Monomer Recovery in Polyolefin Plants

PETROCHEMICALSand GAS PROCESSING Monomer recovery in polyolefin plants

This article outlines the application in polyolefin plants of a membrane based process, VaporSep, for separating and recovering hydrocarbons from nitrogen - in particular, the treatment of resin degassing vent streams Marc L Jacobs DouglasE Gottschlich Richard W Baker MembraneTechnology & ResearchInc (MTR)

hemical feedstocks, or monomers, such as ethylene and propylene, Selective are the single largest operating + cost in the manufacture of polyolefins. Layer Due to the intensely competitive nature of the industry, monomer losses in vent streams are a mqjor concern for produc- Microporous ers. Typical losses for a plant range from <_ 1 to 2 per cent of the feed and can Layer account for 2000 to 4000 tons/year of lost monomer. Assuming a cost of $350/ton, these vent streams represent a significant opportunity for recovery and Support recycling of raw materials web A membrane-based process called 'l VaporSep, to separate and recover Figure The three-layer VoporStepmembrqne hydrocarbons and nitrogen in polyolefin plants, has been developed by free selective layer which performs the and parallel flow combinations to MTR. The process is based on a poly- separation. meet the requirements of a particular meric membrane that selectively perme- After manufacture as flat sheet, the application. ates hydrocarbons, compared to light membrane is packaged into a spiral- gases such as nitrogen and hydrogen. wound module, as shown in Figure 2. Resin degassing vent streams This article describes how the process is The feed gas enters the module and In a typical polyolefin plant, propylene, applied to resin degassing vent streams flows between the membrane sheets. ethylene, catalysts, and solvents are in polyolefin plants. Spacers are added on the feed and contacted at pressure in a reactor to pro- In conventional membranes, the sep- permeate side to create flow chan- duce polyethylene and polypropylene. aration is accomplished primarily by dif- nels. The raw polymer product is produced in ferences in diffusion rates due to As the hydrocarbon preferentially powder form and contains significant differences in molecular size. In con- permeates through the membrane, the quantities of unreacted monomers and trast, the VaporSep membrane separates gas spirals inward to a central collection other raw materials. on the basis of solubility. pipe. Nitrogen and other light gases are These hydrocarbons must be removed This "reverse selective" membrane rejected and exit as the residue stream. before the polymer can be extruded and allows large hydrocarbon molecules to The membrane modules are placed into shipped to the customer. The hydrocar- permeate much faster than smaller pressure vessels and configured in series bons are removed from the powder by molecules such as nitrogen, hydrogen, or methane. The reverse selective effect lllodrle housing is due to the higher solubility of large hydrocarbon molecules in the mem- Feedfloril+ _-+ RBsidueflow brane polymer. Collection pipe -:}* Fermeateflow The membrane is a thin-film compos- --> ite polymer that is 10 to 100 times more Feedflowt' Rssidueflorif permeable to hydrocarbon compounds than to nitrogen. The membrane, shown in Figure 1, consists of three lay- gFacer ers: a non-woven fabric which serves as litembrane the membrane substrate (support web); SFacer a tough, durable and solvent-resistant microporous layer which provides mechanical support without mass trans- fer resistance, and a non-porous, defect Figure2 How the membroneis pockogedinto a spirol-woundmodule PETROCHEMICALSandGAS PROCESSING

Membrane PRUperformance

Recycle l{2 to _ ReFjdue purgehin p91i 1+) Feed composition (#/hr) Ethane 7 Ventstream from purgehin Nitrogen 1730 Propane 230 Compressor Propylene 730 Recoveredpropylene Water {< 0.11tH2} Total 2700

Propylenerecovered (#/hr) 715 Makeuppropylene Propylenerecovery (o/o) 98 refrigerant Nitrogen recovered(#/hr) 1729 Loun Nitrogenrecovery(o/o) 99 pressure N itrogen purity (volo/o) 99 flash Powerconsumption (k\A/) 330 Figure 3 Schemoticof propylenerecovery unit Valueof recovered nitrogen($000/yr)- 300 Valueof recovered using hot nitrogen in a stripper column, stream from the separator (nitrogen and propylene($000/yr)" 1050 also known as the purge bin. In many propylene vapour) flows back through Capitalcost ($000) 1700 plants, the vent gas from the purge bin the BAHX (for refrigeration recovery) is sent to the flare'and both the hydro- and then enters the membrane section. *Basedon $350/tonpropylene and carbon and nitrogen content is lost. The membrane is much more perme- $40/ton nitrogen In an average polyolefin plant, the able to hydrocarbons than to nitrogen, value of the monomers in this stream is so the stream is separated into a purified in excessof $l million/year.ln addition, nitrogen residue stream and a propy- Toble I the value of the nitrogen represents an lene-enriched permeate stream. The per- additional $0.5 million annually. meate stream is recycled back to the nitrogen (less than 0.1 vol%). The per- Beyond these monetary incentives, inlet of the compressor. The residue formance of this PRU is summarised in eliminating or minimising the amount stream is sent back to the purge bin. Table 1 Based on a value of $350/ton for of hydrocarbons to be sent to the flare The advantages ofthis design are that propylene and $40/ton for nitrogen, provides meaningful environmental virtually no refrigeration is required and this project has a payback time of only benefits. that the recovered propylene contains a 15 months. An alternative to the process very low concentration of dissolved scheme in Figure 3 is to provide an Plant application Eight propylene recovery units (PRU) have been constructed to separate and recover propylene and nitrogen in polypropylene plants. The process schematic for one of these units is shown in Figure 3. This scheme is referred to as the self-refrigerating cycle as the recovered liquid propylene is flashed and vaporised at a lower pressure to provide the refrigeration required by the process. The propylene/nitrogen feed stream enters the PRU at atmospheric pressure. The gas is compressed by an oil-flood- PFi Propybn*R€coueryUnit ed, screw compressor to a pressure of approximately 200psig and then fed into a molecular sieve adsorbent dryer to remove moisture. The gas then enters a brazed aluminurn heat exchanger (BAHX) and is cooled to approximately -25'C. A portion ofthe propylene condenses inside the BAHX and the resulting two- phase gas/liquid mixture flows into the i< 0.11!H2! separator. The recovered liquid propylene stream is flashed to lSpsig and vaporised to provide the refrigeration required. In addition, a small amount of additional propylene may be required to make up for heat gain and other system inefficiencies. This make-up propylene is approximately 5 per cent of the propylene in the feed gas. The gas Figure4 Processflow schemeof one of the propylenerecovery units PETROCHEMICALSandGAS PROCESSING external source of refrigeration. The tration solved the temper- propylene is then recovered as a high- ature problem in the com- PRU performance pressure liquid, which may be sent pressor and allowed the directly to the reactor or to a distilla- company to eliminate the tion column for additional purifica- addition of propylene Feedcomposition (#/hr)* PPl PP2 tion. In this case, the recovered vapour. The gas is com- Ethane 8 0 propylene will contain close to 2 volo/o pressed to 400psig, Ethylene 1 0 dissolved nitrogen. cooled, and then partially Nitrogen 1940 1750 The process flow diagram for another condensed in the BAHX. Propane 14 180 PRU is shown in Figure 4. In this case, The liquid and vapour are Propylene 124 1220 the customer had an existing PRU in his separated. Water 0 5 polypropylene plant No. I (PPl). Sever- The liquid is flashed to Total 2087 3155 al years ago, the company decided to I Spsig and returned to the build a new polypropylene plant (PPZ). BAHX to provide cooling Propylenerecovered (#/hr) 107 1180 For this new plant, the company before being returned to Propylenerecovery (o/o) 87 96 wanted to construct a membrane-based the polypropylene pro- Nitrogen recovered(#/hD 1920 1 440 (o/o) PRU and to improve the performance of cess. The vapour is sent Nitrogen recovery 99 82 Nitrogen purity (vololo) 99 the unit for PPl with the addition of a back through the BAHX 99 Powerconsumption (kM/) N/A 300 membrane system. The existing unit for and into the membrane Valueof recovered PPl consists of a three-stage reciprocat- section. nitrogen($000/yr)"* 200 150 ing compressor with a BAHX and uses The purified nitrogen Valueof recovered the self-refrigerating cycle. stream is recycled back to propylene($000/yr)*" 160 1750 The unit recovers 80 per cent of the the purge bin, while the Capitalcost ($000) 400 1000 propylene in the feed, but since the propylene enriched per- *The nitrogen is only purified to 92 vol%, it is meate is sent to the inlet feed compositionfor PPl is for the membrane not pure enough to recycle to the purge of the compressor. sectiononly **Based bin and must be sent to flare. In addi- In summary, the com- on $350/ton propyleneand $25lton nitrogen tion to the 2000 pounds/hour of nitro- bined membrane system gen that is lost, an additional 1,25 provided four important Tqble 2 pounds/hour of propylene is also benefits to the customer. burned. First, propylene andtnitrogen were sepa- compression-condensation with mem- The total value of this stream is close rated and recovered from the new plant, branes as shown in Figure 5. The hydro- to $400000/year. The company also PPZ. Second, the propylene and nitro- carbon/nitrogen mixture enters the experienced difficulties with the com- gen recovery performance of the exist- BRU. The mixture is compressed by an pressor as the hydrocarbon concentra- ing PRU was enhanced. Third, the feed oil-flooded, screw compressor to a pres- tion in the feed gas was too low and gas composition for the PPl compressor sure of approximately 250psig and then caused the compressor to overheat. To was increased from 3O wto/o hydrocar- fed into a shell and tube condenser. A prevent overheating, propylene vapour bon to 40 wto/o, which eliminated the portion of the hydrocarbons condenses was added to the compressor feed. compressor overheating problem. inside the condenser. The two-phase, To avoid the expense of a completely Finally, the payback time for the com- liquid/gas mixture flows into the high new PRU for the new polypropylene bination of PPI and PP2 was less than pressure liquid/gas separator. plant No. 2, the customer planned to one year. The performance of each PRU The gas stream from the separator send the PP2 purge bin vent to the exist- is shown in Table 2. (nitrogen and hydrocarbon vapour) ing PPI PRU. Although the PPz unit had flows into the membrane modules, some excess capacity, it was not enough Polyethylene plant application where it passes across the surface of the to handle the entire stream from PPl. In the manufacture of high density membrane. This membrane separates The VaporSep system was therefore used polyethylene (HDPE), the resin the stream into a hydrocarbon-depleted to concentrate the stream from PPl so degassing vent stream contains a large residue stream and hydrocarbon- that it could be handled by the PPl PRU. quantity of iso-butane. Three butane enriched permeate stream. An additional benefit of this scheme was recovery units (BRU) have been con- The permeate stream is then recycled that adding the concentrated scheme structed to separate and recover iso- back to the inlet of the compressor. The from PP2 eliminated the compressor butane. The process design combines residue stream, which is stripped of overheating problem. The feed stream from PPZ is initially compressed to l4bara and cooled to 35'C. The gas then enters the membrane section where it is separated into Compressor a "q control propylene-enriched permeate and a Vent stream nitrogen-enriched stream. This stream is from purgehin further purified to 99 per cent in a sec- Membrane ond membrane stage and sent back to the purge bin. The permeate from the Gaslliquid second membrane stage is sent to flare. separator The permeate stream from the first membrane stage is mixed with the feed Low from PPl and sent to the existing PRU. pressure Recoverediso-butane By mixing the two streams, the hydro- flash carbon concentration is increased from 30 to 40 wt%. The increase in concen- Figure5 Processdesign combining compression-condensotion with membranes PETROCHEMICALSandcAS PROCESSING

and allowing the membrane completely. The recovery units are sup- feed pressure to vary. The com- plied as complete skid-mounted pack- 1111;1r!:i:1ft4ti]I*:1i!tt3:n?lti9lilii 9:.:1t:Jraln*:+ii:iii;$ry:]i:Lffi::Es*r&r::1 pressor capacity, which is fixed, ages. The systems are simple, requiring at Feedflow rate (#/hr) 4000-8000 is the sum of the feed flow rate most a single stage compressor. Typical lso-butanefeed concentration(Mo/o) 55 and the permeate flow rate. payback times are short ranging from six lso-butanerecovered (#/hr) 2300-3000 As feed flow rate increases, to 15 months. lso-butane recovery(7o) 66-98 the membrane feed pressure Six VaporSep units are currently oper- CO, removal (o/o) 90-98 decreases to reduce the mem- ating in resin degassing service and two Power (kW) 400 brane driving force and thus the units have been running for more than Value of recoveredbutane ($000/yr)* 2200 permeate flow two years. Four additional units are Capitalcost ($000) 1300 Alternatively, as the feed flow expected to come onstream in the next rate decreases, the membrane few months. Other VaporSep applications *Based on $175lton iso-butane feed pressure increases to in polyolefin plants include ethylene increase the membrane driving recovery from reactor purge streams and Tqble 3 force and the permeate flow. A ethylene and propylene recovery from summary of the BRU perfor- distillation column overhead streams. hydrocarbons, is sent to an existing car- rnance is shown in Table 3. The simple bon adsorption unit. To remove dissolved payback time is less than 8 months, indi- This article is basedon a paper presentedat the DeWitt CO, from the recovered hydrocarbons, cating a very attractive investment. World Petrochemical Review, Houston, the liquid from the high pressure separa- Texas,23-24March 1999. tor is flashed to lSpsig in a low pressure Conclusion separator. The vapour from the low pres- VaporSep has been successfully applied in Morclocobs is directorof solesand sure separator is recycled back to the inlet polyethylene and polypropylene plants morketingot MTR,lnc., MenloPork, of the compressor, while the liquid is for the separation and recovery of Californiq,USA. pumped up to 300psig and sent to a stor- monomers and nitrogen from resin DouglosE. Gottschlichis MTR'sproduct age tank before recycle to the po$ethy- degassing vent streams. The systems pro- monagenpetrochemicals, responsible for lene process. vide many important benefits. Hydrocar- solesand new applicationsin the In addition to removal of CO, from the bon recovery is 90 per cent or higher, petrochemicalmorket. iso-butane liquid, the BRU was designed depending on the value of the RichqrdBaker is founderand presidentof to accommodate a feed flow rate rangrng monomers. Nitrogen recovery is opti- MTR.He gained a PhDin physicolchemistry from 4000 to 8000 pounds/hr. This mised, based on its value, and ranges in | 966 ot lmperiolCollege, London, and design feature was accomplished by from 65 to 99 per cent, with purity of gg holdsmore thon 80 potentson membrane maintaining a high condenser pressure vol%t. Flaring is minimised or eliminated processes.