Amine anomaly in a mild hydrocracker
Simulation and operational changes help to crack the riddle of frequent amine carry-over from a high pressure amine absorber.
RAJESH MOHAN, ROHIT KUMAR, HIMANSHU KUMAR GUPTA and BASITH ZOHAIL N
Bharat Petroleum Corporation Limited
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A vacuum gasoil mild hydrocracker (VGO MHC) unit in a refinery treats VGO from the vacuum distillation tower of the crude distillation unit. Treated VGO serves as a feedstock for the FCC unit for (Bharat Stage) BS-IV/VI grade gasoline production. The VGO MHC unit at BPCL Kochi refinery (1.7 million t/y throughput) is operated to produce low sulphur feedstock for the FCC unit, together with conversion of excess VGO to low sulphur diesel. The design maximum feed sulphur in VGO is 1.6 wt%.
The feed is charged to the reactor using charge pumps and is preheated in the reactor effluent feed heat exchangers and the charge heater. The reactor needs hydrogen supplied by the recycle gas compressor (RGC) for hydrocracking and hydrotreating reactions. The effluents from the reactor consist of cracked products, desulphurised VGO, hydrogen sulphide, ammonia, and excess hydrogen which are separated in separators/flash drums into liquid and gas streams. The liquid streams then find their way to the fractionation section, whereas the gas streams are further purified in separators to remove heavier ends and in the high pressure amine absorber to remove H2S. The purified recycle gas leaving the HP amine absorber is then fed to a recycle gas compressor knockout drum (RGC KOD) to remove any amine droplets before being routed to the suction of the RGC. The system pressure of the VGO MHC is 105 kg/cm2 (g) and cracking and treating reactions are adjusted by varying the temperature between 340-400°C. The hydrogen consumed in reactions is replenished by make-up gas compressors. The unit process flow diagram is shown in Figure 1.
The HP amine absorber has a total of nine single-pass fixed valve trays. Recycle gas is scrubbed of H2S using 35 wt% methanol diethanol amine (MDEA).
Cracks and blisters in the amine absorber
During unit turnaround in late 2017, a number of internal cracks and hydrogen blisters were identified in the HP amine absorber when the vessel was offered for inspection. Phased array ultrasonic testing (PAUT) and wet florescent magnetic particle inspection (WFMPI) were carried out to analyse the depth of the internal cracks. Major cracks were observed on the shell internal surface below the trays. Some cracks were observed on the welds of ladder rungs on the shell. Hydrogen blisters were observed on the skim baffles, LT nozzles, and on the longitudinal seam of the absorber column. Multiple major and minor hairline cracks were also found at multiple circumferential seam regions of the absorber column (see Figure 2). Hydrogen blisters with cracks were also observed near the 12in recycle gas inlet nozzle, extending out towards the column shell and on the impingement plate (see Figure 3). Hardness tests were carried out near the cracks and blisters; the result less than 180 BHN (Brinell hardness number). Cracks were eliminated by grinding the blistered surfaces and weld filling the internal cracks followed by post-weld heat treatment. Later, in 2019, an unplanned shutdown was carried out for partial replacement of the bottom portion shell of the amine absorber due to aggravation of cracks that developed on the internal surface of the column shell.
Impact on unit operation
The major reason identified for hydrogen induced cracking and hydrogen blisters was high rich amine H2S loading of more than 0.50 mol/mol of MDEA, against the maximum acceptable industry limit of 0.40 mol/mol of MDEA. This necessitated an increase in lean amine flow to the HP amine absorber. But increased flow led to amine carry-over issues which were as frequent as 10-12 instances during a shift. Amine carry-over from the absorber led to loss of amine and required frequent replenishment in the amine regenerating unit. To reduce the carry-over events, the charge of the unit was reduced to maintain amine loading at 0.4 mol/mol of MDEA. Management finally decided to replace the existing trays of the absorber with high capacity trays, in the hope of a solution to the problem. But the issue became murkier with the change to high capacity trays.
Change to high capacity trays
Because the company’s purchase plans involved purchasing more high sulphur crude, a level higher than the design flow rate of amine would have to be pumped into the column to maintain amine loading at less than 0.4 mole H2S/mole of MDEA. The original trays were designed for a 105% amine flow rate (115 t/h), but with processing of high sulphur VGO in mind along with the existing problem of amine carry-over it was decided to consult with the tray manufacturer for trays with liquid flow rates at 130% turn-up (150 t/h). The manufacturer hinted at downcomer velocity limitations at 130% turn-up for the existing trays and hence proposed mechanical modifications to the column relating to downcomer design along with new trays with Type-I and Type-II fixed valves. The downcomer design was changed from the earlier segmental type to a sloped type. The downcomer top width from the column shell was increased to overcome the limitation in downcomer velocity indicated in preliminary tray rating simulations. The high capacity tray dimensions are shown in Figure 4. A comparison of old and new trays design details is provided in Table 1. But ironically, after the change to high capacity trays, amine carry-over issues and an increase in ∆P across the HP amine absorber were observed at both higher (80-100 t/h) and lower (40-50 t/h) amine flow rates. This aroused the curiosity of the operators who were unable to comprehend the phenomenon.
Amine carry-over at higher flow rates
After the trays in the original column were replaced with high capacity trays, the operators maintained a H2S/amine ratio of 0.4 mole/mole of MDEA during normal operation. But when processing high sulphur VGO, the amine requirement would increase in order to maintain the same mole ratio. This necessitated an increase in MDEA flow. However, beyond an amine flow rate of 80 t/h, the operators observed liquid carry-over from high pressure amine absorber to the knockout drum downstream. Because the amine flow rate could not be increased beyond 80 t/h, the hydrocracker charge had to be reduced to maintain the safe H2S loading limit of 0.4 moles H2S/mole MDEA for increased feed sulphur.
The problem was acute with high sulphur VGO: the charge had to be reduced to even turndown values for longer periods of time. At times, VGO was diluted with diesel to meet the mole ratio. This resulted in inventory build-up of high sulphur VGO, requiring export to sustain refinery operations. Whenever liquid carry-over occurred in the amine absorber, the following changes in the process parameters were observed:
• Increased differential pressure across the amine column from a value of 0.07 kg/cm2 (g) to 0.14 kg/cm2 (g) for nine trays
• Loss of liquid level in the absorber whenever the liquid carry-over problem was about to surface
• A shift in temperature isotherm, with the top vapour outlet temperature from the amine absorber increasing rapidly from 45℃ to 55℃
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