Sep-2003

Crude column revamp using radial temperature profiles

Designers need to be aware of all the factors influencing packed bed performance such as feed zone mixing problems

Glaucia Alves, Da Silva Torres and Silvia Waintraub, Petrobras
Edward L Hartman, Process Consulting Services

Article Summary

Tools for troubleshooting distillation columns include sophisticated techniques such as computation fluid dynamics (CFD) and gamma scans, as well as the more fundamental measurements of temperature and pressure. The temperature variation around the circumference of a distillation column is a powerful diagnostic tool, able to infer composition gradient at a given elevation.

Variations in composition and temperature are consequences of problems with internal liquid or vapour stream distribution, poor mixing of external and internal streams at feed locations, or inadequate circulating reflux (pumparound) stream distribution causing uneven cross-sectional area condensation in heat-removal zones. While none of these can be directly measured, they can be inferred from radial temperature measurements at the same elevation in a packed column. This technique was used to facilitate the revamp scope definition of a large atmospheric crude tower at a Petrobras refinery.

Measuring fractionation performance
Packed column composition and temperature gradients at the same elevation degrade fractionation between products and reduce product yield. Since refiners typically use the gap or overlap between the 95% and 5% distillation temperatures on heavy oil columns as a measure of fractionating performance between adjacent streams, this value varies as fractionation performance changes.

In one case, a refiner changed from trays in their FCC main column to structured packing, with the expectation that the gap between gasoline and LCO products would improve from 24 to 33°C. However, it actually decreased to 0°C because of poor initial liquid distribution into the packed bed. So the apparent fractionation with over 6m of 225m2/m3 surface area structured packing was less than one theoretical stage. In another FCC example, half the spray nozzles above a packed LCO pumparound zone plugged, causing the temperature leaving the bed to vary by over 75°C. Even though the liquid distribution quality was very good into the gasoline/ LCO fractionation bed, 5m of structured packing appeared to have only two theoretical stages. Vapour composition and temperature entering the bed were not uniform, so the fractionation suffered.

Packed column principles
Ideally, each cross-sectional area of a packed bed should have uniform liquid (L) and vapour (V) composition and rate entering it, otherwise parts of the bed will have different L/V ratios and compositions. L/V ratio, vapour composition entering the bed, and packed bed height drive fractionation. In one instance, poor distributor design caused parts of the bed to be nearly dry. Hence, the vapour composition entering the bottom of the bed and leaving was nearly identical in the cross-sections where the liquid rates were low. Poor fractionation, as measured by 95%/5% distillation temperature differences between adjacent products, resulted. 

Packed columns use liquid distributors, collector trays and vapour horns for the initial distribution of liquid and vapour streams, respectively. In addition, collector trays mix the liquid leaving a packed bed prior to redistributing so that composition and temperature gradients are not propagated throughout the column. Packing’s inherent distribution quality will create some liquid maldistribution, even if the initial vapour and liquid distribution to the bed is perfect. So some radial temperature and composition gradient always exist above and below a packed bed. However, the designers need to be aware of the all factors influencing packed bed performance and design the equipment to minimise the effects. Packing itself or typical collector trays will not correct poor initial liquid or vapour distribution, or composition gradient.

Temperature gradient — composition gradient
Figure 1 shows the measured temperatures in an atmospheric crude column after start-up. The revamp replaced standard trays with structured packing to increase capacity and improve distillate yields. During the design phase, two thermowells and TIs were installed 180 degrees apart above and below each packed bed to help monitor liquid or vapour distribution quality. When the revamp did not meet its design distillate yields, radial temperature measurements were essential to identify the root cause. In this case, vapour and liquid distribution quality were very good, yet the apparent fractionation and product yields did not meet the design basis expectations.

Feed zones must adequately mix all external and internal streams. Otherwise, composition gradients are generated. In this case, two feeds, cold flash drum and hot heater outlet streams enter the flash zone through two nozzles. Flashed crude overhead stream was 163°C, heater outlet stream was 374°C, and stripped hydrocarbon vapour was 357°C. The vapour temperature leaving the flash zone directly above the flash drum stream inlet nozzle was 28°C colder than the opposite side of the column. Furthermore, the same temperature pattern remained throughout the three packed beds. Neither the collector trays nor the structured packing corrected the temperature or composition gradient initiated in the flash zone. Although the cold flash drum overhead vapour’s influence on distillate yield was anticipated, the designers assumed flash drum vapour would mix with the heater outlet stream inside the vapour horn. 

During the design phase, the quenching effect of the cold flash drum stream on flash zone vapour rate was anticipated, although the magnitude was not. Flash drum vapour was routed to the flash zone because it occasionally was black prior to the revamp. Since the revamp increased the feed rate, the flash drum overhead vapour contained a significant amount of liquid carry-over due to foaming in the flash drum. Entrained liquid from the flash drum reduced AGO product yield by 2% on whole crude.  When the cold flash drum overhead liquid and vapour entered the flash zone, a significant portion of the vapour from the heater was condensed to provide the heat needed to raise the flash drum vapour and liquid temperature. 

Distillate product yield was low, because the flash drum foam layer was not contained inside the drum. Liquid and vapour from the flash drum and the charge heater outlet streams were not well mixed in the flash zone, therefore temperature and composition gradients were created across the cross-sectional area of the flash zone. These gradients were propagated through three packed beds. At each elevation, temperature indicators 180 degrees apart varied by approximately 28°C. Thus, the liquid and vapour compositions were not uniform. This example shows that packing and well-designed collector trays do not correct feed zone mixing problems. The designer needs to ensure that composition and temperature gradients are not created, because they are not easily corrected by packed column internals.