MERICAT™ J is an advanced liquid treating technology that removes heavy mercaptans in jet fuel and middle distillate streams without using caustic or carbon beds. MERICAT J utilizes Merichem’s FIBER FILM® Contactor as the mass transfer mixing device in combination with the proprietary JeSOL™-9 treating reagent to oxidize heavy mercaptans. Since there is no fixed carbon bed, operators experience no downtime for water or caustic wash maintenance or carbon bed change outs. This significantly increases the on-stream performance and eliminates a refinery waste stream. Additionally, the non-dispersive FIBER FILM® Contactor reduces capital expenditures and overall plant space requirements, making MERICAT J the technology of choice.
76JULY 2015 | HydrocarbonProcessing.com
Gas Treating
• The treated LPG will contain 7 ppm–8 ppm of H2S in
either the two- or three-theoretical-stage scenarios. Both are below the 10-ppm specified maximum specification. • With the inlet LPG containing 0.12 mol% H2S, and the
amine circulation rate at 70 bpd–120 bpd, the calculated rich amine loading was between 0.07 mol/mol and 0.12 mol/mol.
• “Equilibrium loading” refers to the theoretical maximum loading achievable between the H2S and amine. This
loading is dependent on the temperature and pressure conditions in the treater and the H2S concentration of
the inlet LPG. It is normally recommended not to exceed 80% of equilibrium to avoid the risk of corrosion and subsequent fouling. The H2S equilibrium loading was
36%–61%, which was well below the recommended maximum, indicating the tower should not suffer from corrosion. It also indicates that the circulation rate of the amine was appropriate.
The current treater has the capacity to remove the H2S lev-
els to the desired specification of 10 ppm or less. LPG TREATER INTERNAL COMPONENTS
Once it was determined that the absorber had the neces- sary treating capacity for H2S removal, a review of the inter-
nals was necessary.
Treater packing. The packing itself should be 2 in. in diam- eter, to maximize available liquid flow area. At present, the LPG treater uses random packing (high-performance packing in a saddle-type configuration), which is commonly used. For the packing to function correctly, it must be kept clean at all times.
Therefore, full-flow filtration of the lean amine feeding this treater is recommended.
The recommended packed bed height is between 8 ft and 12 ft. The tower’s bed heights are 8 ft. If the packing bed height is too high, then the LPG bubbles can coalesce and form large droplets. This reduces the contact efficiency between the LPG and amine. Each packed section must have effective redistribu- tion to re-disperse the LPG droplets and correct any channel- ing, which may have occurred in the previous bed. The most common packing material is 316 stainless steel. When using metal packing, it is important to ensure that the metal is fully wetted by the amine before bringing any LPG flow into the tow- er, as metal can be wetted by either amine or LPG.
LPG distributors/packing support tray. The distributor/ support tray is very important, not only for supporting the packing that sits on it but also to control the LPG droplet size. The LPG pools below the plate and bubbles upward through the packing. The amine flows downward through several path- ways (downcomers) to below the LPG directly under the plate. The recommended design velocity through the LPG distrib- utor is 70 ft/min with an operational window of 30 ft/min to 75 ft/min. Excessive velocity of the LPG droplets can create emul- sions, whereas low droplet velocity can result in insufficient dis- tribution and entrainment of LPG in the rich amine. The dis- tributor has hole diameters of 124 in. × 0.44 in. The distributor must be checked for an LPG flowrate of 2,000 bpd, or 58.4 gpm. To calculate LPG droplet velocity for this configuration:
Cross-sectional area = (124 × π × (0.44 in./12 in./ft)2/4)
= 0.13087 ft2
Design LPG flow = (58.4 gal/min) × (ft3/7.48 gal)
= 7.81 ft3/min
Design velocity = 11.75 ft3/min ÷ 0.13087 ft2
= 59.68 ft/min
The total LPG orifice velocity is below the maximum accept- able guideline of 75 ft/min. Nevertheless, the LPG distributor drawing states it was designed for an LPG flow of 46.7 gpm. If there is a case where the LPG flow is increased beyond 2,500 bpd (nearing the limit of 75 ft/min), then the plant should con- sider installation of a new inlet distributor plate.
Ladder-type distributors are most commonly used to inject the LPG into the tower, which is then dispersed through the smaller openings on the packing support tray. Adequate space must be available below the distributor and the rich amine level to minimize LPG entrainment. It is recommended that amine have a 10-min residence time in the bottom of the treater before leaving to the downstream flash tank. It was also suggested that the LPG treater be retrofitted with a ladder-type inlet distribu- tor consisting of a series of parallel tubes fed by a central pipe. This is a commonly used design and should be adequate. The orifices in the parallel tubes must be directed downward across the entire cross-sectional area of the packing.
LPG/AMINE SEPARATION, TESTING AND AMINE RECOVERY
Once it was determined that the amine treater did not need to be replaced and had the capacity for higher flowrates, then
TABLE 3. Liquid fl ux conditions using theoretical stages
Two theoretical stages Three theoretical stages LPG fl owrate, gpm (bpd) 55.7 (1,910) 55.7 (1,910) MDEA fl owrate, gpm (bpd) 3.5 (120) 2.04 (70) Total liquid fl ow (LPG + MDEA), gpm 59.2 57.7 Cross-sectional area of 29-in.-diameter treater, ft2 4.58 4.58
Total liquid fl ux, gpm/ft2 12.9 12.6
TABLE 4. Simulated parameters for Increased LPG fl owrates Simulation results—LPG fl ow conditions
Two theoretical stages
Three theoretical stages
30% lean MDEA circulation rate, bpd
120 70
Rich-amine loading at low fl ow, mol/mol
0.07 0.12
H2S equilibrium loading, % 36 61
H2S in treated LPG, ppm 8 7
MDEA in treated LPG, lbmol/hr 0.0046 0.0045
Other sulfur species, ppm 109 109
Note: The maximum values of H2S in both the LRU and crude feeds were used in the
Hydrocarbon Processing | JULY 201577
Gas Treating
the issue of instability in the caustic treater needed to be ad- dressed.1 At maximum flow conditions of 2,000 bpd, the treated
LPG will certainly carry more amine with it.
To address the amine carryover challenges, the plant installed an advanced amine recovery system.3 This recovery system has
two functions: recovery of free and emulsified amine, and recov- ery of soluble amine in the treated LPG. This system would, in turn, protect the downstream caustic treating unit from any amine contamination.1 The advanced amine recovery system, as a skid,
incorporates a filtration section for particle removal followed by water injection, mixing and contacting/separation to extract free, emulsified, and a portion of the dissolved amine from the LPG product.3 The advanced amine recovery process was chosen for
its high efficiency of mass transfer coupled with its ability for high-separation efficiency in a small equipment envelope.3
LPG sampling procedure and MDEA analysis. Once the recovery skid was in place and operating, qualification of its performance was needed to ensure amine would not upset the downstream caustic treating unit.1 As mentioned earlier, de-
tecting amine in LPG is difficult. The sampling procedure was specifically designed to accommodate the analytical method selected for this application. Since there is no direct method to properly quantify amine concentration in LPG, an indirect method was used. The method involves transferring the amine by extraction into a suitable immiscible solvent for analysis by ion chromatography (IC).
The sampling was performed by collecting LPG into stainless steel cylinders filled with a predetermined amount of distilled water. Cylinders were then exposed to low vacuum to properly accommodate the subsequent LPG sample volume. The cylin- ders were equipped with internal mixing elements to maximize mass transfer between the two liquid phases. A total of four cyl- inders were used to collect the LPG samples, at two different water injection rates.
During the initial system operation, the water injection rate was adjusted to 1.5 gpm prior to LPG sample collection. The second sets of LPG samples were collected at a water in- jection rate of 1 gpm. Each cylinder was attached to the sam- pling port and filled with LPG. The water phase was removed from the cylinder, and the remaining LPG was vaporized into a known volume of distilled water (100 ml) to capture any re- sidual amine that might be left. The empty cylinder was further rinsed internally with distilled water (30 ml to 50 ml) to cap- ture any other possible amine traces. All aqueous samples were processed by IC analysis for amine determination. The analy- sis was performed using an advanced column and an isocratic methanesulfonic acid (MSA) eluent.4
SAMPLING AND AMINE RECOVERY RESULTS The results of the various water samples analyzed by IC are shown in TABLE 5. The concentration represents the total
methyl MDEA mass contained in the LPG as determined by combining the results of the extraction water, purge water and
MARCH 1–2, 2016
MOODY GARDENS CONVENTION CENTER