9.1 REVAMP TECHNIQUES 9.1.1 Objectives of Revamp
Some of the common reasons for revamps are: |
(i) Increasing capacity. J
(ii) Reducing reflux and reboil - energy savings. j t
(iii) Accomodating new process conditions - new product 1 specifications, new feed composition.
(iv) Reducing bottom pressure (vacuum applications) - decreasing j polymerization.
TYPES For the purpose of discussion, most revamp objectives can be classified into three types:
A Capacity revamps - the objective of this type of revamp is to increase unit throughput. The number of separation stages remains unaltered or may even diminish with this type of revamp.
B Energy revamps - the objective of this type of revamp is to increase the number of separation stages in the unit. Unit throughput remains unaltered, or may even diminish.
C A capacity/energy revamp - both throughput and separation stages are raised.
EXAMPLES If the reason for a revamp is increasing capacity or reducing energy consumption, classification into the above categories is straight forward.
If the reason for revamp is new product specification, it can be achieved either by using higher reflux and reboil rates (capacity revamp) or by using a greater number of separation stages (energy revamp), or both.
If the reason for revamp is excessive bottom pressure ((iv) above), then either a reduced pressure drop device, or a device allowing a greater throughput because of the lower pressure are required (capacity revamp). If such a device is less efficient, a greater number of stages is also required (energy revamp), making the overall revamp a capacity/energy revamp.
9.1.2 Revamp Techniques Classification Revamp techniques can be classified as follows:
(i) Internal hardware - these involve little change to the column shell and overall process. Typical examples are retraying, re-packing, and changing of tray layouts. This is usually the cheapest class of techniques, and is most effective in cases where columns are either overdesigned, designed for minimum capital, designed with outdated internals, or when some advantage of the existing internals can be readily sacrificed. These are the most common types of revamps practiced in distillation. j
(ii) External hardware - these include replacing the column by a J second-hand column, adding heat exchangers, and adding column j sections. These techniques tend to be more expensive than j internal hardware changes, and are generally practiced when internal hardware changes are unlikely to be sufficiently effective.
(iii) Process changes - these may range from very cheap to very expensive, depending on the process and equipment, and can turn out to be beneficial to the rest of the plant as well. These include unloading the column e.g. by eliminating a recycle stream, replacing a single-feed column by a multi-feed arrangement and changing column pressure.
(iv) Any combination of the above. These techniques are discussed in detail in the following sections.
During revamps, it is important not only to concentrate on the hardware or process revamped, but also on other column internals and equipment in the immediate surroundings. For instance, when a packing is replaced, consideration must also be given to the distributors, re-distributors and other column internals. If pressure is raised in the column, attention must be paid to the feed pump.
The effect on utilities is also important. For instance, when a refrigerated column which has been operating at low rates is revamped with valve trays, reflux rate is reduced because of the higher efficiency. If the refrigeration compressor also operates at low rates, it may need to operate on its minimum flow by-pass after the revamp, in which case, little or no energy savings will result.
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