1.6. STATE OF THE ART OF BIODIESEL PROCESSING
1.6.4 TECHNOLOGIES
individual unit operations of reaction and settling. For such BDI pioneered with centrifuge [133] to reduce the volume and residence time in settlers. The cost of speeding up the throughput was the higher specific consumption of electric energy.
This made that this technique can feasible employed at capacities above 30 kt/y. This limit can be illustrated in taking into account that the cost of centrifugal separation of about 2% water from the biodiesel stream costs -at this scale- ~320000 USD/y [134].
FIGURE 1.10. LURGI BIODIESEL TECHNOLOGY WITH TWO SPECIAL TRANS-‐ESTERIFICATION REACTORS INCLUDING A MIXER AND A COALESCER IN THE DOWNSTREAM SECTION.
Another attempt to shorten the residence time is the approach of Lurgi [135] by employing a coalescer in the downstream section of the two trans-‐esterification reactors (figure 1.10). Beside this merit of enhanced phase separation in a horizontal reactor structured into zones of mixing and coalescing there are a series of weaknesses that make this technology less attractive today.
The most sophisticated and advanced technology to produce standard diesel fuel is represented by high pressure trans-‐esterification operation of Axens (figure 1.11) [136]. It is customary to classify this technology as first representative of second generation units. Publications about this technology are addressing more marketing priorities than scientific aspects. The intellectual property related to this technology is most probably from patent literature with reference to a heterogeneous spinnel catalyst for trans-‐esterification. The same type of catalyst was patented by Südchemie and many other catalyst manufacturers[137]. All claim to have developed efficient heterogeneous catalyst of this type for trans-‐esterification. From the viewpoint of a chemical engineering technology, those severe operational conditions, temperature exceeding 200 °C, pressure levels above 60 bar and the very high inventory of reagent common to heterogeneous catalyst technology systems raise the scale to economy to very high throughputs of the level of 160 kt/y. Feedstock specifications are also strict and demanding. Very low gum and free fatty acid content is required to protect the catalyst. There have been no published data about the losses incurred in feedstock pretreatment operations. “Gossiped” industrial communications state that the loss
exceeds the level of 5-‐8%.
Without sticking to chronological order, published technology scheme of IFP’s next generation unit (Esterfip H-‐ figure 1.11) operates with two consecutive reactors with phase separation in between of these. The scheme shows that equilibrium considerations limit the operation of this second generation heterogeneous catalyst system too. The amounts of oil and methanol fed in these reactors are the same, high specific energy consumption can be calculated for such evaporative duties.
FIGURE 1.11 ESTERFIP-‐H TECHNOLOGY OF IFP
High costs associated to high pressure operations can be hedged (a term borrowed from finance for coupling securities to averse risks of losses) by incorporating secondary use of byproducts or even the main product. Deshpande evaluated the use of this technique in supercritical condition synthesis of biodiesel [138], but this approach is simple an attempt to hide disadvantage of high specific energy costs that will not hide the carbon footprint of the scheme.
Another remarkable and must visit feature of this technology is the statement of producing clean glycerol. This statement provoked my critics. Clean glycerol can only be produced if the feedstock was perfectly refined to contain nothing else but tri-‐
glycerides and fatty acids. This is almost impossible in practice. In this case the question must address the fate of the separated components. In this case if the rule of thumb that 5 times of the amount of polars are separated into the rejected stream the extent of losses must be extremely high. A trivial answer to where to place this byproduct was answered in our research for G-‐phase use as animal feed component.
For technical realization the case can be very similar to pretreatment of gasoline fractions in reforming to protect the high value Pt catalyst from poisoning. If the sorbent for such is inert and inexpensive zeolite than this is an excellent example to industrial ecology serving the agriculture. On this ground the arguments for ecologically acceptable systems must be revisited. The glycerol can only be clean if the
technology converts clean TG into FAME and G. This cannot be the case either with refined or refuse feedstocks.
An independent expert [139] declared the technology of the present thesis to be also of second generation. The basis of evaluation was to take into account the efficiency and specific consumption figures. For an overview of the mostly known technologies offered by vendors table 1.2 was compiled with the main features of the systems.
Early days technologies have almost exclusively scoped trans-‐esterification of refined rapeseed oil. Those most selling technologies of AT Technique, of BDI and of other followers used the basic know how of IFP and contributed to it with some improvements in executing one or more unit operations of the whole process. This contribution has not lacked scandals and juridical processes in which vendors blamed concurrent vendors for breaching intellectual property rights. BDI and AT Techniques sued each other in several cases.
To avoid operational problems most vendors uniformly set strict criteria for clean feedstock qualities. Phospho-‐lipids, gummy substances, free fatty acids and water have been maximized in the feedstock close to the level of refined food grade products.
Vendors of these technologies ask for phosphorous content usually below 10 ppm, for FFA content below 0.5%, or even below 0.1%, water content down to 0.1%. These dictate very high price for agricultural commodities.
Those early day technologies had capacities that did not make feasible to employ a decanter and the rate of volumes of settlers to reactors were in the range of 10-‐15:1.
Processes were only apparently continuous, batch process steps have been harmonized but rare are the examples for use (and presence) of heat exchangers, heat economizers. The only environmental incentive to reduce the footprint of the units was to consider the use of coalescer or decanter.
Colloid chemistry aspects have only been observed and considered right factors that complicate unit operations along with solubility problems, because methanol is not soluble in the starting material triglyceride nor the end products glycerol but the formed fatty acid methyl esters are miscible with methanol. Fatty acid methyl esters do form solution with methanol. It has been reported that at the beginning there is a two-‐
phase system, followed by an almost complete solution. Then as soon as a considerable amount of glycerol is formed, a new two phase system will again prevail.
There is another technology that uses solvent for avoiding phase transfer resistance to trans-‐esterification. This is the BIOX process. The basic difference between the presented and the BIOX technology consists in the selection of the kind of solvent. In BIOX process the polar solvent dissolves both the starting and the reaction products materials. Because of complete solubility without selectivity the solvent does not discriminate between synthesis products. The main differences between the two approaches: Reversible nature of the trans-‐esterification reaction determines that the operation must be conducted in at least two consecutive steps. The solvent must be separated between the two contacting events in order to separate the main and byproduct of the synthesis (figure 1.12)
TABLE 1.2 AN OVERVIEW OF BIODIESEL TECHNOLOGY MARKET Designation Main
operations
Limiting capacity
Critics to features
Time to product(h)
Typical vendor Early day, still
prevailing in the market simple techniques
Mix and settle No known limit, units range from 1-‐3 t/y to 80000 t/y
Vsettlers>>Vreactors
Difficult to control, batch, two stage, apparently continuous
18-‐36 AT Technik, Pacific Biodiesel, Biorafineria SK, etc.
Clean FAME by distillation
Mix, settle and distill
No known limit
FAME distillation is energy
intensive
N.A. DeSmet,
Improved by the
use of centrifuge Mix and settle <30-‐35 kt/y, Additional energy, batch, two stage, apparently continuous
8-‐16 BDI
Improved by built-‐in centrifuge
Mix and settle <80 kt/y Additional energy, batch, two stage, apparently continuous
8-‐16 Lurgi (developed by
Connemann) Improved by
coalescer Mix and settle No limit Batch, two stage, apparently continuous
8-‐16 Lurgi
Modular systems with coalescer
Mix and settle 5-‐8
kt/y/module
Batch, apparently continuous, two stage, parallel operations for large capacities
8-‐16 D1 Oil, BioDiesel Technologies, etc.
Phase transfer avoidance
Mix, distill and settle
No known limit
Batch, two stage, apparently continuous, large inventories
3-‐8 Biox Corp.
Heterogeneous catalyst
React and settle
<160 kt/y No proof for energy, resource efficiencies, not reported for multifuel operation
3-‐5 Axens
Microwave/ultra-‐
sound assisted
Mix and settle Not to industrial scale
Additional energy, batch, two stage, apparently continuous
? Intech
Engenharia &
Meio Ambiente, CPI Critical conditions Mix and settle Not to
industrial scale
Additional energy, batch, two stage,
? Nedo
2nd generation Hydrotreating under severe conditions
>150-‐365-‐
800 kt/y No proof for energy, resource efficiencies
? Canmet,
Neste, UOP
FIGURE 1.12 OPERATIONAL UNITS OF BIOX TECHNOLOGY TE: trans-esterification reactor, SD: solvent distillation unit, SE: settler
In severe hydrotreatment the problem of glycerol byproduct treatment has been eliminated, as in severe hydrogenation every oxygen and other hetero-‐atom containing molecules are converted into pure hydrocarbons with the prevailing reaction schemes of hydrodesulphurization, hydrodeoxygenation, hydrodenitrogenation, hydrocracking, etc. It is known from early days of hydrogenation that at a temperature of 400 °C all sulfur, nitrogen and oxygen atoms can be completely removed [140]. First patent for severe hydrotreatment of vegetable oil was issued to Canmet in 1991 [141] and a series of followers developed commercial technologies, among them Neste Oil and UOP having excellence records. Madsen[142] conducted experiments with a reaction mixture of 90% apolar solvent -‐10% fatty stock, with 5% Pt, Pd and Ni catalysts on alumina substrate with residence time to 10 h. It is a bit unusual to use an apolar solvent in a system in which easily recyclable hydrogen could serve for the same function. Water gas shift reaction (CO+H2 CO2+H2) is also promoted by the catalyst selected and methanation of CO and CO2 to CH4 are additional features of complete conversion of triglycerides. No note on the fate of minor vegetable oil and grease components have been made. With this concern it must be acknowledged that above a certain capacity (around 200 kt/y) this must be the preferred technology in a petroleum refinery.