Can you suggest ways to avoid or deal with asphaltene precipitation in our desalter?Nov-2021
Paul Fearnside, Nalco Champion, firstname.lastname@example.org
Switching to a desalter demulsifier that also acts as a mild asphaltene dispersant is another way to solve your situation. Typically for no additional chemical cost.
Eric Vetters, ProCorr Consulting Services, email@example.com
The best way to avoid asphaltene problems in the desalter is to process a stable crude slate. There are different ways to predict the stability of a crude blend. What is commonly referred to as the Wiehe method, or one of its variations, is most the most commonly used method. Solubility and insolubility blend numbers for individual crudes can be measured and used to predict the stability of a crude blend. Keeping the crude slate stability above a minimum threshold will significantly reduce the chances of having problems in the desalter.
How crudes are blended together is also important. If a heavy crude is batch blended into a light crude, the initial small amount of heavy crude may be destabilised by the large volume of light crude present. Even if the ultimate blend is stable, the transient conditions can lead to asphaltene precipitation. Once destabilised, precipitated asphaltenes are very difficult to resolubilise in the crude.
If getting a sufficiently stable crude blend is not feasible, asphaltene stabilisers can be added to the crude in addition to emulsion breaker to help mitigate any problems in the desalter. Sometimes asphaltenes can be pH sensitive, so if there are consistent problems with particular crudes it might be worthwhile to have the site’s chemical supplier do a desalting study to see if changing the desalter wash water pH might improve desalter operation.
Jari Marci, Petrogenium, firstname.lastname@example.org
Asphaltene precipitation may be related to a number of different mechanisms. One reason could be incompatible crudes. Crude incompatibility may be tested by methods such as the ASTM D4740 (spot test). The solution to this problem lies in shift of the crude oil blend formulation to a stable region. Another mechanism may be the interaction of asphaltenes with calcium naphthenic acid soaps. Naphthenic acids will be captured by the total acid number or TAN (ASTM D 3444), which is commonly reported in crude oil assays. Here again, modification of the crude oil diet will eliminate the issue. If avoiding naphthenic acids crudes is not an option, the problem may be mitigated by reducing the pH of the wash water as, for instance, by dosage of HCl. Reducing the pH of the wash water shifts the soap back to its naphthenic acid form and release of the calcium ion.
Marcello Ferrara, ITW Technologies, email@example.com
Asphaltenes are highly polar components with high molecular weights that are found in crude oil as a stable dispersed system. Asphaltenes are made up of molecules containing paraffinic, naphthenic or aromatic functional groups and polar groups formed by oxygen, nitrogen or sulphur as functional groups or incorporated into the molecule as heteroatoms. Many factors may alter the stability of this dispersed system, including chemical and thermal destabilisation. This is particularly true in the desalter where different processed crudes (with different compositions and compatibilities upon blending) and increased temperature, together with higher residence time, create the conditions for asphaltene destabilisation and precipitation. The result of this destabilisation is the formation of a solid residue called sludge that will impact desalter performance and reduce plant capacity in the refining processes, or impair oil production at the wells.
Additionally, a stable water-in-crude oil emulsion is formed as a result of asphaltene adsorption at the surface of aqueous droplets, leading to stable emulsion which impairs not only desalting operations but the entire production unit and energy efficiency.
Most important, sludge formation inside the desalter has three major negative effects during operations, namely: a) reduction of the effective desalter volume and hence reduction of water and sediment settling time, with related poor desalter performance; b) sludge/oil carry-under in the brine and hence fouling of water preheaters and, most important, of the water effluent treatment system (including, when applicable, de-oiling packings and strippers); c) sludge carry-over in the crude, with related fouling at the preheat.
ITW has developed different patented technologies to address the above problems. Specific patented asphaltenes stabilisers can be added together with the crude to prevent asphaltene precipitation and reduce sludge formation.
The cost of dealing with asphaltene precipitation by continuously injecting asphaltene stabilisers may be prohibitive if the target is the elimination of sludge formation. It is therefore taken for granted that some sludge accumulation will occur inside the desalter (either by adding asphaltene stabilisers or, worse, by not adding any chemical).
In order to deal with the existing sludge, ITW has developed and patented two other technologies, namely ITW Online Cleaning and ITW Onstream Cleaning.
ITW Online Cleaning will clean the desalter in 24 hours on a feed-out/feed-in basis, by using the existing unit layout and circulating a proprietary ITW chemical. ITW Onstream Cleaning will clean the desalter while the unit is running, but plant modifications need to be implemented under ITW licence.
In both cases, multiple desalters can be cleaned at the same time. ITW patented chemicals will dissolve the precipitated asphaltenes, which will be chemically re-peptised, in other words stabilised.
This important characteristic allows for subsequent reutilisation/reprocessing of the washing solution, because the stabilising properties of the chemical avoid reaggregation of asphaltenic clusters, eliminating potential precipitation during storage and/or fouling of process equipment.
Berthold Otzisk, Kurita Europe, firstname.lastname@example.org
Asphaltenes are very complex molecules. Generally, tiny colloidal particles are suspended in crude oil and the molecules are stabilised by natural resins. Removal of small colloidal particles or resins directly results in formation of an agglomeration, followed by precipitation. Asphaltene deposition can have several causes. The composition of crude oil blends, changing temperatures, electrostatic effects, pressure changes, and flow regime have an influence.
As a quick measure, you can try to adjust the composition of the crude blend so that asphaltenes in the desalter show no tendency to precipitate. The crude oil stability can be easily determined with a suitable measuring instrument (for instance, Turbiscan Stability Analyser for Crude oil and fuel oils, ASTM D7061) or by means of a hot filtration test in the laboratory.
Heavy organics such as waxes, paraffins, mercaptans, asphaltenes, or organometallic compounds may exist in the crude oil. Asphaltenes are highly polar compounds and can act as glue and mortar, hardening deposits. Small asphaltene particles can be dissolved, while large molecules may flocculate by forming steric colloids. Flocculation is known to be irreversible, resulting in deposition and fouling. It is better to avoid destabilisation of asphaltenes. Kurita´s asphaltene dispersants are substitutes for the natural resins. By surrounding the asphaltene molecules they stabilise the asphaltene solution and keep the hydrocarbons in a colloidal system and the asphaltenes in a disperse phase to prevent fouling and plugging.
Usually, asphaltene dispersants are dosed with low ppm treat rates into the crude oil after desalting. However, if asphaltenes already precipitate in the desalter, the asphaltene dispersants can also be dosed into the crude oil before desalting to avoid precipitation. At the same time, this measure can have a positive influence on the fouling behaviour of the desalted crude oil, resulting in extended run and improved operation conditions.