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Feb-2017

Balanced distillation equipment design

Fouling resistance and efficiency requirements for distillation equipment are balanced and optimised for reliable unit performance.

Soun Ho Lee
GTC Technology
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Article Summary
Fouling tendency is a critical issue in crude distillation units and should not be overlooked when designing crude distillation columns. Corrosion tendency can influence fouling issues as well. Since fouling resistance has an inverse relationship to efficiency in distillation equipment design, optimising equipment design between fouling resistance and efficiency requirements must be precise. Poor application know-how as well as poor equipment design often downgrade column performance and reduce unit run length.

This article will discuss common fouling issues associated with crude distillation column design. Actual retrofits for crude atmospheric columns are demonstrated through two case studies. These studies examine how fouling resistance and efficiency requirements for distillation equipment are balanced and optimised through careful evaluation and design methodologies.

Case study 1: crude distillation unit description and backgroundThe configuration of the crude distillation unit in this case is illustrated in Figure 1. Fractionated light and heavy kerosene streams through the crude atmospheric column and the side strippers are combined and rundown as a single kerosene intermediate product stream. A diesel intermediate product stream is also formed from a combination of light diesel and heavy diesel streams. These crude atmospheric columns and side strippers were originally designed with conventional movable valve trays, traditionally selected in the past. The exception was the wash section which was arranged with structured packing. Three pumparound circuits are arranged at the heavy kerosene, light diesel and heavy diesel range material locations. The naphtha/kerosene fractionation section is positioned as the crude atmospheric column top section. This column was designed without a top pumparound circuit in order to maximise fractionation between unstabilised naphtha and kerosene at a given column height.1

This crude distillation unit faced two problems: fouling and corrosion of the distillation equipment in the unit were found during a turnaround inspection. Valve perforation hole wearing and corrosion were found in the trays for naphtha/kerosene fractionation. Tray fouling was also identified in the trays for the light kerosene/heavy kerosene and heavy kerosene/light diesel fractionation sections. Kerosene or diesel intermediate product yield limitation was also experienced when kerosene or diesel boiling range material composition was increased in the charged crude slate. Charge crude compositions were frequently varied during operation.

Case study 1: root cause identification
Figure 2 shows that the naphtha/kerosene fractionation trays suffered from valve/perforation hole wearing and corrosion. Some movable valve units dislodged from the tray deck. Perforation hole sizes on the tray deck were increased by wearing and corrosion actions.2 Low column top temperature required for target operation could accelerate hydrochloric acid corrosion and valve/perforation hole wearing. Significant fractionation efficiency loss between naphtha and kerosene was not recognised during the operation. The bulky fractionation nature of crude distillation service might result in fractionation efficiency being insensitive to tray weeping. However, if this valve/perforation hole wearing progresses, significant fractionation efficiency loss will be noticed through substantial weeping.

Fouled trays located for the light kerosene/heavy kerosene and heavy kerosene/light diesel fractionation sections are shown in Figure 3. A tar-like substance was discovered around the periphery of the valve legs. Phosphates used for crude oil production were suspected as the root cause. Boiled phosphates may react with kerosene boiling range material and make fouling deposits.

A dedicated process evaluation for kerosene or diesel yield limitation was conducted. The original column and tray drawing revealed that intermediate side product and pumparound streams were withdrawn from fractionating trays directly. The originally designed side draw configuration is illustrated in Figure 4. Flow from the crude atmospheric column to the side stripper relies on gravity flow. If the liquid head formed on the collector tray is not high enough to overcome total friction losses from the crude atmospheric column to the side stripper, the flow rate can be limited. Moreover, frothy liquid withdrawn from the fractionating tray’s active area can contain vapour. The presence of vapour can limit this gravity flow. Rigorous pipe line hydraulic evaluation revealed that the gravity line hydraulics could be limited at a maximum target draw rate.3

Case study 1: equipment modification
Based on the aforementioned process evaluation and root cause analysis, the original movable valve trays were replaced by fixed valve trays. This tray type conversion improved equipment resistance against fouling and valve/perforation hole wearing.

The original fractionating trays at draw locations were converted to chimney trays to increase the liquid head for gravity flow. This chimney tray conversion also eliminates the chance of yield loss and start-up trouble through fixed valve tray implementation and increases draw liquid residence time for vapour disengagement from liquid. However, this conversion resulted in losing one tray for each fractionation section: light kerosene/heavy kerosene, heavy kerosene/light diesel and light diesel/heavy diesel fractionation. GT-Optim high performance trays with various performance-enhancing features and fixed valves were implemented for the rest of the fractionating trays.

As described earlier, light and heavy kerosene streams are combined and rundown as a single kerosene intermediate product stream. Therefore, fractionation performance between light kerosene and heavy kerosene streams is not critical. The same rundown configuration of light and heavy diesel streams does not necessitate sharp fractionation between the two streams. However, fractionation performance between heavy kerosene and light diesel streams affects rundown kerosene and diesel intermediate product qualities including the kerosene freezing point, one of the key specifications for kerosene rundown. To predict kerosene freezing point change, dedicated sensitivity analysis was conducted.

The aforementioned high performance tray implementation could improve individual tray efficiency. Nevertheless, extra individual tray efficiency improvement was not counted to predict the retrofit heavy kerosene/light diesel fractionation performance.

Case study 1: sensitivity analysis
For reliable sensitivity analysis, simulation modelling was first validated with pertinent unit test run conditions. Simulated kerosene freezing point value was reasonably matched to actual value. The tray efficiency and internal vapour/liquid traffic profile for each fractionation section were quantified through model validation. A constructed kerosene freezing point sensitivity curve per varied theoretical stages is plotted in Figure 5. This curve predicted that the freezing point could be increased by 0.1°F by using a chimney tray conversion scenario. Figure 6 also shows another kerosene freezing point sensitivity curve per heavy kerosene/light diesel fractionation section internal reflux L/V (liquid/vapour) ratio. A freezing point increment of 0.1°F was predicted at a 3% lower heavy kerosene/light diesel fractionation section internal reflux L/V ratio. Undetected kerosene freezing point changes were anticipated through the sensitivity analysis.
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