WO2007042121A1

WO2007042121A1 - Process for the solution crystallization of mixtures

Info

Publication number: WO2007042121A1
Application number: PCT/EP2006/009000
Authority: WO
Inventors: Günter RINKE; Sigrid Kerschbaum; Klaus Schubert; Herbert Holpe
Original assignee: Forschungszentrum Karlsruhe Gmbh
Priority date: 2005-10-12
Filing date: 2006-09-15
Publication date: 2007-04-19
Also published as: CA2620011A1; US20080276525A1; EP1934320A1; DE102005048881A1

Abstract

Process for the solution crystallization of mixtures. The object is to propose a process which, without additives, is suitable for improving the winter stability of, in particular, biodiesel fuels even in the case of large quantities. The object is achieved by a process which comprises providing a liquid mixture (2) comprising at least one subcomponent having an elevated melting point, taking a substream (9) from the mixture (2), feeding the substream to a heat exchanger (16), with the substream flowing through the heat exchanger, cooling the substream in the heat exchanger to a temperature below a crystal formation temperature of only the subcomponents of the mixture, with the temperature being set so that crystals (19) of the subcomponents form in the substream directly at the outlet from the heat exchanger (16), mixing the substream comprising the crystals into the mixture (2), with the crystals selectively binding further substances in the mixture by crystal growth, and sedimenting and separating off the crystals in the mixture.

Description

Process for the solution crystallization of substance mixtures

The invention relates to a process for the solution crystallization of. liquid mixtures according to the first claim.

In general, there is the problem in liquid mixtures of substances that sub-substances which have a higher melting point than the other substances in the substance mixture crystallize out with increasing cooling of the substance mixture. The mixture of substances thus becomes a suspension, which may lead to handling problems of the substance mixture with increasing crystallization of one or more partial substances.

A typical example from the everyday practice to do this is diesel fuels in which certain sub-substances crystallize at low winter temperatures, the mixture thereby macroscopically occupies a gel-like substance with an increased viscosity and thus significantly hindering handling in injection components such as nozzles in diesel engines. This restriction can be observed especially with so-called biodiesel, in which the substance mixture is made up of oils of different origin or production as partial substances. Crystallization is usually hampered by the addition of appropriate additives, such as in the so-called. Winter diesel usual, mixed with the mixture.

The production of biodiesel has meanwhile achieved a permanent place in the provision of regenerative liquid energy sources. In its classical form, biodiesel is obtained from the transesterification of rapeseed oil as rapeseed methyl ester (RME). However, biodiesel can be produced cost-effectively from used cooking oil (used cooking oil = AME). Because of the much higher concentration of saturated fatty acid methyl esters in AME, this biodiesel can only be used in summer.

To avoid the above-mentioned crystal formation in biodiesel, the cold resistance of biodiesel based on used oil base (AME) is attempted to be improved by so-called winterization. The cold strength describes the temperature at which a substance from the liquid in passes the solid state and corresponds / if crystals are formed, the melting temperature of the substance. Practically, this means that the fluidity of the fuel is limited by increasing gel formation and the formation of crystals in the product and prevents motor use. In addition to the properties of fatty acid methyl esters, accompanying substances such as waxes, residues of undesirable reaction products such as glycerides and glycerol contribute to this behavior.

Previous methods of winterizing biodiesel rely on e.g. on mixing with mineral diesel or adding additives to RME. The use of additives is currently the route of choice, at least for rapeseed oil.

The mode of action of such products is based on the formation of a large number of crystallization seeds with a size of less than 50 μm, which prevent agglomeration in the biodiesel. Although this effectively counteracts gel or crystal formation, so that the requirements of winterization (CFPP measurement according to DIN EN 116) can be formally fulfilled, the problem is not fundamentally solved, since a crystallization of the product extremely delayed, but not prevented becomes.

In addition, available additives have only been developed specifically for RME, while they do not work in the same way at AME. Special additives for AME that meet the given commercial fuel framework are not known.

Alternatively, the fractional winterization and physical filtering off open ^¬ discloses crystallized components in US 4,265,826, in which the high boiling components to crystallize out according to their boiling points. The mixture is cooled until crystals form in a filterable size. Be ^¬ ferred cooling rates are between 0.5 and 5 ° C / h, optio ^¬ nal at certain temperature levels residence times between 2 - h are provided 16th The total yield can vary depending on the requirement of the Cold strength of the product below 30%, based on the starting material, fall and is therefore not economical.

Filtration processes, however, have only limited effectiveness if the viscosity of the still liquid substance mixture content increases and the separation effect in particular of the gel-like crystals decreases. The aforementioned overall yield is also adversely affected by the liquid constituents of the substance mixture incorporated in the crystals. Even a very long process time of up to 100 hours and the large amount of the substance mixture to be cooled in a defined manner, especially in the case of diesel fuels, make this approach appear technically less practicable.

DE 34 44 475 C2 also describes a device for separating crystals by means of a vacuum belt filter for the dry fractionation of fats and oils, which moves in the area of classic winterization. The process works batchwise and requires no additional chemicals.

Based on this, the object of the invention is to propose a process for the solution crystallization of mixtures of substances, in particular of fuels such as biodiesel, which makes do without additives and is also suitable for increasing the winter strength of biodiesel fuels for larger quantities.

This object is achieved by a method having the features of the first patent claim. The dependent claims give advantageous embodiments of this method again.

The solution is based on a process for the solution crystallization of mixtures, in particular fuels such as biodiesel. In this case, a partial stream is taken from a liquid substance mixture which contains at least one sub-substance with an increased melting point. This sub-stream contains all sub-substances occurring in the substance mixture and is preferably representative of the substance mixture, ie it holds the sub-substances in the same ratio to each other as the mixture. The partial flow is then a heat exchanger - A - zugefuhrt and introduced into the Fluidfuhrungen of the heat exchanger, the partial flow flows through the heat exchanger. In the heat exchanger, the partial flow is cooled to a temperature below a crystallization temperature of only the partial substances of the substance mixture, while the other partial substances remain unchanged in the partial flow in the liquid phase. In this case, the temperature and the flow rate are adjusted so that crystals of the crystallized partial substances form in the partial flow directly at the outlet from the heat exchanger in the partial flow. The location of the crystal formation by means of a relaxation in the fluid, such as through a nozzle or a pressure reducer set additionally targeted. This is followed by a re-mixing of the partial flow with the crystals formed of one or more sub-substances in the mixture, wherein the crystals in the mixture further substances selectively bind by crystal growth itself. In this case, for improved crystal growth, the mixture of substances is either not rolled over (sedimentation) or mixed and cooled and held on the so-called cloud point, as the temperature at which turbidity and incipient crystallization can be detected. After sedimentation and reaching a certain filterable size, the crystals are separated by a separation process from the mixture, for example by filtration or ultracentrifuge

The method will be explained in more detail with reference to exemplary embodiments, experimental examples and the following figures. Show it

Fig.l the schematic structure of an exemplary embodiment for carrying out the method on a laboratory scale,

2 shows the time course of the AME concentration and the tempera ^ture in the storage vessel according to the first experimental example,

3 shows the time course of the AME concentration and the temperature in the storage vessel according to the second experimental example and

4 shows the course of the AME bath temperature as a function of the sum concentration of saturated fatty acid methyl esters, as well as the empirical Determined course of CFPP values (CFPP = CoId Fil- ter Plugging

Point; Measurement acc. DIN EN 116) as a function of the sum concentration of saturated fatty acid methyl esters.

The exemplary embodiment acc. Fig.l comprises a storage vessel 1 for about 5 liters of biodiesel 2 with temperature sensor 32, and a circuit with suction 9, a pump 10 (gear pump), heat exchanger 16 (heat exchanger, in the exemplary embodiment, a cross-flow micro waremubertrager) and return 20 for the branch partial crystallization and recycling of the partial flow from and into the Vorratsgefaß. The storage vessel 1, e.g. a 5-liter beaker, includes a tempering, in the exemplary embodiment, a wrapping with a PVC hose as Kuhlmantel 3, which is connected as part of a closed Kuhlmittelkreislaufs Kuhlmittelzuleitung 7 and a Kuhl- medium pressure line 8 to a thermostat 5. The coolant 6 is a volumetric 1: 1 mixture of ethylene glycol and water. For better temperature control, the storage vessel 1 and the Kuhlmittelzuleitung 7 are surrounded with insulation material. The biodiesel 2 in the storage vessel is optionally kept in motion by means of a magnetic stirrer 4.

The partial flow is sucked by the pump 10 via the suction line 9, in the exemplary embodiment, a metal hose or Säugrüssel, preferably from the lower third of the Vorratsgefaßes 1 and discharged via the return 20 into the upper third again. In this way, a large part of the biodiesel 2 in the storage vessel 1 is rolled over. The pump 10 forwards the partial flow via a heat exchanger feed 12 (eg metal hose) to the heat exchanger 16 with a microfilter 11. The Warmeubertragerzufuhrung is equipped to maintain a kon ^¬ constant heat input entering temperature and to avoid premature crystal formation in the partial flow before and in the heat exchanger 16 with a tempering device for the partial flow. This includes in laboratory-oriented exemplary embodiment filled with water 14 as a tempering medium and connected on both sides of a water thermostat 13 hose 15 (PVC hose with hose winding to the Warmeubertragerzufuhrung). With the entry of the partial flow into the heat exchanger 16 (cross-flow heat exchanger), this is subdivided into the fluid passages of the cross-flow heat exchanger and cooled in crossflow by a cooling medium 22 (ethylene glycol / water). The Kuhlmedium is guided by a Kuhlmediumthermostaten 21 by means of a Kuhlmediumpumpe 24 horizontally by their own Fluidfuhrungen for the Kuhlmedium in the heat exchanger 16. Heat exchanger, Kuhlmediumthermostat, Kuhlmediumpumpe and a Kuhlmittelfilter 25 are connected via a Kuhlmittelzuleitung 23, a Kuhlmittelleitung 26 Kuhlmittelableitung 27 and other lines (each polyethylene hoses) to a closed Kuhlmittelkreislauf.

The heat exchanger used in the exemplary embodiment is a micro-Kreuzstromwaremubertrager with a variety of Fluidfuhrungen with diameters or maximum cross-sectional dimensions less than 1 mm. Due to its fineness, the heat exchanger is characterized by a high specific heat transfer, i. large specific heat transfer planes and small heat transfer paths. The fluid guides are subdivided into two fluid guide fractions, which are layered in alternating order and cross each other, preferably at right angles to one another. Preferably, a micro-heat exchanger is used, which comprises a layered structure with a metal foil stack, wherein the Fluidfuhrungen are incorporated as kanalformige depressions in the stacked and interconnected by diffusion bonding fluid-tight individual foils. Each of these individual films has a plurality of similar, preferably parallel to each other Fluidfuhrungen only one Fluidfuhrungsfraktion.

After passing through the heat exchanger of the now cooled biodiesel from ^¬ reaches the heat Uber exit point 17. In this area form the aforementioned crystals 19 methyl esters from saturated Fettsaureme-. This is supported by the fact that the partial flow cross section expands abruptly with the leaving of the Fluidfuhrungen and thus relaxes the partial flow spontaneously. To judge the amount and appearance of the crystals 19 in the partial flow, is to visual Control (eg by subjective visual inspection odör by an optical method such as extinction measurements or transmission measurements) between the heat exchanger 16 and 20 return to the storage vessel 1 a sight glass 18 used. What is desired here is a high nucleation rate, ie the smallest possible and finely distributed crystals in the partial flow. These are introduced via the return to the storage vessel in the upper third and serve as seed crystals for the biodiesel located there.

For precise determination of the crystallization conditions, the temperature at the inlet and outlet of Biodiesel and Kuhlmediumpassagen the heat exchanger 16 with the help of Temperaturmessfuhlern (temperature sensors on the heat transfer inlet 28, the Warmeubertrageraustritt 29, the Kuhlmediumzulauf of the heat exchanger 30 and Kuhlmediumablauf the heat exchanger 31) measured and therefrom the temperature gradient determined in the heat exchanger. A further temperature sensor 32 is located in the interior of the storage vessel 1. In addition, 16 pressure sensors (pressure sensor on the heat transfer inlet 33 and the heat exchanger outlet 34) are mounted on both passage inputs of the heat exchanger to detect an incipient possible blockage of the heat exchanger.

This device allows the above-mentioned winterization of biodiesel fuels in two different ways as follows:

Experimental Example 1

The temperature of the biodiesel is in the heat exchanger 16 abge lowered by adjusting the temperature in Kuhlmittelthermostaten 21 ^¬ until 18 crystals 19 show in the sight glass. Thereafter, the temperature of the Kuhlmantels 3 of the Vorratsgefaßes 1 is slowly lowered by adjusting the temperature in the thermostat 5. The temperature of the cold medium circuit in the heat exchanger 16 is kept constant. The crystals formed are passed through the partial flow circuit in the storage vessel 1 and sediment there to the ground, be but whirled up again by the magnetic stirrer 4, so that the crystals are distributed as crystallization nuclei in the entire storage vessel. By regular sampling, the decrease of the saturated fatty acid methyl ester in the liquid biodiesel can be determined by means of gas chromatography. The temperature in the storage vessel is then lowered until the desired composition of the biodiesel 2 is reached.

Experimental Example 2

The temperature of the biodiesel is lowered in the heat exchanger 16 by adjusting the temperature in the Kuhlmittelthermostaten 21 until 18 show crystals 19 in the sight glass. Thereafter, the temperature of the Kuhlmantels 3 of the Vorratsgefaßes 1 is slowly lowered by adjusting the temperature in the thermostat 5. After a certain time for crystal formation and crystal growth (about 5 hours), the two pumps IO and 24 and the magnetic stirrer 4 are switched off. In the storage vessel 1 crystals formed sink down, during which the biodiesel is sampled in the storage vessel at regular intervals and analyzed. If the concentration of the saturated fatty acid methyl ester does not change any more or only slightly, the temperature of the storage vessel 1 is further lowered via the thermostat 5. This process is repeated until the desired composition of the biodiesel is achieved.

In the present exemplary embodiment, biodiesel from used cooking oil (AME) was used whose composition was previously determined. The proportion of saturated fatty acid methyl ester at the beginning of the experiment was 22%.

Figures 2 and 3 show the time course of the cumulative Kon ^¬ concentration (concentration curve 38) of the saturated Fettsauremethylester of AME and the temperature in Vorratsgefaß 1 (tempera ^¬ turverlauf 39) as functions of time for the Experimental Example 1 (Fig.2 ) or Experimental Example 2 (Fig.3). In both diagrams is the Biodiesel bath temperature 35 in ° C and the concentration of AME 36 in wt.% Over time 37 in minutes applied.

It should be noted, however, that the equilibrium temperature had not always set when sampling. From these diagrams, a relationship between temperature and decrease of the saturated methyl ester in the storage vessel 1 can be seen. Over the entire measuring period, a cooling rate of 0.02 ° C./h and a decrease in concentration of 0.03% by weight / h were obtained for Experimental Example 1; for Experimental Example 2, a cooling rate of 0.04 ° C./h and a decrease in concentration of 0.06 wt% / h. So you get the same result in half the time. This result leads to the conclusion that the time required for winterization can be significantly reduced by optimizing the cooling rate.

FIG. 4 shows the temperature profiles for both experimental examples (via the concentration 41, in each case as a point quantity and as a geometrically-averaged straight line, straight line of test example 1 lies below that of test example 2) in the storage vessel of AME (biodiesel temperature 35) and the empirical course of FIG CFPP values 42 (CFPP value 40) as functions of the sum concentration of saturated fatty acid methyl esters (concentration 36). The CFPP is a measure of the winter strength of biodiesel. It increases with increasing concentration of saturated fatty acids. During the measurements, care was taken in both of the above experimental examples that bath temperatures were above the CFPP values for the same concentration of the saturated fatty acids.

An advantageous development of the method is the use of a scattered light photometer to uwa monitor the crystal formation exactly. When light scattering photometer light is coupled through an optical window in the biodiesel, the measured gestreu ^¬ on the crystals te light and it is true, the concentration of the crystals be ^¬. This allows for example the flow rate or the temperature so Gere ^¬ be gel that always a constant concentration of crystals is present. For cooling the heat exchanger 16 instead of a liquid coolant, in this case an ethylene glycol-water mixture, it is also possible to use another cold medium, for example R134a, which evaporates within, ie in the cold medium fluid ducts, of the heat exchanger. In order to avoid cold medium losses, the cold medium circuit must necessarily be connected in a fluid-tight manner, wherein the temperature during the evaporation can be adjusted via the pressure in the cold medium.

A further advantageous embodiment comprises a use of countercurrent micro-heat exchanger (countercurrent Mikrowarmeübertragern) instead of the cross-flow described in the exemplary embodiment Mikrowarmeubertragers. This allows a gleichmaßigere temperature distribution can be achieved at the output of the heat exchanger.

LIST OF REFERENCE NUMBERS

1 storage vessel

2 biodiesel

3 cooling jacket

4 magnetic clocks

5 thermostat

6 coolants

7 coolant supply line

8 coolant pressure line

9 suction line

10 pump

11 microfilters

12 heat exchanger feed

13 water thermostat

14 water

15 hose

16 heat exchangers

17 Warloader Carriage

18 sight glass

19 crystals

20 return

21 Cooling medium thermostat

22 cooling medium

23 cooling medium drain

24 cooling medium pump

25 Cooling medium filter 26 Cooling medium line

27 Kuhlmediumzulauf

28 Temperature sensor at the heat transfer inlet

29 Temperature sensor at the heat exchanger outlet

30 Temperature sensor on the cooling medium inlet of the heat exchanger

31 Temperature sensor on the cooling medium outlet of the heat exchanger

32 temperature sensor in the storage vessel

33 Pressure sensor on the heat transfer inlet

34 Pressure sensor at the heat exchanger step

35 Biodiesel temperature Concentration of AME time temporal concentration course temporal temperature profile CFPP value temperature profile over the concentration empirical course of the CFPP value

Claims

Patent claims:

1. A process for the solution crystallization of mixtures of substances, comprising the following process steps: a) providing a liquid mixture (2) comprising at least one partial substance having an increased melting point, b) taking a partial flow (9) from the mixture (2), c) supplying the Partial flow to a heat exchanger (16), wherein the partial flow flows through the heat exchanger, c) cooling of the partial flow in the heat exchanger to a temperature below a crystal formation temperature only the partial substances of the substance mixture, wherein the temperature is adjusted so that in the partial flow directly at the exit from the Heat exchangers (16) form crystals (19) of the sub-substances, d) mixing the partial flow with the crystals in the mixture (2), wherein the crystals in the mixture selectively bind other substances by crystal growth and e) sedimentation and deposition of the crystals in the mixture.

2. The method of claim 1, comprising a thorough mixing after mixing of the partial stream and prior to sedimentation

3. The method of claim 1 or 2, wherein the partial flow is guided in the heat exchanger in a plurality of Fluidfuhrungen with a diameter of less than 1 mm.

4. The method according to any one of the preceding claims, wherein the mixture is relaxed at the outlet.

5. The method according to any one of the preceding claims, wherein the heat exchanger is tempered with a fluid.

6. The method of claim 5, wherein the fluid is a mixture of ethylene glycol and water.

7. The method of claim 5, wherein the fluid has a boiling temperature which corresponds to the temperature below the crystal formation temperature of only the partial substances and evaporates during the process in the heat exchanger (16).

8. The method according to any one of the preceding claims, wherein the substance mixture (2) during the crystal growth is not mixed and cooled separately.

9. The method according to any one of the preceding claims, wherein the mixture of substances (2) consists of a mixture of saturated and unsaturated methyl esters.

10. The method according to any one of claims 1 to 7, wherein the mixture of substances (2) is a fuel.

The method of claim 9, wherein the fuel is a biodiesel of used cooking oil.

12. The method according to any one of the preceding claims, wherein the heat exchanger (16) is a micro-Kreuzstromwaremubertrager or a micro- Gegenstromwaremubertrager