Process for producing biodiesel and related products

Abstract

There is described a process for producing biodiesel and related products from mixtures. There is also described a process for producing precursors and feedstock materials for producing biodiesel and related products. The processes use esterification and trans-esterification, separation and purification. Other process steps such as acidification and distillation can also be used.

Claims

1. A process for esterifying a mixture comprising between 10% and 20% by weight free fatty acids (FFAs), said process comprising the steps of: (i) providing at least a first portion of the mixture to at least a first reaction vessel; (ii) heating the first portion of the mixture to a reaction temperature for esterification, the reaction temperature being between 71 C. and 76 C.; (iii) introducing esterification conditions to the first reaction vessel, comprising adding an esterification catalyst and an alcohol to the reaction vessel, the esterification catalyst being sulphuric acid; (iv) stopping the heating of the first portion of the mixture; (v) recirculating the first portion of the mixture by removing it from the first reaction vessel and returning it to the first reaction vessel without further external heat being applied; (vi) esterifying the FFAs in the first portion of the mixture; (vii) providing at least a second portion of the mixture to at least a second reaction vessel; (viii) heating the second portion of the mixture to a reaction temperature for esterification, the reaction temperature being between 71 C. and 76 C.; (ix) introducing esterification conditions to the second vessel, comprising adding an esterification catalyst and an alcohol to the reaction vessel, the esterification catalyst being sulphuric acid; (x) stopping the heating of the second portion of the mixture; (xi) recirculating the second portion of the mixture by removing it from the second reaction vessel and returning it to the second reaction vessel without further external heat being applied; and (xii) esterifying the FFAs in the second portion of the mixture; wherein the recirculation of the first portion of the mixture is configured to maintain a reaction temperature suitable for esterification of the FFAs in the first portion of the mixture, the reaction temperature being between 71 C. and 76 C.; wherein the recirculation of the second portion of the mixture is configured to maintain a reaction temperature suitable for esterification of the FFAs in the second portion of the mixture, the reaction temperature being between 71 C. and 76 C.; and wherein the mass of esterification catalyst used relative to the % by weight FFAs in the mixture before esterification is from 26 kg catalyst per % by weight FFAs to 32 kg catalyst per % by weight FFAs, such that the amount of FFAs in the mixture is reduced to 3% by weight or less.

2. A process as claimed in claim 1, wherein the esterification of the first portion of the mixture and the esterification of the second portion of the mixture are at least partly concurrent.

3. A process as claimed in claim 1, wherein the reaction temperature is between 72 C. and 75 C.

4. A process as claimed in claim 1, wherein the mixture comprises between 10% and 15% by weight FFAs before esterification.

5. A process as claimed in claim 1, wherein the alcohol is methanol.

6. A process for esterifying a mixture comprising between 10% and 20% by weight free fatty acids (FFAs), said process comprising the steps of: (i) providing at least a first portion of the mixture to at least a first reaction vessel; (ii) heating the first portion of the mixture to a reaction temperature for esterification, the reaction temperature being between 71 C. and 76 C.; (iii) introducing esterification conditions to the first reaction vessel, comprising adding an esterification catalyst and an alcohol to the reaction vessel, the esterification catalyst being sulphuric acid; (iv) stopping the heating of the first portion of the mixture; (v) recirculating the first portion of the mixture by removing it from the first reaction vessel and returning it to the first reaction vessel; and (vi) esterifying the FFAs in the first portion of the mixture; (vii) providing at least a second portion of the mixture to at least a second reaction vessel; (viii) heating the second portion of the mixture to a reaction temperature for esterification, the reaction temperature being between 71 C. and 76 C.; (ix) introducing esterification conditions to the second vessel, comprising adding an esterification catalyst and an alcohol to the reaction vessel, the esterification catalyst being sulphuric acid; (x) stopping the heating of the second portion of the mixture; (xi) recirculating the second portion of the mixture by removing it from the second reaction vessel and returning it to the second reaction vessel; and (xii) esterifying the FFAs in the second portion of the mixture; wherein the recirculation of the first portion of the mixture is configured to maintain a reaction temperature suitable for esterification of the FFAs in the first portion of the mixture, the reaction temperature being between 71 C. and 76 C.; wherein the recirculation of the second portion of the mixture is configured to maintain a reaction temperature suitable for esterification of the FFAs in the second portion of the mixture, the reaction temperature being between 71 C. and 76 C.; wherein the mass of esterification catalyst used relative to the % by weight FFAs in the mixture before esterification is from 26 kg catalyst per % by weight FFAs to 32 kg catalyst per % by weight FFAs, such that the amount of FFAs in the mixture is reduced to 3% by weight or less; and wherein the esterification of the first portion of the mixture and the esterification of the second portion of the mixture are at least partly concurrent.

7. A process as claimed in claim 6, wherein the reaction temperature is between 72 C. and 75 C.

8. A process as claimed in claim 6, wherein the mixture comprises between 10% and 15% by weight FFAs before esterification.

9. A process as claimed in claim 6, wherein the alcohol is methanol.

10. A process as claimed in claim 6, wherein the esterification catalyst is 96% sulphuric acid.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the invention will now be described, by way of example only, with reference to the drawings, in which:

(2) FIG. 1 is schematic diagram of a process in accordance with one embodiment of the invention; and

(3) FIG. 2 is a flow diagram which illustrates a process in accordance with one embodiment of the invention.

DETAILED DESCRIPTION

(4) The biodiesel production process is broken down into two areas, namely esterification (referred to as pre-esterification) and trans-esterification. The processes as further described below are summarised in the flow diagram of FIG. 2.

(5) The processes described are based on approximately 38 tonnes of feedstock (including recovered FFAs) being used.

(6) Pre-Esterification

(7) Pre-esterification is a preconditioning step that produces useable oil for the trans-esterification process. It involves the catalysed esterification of free fatty acids (FFAs) with an alcohol to provide a fatty acid ester.

(8) Referring to FIG. 1, there is shown at 100 an apparatus for producing biodiesel. The apparatus comprises a feedstock tank 10 from which tallow (animal fat), which comprises FFAs, and recovered FFAs from the by-product stream of an earlier biodiesel production process, are pumped via pump 20 into a first pre-esterification reactor 30. The total amount of FFAs in the feedstock is typically between 10% by weight and 20% by weight. Under normal operation conditions, the combined amount of FFAs (i.e., when tallow and recovered FFAs are combined) is 10% by weight to 15% by weight, but may also be 13% by weight to 15% by weight. In one embodiment, the total amount of FFAs in the feedstock is 14% by weight.

(9) 100 kg of methanol (an alcohol) per tonne of feedstock is then added to the feedstock in the first pre-esterification reactor 30, followed by the addition of 96% by weight sulphuric acid (an esterification catalyst). A range of methanol can be used, varying between 80 kg and 120 kg per tonne of feedstock, and usually between 90 kg and 110 kg per tonne of feedstock. The so-formed mixture is then heated for at least 90 minutes with agitation to a reaction temperature of approximately 73.5 C. The temperature can be between approximately 71 C. and 76 C., and is typically is from between 72 C. and 75 C.

(10) Once the reaction temperature has been reached in the first pre-esterification reactor 30, the mixture is removed from and then returned to the first pre-esterification reactor 30 by recirculation of the mixture such that the reaction temperature is maintained. The recirculation and reaction continues with agitation for approximately 150 minutes, which ensures that the reaction is complete. This so-formed mixture is then allowed to settle for 90 minutes before the water phase is removed.

(11) Once the reaction temperature is reached in the first pre-esterification reactor 30, the feedstock is diverted to a second pre-esterification reactor 40.

(12) 100 kg of methanol (an alcohol) per tonne of feedstock is then added to the feedstock in the second pre-esterification reactor 40, followed by the addition of 96% by weight sulphuric acid (an esterification catalyst). A range of methanol can be used, varying between 80 kg and 120 kg per tonne of feedstock, and usually between 90 kg and 110 kg per tonne of feedstock. The so-formed mixture is then heated for at least 90 minutes with agitation to a reaction temperature of approximately 73.5 C. The temperature can be between approximately 71 C. and 76 C., and is typically is from between 72 C. and 75 C.

(13) Once the reaction temperature has been reached in the second pre-esterification reactor 40, the mixture is removed from and then returned to the second pre-esterification reactor 40 by recirculation of the mixture such that the reaction temperature is maintained. The recirculation and reaction continues with agitation for approximately 150 minutes, which ensures that the reaction is complete. This so-formed mixture is then allowed to settle for 90 minutes before the water phase is removed.

(14) The amount of pre-esterification catalyst used is from approximately 26 kg catalyst per % by weight FFAs to approximately 32 kg catalyst per % by weight FFAs.

(15) The quality of the feedstock may deviate on a day to day basis, which can have a significant impact on the following steps of the biodiesel production process to the extent that in known prior art processes such steps may not be viable. For example, if too many impurities are present this can prevent different phases from being separated and/or this can lead to a very poor quality end product.

(16) Thus, fluctuations in FFA content must be taken into account to establish the amount of catalyst that is required to achieve a feedstock suitable for further processing (i.e., suitable for trans-esterification). A suitable endpoint may be when the total amount of FFAs in the mixture is reduced to 3% by weight or less. The inventors have found that a range of pre-esterification catalyst of from approximately 26 kg catalyst per % by weight FFAs to approximately 32 kg catalyst per % by weight FFAs enables a successful pre-esterification to take place.

(17) The pre-esterification process outlined above enables two pre-esterification reactions to take place at least partly concurrently. In particular, once the contents of a pre-esterification reactor reach the reaction temperature, the heat input is removed. The contents of the pre-esterification reactor are then recirculated, the residual heat generated by the reaction enabling the reaction to continue to completion without further external heat being applied. Removing the heat input from one pre-esterification reactor enables a further reaction in another separate pre-esterification reactor to be initiated almost immediately. These improvements reduce the time taken for the pre-esterification process by at least 120 minutes, as compared with using only one pre-esterification reactor of equivalent size. Thus, the process outlined provides a significant improvement in the speed of throughput. Furthermore, the process outlined uses less energy than known pre-esterification processes, due to the use of residual heat of reaction and recirculation of the reaction mixture to maintain the reaction temperature.

(18) Trans-Esterification

(19) The trans-esterification process exchanges the R group on an ester for the R group from an alcohol. In the biodiesel production process it is used to convert triglycerides to fatty acid methyl esters.

(20) Referring once more to FIG. 1, the mixture from the pre-esterification reactors 30, 40 is transferred to a trans-esterification reactor (reaction vessel) 50. The mixture in the trans-esterification reactor 50 is agitated, and recovered methyl ester is added. The recovered methyl ester is obtained later in the process from the separation and purification of the by-products.

(21) 2,900 kg of potassium methoxide and 2,500 kg of methanol are added to the mixture in the trans-esterification reactor 50, and the reaction temperature is adjusted to approximately 55 C. The amount of methanol may vary between approximately 2,000 kg and 3,000 kg. The amount of potassium methoxide used above is based on 2% by weight of FFAs in the mixture. However the amount used can be between 1,100 kg per % by weight of FFA and 1,500 kg per % by weight of FFA, typically 1,150 kg per % by weight of FFA and 1,450 kg per % by weight of FFA. Also, the reaction temperature is a reaction temperature for trans-esterification and can be between approximately 48 C. and approximately 62 C., and is typically between approximately 52 C. and approximately 58 C.

(22) Where the catalyst is methoxide in methanol, the amount of methoxide is 12% to 14% by weight methoxide, typically 12.5% by weight to 13% by weight methoxide. The conditions described above are trans-esterification conditions and, specifically, are used to trans-esterify triglycerides in the mixture.

(23) Using the amounts of potassium methoxide noted ensures that the reaction mechanism is driven to completion i.e., methyl ester is formed and glycerol is removed. Whilst in this example potassium methoxide catalyst and methanol in excess is used, other suitable methoxides can be used such as, for example, sodium methoxide.

(24) A second trans-esterification is then carried out in the trans-esterification reactor 50 using a smaller quantity of potassium methoxide catalyst (approximately 600 kg) and methanol (approximately 1,000 kg). The amount of potassium methoxide catalyst may be between approximately 550 kg to approximately 650 kg. The amount of methanol may be between approximately 900 kg to approximately 1,100 kg. The second trans-esterification reaction ensures that substantially all of the triglycerides in the mixture are converted to esters, and that substantially all of the glycerol is removed.

(25) The reaction temperature is adjusted to approximately 55 C., and the reaction is run for around 300 minutes. The reaction temperature is a reaction temperature for trans-esterification and can be between approximately 48 C. and approximately 62 C., and is typically between approximately 52 C. and approximately 58 C.

(26) An optional separation step can be used to aid settling, particularly if the aqueous and non-aqueous phases of the mixture prove difficult or very time-consuming to separate. The separation step involves spraying an aqueous solution comprising 1,000 kg of recycled water and 10 kg to 18 kg of 75% by weight phosphoric acid onto the surface of the mixture in the trans-esterification reactor 50 at a temperature of approximately 70 C. The temperature may be between approximately 60 C. and approximately 80 C. The acid solution percolates through the mixture and the contents of the trans-esterification reactor 50 are allowed to settle for 60 minutes before the so-formed aqueous phase is discharged into a glycerine (aqueous) phase collection tank 90. This process removes impurities and better enables separation of soaps.

(27) Based on the amounts stated above, the acid solution used in the separation step is 1.0% by weight acid to 1.8% by weight acid, but can be from 1.2% by weight acid to 1.6% by weight acid, typically 1.4% by weight acid.

(28) The mixture, still in the trans-esterification tank 50, is then subjected to a first purification (washing) step. The first purification step involves adding approximately 500 kg of water to the mixture, and mechanically agitating the mixture combined with the water, before allowing to settle for approximately 100 minutes. The amount of water used may vary between 400 kg and 600 kg. An aqueous solution comprising 250 kg (150 kg to 350 kg can be used) of recycled water at a temperature of 70 C. (a temperature of 60 C. to 80 C. can be used) is then sprayed onto the surface of the mixture in the trans-esterification reactor 50 before allowing to settle for approximately 140 minutes. The water percolates through the mixture and the contents of the trans-esterification reactor 50 are allowed to settle for 60 minutes before the so-formed aqueous phase is discharged into a glycerine (aqueous) phase collection tank 90.

(29) The mixture, still in the trans-esterification tank 50, is then subjected to a second purification (washing) step. The second purification involves spraying an aqueous solution comprising 1,000 kg of heated (recycled) water and 14 kg of 75% by weight phosphoric acid onto the surface of the mixture in the trans-esterification reactor 50. The temperature of the solution is 70 C. (a temperature of 60 C. to 80 C. can be used). The acid solution percolates through the mixture and the contents of the trans-esterification reactor 50 are allowed to settle for 60 minutes before the so-formed aqueous phase is discharged into a glycerine (aqueous) phase collection tank 90. This process removes impurities and neutralises residual potassium soaps.

(30) Based on the amounts stated above, the acid solution used in the second purification (washing) step is 0.8% by weight acid to 1.6% by weight acid, but can be from 1.0% by weight acid to 1.4% by weight acid, typically 1.2% by weight acid.

(31) A further (third) purification (washing) step is applied to the mixture in the trans-esterification reactor 50. The third purification step involves spraying an aqueous solution comprising 500 to 1,000 kg of recycled water at a temperature of 70 C. (a temperature of 60 C. to 80 C. can be used) onto the surface of the mixture in the trans-esterification reactor 50. The water percolates through the mixture and the contents of the trans-esterification reactor 50 are allowed to settle for 60 minutes before the so-formed aqueous phase is discharged into a glycerine (aqueous) phase collection tank 90. This third purification process ensures that the vast majority of impurities, free glycerides and soaps are removed prior to the non-aqueous phase of the mixture (which is now crude biodiesel) entering a flash vessel 60.

(32) The percolation of the aqueous solutions through the mixture in the trans-esterification reactor 50 facilitates the removal of impurities from the mixture. The impurities removed include, for example, the following: glycerides, glycerol, methanol, water, salts, acids, bases, condensed volatile compounds and soaps.

(33) Depending on the feedstock material used, some or all of the separation/purification steps described above are required. The separation/purification steps ensure that the process is not unduly lengthy. For example, in known prior art processes settling problems may be encountered, which can then lead to excessive waiting times for the separation of the aqueous and non-aqueous phases. The separation step described above helps to overcome such settling problems should they occur. Overall, the separation/purification (washing) steps described can reduce by around 5 hours the time taken to obtain crude biodiesel from the trans-esterification process.

(34) The crude biodiesel mixture obtained from the separation/purification steps is provided to the flash vessel 60, where the crude biodiesel (fatty acid methyl ester mixture) is steam treated (steam injection) under vacuum to remove water, volatile compounds, and free glycerides, all of which may impact on the final quality or the operation of the vacuum system in the distillation step which follows.

(35) From the flash vessel 60, the mixture is transferred to a distillation apparatus 70, where remaining impurities (such as glycerides, water and sulphur or sulphur containing materials) are removed under vacuum, the pressure being set to approximately 0.1 millibar (10 Pa) to approximately 3 millibar (300 Pa).

(36) The distillation takes place is two stages. The first stage has the following conditions. Steam is injected at a pressure of from approximately 3.5 bar (350 kPa) to approximately 9 bar (900 kPa).

(37) The second stage has the following conditions. Steam is injected at a pressure of from approximately 4.5 bar (450 kPa) to approximately 11 bar (1,100 kPa).

(38) The product produced from the distillation apparatus is purified biodiesel, which is transferred to biodiesel tank 80.

(39) The purified biodiesel may be used as a fuel, or may be blended with other products to produce a fuel.

(40) Impurities are periodically removed from the process. For example, the aqueous (glycerine containing) phase from the trans-esterification is periodically separated into the glycerine phase tank 90. This aqueous phase may contain glycerol, methanol, water, salts, acids, bases, condensed volatile compounds and soaps, for example.

(41) The aqueous phase is further transferred to an acidulation tank 110 to which acid water waste is added. The aqueous phase is then separated into recovered FFAs (recovered FFA tank 120), potassium sulphate (potassium sulphate tank 130) and glycerine/methanol (glycerine/methanol tank 140). The glycerine methanol is further separated into purified glycerine (purified glycerine tank 150) and recovered methanol (recovered methanol tank 160). The materials isolated from the impurities can be reused in a subsequent process for producing biodiesel.

(42) The improved processes as described herein enable the use of impure and poor quality feedstocks to produce high quality biodiesel. To date, it has been otherwise impracticable to use such feedstocks as a source of fats and oils (and greases) for producing fuels such as biodiesel.

(43) In addition, the improved processes as described herein enable a reduction in the throughput time for the production of biodiesel from a feedstock. This results in an increased amount of biodiesel production, which is of benefit. For example, the present inventors have found that their improved processes enable the production of biodiesel in approximately 7 to 8 hours less than the time taken using existing processes. For the present facility used by the inventors, this represents an additional 38 tonnes of biodiesel produced in a 24 hour period.

(44) While this invention has been described with reference to the sample embodiments thereof, it will be appreciated by those of ordinary skill in the art that modifications can be made to the structure and elements of the invention without departing from the spirit and scope of the invention as a whole.