PROCESS AND APPARATUS FOR CONVERTING STARCH TO GLUCOSE

Abstract: In a process of converting starch to glucose in a reactor (6) the starch slurry used as starting material is pre-heated in at least three stages (2, 3, 4). In the first and third of these stages (stages 2 and 4) the slurry is heated by heat exchange with a flow of converted liquor from the reactor (6), whereas in the second pre-heating stage (3) the slurry is heated by heat exchange with pressurised steam which heats the slurry through the peak of the viscosity/temperature curve sufficiently rapidly to avoid difficulties due to clogging of the plant.

Description: •PROCESS AND APPARATUS FOR CONVERTING STARCH TO GLUCOSE This invention relates, to a continuous process of converting starch to glucose wherein a pre-heated starch slurry is conducted through a reactor in which conversion takes place under heat and pressure. The term "glucose" is here used in a general sense and includes not only dextrose but also other hydrolysis products resulting from different degrees of conversion of starch. The invention also relates to apparatus for use in such a conversion process. Examples of processes and apparatus of this kind are described in United States patent specification No 2 735 792, and in United Kingdom patent specification Nos 836 764 and 1 124 747. The convertible starch slurry used in a said process can be an acidified or an enzyme-containing slurry. The temperature to which the slurry is heated for effecτing the conversion is a factor which influences the speed and degree of hydrolization of the starch. Other influential factors include the type of starch used as starting material and the pH value of the slurry. The conversion level is a matter for choice within certain limits. The temperature of the converted starch liquor leaving the reactor is therefore liable to vary from one plant to another but it is in any case appreciably higher than 100 C. and generally exceeds 130 . The converted liquor leaving the reactor has to be cooled for the purpose of further processing, which can include neutralisation, decolorising and concentration stages. Economic considerations make it very desirable to use as effectively as possible the BAO ORIGINAL j heat extracted from liquor. The importance of productive heat utilisation becomes more important as world energy prices increase. The liquor leaving the reactor is usually depressurised in a flash vat. In some cases the developing vapour is released into the atmosphere. This involves a large energy wastage. As an example, the heat recoverable by reducing the temperature of such a vapour from 140° to 85 C is about 370 kJ per kg of glucose produced. Taking steam cost to be £6.00 per ton, this energy represents a cost of about £50,000 per annum in a plant producing 50,000 tons of glucose per annum. - In order, to make productive use of-heat energy of the vapour it is known to .use it as heating medium in the evaporator for concentrating the converted glucose- containing liquor. By effecting condensation of the vapour under vacuum its temperature can be reduced in the evaporator to the temperature, e.g. about 65°C, at which the concentrated liquor is subjected to the next step in the glucose production process. However, use of the flash vapour as a heating medium entails a number of disadvantages. It is difficult to achieve the flash-off in such a way that there is no carry over of glucose by the vapour. The difficulty is aggravated by the tendency for froth formation to occur in- consequence of the depressurising of the liquor. Entrainment of glucose in the vapour . means a loss of yield of the plant unless the very dilute vapour condensate is subsequently re-evaporated to permit recovery of the entrained glucose. Moreover when using the flash vapour in a heat exchanger the vapour " generally has to be mixed with resh vapour rom another source in order to maintain the necessary balance between the heat of the vapour and the heat demand in the exchanger. In these circumstances a larger volume of condensate must be re-evaporated for recovery of the glucose content if this is-not to be allowed to run to ' waste. The present invention departs from the known practice of flash-vaporising liquid from the reactor outflow and using such vapour as a heat- exchange medium. The invention involves use of out-flow liquor from the reactor as a heat exchange medium for heating the starch slurry preparatory to its conversion in the reactor, and enables the energy consumption of the conversion plant to be substantially reduced. The said use of outflow liquor as heat exchange medium is effected in such a manner that a substantial amount of the heat content of the entire volume of outflow liquor from the reactor can be trans¬ ferred to the inlet flow without increasing the problems associated with the well known tendency of a starch . slurry to undergo rapid gelaτination during heating through a certain temperature range. According to the present invention, there " is. provided a continuous process of converting starch to glucose wherein a pre-heated starch slurry is conducted through a reactor in which conversion takes place under heat and pressure, characterised in that before said conversion the slurry is pre-heated i'n three successive stages in the first and third of which stages said slurry is heated by heat exchange with a flow of converted liquor from the reactor, and in the second of which three stages the slurry is heated .by another heat source, which is at a higher temperature OMP than said converted liquor, through a temperature range in which the slurry exhibits a marked increase and then a decrease in viscosity. The abrupt and marked increase in the viscosity of a starch slurry during its heating in a glucose prod¬ uction plant gives rise to problems unless counter- measures are taken to prevent clogging of the plant. It is known that in a continuous glucose production plant the clogging of the passageways in which the slurry stream is heated can be prevented or reduced by ensuring that the slurry is heated sufficiently through the critical temperature range. As referred to in United States Patent 2 735 792 already referred to, the exertion of pulsed pressure and/or a kneading action on the slurry during its rapid heating through the critical range can also assist in preventing problems caused by gelatination. A high flow velocity can likewise contribute to that end. In the second preheating stage in a process according "to the present invention the starch slurry can be. heated sufficiently rapidly to avoid any or any -serious depositions of gel. The heating of the slurry in this second stage can be confined to a narrow temperature interval, say of 35 or less, and the heat consumption in this stage can therefore be modest. It is very suitable for the second stage pre-heating to be performed in a heat exchanger using steam under pressure as the 2 heating medium,e.g.steam at8-12 Kg/cm with a condensation temperature of 170-190 C, in the same way as in a conventional plant in which the heating of the slurry ' " and its subsequent conversion takes place in a tubular convertor. In carrying out the present invention, measures additional to the use of the relatively high BAD ORIGINAL D . temperature heat exchange medium in the second pre¬ heating stage, for example an increased pumping pressure, pressure pulsations and/or the maintenance of mixing or shear forces within the slurry stream, can be adopted if required in order to assist in preventing obstruction or excessive flow resistance as a result of gelatination. Generally, all that is required is to maintain a sufficiently high flow velocity through the second pre¬ heating stage but a directional change along the slurry flow path can also assist. It would not be possible to employ the outflow liquor from the reactor for heating the starch slurry through the critical temperature range because its temperature ievel would be insufficient for heating the initial starch slurry through that range with sufficient rapidity for the purpose in view. In a process according to the present invention, considerable use is nevertheless made of the potentially valuable heat content of the converted liquor for heating the slurry. This is made possible by transferring heat from the liquor to the starch slurry before and after a further heating stage in which another and higher temperature heat source is used for raising the temperature of the slurry through its viscosity maximum on the viscosity/temperature curve. The critical temperature range of a starch slurry depends upon various factors, including the kind of the starch, its pH value and its solids concentration, but in all practical cases the heat required to bring the temperature of the slurry to a value well within the critical temperature range is much less than the available heat content of the outflow liquor from the BAD ORIGINAL reactor. The yield of heat from the liquor in the first heating stage must therefore be restricted, but the liquor is caused to yield further heat to the starch slurry after it has passed- the most critical temperature. 5 The extent of that further yield does not of course have to be-restricted to avoid problems caused by gelatination. As compared with a conversion plant wherein vapour flashed from the liquor is released into the atmosphere, 10 a plant according to this invention can be operated with a steam saving of 50% or more. The heat exchangers used can be of simple design. By using efficient heat insulation, heat losses therein can be kept very low, e.g. a fraction of 1%. 15 Preferably the temperature of the slurry is increased by the steam or other higher temperature heat source through the entire temperature range corresponding with the viscosity peak on the viscosity/ temperature curve. That is the preferred procedure 20 because in those circumstances it is much easier to avoid interference with the smooth continuance of the process and impairment of product quality. However the invention is not restricted to that optimum procedure. For example the temperature of the slurry 25 can be increased in the first stage to just above the lower end of the temperature range corresponding with the viscosity peak or the slurry can leave the second of the speci ied three heating stages at or before its temperature has reached the upper end of that 30 range. Although less satisfactory those alternative procedures would enable some benefit to be realised * BADORIGINAL f) O.ΛP from the three-stage heating procedure; Any tendency for clogging to occur would depend upon circumstances from case to case and could be kept very low e.g. by ensuring that the heating function of the reactor outflow does not encroach upon the critical temperature range of the starch to any significant extent or by exerting sufficient pressure or shear forces on or within the slurry stream during the temperature interval(s) in which the heating rate would otherwise be too low to prevent objectionable flow resistance or obstruction. In all cases however the higher temperature heat source should be relied upon for increasing the temperature of the slurry through at least the greater part of the range corresponding with the said viscosity peak, including of course the temperature corresponding with the viscosity maximum. A particularly valuable field of use of the invention is the conversion of maize starch to glucose. The critical temperature range of maiz« starch slurries mostly used as feedstock in glucose production plants generally lies within the range 60 to 90 C, its actual limits depending upon the pH of the slurry and its solids concentration. In preferred embodiments of the invention the starch slurry is heated in the first and third of the aforesaid heating stages by a flow of converted liquor which is conducted from the reactor first through the said third and then through said first stage. This procedure is conducive to the most efficient utilisation of heat content of the converted liquor. It is advantageous for the first and third heating stages to take place in different parts of a common heat exchange 5UB5T.TU1 * £ S apparatus through which the reactor outflow stream' used as heating medium flows to transfer heat to the starch slurry. As herein exemplified, the three specified heating stages can take place in a single heating unit combining three heating sections. The invention includes apparatus suitable for use in carrying out a starch conversion- process according to the invention as hereinbefore defined. Apparatus according to the invention comprises means for pre- heating a starch slurry stream preparatory to conversion of the starch in a reactor in which said conversion can take place under heat and pressure conditions, and means for. continuously passing the preheated slurry stream through said reactor, and is characterised in that said pre-heat-ing means comprises heat-exchangers for pre-heating the starch slurry stream in three successive stages; the first and third stage heat exchangers are connected to the reactor to cause outflow liquor from the reactor to flow through said first and third stages in heat exchange relationship to said slurry stream, and the second stage heat exchanger has means for connecting it to a source of heating medium at higher temperature than said outflow liquor to cause such higher temperature medium to flow through said second stage in heat exchange relationship to said slurry stream. Apparatus according to the invention can include any feature or features appropriate for making use of one "or more of the various optional process features hereinbefore referred to. For example the ' slurry pre-heating means may be arranged for conducting said " stream of outflow liquor first through said third and then through said first stage heat-exchangers. OMP Reference is now made to the accompanying ■ ■ drawings, in which Fig.l represents an example of a starch conversion plant for use in carrying out the invention;. Fig.2 is a diagrammatic ' cross-sectional elevation of a heater integrating three of the heating stages shown in Fig.l; and Fig.3 is a viscosity/ temperature graph. In the plant shown in Fig.l an initial starch slurry feeding a starch slurry pump 1 passes through heat exchangers 2,3,4- and 5 before entering a reactor 6 in which the conversion to glucose takes place. The three pre-heating stages hereinbefore specified take place in heat exchangers 2,3 and 4. The heating effected in.exchanger 5 is an additional heating stage for bringing the slurry to reaction temperature. In heat exchangers 2 and 4 the slurry stream is heated by heat exchange with the converted outflow liquor from the reactor 6. Such outflow is conducted by line 7 to heat exchanger 4 and on leaving that exchanger the outflow stream is conducted by line 8 to • heat exchanger 2. The cooled outflow liquor leaves exchanger 2 via line 9. After depressurisation in reduction value 10 it is conveyed for further processing in manner known per se. After its first stage heating in heat exchanger 2 the starch slurry passes via line 11 to heat exchanger 3. In this heat exchanger the starch slurry is rapidly heated by a relatively high temperature heat source HS (in this instance steam) through a temperature range in which the slurry undergoes a rapid increase and then a decrease in its viscosity. In the heat exchanger 3 the slurry stream undergoes a directional change. -The rate of heat transfer in this heat exchanger combines with the adequately high flow velocity to control gelatination so as to avoid clogging of the apparatus or excessive flow resistance. Condensed steam leaves the heat exchanger 3 at 12. ' The starch slurry leaving heat exchanger 3 is conducted via line 13 to heat exchanger 4 and it then passes via line 14 to heat exchanger 5 in which it is further heated by steam under pressure to bring it to an appropriate temperature preparatory to feeding into the reactor 6 via line 15. The .temperature in this reactor, and also the pH of the slurry and.its residence time in the reactor, influence the degree of conversion " of the starch. The conditions imposed for avoiding clogging, which includes a high rate of heat transfer are confined to the heat exchanger 3 which is only responsible for supplying about one third of ' the heat demand of the slurry prior to the conversion reaction. The balance of that heat demand is supplied in heat exchangers 2,4 and 5 which are of a very simple design and offer less flow resistance. The requirement for at least three heating stages does not necessitate an equal number of physically separate heaters. For example the three heat exchangers 2,3 and 4 in the plant shown in Fig. 1 can be combined in a single unit. An example of how this can be done is shown in Fig. 2. The unit is of generally cylindrical form and comprises two axially aligned elongate sections 16, 17 joined by flanges 18,19. A manifold cap 20 is joined to section 17 by flanges 21,22. The manifold cap 20 provides the inlet and outlet passages for the starch slurry. These passages are separated within the manifold cap by a partition 23 which also extends along and divides the elongate section 17 up to a position short of the remote end of that section. Each of the conjoined sections 16 and 17 provides' a part of the outer shell of the unit and within each shell part there is a plurality of spaced tubes 24. The tubes in section 17 communicate with the interior of the manifold cap 20 and with the tubes in section 16. The cool starch slurry entering the manifold cap 20 through its inlet port (the lower port in Fig. 2) flows along the unit and back again, through the tubes 24. During this flow through the unit, the slurry is successively heated first in section 17 by outflow liquor from the reactor 6, then by steam in section 16, " and then in section 17, again by outflow liquor, before leaving the unit through the outlet port of the manifold cap. The outflow liquor from the reactor, used as heating medium in section 17 of the unit enters that section through the top inlet port near the end where the slurry inlet and outlet passages are located and fills the free space surrounding the tubes 24. The outflow liquor flows around the tubes 24 on the slurry exit side of the partition 23, which forms a kind of baffle, before flowing downwardly past the free end of that partition into the lower part of the shell. In consequence the cool starch slurry entering the unit is heated by converted liquor which has already yielded part of its heat to the slurry in the hottest section of the unit. The thermodynamics of the pre-heating stages in the unit are therefore similar to those of stages 2,3 and 4 in Fig.l. The following is an example of a process according to the invention: BAD ORIGINAL Example A maize starch slurry having a pH of 1.25 and a solids concentration of 40% by weight was used as feedstock in a glucose production plant as represented 5 in Fig. 1. Fig. 3 shows the viscosity temperature graph of the slurry in which the Brookfield viscosity in centipoises is plotted on the ordinate against the temperature on the abscissa. It will be seen from this graph that the peak- of the viscosity/temperature curve 10 extends from approximately 70 to approximately 90?C. That is the critical temperature range. The viscosity maximum is at approximately 78°C. The starch slurry entered heat exchanger 2 at 25°C. The starch was converted to glucose in reator 6. In 15 that reactor the acidified starch slurry was maintained at a temperature of 140 to 145 C and under a pressure 2 between 4 and 5 Kg/cm and the residence time of the slurry in the reactor was from 5 to 20 minutes, sufficient for achieving the required degree of conversion. 20 Tbe outflow stream of converted liquor flowing along line 7 entered the heat exchanger 4 at a temperature of approximately 140 C and under a pressure of 4 Kg/cm or more. On leaving heat exchanger 4 and entering heat exchanger 2 the liquor had a temperature of approximately 25 115 C. On leaving heat exchanger 2 the liquor had a temperature of approximately 85 C which was suitable for the next processing step in the glucose production. Heat losses in the heat exchangers were so small as to be negligible. The heat transferred to the initial 30 starch slurry during its passage through heat exchanger 2 increased its temperature from 25°C to about 55°C. In the heat exchanger 3 in which the starch slurry was heated by steam at 8 to 12 Kg/cm 2 (condensation ' V temperature of 170 to 190°C) the temperature of the starch slurry was rapidly increased to about 95°C. It will be noted from the graph (Fig. 3) that this rapid heating of -the slurry raised its temperature through the 5 entire peak of its viscosity/temperature curve. The heat transferred to the slurry in heat exchanger 4 raised the temperature of the slurry to about 120 C and in heat exchanger 5, in which the heating 2 medium was again steam at 8-12 Kg/cm , the slurry was 10 further heated to 140 C preparatory to entering the reactor 6. The process proceeded very smoothly without problems caused by starch gelatination. The steam consumption of the glucose production 15 plant was less than half that of a plant in which the slurry is heated by steam through the entire temperature range. It will be noted that because the converted outflow liquor from the reactor 6 was used as heating medium 20 without depressurisation and was cooled in heat exchangers' 4 and 2 to below 100°C the disadvantages of foam formation and glucose losses encountered in prior processes described earlier in this specification are avoided. 25 The described and illustrated embodiments of the invention illustrate the fact that the three pre-heating stages specified as the first, second and third pre-heating stages in claim 1 of this specification need not be the only pre-heating stages. In the 30 ' illustrated embodiments there is a fourth pre-heating stage, namely that which occurs in heat exchanger 5. Furthermore, it is not absolutely essential for the said specified first, second and third pre-heating / f stages to be in immediate succession. A further ^ *■ '-* "". BAD ORDINAL < OMPI pre-heating stage could intervene between the first and second -or between the second and third of the said specified three pre-heating stages. But it is much more satisfactory for the said specified three pre-heating stages to be in immediate succession and • for any supplementary pre-heating stage to follow those three stages. The invention has been particularly exemplified above • as applied to the processing of maize starch. The invention can be applied in a similar manner in the conversion of other kinds of starch products to glucose, for example in the conversion of wheat starch or potato starch in which problems associated with gelatination over a certain heating temperature range also occur to a greater or lesser extent. The choice of an enzyme-containing starch slurry as starting material instead of an acidified slurry entails no change in the principles involved in carrying out the present invention and it can be practised also in such cases to achieve important energy savings. BAD ORIGINAL -

Claims: Claims 1. A continuous process of converting starch to glucose wherein a pre-heated starch slurry is conducted through a reactor (6) in which conversion takes place under heat and pressure, characterised in that before said conversion the slurry is pre-heated in three successive stages (2,3,4) in the first (2) and third (4) of which stages said slurry is heated by heat exchange with a flow of converted liquor from the reactor (6) and in the second (3) of which three stages the slurry is heated by another heat source (HS) , which is at higher temperature than said converted liquor, through a temperature range in which " the slurry exhibits a steep increase and then decrease in viscosity. 2. A process according to claim 1, wherein said flow of converted liquor fro the reactor is used as heating medium first in said third pre-heating stage (4) and then in said first pre-heating stage (2). 3. A process according to claim 1 or 2, wherein _n said second pre-heating stage (3) the temperature of the slurry is raised through the entire temperature range corresponding with its viscosity peak on the viscosity/temperature curve. 4. A process according to any preceding claim, wherein the slurry is heated in said second pre-heating stage (3) through a temperature interval of 35 C or less. 5. A process according to any preceding claim, wherein the heat transferred to the slurry by said converted liquor in the first and third pre-heating stages (2 and 4) reduces the temperature of said liquor from its value on leaving the reactor. to a temperature at which the liquor is subjected to vthe next processing step. OMPI W 6. A process according to any preceding claim, wherein the heat source (HS) used for heating the slurry in the second pre-heating stage (3) is steam under super-atmospheric pressure. 7. A process according to any preceding claim, wherein the three said pre-heating stages (2,3,4) take place in a single heating unit (16,17)- combining three heating sections. 8. A process according to any preceding claim, wherein the starch slurry is a maize slurry. 9. A process according to claim 8, wherein the slurry is an acidi ied maize starch slurry and wherein in said second pre-heating stage (3) the temperature of the slurry is raised from not more- than 70 to at least 90°C. 10. A process according to any preceding claim, wherein the said flow of converted liquor is used without depressurisation as heating medium in said first and third pre-heating stages and is cooled in such stages to below 100 C. 11. Apparatus for use in converting starch to glucose, comprising means (2-5) for pre-heating a starch slurry stream preparatory to conversion of the starch in a reactor (6) in which said conversion can take place under heat and pressure conditions, and means (15) for conducting the pre-heated slurry stream through said reactor, characterised in that said pre-heating means (2-5) comprises heat-exchangers (2,3,4) for pre-heating the starch slurry stream in three successive stages; the first and third stage heat exchangers (2 and 4) are connected to the reactor (6) to cause outflow liquor from the reactor (6) to flow through said first and third stages (2 and 4) in heat exchange relationship to said slurry stream,, BAD ORIGINAL OMP and the second stage heat exchanger (3) has means (25) for connecting it to a source (HS) of heating medium at higher temperature than said outflow liquor- to cause such higher temperature medium to flow through said second stage in heat exchange relationship to said slurry stream. 12. Apparatus according to claim 10, wherein said first, second and third stage heat-exchangers (2,3,4) are combined in a single heating unit (16,17,20). 13. Apparatus according to claim 10 or 11, wherein the second stage heat-exchanger (3,16) imposes directional change on the starch slurry during its flow through said heat-exchanger. 14. A continuous process of converting starch to " glucose, substantially according to the Example herein. WIP

Inventors: RIEL JACOBUS ADRIANUS WILHELMU, SCHAFT CORNELIS WILLEM, RIEL JACOBUS ADRIANUS WILHELMUS MARIA VAN
Applicants: RIEL JACOBUS ADRIANUS WILHELMU, SCHAFT CORNELIS WILLEM, ZETMEELBEDRIJVEN BIJENKORF BV, RIEL JACOBUS ADRIANUS WILHELMUS MARIA VAN
Application year: 1982
Main classification: C13K1
IPC codes: C13K1