Our cheaper crude intake has relatively high chloride levels at the cost of increased preheat exchanger fouling. What measures can we take to minimise the problem? (PTQ Q&A)
Responses to a question in the Q3 2020 issue of PTQ
Various from G. W. Aru, BASF, SUEZ Water Technologies & Solutions, NALCO Water and Chimec.
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Tom Ventham, Sales & Technical, Europe and Africa Unicat BV/G. W. Aru, LLC - firstname.lastname@example.org
CJ Farley, Senior Technical Services Engineer, G. W. Aru, LLC - email@example.com
Natalie Herring, Director of Technology and Business Development, G. W. Aru, LLC - firstname.lastname@example.org
A further problem caused by high levels of chloride in crude can be witnessed in the FCC unit. Chlorides are often found in heavy portions of crude, meaning they end up in the FCC unit, either in residue or VGO cuts. In the FCC, chlorides can affect product yields and are highly deleterious in both of the process outlets. When chlorides enter the FCC, it can be observed that hydrogen yield increases and coke selectivity deteriorates. Several documented cases point to the issue of chloride increasing the activity of contaminant nickel in the FCC unit when such effects are observed without a subsequent increase in metals loading. Routine analysis is advised in the event this phenomenon is seen, but as chlorides often enter the FCC in brief slugs, reactivating the nickel present, it is rare to be able to capture these effects within the normal sampling schedule.
Focusing on the hydrocarbon outlet, where chlorides leave with cracked products, serious problems can be found in the upper sections of the main fractionator and in the gas plant. In the main fractionator, chloride combines with ammonia, which is typically found in excess due to the cracking of amines in feed, to form ammonium chloride. Ammonium chloride salt deposition may occur under certain conditions as a function of partial pressure of NH3 and HCl and the dewpoint of water. Where NH4Cl deposits form on trays, increased pressure drop or tower flooding issues can occur. This situation is typically seen towards the top of the tower, especially when a side cut naphtha stream is taken, such as heavy cut naphtha (HCN), which results in a cooler tower top temperature of <120°C. A water wash in the reflux can help alleviate this situation as NH4Cl is highly soluble in water. However, if the material has already solidified, this is unlikely to help. Further, amine filmers, which are supplied by water treatment companies, can be used to add a protective layer on the trays, but this is more effective for clean trays as a preventative measure. Typically, mechanical removal of the deposits is required following a unit shutdown.
Additionally, and at higher tower top temperatures, corrosion problems can be seen further downstream, such as chloride stress corrosion cracking (SCC) in the wet gas compressor and interstage cooling system. A suitable water wash system is required to minimise corrosion in this vulnerable area.
On the regenerator side, chlorides also cause corrosion problems through SCC in flue gas ducting and other downstream systems. In the regenerator itself there are cases where combustion problems manifest as a result of high chlorides, leading to rapid and severe CO excursions, with the associated environmental compliance and afterburn implications.
Good desalting is essential to remove as much chloride as possible from the eventual FCC feed components. If an exhaustive investigation of typical sources of chloride has not explained an increase, FCC catalyst should also be considered. It is known that FCC fresh catalyst can release chlorides when injected to the FCC1 due to incomplete calcination of the FCC catalyst before leaving the supplier’s factory. These effects are typically observed on the regenerator/flue gas side where catalyst is exposed to high temperatures. Retention of fresh catalyst samples for retrospective testing is advised when a history of chloride corrosion has been experienced.
1 Salt deposition in FCC gas concentration units, Michel Melin, Colin Baillie and Gordon McElhiney, Grace Davison Refining Technologies Europe, PTQ Q4 2009.
Melissa Clough Mastry, Technology Manager EMEA, BASF, email@example.com
Another point to consider is if the chlorides are making their way past crude processing, there is a chance for chlorides to also enter the FCC if your refinery is configured with one. Once chlorides enter an FCC, they may cause problems downstream, including fouling of the main column overhead system (NH4Cl deposition) or even within the FCC by reactivating old contaminant nickel (leading to higher hydrogen and coke). The preferred option to minimise the problem is to avoid all sources of chlorides – so optimising the crude desalting processing or using an FCC catalyst that has zero chloride content. (It is known that if present in the fresh catalyst, not all the chlorides will leave from the regenerator stack, but some will entrain to the riser side.)
Xiomara Price, Senior Product Analytics/Support Manager, SUEZ – Water Technologies & Solutions, firstname.lastname@example.org
Salt and solids fouling can occur either in the cold preheat exchangers before the desalter or the hot preheat exchangers after the desalter. This type of cold preheat fouling can be typically addressed by adding a portion of the desalter water wash to the front end of the preheat exchangers.
The desalter is the primary defence against salt and solids fouling in the hot preheat exchangers. Desalter performance optimisation is essential to mitigate the fouling. It can be achieved by adjusting any number of variables depending on your specific system design, operation, and limitations. Some of those variables include percent wash water, mix valve pressure drop, water wash quality, desalter temperature, slop addition, chemical addition, and desalter level.
Your service provider should be able to provide you with specific guidance on what variables are negatively impacting your system, and how to best improve them. If for some reason the desalter operation cannot be further optimised due to design or operational restrictions, an effective dispersant can help reduce fouling until the design or operational problem can be resolved.
Chris Claesen, Director, Technical Consulting, NALCO Water, email@example.com
If you have not already done so, I suggest doing a root cause investigation. Find the exact location of the fouling: is it in the cold or hot preheat and which specific exchangers are fouling? Running a rigorous preheat monitor model such as Monitor can help with this. If possible, try to get a deposit sample and have it analysed, and the fouling mechanism determined or confirmed. Also perform the necessary analyses to determine if the increased chloride levels are due to inorganic chlorides (salt) or organic chlorides.
Check with the company that treats your desalters and overhead if they have processing experience with the specific crude and if they have seen any problems associated with it. Having access to a database with processing experience such as the Nalco CrudeFlex database that points out potential problems linked to a crude can certainly help with the investigation.
Look at the crude stability and blending stability; the crude may already be self-unstable or blending with other crudes can cause instability of asphaltenes which can lead to increased preheat fouling.
Check if the increased chloride content is linked with an increase in other contaminants such as increased water and solids content. If these have increased above your normal values, a crude tank settling programme using crude tank demulsifiers can help reduce them.
Verify how the desalter is responding to the increased chloride levels. Have the chloride levels (salt levels) of the desalted crude increased? Refiners often respond to increased desalted crude chloride levels by increasing the caustic dosage after the desalter and this may lead to increased fouling in the hot preheat exchangers. Caustic usage should be minimised by optimising desalter performance, for example by increasing washwater to the desalters and optimising washwater distribution over the cold preheat and upstream the mix valve, and optimising mix valve pressure drop.
If not controlled properly, the increased desalted crude chloride content may lead to higher levels of hydrochloric acid going to the overhead and an increased overhead salt formation potential; deposited salt decreases heat exchange in the overhead/cold crude heat exchangers. If this happens, it would normally also be noticed by an increased pressure drop over these exchangers on the overhead side and an increased tower top pressure.
If the preheat exchanger fouling cannot be controlled by changes in pretreatment, blending, or desalter operation, an antifoulant programme may help reduce the fouling. To select a suited antifoulant, the fouling mechanism must have been confirmed, as suggested earlier.
Marco Roncato, Senior Product Manager Process Development & Marketing, Chimec, firstname.lastname@example.org
Refinery crude feeds contain water and inorganic salts such as sodium, magnesium, and calcium chloride. In the case of cheap crudes, the water content – hence the salts content – can be higher.
The first measure in order to manage this higher content is obviously to maximise desalting efficiency. But after this first step, despite good desalting efficiency, some inorganic chlorides remain in the desalted crude and it is well known that they can hydrolyse, generating HCl. In order to minimise this phenomenon, a common practice is to inject NaOH into the desalted crude. Caustic injection downstream of the desalter is recognised as a cheap, effective method to reduce overhead corrosion. Unfortunately, at the same time, NaOH injection into the desalted crude can be detrimental for the following reasons:
• Concentrated caustic solutions can cause general corrosion of carbon steel equipment at these temperatures. Additionally, caustic can cause caustic stress corrosion cracking (CSCC or caustic embrittlement) of non-post weld heat treated carbon steel and of austenitic alloys including stainless steels and nickel alloys such as Alloy 825 (UNS N08825).
• If allowed to precipitate (usually in the hottest CDU heat exchangers, where water flash occurs), caustic/salt reaction products (calcium and magnesium hydroxide) and unreacted caustic can cause plugging of heat exchanger tubes (inorganic fouling). This leads to losses of heat exchanger efficiency (lower energy saving and higher maintenance costs) and increases the ΔP of the equipment.
• NaOH can contaminate the bottom streams affecting downstream units:
ν Catalyst poisoning in the downstream catalytic plants – the FCC unit, hydrocracking unit, residue desulphurisation and so on
ν Increased coking rate in the downstream unit furnaces; for instance, the vacuum and visbreaker units
ν Low quality produced fuel oil (fouling problems in the burners, for instance in a power station or in the fuel oil furnaces)
In order to manage these issues, Chimec has developed Chimec 3034 – Caustic Replacer to substitute completely or partially the injection of NaOH downstream the desalter; the overall effect is the reduction of the sodium content in the atmospheric residue.
• Lower catalyst poisoning (hence deactivation) in the downstream unit
• Lower coking rate catalysed by Na in the downstream furnaces
• Higher fuel oil quality
• Lower Cl level in the overhead systems, thus lower corrosion rate
• Lower risk of Na embrittlement
• No risk of NaOH induced fouling in the hot preheat train
Chimec 3034 is an oil soluble blend of high molecular weight polyamines able to neutralise free chlorides in the desalted crude oil, thus forming salts that show higher thermal stability compared with the inorganic chlorides, especially magnesium and calcium that hydrolyse at the hot train preheat exchangers and furnace outlet temperatures. This would increase the HCl that distils at the top of the pre-flash and main fractionator columns in CDUs.
Furthermore Chimec 3034, compared with most commercially available caustic replacers based on other lighter amines and diamines, ethylenediamine, diethylenetriamine, diethanolamine, triethanolamine, triethylenetetramine, offers the following advantages:
• Its oil based formulation conveys the active compounds in the crude oil matrix in a more efficient way
• The higher boiling point range of the active components decreases vaporisation, thus preventing salts formation on the columns’ trays
• The higher thermal stability of the salts produced by the reaction with hydrolysed chlorides ensures a reduction of chlorides ending up in the overhead system of the CDU preflash column and main fractionator, in turn decreasing the corrosion rate
• It is completely organic and metal free, i.e. no impact on the coke promotion and on the catalyst deactivation)
• It has a fast reaction
Chimec 3034 has to be injected downstream of the desalter. If NaOH will be only partially substituted, it must be injected in a separate line with respect to the NaOH, being in a different solvent.
Chimec 3034 dosage strongly depends on the quantity of salts in the desalted crude. Approximately 1 ppm of NaOH (considered as pure NaOH) can be replaced by 7 ppm of Chimec 3034.
To confirm the results of applying Caustic Replacer, the dosage of Chimec 3034 should be kept constant for 12-24 hours and analyses should be repeated on:
• The desalter inlet and outlet crude
• Overhead tail water
Optimisation of the dosage can be performed as outlined in Figure 1.
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