Energy-Saving UHMW Polymeric Flow Aids: Catalyst and Polymerization Process Development

: Crude oil and refinery products are transported worldwide to meet human energy needs. During transportation via pipeline, huge pumping power is required to overcome the frictional pressure drop and the associated drag along the pipeline. The reduction of both is of great interest to industry and academia. Highly expensive ultrahigh molecular weight (UHMW, MW ≥ a million Dalton) drag reducing polymers (DRPs) are currently used to address this problem. The present paper, therefore, emphasizes particularly the development of a high-performance catalyst system that synthesizes DRPs (using higher alpha-olefins)—a highly promising cost reduction alternative. This homogeneous catalyst system features a new concept that uses a cost-effective titanium-based Ziegler–Natta precatalyst and a cocatalyst • Lewis base complex having both steric hindrance (around N heteroatom) and electronic effect. This novel work, which involves precatalyst–cocatalyst molecular separation and cocatalyst • monophenyl amine association-dissociation phenomena, already generated several US patents. The subject catalyst prepares UHMW DRPs at room temperature, avoiding the use of zero and sub-zero temperatures. The resulting product almost tripled the rate of transportation of a selected grade of refinery product and saved about 50% pumping energy at ppm level pipeline concentration. It is also very easily soluble. Hence, massive modification of existing pipeline will be unnecessary. This will save additional infrastructure cost. This paper also summarizes challenges facing the development of improved heterogeneous catalysts, dispersed polymerization process, molecular simulation-based DRP product formulation, and model/theory of turbulent mixing and dispersion in the transportation pipeline setting. precatalyst–cocatalyst–donor


Introduction
Crude oil and refinery products are transported using pipelines. Figure 1 shows the very high production level of global 2018 crude oil and refined product productions [1,2]. During transportation, the pressure drastically drops due to interfacial friction between the pipe wall and the flowing fluid. Consequently, the desired throughput cannot be maintained. Increasing the input pressure and introducing higher flow rates seem to be quick and facile solutions. However, design limitations on pipelines limit the level of pressure that can be applied. The pressure further drops when fluids are transported over long distances. Consequently, the equipment and operational costs increase. Therefore, the efficient transport of crude oils and refinery products is a great challenge [3][4][5][6][7]. To overcome this challenge, highly expensive ultrahigh molecular weight (UHMW, MW ≥ a million Dalton) drag reducing polymers (DRPs), which are commercially available, are commonly used. The DRP literature can be mainly divided into journal publications and patents. The first publication originated from Toms in 1949 [8]. He reported in the First International Rheology Congress that long-chain polymethyl methacrylate (PMMA) reduced the single-phase turbulent wall friction by 80%. This increased the fluid flow rate at constant pressure gradient. Since that time to the present, DRP-mediated fluid flow remains a very active research area. Consequently, DRPs are currently used to transport fluid in very long-distance intra-and trans-country pipe lines [9][10][11][12][13][14]. The subsequent journal publications mostly report about the hydrodynamic theories of drag reduction (DR) and the effects of DRP MW and stability, rheology, multiphase flow, hydrodynamic variables, conduit configuration, surfactant-based product formulation, etc. on the performance of commercial DRPs [14][15][16][17][18][19][20][21][22][23]. These references and the citations therein can be pursued for the details.
Commercial DRPs are made catalytically, mostly by polymerizing linear alpha-olefins. Therefore, we review catalyst development research in this area as follows. The DRP catalyst research is mostly covered by patents. The central concept comprises using the second generation α-TiCl3 Ziegler-Natta (Z-N) precatalyst with an organoaluminum cocatalyst [24,25]. Note that β-TiC13 only polymerizes diolefins but not mono-olefins, such as linear alpha-olefins [26]. Figure 2 shows the crystal structures of both forms of TiC13. All the patents modified the above basic catalyst system by introducing a Lewis base (LB) electron donor to block the active centers that catalyze the formation of undesirable low molecular weight poly(alpha-olefin) backbones. The referenced LBs can be divided into Type I LBs and Type II LBs. Type I LBs are non-phosphorous and Type II LBs are phosphorous. Type I includes ether [28], ketone [29,30], pyridine and piperidine [31], etc. Type II lists phosphine, phosphite, etc. [32]. Additionally, a third family of patents uses aluminoxanes alone, or combined with Z−N alkyl aluminum cocatalysts [33][34][35][36][37]. All these patents showed variation in components and composition. The resulting poly(alpha-olefin)s were mostly characterized by measuring the standard solution viscosity, which was used to confirm the formation of UHMW product. The cited patents show the following:  Low catalyst activity;  Requirements of 0 °C and below, and very long polymerization time (≥12 h) to synthesize the DRP;  Low drag reduction (∼13.4%); and  Use of hazardous catalyst adjuvant(s), and costly gel-prone aluminoxane cocatalysts.
The patents also did not report any catalyst characterization work, fundamentals, or insight. Therefore, enough room to develop high-performance catalyst and DRP polymerization process exists. Note that a high activity catalyst is a pre-requisite to develop cost-effective commercial DRP process because it increases yield and production volume. This is why the present study particularly addresses how to enhance DRP catalyst activity.

Present Patented Catalyst Design Concept and Composition
The catalytic synthesis of UHMW DRP has two requirements. One is kinetic and the other is thermodynamic. The kinetic requirement demands that the rate of chain propagation Rp should be, on principle, exceedingly large. On the contrary, the rate of chain termination Rt (originating from various active chain transfer reactions) should ideally approach zero. The thermodynamic requirement, in terms of activation energy, is just the opposite. See Figure 3. An embedded challenge in this context is the following. In organometallic-catalyzed olefin polymerization, the catalyst activity and the resulting MW are inversely related. How to strike a balance between both poses a challenge. Our patented catalyst design concept attempts meeting the above requirements and overcoming this challenge [23−25]. The above catalyst design concept is summarized here. To meet Figure 3 kinetic and thermodynamic requirements, we proposed to use (nucleophilic) Lewis base electron donors that featured the following [38][39][40]:


They simultaneously show steric hindrance and electronic effect. Steric hindrance is achieved by the bulky substituents around the heteroatom, which reduces the access of the related base functionality to the Lewis acid alkyl aluminum cocatalyst.
 Electronic effect is obtained by placing electron-withdrawing substituents on the heteroatom to reduce its electron density. Consequently, the (electrophilic) Lewis acid alkyl aluminum cocatalyst•Lewis base complex presumably dissociates (in a nonpolar medium) under the DRP synthesis condition, and consequently the catalyst productivity increases. The introduction of this dissociation phenomenon and the control of precatalyst-cocatalyst molecular separation are the key to the subject novel catalyst design. Aromatic substituents are especially useful because they are relatively inert toward other catalyst components.
Our patents illustrate the above overall catalyst constitutive coordination/complexation and precatalyst-cocatalyst molecular separation concept and structure, first by forming an alkylaluminum cocatalyst•Lewis base (LB) complex, and next by contacting this complex with the transition metal precatalyst to make the final catalyst [23][24][25]. A preferred example of the above LB is a tertiary monophenyl amine, belonging to the group comprising N,N-diethylaniline, N-ethyl-Nmethylparatolylamine, N,N-dipropylaniline, N,N-diethylmethylamine, and combinations thereof. Figure 4 shows the reference versus our patented catalyst structure, and compares the productivity of a given example of the former with that of the latter. The patented catalyst formulation PatCat, prepared as per the novel concept reported in Section 2.1, shows to be 1.4 times more productive than the reference catalyst RefCat. This can be explained based on the following postulations. First, the steric hindrance, due to N,N-diethylaniline, widens the precatalyst-cocatalyst molecular separation. The larger is this separation, the lower is the energy barrier associated with each copolymerization insertion and propagation step. This situation is analogous to ion-pair separation that (i) concerns metallocenium cation and methylaluminoxane or borate anion, and (ii) applies to olefin polymerization [41,42]. Second, the electron-withdrawing phenyl group decreases the electron density on N, which makes (diethyl aluminum chloride) DEALC•N,N-diethylaniline complex prone to dissociate. Resultantly, access of monomer (preferably, 1-hexene because of the smaller size) to the vacant coordination site, its insertion into the growing copolymer chains, and chain growth-all concertedly increase. See Scheme 1. Therefore, PatCat showed higher productivity than RefCat, and the resulting copolymer chain length also enhanced. Note that increased catalyst productivity increases production volume, which consequently reduces production cost. Therefore, our patented catalyst adds value to DRP process development and economics. Scheme 1. Insertion, propagation, and chain growth in 1-hexene-1-dodecene copolymerization mediated by PatCat. Figure 5 shows the effects of cocatalyst formulation type, and DRP MW and microstructure (from a qualitative perspective) on DR%. By DRP microstructure, we mean monomer/comonomer sequence and side chain distribution in the copolymer backbone. The following important findings are summarized below. First, compare the DR% of PatCat DRP with that of RefCat DRP. The former approaches that of the latter even for a much lower UHMW value. The MW of PatCat DRP is 1.70 million Dalton while that of RefCat DRP is 2.37 million Dalton. Scheme 2 illustrates the related major β-hydride chain transfer reaction that terminates the 1-hexene−1-dodecene copolymer chain length. The overall electronic environment and the steric hindrance around Ti affect the suppression of the above chain transfer reaction. This means that the patented catalyst is capable of regulating the microstructure of the 1-hexene−1-dodecene copolymer in such a manner that commensurate DR% can also be achieved using lower UHMW value DRP. Increased UHMW value DRP has been reported as a pre-requisite to improving DR% [5,8,9,12,13,15]. This work does not favor this argument. To the best of our knowledge, such a finding has not been reported in the literature.
The above finding has significant scientific value. This will motivate the concerned researchers to revisit the current mechanism of turbulent drag reduction and DRP rheology, which will set new directions to fundamental and applied research in this important subject. The 45% DR of the PatCat DRP signifies the following. The resulting product almost triples the rate of transportation of a selected grade of refinery product and saves about 50% pumping energy at ppm level pipeline concentration. The flow rate increase (FRI) was calculated using the following relation [26]: 1

100.
(1) Additionally, the use of increased UHMW DRPs has the following practical disadvantages. They dissolve less readily in hydrocarbon fluid than the lower UHMW value DRPs. The solution viscosity also increases with MW. Therefore, the injection procedure becomes cumbersome and more energy consuming. Therefore, we show how these disadvantages can be overcome through designing the catalyst in an appropriately novel fashion.
Second, evaluate the DR% performance of PatCat DRP against that of CP LP100 and SP PIB. CP LP100 is a poly(1-decene)-based commercial DRP formulation whereas SP PIB is an UHMW poly(isobutene) from Scientific Polymer Products Inc. Both products are homopolymers. This evaluation establishes that the as-synthesized PatCat DRP, despite having lower UHMW, shows comparable to higher DR% without making a DRP formulation. This is another manifest of the merits of our patented catalyst and the UHMW 1-hexene-1-dodecene DRP it synthesized. This improved PatCat DRP performance may be attributed to the important role that particularly the longer and bulkier n-decyl branch of the above 1-hexene-1-dodecene copolymer plays, under such a situation, in the following areas [5,8,9,12,13,15]: The present catalyst-mediated copolymerization route is apt to shift the DRP synthesis process from 0 °C and below to room temperature and above.

Materials and Methods
All the procedures, related to catalyst formulation, DRP synthesis, and drag reducing (DR) performance evaluation, are detailed in our patents [38][39][40]. Therefore, they were only summarized here (as appropriate). A local kerosene was used as the transportation fluid.
Toluene was demoisturized by contacting it overnight with 4A molecular sieve, activated at 230 °C. The toluene Schlenk flask (containing the molecular sieve) was shaken from time to time, evacuated, and purged with argon until the moisture level dropped to ≤10 ppm.
These DRPs were synthesized using a computer-aided AP Miniplant polymerization reactor equipped with a 1 L Büchi glass reactor (Flawil, Switzerland). First, the reactor was thoroughly cleaned and baked at 120 °C for 2 h; then, it was cooled to room temperature. The required amount of toluene was added to the reactor by a syringe. Of 1.0 M triisobutyl aluminum (TIBA) 1.0 mL was added to toluene as the scavenger. The precatalyst solution and the cocatalyst formulation were prepared in dried toluene inside a highly inert glove box. All additions and manipulations were done under inert argon environment inside the reactor. In the RefCat [9,10], the DEALC:(TiCl3•1/3AlCl3) ≅ 2-4 while in the PatCat, an equimolar N,N-diethylaniline was premixed with DEALC. The copolymerization of 1-hexene with 1-dodecene was conducted at equimolar ratio at 20 °C for 5 h. The reaction was quenched using acidic methanol. The copolymer concentration in the reaction mixture was determined by drying a given volume of it at 40 °C in a vacuum oven.
A Polymer Laboratory gel permeation chromatography (GPC) instrument (Salop, UK) measured the average molecular weight of the as-synthesized DRPs. The column temperature was set at 135 °C.
The DRP sample (about 1.0 mg), taken in a 1 mL vial, was dissolved in 1.0 mL butylated hydroxy toluene BHT-stabilized 1,2,4 trichlorobenzene (TCB) as follows. The vial was shaken first in a regular hot-plate shaker, then in the warming compartment of the GPC instrument-both at 135 °C-for about 5 h to completely dissolve the sample.
Before injecting a sample, the differential refractive index (DRI) detector was purged for 4 h using TCB (1 mL/min) to obtain stable baseline. The inlet pressure (IP) and the differential pressure (DP) outputs were also purged for 1 h. The above flow rate of TCB was used, and each sample was analyzed for 35 min.
The instrument was calibrated using nine polystyrene standards whose peak molecular weights ranged from 1530 to 15 million Dalton. The MWs of the as-synthesized DRPs were determined using this calibration curve, the corresponding chromatograms, and Polymer Laboratory GPC software.
where Tg1 and Tg2 are the glass transition temperatures of poly(1-hexene) and poly(1-dodecene), respectively. MW1 is the molecular weight of 1-hexene (84.16 Dalton) whereas MW2 is that of 1dodecene (170.33 Dalton). The molecular size of 1-dodecene is twice that of 1-hexene. Therefore, the insertion of 1-dodecene in the growing copolymer chain will be restricted due to diffusion limitation. A maximum of 5 mole% average 1-dodecene incorporation will be a reasonable assumption. Hence, the Tg of the assynthesized 1-hexene-1-dodecene copolymers are expected to be limited as −52.00 °C ≤ Tg ≤ −48.60 °C.  The DR performance of the experimental α-olefin copolymers was evaluated, using the above test facility, as follows. A diaphragm pump injected the concentrated DRP master solution into the pipeline transportation liquid (a local kerosene). The DRP was carefully introduced after the oil pump to avoid polymer degradation (by the rotating pump impeller) and achieved good mixing before entering the pressure measurement section. A Vortex Flowmeter (DN15 Sandwich) measured the volumetric flow rate of the kerosene up to 85 L/min. A check valve, placed after the flow meter, prevented the liquid back flow. On the other hand, a needle flow control valve, installed before the flowmeter, adjusted the flowrate. A built-in data acquisition system recorded the pressure drops (at 1.5 m and 1.0 m length along the flow pipe), temperature, and the corresponding volumetric flowrate.
Before DRP performance evaluation, the test loop was calibrated to evaluate system reliability, using a selected grade of kerosene. Figure 8 shows the related calibration curve that plots the pressure drop per unit length (ΔP/L) as a function of the Reynolds number (Re). It has two parts-linear laminar region for Re < 2000 and the nonlinear turbulent region for Re > 3000. The linear laminar flow behavior confirms the accuracy of the pressure transducer at very low flowrate. However, it does not apply to evaluate the performance of DRP, which reduces the frictional drag only in the turbulent flow region. ΔP/L was calculated assuming a fully developed steady state Hagen-Poiseuille flow in the loop pipe and using Equation (6) [48]: where f is the friction factor; D is the tube inner diameter (ID); ρ is the fluid density; and V is the average fluid velocity in the pipe. Equations (7) and (8) where ks is the surface roughness of the stainless steel pipe which was provided by the manufacturer (AP Miniplant Germany). Equation (9) defines Re as follows: where μ is the dynamic viscosity of the experimental liquid. The thermocouple, placed at the tube discharge and connected to the data acquisition system, measured the corresponding temperature.
The dynamic viscosity at this temperature was used to calculate Re.  Figure 9 shows the calibration curves without and with a polyisobutene (PIB) DRP. In both figures, the experimental (measured) and calculated ΔP/Ls (estimated using Equations (6)-(9)) matched very well. The dosage pump was calibrated from the lowest to the maximum range before the experiment. The DRP performance was evaluated at 10 L/min, which gives an average velocity of about 2.1 m/s (6.8 ft/s) in the pipe and Re = 18,850. This corresponds to highly turbulent flow, typical of a fluid transportation pipe. The DRP flow rate was gradually increased by applying the dosage pump. The concentration of the experimental DRP in the pipeline was calculated using the following: The %DR was determined using the following pressure drop relation [50][51][52]: % Drag reduction = 100 where ∆PRDF and ∆PPF are the pressure drops (corresponding to the same flow rate) over a given length of pipe for the experimental fluid with and without the DRP, respectively.

Future Challenges and Research Niches
This section addresses future challenges and research niches in DRP catalyst and process development, surfactant-based product formulation development, and turbulent mixing and dispersion models in transportation pipeline and DRP injection. These are categorically summarized below.

DRP Catalyst and Process Development
The developments of high-performance DRP catalysts and processes are inter-related. The current process and catalyst mostly synthesize a single-phase solution product. They require a very low temperature and lengthy polymerization time. Undesirable rod-climbing effect occurs with progression of polymerization due to stirring as viscosity develops and normal stress exceeds tangential stress. This adversely affects macro-and micromixing of the highly viscous reaction mixture. Consequently, consistency in product quality deteriorates. The resulting product dissolves in the alpha-olefin comonomers and the solvent used. This prevents achieving in-situ particulate morphology (during polymerization), which is a major catalyst-cum-process development challenge. Such an as-synthesized DRP morphology can make significantly improved product formulation.
Comprehensive investigation of precatalyst-cocatalyst-donor complexation and dissociation behavior, precatalyst-cocatalyst molecular separation, and product microstructural characterization can further elucidate the catalyst and DRP process performance. These subjects have been either unfortunately undone or unpublished.

Surfactant-Based DRP Formulation Development
The UHMW DRP, dissolved in a suitable solvent, is a highly viscous product. The product development challenges list the following:


Viscofication-free pumpable and stable slurry;  Tendency to aggregate, gel, and layer with the passage of time;  Phase separation instability;  Transportation inconvenience;  Inadequate DRP dissolution within the turbulent mixing length; and  Difficulty in achieving very high extensional viscosity. The currently used cryogenic and grinding process-a prerequisite to DRP formulation-is very cumbersome and costly. This requires maintaining the storage and transportation temperature above certain critical value. Heated injectors are also to be often used to introduce the DRP into the transportation line. Dipropylene glycol monomethyl ether (DPGME) is widely used. This oxidizes readily in air to form unstable peroxides that may explode spontaneously. Most seriously, cryogenic grinding destructs the UHMW polymer backbones. Consequently, the DRP performance significantly decreases.
To overcome the above problems, the DRP must be transformed into a formulated dispersion/emulsion that should comprise the following components. The DRP, devoid of the O-H bond, itself makes the dispersed nonpolar phase. This is, in turn, mixed with a liquid carrier (LC) and dispersion aids. The LC is polar (having an O-H bond), and it does not dissolve the DRP but may swell it. It consists of several mutually miscible constituents such as alcohol, glycol, and glycol ether, all of which contain the O-H bond. The glycol and glycol ether may act as co-dispersant, too. The dispersion aids include partitioning agent, wetting agent, dispersant, and nonionic surfactant. Depending on the physical form, chemical structure, and temperature, the dispersion aids may be miscible to soluble in the polar liquid carrier. Therefore, the overall product formulation, under such a situation, is a nonpolar-polar (O/W) emulsion/dispersion. As per the above summary, the interaction among a selected nonpolar DRP, polar liquid carrier, and dispersion aids must be evaluated as a function of composition and temperature. The interaction between a liquid carrier and a dispersion aid can be a sub-set of this study. In particular, the following should be determined or modeled [53]:

Turbulent Mixing and Dispersion Models in Transportation Pipeline and DRP Injection
The DRP pipeline injection problems include the following:  Inconsistent pumping and feeding;  Inadequate dispersion and mixing under turbulent fluid flow; and  Turbulence-and shear-induced mechanical degradation. Models that can illustrate particularly inter-phase interaction, dispersion, and mixing of DRP with the fluid of interest under turbulent flow dynamics are fairly inadequate [54,55]. Research in this area can significantly improve DRP injection methodology.

Conclusions
Crude oil and refinery products are transported worldwide to meet human energy needs. During transportation, the fluid pressure drastically drops in the pipeline due to interfacial friction between the pipe wall and the flowing fluid. Highly expensive ultrahigh molecular weight (UHMW, MW ≥ a million Dalton) drag reducing polymers (DRPs) are currently used to address this problem. DRPs are synthesized using linear alpha-olefins and second generation Ziegler-Natta catalysts. A high performance catalyst can develop a cost-effective commercial DRP process. This is why the present study particularly addresses how to enhance DRP catalyst activity.
The novel concept underlying the present catalyst system, which involves precatalyst-cocatalyst molecular separation and cocatalyst•monophenyl amine association-dissociation phenomena, has been patented, schematically illustrated, and experimented. The premier contributions of this catalyst system are as follows. Its activity was 1.4 times that of the reference catalyst. The resulting 1-hexene-1-dodecene DRP copolymer almost tripled the rate of transportation of a selected grade of refinery product and saved about 50% pumping energy at ppm level pipeline concentration. This catalyst also regulated the copolymer microstructure in such a manner that a lower MW value UHMW product could also achieve commensurate DR%. This finding will set new directions in drag reduction and DRP rheology research. The current catalytic copolymerization shows promise to shift the DRP synthesis process from 0 °C and below to room temperature and above. Future research challenges and opportunities in overall DRP research have been outlined.

Perspectives
The background and importance of this research well aligned with a key part of Saudi Arabia's Vision 2030-conversion of local raw materials into value-added products. Saudi Aramco at present imports a huge volume of highly expensive DRPs for transporting crude oil and refinery products despite several basic materials being locally available. Almuttahida (Jubail United Petrochemical Company) produces the required alpha-olefins and Saudi Organometallic Chemicals Company (SOCC Al-Jubail) makes the related alkyl aluminum cocatalyst. Hence, several compelling success factors are already available. To develop, therefore, an indigenous DRP technology using Almuttahida alpha-olefins and SOCC cocatalyst is appropriate. We envisioned to build a world-class DRP research program at KFUPM (to be led by the Center for Refining and Petrochemicals). This will eventually serve a great national cause. We anticipated Saudi Aramco would continue to play a critical collaborative role in this regard. It may be noted that KFUPM and Saudi Aramco (currently holding 70% of SABIC share) are neighbors. Both are under the Saudi Ministry of Energy, and DRP research concerns energy conservation.

Funding:
We highly appreciate Saudi Aramco for supporting this research through PN CRP02253 under University Collaboration Program (UCP). The donation of selected higher alpha-olefins (1-hexene and 1dodecene) by Almuttahida SABIC is also thankfully acknowledged.