Hydrothermal Carbonization of Waste Plastic & Syngas Plant Design

HTC - A Solution to a never ending wave of plastic waste?

This plant aims to reduce the impact of plastic waste on the environment by converting it into a char that can be sequestered in the ground. Of the 3 Million tonnes of plastic waste produced by Canadians annually, only 8% of it is of recyclable quality and 5% is incinerated. The remaining 86% or 2.6M tonnes of plastic waste is dumped into landfills. Plastic in landfills eventually breaks down into a mixture of microplastics and methane gas, which contribute to global pollution and increased greenhouse gas emissions.

The typical reason these waste plastics are considered "unrecyclable" quality is because they are dirty or contaminated, and sorting, pre-processing and cleaning them is just not commercially viable at scale.

The solution to this is to process and manage this plastic waste responsibly. This design proposes using Hydrothermal Carbonization as a method to convert the plastic into an inert form (a char) while simultaneously producing a hydrogen-rich syngas as a byproduct. The resultant carbon-rich char is inert and can be sequestered or used in a variety of filling applications. The main value of the process comes from the hydrogen-rich syngas which can easily be commercialized.

Background and Motivation for Plant Design
Hydrothermal Carbonization (HTC) Process Inputs and Outputs
Block Flow Diagram of HTC Plant

HTC Process Overview & BFD

The BFD shows the overall process flows of the system and the key design choices. A Limestone feed is required to neutralize the chlorine in the PVC. Despite the large water requirements of the system, we are able to minimize our utility costs by a 99% recycle ratio of water. This enables us to retain the heat in the water at a temperature of 300 °C.

PFD of Specific Unit Operations

This Process Flow Diagram (PFD) shows refinements and synergies captured in the design. Streams 201 and 207 run parallel to split the water flow from the boiler before combining again at the reactor. This split is to minimize the size of the mixer M-200. A dryer and conveyor belt were added to automate the extraction and transportation of char from screen filter F-500 to storage.

Piping & Instrumentation Diagram (P&ID) of Storage Tank Unit Operation
Piping & Instrumentation Diagram (P&ID) of Reactor Vessels

P&ID of Reactor Vessel

The detailed Piping and Instrumentation Diagram (P&ID) of the reactor vessesl indicates key control systems required for the safe and efficient operation of the plant. These systems inlcude flow control systems, level monitoring systems, pressure monitoring and temperature control and monitoring systems. Some safety features are the redundant pressure release valves and flow bypass valves.

Detailed Reactor Drawing & Design

The reactor is fitted with an ellipsoidal head to handle high pressures and a conical bottom to prevent settling of the fluid [4]. Due to gas generation from reaction, it is designed for liquid level to be ⅔ of the reactor’s height. A “skinny” reactor with 4:1 ratio of height to depth was chosen to ensure even mixing and more easily maintain the solids in a fluidized state. Reactor wall thickness was determined using the  hoop stress equation, dependent on the maximum allowable pressure for the material of construction, as outlined in the ASME VIII codes for high pressure vessels. A shell of 1060 Carbon Steel provides strength, while an inner glass lining prevents leaching of metal ions into reacting fluid. A demister is located at the top of the reactor head to precipitate water droplets trapped in the gas phase. Since the pellets/char are of a lower density than water, they would tend to float to the top of the reactor. To keep the pellets well mixed in the water and pushed down, a 3 impeller configuration with Pitched Blade Impellers (PBT) is used in a downward orientation.

Detailed Reactor Drawings & Design
Advantages of HTC Over Conventional Pyrolysis Methods

Why HTC? Are there alternatives?

Other methods exist to convert waste plastic into inert char. The alternative plastic pyrolysis methods exist but have their own disadvantages such as the production of oil and tar and requiring large amounts of inert gas, very high operating temperatures and incredibly low vaccuum pressures to operate. However, HTC is not perfect either, and is a much newer less studied process than conventional pyrolysis.

Final Report & Recommendations

The process includes the inputs of plastic waste (50 tonnes/day), water, and limestone, and targets a production rate of 35 tonnes/day of hydrochar with an overall yield of 70% through HTC. The byproduct, syngas, will be produced at a rate of 28.7 tonnes/day with this yield. This design incorporates configurations and decisions such as wastewater treatment, safety configurations for the equipment units, recycling the unreacted feedstock and cleanly burning any syngas.

After outlining the scope and the key units through a block flow diagram (BFD), a process flow diagram (PFD) was constructed to display all of these elements. A mass balance and extensive equipment sizing was done in accordance with this PFD. A piping and instrumentation diagram (P&ID) was constructed for the HTC reactor. A list of design and plant operation specifications was developed through applying occupational health and safety regulations as well as environmental regulations. 

The plant will be located in Mississauga, Ontario near the Disco Transfer Station, with site dimensions of 35 meters by 50 meters. A top view site layout was constructed depicting the equipment units, building area and production area.

An economic evaluation of the plant was completed to determine the profitability of the project. With a project life of 15 years, the predicted NPV is around $25.94M with an internal rate of return of 17.25% and a payback period of 2.1 years. The estimated plant capital expenditure (CAPEX) is $15.05M (including working capital) and the total cost of operation is $29M per year. The projected annual revenue is $42.4M per year. As this process generates a product with a higher NCV than its input (43 MJ/kg output; 35 MJ/kg input), meets all safety and environmental standards and regulations, and generates a positive NPV, CharTime recommends proceeding with this design.

CHE430 Group G18 - Final Report

If you would like to take a look at the mass balances or learn more about this project, please feel free to contact me at my email address down below!