Oct-2000

Improved reactor internals for HGO hydrotreaters

Reactor internals of a plant’s heavy gasoil hydroprocessing units, which became inadequate for the operations they were originally designed for, have been replaced with a system which has significantly improved all-round performance

F Emmett Bingham, Haldor Topsoe
Edwin Chan, Tony Mankowski and Peter Hubbard, Syncrude Canada

Article Summary

Continuous expansion of the capacity of Syncrude Canada’s heavy gasoil (HGO) hydroprocessing units at the Mildred Lake ugrader led to the current operation differing significantly from the original design. As a consequence, the existing reactor internals no longer provided adequate vapour/liquid distribution, quench mixing or redistribution. New reactor internals designs were evaluated and Topsoe’s Vapour-Lift distribution trays and Vortex mixing chambers were selected to improve the HGO reactor performance.

Plant 15-1 and Plant 15-2 were originally commissioned in 1978, to process a blend of coker gasoils from Tar Sands bitumen. Each of the plants has two parallel process trains: the reactor configuration in each train is comprised of a single-bed guard reactor and a three-bed main reactor.

The original reactor internals were bubble-cap liquid distribution trays and impingement-type, quench mixing chambers, similar to those described in US Patents Nos. 3218249 and 3502445 ( Ballard, et al , Union Oil of California). (Figure 1). The original thermometry in the main reactors included three vertical thermowells running the length of the reactors and horizontal thermowells traversing the top and bottom of each catalyst bed.

Since startup, Syncrude has successfully modified the units to increase the throughput of Plant 15-1/Plant 15-2 by approximately 35–40 per cent above the original 45200bpsd nameplate capacity. At the higher throughput, the impingement-type quench mixing chambers and bubble-cap distribution trays could no longer sustain adequate mixing and even flow distribution across the catalyst beds. The degradation in performance of the reactor internals resulted in wide radial temperature variations ranging from 7–10°C at the catalyst bed inlets to 15–40°C at the catalyst bed outlets.

The flow/temperature maldistribution is documented by temperature profiles plotted from the thermocouple measurements (Figure 2). The temperature readings for the vertical thermo-wells are designated TA, TB and TC, and the temperature readings for the horizontal thermowells are designated TX, TY and TZ.

The temperature profile shown for Plant 15-1B main reactor is representative of the temperature profiles observed in all the main reactors.

As seen, the temperatures measured by the vertical thermowells were significantly different from those measured by the horizontal thermowells. The radial temperature differences measured by the thermocouples in the vertical thermowells, especially for the middle catalyst bed in each of the main reactors, were quite large.

The temperature differences measured at the top of the catalyst beds indicated that the quench boxes were not mixing the liquid to an equilibrium temperature. Furthermore, the vapour-liquid mixing through the distribution trays was apparently inadequate to further quench the liquid to an equilibrium temperature before it was distributed over the following bed.

In addition to the poor apparent performance of the reactor internals, catalyst dump tubes and vertical thermowells, penetrating the quench zones, may also have been a source of potential bypassing of hot reactants around the quench section internals. Regardless of the cause, the temperature of the liquid redistributed across the lower catalyst beds was hotter in some regions relative to the others.

In addition to the obvious temperature maldistribution, the original redistribution trays were not providing an even liquid flow onto the catalyst beds. This might have been the result of poor levelness of the distribution trays or inadequate distributor coverage, especially adjacent to the reactor wall. The poor flow distribution is indicated by the widening of the radial temperature differences measured by thermocouples descending through the beds.

Large radial temperature differences are problematic because they are symptomatic of poor catalyst utilisation, which results in high average bed temperature, and rapid catalyst deactivation, all of which reduce the operating cycle between turnarounds. High peak temperatures adjacent to the reactor wall lead to premature unit shutdown when the temperature approaches the reactor design limit. Furthermore, because of the relatively poor thermocouple coverage, there is real concern that the existing temperature indicators might not represent the actual hot spot in the catalyst bed, and that damage to the reactor wall could occur unknowingly. Because of this problem, the company decided it was necessary to replace the reactor internals in the main reactors, prior to attempting any further increase in the unit throughput.

Criteria and evaluation
Enquiries were sent to several licensors and engineering contractors, providing them with sets of temperature data measured by the reactor bed thermocouples. Based on an evaluation of this data, the respondents were requested to submit recommendations for improving the flow distribution and corresponding temperature profiles through the main reactors. Syncrude employed the following criteria in its evaluation of the new reactor internals proposals:
Process criteria
- Quench section should thermally homogenise reactants by: intra-phase liquid-liquid mixing and vapour-vapour mixing, and by inter-phase vapour-liquid mixing
- Pressure drop should be minimised
- Performance should be stable over a wide operating range
Mechanical criteria
- Vertical height of tray elements should be minimised (reduce catalyst loss)
- Tray weight should be minimised (ie cost)
- Welding should be avoided in the reactor shell
- Trays should be suitable for revamp applications
- Accessibility for inspection and catalyst changeout should be easy
- Assembly in shop or field should be easy
- Trays should have adequate sealing to prevent bypassing.