Practical Oil Spill Recovery by a Combination of Polyolefin Absorbent

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Practical Oil Spill Recovery by a Combination of Polyolefin Absorbent and Mechanical Skimmer Changwoo Nam, Houxiang Li, Gang Zhang, Logan Robert Lutz, Behzad Nazari, Ralph H. Colby, and Mike Chung ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b02322 • Publication Date (Web): 24 Jul 2018 Downloaded from http://pubs.acs.org on August 1, 2018

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ACS Sustainable Chemistry & Engineering

Practical Oil Spill Recovery by A Combination of Polyolefin Absorbent and Mechanical Skimmer Changwoo Nam†‡, Houxiang Li†, Gang Zhang†, Logan R. Lutz†, Behzad Nazari†, Ralph H. Colby†, T. C. Mike Chung*,† †Department of Materials Science and Engineering, The Pennsylvania State University, 302 Steidle Building, University Park, Pennsylvania, 16802, United States ‡Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), 409 Environmental Building, Pohang 37673, Republic of Korea

*E-mail: [email protected] Telephone: 814-863-1494 Fax:814-863-3457

Keywords: LLDPE, i-Petrogel, Oil absorbent, Oil spills, Oil recovery, Oil/Water separation.

ABSTRACT The objective of this research is to develop a practical and efficient oil recovery method that can be applied in large-scale crude oil spills in open water. The method is centered on a newly developed polyolefin oil-superabsorbent, called "i-Petrogel", in conjunction with existing mechanical (skimmer) recovery method. The i-Petrogel absorbent can be produced in large scale (>90 kg in our laboratory) by mixing two polyolefin polymers, including a semicrystalline

LLDPE

thermoplastic

and

a

thermally-crosslinkable

poly(1-decene-co-

divinylbenzene) (D-DVB) elastomer, to form an interpenetrated network (IPN) structure with porous morphology. In two practical tests, i-Petrogel with a specific composition (LLDPE/DDVB:1/1 weight ratio) shows selective absorption of ANS crude oil on open water surface with fast kinetics and high absorption capacity of more than 40 times that of the polymer weight. In fact, i-Petrogel effectively stops the crude oil weathering process (evaporation, emulsification, and spreading) in open water. Furthermore, based on our rheological findings

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on the resulting gel adducts, we propose that one can effectively recover these gels by a drum-skimmer in both warm and cold condition. The recovered oil/i-Petrogel adducts (hydrocarbons) contain almost no water, which can be refined as the original crude oil using regular refining processes (instead of chemical wastes). Overall, i-Petrogel technology potentially provides a comprehensive solution for combating crude oil spills in open waters, with dramatic reduction of environmental impact.

INTRODUCTION When an oil spill occurs on the water, the first hours of action are most critical in determining the effectiveness of later recovery, removal, and disposal, as well as the level of environmental impact.1,2 The ideal action shall immediately halt oil weathering (i.e. spreading, evaporation, and emulsification).3 Currently, the most common first action is the deployment of floating booms, which cannot prevent the evaporation of lighter or more volatile hydrocarbons within the oil mixture nor emulsification to form small oil/water droplets, hampering recovery and cleanup processes.4 Furthermore, the efficiency of both booms and skimmers are highly dependent on weather conditions.5 While most booms perform well in gentle seas, the forces exerted by currents, waves, and wind may impair the ability of a boom to hold oil. In choppy water, skimmers tend to recover more water than oil. During the 2010 BP Deepwater Horizon oil spill in the Gulf of Mexico, the decision to use 2 million gallons of dispersants in the Gulf amounted to an environmental trade-off with long term concerns below the surface.6 Based the estimations, the majority of the BP oil spilled remained in the water and conceded as pollutants. Only about 10 % of spilled oil was removed by mechanical recovery. The recovered oil itself generated about 80,000 tons of solid waste from soiled booms and more than 956 bbl of oily liquid waste (mixed with water).7 According to several 2


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reports, bacteria played a pivotal role in breaking down oil deposits in the Gulf of Mexico.8 Thanks to warm conditions, the bacteria was able to work much faster.9 However, in colder temperatures (near or below 0 °C) the oil-eating bacteria does not show any detectable hydrocarbon-degradation activities. The application of dispersants shall not be an option in icy cold water environments.10,11 With the increasing oil exploration activities in Artic circle, it is important to improve the recovery technology that can actually remove the spilled oil from the waters.12 Oil sorbent provides a physical method to remove spilled oil from water. Various sorbent materials have been applied in oil recovery, often used to remove the oil in areas that cannot be reached by skimmers. Most of them show limited oil sorption capacity and also absorb water, thus making the recovered solids unsuitable for calcinations and are often disposed on land. It would be ideal to be able to recover oil from spills and reuse it as the regular crude oil.13,14 Due to economic and environmental concerns, the most common sorption materials used are natural organics (straw, wood/cotton fibers, wool-based materials, etc.), natural inorganics (i.e. zeolites, silica aerogel, calcium fly ash, etc.)

15–20

biopolymeric and

supramolecular sorbents.21–25 However, they show limited oil sorption capacity and also absorb water, most of them end up in the landfills. In synthetic polymer sorbents, melt blown polypropylene (PP) pads and booms are the most commonly used oil sorbent materials, which can be cleaned and reused several times.26,27 However, due to highly crystalline polymer with impenetrable crystalline domains, PP sorbents exhibit adsorption mechanism with sorption mostly happened on fiber surfaces with limited sorption capacity. In the past decade, a great deal of research efforts has been devoted in developing new sorbent materials with some success.28–32 However, they are still far from the practical application for large-scale crude oil spill recovery in the open waters. Few of them had ever been scaled up for field tests. In our previous paper, we showed a systematic study to examine hydrocarbon (oil) 3



absorption and swelling behaviors in polyolefin (pure hydrocarbon) networks, without detectable water absorption.33,34 However, the trend doesn’t follow with complicate hydrocarbon mixtures, especially crude oils. Most of crude oil absorption curves were collapsed into a low absorption profile, with 40 times that of polymer weight. This polyolefin-based absorbent has also been scaled up to a few hundred kilograms quantity for conducting several large-scale field tests, especially examining its crude oil absorption profiles under various weather conditions and the mechanical recovery capability of the resulting oil/polymer (gel) adducts by using a commercial drum-skimmer. We correlated the rheology of this oil/polymer gel adducts with skimmer recovery efficiency. Furthermore, we also examined the effects of iPetrogel absorption in stopping the weathering of spilled crude oil on water surface, as well as the recyclability of the recovered oil/polymer adducts. Our objective is to develop a practical and efficient recovery technology for mitigating the environmental impacts due to large-scale crude oil spill in open waters. EXPERIMENTAL SECTION Materials and Instrumentation. The following reagents, including TiCl3.AA, AlCl2Et (25 wt% in toluene), methanol, and HCl were used as received. Both 1-decene and divinylbenzene (DVB) were distilled over CaH2 reagent under vacuum. Toluene was used after drying via solvent purification system (Puresolv MD5, Inert Company). Petroleum was 4


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purchased from Sigma-Aldrich (Switzerland) and used without additional purification. Linear low-density polyethylene (LLDPE) copolymer were kindly provided by DOW chemical company. Poly(1-decene-co-divinylbenzene) (D-DVB) copolymers (laboratory scale) were prepared by following the same procedures reported in our previous paper.35 Alaska North Slope (ANS) crude, provided by US Department of Interior, Bureau of Safety and Environmental Enforcement (BSEE), was used as received. All 1H spectra were recorded on a Bruker AM-300 instrument in chloroform-d at room temperature. The thermal properties of the polymers were measured by differential scanning calorimetry (DSC) using a PerkinElmer DSC-7 instrument controller with a heating and cooling rate of 20 °C/min under nitr ogen. The molecular weight of the polymer was determined using a Waters GPC. The columns used were Phenomenex Phenogel of 105, 104, 103, and 500 Å. A flow rate of 0.7 ml/min was used, and the mobile phase was THF. Narrow molecular weight polystyrene standards were used to estimate the molecular weight. Scale-up synthesis of D-DVB copolymer. The copolymerization was conducted in a 20 L autoclave equipped with a mechanical stirring. Under nitrogen atmosphere and ambient temperature, the reactor was charged with 10 L of toluene, 3 L of 1-decene, and 20 ml of divinylbenzene (DVB). About 5 g of TiCl3(AA) and 40 ml of AlCl2Et in 150 ml of toluene were stirred for about 20 minutes in an argon-filled dry box and then was introduced under nitrogen pressure to the reactor to initiate the polymerization. After about 1 hour at 45 °C, the polymer solution was discharged into a vessel containing 5 L of dilute HCl solution in methanol to terminate the reaction. The precipitated polymer was isolated by filtration and was washed completely with methanol and dried under vacuum for about 8 hours. The overall polymer yield was 84%. The resulting D-DVB copolymer was completely soluble in common organic solvents, including toluene and decalin. Its molecular structure was determined by a combination of 1H-NMR (Bruker AM-300 instrument in chloroform-d) 5



and GPC measurements (Waters 1515 Isocratic HPLC pump with Waters 2414 refractive index detector). 1H-NMR spectrum shows the composition of 1-decene/divinylbenzene mole ratio = 99.53/0.47 (Figure S1), and the GPC curve indicates the copolymer has a weight average molecular weight of above 500 Kg/mol and a polydispersity index (Mw/Mn) of about 3.5. Fabrication of i-Petrogel absorbent. In a typical fabrication of i-Petrogel film, a specific quantity of D-DVB copolymer was dissolved in toluene and then mixed with LLDPE (20 wt % in toluene) with a pre-decided weight ratio under an elevated temperature condition. The homogeneous solution was casted into films and dried in a dry oven at 150 °C under air for 15 min, then cooled down to ambient temperature. Under the elevated temperature condition, the DVB units located along the D-DVB polymer chain engaged in a thermal cyclo-addition reaction to form a crosslinked polymer network. On the other hand, the LLDPE polymer chains presented in the formed crosslinked x-D-DVB network were crystallized during the cooling process, which formed many small crystallites in the LLDPE polymer chains to serve as physical cross-linkers to create another polymer network. In other words, during the film fabrication process both x-D-DVB and LLDPE polymer chains are intertwined and in situ crosslinked to form an IPN structure. The resulting i-Petrogel material is not soluble (but swellable) in the hydrocarbon media. The thickness of the prepared film samples was between 0.2-0.3 mm. Typically, the film was cut into several pieces (about 0.2g /each) for oil absorption testing. Rheology. To conduct the rheology tests, i-Petrogels with specific compositions were swollen in petroleum, mostly with swelling ratio of 40 and a few with swelling ratio 20 for comparison. The rheology measurement was conducted in a Discovery Hybrid Rheometer (DHR3) from TA Instruments with a concentric cylinder’s geometry (bob diameter: 28.03 mm and cup diameter: 30.37 mm), as well as a cone-plate geometry (diameters 60 mm, cone 6


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angle 1°, and truncation gap 28 µm). The goal was to rheological test multiple samples in order to determine the best i-Petrogel composition and structure that is not only offering good oil absorption capacity but also the desired viscoelastic characteristics for mechanical recovery using drum skimmer. Oil adsorption capacity test. Oil absorption evaluation was conducted in the laboratory following ASTM F716 method. About 0.2 g of i-Petrogel flakes were immersed into a vial with 20 ml of ANS crude oil. The solid flakes were quickly swelled with expanding volume to become gel material. After 2, 4, 6, 8, and 24 h, respectively, the free oil, not absorbed, was drained from the vial and the vial was weighed on a balance (W2). The maximum oil capacity was measured at 24 h, in which the absorption capacity reaching to stable state. The capacity was calculated by the weight ratio between the adsorbed oil (W2-W1) to the originally unabsorbed i-Petrogel material (W1). In order to obtain the profile of oil swelling kinetics, the above measurements were carried out from time to time. Field test at Ohmsett facility. The field test was conducted in Ohmsett (National Oil Spill Research & Renewable Energy Test Facility located in New Jersey, USA) under various operational conditions, simulating the weather conditions that i-Petrogel might encounter in the large crude oil spill on the ocean. The objective was to evaluate its oil absorption properties and its ability to be recovered by mechanical oleophililc drum skimmer. Known volume of Fresh ANS oil was dispensed onto the sea water surface. A known weight of iPetrogel flakes (with weight ratio of oil/i-Petrogel= 40:1) was then administered to the oil surface, then i-Petrogel was allowed to absorb oil for a pre-determined time. After specific time, an Elastec drum-skimmer was utilized to recover the oil/polymer adducts. RESULTS AND DISCUSSION As discussed, polyolefin (pure hydrocarbon polymer) is suitable for selective absorption

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of low molecular weight hydrocarbons in water. However, both polyolefin networks along, either semi-crystalline thermoplastics or amorphous crosslinked elastomers, showed significant limitations with crude oils (viscous complex mixtures) due to slow kinetics. In semi-crystalline polymers (such as LLDPE copolymers), the crystalline domains provide stable structural junctions between polymer chains, but also limit its absorption (swelling) capacity. On the other hand, the lightly cross-linked amorphous D-DVB elastomer network can offer the maximum oil swelling ability and achieve high oil absorption capacity, following Flory-Rehner polymer swelling theory.36 However, it is too soft (somewhat sticky) and cannot form stable porous morphology or fine powders to improve the absorption kinetics with complex or viscous crude oils. This paper discusses a new approach investigating

a

homogeneous

polyolefin

blend

between

LLDPE

semi-crystalline

thermoplastic and D-DVB cross-linkable elastomer. They form an IPN structure as illustrated in Figure 1. Both LLDPE thermoplastic and D-DVB elastomer polymer chains are homogeneously intertwined and crosslinked (chemically or physically) to prevent phase separation and dissolution in low molecular weight hydrocarbons (solvents, refined oil products, or crude oils).

Figure 1. A schematic representation of i-Petrogel IPN molecular structure. Preparation of i-Petrogel absorbent. One of the major tasks in this research program is to

8


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prepare i-Petrogel with IPN structure and scale-up the material production for the practical evaluation of oil spill recovery at the Ohmsett test facility. The preparation of i-Petrogel absorbent involves two steps, including the synthesis of D-DVB copolymer and the subsequent mixing/crosslinking between D-DVB copolymer and commercially-available LLDPE to form an IPN structure with porous morphology. The copolymerization reaction between 1-decene (D) and divinylbenzene (DVB) was carried out by using heterogeneous Ziegler-Natta catalyst (TiCl3.AA/AlCl2Et; AA: activated by aluminum). As discussed, the desirable D-DVB copolymer shall exhibit high molecular weight with very low DVB (crosslinker) content, only few DVB units per polymer chain. Table 1 summarizes a set of comparative reactions that were carried out under various reaction conditions, with N2 atmosphere at ambient temperature for one hour. They include a small-scale (laboratory) reaction (run 1) and three pilot-scale reaction runs (runs 2-4) using a 20 L autoclave reactor that is equipped with several control units for chemical feeding, heating/cooling, and agitation. In these pilot-scale reactions, the amount of Ziegler-Natta catalyst was systematically reduced to control the reaction temperature and understand the suitable quantity required for this copolymerization reaction.

Table 1. Summary of 1-decene/DVB copolymerization reactions to form D-DVB copolymers. Run 1 2 3 4 a. b. c.

TiCl3.AA (g) 0.1 7.5 5.0

AlCl2Et (ml) 0.8 60 40

Reaction Conditions Cat. 1-D a ratio (L) 100% 0.1 75% 10 50% 10

2.5 20 25% 1-D: 1-Decene Determined by 1H NMR spectrum. After 1 hr reaction.

10

DVB (ml) 0.2 20 20

Temp/Time (°C/hr.) 45/1 60/1 62/1

20

45/1

Polymerization Results 1-D a DVB Yield (mol%)b (mol%)b (%)c 99.74 0.26 70 99.44 0.56 68 99.50 0.50 87 99.68

0.32

84

Usually, the formulation and reaction condition in a small lab-scale reaction is difficult to

9



directly transfer to the pilot-scale reaction, due to heat transport, mixing, impurity content, etc. Our attempts were to have a good control of reaction temperature and agitation in the pilot reactor, so that we could consistently prepare large-scale D-DVB copolymers with predictable mass yield and copolymer composition. All resulting D-DVB copolymers were examined by 1H NMR and GPC to determine their compositions and polymer molecular weights, respectively. They are all high molecular weight polymers with Mw >500,000 g/mol and Mw/Mn ~4. However, the DVB cross-linker concentration in the copolymer is directly associated with the reaction temperature, higher the reaction temperature higher the DVB incorporation. Figure 2 compares the reaction profiles (reaction temperature and polymer yield) of four copolymerization reactions shown in Table 1, with the reduced catalyst concentration in the scale-up runs. Comparing with Run 1 (laboratory-scale reaction), the catalyst ratio was 75 % in Run 2, 50 % in Run 3, and 25 % in Run 4, respectively.

Figure 2. The kinetic profiles of (a) reaction temperature and (b) polymer yield after one hour during the D/DVB copolymerization with different catalyst concentration. As expected, the reactor temperature was raised rapidly in the beginning of Ziegler-Natta mediated polymerization, due to the highly exothermal α-olefin polymerization reaction. The sharp rates increase in Runs 2 and 3 are indicative of vigorous polymerization reactivity. The reaction temperature becomes constant after reaching to high monomer conversion (about 10


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one hour). It is essential to lower the reaction temperature to obtain high polymer molecular weight and the reduced DVB content in D-DVB copolymer. Despite the combination of good agitation and cooling system in the pilot reactor and the reduction of catalyst concentration, we were only able to control the steady temperature at ~60 °C in Runs 2 and 3, which were higher than that (52 °C) in Run 1 small-scale reaction. As the consequence, the DVB content in the resulting copolymers was 0.42 and 0.46 mol% for Runs 2 and 3, respectively, which are more than 50% higher than that (0.26 mol%) in Run 1 small scale reaction. Thus, we further reduced the catalyst concentration in Run 4 to only 25% of Run 1, in which we were able to control the entire polymerization temperature

Practical Oil Spill Recovery by a Combination of Polyolefin Absorbent

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