(60c) Modeling and Simulation of a Process That Converts Ethane to Low Density Polyethylene

Low-density polyethylene (LDPE) is a versatile plastic that has become an essential material in daily life. Its importance lies in its unique properties, which make it suitable for a wide range of applications. LDPE is known for its flexibility, toughness, and resistance to chemicals, moisture, and UV radiation. It is used extensively in the packaging industry, including food packaging, as it provides an excellent barrier against moisture, air, and light. It is also used in the manufacture of household items such as garbage bags, toys, and even medical equipment. The global LDPE market was valued at approximately USD 33.3 billion in 2020 and is projected to reach USD 43.8 billion by 2027. The increasing demand for sustainable packaging materials and the rising demand for LDPE in the healthcare industry are some of the factors driving the growth of the LDPE market ^1,2 .

The feedstock, ethylene, for LDPE has traditionally been obtained from naphtha cracking. However, with the recent discovery of shale gas in the United States, there is an increasing interest in manufacturing LDPE from ethane, which is a significant part of the natural gas liquids in shale gas. In this research, a steady-state model is developed in the ASPEN Plus ³ environment that utilizes ethane as the primary feedstock to produce ethylene ⁴ and the subsequent polymerization of ethylene to low-density polyethylene (LDPE). Several simulations are conducted to determine the effect of feed conditions on the quality and quantity of LDPE. Heat integration tools are utilized to reduce the hot utility and cold utility usage in this integrated plant that leads to optimal operation ⁵ . The feed to the process is 150 tons per hour of ethane and 50 tons per hour of steam. The ethane is cracked to form ethylene at a temperature of 840 °C and pressure of 3.2 bar. Some methane and hydrogen is also formed. The outlet of the cracker is sent through a series of distillation columns to remove the excess water, methane, water, and hydrogen. The unreacted ethane is recycled back to the cracking reactor. The ethylene stream is sent to the LDPE reactor system for polymerization. The LDPE reactor system consists of four reactors in series that operate at a temperature ranging from 200 °C to 270 °C and a pressure ranging from 1700 bar to 2000 bar. Since the final product LDPE can get very viscous at high conversions, the reactors are designed to keep the conversion at 20%. The outlet stream from the reactor system goes through a two-stage pressure separation system where the pressure is dropped form 1700 bar to 1 bar. The final product composition is 99.934% LDPE and 0.066% ethylene. The final product will later be degassed of any residual ethylene then fed through an extruder to be pelletized. This process was simulated in ASPEN Plus. The thermodynamic property method used was Grayson Streed for the process to convert ethane to ethylene and Sanchez-Lacombe Polymers for the process to convert ethylene to LPDE. A base case simulation with no heat integration shows that the feed stream of 150 tons per hour of ethane and 50 tons per hour of steam results in a final product stream that consists of 15 tons per hour of LDPE that has a purity of 99.934%. It is observed that the hot utility requirement is 119 tons per hour and the cold utility requirement is 5150.10 tons per hour. A major advantage of considering an integrated plant is that it is possible to utilize heat integrating technology, to reduce the cost of hot and cold utilities. In particular, the use of a heat exchanger network (HEN) is considered in this research. The optimization of HEN is based on the objective function of minimizing the total utility costs and the minimum temperature approach adopted is 1°C. To optimize the heat exchanger units, heat exchange area and loads on each utility a mixed-integer-linear-programming (MILP) approach is used. Several feasible solutions are obtained using the MILP heat integration tool that is built-in ASPEN Energy Analyzer. Several simulations are conducted in this heat-integrated plant to study the effect of changing feed conditions on the product quality and quantity.

References
[1] American Chemistry Council. 2020. “Ethylene production in the United States from 1990 to 2019"; Guide to the Business of Chemistry 2020.
[2] American Chemistry Council. 2020. “Polyethylene production in the United States from 1990 to 2019"; Guide to the Business of Chemistry 2020.
[3] Aspen Technology. 2019. Low-Density Polyethylene High Pressure Process. Bedford, MA: Aspen Technology.
[4] Rosli, M N, and N Aziz. 2017. "Simulation of Ethane Steam Cracking with Severity Evaluation"; Second International Conference on Chemical Engineering 162.
[5] Zhu, Xin X. 1997. "Automated design method for heat exchanger network using block decomposition and heuristic rules"; Computers & Chemical Engineering Volume 21 1095-1104.