PTQ Q3 2024 Issue

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

T he 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 bio - genic 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 narra - tive, only the case involving palm oil mill effluent (POME) will be discussed, widely recognised as the most challeng - ing biofeedstock due to its high concentration of contami - nants (see Table 1 ). The findings from the ionic survey indicate that with a 50% POME incorporation, a NH₄Cl b -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 contin - uous 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 imple - mentation. 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 b -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 chlo - rides is incompatible with carbon steel and stainless steel exchangers and piping. Therefore, dry chloride salt deposi - tion 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

Biogenic feedstock contaminants

UCO 500 500

Animal fats

POME 1,500

Nitrogen-based, ppmwt Sulphur-based, ppmwt Chlorides, ppmwt Oxygenated, wt% Phosphorus, ppmwt

1,000

800

50/150 *

25/50 *

25/100 *

5 ** 50

5 **

10 ** 150

350

Metals, ppmwt

150 ***

750 ***

1,000 ***

FFA, wt%

5 4 1 5 1

Hydrogen, wt% Methane, wt% Propane, wt%

4 1 5 1

Carbon monoxide, wt% Carbon dioxide, wt%

4 4 * Both organic and inorganic chlorides. ** Both water and peroxides. *** Including Ni, Va, Na and Fe. 4

Table 1

PTQ Q3 2024