In our previous article, “Distill your quote request” (June, p. 30), we discussed how to prepare requests for quotation (RFQs) to get adequate and comparable quotes from tower internals vendors. It is common to obtain quotes from more than one supplier for price comparisons and checking each for correct device model names/numbers is easy with standard internals types. But if the project involves high capacity or proprietary devices, you will need to compare the offerings from the different vendors in terms of relative capacity, efficiency and pressure drop. You also must be prepared to assess any performance claims or recommendations from the vendors as well as suggested alternatives to the base case.
If your RFQ included a detailed base configuration, you probably have already checked that the requested device(s) will meet your process requirements. In this case, evaluation and comparison of the vendor base quotes is relatively straightforward. However, if the RFQ base configuration was unspecific or you are offered one or more alternative configurations, the first step in comparing the vendor quotes is verifying that the offered device(s) will meet your process needs.
Verify the device
When attempting to verify the suitability of an internal device in a particular distillation service, the best method by far is to have independent, unbiased tests of device capabilities. Such tests are available through organizations sponsored by industry such as Fractionation Research, Inc. (FRI) or academia such as the University of Texas’ Separations Research Program (SRP). Of course, your company must belong to an organization to be able to use its resources.
Lacking access to FRI or SRP, you still have some ways to check the applicability of proposed internals. However, these methods depend more heavily upon vendor representations and should not be considered independent assessments of vendor wares.
The first approach is to request actual plant data from a commercial application of the device. Keep a few critical points in mind: First, the tower service and conditions should as closely as possible match your proposed application and the tower configuration (number of trays or packed beds, tray or packing types and other internals items) should be provided. Second, get complete material balance information, including analyses of all feeds and products. Third, have the vendor supply unreconciled heat-balance information, including all redundant ways of measuring heat input and removal. You should plan on developing a simulation model of the tower to find the magnitude of the plant heat-balance error. This lets you determine the effect on the calculated device efficiency when the error is assigned alternatively to the overhead condenser/reflux and then the reboiler — that is, complete heat-balance data allow you to perform a sensitivity analysis.
Plant test data with complete heat-balance information often are very difficult to obtain. As a last resort, ask the vendor to share its in-house test results under a secrecy agreement. Usually these data are for air/water or air/isopar systems. (Isopar is a high-flash paraffin oil from ExxonMobil that has a surface tension of about 26 or 27 dyne/cm at ambient conditions, versus water’s 60 dyne/cm; air/isopar data suit hydrocarbon or other low surface tension applications.) Of course, air/water or air/isopar data only give capacity and pressure drop information, not separation efficiency. Exercise caution when interpreting vendor in-house data because results from only the most favorable runs might have been provided.
Also, talk to competing suppliers about known failures of a device. A rival vendor sometimes may offer such information to promote its own product. Once again however, you must be careful about the details because failures can occur for a number of causes not involving the device itself, such as improper design of feed locations or draws, or inadequate ancillaries such as condensers, reboilers or pumps.
For packing applications, the vendor should offer the most efficient packing that meets the capacity requirements. Structured packing is preferred for sub-atmospheric pressure applications due to its low pressure drop, but should be avoided in hydrocarbon services above 100 psia due to erratic height equivalent to a theoretical plate (HETP) performance known as the hump effect.
Other process requirements
After verifying that the device(s) offered satisfy your capacity, efficiency and pressure-drop criteria, compare the base offering with other requirements set forth in the RFQ to confirm that all important criteria have been met. Considerations may include:
• process flexibility (turndown);
• mechanical strength;
• material type and thickness; and
• fouling resistance.
For packing applications involving a gravity distributor, a distributor test plan and cost should be included.
If you have alternative proposals in addition to the base proposal, the above checks should show if an alternative is better than the base proposal in ways such as fouling resistance, capacity, efficiency and/or pressure drop. They also should identify the process advantages necessary to justify additional spending for costlier alternatives.
Scope of supply. This should be checked carefully, especially when comparing quotes from different vendors. Gaps generally reflect details that may not have been spelled out or were simply overlooked. Here are several commonly missed items:
• new internal piping from existing nozzles, where necessary;
• gaskets for all pipe flanges (and correct gasket materials);
• undesirable “plate flanges” in pressure-containing piping, such as spray headers;