Sound purchasing decisions depend upon properly soliciting and evaluating proposals from various vendors. This article will focus on the first step: assembling a request for quotation (RFQ) that specifies your requirements in a manner that assures you will receive offerings that meet your needs and that you can compare. A follow-up article will look at evaluating the quotations you receive. While we will focus on distillation/absorption/stripping column internals, many of the general principles apply to all types of process equipment. Some, as we will point out, relate particularly to new units and others to retrofits/revamps.
If you want to have any hope of comparing quotations, the RFQ must be specific enough so that all offerings are on a common basis. In addition, management must clearly define what you absolutely must achieve as opposed to what is desired. Penalties for missing both throughput and product quality specifications must be identified. In the throughput case, does the amount reflect what marketing believes it can sell, or what a signed contract, with penalties, calls for? In the quality case, is the specification what is desired, or is it a threshold value that must be achieved to meet the definition of product? In some instances, an upper limit on total cost or downtime can be a valuable aid in the screening process.
Are you looking at packing, trays or both? A detailed treatment of this question is far beyond the scope of this article. In general, trays tend to be more predictable, whereas structured packing wins at vacuum; it can be a toss-up between the two at atmospheric pressure. High pressure and/or high liquid loads favor trays and random packing. Some special considerations that can influence the choice of device are the corrosiveness of the system and whether it contains suspended solids or is otherwise fouling. Vendors need to know if the material is non-Newtonian because design models are normally based on Newtonian fluids.
For either a new column or a retrofit/revamp, you need a set of design loadings. The most common way of obtaining them is from a simulation. However, make sure you can really believe the simulation [1, 2]. (Also, when running the simulations, determine the sensitivity of the separation to reflux versus the number of stages so high-efficiency options can be evaluated.)
Don’t base actual design loads on the simulation alone, though. Consider additional factors. For example, how certain are you of the physical properties of the system? Are they well-known hydrocarbons, or an exotic collection of components whose properties must be estimated by some esoteric means? Is the feed composition or quantity likely to change as the upstream catalyst ages? Is there a startup case? Is there an alternative feed case? Is there a future case? The minimum rate that realistically might occur can be important. Trays not only might weep, but can lose their downcomer seal. Excessive weeping has been blamed for vibration leading to tray damage. A minimum rate to a packed tower might affect distributor performance.
Size up the column
For a new column, it is always a good idea to do your own preliminary (or final) column sizing. Excellent sources for sizing methods for nonproprietary hardware are available [3, 4]. Members of Fractionation Research Inc. (FRI) have access to its rating methods. The major vendors, such as Koch-Glitsch and Sulzer, offer user-oriented rating programs via their Web sites. An important point to bear in mind on new installations is that the cost of a shop-fabricated column with its trays or packing (in ordinary metallurgy) is usually less than half the total erected cost of the column, reboiler, condenser, instruments, pumps, piping, etc. Thus, modern high-capacity devices that save six inches or so on the diameter of a new column might not be justified.
For a retrofit/revamp, find out the original design and rating, if possible. When you hear: “My device will give you 25% more capacity,” you should ask: “25% more than what?” Most likely the column was designed for about 75% of flood; this has been standard practice for more than 40 years. Therefore, the tower shell with redesigned, modern internals might be good for a significant throughput increase without going to the more exotic proprietary internals. Hopefully, plant records are good enough to check for observed bottlenecks. A talk with operators who have been around for a while invariably is a good idea, and a test run, if possible, is always useful.
Also, make sure to check the auxiliaries. Simulations will give you the required condenser and reboiler duties at the desired new conditions. It is pointless to spend a lot of money on new internals if the reboiler cannot handle the required duty. This is when it is particularly useful to have determined the reflux/theoretical-stage relationship necessary to achieve the desired product rate and purity. If the desired new conditions make the existing column shell and auxiliaries very tight, consider all options. High-efficiency internals might allow reflux reductions. Conversely, high-capacity devices at slightly lower efficiency might save a column shell if the condenser and reboiler are adequate. Low-pressure-drop packings, where trays might otherwise do the job, might ease reboiler approach temperatures.
Check the system limit
In many cases for a retrofit/revamp, assessing the system limit or ultimate capacity of the column shell can be very useful. The concept is quite simple: In a turbulent vapor/liquid flow field, there is a maximum, stable drop size that is a function of system properties only. If that drop size is less than or equal to the size that will be entrained due to the velocity of the rising vapor, all the liquid will be blown out of the column. This is a hardware-independent value; it can govern for devices that depend upon gravity separation, particularly in low-surface-tension applications, such as hydrocarbon services.