Figure 1: Exelys Continuous Thermal Hydrolysis System
Exelys is a continuous process that handles biosolids with a dry solids (DS) content greater than 20% w/w. The high solids content leads to a considerable savings of steam required to reach hydrolysis conditions, while the continuous operation allows for optimal energy recovery. Relative to traditional batch thermal hydrolysis, the system provides the same process conditions and hydrolysis product, but in a much more energy efficient manner.
Process Flow
The Exelys process is shown in Figure 1. Dewatered biosolids from a storage silo or hopper is continuously fed to the process via a progressive cavity pump. As the biosolids enter the system, steam is continuously added to the system through a series of injection nozzles. As the mixture moves up the steam condenser section, the steam condenses transferring heat to the biosolids and raising the temperature to a required level for thermal hydrolysis to occur.
The heated biosolids pass through a self-cleaning static mixer which ensures that the heat is distributed evenly resulting in a homogenous biosolids/water mixture. The static mixer will also catch any steam that has not yet condensed. This ensures that all of the energy available from the steam injected is utilized effectively, maximizing the energy efficiency of the process.
After the biosolids are at the required temperature and pressure for thermal hydrolysis, the biosolids enter the reactor and flow at a low velocity resulting in plug flow. This plug flow ensures there is no short-circuiting and all the biosolids are under thermal hydrolysis conditions for the required amount of time.
After the reactor, the biosolids enter a heat exchanger system where they cool to a suitable temperature for the downstream mesophilic or thermophilic digestion process without requiring supplemental heating in the digesters. The heat exchangers are a tube-in-tube design which allows for easy cleaning and maintenance. The cooling water from the heat exchanger system can be used for pre-heating boiler water, building heat or for other low temperature heat sources at the plant.
After the heat exchanger system, there are provisions to inject treated wastewater into the biosolids. The water injection can be used to dilute and cool the biosolids as required by the downstream digestion process.
The final component of the system is the pressure holding pump. This pump is controlled to maintain a pressure set point in the system. In most cases the pressure holding pump can also be used to feed the downstream digestion process.
