Co-processing renewable feeds in hydrodesulphurisation units: Part 2

Part 2 of the study assesses the behaviour of diesel hydrotreaters when incorporating different biogenic feedstocks and rates, with a minimum target of 10 wt%.

Cristian S Spica
OLI Systems

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Article Summary

The first phase evaluation outlined in Part 1 of this article, published in PTQ Q2 2024, discussed the success of a project evaluating biogenic feedstock monetisation benefits from ionic modelling’s contribution to designing crucial modifications in hydrodesulphurisation (HDS) process units, such as diesel hydrotreaters (DHTs).

The second phase of the project discussed herein assesses unit behaviour when incorporating different biogenic feedstocks and rates, with a minimum target of 10 wt%. The feasibility study results revealed that achieving the targeted minimum biofeedstock incorporation rate of 10% was not feasible with the current DHT configuration, necessitating significant capital expenditure (Capex) for implementation. In the interest of streamlining the narrative, only the case involving palm oil mill effluent (POME) will be discussed, widely recognised as the most challenging biofeedstock due to its high concentration of contaminants (see Table 1).

The findings from the ionic survey indicate that with a 50% POME incorporation, a NH₄Cl β-solid phase forms at 270⁰C in HEX 06, remaining stable in HEX 07 and HEX 08 before transitioning to the NH₄Cl α-solid phase in HEX 09 and HEX 10 with calculated relative humidity (RH) ranges from 20-50%, necessitating significant modifications to the unit. This includes incorporating two additional continuous wash water injection points, injecting a large volume of water due to the higher temperatures in the streams, installing intermediate separators for water separation, increasing the size of piping, and upgrading the metallurgy for the HEX tube bundles.

Furthermore, increased CO₂ generation raises the risk of carbon steel alkaline stress corrosion cracking (ASCC) due to the elevated carbonate ion concentration at high pH (see Figure 1), predicating a metallurgical upgrade (carbon steel to SS 316L or high-chrome alloys).

Moreover, the rise in CO, CH₄, and C₃H₈ production led to a decline in make-up H₂ purity recycled back to the reactor feed, as well as an increase in the recycled gas rate, which poses a constraint on the compression stage. Additionally, the augmented load of light ends created a bottleneck in the stripper overhead section.

It, therefore, became apparent that incorporating a 50% POME rate was not feasible with the current plant configuration, predicating significant Capex for its implementation. Consequently, two additional scenarios have been investigated with POME incorporation rates of 25% and 10%.

The results of the ionic survey indicated that with 10% POME co-processing, a NH₄Cl β-solid phase formed at 230⁰C in the outlet of HEX 07, remaining stable in HEX 08 before transitioning to the NH4Cl α-solid phase in HEX 09 and HEX 10. Consequently, the number of HEX units impacted by NH₄Cl salt deposition was limited to HEX 08, 09, and 10, like the scenario with 100% fossil feed in the base case. Additionally, the overall calculated RH was within the Iow set for this unit (that is 15%, resulting in more fouling than corrosion issues).

Restoring HEX efficiency
As previously mentioned, the formation of dry salt deposits leads to exchanger fouling. To restore exchanger efficiency, intermittent water washing is necessary to dissolve these salts. However, the resulting acidic water containing chlorides is incompatible with carbon steel and stainless steel exchangers and piping. Therefore, dry chloride salt deposition temperature should be kept in the temperature range of the HEX 09 outlet, where an intermittent water wash system is currently available, by limiting the contaminants load (such as organic and inorganic chlorides by means of a pretreatment system or a quality control system of the feedstock).

Alternatively, two new intermittent wash water injection facilities should be installed upstream of HEX 06 and HEX 07, providing that the co-processing is intermittently operated. The material of construction (MoC) for piping and tube bundles should be upgraded from carbon steel or stainless steel to a corrosion-resistant alloy (CRA) to withstand chloride water corrosion during intermittent washing. Additionally, intermittent water injection should be as close to the continuous water injection point as possible to minimise the impacts of the intermittent washing procedure on unit operation. Furthermore, the issues of ASCC and CO₂ build-up in the amine absorber, as well as the increase in light ends, continue to be a concern, necessitating an upgrade of the current metallurgy.

General corrosion risk
A corrosion risk assessment was conducted using OLI Studio’s proprietary Stream & Corrosion Analyser to evaluate the suitability of MoC with the new configuration for the intermittent and continuous new wash water injection facilities to be installed upstream of HEX 06 and HEX 07.

The Corrosion Analyser, a first principles corrosion prediction tool, was used to perform the corrosion analysis. It is used to predict the rate of general corrosion, the propensity of alloy to undergo localised corrosion (pitting), depletion profiles of heat-treated alloys, and thermodynamic stability of metals and alloys.
Corrosion is calculated by quantifying the bulk chemistry, transport phenomena, and surface reactions through a thermophysical and electrochemical module. Figures 2-4 show the results of the corrosion risk assessment carried out for the 10% POME incorporation rate case: HEX 06 to HEX 07 uniform corrosion risk assessment for carbon steel (see Figure 2), stainless steel SS316L (see Figure 3), and Hastelloy C-276 (see Figure 4).

It is evident that the corrosion rate exceeds the acceptable limits for long-term continuous service for carbon steel material. Therefore, chloride stress corrosion cracking (CSCC) becomes the predominant damage mechanism. As a result, the MoC for tube bundles and piping systems should be upgraded to SS316L or Hastelloy C-276.

Localised corrosion risk
Using the proprietary Corrosion Analyser, it is also possible to calculate the localised corrosion risk of alloys by determining the corrosion and re-passivation potential:
· Corrosion potential (Ecorr): Electric potential where anodic reaction rates = cathodic reaction rates. This results in no net current flowing into or out of the electrode. This potential determines the likelihood of metal corrosion in the specified environment. Consider this potential as the inherent tendency of a material to corrode.
· Repassivation potential (Erp): Also called the protection potential, the Erp is the potential at which a stably growing pit or crevice corrosion will cease to grow. At potentials below Erp, localised corrosion is unable to progress. It can be considered the threshold needed for a pit or crevice to cease its corrosion activity.
The wider the Ecorr – Erp difference, the greater the propensity for localised corrosion. This delta is referred to as the Maximum Pit Current Density and is measured in A/m², where A is amperes and m is metres. The general rule of thumb is that 1 A/m² corresponds to the potential for a pit of 1 mm of depth, as the HEX 06 to HEX 07 localised corrosion risk assessment shown in Figure 5 for carbon steel, stainless steel (see Figure 6, SS316L), and Hastelloy (see Figure 7, C-276).

It is evident that the localised corrosion risks mostly affect carbon steel. By using Ni-Cr-Mo-containing alloys, the pitting resistance is dramatically enhanced. Nevertheless, upgrading to austenitic SS316L does not always provide sufficient protection against pitting corrosion. Hence, if an increased share of biogenic feedstock is processed or for intermittent water washing, it is suggested that the piping systems and HEX tube bundles be upgraded to Hastelloy C-276. Table 2 is the complete set of data for all streams at risk of corrosion.

As a result of the study, it appears that co-processing of 10% non-pretreated bio-feeds would not be possible. The two main issues the unit may face are delta P in the top of reactors and corrosion related to chloride content. The most problematic issue would be corrosion, as the unit is already experiencing bottlenecks in chloride salt deposit, wash water facilities, air cooling capacity, and separation in the cold HP separator during current operation (100% fossil). Therefore, any increase in chloride intake does not seem feasible.

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