F. OVAICI, Al-Ghurair Energy, Dubai, United Arab Emirates
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n this article, a refiner needed a solution to recover export-ori-ented naphtha contaminated by methanol (MeOH). This is an actual case in which one of three 30,000-m3 capacity tanks used to store naphtha export was found to be severely contaminated with MeOH. A water-washing solution was applied to reprocess the naphtha in-situ and to remove MeOH from the naphtha, along with returning the storage tank back to continuous operations.Problem-solving approach. The approach to solving prob-lems such as this type is threefold. First, understand the root cause of the incident and rectify it. Second, develop a theoretical basis to resolve the problem and validate it. Third, evaluate and imple-ment a practical solution.
Contamination mishap. Naphtha contamination with oxygenates, such as MeOH, is a costly problem for any refinery.
Reprocessing or re-blending naphtha is a risky proposition, espe-cially when only limited storage is available. A creative and scien-tific water-washing solution was identified to remove oxygenates from the naphtha. Understanding the chemistry of the problem is only the first step. Substantial laboratory and engineering work was necessary to successfully identify and to validate the solution.
PROBLEM: MeOH CONTAMINATION IN NAPHTHA TANK This Middle East refinery exports straight-run naphtha (SRN) from the crude distillation unit (CDU). There are three storage tanks available to store, blend and certify the naphtha prior to shipping. The refinery also operates a tertiary amyl methyl ether unit that uses imported MeOH as a feedstock.
Following a routine MeOH unloading at the refinery, 25,000 m3 of SRN product in an export 30,000-m3 storage tank was later found to be contaminated. This naphtha failed a product-certification test. Contamination results were further confirmed by a third-party laboratory.
Test results of the SRN showed an oxygenate content of 240 ppm against the maximum acceptable level of 50 ppm to obtain a quality certificate. Unfortunately, the SRN of this tank could not be exported. Now, there were only two naphtha tanks left in operation. The quantity of material and oxygenates prevented meaningful re-blending and reprocessing. Contamination of the other two tanks remained a real and immediate threat to plant operations. During the incident, naphtha rundown from the process plant was continuously showing acceptable oxygenate content that was less than 50 ppm.
Step 1: Identify and rectify root cause. The investigat-ing team successfully determined the root cause for the MeOH contamination. The investigation revealed that both the meters installed on the line to the jetty for naphtha export and the meter on the MeOH import/unloading line share the same meter prover.
A single cross valve on the meter prover was mistakenly left open.
Pressure differential allowed imported MeOH from the vessel to flow unimpeded to the naphtha-product line from the CDU to the storage tank during unloading of the cargo vessel.
Rectification of the problem involved strict new operating procedures for the meter and meter prover. Operator training was improved, and it focused on careful handling of equipment with checklists and verification by foremen. For the long-term solution, a separate meter and meter prover would be installed.
Different ways of disposing the off-spec naphtha were stud-ied. However, disposal could impose higher financial losses to the company due to contamination of the naphtha lines to the jetty. Moving the material to other tanks had a higher inherent risk of cross-contamination for the remaining product-storage tanks. At that time, no buyer could be found to purchase this
8+
+ +
Oxygen atoms
More negative changes– –
Water is a polar molecule with positive charges on one side and negative charges on the other.1
FIG. 1
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APRIL 2012 HydrocarbonProcessing.combatch of off-spec naphtha. Unfortunately, the MeOH-contam-inated naphtha remained in the tank for several months as vari-ous options were considered.
Step 2: Theory and validation. The rule for determining if a mixture becomes a solution is that polar molecules will mix to form solutions and nonpolar molecules will form solutions, but a polar and nonpolar combination will not form a solution.
Both MeOH and water are polar. So extraction of MeOH in an aqueous solution is a feasible pathway. The geometry of the atoms in polar molecules is such that one end of the molecule has a positive electrical charge and the other side has a negative charge.
Nonpolar molecules do not have charges at their ends. Mixing
molecules of the same polarity usually results in the molecules forming a solution.
Low-molecular-weight alcohols, such as MeOH, are com-pletely soluble in water. Because of their polar structure, the alcohol molecules actively associate with water molecules through the hydrogen bonds. The hydrogen bonds are strong enough to prevent separation of the water/alcohol mixture by distillation, as shown in Fig. 2.2
Various molecules may mix and dissolve in each other if they have approximately the same polarity. In the case of water and MeOH, this is the situation. The hydrogen of the –OH group on the alcohol is polar in the same manners as the water molecule.
Solubility of MeOH in naphtha. In terms of polarity, MeOH is a strong polar molecule, and aromatics, such as toluene, are slightly polar. Paraffins, such as hexane, are nonpolar. Aromat-ics will be temporarily polarized within the vicinity of a polar molecule (MeOH), and the induced and permanent dipoles will be mutually attracted (Debye Interactions). However, MeOH is not completely soluble in streams, such as SRN that contain low levels of aromatic compounds. Paraffinic/naphthenic hydrocar-bons (HCs) comprise 90 wt% of the SRN, and the remaining TABLE 1. SRN water-washing effects on T6217C
Mixing with magnetic stirrer Without mixing Water inject, Oxygenates Methanol Moisture, Water inject, Oxygenates Methanol
vol % content, wt ppm content, wt ppm ppm Remark vol % content, wt ppm content, wt ppm Remark
0 220.4 196.5 140.7 Top/Mid/Btm: 190.4/ 0 220.4 196.5
188.8/151.5 (MeOH)
1 44.1 22.7 147.8 Color: No change – – –
5 30.9 12.8 165.7 Color: No change 5 160.9 141.6
10 18.2 1.4 170.8 Color: No change 10 90.2 72.6
20 17.1 1.8 177.0 Color: No change 20 98.0 80.5
• Magnetic Stirring Duration 10 min
• Settling Duration 1 hour (end table 1)
TABLE 2. Effect of MeOH removal from naphtha at different water addition rates with/without mixing
SR naphtha (T6217C) water-wash oxygenate result Sample date: Feb. 15, 2008
With H2O wash With H2O Wash
Before H2O wash (10 mins mixing/1 hour standby) (no mixing) Case 1, Case 2, Case 3, Case 4, Case 5, Case 6, Case 7, 1 vol% 5 vol% 10 vol% 20 vol% 5 vol% 15 vol% 25 vol%
Component, wt ppm Top Mid Bot Comp H2O H2O H2O H2O H2O H2O H2O
Ethyl methyl ether 3.0 2.7 2.7 2.7 2.3 2.4 2.3 2.4 2.2 2.3 2.3
MTBE 1.8 1.8 1.8 1.8 1.4 1.4 1.4 1.5 1.3 1.3 1.4
Isopropyl ether 0.0 0.8 0.8 0.9 0.0 0.0 0.0 0.0 0.8 0.0 0.0
Butyl methyl ether 0.9 0.9 0.8 0.9 0.0 0.0 0.0 0.0 0.0 0.0 0.0
Methanol 190.4 188.8 151.5 196.5 22.7 12.8 1.4 1.8 141.6 72.6 80.5
Acetone 1.0 1.0 0.9 1.0 0.8 0.6 0.4 0.3 0.8 0.6 0.7
2-Butanone 8.6 8.3 8.1 8.3 7.0 6.8 6.2 5.5 7.1 6.6 6.6
Methyl butyrate 1.7 1.5 1.5 1.6 1.4 1.3 1.3 1.3 1.4 1.3 1.3
Hexanol 1.6 1.4 1.4 1.4 1.2 1.2 1.2 1.1 1.2 1.2 1.1
2-Pentanone 3.1 3.0 2.9 2.9 2.5 2.5 2.5 2.4 2.5 2.5 2.4
2-Butanol 1.5 1.5 1.5 1.5 1.2 1.0 0.7 0.0 1.2 1.1 1.0
Tertiary amyl methyl ether/alcohol 1.0 1.0 1.0 1.0 0.8 0.8 0.8 0.8 0.8 0.8 0.8
Total oxygenates 214.5 212.6 174.9 220.4 41.1 30.9 18.2 17.1 160.9 90.2 98.0
TABLE 3. Water requirement for each water-wash cycle
Basis—Based on Option 1
Description Remarks
Contaminated naphtha in T6217C 25,000, m3 Demin. water for water wash 1%
Demin. water for water wash 250, m3 ~ 10 m3/hr will be required
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11910% are aromatic HCs. Therefore, the MeOH and naphtha are not soluble in any large ratios.
SRN, depending on the crude type processed, normally con-tains 8 wt%–10 wt% of aromatics. MeOH solubility in aromat-ics is temperature dependent. Essentially above 0°C, for every percentage of aromatics present, 0.5% of MeOH will be soluble.
Following this rule, it is expected that the SRN can dissolve up to a maximum of 4 wt%–5 wt% MeOH.
Laboratory testing was proposed and arranged. Test samples with different water concentrations were added to known volumes of the off-spec naphtha—0% water content in naphtha was the control sample with 1%, 5%, 10% and 20% water concentration standards tested. To investigate the effect of thorough mixing, the samples were analyzed with and without a magnetic stirrer used.
Table 1 summarizes the lab results.
Another set of tests was done on the samples from the con-taminated tank to measure the effect of water washing at different vol% of water to remove the various oxygenates from the con-taminated naphtha. Test results show that water washing removed the majority of the MeOH content from the naphtha while other oxygenates were not affected. Table 2 lists these test results at
dif-ferent water-wash volumes with and without mixing. Fig. 4 shows the appearance of the SRN after water washing at different vol%
of water with a one-hour settlement time and the settled water drained from the sample. These tests showed that there was not much difference in haziness of the naphtha when different volumes of wash water were used. The lab report can be summarized as:
• MeOH and total oxygenate content decrease dramatically to within specs (50-wppm maximum) when the contaminated naphtha was water-washed with subsequent mixing (by a mag-netic stirrer similar to actual tank mixing). The MeOH content remained high when mixing was not done.
• There was no change in color and the product was not hazy.
• There is only a slight increase in water content after the sample remains stagnant if water is not drained.
H
Source: C. Ophardt © 2003
MeOH-hydrogen bonding
C C
MeOH hydrogen bonds and polarity.2 FIG. 2
Lab results of water washing of contaminated SRN.
FIG. 3
Haziness of treated naphtha after water washing with different water volumes.
FIG. 4
TABLE 4. Estimated time for each activity during each water-wash cycle
Timeline
No. Activity Remarks
1 Time for filling water, @ 10 m3/hr 25 hr 2 Mixing by tank mixer 24 hr 3 Settling time, maximum 24 hr 4 Water draining time, @ 25 m3/hr 10 hr
5 Sampling and analysis 0 hr Sampling and analysis to be done during settling
and draining
Total time 83 hr
3.5 days
DW tanker DW was pumped using the pump of a water tanker
Process scheme for Option 1: Direct water injection to tank.
FIG. 5
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APRIL 2012 HydrocarbonProcessing.comThe tests also confirmed the understanding that, if water wash-ing is done together with mixwash-ing, MeOH removal would be more efficient. Based on these results and the lab report, it was also decided that the contaminated naphtha should be washed with demineralized (DM) water.
Lab results showed that 10% water addition to the naphtha with mixing would reduce the MeOH content from 190 ppm to 1.4 ppm, while 1% water can reduce it from 190 ppm to 22.7 ppm. Subsequently, it was decided to inject only 1% of water wash and to drain after mixing, and repeat several times, until the total oxygenate concentration dropped to less than 50 ppm. Using this method, less DM water would be used, thus, limiting cleanup costs and time to recover the product naphtha.
Step 3: Effective implementation. Now that a lab-scale solution was available, the emphasis shifted to execution. Several ideas were considered, with three of the most viable choices listed here:
Option 1. Direct water injection to tank. Water can be pumped directly into the tank T6217C. After injection, the SRN could be mixed with the aid of an available tank mixer. Advantages of this process were:
• Water can be introduced through a larger nozzle (4-in. size).
TABLE 6. Moisture content of naphtha water settlement
Moisture content
Top Middle Bottom Feb. 28 @ 11:30 H 194 200 200 Feb. 29 @ 11:00 H 171 187 184 March 1 @ 08:10 H 194 200 215
March 4 @ 13:45 H 1,640 1,700 1660 Free water was detected March 4 @ 20:45 H 181.3 182.5 176.2 After 7 hr settling @ lab March 4 @ 22:45 H 1,82.7 187 187.8 After 9 hr settling @ lab
TABLE 5. Water-washing effect on SRN in T6217C SRN
Test parameter
Date T6217 SRN Oxygenates, ppm MeOH, ppm Chlorine, ppm Remarks
Feb. 26 @ 0400 H Top 242 219 < 1 Before water washing
Middle 242 220 < 1 DM water 3.4/1.7/Nil(4) Cl ppm
Bottom 245 223 < 1
Composite 242 220 < 1
Feb. 28 @ 11:30 H Top 64 47 1st water washing
Middle 48 30
Bottom 53 36
Composite 54 36
Feb. 29 @ 11:00 H Top 61 42 1st water washing
Middle 61 43
Bottom 58 41
Composite 60 41
March 1 @ 08:10 H Top 59 41 1st water washing
Middle 60 42
Bottom 62 44
Composite 60 41
March 4 @ 13:45 H Top 46 23 2nd water washing
Middle 37 16 9 hr settling @ lab
Bottom 41 17
Composite 41 19
March 5 Top
Middle
Bottom
Composite
Tank mixing patterns for Option 1: A and B.
FIG. 6
9 hours
6 hours
3 hours Tank mixing pattern
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121• The associated lines would not be contaminated.
• The procedure can be done several times. In case of failure, the other two naphtha tanks would remain available for rundown and dispatch.
The disadvantages included:
• Mixing will require a longer time.
• Mixing may not be as effective as circulating the SRN to and from the tank.
The tentative time required for each cycle of water, assuming 1% DW will be mixed to the naphtha tank and drained after mix-ing and settlement, are summarized Tables 3 and 4.
Option 2. Water injection via export piping. Naphtha inven-tory of the tank can be circulated by way of marine-loading pump.
Water can be put into the suction line of the pump—0.75-in.
nozzle with two nozzles with a capacity of 3 m3/hr per nozzle.
The resulting mixing could be done by the pump itself and would not rely on the effectiveness of the tank mixer. The advantages of this option include:
• There is thorough mixing of SRN and water
• The mixing time will be shorter
• The experiment can be carried out several times. In case of failure, the other two tanks will be available for rundown and dispatch.
However, the disadvantages are:
• Associated pipelines will have to be flushed thoroughly with on-spec naphtha
• Limitations would have to be imposed on the scheduling of naphtha shipments.
Option 3. Water injection and mixing using remaining tanks. Water can be sent to one of the other tanks (T6217 A/B.
The T6217C can be transferred to it. The advantages from this option include:
• There is thorough mixing of SRN
• The mixing time will be less, as the SRN can mix while it is filling the tank.
Conversely, the disadvantages are:
• Only one tank will be available for operation.
• If the procedure fails for any reason, then the additional tank also contains contaminated naphtha.
• Associated pipelines will have to be flushed thoroughly with on-spec naphtha
• Naphtha shipment schedules would be affected.
SOLUTION
Option 1 was selected as the preferred method. As per the plan, 1% DW or 250 m3 of DW would be injected directly to the tank.
The tank mixer would be used to mix the SRN and DW, followed by tank settling and draining of settled water. This procedure would be repeated as required until the naphtha is completely washed and meets all oxygenate specifications.
Successful water-washing plan. Water injection to the tank started on Feb. 26 during the day shift. Table 5 shows the result of oxygenates, MeOH and chlorine content of the naphtha before and after the water washing. Water draining started right after nine hours of settling. After the first water wash, the oxygen-ate level dropped to 60 ppm, close to spec, from the average result of 240 ppm. Therefore, after the water was drained, a second water-wash operation started on March 4, after which the total oxygenates dropped to 40 ppm; both were acceptable and on-spec.
Fig. 8 shows how the oxygenate level changed with water washing.
Table 6 summarizes the moisture content of the SRN before and after the water-washing operations. The SRN then received a qual-ity certificate and it was successfully exported. The refinery contin-ues to successfully operate with all three naphtha tanks in service, and with no further incidents of MeOH contamination. HP
LITERATURE CITED
1 http://www.school-for-champions.com/chemistry/polar_molecules.htm.
2 http://www.elmhurst.edu/~chm/vchembook/162othermolecules.html.
Farzad Ovaici received his MSc degree in chemical engineering from Shiraz University in 1978. In 1979, he began his career with Bandar Imam Petrochemical Co. in Iran. In 1980, he moved to the Isfahan refinery. Mr. Ovaici was later responsible for reconstruction and rehabilitation of Abadan refinery, a 630,000-bpd refinery. This refinery severely damaged due to the Iran-Iraq War. In 1992, he joined Tabriz Petrochemical Co., and was assigned as project director for EB/SM, and different polystyrene plants. Later, he was assigned as chairman and managing director of Tabriz Petrochemical Co. In 2000, he became the managing director of Kala Naft Canada Ltd. Mr. Ovaici received an M. Sc. degree in engineering from the Chemical and Petroleum Engineering School of University of Calgary. He is a member of the Association of Professional Engineers Geologists and Geophysicist of Alberta Canada. In 2005, he moved from Canada to Oman and joined Oman Refinery Co.
as the general manager, of the Mina Al-Fahal Refinery. Later, he was promoted to general manager of the two refineries in Oman Refineries and Petrochemical Co. Mr.
Ovaici joined Al-Ghurair Energy as the managing director, of refining and petrochemi-cals and is based in Dubai, UAE. In addition to his position in Al-Ghurair Energy, Mr.
Ovaici is currently chief executive officer of Libyan Emirates Oil Refining Co.
DM water
Processing scheme for Option 2: Water injection via export piping.
Oxygenates, ppm Chlorine, ppm200
250 Effect of water washing, T6217C
Oxygenate content changes over time with water washing.
FIG. 8
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