4usesCumene is mainly used for the manufacturing of Phenol and Acetone. Cumene is oxidized with air to get Cumene Hydro Peroxide (CHP). This CHP is cleaved to get Phenol and Acetone. Cumene is also used in small quantities for the manufacturing of Pesticides.
5Process ChemistryBenzene is Alkylated with Propylene in the Alkylation reactor to produce Iso Propyl Benzene or Cumene. Zeolite catalyst from EXXON MOBIL is used as the catalyst for the Alkylation reaction. Reaction takes place in liquid phase. Benzene and Propylene are fed to the reactor with a molar ratio of 2:1 Excess Benzene is recycled to the reactor through Benzene column. Di Iso Propyl Benzene(DIPB) and Tri Iso Propyl Benzene(TIPB) are also formed as side reaction products in the reactor. These are collectively called PIPB. DIPB is formed by the reaction of Propylene with Cumene and TIPB is formed by the reaction of Propylene with DIPB. Both DIPB and TIPB are converted to Cumene in the Transalkylator reactor by reaction with Benzene.
10Process Description Major equipments in Cumene plant are: Alkylation ReactorBenzene columnCumene columnPIPB columnTransalkylation ReactorPropylene TreatersClay TreatersSecondary Benzene TreatersActive Carbon bed
11Alkylation ReactorAlkylation reactor consists of 4 beds; each one filled with Zeolite catalyst(EM-5100) supplied by Exxon Mobil.Recycle Benzene from Secondary Benzene Treater is fed to the first (top) bed of the Reactor. Propylene and Alkylation Effluent Recycle are fed to all the four beds of the reactor.In the Alkylator, Benzene reacts with Propylene to produce Cumene. Some DIPB and TIPB are also produced as side reaction products;Reaction between Propylene and Benzene is exothermic. Part of the reactor effluent is cooled and recycled to the reactor as quench flow to all the four beds to control the catalyst bed temperature.Alkylator effluent is then sent to Benzene column for separating and recycling unreacted Benzene.
12BENZENE COLUMNThe primary function of the Benzene Column, is to recover all of the unreacted benzene contained in the Alkylation Reactor effluent and Transalkylation Reactor. Benzene is collected as a side draw from Benzene column and recycled to Alkylator and Transalkylator. The column removes the non aromatic components that are contained in the fresh Benzene feed to the plant by purging from Benzene column reflux pump discharge and Benzene recycle pump discharge. These purges are incinerated in the light incinerator Benzene column also serves as a Benzene dryer by removing the moisture present in the fresh benzene. Water present in the Benzene feed will get separated and collected in the reflux drum boot and from there it is drained to effluent water treatment system.
13BENZENE COLUMN (contd..) Benzene and Toluene levels in the bottom stream from the Benzene column are to be maintained at very low levels as any Benzene or Toluene present in the bottom stream of Benzene column will appear in the product Cumene. Propane and other inerts present in the feed are vented to flare from Benzene column under pressure control. Benzene column bottom stream mainly contains Cumene, DIPB, PIPB, heavies etc which is fed to Cumene column for recovering pure Cumene.
14CUMENE COLUMNCumene Column separates butyl benzene, cymenes, DIPB, TIPB and heavier components to the levels required to produce specification grade cumene product. The cumene product is recovered mainly as a sidedraw from tray # 50 and partly as a small distillate stream from reflux pump discharge. Product is directed to one of the two rundown tanks after cooling and from there transferred to Cumene main storage tank. There is one off spec storage tank to receive product when the plant is under start up or whenever plant is not stable. This off spec material can be fed to Benzene column for reprocessing. DIPB, TIPB, Cymenes and heavier compounds are removed in the bottom stream from the column. Bottom from Cumene column is directed to PIPB column.
15PIPB COLUMNThe PIPB Column operates under vacuum condition, recovers di-isopropylbenzene (DIPB) and tri‑isopropyl benzene (TIPB), collectively referred to as polyisopropylbenzene (PIPB), as a side stream.The PIPB recycle from PIPB column side draw is fed to the Transalkylation Reactor, where both DIPB and TIPB are converted to cumene.Light components (mainly cumene, butylbenzene,and cymene) are separated as a distillate purge stream and send to incinerator.Heavy components (mainly dicyclic aromatics such as diphenyl propane) leave the column from the bottom and send to incinerator.
16TRANSALKYLATION REACTOR The Transalkylation Reactor serves to convert the PIPB back to cumene to increase the overall cumene production. Transalkylation reactor is filled with Exxon Mobil Zeolite catalyst. Part of the recycled benzene stream from the Benzene Column is combined with the recycled PIPB in a 1:1 ratio to feed to the Reactor. The Transalkylation Reactor converts about 50% of the feed DIPB back to cumene and the resultant effluent is fed to the Benzene Column for product recovery.
17: Propylene TreatersPropylene treaters are used to remove Sulfur compounds, Arsine and Phosphine from Propylene feed before admitting it to Alkylation Reactor because these are poison to the Alkylation catalyst. It uses BASF R3-12 as catalyst. This is a mixture of CuO, ZnO and Al2O3. Treaters are used in series with lead and lag positions. Each Treater catalyst has a total life of 2 years with first year in lead position and second year in lag position. Total Sulfur at the outlet of Propylene Treater should be ppm.
18Clay TreatersClay Treaters are used to remove Basic Nitrogen compounds from fresh Benzene feed.It uses Englehard F-25 Activated clay as catalyst.Each Treater catalyst has a total life of 6 months with 3 months in lead position and next 3 months in lag position.Basic Nitrogen at the outlet of Clay Treaters should be less than 0.03 ppm.
19SECONDARY BENZENE TREATORS Secondary Benzene Treators are used to remove polar Nitrogen compounds from the Recycle Benzene stream which are poisonous to the Zeolite catalyst.Each treator is filled with the a single bed of type 13X Molecular sieve adsorbent.Expected life of the catalyst is 3 months in the lead position and 3 months in the lag position based on a polar nitrogen concentration in the fresh benzene at 0.5 ppmwTotal Nitrogen at the outlet of Secondary benzene Treater shall be 0.03 ppm.
20ACTIVE CARBON BEDActive carbon bed is used to remove Hydrocarbon from PIPB column vacuum pump discharge vent before releasing to atmosphere. Life of the Carbon bed is 6 months.
22ALKYLATION REACTORIt is important to keep the reactor pressure sufficiently high to keep all the components in liquid state. Low pressure will lead to flashing of Propylene which will result in Propylene oligomerization and deactivation of the catalyst. Reactor pressure is maintained by controlling the reactor effluent flow to Benzene column. Reactor inlet temp is important to get full conversion of Propylene. Higher temp will lead to more generation of N Propylbenzene and heavies. Low temp will lead to reduced activity of catalyst. Inlet temp need to be increased as the catalyst ages. Temperature is maintained by controlling the reactor effluent recycle stream. Temp rise across each bed shall be kept low to prevent catalyst deactivation and to reduce side reactions. This is achieved by controlling the Alkylator effluent recycle flow Benzene to Propylene mole ratio shall be maintained to get Cumene quality and to reduce side reactions. This is achieved by controlling the Recycle Benzene flow to the reactor.
23Benzene columnBenzene column pressure is maintained low to allow flashing of Benzene from reactor effluents. Pressure is maintained by controlling vent to flare and by Nitrogen make up to the column. Tray #3 temp is maintained to get proper separation of Benzene from other constituents. Low temp will lead to slippage of Benzene and Toluene to Cumene column. High temp will increase the Cumene content in recycle Benzene to Alkylator and Transalkylator which will lead to more heavies generation. Temp is maintained by controlling HP steam flow to the reboiler and by adjusting top reflux and side draw reflux flow . Purge flow from Benzene column overhead and side draw to incinerator are to be maintained to maintain non aromatic concentration in recycle benzene.
24Cumene columnCumene column tray #7 temp is maintained to get proper separation between Cumene and heavier components. Higher temp will lead to presence of DIPB and Cymene in Cumene product. Low temp will lead to Cumene slippage to PIPB column. Temperature is maintained by controlling HP steam flow to reboiler and by controlling reflux flow. Column pressure is maintained by venting to flare header and by making up with Nitrogen. Maintaining column pressure is important to keep the temperature profile steady. A small purge flow from Cumene column overhead is maintained to prevent accumulation of lighter ends at the top section of the column.
25Pipb columnPIPB column is operated under vacuum to reduce the boiling temperature of DIPB and TIPB. Column bottom temp is maintained to get proper separation between PIPB and heavy aromatics by controlling HP steam to the reboiler. Low temp will lead to loss of PIPB to incinerator from column bottom. High temp will lead to presence of heavier impurities in PIPB recycle stream. Column top reflux and side draw reflux are also to be maintained to get proper separation. PIPB recycle flow to Transalkylator has to be maintained to prevent accumulation of PIPB in the system.
26transalkylatorTransalkylator reactor pressure has to be maintained by controlling the reactor effluent flow to the Benzene column. Controlling the pressure is important to keep all the constituents in liquid medium. Benzene to PIPB ratio shall be maintained to get maximum yield from PIPB. Recycle Benzene flow shall be adjusted to get this ratio. DIPB conversion shall be maintained by adjusting reactor inlet temp. Higher conversion will lead to more byproducts and low conversion will lead to accumulation of PIPB in the system and will come out through Cumene product.
28HIGH BENZENE IN CUMENE PRODUCT CAUSE Low Benzene column bottom temperature. CORRECTIVE ACTION Increase steam flow to reboiler and maintain temp. Tube leak on Cumene product/Recycle Benzene exchanger Confirm the leak by sampling and rectify the leak.
29High toluene in cumene product CAUSE Low Benzene column bottom temperature. CORRECTIVE ACTION Increase steam flow to reboiler and maintain temp High toluene in the Benzene feed (more than 300ppm) Take corrective action to improve the feed quality
30High Ethyl benzene in cumene product CAUSEHigh Ethylene in propylene feedCORRECTIVE ACTIONTake corrective action to improve the feed qualityEthyl Benzene is produced in Transalkylator because of high tempReduce temp in Transalkylator.
31High NPB in cumene product CAUSE High temp in Alkylator and Transalkylator CORRECTIVE ACTION Reduce reactor temperature High Cyclo Propane in the Propylene feed. Take corrective action to improve the feed quality.
32High NPB in cumene product (contd) CAUSE Low B/P ratio in Alkylator feed CORRECTIVE ACTION Increase recycle Benzene flow to Alkylator Low Benzene to PIPB ratio Increase recycle Benzene flow to Transalkylator
33High butyl benzene in cumene product CAUSEHigh Butylene in propylene feedCORRECTIVE ACTIONTake corrective action to improve the feed qualityCumene column is not operated properly.Reduce Cumene column bottom temp and increase reflux flow.
34High cymene in cumene product CAUSE Cymene is produced by reaction of Propylene with Toluene. Toluene enters the section through fresh benzene feed. CORRECTIVE ACTION Take corrective action to improve the feed quality Cumene column is not operated properly. Reduce Cumene column bottom temp and increase reflux flow.
35High cymene in cumene product (contd..) CAUSE Cymene is not purged out from the plant. CORRECTIVE ACTION Increase aromatic purge from PIPB overhead.
36High non aromatics in the cumene product CAUSE High non aromatics in the Benzene feed CORRECTIVE ACTION Take corrective action to improve the feed quality. Increase non aromatics purge from Benzene column overhead and side draw. Alkylation reactor catalyst is getting aged. Increase reactor inlet temp. Increase non aromatics purge.
37High dipb in cumene product CAUSE Cumene column is not operated properly CORRECTIVE ACTION Reduce temp and increase reflux to Cumene column. Conversion in Transalkylator is not sufficient and DIPB is accumulating in the system Increase transalkylator inlet temp to get 50% DIPB conversion.