Are there options for online cleaning a preheat exchanger network?Jun-2021
Ramon uzcategui, Refidomsa, firstname.lastname@example.org
The use of washing with kerosene and diesel is one of the best practices that we has been applied in distillation units in order to remove asphaltenes and order particles as part of procedure in the preheat exchangers during the process of shutdown. This practice let us to recover the heat transfer in 80-85% without open the equipments.
Rob Turley, KBC, Rob.Turley@kbc.global
Yes. Chemicals or process fluids (such as LCO) can be used for on-line heat exchanger cleaning. Cleaning chemicals can be injected into the process fluid upstream of fouled exchangers at a specified dosage to dissolve and disperse foulants such as asphaltenes. These chemicals, however, can be harmful and so must be properly disposed of – which can prove costly.
Likewise, process fluids can be used to dissolve foulants – with paraffinic fluids being most effective at removing waxy components, whilst naphthenic/aromatic fluids are most effective at dissolving asphaltenes. Fouled exchangers are drained, before a suitable solvent oil is added and left to soak for a sufficient time to dissolve the fouling material. The solvent oil is then removed from the exchanger, and with it the dissolved material and any loosened solids deposits are reprocessed by conventional refinery operations.
An alternative option for online heat exchanger cleaning is back flushing. A back flushing valve can be installed which, when in use, reverses the direction of flow through a heat exchanger. This allows fouling material that has built up in the inlet channels of the exchanger to be flushed out through the same path that it entered.
Online cleaning can extend unit run lengths and reduce energy and maintenance costs. KBC’s HX-Monitor utility in Petro-SIM can be used to quantify the benefits associated with different cleaning methods and recommend when is the right time to clean fouled exchangers.
Berthold Otzisk, Kurita Europe, email@example.com
The options of online cleaning of preheat exchangers are often limited because larger product quantities are flowing through the heat exchangers. Because of high treat rates, online cleaning quickly becomes commercially uninteresting compared to mechanical cleaning, when individual exchangers can be separated.
Nevertheless, there are various cleaning methods and possibilities to clean entire heat exchanger networks during operation. Experience has shown that coke deposits or completely polymerised deposits cannot be removed, or usually only to a small extent. If organic salt deposits or corrosion products are the cause of fouling, these deposits can be removed quickly and easily using Kurita´s ACF technology. Water-soluble ACF salts are mobilised and transported with the product stream. Mobilised metals then remain in the residue.
If asphaltenes, maltenes or other tenacious hydrocarbon deposits are causing the fouling, these deposits can be removed with a cleaning product from the Kurita CD series or an asphaltene dispersant. The cleaning product dissolves hydrocarbon deposits. An asphaltene dispersant from the Kurita AP series transports smaller particles and prevents them from agglomerating into larger molecules to avoid precipitation.
Online cleaning requires much higher dosing rates in the range of 20 - 200 ppm or more compared to continuous antifoulant treatment.
Chris Claesen, Nalco Water, Chris.Claesen@ecolab.com
The individual exchangers can be cleaned with a chemical programme such as Nalco Water Enterfast, while bypassed on the fouled side, if they can be isolated and have access nozzles for the cleaning programme. The advantage of such cleaning is that it can be done in a short time span and without opening and pulling the exchanger. This will also reduce safety and environmental risks and will typically be a lot cheaper than mechanical cleaning. Using heat exchanger modelling software such as Monitor can help evaluate which exchanger is most fouled and what the energy gain will be if a particular exchanger is cleaned. This will also give the benefits in terms of CO2 emission reductions.
Steve DeLude, Becht, SDeLude@becht.com
There are a number of options either using chemical additives/solvents or mechanical means available that can be used to on-line clean preheat exchanger networks.
For chemical or solvent approaches, the most appropriate choice of cleaning option and its effectiveness is dependent on the nature of the fouling that has occurred in the exchangers and the ability to ensure flow through all the exchanger tubes. Your chemical suppliers should be involved and can provide any information from past turnarounds on the deposit chemistry/characteristics to determine the most appropriate on-line cleaning chemistry to apply. The chosen solution will often be a combination of proprietary chemical formulations customised by your chemical supplier for your specific situation.
Care should be taken to ensure that chemical and deposits removed from the exchanger network does not have an adverse impact on downstream equipment or catalyst.
If the fouling is caused by deposition of asphaltenes or other heavy hydrocarbons, then a highly aromatic stream or solvent additive can assist in redissolving and dispersing the heavy deposit. If the fouled exchanger is upstream of the desalter, it may be possible to remove some of the heavy deposit with the desalter mud. Temporarily bypassing or shutting down one side of parallel banks of exchangers can increase flows and temperatures to also help move very heavy deposits.
If the deposit is mainly corrosion products, these can sometimes be removed by acidic or other chemical reactants additives.
If the deposit is an inorganic salt, a water wash may temporarily improve the exchanger heat transfer but extreme care is required to not cause any materials damage due to rapid corrosion that can occur with wet salts if they are not completely removed. This can be especially true where the exchanger pressure drop is high and there is evidence of partially plugged tubes.
However, even with the correct chemistry, there will always be some risk that the on-line cleaning will not be fully effective due to an inability to get good flow through all the fouled areas of the exchanger on both the tube side and the shell side of the exchanger.
In addition to chemical cleaning methods, several mechanical cleaning methods can also be utilised often in conjunction with chemical cleaning methods. Such methods include, attaching ultrasonic transducers to the exchanger tubesheet. This method utilises ultrasonic transducers to generate vibration, acoustic streaming and cavitation to “shake loose” deposits from the exchanger surface.
Another option includes the use of internal vibrating systems to keep the inside of the exchanger tubes clean by installing a vibrating device or spring secured on both tube ends by a fixing wire. The energy of the fluid flow in the tubes causes these devices to vibrate creating a continuous online mechanical cleaning effect of the tube interior.
In cases where on-line cleaning is not completely successful, then improved techniques for off-line cleaning such as those applying ultra-sonic cleaning technology can be applied to regain most of the original heat transfer performance of the equipment.
Marcello Ferrara, ITW Technologies, firstname.lastname@example.org
One common cause of corrosion is the formation of heat stable salts (HSS). HSS cause an irreversible consumption of amine with subsequent loss in acid loading capacity and increasing the level of contaminants, causing corrosion in the amine unit and operating problems in the SRU.
Additional corrosion is due to HSS penetrating the protective FeS layer on the metal surface exposing the metal to further attack. Several HSS anion species have been directly related to increased corrosion in carbon steel in amine solvent systems; in particular they appear to act as corrosion accelerators by displacing the iron sulphide film.
The corrosion rates increase with higher concentrations of all of the HSS species. The corrosion from HSS occurs in hot areas of the plant, wherein the liquid and vapour phases are present: the reboiler, reboiler outlet line, the regenerator column between the lean amine level and the bottom trays. HSS start causing evident corrosion problems when the total HSS level exceeds 2%.
ITW has developed and patented technologies, together with proprietary know-how, to dissolve HSS and hence remove them from the equipment.
A pro-active approach has also been developed for preventing HSS formation.
ITW technologies’ applications range from normal run to turnaround.
Equipment can be cleaned online without having to open the same. This is particularly beneficial in severe applications such as heat exchangers placed at elevated location or columns/vessels.
The ITW cleaning method involves the injection of a proprietary chemical that transforms the fouling into a fully reusable liquid, thereby removing deposits from metal surfaces.
Avoidance of opening the equipment is particularly important at unit start-up and during maintenance turnarounds because of pipe scale/rust formation; this readily forms when carbon steel is exposed to air, especially in the presence of moisture. Any pipe scale or rust is immediately converted to iron sulphide when H2S is introduced in the unit. Because FeS is not bonded to the pipe it will be carried by the circulating solution and contribute to fouling.
This is an important additional benefit of ITW Online Cleaning in that it will improve performance upon restarting.
Barry Dallum, ECubed, email@example.com
Oxidative desulfurization (ODS) may be an option for you. You would have to test your samples. It is much lower investment cost than hydrodesulfurization and much lower operating cost besides being safer & more environmentally friendly. ODS reduces sulfur in middle distillates, heavy fuel oils, lubricating oils, pyrolysis oils and some other hydrocarbon blends.