9.3.1 External Hardware Changes
ADDING COLUMN SECTION IN SERIES Adding a section to the column in series is simply a means of increasing the number of trays. The new section may be added either directly to the existing column, or as a separate column in series with the existing column. Adding a new section at the top of an existing column is usually preferred, unless there is a height limitation or the added section is large. Care must be taken to ensure the feed enters the modified column in the right location.
This technique can be used for increasing capacity, reducing energy | consumption or both. Best results are obtained when the ratio of I reflux to minimum reflux is high. If this ratio is close to unity, this technique suffers from a marginal returns situation.
The main disadvantage of this technique is its high cost.
ADDING COLUMN SECTION IN PARALLEL Adding a section' to the column in parallel is simply a means of increasing its capacity. This is usually carried out by adding a separate column (often a duplicate). Only the section that suffers a capacity limitation needs to be added. This technique is most effective where a small section bottlenecks a large column. A common application is fouling services, where the bottom section fouls over a period of time and bottlenecks column throughput. In such a case, only the bottom section needs to be duplicated.
REPLACING THE COLUMN BY A SECOND-HAND COLUMN This is another expensive revamping technique. Depending on the column available, this technique can be used for increasing capacity, reducing energy consumption, or both.
Several pitfalls exist when a column is replaced by a second-hand I one. First, the temperature and pressure rating of the column must , be compatible with the process. Second, either column diameter, or number of trays, or both, must be larger than that of the existing column. Thirdly, the internals of the new column must be checked.
It is usually best to re-design the internals of the second-hand column, and one must be able to do this without extensive work on the column shell. For instance, a trayed column with 16 inch tray spacing may not be suitable for spray regime operation. Increasing the spacing from 16 to say 24 inch may involve a large amount of work on. the shell and this may not be worthwhile. On the other hand, if a tray spacing of 16 inch is acceptable for the new application, the effort involved in replacing the internals may be minor.
Another pitfall is the ability of the second-hand column to use the same foundations as the existing column. If new foundations are required, the cost may escalate. This problem is aggravated if the column is supported on a platform. The platform may not be strong enough to support the new column.
Most important is the condition of the new column. If metal thinning is significant, it is often not worth the trouble.
Access to the new column is also important. If the column is to be installed in a congested area, a larger column may be very difficult to maintain.
One extreme example of an application in which new and second-hand sections were added together to make up a fractionator is the C. splitter in Mobil's Beaumont, Texas ethylene plant (9). This fractionator is made up of seven different sections, added in series and in parallel (Figure 9.7). Some of the column sections also operate at different pressures (9).
ADDING AND MODIFYING HEAT EXCHANGERS This technique is commonly used for energy saving revamps, but may also be effective in capacity revamps. The main types of exchangers concerned are interreboilers, intercondensers, feed preheaters, feed precoolers and feed-bottom interchangers. Adding or modifying these exchangers is relatively cheap compared to other external hardware changes.
Application of these exchangers for energy savings has been discussed in Section 4.1. In general, interreboilers and feed preheaters will significantly save energy if a heating medium cheaper than the reboiling medium is available at a temperature and flow rate sufficiently high to reduce reboiler heat duty. Similar considerations apply to the cooling medium used in a feed subcooler or an intercondenser.
Feed precoolers and preheaters can also be used to increase capacity. Adding a preheater or increasing the capacity of an existing one unloads the bottom section at the expense of loading up the top section. A precooler reverses this function. If one of the sections in the column is loaded up, while the other is not, capacity may be increased by adding a heat exchanger in the feed stream. Adding these exchangers to improve capacity may increase or decrease column energy usage, depending on the heating and cooling media used. A change in column feed point may be required when these exchangers are added.
Interreboilers and intercondensers are rarely used for capacity revamps. Although the liquid and vapor traffic below an interreboiler may be reduced when the exchanger is added, the section between the exchanger and the feed tray is still highly loaded, and there is a need to add separation stages» Despite this, at least one source (10) reports favorable results with this technique.
Ethylene distillation train
FIGURE 9.7 REVAMPED C2 SPLITTER COLUMN
(From Chou, A, A M Fayon, and B L Bauman, Chemical Engineering, June 7, 1976; by Courtesy, KcGraw-Hill Book Company)
MULTI-FEED ARRANGEMENTS This technique can be applied when streams from different sources are combined before they enter the column. The feed piping can be changed so that the feed stream from each source enters at a different location along the column height. This arrangement reduces reflux, reboil and energy requirements, thereby also increasing capacity. The capacity increase achieved by this method is, however, relatively small.
COLUMN UNLOADING This technique may lead to significant savings in both capacity and efficiency. One method of achieving this is by installing a simple distillation facility upstream, such as a flash drum, a series of flash drums or a stripper. Another method is that of installing a separation facility that enables bypassing a stream around the column; for example, when a column is used to dehydrate two streams, one of which contains a small quantity of water compared to the other, it may pay to install a dryer on the stream containing little water, and to bypass it around the column. The ability to use this technique depends on the process.
CHANGING PRODUCT SPECIFICATIONS This technique increases capacity and reduces energy consumption in the column, but further processing may be required downstream. Sometimes it may be easy to install an additional separation facility downstream of the column.
CHANGING PRESSURE The effect of pressure on capacity and energy consumption has been discussed in Sections 5.2.8 and 4.2.4 respectively. At low pressures, raising the column operating pressure can be used to increase column capacity, but at the expense of higher energy consumption. Best results are obtained with this technique when relative volatility is high, its rate of reduction with pressure is slow, and when the ratio of reflux to minimum reflux is low. Guidelines defining the range of application of this method are shown in Figure 5.17.
Lowering the pressure is an effective means of reducing energy consumption at all pressures, and of increasing capacity at high pressure. Best results are obtained when relative volatility is low, its rate of reduction with pressure is high and the ratio of reflux to minimum reflux is high.
There are wide application ranges over which changing pressure is ineffective in changing capacity and energy consumption. These are discussed in Sections 4.2.4 and 5.2.8 and their associated references.
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