Large part of HTL research has been conducted in batch reactors. They represent the easiest form for experimentation, as they are very robust and can operate under several reaction conditions. Among their many advantages, an important one is that they can be operated with reduced amounts of feedstocks, which can even be in the order of a few grams, which is not generally possible for continuous systems. Additionally, batch experimentation does not resent of other issues that are more typical of continuous systems, for example it does not need to deal with the pumpability of reaction slurry.
At the same time, batch systems have some relevant disadvantages:
- The recovery of the product after reaction can be quite laborious and involves a number of different operations, such as extraction with solvents followed by their removal by distillation, filtration to separate solids from biocrude, etc. These operations can potentially affect the uncertainty of mass balances and, in some case, even alter the chemical composition of the products.
- Batch systems display some kind of thermal transient, i.e. the time needed to go from ambient to reaction temperature. According to the size of the unit, this transient might be considerably long and even longer than the desired reaction time, thus resulting in a poor control over the actual reaction conditions. This drawback can be minimized by reducing the size of the unit down to the microscale.
There are mainly two types of batch reactors commonly used in research practice: micro-batch reactors and autoclaves.
Micro-batch reactors represent a very practical and affordable way for testing, also allowing a certain flexibility. Micro-batch reactors are essentially small metal pipes, sealed at both ends, which can be filled with the reacting mixture (biomass, water and possibly catalyst). These devices can then be placed in a heat source, which can be an oven or, more often, a sand bath, where they can be heated to the desired temperature. The pressure inside the units is a function of the temperature and the amount of added water (autogenic pressure): the desired pressure can be determined before the experiment by calculations according to thermodynamic properties of water. After the desired time has elapsed, micro-batch reactors can be quenched in cold water and then opened to collect the reaction product.
An example of micro-batch reactors are those utilized at Aalborg University (Denmark). In this case, they consist of a bottom part (the reactor itself), represented by a metal pipe sealed at the bottom, and an upper part, featuring a long capillary tube, with a valve and a pressure transducer at the top. In a typical experiment, the lower part of the reactor is loaded with the feed and then it is connected to the top part. The capillary allows filling the reactor with N2, which is useful to leak-test the unit. Subsequently, the valve is closed and the lower part of the reactor can be dipped in a sandbath, while the top part is attached to an oscillating rack, that shakes the whole reactor to provide effective mixing. Meanwhile, the pressure transducer is connected to a datalogging system, which allows continuous monitoring of the reaction pressure. After reaction, the reactor is detached from the rack and the lower part is quenched in water. By carefully opening the top valve, it is possible to sample process gases, before opening the reactor to collect liquid and solid products.
This system has the great advantage to minimize the heat transient: reaction temperature and pressure are reached in a few minutes. On the other hand, their reduced size implies very low amounts of reactant (typically a few grams), which complicates product recovery and can potentially result in higher uncertainties in mass balances.
Another widely used typology of batch reactors are autoclaves. These are more classic devices for lab experimentation and consist of a reaction vessel surrounded by heating elements and closed on the top by a sealed lid. An impeller is normally present, which provides the necessary mixing. The reactor lid presents a number of connections, both for inlet/outlet lines (for gases or liquids) and to connect measuring instruments, such as pressure transducers and thermocouples. An internal coil with a circulating cooling fluid can be present to allow quenching after reaction.
Autoclaves are commercially available in many volumes, from 50 mL to even more than 1 L. Therefore, they allow processing relatively large amounts of feedstock, with the possibility of producing larger amounts of products for characterization and further processing (upgrading). However, their disadvantage is mainly represented by the long heat transient, which can lead to uncertainties in determining the actual reaction time and in products that are not indicative of continuous reactors.
Despite batch reactor may represent a very versatile solution, they are not suitable for industrial production of biofuels, where volumes of thousands of barrels are involved. Therefore, a continuous HTL process is a necessity for the scale-up of such a plant. As the process takes place at high temperature (220 ˚C – 400 ˚C) and pressure (180 bar to 300 bar), the safety of the reactor is of vital importance. The reactor design is generally less complex than for the pyrolysis process. Usually, the reactors are long pipes behaving as a Plug Flow Reactor (PFR). Nevertheless, the use of a PFR increases the risk of plugging issues in the reactor due to the high pressure viscous multiphase slurry.
Pressure let down and separation units are often very challenging in this process due to the presence of multiple phases at high pressure and temperature. Specifically, the presence of solids at high pressure often leads to wear on the components such as back pressure valves which induces the need for a robust pressure let down system.
Continuous stirred tank reactor and plug flow reactor combined in the HTL process
A Continuous Stirred Tank Reactor (CSTR) is a reactor tank equipped with an impeller to ensure mixing of the slurry. Moreover, the CSTR is in this case equipped with an electric heater which ensures that the specified reaction temperature is achieved and maintained. The mixing of the slurry increases the turbulence of the flow and thus increases the heat transfer in the reactor as well. Consequently, the mixture is more easily heated in the CSTR in comparison to the Plug Flow Reactor (PFR). However, the theoretical conversion of the biomass into biocrude is lower in the CSTR in comparison to solely using the PFR due to short residence time in the reactor. Thus, the residence time can be extended by implementing a PFR in series with the CSTR (see below) and thereby increasing the bio-crude yield. As a result of the flow mixing in the CSTR, plugging issues in the PFR are minimized, especially with lignocellulosic feedstocks (typically woods). Consequently, the combination of a CSTR and a PFR shows great potential in the process of HydroThermal Liquefaction (HTL).
Hydraulic oscillation in the HTL process
Hydraulic oscillation in the HTL process can be another solution to reduce the risk of plugging in the PFR. A patented hydraulic oscillation method invented by Ib Johannsen is implemented to increase turbulence in the flow which leads to improved heat transfer and decreases the viscosity in a part of the production line while the average velocity and thus the reaction time remains unaffected (see diagram below).
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The hydraulic oscillation system consists of two pistons as shown below. Each piston has two separated sections. One section is subjected to the HTL slurry whereas the other side is linked to a high-pressure hydraulic oil system. The hydraulic site controls the pistons to make sure that when one piston is pushed down, the other retracts. This must be performed continuously in alternation to ensure that the mixture in the reactor behaves as presented in the diagram above.
Results from test facilities show that heat recovery of up to 80% is obtainable. Furthermore, results showed that the chemical energy recoveries in the bio-crudes were between 33.6% and 48.1% depending on the feedstock. So far, no improvement in bio-crude quality or yield has been documented by the use of hydraulic oscillation.
A recovery system for the aqueous phase is crucial for the performance and economy of the HTL system. The recovery system patented by Steeper Energy recirculates the aqueous phase which contains liquid organic compounds and dissolved salts such as homogeneous catalysts in the form of potassium and sodium. Furthermore, a bleed stream is introduced to prevent trace elements like chloride to accumulate in the system. Thereby, the carbon recovery in the bio-crude is increased and the water containing catalysts are recycled which benefits the economy of the system.
To recycle the aqueous phase, a 3-phase separator must be included. The patent includes a 3-phase separator presented in the figure below. The concept is that the reactor products enter the tank through a distributor, for instance, a diffuser to separate the liquids from the gaseous phase. Due to its low density, the gaseous phase will move upwards and out through the top of the tank. The aqueous phase and the bio-crude will also separate because of their difference in density. Eventually, there will be an overflow of bio-crude as indicated in the figure. To enhance the separation of the bio-crude and the aqueous phase perforated baffles, lamella plates or meshes can be inserted to calm the flow, whereby a more efficient separation is obtained.
Experiments have shown that the bio-crude yield can be enhanced with approximately 4 wt. % while the composition of the bio-crude mainly remains unchanged with the use of this invention. Furthermore, the higher heating value of the bio-crude shows potential to increase by more than 1 MJ/kg as a result of the aqueous recirculation.
Pressure reduction in HTL cannot happen through a single valve due to the high velocities and abrasive components, which increases the risk of wear on the valve and the seat etc. Thus, the patented pressure let down system by Steeper Energy has been suggested as a solution to the pressure reduction. Basically, the technology implies that the reaction products flow through a series of long tubes, and hence, the pressure is reduced due to head losses. Thereby, the pressure loss depends strongly on the flow velocity, the diameter and length of the tubes, the density, and the viscosity of the reaction products. It is suggested that the velocity in the tubes preferably is 10 m/s and the length of the pipes should be within the range of 10 to 100 m with a diameter of 2.5-10 mm. Furthermore, it is proposed that the tubes are chosen to be shaped as a helix in order to reduce the space needed for the pressure reduction system.
With fixed dimensions of the tubes, a stepwise pressure reduction is advantageous due to variations in the composition and thus the condition of the reaction products including the density and viscosity. Since pressure loss depends on the density and viscosity of the fluid, the pressure loss is not necessarily consistent at any given time. Thus, a pressure let down system has been developed to reduce the pressure stepwise as presented in the figure below, which shows one of the repeated sections in the pressure reduction system. Each pressure let down step consists of a pressure reduction device and a bypass piping controlled by an on/ off closing valve. Furthermore, a pressure sensor communicates with the control unit which determines the opening/closing of one or more valves based on predetermined pressure limits. Thereby, the pressure is gradually decreased, and the risk of wear is significantly reduced.
- Plugging in the reactor is a common problem in HTL which can be solved either by the implementation of oscillators or a CSTR in combination with a PFR.
- Aqueous recovery benefits the final bio-crude product as the bio-crude yield increases along with the higher heating value while the biocrude composition mainly remains unchanged.
- Pressure reduction after the reactor can be done by the use of multiple tubes to utilize head losses and avoid wear on valves.