9.2.1 Change of Tray Layout
WHEN EFFECTIVE A quick browse through Sections 5.2.3 and 6.3.4, which list factors affecting capacity and efficiency, can define the effectiveness of this technique. Changing layout may be used effectively to increase column capacity, but the technique is rather limited in its ability to significantly improve column efficiency. It is thus suitable for capacity revamps only.
METHODS The methods commonly used depend on the flooding mechanism which restricts capacity, and can simply be deduced from the discussion in Section 5.2.3 and Table 5.2. The methods commonly used I include: I
(i) Change downcomers from straight to sloped (Figure 9.1). Keep top downcomer area constant, or even reduce it when entrainment flooding is limiting (Figure 9.1a). Increase downcomer top area when downcomer flooding is limiting (Figure 9.1b).
Best results are achieved with this method when the downcomer is large and entrainment flooding is limiting, or when the downcomer is small and downcomer flooding is limiting. Poor results are achieved if the above are reversed, because of a marginal returns situation.
(ii) Increase fractional hole area. This reduces the entrainment flooding tendency, entrainment, and pressure drop. It may cause a greater weeping tendency and a slight reduction in efficiency in the froth regime. Best results are achieved in the spray regime, where entrainment or spray-entrainment flooding is limiting, and when fractional hole areas are small (<8%) .• Poor results are achieved when fractional hole areas are large (>11%) because of diminishing returns, and in the emulsion regime because of excessive weeping.
(iii) Increase clearance under the downcomer. Only effective for downcomer backup flooding. Not recommended unless clearances are extremely small (<1 inch) or when the downcomer apron suffers from a buildup of solids.
(iv) Use swept-back weirs (Figure 9.2) . This is similar to sloping downcomers, but its effectiveness is generally more limited.
(v) Increase the number of passes. Using a greater number of passes reduces the liquid load and increases downcomer area at the expense of some efficiency reduction (shorter liquid flow path). Best results are achieved at high liquid rate applications and downcomer limitations. Poor results are achieved at low liquid loads.
Caution is required with this type of revamp with small-diameter columns (<6 feet diameter). Under these conditions, the reduction in efficiency caused by the shorter path length can be significant. One case study was reported (15) in which an absorber was revamped by replacing single-pass by two-pass trays. Although it achieved the predicted capacity increase, the reduction of flow path length from 36 to 18 inches caused a loss of efficiency large enough to justify re-installing the original trays.
Figure 9.1 Sloping the down comer
Figure 9.1 Sloping the down comer
00 To INCREASE TRfly CflPftCHy
(I) TO- WCREftSE DOWMCOMÊR CfiPflCITy .
(a) TO WCRERSE DOWNCONER CflPfiCIT/
(b) TO IHCREftSe TUfí/ CltfflCIT/
9.2.2 Change of Common Tray Type
Section 5.1.2 and Table 5.1 set the guidelines for the effectiveness of changing the common tray types.
FROM BUBBLE-CAPS Changing to sieve or valve trays generally increases both capacity and efficiency and reduces pressure drop and entrainment. Capacity improvement is achieved under most conditions. Energy savings may be large if the reflux to minimum reflux ratio is not close to unity. The only disadvantage of this type of revamp is a greater rate of leakage to the tray below.
FROM SIEVE TRAYS Changing to valve trays may sightly increase efficiency and capacity, but generally this increase is relatively small. Revamping from sieve to valve trays is usually not justified, unless long periods of low-rate operation are anticipated and turndown improvement leads to energy savings.
TO DUAL FLOW TRAYS Changing to dual-flow trays considerably increases tray capacity, but at the expense of some efficiency and turndown loss. Best results are obtained with columns operating under constant load when some efficiency can be sacrified, or with columns which suffer from fouling.
SUMMARY Revamping from bubble-caps is generally effective for capacity improvement, energy savings and pressure drop reduction. Revamping from sieve to valve trays is effective for energy savings at low rates. Revamping to dual flow trays is effective for capacity improvement only.
9.2.3 Change of Packing Type
Section 7.1.4 and Figures 7.2 and 7.3 set the guidelines for the effectiveness of changing packing types.
CHANGING METAL PACKINGS Changing metal packing from Raschig or Lessing rings to Pall rings or Metal IntaloW©packings considerably improves both capacity and efficiency. If the capacity increase is to be maximized, Pall rings or Metal Intaloj^packings of a larger size than the existing Raschig or Lessing rings can be used, and this would trade off the efficiency improvement for a further capacity increase. Similarly, if the efficiency improvement is to be maximized (energy saving revamp), a smaller size packing would trade off the capacity gain for an efficiency gain.
Pressure-drop reduction revamps follow reasoning similar to capacity-increase revamps, and pressure drop can be reduced by replacing Raschig and Lessing rings by Pall rings or Metal Intalox— packing.
Changing Pall rings to Metal Intalox packing only achieves a small enhancement in capacity and a small reduction in pressure drop, which are somewhat uncertain. The efficiency remains virtually the same. Replacing Pall rings with Metal Intalox^packings is therefore not likely to achieve much.
CHANGING CERAMIC PACKINGS Changing Berl Saddles to Intalox saddles significantly improves efficiency but has a relatively small effect on capacity. This change is therefore effective for energy saving revamps. For capacity and pressure drop revamps, the improved efficiency of the Intalox saddle may be traded off for capacity enhancement or pressure drop reduction by going to a larger size of packing.
CHANGING MATERIAL This depends to a large extent on the chemicals processed. In general, changing material can be most beneficial when the column contains ceramic packings. Changing from ceramic to either metal or plastic generally improves both capacity and pressure drop (20). Changing from ceramic to metal is likely to also improve efficiency. One of the main reasons for this improvement is the availability of the Pall-ring shape in both metal and plastic, but not in ceramic. This geometric shape gives better capacity and efficiency than most other common shapes (7.1.4). Capacity gains by changing from ceramic saddles to plastic or metal Pall rings are of the.order of 20-30% (20).
In addition to this capacity increase, there is a further improvement in capacity and pressure drop resulting from elimination of the breakage problem which commonly occurs with ceramic packings. Caution, however is required, as neither metal nor plastic generally match the chemical inertness of ceramic. Plastic in particular can be troublesome when processing hot fluids, when heat of mixing is high, in oxidizing atmospheres and in the presence of solvents. Additional discussion is presented in Section 7.1.5 and Ref. 20.
SUMMARY Changing from Raschig and Lessing rings to Pall rings or Metal Intalox is effective for increasing capacity or reducing energy consumption. Changing from Berl Saddles to Intalox saddles is effective for increasing either efficiency or capacity and pressure drop. Changing material from ceramic to metal or plastic is effective for increasing capacity, reducing pressure drop, and often improving efficiency. Changing Pall rings to Metal Intalox may bring about a slight capacity improvement but is unlikely to be an effective revamp method.
9.2.4 Replacing Trays with. Packings
The introduction of the^high capacity, high efficiency Pall ring, and ! later the Metal Ir.talox^packir.g, have increased the popularity of j using packings for revamps of trayed columns. Several literature I sources (for instance, 1-3) have pointed out the advantages of | packings. A comparison of trays and packings is presented in j Sections 7.5.1 and 7.5.2.
i It is difficult to directly compare the capacity and efficiency cf j trays and packings, but a recent analysis by Dolan and Strigle (3) ; permits at least a crude comparison, which is suitable only for j deriving general guidelines.
j CAPACITY COMPARISON In order to compare the capacity cf trays and packings, a similar analysis was carried out here. The results of this analysis are shown in Figure 9.3, which compares the capacity of Metal Intalox®packing (IMTP) to sieve tray capacity. The analysis is crude, and is suitable only for deriving very general guidelines.
IMTP was selected to represent packing capacity because it appears tc have higher capacity than other common packings (See,Figure 7.3, Section 7.1.4). It should be noted that Pall rings have a capacity only about 5-10% lower (Section 7.1.4), and comments made here are generally applicable for this packing type as well.
The values compared are the design values of C , based on the total column cross-section area. IMTP data were obtained from the revised Strigle and Dolan design correlation (11) , and those for sieve trays were derived from Fair's flooding correlation (6), using the procedure described in Reference 3. '
Was this article helpful?