A similar process is in use today. Cellullose molecules are polymer chains of different forms of cellulose bound together with lignin. The process works by de-polymerizing the lignocellulose, freeing the cellulose from the lignin, which is then hydrolyzed to the simpler sugars for fermentation to alcohol. The process uses acid as a catalyst.
Dilute acid may be used under high heat and pressure, or concentrated acid can be used at lower temperatures and pressure. The mixture must be neutralized and cleaned, and yeast fermentation is used to produce alcohol.
Many chemical processes work better using subcritical or superheated water under pressure. These conditions have been used in the chemical and food industry for more than 180 years. Examples include dilute-acid hydrolysis of cellulose and starch to saccharides, the extraction of instant coffee, extracting indigo dye from woad, and treating wastewater sludge through wet-air oxidation.
In all of these applications, the process used has generally remained a batch procedure where the water is pumped into a pressure tank and a heat exchanger. After treatment the resulting liquid is returned through the heat exchanger, which pre-heats the inflow which moves through a pressure-regulating valve before being released to normal pressure. The process depends on energy- intensive mechanical and electrical pumps and pressure tanks. It has mostly been used for small-scale production since the mixing requirements and the need to add chemicals while maintaining temperature and pressure limits the potential for scale-up.
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In 1967 James Titmas modified the process with the aim of making the best use of the pressure and heat from the subcritical water process. His goal was to convert biomass to useful materials using wet oxidation, pyrolysis and hydrolysis. To accomplish this, he placed the pressure vessel below ground in a borehole. Using gravity and heat from the process minimizes the amount of energy needed and creates continuous flow. Another advantage is that being underground creates an environment for efficient thermal insulation, a small plant footprint, and improved health and safety.
To obtain the natural pressure needed to maintain the temperature in subcritical water, the reactor has to be placed no more than 7,200 feet underground. This is within the capabilities and expertise of the oil drilling industry. The accuracy and skill of drilling wells vertically, straight and lining them to preserve water aquifers is also well proven.
The technique has been proven in use. The U.S. EPA and Bow Valley Energy used a 4,200-foot deep vertical-tube reactor based on a 1982 patent for the wet-air oxidation of sewage sludge with heat recovery at Longmont, Colo. After modification, parts of this plant were moved to Apeldoorn, Netherlands, where it was used to treat sewage sludge from 1992 to 2004. The plant out-performed its design expectations. In its later years the use of Taylor bubble and heat recovery was abandoned in favor of product recovery following the Titmas approach.
The gravity pressure vessel provides a simple way of making the subcritical water process continuous. It uses the heat released from the controlled wet oxidation of process contaminants to drive the water flow, much in the same way as an autogenic thermal airlift pump. This greatly increases production capacity because the gravity pressure vessel works as a continuous, linear, plug flow reactor with high internal heat and pressure recovery and no moving parts. This makes the process easy to control and scale-up without the need for multiple arrays of pumps, pressure tanks or complex controls.
The gravity pressure vessel is comprised of a long steel pipe, shaped like a test tube, of a fixed diameter between 12 and 24 inches. The annulus of an open-ended steel pipe creates updraft and is suspended within the test tube. This updraft protrudes above the test tube and descends to within a few feet of its concave bottom. Small bore steel pipes are suspended in the updraft to inject steam and chemicals, for temperature control, cathodic protection and cleaning.
The diameter of the tube and updraft pipes is governed by hydraulics of the supercritical water and the need for a self-cleansing velocity as well as the small bore pipes.
The entire gravity pressure vessel is freely suspended inside a steel-lined borehole, which is cemented into the ground. A pressure cap is placed over the space between the gravity pressure vessel and the borehole and a vacuum is applied to the void between the enclosed space to form a thermal barrier between it and the borehole. Through the top of the gravity pressure vessel the pipes connecting to the gravity pressure vessel includes a feed solution to the annulus formed between the updraft and the test tube. A pipe is used for discharging the treated solution from the updraft with the smaller pipes at the top.
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