engel - solid contamination evaluation in refinery tail gas treating units

7/26/2019 Engel - Solid Contamination Evaluation in Refinery Tail Gas Treating Units

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Solid Contamination Evaluation in Refinery Tail Gas Treating Units

David B. Engel, Ph.D.

Nexo Solutions

Global Process Efficiency

9391 Grogans Mill Road, Suite A-6

The Woodlands, Texas 77380 (USA)

Abstract

An amine tail gas treating unit (TGTU) using MDEA solvent was experiencing problems with high

suspended solids and sludge formation (“Shoe Polish” material) in its rich amine circuit. The

sludge was accumulating in the rich amine stream filtration system causing saturation and

rapidly plugging of the filter elements. Testing was performed in addition to an extensive

analytical investigation of the sludge material and amine solution in order to discover

composition and possible root-causes of the contamination. Techniques utilized include

microscopy, acid and solvent solubility studies, filtration testing, infrared spectroscopy, energy

dispersive x-ray analysis and water analyses. The results revealed the sludge contained high

amounts of water soluble components and heat stable salts with iron salts and traces of

hydrocarbons. This is generally associated with amine units connected to FCC and Coker units.

Further investigation revealed a cross-contamination of amine solution form the main aminesystem to the TGT unit.



Case Study Solid Contamination Evaluation in Tail Gas Treating Units

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1. Introduction

Background

Amine Units and to some extent Tail Gas Treating units (also called SCOT: Shell Claus Off Gas Treating

Units or TGTU) experience from time to time the so called “Shoe Polish” sludge contamination. This will

cause a series of detrimental effects ranging from fouling to filtration overload (and high filter use). In

such cases filtration is not the proper solution; however, it does alleviate the effects of this aggressive

gel-like contamination. Filter media is rapidly sealed (reducing porosity and permeability), causing fast

differential pressure increase, leading to frequent filter maintenance and high costs. To date there is

not comprehensive study on what exactly this semi-solid residue is, and what the root-cause for its

formation is. There are several and mostly anecdotal perspectives about the composition of the

material, most of them lack hard data. Many believe it is formed by heavy hydrocarbons because of the

black color and its nearly gelatinous consistency. Therefore, we felt it was important to explore this

“Shoe Polish” material and gain some insights on its composition and formation pathways. We selected

a real case encountered during 2013 in a TGT unit. Granted that TGT units do not encounter this type of

contaminants, hence, this was one of the main reasons for this investigation.

An amine tail gas treating unit (TGTU) at a US refinery using MDEA (methyl di-ethanolamine)

experienced considerable suspended solids content in its amine solution. The material was consistent

with the so called “Shoe Polish” material and was present in the rich amine circuit. This sludge was

accumulating in the rich amine filters. Figure 1A shows the comparison of the lean and rich amine fluids.

It can be seen the dark, opaque color of the rich amine compared to the lean amine. Figure 1B shows

the sludge accumulation at the filters. The residues completely saturated the filter elements.

Figure 1. A) Lean and rich amine solvent samples. B) Rich amine filters with sludge accumulation.

As the rich amine filters were inspected, a sample of the sludge was removed from the filtration system,

placed in a 500 mL plastic container and saved for further analysis. The sludge had a black aspect




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consistent with an amorphous solid (similar to a wax). This is known in the industry as the “Shoe Polish”

residue and is found in many refinery amine units. This residue is not typical in TGT units, however. An

investigation was therefore performed in order to determine the composition and possible origin of the

show polish deposit. The sample also had an amine-like odor caused by the presence of amine solution

in the residue.

Figure 2. Sample of the “Shoe Polish” sludge material in a 500 mL plastic container.

2. Acid Solubility

A sample (0.5 grams) of the sludge was placed in an aluminum pan and treated dropwise with

concentrated HCl (5 mL). Upon exposure to the acid, gas evolution was observed after a few seconds

and the pan heated at the bottom indicating gas and energy release. A wet pH paper was exposed to

the evolved gases from the sample (Figure 3). The red color change indicates the formation of acid

vapours. This is consistent with H2S formation from the exothermic reaction of FeS residues with HCl

generating H2S and FeCl2. The reaction did not start initially upon contacting with the HCl solution, but

rather at the on-set of gas evolution; this can be explained by the excess amine and hydrocarbon

residues coating the FeS particles. The amine required neutralization, and the hydrocarbon traces had to

be removed from the agglomerated material to expose FeS to the acid.




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Figure 3. Sample of the “Shoe Polish” sludge material exposed to concentrated HCl and using pH

indicator (damped in water) for gas emanation pH detection.

3. Solvent Solubility

Solvent solubility is a way to understand the nature of solid and liquid materials in terms of their general

properties and polarity. In this case, the solubility was evaluated in water (polar solvent), methanol

(polar organic solvent), naphtha (organic solvent, non-polar) and toluene (organic solvent, aromatic).

Initial Solubility Tests

A small sample of the sludge (0.3 grams) was placed in glass vials (Figure 4) and treated with differentsolvents (5 mL). Results are indicted below.

Water: partially soluble (high) and disaggregation – black suspension

Naphtha: insoluble – black residue

Toluene: insoluble – black residue

Methanol: partially soluble (low) – black residue with suspension

Time Elapsed Solubility Tests (after 14 days)

A small sample of the sludge (0.3 grams) was placed in vials (Figure 4) and treated with different

solvents (5 mL). The sample was allowed to stand for 14 days. Results are indicted below.

Water: partially soluble (high) and disaggregation – yellow suspension

Naphtha: insoluble – black residue with pale yellow supernatant

Toluene: insoluble – black residue with pale yellow supernatant

Methanol: partially soluble (low) – black residue with suspension and pale yellow supernatant




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Figure 4. Solubility of the “Shoe Polish” sludge material in various solvents A) initial solubility and B) time

elapsed (14 days). Solvents: 1) water, 2) naphtha, 3) methanol and 4) toluene

As observed from Figure 4, it can be noted that only water and methanol were capable of suspending

and/or dissolving the sample (partially). Both organic solvents (naphtha and toluene) were incapable of

any interaction with the sample initially. The time elapsed test (Figure 4B) revealed that the water

sample turned yellow/orange with orange suspended solids. The other samples did not display major

changes besides some limited solubility and color changes. It should also be noted that solvent was lost

due to evaporation.

4.

Filtration Test

A sample of the TGT unit sludge (2.0331 grams) was transferred to an aluminum pan and dried in an

oven at 60 oC for 8 hrs. The resulting material (1.554 grams) was dissolved in water (400 mL) and stirred

for 10 min. The mixture was filtered using a 0.45 PVDF pre-weighted membrane. The suspended

material had a total mass of 0.5011 g (32%). This indicated that about 68% of the sample was soluble in

water (for this particular sample). The suspended solids were analyzed by infrared spectroscopy and x-

ray techniques. The liquid filtrate was analyzed for a number of species such as anions, cations and

other species found in amine samples.

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5. Infrared Analysis of Filtered Material Residue (from suspension in water)

Figure 5. Suspended solids IR analysis of the sludge (after removal of water soluble components)

Table 1. Infrared Spectral Analysis

Wavelength (cm-1

) Vibration Type Comments

3334 N-H/O-H Amine and water

2848 C-H (alkyl) Hydrocarbons and/or organic acids

1565 C=C (aromatic) Aromatic hydrocarbon bonds

1448 CH2 (alkyl) Organic structures

1346 CH3 (alkyl) Organic structures

1061 C-O-C Glycol C-O bonds

1012 C-OH Alcohols




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6. EDS Analysis of Filtered Material Residue (from suspension in water)

Figure 6. EDS analysis of the sludge (after removal of water soluble components)




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7. Water Analysis of Sludge Water Soluble Components

Table 2. Water-Soluble Components Analysis in the Sludge Material

Parameter Concentration Parameter Concentration

Alk,% 0.73 Fe, ppm 19.7

pH 9.02 Cr, ppm <0.10

H2O, % 99.13 Ni, ppm <0.10

CO2, % 0.000 Mg, pp, <0.10

H2S, % 0.000 Na, ppm 3.0

Acetate, ppm 89 K, ppm <0.10

Glycolate, ppm 68 Ca, ppm <0.10

Formate, ppm 346 Sulfide, ppm <0.05

Cl, ppm <30 HC, ppm <10

Sulfate, ppm 52 Foam Tendency slightOxalate, ppm 45 Appearance Yellow

Thiolate, ppm <30 MDEA % 0.80

Thiocyanate, ppm <30 Bicine % 0.000

HSS, % 0.06 THEED % 0.000

8. Microscopy of Suspended Solids in the Sludge Material (in water)

Figure 7: Microscopy analysis of the sludge material (after removal of water soluble components)




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Material Residue (from suspension in water)

Figure 8: A) fresh suspended residue from filters. B) suspended residue filtered and exposed to air.

9.

Data Interpretation

Pure water is capable of disassociating the liberated insoluble iron sulphide grains (that turns to rust-

colored Fe2O3 upon exposure to air). This oxidative reaction also takes place in the liquid phase to some

extent in all tested solvents, but mostly with polar components such as water and methanol.

X-ray techniques indicated iron sulfide as the predominant compound in the solid residues in the sludge.

Infrared spectroscopy showed minor hydrocarbon contents and amine residues. Sludge exposed to

strong acids liberated acidic gaseous materials by indication of the pH paper (red). This gas material is

likely to be H2S.

The analysis of the water soluble species indicated the presence of anions consistent with heat stable

salts. Heat stable salts are not often encountered in TGT units unless SO2 breakthrough, however, the

specific heat stable salts detected in this case are rarely found in TGT units.

The suspended solids in the sludge upon exposure to water and analyzed by microscopy displayed a very

narrow particle composition with the majority of them below 10 microns with only a few larger particles

in the order of 20-30 microns.




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10. Conclusions

The performed tests and analysis indicated that for this particular case, the “Shoe Polish” sludge

material composition was consistent predominantly with insoluble and crystalline iron sulfide clusters

encapsulated in a matrix of water soluble polymeric iron sulfides, heat stable salts, amine residues and

minor hydrocarbon traces. We believe that this characterization is representative of similar materials in

other amine units and some TGT units. However, caution has to be used as these process units can

experience considerable variability and the composition of these materials might vary somewhat.

Interestingly, only water and other polar-hydroxyl solvents were capable of disrupting the amorphous

association within the sludge and partially dissolve certain sections of the nearly gelatinous material.

This liberated the solid iron sulfur particles and other water insoluble components. However,

hydrocarbon-based solvents were incapable of similar solubilisation. Thus, hydrocarbons are actually a

minor component in the “Shoe Polish” sludge material. Perhaps the most important information gained

was revealed by the type of the water soluble species present in the residue. These were found to be

similar species to what is found in amine units contaminated with heat stable salts (typically associated

with FCC and Coker refinery units). Minimization of the problem is related to the removal of heat stable

salts, enhancing filtration while monitoring soluble iron levels in lean amine circuit.

Finally, the fact that this particular TGT unit presented the “Shoe Polish” contamination material

combined with all the analysis performed; suggested that somehow contaminated amine solution

(contaminated with heat stable salts) originating from main amine units was somehow being routed to

the TGT unit. This is not a common scenario; however, it is about the only explanation as to why the TGT

unit presented the sludge “Shoe Polish” material, and that is contained the specific anionic composition

analyzed. Further performance simulation studies (using Protreat™ software) confirmed that the TGT

unit operating with MDEA amine solvent was contaminated with DEA (diethanolamine) from the main

amine units in the plant.

engel - solid contamination evaluation in refinery tail gas treating units

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