Evaluation of the alternate design. The alternate design is attractive and can save considerable capital compared to the preliminary design, but the user must be convinced that it will work before proceeding with it.
The third-generation packings used are of the same nominal size as the Pall® rings. Looking at Fig. 8.7, this suggests lower packing factors, but much the same packing surface area, and therefore, much the same efficiency as the Pall® rings. Yet, this is not so; the efficiency predicted for the Pall® rings is 23 percent lower than that predicted for the third-generation packing. The difference is associated with the basis of the HETP prediction. The key for acceptance of the alternate quote here is, therefore, being convinced that the supplier's basis for the lower HETP is correct.
Normally, the HETP used by the supplier was derived from tests or experience in similar towers. The user needs to confirm that this ex perience was correctly and conservatively translated into the design HETP for the new tower. It may be possible to obtain actual test data from an existing tower; these would provide a very reliable check. Unfortunately, data are often unavailable, and the user needs to trouble-shoot the design without them. Asking the right questions is important. Here are some:
■ What HETPs were used in the previous designs? What was their basis?
■ Were there performance tests that confirmed the design HETPs? Were the performance tests conducted rigorously (see Ref. 40 for good testing practices), or did they simply consist of taking sets of readings?
i Was a sensitivity analysis conducted on the effects of measurement errors on HETP (see Ref. 40)? What variables were the data sensitive to?
■ Was the VLE used in the test data analysis the same as the VLE used in the current design? If not, does analyzing the test data using the current VLE lead to higher HETP (see Sees. 7.3.1 and 7.3.6).
■ Were the packings (type and size), distributors, and redistributors similar to those that will be used in the current design? If not, how does the design HETP account for the differences?
■ Was the column used to derive the data of the same scale (diameter, bed heights) as will be used in the new design? If not, are there any scaleup effects that should be allowed for?
■ Are there any differences in operation—such as the nature of the feed mixture, pressure, temperature, etc., that may affect the scaleup of HETP to the new design?
Modification of the alternate design. Once the replies to the above questions are evaluated, a new design basis emerges. This basis is established by closely working with the supplier until an HETP is derived with which both the user and the supplier are comfortable.
9.4.0 Column sizing example: trays or packings?
Example 9.2 asked (Sec. 9.4.2) whether a packed tower would be better than trays for the depropanizer. The design with trays is in Sec. 6.5.11. For the comparison, it is assumed that after evaluation of the alternate design (with third-generation packing; Sec. 9.4.9) it was found to be sound and not too tight.
Tray tower Packed tower
Column diameter, ft 6
Top section: No. of trays 11
Tray spacing, in 21
Packed height, ft Allowance for distributor, ft Allowance for support or holddown, ft
Total height, ft 17.5
Bottom section: No. of trays 18
Tray spacing, in 24
Packed height, ft Allowance for distributor, ft Allowance for redistributor, ft Allowance for support or holddown, ft
Total height, ft 34
Middle section (between top and bottom)
Height, ft 3.5
Height of packed or trayed 55
portion of column, ft
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