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 poly- olefin 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 pres- sure 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 propy- lene 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.