WO2014004933A1

WO2014004933A1 - Zonal mixing for tubular plug flow digesters

Info

Publication number: WO2014004933A1
Application number: PCT/US2013/048380
Authority: WO
Inventors: T. Guy Roberts; Ryan KOLOSKI; Joel Van WYCK; Peregrine Scofield; Tim BESSETTE
Original assignee: Avatar Energy Llc
Priority date: 2012-06-29
Filing date: 2013-06-27
Publication date: 2014-01-03

Abstract

A system for zonal mixing in a tubular digester pumps biogas from the digester's headspace back into the bottom of the digester. The rising bubbles of biogas create upward and downward currents capable of thoroughly mixing the digester contents in a zonal manner. Biogas is blown from the gas outlet in the upper portion of the hull through biogas bubble bars located inside the digester hull. Check valves are positioned along the biogas bubble bar to allow the biogas to flow out while the blower is running, but prevent the liquid contents of the digester from flowing into the biogas bubble bar when the blower is turned off. A pipe assembly joins the various components together.

Description

TITLE: Zonal Mixing for Tubular Plug Flow Digesters INVENTORS:

T. Guy Roberts, Richmond, VT, US

Joel Van Wyck, Vergennes, VT, US

Tim Bessette, South Burlington, VT, US

Ryan Koloski, Montpelier, VT US

Peregrine Scofield, South Burlington, VT, US

CROSS-REFERENCE TO RELATED APPLICATION

The application claims benefit of priority to US provisional patent application 61/666,758, filed June 29 2012.

FEDERALLY-SPONSORED RESEARCH: None

BACKGROUND: FIELD OF THE INVENTION

The system described in this application relates to the of field anaerobic digesters which process waste matter. The need for mixing of the contents of anaerobic digesters depends on the type of digester being considered. One of the most common types of digesters, the complete mix digester, is routinely mixed on a frequent basis to maintain a homogenous feedstock. This also maintains homogenous distribution of microorganisms, digestion substrates and digestion byproducts. Mixed digesters are not limited by the rate of diffusion of small molecule intermediates of digestion; however, mixing itself may have negative impacts on digestion. Mixing systems for plug flow digesters are useful, not only for creating a means of overcoming diffusion rates, but also for reducing stratification of materials within the digester.

In order to maintain the advantages of plug flow digestion, the mixing must be incomplete; otherwise the "first in, first out" nature of plug flow digestion would be destroyed. Within a tubular plug flow digester, the mixing must be "zonal", meaning that along the digester's hull there are mixing zones that are approximately the size of a feed plug. The contents of each zone are uniformly mixed, but zones are not cross-mixed, ensuring that each plug of feedstock remains mixed, but that there is no short circuiting of materials.

BACKGROUND: BRIEF SUMMARY OF THE INVENTION

The preferred embodiment of zonal mixing in a tubular digester is to pump biogas from the digester's headspace back into the bottom of the digester. The rising bubbles of biogas create upward and downward currents that can thoroughly mix the digester in a zonal manner. This is done via routing biogas from the gas outlet in the upper portion of the hull, through biogas bubble bars located inside the digester hull.

In this preferred embodiment, a biogas rated blower or pump pulls the biogas from a "tee" near the base of the biogas outlet and pushes the biogas back into the digester through a manifold that attaches to through-hull fittings connected to interior biogas outlet pipes running along the length of the digester's floor.

Check valves are positioned along the biogas bubble bar to allow the biogas to flow out while the blower is running, but prevent the liquid contents of the digester from flowing into the biogas bubble bar when the blower is turned off. The number of check valves positioned longitudinally on the biogas outlet pipes determines the thoroughness of mixing; where a greater number can create a nearly continuous longitudinal mixing and a smaller number creates mixed zones separated by unmixed zones. In the preferred embodiment, the check valves are positioned along the biogas bubble bar spaced 1 to 3 feet from each other, which results in a uniformly, but zonally mixed digester. Where there are more than one biogas bubble bar, the outlet hole spacing on each pipe is staggered relative to one another to provide more uniform mixing along the length of the digester.

An alternate embodiment uses pumped recirculation between paired ports to mix the digester contents. One means of mixing a section of a tubular digester is to install paired ports along the digester that serve as the outlet and inlet for a pump. When material is pumped out of one of the ports and back into the other, a circular flow pattern develops that turns over the contents of the digester. If these ports are offset slightly, a spiral flow pattern will develop when pumped which may be beneficial in some circumstances.

In this configuration, mixing is restricted to the vicinity of the paired ports, which means that enough pairs of ports must be installed to effectively mix the entire length of the digester. If thorough mixing is desired, a pump for each pair of ports is required. BRIEF DESCRIPTION OF THE FIGURES

Fig. 1 shows a front perspective view of the digester system described herein as the preferred embodiment.

Fig. 2 shows a side perspective view of the digester system described herein.

Fig. 3 shows both a perspective view of the components of the flapper check valve and a perspective view of the assembled valve on the bubble bar.

Fig. 4 shows perspective view of the digester hull with the mixing port and pump assembly.

Fig. 5 shows a section view of the digester hull with the mixing port and pump assembly.

DETAILED DESCRIPTION OF THE INVENTION

In an anaerobic digester, there are several primary benefits of mixing. One benefit is_overcoming rate-limiting diffusion. Under unmixed conditions a plug flow digester operates at the rate of diffusion from the channelized mobile phase to the peripheral stationary phase. The most active zones of methane production are peripheral in a plug flow digester, with particularly high densities of methanogens occurring at the surface of the digester where gas formation has lifted organisms to the surface in the process of crust formation. Acetate and other nutrients must diffuse from the mobile phase to reach the peripheral organisms. By mixing zonally, the organisms and nutrients are brought in close contact, effectively reducing the retention time required for equivalent digestion.

Another benefit to mixing is overcoming stratification, or the natural tendency of the feedstock stratifying into separate layers, based on density. These layers include 1. A sandy layer along the bottom of the digester, depending on the source of the feed. 2. A heavy sludge that forms above this layer that includes water-logged, dense materials. 3. A watery layer that forms at mid-levels that is the most mobile layer and which represents the mobile phase and is whose volume is equal to the actual retention time of the digester. 4. A fibrous, foamy layer floats above the watery layer that forms as microbes produce gas bubbles. These bubbles temporarily remain attached to microbes and the fibers that microbes attach to, causing the fibers and gas-forming microbes to float to the top of the digester surface. The bottom of this layer is very active in producing biogas, although the top of this layer eventually becomes too dried out to support the growth and survival of the gas-forming microbes.

Breaking up these layers by mixing reestablishes the uniformity of the feedstock and the gas forming organisms are once again in proximity with the organisms that produce hydrogen, carbon dioxide and acetate and other small molecule substrates. By regular mixing, a greater part of the cross-section of the digester hull remains in the mobile phase and the retention time is lengthened due to the increase in the volume of the mobile phase.

Further considerations include overcoming channelization and using the full volume of the digester for retention time. As described above, mixing along the length of the digester overcomes the stratification of the feedstock, creating a uniform consistency. As new feedstock is added, the material displaced is representative of the full digester diameter, not just the channelized material. Peripheral materials are brought back into the mobile phase of the digester.

Another consideration relates to preventing 'washout' of microbes. There is a layer along the hull walls and feedstock surface that does not get pushed forward with each feed. This is due to friction and viscosity. This layer provides a reservoir of organisms that reinoculate the feedstock material as it gets pushed along the length of the digester. The mixing system will introduce some of these organisms into the center material and will introduce fresh feedstock into the stationary layer to keep this peripheral community active. Overall this benefits the system by preventing the organisms from being swept or washed further down the length of the digester until all the organisms have been pushed out.

Another consideration is mixing frequency. The digester contents should be mixed at a frequency that is high enough to prevent feedstock stratification, but should not be so frequent to inhibit or negatively impact microbial growth. Prior art establishes that mixing can be detrimental to microbial activities, probably through shear forces exerted on methanogens. Given that many methanogens develop colonial structures, mixing with too much force, frequency or duration will impair the benefits of colonial growth.

Turning now to the system that is the subject of this application: In the preferred embodiment, the digester contents are zonally mixed by recirculating biogas within the digester (Figures 1 and 2). A blower (20) rated for use with biogas pulls biogas from the gas mast (22) into the blower feed line (24). Collectively, gas mast (22) and the blower feed line (24) are referred to as the gas outlet. In a preferred embodiment, this line contains a water trap (26) that catches particles and moisture before the line proceeds to the blower (20). The blower is powered by an electric motor (28) that allows the mixing system to be controlled by a programmable logic controller (PLC, not shown). For example, the blower motor can be turned on by the PLC on a regular schedule or can be timed to mix the digester before, after or during the injection of feedstock into the digester hull (30). Biogas from the blower is pushed into the Blower outlet line (32). The blower outlet line connects to the blower manifold (38); these two components (32) and (38), are referred to collectively as a pipe assembly. The blower manifold links to biogas bubble bars (40) that are located inside the digester hull (30). Positioned at short intervals along the lengths of the bubble bars are biogas check valves (42) that allow biogas to be released into the digester hull. The number and spacing of the biogas check valves influences the force and pattern of mixing.

In order to adjust the volume of biogas being pumped into the mixing system, a bypass loop (34) is installed to recycle gas into the blower. A blower recycle valve (36) is installed in the bypass loop to adjust the amount of biogas that is pushed through the line and the force of mixing.

Digester contents are prevented from entering the bubble bar (40) by placing check valves over the bubble bar's outlet holes. A preferred embodiment of a check valve is a flapper valve as shown in Figure 3. A small sheet of rubber, preferably reinforced with nylon or similar thread, and/or a sheet of plastic or thin metal, is cut of a size adequate to extend at least an inch beyond the edges of the outlet hole (60) to create a valve sheet (62). A U-shaped cut is made in the center of the valve sheet to create a flapper (64). For best results, the cut should be made at an angle, so that the flapper can only be pushed one direction and is seated snugly when lying flat. The flapper should be large enough to cover the outlet hole and extend beyond the outlet hole's edges by at least ½ of the radius of the outlet hole. This sheet may be attached directly to the bubble bar by gluing, welding, or clamping. In the preferred embodiment of the flapper valve a curved or flexible sheet of plastic or metal is clamped over the valve sheet to create a valve retainer (66). A U-shaped hole (68) that is slightly larger than the flapper is cut into the center of the valve retainer. The material surrounding the flapper in the valve sheet serves as a gasket when the valve retainer is clamped in place over the valve sheet using stainless steel band clamps (70) or other suitable clamps. Of course, other suitable types of check valves may also be used.

During operation of the biogas blower, biogas is forced from the outlet hole, displacing the flapper in the check valve. Once the biogas blower is turned off, the pressure and weight of the digester's contents reseats the rubber flap, covering the outlet hole and preventing digester contents from entering the bubble bar.

An alternate embodiment for mixing digester contents is to use pairs of ports and circulation pumps to induce mixing in the digester hull (Figures 4 and 5). Mixing ports (80) with valves (82) are positioned in pairs along the length of the digester hull (84). A pump (86) is positioned between each pair of ports. When the pump (86) has a direction of flow that is upward, the contents of the digester is pulled into lower or 'suction' port (88) and flows out of the upper or 'discharge' port (90). Provided that the pump is powerful enough, the contents of the digester are rolled over by the force of the stream from the discharge port, as shown in Figure 5. The flow into the suction port helps to complete the circular flow within the digester (92). By positioning the discharge port above the suction port and just the below the surface of the digester's liquid contents (94), the full force of the pumped material should encounter the surface of the digester's liquid contents and act to break up floating crusts.

This second method of mixing is more suitable for feedstock slurries with percent solids in the range of 5-10%; higher concentrations of solids may lead to the formation of crusts and strata that are too thick to be disrupted by the action of pumped streams.

Although the present system has been described with respect to one or more embodiments, it will be understood that other embodiments of the present system may be made without departing from the spirit and scope of the present system. Hence, the present system is deemed limited only by the appended claims and the reasonable interpretation thereof.

Claims

What is Claimed

1. A device for mixing material in an anaerobic digester, comprising:

a tubular hull with endcaps on each end;

at least one gas outlet on said hull;

a pipe assembly connecting said hull gas outlet to at least one biogas bubble bar, where said bubble bar is located inside said hull;

a blower connected to said pipe assembly;

at least one check valve located on said bubble bar;

wherein said blower recirculates biogas from said gas outlet through said pipe, out said checkvalve, and back into said hull.

2. The device of claim 1, in which a single gas outlet connects to two or more pipes, said two or more pipes further connecting to two or more bubble bars positioned inside said hull.

3. The device of claim 1 which further includes a bypass loop and a blower recycle valve connected to said pipe.

4. The device of claim 1 which further includes a trap in said pipe assembly.

5. The device of claim 1 which further includes multiple check valves positioned longitudinally on each said bubble bar.

6. A method of mixing material in an anaerobic digester, comprising;

venting biogas from a gas outlet on a tubular hull, where said hull has endcaps on each end;

blowing biogas in said hull with a blower through a pipe assembly into at least one biogas bubble bar, where said bubble bar includes at least one checkvalve and said bubblebar is positioned inside said hull;

recirculating said biogas from said gas outlet through said pipe, out said checkvalve, and back into said hull.

7. The method of claim 6, in which a single gas outlet connects to two or more pipes, said two pipes further connecting to two or more bubblebars positioned inside said hull.

8. The method of claim 6 which further includes a bypass loop and a blower recycle valve connected to said pipe.

9. The method of claim 6 which further includes a trap in said pipe assembly.

10. The method of claim 6 which further includes multiple check valves positioned longitudinally on each said bubble bar.