Trays, sieve costs

Distillation towers feed-tray location for, 10 optimum reflux ratio for, 371-376 specifications for, 16 (See also Bubble-cap contactors, Packed towers. Sieve trays, and Valve trays) Distribution costs, 194, 196, 207, 211 Distribution in statistical analyses, 745-746 Dividends, tax exemptions for, 259 Documentation, 137-149,452-476 Double-entry bookkeeping, 143-144 Downcomers in tray columns, 684-686 Drives, cost of 532-533 Dryers, cost of 713-716... 

Air stripping is used to remove 90% of the toluene (molecular weight = 92) dissolved in a 10 kg/s (159 gpm) wastewater stream. The inlet composition of toluene in the wastewater is 500 ppm. Air (essentially free of toluene) is compressed to 202.6 kPa (2 atm) and bubbled through a stripper which contains sieve trays. In order to avoid fire hazards, the concentration of toluene in the air leaving the stripper is taken as 50% of the lower flammability limit (LFL) of toluene in air. The toluene-laden air exiting the stripper is fed to a condenser which recovers almost all the toluene. A schematic representation of the process is shown in Fig. 2.11. Calculate the annual operating cost and the fixed capital investment for the system. The following physical and economic data are available ... 

The cost,, of the stripper (including installation and auxiliaries, but excluding the sieve trays) is given by... 

In comparison with tray towers, packed towers are suited to small diameters (24 in. or less), whenever low pressure is desirable, whenever low holdup is necessary, and whenever plastic or ceramic construction is required. Applications unfavorable to packings are large diameter towers, especially those with low liquid and high vapor rates, because of problems with liquid distribution, and whenever high turndown is required. In large towers, random packing may cost more than twice as much as sieve or valve trays. 

Fixed valve and sieve trays prevail when fouling or corrosion is expected, or if turndown is unimportant. Valve trays prevail when high turndown is required. The energy saved, even during short turndown periods, usually justifies the small additional cost of the moving valve trays. 

Only tray-type columns were considered because of the difficulty of incorporating an effective cooling circuit into a packed column. Sieve trays (as opposed to bubble or valve-type trays) were preferred because of the ease of installing cooling coils and also their low unit cost. Details of tray selection are included in Appendix G.l. 

The costs of the two sieve-plate columns (absorption column and product bleaching column) are estimated according to the recommendations presented in Ref. CE9 (p.389). The columns are costed according to their diameter, operating pressure, and the number of sieve trays. 

Equation (3.89) is the sieve tray liquid entrainment flood gas loading equation. Equation (3.89) sets the maximum gas rate VM- At a higher Vm, excess gas-liquid froth buildup would reach the tray above and recycle liquid to it. This liquid recycle would build up to a point at which the liquid would block any vapor passage, resulting in a flooded column and costly shutdown. 

Fjj is the cost factor for tray type (valve, grid, bubble cap, sieve) ... 

The cost of bubble-cap trays is by far the highest. Sieve trays are the least expensive, but valve trays are only slightly more expensive. 

Cost Low About 20 percent higher than sieve trays (11) High, About 2-3 times the cost of sieve trays (10,11) Low... 

For most other services, either sieve or valve trays are the best choice, Sieve trays are at an advantage when the service is fouling, or corrosive, or when turndown is unimportant, while valve trays are preferred when turndown is important. With high energy costs, the energy saved during even short turndown periods usually justifies the relatively low cost difference between valve and sieve trays. This has made valve trays most popular. 

Discuss the applicability of sieve trays. The sieve tray is probably the most versatile contacting device. It should be considered first for the design of a tray column. It has the lowest installed cost of any equilibrium-stage-type device, its fouling tendencies are low, and it offers good efficiency when properly designed. 

Sieve trays, 651-652, 662, 681-686 allowable velocities in, 656-661 cost of 709 efficiency of 661-667 pressure drop over, 667-679 Silicon rubber, 435 Silver, 432... 

Static extraction columns Spray column Baffle column Packed column (random and structured packing) Sieve tray column Deliver low to medium mass-transfer efficiency, simple construction (no internal moving parts), low capital cost, low operating and maintenance costs, best suited to systems with low to moderate interfacial tension, can handle high production rates Petrochemical Chemical Food... 

A number of different constructions have been developed for tray columns the essential ones are still bubble-cap, valve, sieve and grating plates. Bubble-cap plates are the oldest development and, due to the high production costs, they are rarely used today. 

Sieve and valve trays have replaced bubble cap trays in many new construction projects because of lower cost. Yet bubble cap trays are still often used for the superior characteristics mentioned earlier. 

For new tower design, without overriding process factors, the cheapest overall system (tower, foundation, internals) is normally a sieve or a valve tray. Both have essentially the same capacity. The driving force behind valve tray selection is the flexibility benefit of the valve tray at little additional cost. However, for larger diameters and higher liquid rates, the flexibility advantage of the valve tray is more illusory than factual.f ... 

The mass-transfer devices may be sieves (holes), fixed valves, moveable valves, or bubble caps. Fig. 2 shows a selection of mass-transfer devices. The purpose of the device is intimate mixing of the vapor and liquid on the tray deck. An ideal device has high capacity, high flexibility, low leakage, low pressure drop, and low cost. 

---