco TM-1105) may be recommended to achieve low pressure drop. To gain higher efficiency without much cost in terms of pressure drop, a possible solution is a dual-density mesh pad. In such a pad, the downstream layer has higher density than the upstream layer. The result is higher separation efficiency with only slight increase in pressure drop.
Throughput exceeding design capacity
In designing compressor knockout drums, like other gas-liquid separator vessels, conventional mist eliminators are generally selected and sized with a margin of about 10% above the design throughput. Flow rates beyond the upper operating limit may allow liquid carryover due to high velocities that cause re-entrainment from the mist eliminator element.
More specifically, mist eliminators are typically sized for cross-sectional area to achieve a design velocity according to the Souders-Brown vapor load factor K:
Conventional horizontal mesh pads are traditionally sized for a K factor of 0.35 feet per second, which corresponds to a velocity of 10 feet per second in the reference case of water and air at room conditions.
When a knockout drum’s throughput has grown to exceed its capacity, there are generally two options:
1. Replace the vessel with a larger one to allow a mist eliminator with greater cross-sectional area, thus reducing the
velocity.
2. Replace the mist eliminator in the existing vessel with one using the latest technology to maintain efficiency
with higher throughput.
Option 2 is generally much more cost-effective and often does not require prohibitive down time. In a traditional vertical cylindrical vessel, the traditional horizontal orientation is no longer the only solution. Compressor knockout drums can be retrofitted for capacity increases using any of the following techniques:
1. Vertical mist eliminator elements with horizontal flow (K = 0.42 for mesh pads, K = 0.65 for vane units)
2. Properly engineered baffling for even velocity profiles with close spacing
3. Horizontal mesh pads with drainage layers or multiple zones that can increase capacity by 10% to 12% (K = 0.40)
4. Amistco Double-Pocket Vanes that can double the capacity of a conventional vane unit (K = 0.8 to 1.1)
5. Mesh-vane combinations that can increase efficiency and capacity by 10% to 25% (K = 0.5 to 0.65)
6. Mesh agglomerator followed by Double-Pocket Vanes for highest efficiency (99.9% of 2-micron droplets) and greatest capacity increase
7. Two-bank or four-bank configurations that allow mist eliminator elements of greater cross-sectional areas
 Figure 8 illustrates several of these possibilities: horizontal flow through vertical mist eliminator elements, use of mesh pads to agglomerate fine mist into large droplets that are removed by vane units, and a double-bank configuration. Amistco’s design specialists can help apply such advanced means of optimizing the efficiency and capacity of existing knockout drums to create optimal solutions for particular applications.
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