Tower rating assessment

Tower simulation is only mimicking the current operation and can provide the vapor and liquid loadings as the basis for rating assessment while rating assessment will teU 

Tower rating is applied to assess the effects for changing process conditions on tower performance. It can also be applied when an existing tower is considered to be used for a new service. Briefly, a tower rating can answer three questions  

The criteria can be established as necessary and sufficient conditions for the suitability of an existing tower for changing conditions or new services  

When these two conditions are fulfilled, an existing tower is suitable for different conditions for which it is rated. When the process conditions undergo significant changes, the rating assessment should be performed to make sure the tower can perform the task satisfactorily imder new conditions. Otherwise, either operating conditions should be altered or modifications to the existing tower need to be determined. 

The rating assessment software will indicate the current operating point in relation to the operating window and thus reveal what operating limits the tower is encountered. Performance diagrams, if plotted with vapor and liquid volume loadings, can represent tray performance independent of operating pressure and composition. 

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A benchmarking efficiency for a tower should be established. By eomparing the actual efficiency with the benchmark efficiency, we can obtain a trend of efficiency over time and see the sign of poor separation. A good efficiency indicates a healthy operation of the tower in general, while a poor effieiency identifies signs of unstable operation, which warrants a tower rating assessment to reveal root causes of abnormality and thus determine actions for corrections. 

A drop tower has been shown to be capable of performing high rate SENB tests and the associated tests such as elastic modulus and contact stiffness. Fracture toughness was assessed... 

Corrosion rates underground have been measured using the Stern-Geary linear polarization method (Section 5.8) as well as weight loss [4]. The former method has been useful, for example, in assessing the corrosion rates of footings of galvanized-steel towers used to support power flues [5]. 

Pump-Around Many fractionation towers have pump-arounds to remove excess heat in the key sections of the tower. The effect of increasing pump-around rate is reduced internal reflux rate in the trays above the pump-around, but increased internal reflux rate below the pump-around. Thus, change in pumparound duty affects fractionation. On the other hand, pump-around rates and return temperature have effects on heat recovery via the heat exchanger network. It is not straightforward in optimizing pump-around duties and temperamres since the effects on both fractionation and heat recovery can only be assessed in a simulation model. An APC application incorporated with process simulation should be able to handle this optimization. 

The key role of equipment rating analysis is to assess equipment performance and identify equipment spare capacity and limitations. Utilization of spare capacity can allow capacity expansion up to 10-20% in general and accommodate improvement projects with low capital cost. When equipment reaches hard limitations, for example, a fractionation tower reaches the jet flood limit, or a compressor at the flow rate limit, or a furnace at the heat flux limit, it could be expensive to replace them or install new ones. The important part of a feasibility study is to find ways to overcome these constraints, which is accomplished in the succeeding steps. 

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