462318 Improved Olefin Polymerization Process with Reduced Energy Consumption
Carrying out olefin polymerization reaction in solution has some distinct advantages such as a homogenous environment to facilitate consistent fast reaction; solvent acting as natural heat sink to absorb heat of polymerization, and fast product grade transition. On the other hand, the need to remove and recover large amount of solvent used in solution polymerization reaction, and the requirement to meet target solvent residues in the final product, greatly increases the capital expenditure and energy consumption of the designed asset.
The conventional method to remove solvent in a solution polyolefin process is by heating up the polymerization reactor effluent to desired temperature, then flashing off the solvent through vapor liquid equilibrium (VLE) using multiple devotalization stages at different operating pressures, which normally require deep vacuum in the final vapor liquid separator. The amount of solvent that needs to be processed by the devotalization and solvent recovery section of the plant dictates the maximum production throughput, and leads to higher energy cost than other alternative olefin polymerization process.
The approach of applying Lower Critical Solution Temperature (LCST) behavior of polymer solution2, and subsequently inducing a “front end” liquid-liquid separator of polymer solution followed by one or two finishing vapor liquid separator, has been explored in the recent years. Under carefully designed temperature, pressure and solvent environment, a homogenous polymer solution may be split into two liquid phases: a polymer rich phase and a solvent rich phase. This almost isothermal separation process1 has some unique characteristics that can be advantageous to designing an energy efficient solution polyolefin manufacturing process. However, achieving sufficient and reproducible liquid-liquid separation can be difficult if the solvent composition does not possess the needed high volatility, or the system temperature is not substantially higher than the Lower Critical End Point (LCEP) of the reactor effluent.
This presentation explored the key thermodynamic aspects of polymer solution Liquid-Liquid Equilibrium (LLE), and described a unique approach to achieve effective liquid-liquid separation. Unlike the conventional process of using solvent with a fixed boiling point, It applies a mixed solvent composition of one high boiling point solvent, and a low boiling point solvent. The low boiling solvent acts as the anti-solvent that significantly alters the LCST behavior of the polymer solution and reduces the LCEP of the system by 40 ~ 60 °C. This mixed solvent approach enables effective polymer-solvent separation inside the liquid-liquid separator without the need to put out energy to heat up the polymer solution stream exiting the reactor. In addition, the amount of low boiling solvent (anti-solvent) can be adjusted and used as a control knob to ensure a consistent liquid-liquid separation outcome (i.e., similar polymer concentration in the polymer rich phase). The amount of low boiling solvent can also be used to adjust for changes in compositions or reactor conditions (e.g. different product design or buildup of inerts).
This new liquid-liquid separation based Olefin Polymerization Process3 brings a 35% reduction of the total plant energy input as compared to conventional technology using vapor liquid separator alone, because heat is added only to the polymer-rich phase that is downstream of the Liquid-Liquid Separator (LLS). In addition, the cooling requirement for the solvent-rich phase downstream from the LLS is also lower as no heat was used to heat up the reactor effluent. Moreover, this new design also brings capital savings as it enables the use of lower cost adiabatic reactors operating at lower polymer concentration.
1. TG. Gutowski, NP Suh, C Cangialose, and GM Berube, “A Low-Energy Solvent Separation Method,” Polym Eng & Sci 23:230-237, 1983
2. Freeman, P.I. and J.S. Rowlinson, Lower critical points in polymer solutions. Polymer, 1960. 1: p. 20-26.
3. P. Jog, Midland, R. Swindoll, N. Mead, P. Jain, A. Wang, J. Guzman, United States Patent 9018328, Polymerization process for olefin-based polymers.
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