QUESTION FROM OCTOBER'S CHEMICAL PROCESSING
|We use a light stripping column for removing methane and lighter components from natural gas liquids (NGL). We are having flooding problems with the column that we think might be due to water (the feed is saturated). Simulation shows all the water should go overhead and no hydrates should form. The first tower has a condenser temperature of -20°C (-4°F), a bottoms temperature of 84°C (183°F) and an operating pressure of 20 bar (295 psig). Could our simulation be wrong? What else could be going on?|
Check the bypass valve
It appears that the hot-vapor bypass valve at the reflux drum is leaking, increasing the amount of condensate in the condenser. This reduces the heat-transfer surface area and the rate of condensation, which will cause the tower pressure to rise (295 psig). This increased tower pressure will result in decreased pressure drop and vapor velocity across the trays. The liquid on the trays then &ldquoleaks&rdquo backward through the sieves and onto the lower trays; hence, your column appears to be flooding.
Likewise, at 183°F no water is carried over. But if it is, it&rsquos freezing in the condenser (-4°F) and restricting vapor flow. This will cause the same effects as mentioned above: increased tower pressure.
Ron Johnson, process engineer
Alpharma Pharmaceuticals, Baltimore
Keep ice at bay
My guess is that ice is forming on the trays at the top of the stripper. This will restrict vapor flow and liquid flow in the downcomers, leading to a combination of jet and liquid flooding.
Don Pearson, technical team leader, derivatives
PPG Industries, Lake Charles, La.
Pack your column
What type of internals do you have: packing or plates? What is the design liquid-to-gas ratio? What is the design liquid flow? Are you cooling at the top portion of the tower?
Without knowing these answers, my best guess is that you need packing with 95-98% free void space to reduce or eliminate flooding.
Tom Murphy, P.E.
Remove the water
Your problem is most likely due to vapor or liquid flow restrictions arising from accumulation of frost and/or ice formation in the column since the reflux temperature is below the ice point (32°F/0Â° C).
The solution to the problem would be the elimination of water from the unit. Since this is an existing column, the best way to remove the water will depend on the type of column construction and the product requirements.
1. You could raise the condenser temperature above the freezing point. This would allow moisture to leave the unit with the overhead vapors. Unfortunately, the higher temperature would also increase the carryover of NGL components, which could reduce yield. The amount of moisture in the overhead vapor will also be limited by the partial pressure of water. So, unless there is a high fraction of light components to carry away the moisture, liquid water might accumulate in the column and create other problems.
2. You could remove any moisture directly from the column feed. This would necessarily involve another operation.
3. If a trayed column is employed, you could process a side draw from a downcomer. The liquid sidedraw can be sent to a coalescer and decanter to allow phase separation and accumulation of a water phase that can simply be drawn away. The overhead hydrocarbon-rich phase would be returned to the column.
Dr. Donald J. Stookey, director, industrial technology
Compact Membrane Systems Inc., Wilmington, Del.
Raise the temperatures
1. You probably have hydrates in the condenser, even if the simulation doesn&rsquot indicate their presence. Most hydrocarbon gas streams will form hydrates at temperatures below 50°F or 60°F. At your operating temperatures, you should dry the feed upstream of the column or raise the condensing temperature to 60°F or 80°F.
2. The feed is saturated with water: How are you taking it out of the column? The overheads are too cold and bottoms are too hot. You need a water draw-off tray. Otherwise the water will build up on the trays (especially if they have weirs or, worse yet, sumps) and cause flooding.
Larry Tarkington, P.E.
Project Engineering of Texas, San Antonio
Add more trays
You should consider increasing the number of bubble trays and also adjust the velocity of the feed. It appears that there is not enough entrainment of water droplets, whereby in a saturated atmosphere the water droplets flood the stream. The residence time for water vapor to condense and drop out may not be sufficient.
Another option would be to inject diethylene glycol into the feed stream prior to introducing feed in the stripping column.
Uruj Kirmani, consultant
Add some heat
I suspect ice is building up in the column on any tray below 0°C. This is the result of mixing the -20°C reflux and the overhead vapor. Although the composition of the feed and bottoms is not given, NGL saturated with water will probably hold more water than can be held by the small quantity of overhead liquid methane in contact with ice at -20°C.
David A. Milligan, P.E., C.C.E., principal engineer
Matches, Edmond, Okla.
We recently had a spill at our chemical plant during the night. The night-shift supervisor didn&rsquot write it up, so we weren&rsquot sure of the exact location. Since our plant is in the Southeast, the soil is sandy and we were worried about the spill contaminating any water wells in the area.
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