Technical area

The present invention concerns a system and a method for the regulation of a continuous vapour phase digester that is used for the production of cellulose pulp.

The prior art

In existing systems for a continuous vapour phase digester with an inlet at the top for finely divided cellulose material and an outlet at the bottom for cooked pulp, the vapour phase digesters have always been dimensioned based on the criterion that the digester is fully filled with fluid. This dimensioning technology has been the rule for what are known as "Kamyr digesters" of vapour phase type since Metso Fiber Karlstad AB (then under the name Kamyr AB) installed the first digester in Fengerfors in 1950 with a capacity of 50 ADMT/24 hours, until the vapour phase digesters that were installed in 2007 with capacities approaching 4,000 ADMT/24 hours in Indonesia in lndah Kiat, Jambi and Lontar Papyrus.

When continuous operation has been established, this has meant that the digester has been over-dimensioned, since the fluid level during normal operation is established a considerable distance below the top of the digester. In large digester plants, the fluid lies approximately 10 metres below the top of the digester when operation has been established, which corresponds to a reduced static pressure on the digester shell of 1 bar. The digester is dimensioned also for a nominal target value for the pressure in the gas phase, which may be a pre-determined target value in the interval 3-8 bar. In a digester of height 100 metres with a pressure in the gas phase of 5 bar, the design pressure will be the sum of the static pressure from the fluid-filled digester and the pressure of the gas phase, total 15 bar.

The dimensioning of the digester shell thus is based primarily on a fluid-filled vessel and the relevant target value for the pressure in the gas phase, and secondarily on the relevant temperature, which determines the heat load on the digester vessel. There may also be considerations of the fact if the digester is installed in a geographical zone that is subject to earthquakes. All of these dimensioning criteria entail the thickness of material being increased not only in order to satisfy the primary dimensioning criteria but also to satisfy the secondary dimensioning criteria, which further increases the consumption of material and costs. If it is possible to influence, for example, the primary dimensioning criteria, then it will be possible to manuTacture the complete digester with significantly lower consumption of material and investment costs.

The purpose of the invention A first purpose of the invention is to manufacture a continuous digester for predetermined operational parameters, where the cost of the digester can be reduced by an amount approaching 10%, while still satisfying the dimensioning criteria.

A second purpose is to reduce the consumption of sheet metal in order to achieve an economical manufacture of the digester that does not place as great a load onto the available supply of raw materials. A reduced load on our natural resources has become evermore important during the past decades. It is required that certain requirements for economical equipment be satisfied for certain deliveries.

A third purpose is to ensure a margin against extreme operating conditions of the digester that deviate from the nominal operating conditions during established stable operation.

Brief description of the invention

The invention concerns a system to regulate a continuous vapour phase digester, with an inlet at the top for finely divided cellulose material and an outlet at the bottom for cooked pulp. The relevant process inflows, such as the inflow of chips and associated cooking fluids are regulated by a process regulator, depending of the current process outflows, such as the outflow of cooking fluids and the outflow of cooked pulp. The regulation takes place in order to establish a gas phase above a level with cooking fluid in the digester and a level of chips in the digester that lies above the level of fluid at least during established continuous operation, which level of chips can be regulated in order to adjust the downwardly directed force on the column of chips in the digester.

The process regulator has a first set of sensors that are connected to the process regulator and are used to control a first set of regulators in the inputs of finely divided cellulose material and cooking fluids to the digester.

According to the invention, the system comprises a load controller that is independent of the process regulator, which load controller has a second set of sensors that are connected to the load controller, and used to control a second set of regulators in the inputs of finely divided cellulose material and cooking fluids to the digester. The process regulator regulates the pressure in the gas phase, the level of chips and the level of fluid with respect to a first set of target values in the process regulator, while the load controller monitors the pressure in the gas phase, the level of chips and the level of fluid with respect to a second set of threshold values in the load controller, which give in their interaction a higher load on the digester vessel than the control of the process regulator gives with respect to the relevant target values.

When these threshold values are exceeded, the load controller closes at least one of the relevant inflows with the second set of regulators that are connected to the load controller.

The second set of regulators, which are connected to the load controller, are preferably constituted by open flow valves during normal established operation, which valves are closed when the threshold values of the load controller are exceeded. These flow valves are independent of the regulation exerted by the process regulator on the outflows from and inflows into the digester.

The second set of regulators, which are connected to the load controller, may alternatively be constituted by current controllers for flow-promoting machines that convey the inflows or outflows to or from the digester. These current controllers interrupt the operating power to the flow-promoting machines when the threshold values of the load controller are exceeded. Also these current controllers are independent of the regulation exerted by the process regulator on the outflows from and inflows into the digester.

The method according to the invention concerns the regulation of a continuous vapour phase digester, with an inlet at the top for finely divided cellulose material and an outlet at the bottom for cooked pulp. The relevant process inflows, such as the inflow of chips and associated cooking fluids, are regulated by a process regulator depending on the current process outflows, such as the outflow of cooking fluids and the outflow of cooked pulp. The regulation takes place in order to establish a gas phase above a level with cooking fluid in the digester and a level of chips in the digester that lies above the level of fluid at least during established continuous operation. The level of chips can be regulated in order to adjust the downwardly directed force on the column of chips in the digester

According to the method, the inputs of finely divided cellulose material and cooking fluids to the digester are controlled with respect to a number of first process parameters for a first dedicated process regulator, while the load on the digester vessel is monitored by a load controller that is independent of the process regulator. The process control regulates the pressure in the gas phase, the level of chips and the level of fluid with respect to a first set of target values in the process regulator, while the load controller monitors the pressure in the gas phase, the level of chips and the level of fluid with respect to a second set of threshold values in the load controller. The pressure in the gas phase, the level of chips and the level of fluid give in their interaction a higher pressure load on the digester vessel than the control of the process regulator with respect to the current target values gives, and when these threshold values are exceeded the load controller closes at least one inflow to the digester, independently of the regulation that the process regulator exerts on this inflow. It is possible with this division into two independent systems in the form of a process regulator and a load controller, each one of which has independent sensors and regulators, to operate the digester plant with respect to the current target values in the process regulator, and to have an independent safety system for the load controller, which allows the complete digester to be dimensioned with a highest possible total pressure load in the digester.

In one preferred embodiment of the method, the process regulator controls the process with respect to a first target value for the level of fluid and for the pressure at the top of the digester, where the total pressure load onto the digester vessel derived from the current process parameters is given by the static height of the fluid level plus the pressure in the gas phase at the top of the digester, and that the load controller has at least one pre-determined threshold value for the total pressure load that is defined by at least one pre-determined fixed threshold value for the level of fluid in the digester, and a fixed pre-determined threshold value for the pressure in the gas phase. In this variant the threshold value for the load at the bottom of the digester can correspond to a load from a fluid level that lies 10 metres below the top of the digester, at a pressure in the gas phase of 5 bar, and the load controller will be activated if any one of these fixed threshold values are exceeded.

In a second preferred embodiment of the method, the process regulator is controlling the process with respect to a first target value for the level of fluid and for the pressure at the top of the digester, where the total pressure load onto the digester vessel derived from the current process parameters is given by the static height of the fluid level plus the pressure in the gas phase at the top of the digester, and that the load controller has at least one threshold value for the total pressure load that is defined by at least one pre-determined fixed threshold value for the load at the bottom of the digester, given by the sum of the load from the level of fluid in the digester and the pressure in the gas phase. In this variant, the threshold value for the load at the bottom of the digester can correspond to a load from a level of fluid that lies 10 metres below the top of the digester at a pressure in the gas phase of 5 bar, but if the pressure in the gas phase were to be reduced, the threshold value for the level of fluid is raised by a corresponding degree.

In one application of the invention on a continuous vapour phase digester with a safety level of fluid that lies 11 metres below the top of the digester and with a total built height of 72.5 metres with a diameter at the bottom that is greater than 12 metres, and with pre-determined operating parameters with respect to top pressure, capacity and temperature, the total cost (using material prices as at April 2008) can be reduced from approximately SEK 23.5 million to SEK 21.2 million. This is thus a saving of SEK 2.3 million, corresponding to approximately 10%.

Description of drawings Figure 1 shows in principle a system for the regulation of a continuous digester according to the invention; Figure 2 shows an alternative or complementary load controller according to the invention; and

Figure 3 which shows a variant of Figure 2.

Detailed description of the invention The concepts "target value" and "threshold value" will be used in the following detailed description.

The term "target value" is here used to denote a target value of a process parameter against which a regulatory system regulates in order to maintain this value, but where the actual process parameters may deviate to a greater or lesser extent from these target values.

The term "threshold value" is here used to denote a threshold value for a process parameter that may not be exceeded during regulation of a regulatory system with respect to the target values described above. Thus the threshold value is a safety limit for the regulation of the digester.

THE SYSTEM

Figure 1 shows a system to regulate a continuous vapour phase digester 50, with an inlet 51 at the top for finely divided cellulose material and an outlet 52 at the bottom for cooked pulp. The inlet 51 is here constituted by a top separator with upward feed. The digester comprises in a conventional manner a number of digester flows with withdrawal strainers 60/63, pumps 61/64, and with central pipe returns 62/65, but these flows do not constitute any part of the invention as such. Any withdrawal flows from these flows, however, may be monitored as partial outflows from the digester, together with other withdrawal flows.

A process regulator PC regulates the relevant process inflows such as the inflow of chips CHi _N and associated cooking fluids ST, WL, DL, depending of the current process outflows, such as the outflow of cooking fluids LIQ _RE τ, and the outflow of cooked pulp CHOUT- It is appropriate that the inflow of a chips CHIN be measured by the process regulator PC with a flow meter 31 , alternatively by a pump or sluice monitor in the inlet line, and the regulation of this flow can be controlled by the process regulator using a valve 13.

The inflow of cooking fluids ST, WL, DL (steam, white liquor and dilution fluid, respectively) can be controlled by the process regulator PC with the valves 12, 11 and 10, respectively. The regulation by the process regulator takes place in order to establish a gas phase STp above a level LIQ _LE v with cooking fluid in the digester and a level of chips CHLEV in the digester that lies above the level of fluid at least during established continuous operation, which level of chips can be regulated in order to adjust the downwardly directed force on the column of chips in the digester. It is appropriate that the level of chips be detected by the process regulator by the chip level sensor 43.

The process regulator PC has a first set of sensors 40, 41, 43 that are connected to the process regulator and are used to control a first set of regulators 10, 11 , 12, 13 in the input of the finely divided cellulose material and cooking fluids to the digester. The sensors 40, 41, 43 are constituted by pressure sensors, temperature sensors and fluid levels sensors, respectively, in the digester.

It is preferable that the regulators 10, 11 , 12, 13 be regulator valves that regulate the inflow of dilution or cleaning fluid DL, white liquor WL, steam ST and the inflow of chips CHiN. According to the invention, the system comprises a load controller LC that is independent of the process regulator PC, which load controller has a second set 42 of sensors that are connected to the load controller and used to control a second set of regulators 20, 21 , 22, 23 in the inputs of finely divided cellulose material and cooking fluids to the digester.

The process regulator regulates the pressure in the gas phase, the level of chips and the level of fluid with respect to a first set of target values in the process regulator, and the load controller monitors the pressure in the gas phase, the level of chips and the level of fluid with respect to a second set of threshold values in the load controller, which give in their interaction a higher load on the digester vessel than the control of the process regulator with respect to the relevant target values gives. When these threshold values are exceeded, the load controller closes at least one of the relevant inflows with the second set of regulators 20, 21 , 22, 23 that are connected to the load controller. The second set of regulators 20, 21 , 22, 23 that are connected to the load controller are constituted by open flow valves that are closed when the threshold values of the load controller are exceeded, and where these flow valves are independent of the regulation of the inflows and outflows to the digester performed by the process regulator.

The second set of regulators 20, 21 , 22, 23, which are connected to the load controller, may also be constituted by current controllers for flow-promoting machines that promote the inflows or outflows to the digester, which current controllers interrupt the operating power to the flow-promoting machines when the threshold values of the load controller are exceeded, and where these current controllers are independent of the regulation of the inflows and outflows to the digester by the process regulator.

The sensors 40, 41 , 42, 43 in both the process regulator PC and the load controller LC are normally installed in at least sets of pairs, in order to achieve increased security in both the regulation and the load control. Only one position is, however, specified in the drawing, while several sensors can be located at each position in order to achieve this redundancy. For example, 3-5 sensors for the level of chips 43 are usually installed in order to obtain a secure indication and check that the sensors are working correctly.

For increased redundancy at a second level, it is possible also that a check that the sensors 42 for the load controller LC are functioning correctly by their validation with respect to signals from the process regulator through a suitable transfer of signals shown as X in the drawing.

THE METHOD

The method according to the invention regulates a continuous vapour phase digester 50 with an inlet 51 at the top for finely divided cellulose material and an outlet 52 at the bottom for cooked pulp. A process regulator PC regulates the relevant process inflows such as an inflow of chips CH| _N and associated cooking fluids ST, WL, DL, depending of the current process outflows, such as the outflow of cooking fluids LIQRET, and the outflow of cooked pulp CHOUT- The regulation takes place in known manner in order to establish a gas phase STp above a level LIQLEV with cooking fluid in the digester and a level of chips CHLEV in the digester that lies above the level of fluid at least during established continuous operation. The level of chips can be regulated in order to adjust the downwardly directed force on the column of chips in the digester.

According to the invention, the inputs of finely divided cellulose material and cooking fluids to the digester are controlled with respect to a number of first process parameters for a first dedicated process regulator PC, while the load on the digester vessel is monitored by a load controller LC that is independent of the process regulator.

The process regulator PC regulates the pressure in the gas phase, the level of chips and the level of fluid with respect to a first set of target values in the process regulator. The load controller monitors the pressure in the gas phase, the level of chips and the level of fluid with respect to a second set of target values in the load controller, which give in their interaction a higher load on the digester vessel than the control of the process regulator with respect to the relevant target values gives. When these threshold values are exceeded, the load controller closes at least one inflow to the digester, independently of the regulation of this inflow exerted by the process regulator on this inflow.

THE METHOD WITH FIXED THRESHOLD VALUES FOR FLUID LEVEL AND PRESSURE

According to one preferred embodiment, the process regulator regulates the process with respect to a first target value for the level of fluid and for the pressure at the top of the digester, where the total pressure load exerted on the digester vessel from the current process parameters is given by the static height of the level of fluid plus the pressure at the top of the digester. In this implementation, the load controller has at least one threshold value for the total pressure load, which is defined by at least one pre-determined fixed threshold value for the level of fluid in the digester, and one pre-determined fixed threshold value for the pressure in the gas phase. In an example of the implementation in a digester of height 100 m, the process regulator can regulate the process with respect to target values of the level of fluid that lie 15 metres below the top of the digester and a target value of 5 bar in the vapour phase. The regulation of the inflows and outflows to the digestion process thus takes place such that these target values are achieved, but where the relevant values for these process parameters are allowed to deviate during the regulation. The load controller, however, may in this case have a pre-determined threshold value for the level of fluid that lies 10 metres below the top of the digester and a threshold value of 8 bar in the vapour phase. As soon as any one of these threshold values is exceeded, the load controller activates a forced closure of at least one inflow. It is possible in this way to achieve load security of the digester vessel that is independent of the regulation of the digestion process.

THE METHOD WITH FIXED THRESHOLD VALUES FOR THE TOTAL LOAD

According to a second preferred embodiment of the invention, the process regulator regulates the process with respect to a first target value for the level of fluid and for the pressure at the top of the digester, where the total pressure load exerted on the digester vessel from the current process parameters is given by the static height of the level of fluid plus the pressure at the top of the digester.

In this implementation, the load controller has at least one threshold value for the total pressure load, which is defined by at least one pre-determined fixed threshold value for the load at the bottom of the digester, given by the sum of the load from the level of fluid in the digester and the pressure in the gas phase.

In an example of the implementation in a digester of height 100 m, the process regulator can regulate the process with respect to target values of the level of fluid that lie 15 metres below the top of the digester and a target value of 5 bar in the vapour phase. The regulation of the inflows and outflows to the digestion process thus takes place such that these target values are achieved, but where the relevant values for these process parameters are allowed to deviate during the regulation. The load controller, however, may in this case have a pre-determined threshold value for the total pressure load that corresponds to a situation in which the level of fluid lies 10 metres below the top of the digester and a threshold value of 8 bar in the vapour phase. The load at the bottom of the digester, however, is given by the sum of the load from the level of fluid in the digester and the pressure in the gas phase, which in this case corresponds to a column of fluid of height 90 metres and a pressure of 8 bar in the gas phase, i.e. 9 plus 8 bar, giving a total of 17 bar. These 17 bar will then be the representative threshold value. Under these conditions, it is possible for the level of fluid in the digester vessel to be increased somewhat if the pressure in the gas phase is at the same time reduced, as long as the load does not exceed 17 bar. If, for example, the pressure in the gas phase were to fall by 0.5 bar to 7.5 bar, the level of fluid can be allowed to increase to a level of fluid that lies 5 metres below the top of the digester. Alternatively, the pressure in the gas phase can, in these conditions, be somewhat increased if the level of fluid in the digester vessel is at the same time reduced, as long as the load does not exceed 17 bar. If, for example, the pressure in the gas phase were to increase by 0.5 bar to 8.5 bar, the level of fluid cannot be allowed to increase to a level of fluid that lies 15 metres below the top of the digester.

As soon as the threshold values of 17 bar is exceeded, the load controller activates a forced closure of at least one inflow. It is possible in this way to achieve load security of the digester vessel that is independent of the regulation of the digestion process.

REGULATORY MEASURES

Several regulatory strategies can be used when a relevant threshold value is exceeded.

A simple implementation carries out all regulatory measures at the same time as soon as the threshold value is exceeded. This security strategy is often easiest to validate during system installation. It may mean that all inflows that can be shut down are shut down immediately. The flow of chips CHIN, the flow of white liquor WL, the flow of dilution fluid DL and the flow of steam ST, for example, may all be closed down instantaneously. Alternatively, the number of inflows that are closed down can increase in stages, and depending on the margin by which the threshold value has been exceeded, or if a regulatory effect has not been achieved within a certain activation period. A first such stage may be the closure of the flow of chips CH| _N , followed in sequence by the flow of white liquor WL, the flow of dilution fluid DL, and the flow of steam ST.

If a pre-determined fixed threshold value for the gas pressure is exceeded, the parameters that have a dominating influence on the gas pressure may be closed instantaneously or in sequence, such as the flow of steam ST, the flow of chips CHIN, and the flow of white liquor WL.

If a pre-determined fixed threshold value for the level of fluid is exceeded, the parameters that have a dominating influence of the level of fluid may be closed instantaneously or in sequence, such as the flow of white liquor WL, the flow of dilution fluid DL, the flow of chips CHIN, and the flow of steam ST.

In addition to closure of the inflows, the outflows from the digester, such as the outflow of cooked pulp CHOUT, may be increased in the regulatory measures specified above, as may also the withdrawal flows REC from the digester flows. "REC" here is used to denote the withdrawal of cooking fluid consumed in the digester, which can be passed directly to a recovery process or may pass through a black liquor impregnation that lies before the digester.

COMPLEMENTARY OR ALTERNATIVE LOAD CONTROL

Figure 2 shows a load controller that may be present in addition to the system that is shown in Figure 1 , or that may fully replace the load controller shown in Figure 1.

A dumping line 70 for cooking fluid is present in this variant, which line is controlled by a valve 71 that is normally closed. This dumping line is connected in the drawing to a withdrawal compartment 73 behind a strainer section 74 such that the inlet to the line is protected from clogging by chips. The withdrawal compartment 73 may be part of an existing digester flow, and where the withdrawal compartment 73 may possibly be supplemented by reverse rinsing at regular intervals or on an intermittent basis. The valve in this variant is controlled by a pressure sensor 42 and the valve 71 can be opened by the load cell DP ₂ when the pressure exceeds the pre-determined level. A pump 72 may also be installed in the line 70 in order to promote the dumping flow, which pump is activated in parallel with the opening of the valve 71.

The line 70 can be laid to a suitable position in the digestion plant such as, for example impregnation fluid to a preceding impregnation vessel of IMPBIN™ type, or it may be sent to flash tanks and subsequently to a recovery process.

An alternative to Figure 2 is shown in Figure 3. The dumping line 70 for cooking fluid in this variant of load control is connected to a chamber 75 that is in direct communication with the internal space of the digester, preferably through a strainer section in this chamber in the wall of the digester. The pressure sensor 42 is here also connected to this chamber 75. Both the inlet to the line and the pressure sensor 42 can in this way be protected from clogging by chips.

In the simplest form of load controller in Figures 2 and 3, the valve 71 can be replaced by a mechanical rupture disc that ruptures when the pressure exceeds the pre-determined level. As is the case for previous pressure sensors, the pressure sensor 42, the dumping line 70 and the valve 71 may be present in several parallel sets in order to increase the reliability of the safety system.

The pressure measurement by the sensor 42 is preferably located in all illustrated embodiments as close as possible to the level for the target value for the level of fluid, either in the region 0-5 metres above the level corresponding to the target value of the level of fluid, as is shown in Figure 2, or in the region 0-5 metres below the level corresponding to the target value of the level of fluid, as is shown in Figure 3. If the measurement of pressure were to take place at an excessive distance from the top of the digester, the risk arises that the chips in the digester form a suspended plug that insulates against pressure differences, and that separates the upper part of the digester from the lower part of the digester, and it is for this reason that detection of pressure far down in the digester may not be representative for the pressure level established at the top of the digester.

It is appropriate that the threshold value for the level of fluid for the load controller in all embodiments correspond to a level at least 5 metres below the top of the digester when the gas pressure in the top of the digester is at the relevant target value for the gas pressure. If the threshold value is set to, for example, 10 metres below the top of the digester and the target value for the gas pressure is 4 bar, then the level of fluid can be allowed to be raised to 5 metres below the top of the digester if the gas pressure falls 0.5 bar below the target value.

The invention can be varied in a number of ways within the framework of the patent claims. The critical characteristic is that the independent safety function of the load controller takes place.