Riser overflow

Chimneys (risers) blocking flow to draw sump forcing liquid to overflow prematurely. Flooding of trayed section below pump around. Lack of response to pump around flow changes. Design error. 

The principle is basically the same as any coffee pot a paper thimble is filled with the substance to be extracted (F) and a loose plug of cotton is placed (E) over the top. The Soxhlet apparatus is attached to a flask containing the proper solvent (if the solvent is not given in the formula, then usually you must find a solvent in that either the desired substance or the impurities are insoluble in). Attach a condenser to the Soxhlet tube (B). The solvent is boiled causing vapor to rise and pass through the holes (C) into the condenser where it is turned back into liquid. The liquid drops down into the thimble and solvent When the solvent level exceeds the top of the riser tube (D) the solvent overflows back into the boiling flask (G) and the process is recycled or continuous. 

Fig. 7. Schematic drawing of continuous laboratory cell. 1, Boiler 2, riser 3, condenser 4, disperser 5, cathode when Hg 6, catholyte solution 7, anode chamber 8, diaphragm 9, lead to cathode 10, lead to anode 11, thermometer 12, inlet tube 13, stirrer 14, catholyte level 15, level of supernatant AN 16, washer 17, disperser 18, water level 19, supernatant level 20, stopcock 21, overflow tube 22, overflow tube and 23, water inlet. From Baizer.58...

A = Storage vessel for intake, B = Riser tube, C = Distillation vessel, D = Electrical heater, E = Aluminium block, F = Descending capillary tube, G = Receiver with overflow, H = Mercury float valve with connection to diffusion pump... 

A vapor-distributing support (Fig. 3.10) is a flat perforated plate containing perforated vapor risers. Liquid descends through the floor perforations, while vapor rises through the riser perforations. The bottom portion of the vapor risers is unperforated, so that vapor is injected above the liquid pool on the plate. Sumps are optional and recommended (289) where liquid inventory is to be reduced. The vapor-distributing support combines two internals (a vapor distributor and a support plate) into one. Compared to a vapor distributor, this saves both vertical space and internals costs. On the other hand, obstruction of perforations by pieces of packing and possible liquid overflow into risers may make its vapor distribution quality somewhat lower. 

An excessive number of risers should be avoided, because they will obstruct liquid flow and form a hydraulic gradient on the chimney tray. With short risers, this can provoke liquid overflow into risers. Caution is required to prevent any row of risers from restricting liquid flow. The hydraulic gradient can be estimated by techniques similar to those used for estimating hydraulic gradients on bubble-cap trays (48, 257, 319, 371). 

An overflow pipe (or downcomer), with an opening located below the top of the risers, is recommended for preventing liquid from overflowing the risers at high liquid levels (Fig. 4.10). The overflow pipe should be liquid-sealed at the bottom to avoid vapor rise through it. Two experiences have been described (57, 237) where failure to provide an overflow pipe caused liquid to overflow the risers and prematurely flood the column section above the chimney tray. In a third case (334), liquid overflowing the risers caused entrainment (Sec. 8.2). 

In packed columns, liquid overflowing the chimneys may preferentially descend into some risers (often the peripheral), with vapor ascending through the others (often the central risers). This may cause severe maldistribution of vapor to the bed above. 

I-beam interference can be just as troublesome in the space above a chimney tray. In one case history contributed by D. W. Reay (334), this interference is believed to have led to severe vapor maldistribution in a refinery vacuum tower (Fig. 8.66). The maldistributed vapor profile was displayed as a carbon deposit on the siuTace of the bottom packing. The deposit formed an annular ring about 5 ft wide that extended about 1 in into the bed. In that case, liquid was known to overflow the chimneys for several months because of an incorrect location of level tappings. This overflow caused liquid entrainment. Some entrained droplets ultimately carbonized on the base of the bed. Had the vapor profile been uniform, entrainment (and therefore deposit laydown) would have been more uniform. It is believed that vapor from the side chimneys was blocked by the beams and preferentially ascended around the periphery. If liquid overflow (down the risers) had been uneven, the maldistribution could have been further aggravated. 

Examples of major accidents that have happened in the industry are the Piper Alpha platform tire and explosion in 1988 that was caused by permit to work (PTW) system failure and layout issues where 165 people died the Bombay High platform, where in 2005 a multi service vessel (MSV) collided with the platform, causing riser damage and a fire that killed 22 people and the Texas City Refinery, where in 2005, maintenance and process start-up flaws caused a petroleum distillate to overflow, causing an explosion and a flash fire that killed 15 people. 

The large liquid volume required can be obtained by using a deep liquid level on the plates or by having the overflow from a plate pass thro igh a holding tank for the chemical reaction before it is added to the plate below. Deep liquid levels give undesirable action with normal-type bubble plates, but tall caps and risers can be used to obtain satisfactory operation. 

Risers and FVO/LVOs should be located as far away from the Safe Area as practical. To prevent damage to valves, acmators and instrumentation, FVOs and LVOs should be located in areas to minimize potential risks from falling objects, liquid hydrocarbon overflow, explosions, or flame impingement. 

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