Trap-out tray

A viable alternative is to place a large total trap-out tray below the condenser that can serve as an internal reflux drum. Liquid reflux can be taken from this trap-out tray and fed to the top tray through a control valve. This modihed system requires additional column height, which means higher capital investment. But its dynamic controllability is much better. 

The liquid level in the base of the rectifier corresponds physically to the total liquid trap-out tray. A pump and two parallel lines with control valves in each are installed. Since the flow rate to the sidestream side of the wall is the larger of the two, the level on the trap-out tray is controlled by manipulating the control valve in the liquid line to that side of the wall. A ratio scheme then adjusts the other control valve to maintain the desired liquid split. The liquid flow rate to the sidestream section is measured, and this signal is sent to a multiplier whose other input is adjusted to give the desired liquid split. The output of the multiplier is the set point signal to a flow controller that manipulates the valve in the liquid line to the prefractionator to achieve the specified flow rate. Note that this ratio is changed by the composition controller in the prefractionator. 

A third alternative also exists that has the condenser on top of the column but also collects the liquid reflux on a trap-out tray (Fig. 3). Vapor goes up through a riser in the trap-out tray to the condenser. Liquid level is measured on the trap-out tray and is controlled with the coolant flow. Then the reflux comes off of the trap-out tray through a flow control valve and is sent to the trays below. We need no reflux pump or drum, but we will need to add height to the column and also to have an additional control valve. Yet the advantages of this trap-out tray arrangement are clear in terms of having a direct measurement and control of colunm reflux flow. 

This temperature mismatch problem can sometimes be overcome by considering an alternative process flowsheet that feamres a distillation column linked with several external side reactors. The column operates at a low pressure and temperamres favorable for separation. Liquid side streams from trap-out trays at intermediate locations in the column are pumped to external reactors operating at higher pressure and temperamres that are favorable for chemical kinetics. 

One approach to overcoming the temperature mismatch is to consider a flowsheet that features a distillation column with external side reactors. The column is operated at a pressure that is favorable while stiU permitting separation, which is typically as low as possible for the use of cooling water in the condenser. There is no catalyst on the trays in the column, so there is no chemical reaction occurring in the column. Several total liquid trap-out trays are located in the middle section of the column. Liquid is collected and pumped up to a sufficiently high pressure so that it remains liquid when heated to the higher temperatures that are favorable for reaction. This configuration permits the temperature for separation and the temperature for reaction to be set independently. 

The distillation column operates at its optimum pressure and temperature for separation (320 K in the reflux drum). The column is fed with two pure reactant fresh feedstreams Fqa and Fob- The column has three zones. There are Ns trays and a partial reboiler in the stripping section and a rectifying section with Nr trays and a total condenser. As shown in Figure 16.1, a number of total liquid trap-out trays are installed at several intermediate trays between these two zones. There are Nm trays in the middle of the column. No reaction occurs anywhere in the column. Reaction only occurs in the external side reactors. 

The column/side reactors process has a large number of design optimization variables. A number of specifications and assumptions ate made to leduee this number to a manageable level. The design objective is to obtain 95% conversion for fixed fresh feed flowrates of 12.6 mol/s of each reactant. The product purities of distillate stream x c and bottoms stream xb,d are both 95 mol%. The two fresh feedstreams of reactants A and B ate fed into the column at the trays immediately above the lower and upper trap-out trays, respectively. 

The reactors are assumed to be adiabatic, and the holdups of each reactor are the same. We assume that the number of trays between each liquid trap-out tray is the same. Other assumptions are theoretical trays, equimolal overflow, saturated liquid feeds and reflux, total condenser, and partial reboiler. 

The tray temperatures of the trap-out trays are the inlet temperatures to the external reactors. The ordinary differential equations of the plug flow reactor for steady-state adiabatic operation are integrated from 0 to the total reactor volume Vr, keeping track of how the component compositions and temperature change along the reactor. These equations are the following ... 

Fix the number of trays IVlt between each liquid trap-out tray at a small value. 

During PFR operation it also supplied sodium coolant to the core melt-out tray situated below the diagrid in the reactor vessel. During the current decommissioning period it remains in use for cold trapping, if necessary, and measuring impurity levels in the primary sodium. The PCTL is situated in a shielded air-cooled concrete vault adjacent to the reactor vessel, as shown in Figure 5.2. 

Often, there is no process reason for the use of inlet weirs, especially at higher liquid rates. Then the inlet weirs may be removed. But some tray types, such as "Exxon Jet Tab" trays or total trap-out chimney trays with no outlet weir, absolutely require the use of inlet weirs. 

If inlet weirs are used they should have at least two slots %-in. by 1-in. flush with the tray floor to aid in flushing out any trapped sediment or other material. There should also be weep or drain holes below the downcomer to drain the weir seal area. The size should be set by the type of service, but a minimum of M-in. is recommended. 

Elemental mercury readily evolves the vapour which constitutes a severe cumulative and chronic hazard. No mercury surface should ever be exposed to the atmosphere but should be covered with water. All manipulations involving mercury should be carried out in a fume cupboard and over a tray to collect possible spillage. Spilt mercury is best collected using a glass nozzle attached to a water suction pump via a bottle trap the contaminated areas should be spread with a paste of sulphur and lime. 

Bubble-Cap Trays (Fig. 14-27a) These are flat perforated plates with risers (chimneylike pipes) around the holes, and caps in the form of inverted cups over the risers. The caps are usually (but not always) equipped with slots through which some of the gas comes out, and may be round or rectangular. Liquid and froth are trapped on the tray to a depth at least equal to the riser or weir height, giving the bubble-cap tray a unique ability to operate at very low gas and liquid rates. 

During adjustment of pressure setting of the regulating valve in supply argon system (May 88) about 2 1 of NaK backed up and leaked out from the NaK bubblers provided for supply argon purification. The NaK leak was carefully collected in a tray covered with DCP and safely disposed off vwthin 30 min. As a remedial measure, a backflow trap of 50 1 capacity was introduced on the upstream side of bubbler and a pressure equalising valve was provided across it. ... 

The vessel or work area must be drained as completely as possible. Often liquid will remain trapped on vessel internals, such as pieces of packing or tray parts. In such cases, water can be added to the vessel such that the process material is either dissolved or floated off. Alternatively, all large openings such as manholes can be opened, and the vessel can be steamed out. 

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