PETROCHEMICAL DEVELOPMENTS
Distillation in wax vessel. Dur-
ing the plant shutdown, reactors and the suspension receiver are emptied into the wax distillation vessel V-342 to minimize inventory. The slurry transferred to V-342 is distilled with live steam. Since there are no high-level indicators in the vessel, filling is done manually. There is always the possibility of over-filling the vessel. During distillation, if the live steam flowrate is too high, powder and wax can be entrained along with vapor that is collected in the distilled mother-liquor vessel, V-344, through condenser E-341/E-341A. During a recent event for condenser E-341/E-341A, fouling along with choking in pump P-342’s discharge line proved powder and wax carryover possibility. Since discharge from pump P-342 (fed from vessel V-344) goes directly to tank T-412, this can support the root cause for powder and wax accumulation in tank T-412. A high-level indicator is
Provision of woven strainer/impingement baffles
Wax distillation vessel V-321
Wax drain
Hexane vapor
TC
Wax distillation vessel V-342
Waste hexane from catalyst preparation vessel
Waste hexane separation vessel V-314
Waste hexane storage tank V-343
Distilled mother liquor collection vessel V-344
E-341A
trifugal decanter separates polymer powder from the mother liquor. Improper separation can entrain powder in the mother liquor. After separation, the mother liquor is sent to a feed vessel, V205. The mother liquor is fed into the reactors, and the remainder is sent to a storage tank, T-412. Periodic sampling of the mother liquor can detect polymer powder at levels reaching 0.15 wt%. Because the process is a closed-loop system, powder can accumulate in storage tank T-412 due to poor separation. This powder can potentially choke the adsorption columns. Powder entrainment in the mother liquor occurs when the polymer powder has a low bulk density, which depends on processing conditions. Another factor is adequate residence time, which is determined by decanter loading. The slurry-feed valve of the decanter should open gradually to increase retention time and reduce powder carryover into the Hexane mother liquor. Adjusting the overflow weir vapor of the decanter can increase retention time.
Provision of high-level switch
Wax drain TC
Condensed hexane
Powder carryover into the mother liquor. The cen-
E-341
Regeneration of adsorption column follows a cycle of heating with air, nitrogen and steam. Coke, if present, is oxidized by air. If the oxidization process is properly completed, then no coke should remain. As exposure to higher temperatures reduces the service life of the adsorbent pellets, regeneration durations must be extended to ensure complete oxidation of coke content. Carbon monoxide (CO) and carbon dioxide (CO2) analyzers indicate oxidation level of the carbon content. These analytical instruments are often not properly functioning. Periodic calibration of the CO and CO2 analyzers is necessary.
Evaporation step. The mother liquor is fed into separator V-311. By thermosiphoning, it is sent to the tube side of evaporator E-312. The level in separator V-311 is controlled by injecting steam on the shell side of the evaporator E-312. The evaporator performs well at level ranges of 20%–25%. If the level decreases, then disengagement space above the liquid level increases and causes sudden vapor expansion. Such vapor expansion causes higher velocity, thus entraining wax and powder downstream with accumulation in the adsorption column. Maintaining proper level control of the evaporator/separator has a great impact on the adsorption-column availability. This is very crucial during pipegrade polymer runs, which generate a huge amount of byproduct wax. Jacket pressure of evaporator E-312 in the pipe-grade run can increase over 3 barg (usually 1.5–2 barg), thus indicating a high concentration of wax. If the evaporator level is not properly maintained, it may cause wax carryover with the vapor. Evaporator-level control is the key to avoiding wax and powder carryover, along with hexane vapor. Installing an inline strainer on the vapor line to condenser E-313 can prevent this carryover. Field observation shows that pump P-311 and P-312 suctions were fouled on several occasions with waxy materials. A strainer is required at the suction of both pumps, along with a current monitor that is tied to the distributed-control system.
E-322
Inadequate regeneration of adsorption column.
needed for vessel V-342; to prevent excess slurry loading. During distillation, control of the dosing steam could prevent powder carryover with the steam. A woven strainer or impingement baffles installed the vapor line would mitigate powder entrainment further downstream (Fig. 5).
Condensed hexane
heavy compounds at high temperatures during regeneration. To eliminate wax carryover, woven strainers or impingement baffles can be installed on the wax-distillation-vessel vapor lines. For further safety, suction strainers can be installed on the mother-liquor transfer pump (P-342) to prevent wax carryover to the storage tank (T-412). All of these suggested changes are shown in Fig. 5.
Provision of temperature controllers
Pump P-342
Mother liquor to tank T-412
Provision of suction strainer FIG. 5
Flow chart of the suggested changes in the wax-distillation unit to prevent ingress of powder and heavy hydrocarbons into mother liquor. Dashed line blocks indicate the waste-hexane unit. HYDROCARBON PROCESSING MAY 2012
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