Q & A > Question Details
We want to by-pass our de-salters in order to check the consequences with and without desalters on CDU. Moreover we have stopped de-emusifier dosing prior to desalters. What impacts are anticipated in your opinion and what parameters to be monitored in case when there is no desalter in crude preheat trains?
02/07/2010 A: RAUL FLORES, PDVSA INTEVEP, fraulf@hotmail.com
1. A desalter de-salts; that is eliminates part of the CaCl2 and MgCl2 occluded in the crude. If you do not do this, well, both of this salts (none the NaCl) decomposes with the water and heat (120 - 370 °C) to HCL, hydrochloric acid, which is "mortal" to the walls of the CDU mainly at the head - coolest - zone (water condensation). In the other hand, a desalter helps to decrease the water and solids/dirt content in the crude. If you do not do so, you will get troubles on the heated zones of the furnace and the CDU (fouling, over boiling, erosion and corrosion)
2. A demulsifier de-emulsify; that is to break the emulsion crude-water. If you do not do this you will increase dramatically the water content in the crude; the result: instability in the CDU operation (over boiling)
23/02/2010 A: Virendra Kapoor, Petroleum Refining Consultants, vkkapoor9@yahoo.com
Consequences of bypassing desalter are already well known. What prompts you to do so? Demulsifiers do great job in water separation.
11/02/2010 A: AMIYA LAHIRI, AK Associate, lahiri04k@rediffmail.com
Answer to question lies in “why desalting is done”. Crude coming to the refinery contains water, some free and some as emulsion. The water also has dissolved salts in it and possibly suspended solids, depending on source of crude. In desalter the water along with salts are knocked out. Not using desalter is not a good practice and cannot be recommended. Literature deals in detail about it and should be consulted.
In case non-desalted crude is used as feed in CDU, serious problems can be encountered depending on quantity and quality water coming with the crude. Anticipated major problems are:
1. Process upsets
2. Fouling of exchanger train, erosion of heater tubes and transfer line by dissolved / suspended salts
3. Upset and lack of consistency in the O/H corrosion control
All these in long run would damage equipment and result in loss of production and high cost of maintenance.
Answer to what parameters to be monitored in case when there is no desalter in crude preheat trains are:
1. By-pass desalter only if it is required for maintenance
2. Ensure max. drainage of water from the feed crude tank bottom before it is used as feed,
3. Monitor out let and inlet temperatures of feed at critical locations in the feed exchanger train to see if desired degree of temperature rise is achieved or not. This would indicate extent of fouling, if any, and also identify affected exchangers.
4. Operation would have to be vigilant to avoid upsets in distillation column in case of entry of large quantity of water.
5. As there would be large variations (fluctuations) in HCl going in the O/H, the O/H corrosion control system cannot be manually operated and advanced automatic system will have to be installed.
6. Any increase in erosion / corrosion of heater tube and transfer lines should be monitored.
09/02/2010 A: keith bowers, B and B Consulting, kebowers47@gmail.com
Perhaps the crude you process is extremely light, clean, and does not have significant chloride salts.
However, the industry trend is toward significantly increasing the performance of their de-salter operations.
Dirt (fine particles from the producing formation), water-usually a complex brine, rust particles, and other miscellaneous deleterious matter normally MUST be removed from the crude oil before being heated to above about 200C. This is the approximate temperature at which the chloride salts dissolved in the inevitable brine in the crude begin to decompose into insoluble deposits and chlorine -which immediately turns into hydrochloric acid--which has a ferocious appetite for steel and its alloys.
The entrained dirt and rust particles will drop out of suspension, depositing in all the quiet regions on the trays, packing, horizontal surfaces,etc. plugging them up You may easily calculate the volume and mass of these potential deposits from the BS&W analysis of the crude. If those are not removed by the desalter, they WILL remain inside the heat exchangers and distillation tower.
IF the crude you process is similar to MOST, and you cease desalting, I would expect to find much more rapid decline in heat transfer coefficients on the pre-heat exchangers, requiring more severe firing of the furnace--resulting in more coke formation and higher pressure drop there, greatly increased corrosion rates on all the metals in the top section of the tower and all the associated equipment (HCL is not picky about what it eats up), and within a few weeks/months, the fractionation performance of the tower itself will have noticeable decline, requiring higher reflux ratios to maintain product distillation specifications. All of these effects significantly lower the operating efficiency of the unit.
09/02/2010 A: Ralph Ragsdale, Ragsdale Refining Courses, ralph.ragsdale@att.net
Excellent answers, in my opinion. I would add the obvious: If the salt in the crude is 1 lb/1000 bbls or less, the desalter is needed only to remove BS&W, which in itself can cause fouling, but is not as critical as the corrosive effect of higher salt levels.
09/02/2010 A: RAJESWARA RAO, REFINERY/INDIA, abrrao143@yahoo.co.in
1. If you bypass desalters, salt,sediments and water content can't be separated from crude.You can't add wash water to crude if desalter is bypassed and salts will carryover to preheat train and foul the exchangers. Caustic and magneesium salts will decompose above 200 C forms Hydrochloric Acid in Column overhead system which in turn increases corrion in overhead system.
2. If you stop de-emulsifier, water in emulsion phase will not be separated even with Electric field. De-emulsifier reduces the surface tension between crude and water and helps to separate water from crude.
3. If Desalter bypassed, post desalter preheat train fouling will increase and pressdrop across each exchanger increases which can be observed at last exchangers (>200 C). CDU colum overhead system pH will drastically drop to 3 to 4 because of HCl in aqueous form and rate of corrosion will be increased. Overhead chlorides will be increased which is a direct indicates of salts carryover in crude to preheat train and heater.
09/02/2010 A: Celso Pajaro, Sulzer Chemtech, celso.pajaro@sulzer.com
you should verify the following parameters:
Chlorides content in overhead water draw
Iron content in overhead water draw (to try to measure an increase in corrosion)
Sodium in the residium (If sodium if above 20 ppm it will speed up the coke formation in the fired heater of the vacuum column)
monitored the fouling on the heat exchangers downstream (this will change within a long period of time)
If you have a coker or visbreaker unit, also monitored the sodium in the feed to these units.
09/02/2010 A: Alan Goelzer, Jacobs Consultancy, alan.goelzer@jacobs.com
Bypassing Desalters is a BAD idea within the context of CDU+VDU operations. This will cause accelerated fouling of the hotter crude preheat exchangers plus salt fouling and accelerated coking within the Crude Heater plus accelerated deposition + fouling + corrosion of the top trays [assuming that CDU is not best practice with "double overhead drum system" or "high recirculation warm TTPA and single overhead drum system".
Desalters are intended to water wash the crude oil to remove dirt and rust and particulates and as well as lower the salt levels [both in emulsified brine form and micro-particulates].
The only plausible reason to bypass a desalter---except temporarily to fix or upgrade its internals or to retrofit a second stage desalter---is that the API Gravity of the crude mix is less than 19 degrees. Even then, best practice calls for recycling overhead naphtha or TTPA naphtha to bring the API Gravity seen by the desalter to 22 degrees or higher.
09/02/2010 A: sam lordo, Becht Engineering, salordo@comcast.net
this is not a very wise move in my opinion, besides the potential increase in fouling of not only your preheat train but also your furnace due to salts and solids you can count on increase in corrosion concerns in the top of your cdu tower... suggest that you review your operations and ask yourself:
1) what am I trying to achieve, by shutting down the desalter, are the financial gains from shutting down the desalter at the CDU offset from all of the other downstream cost impacts?
2) is the desalter working to its fullest potential, has it been optimized?
Besides the immediate impact on preheat, furnace and tower over corrosion you will see much larger losses in energy and due to the impact anticipated on downstream processes (if they exist), for example a slight increase in water can increase the energy loss by >$500 K USD/per year; increases in salt can result in FCCU catalyst impacts that can be >$1 M USD; there are similar impacts on thermal conversion units also.