US20130283829A1

US20130283829A1 - Method for providing a refrigerant medium in a secondary cycle

Info

Publication number: US20130283829A1
Application number: US13/869,260
Authority: US
Inventors: Raymund KOMPA; Markus Foerster; Andreas Walter
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2012-04-25
Filing date: 2013-04-24
Publication date: 2013-10-31

Abstract

A method for providing a refrigerant medium having a controlled flow temperature in a secondary cycle (20), wherein the refrigerant medium in the secondary cycle (20) takes up heat from one or more process coolers (22) and then gives off heat to primary water in a primary cycle (10) before it flows back to the process coolers (22), wherein at least two primary heat exchangers (12, 14) are present for cooling the refrigerant medium, in addition a bypass line (26) in the secondary cycle (20) branches off after exit from the process coolers (22) for bypassing the primary heat exchangers (12, 14), and the temperature in the secondary cycle (20) in the flow to the process coolers (22) is controlled via the setting of the bypass stream.

Description

The present invention relates to a method for providing a refrigerant medium having a controlled flow temperature in a secondary cycle, wherein the refrigerant medium in the secondary cycle takes up heat from one or more process coolers and then releases heat to primary water in a primary cycle before it flows back to the process coolers. In addition, the invention relates to apparatus for carrying out the method according to the invention, wherein the apparatus comprises one or more process coolers in the secondary cycle and also at least one temperature sensor in the flow to the process coolers.
In many engineering processes it is necessary to remove heat from apparatuses or plant components. For this purpose primary water is frequently used which, in particular in the case of large-scale processes, is available as return cooling water, river water or sea water. From the viewpoint of process control, it is desirable that the apparatuses or plant components are cooled at a temperature that is as constant as possible. From the viewpoint of safety and environmental protection, in the event of leaks for example, substances that may be harmful must be prevented from passing into natural water bodies. These requirements can be met by the primary water not coming into direct contact with the apparatuses that are to be cooled, but a refrigerant medium is used in a closed intermediate cycle, also termed a secondary cycle. The refrigerant medium is chosen in accordance with the respective requirements. A common refrigerant medium is water, other examples are glycol-water mixtures or methanol.
The refrigerant medium in the secondary cycle flows at a defined flow temperature into one or more process coolers and there takes up heat from the process, wherein it is heated. In order to bring the refrigerant medium back to the desired flow temperature, it is fed to one or more heat exchangers in which it is cooled by primary water. The circuit of primary water, for example return cooling water, river water or sea water, is designated the primary circuit. Correspondingly, the heat exchangers in which the refrigerant medium is cooled by the primary water, are termed primary heat exchangers.
The division of the cooling into a primary cycle through which primary water flows and a secondary cycle through which refrigerant medium flows is an established technique and offers a number of advantages. In the case of a leak at an apparatus or a process cooler, although possibly harmful substances can pass into the refrigerant medium, the primary water in the primary circuit remains protected from a contamination. Since the secondary cycle is closed, in addition the process coolers in the plant are less fouled than in the case of direct cooling with river water, for example.
This procedure also makes it possible, independently of fluctuations in the primary water due to time of day and season, to set the flow temperature to the process coolers to a desired value. Usually, the primary heat exchangers, with respect to number and dimensioning, are designed in such a manner that they can withdraw sufficient heat from the refrigerant medium in the high load case. For definition of the high load case, a state is assumed in which the temperature difference between the refrigerant medium entering the heat exchanger and the incoming primary water has a defined minimal permissible value. At a given value for the incoming refrigerant medium, this gives a maximally permissible value for the incoming primary water. In Central Europe, the high load case is therefore generally in the summer months in which river water can achieve temperatures of 28° C. or more, for example.
In contrast, a low load case is taken to mean when the temperature difference between the refrigerant medium entering into the primary heat exchangers and incoming primary water has high values. In Central Europe, this case customarily occurs in the winter months when the temperature of river water can fall, for example, to values of 4° C. or below. In addition, a low load case is considered to be when only very little heat needs to be transferred from the refrigerant medium in the secondary cycle to the primary water, for example when a plant is not operated at maximum capacity or is entirely shut down, such that in the primary coolers less heat is transferred from the refrigerant medium to the primary water. In order, in these cases, to keep the flow temperature of the refrigerant medium to the process coolers at the same value as in the high load case, less primary water is required, such that customarily the flow rate of primary water to the primary heat exchangers is reduced.
This procedure is disadvantageous in that, in the low load case, owing to the low flow velocities, the primary heat exchangers on the primary water side have a tendency to fouling. In addition, control of the flow temperature to the process coolers via the primary water feed stream to the primary heat exchangers is sluggish.
The object was to provide a method of the type in question for cooling, in which fouling of the primary heat exchangers can be reduced and the flow temperature to the process coolers can be controlled rapidly and robustly.
This object is achieved by a method according to the invention as claimed in claim 1 and also by apparatus according to the invention as claimed in claim 7. Advantageous embodiments of the invention are specified in dependent claims 2 to 6 and 8, respectively.
In the method according to the invention for providing a refrigerant medium having a controlled flow temperature in a secondary cycle, the refrigerant medium in the secondary cycle takes up heat from one or more process coolers and then gives off heat to primary water in a primary cycle before it flows back to the process coolers. At least two primary heat exchangers are present for cooling the refrigerant medium. After the exit from the process coolers and before the intake into the primary heat exchangers, in the secondary cycle a bypass line branches off for bypassing the primary heat exchangers. The bypass line opens out into the conduit of the secondary cycle from the primary heat exchangers to the process coolers. According to the invention, the flow temperature to the process coolers in the secondary cycle is controlled via the setting of the bypass stream. “Flow temperature” here and hereinafter is taken to mean the temperature in the secondary cycle in the flow to the process coolers. “Flow to the process coolers” designates the section of the secondary cycle which is situated between the intake of the bypass line into the secondary cycle and the first process heat exchanger. “Return” designates the section of the secondary cycle which is situated between the outlet from the at least one process heat exchanger and the branch of the bypass line. In the case of a plurality of process heat exchangers, “return” designates the section of the secondary cycle which is situated between the union of the exit lines from the process heat exchangers and the branch off of the bypass line.
In a preferred embodiment of the invention, a temperature sensor is present in the flow to the process coolers, and the bypass line is provided with an actuator, using which the flow temperature to the process coolers is controllable in the secondary cycle. A plurality of temperature sensors can alternatively be provided in the flow to the process coolers, for example in order to achieve redundancy in measurement. Suitable temperature sensors, for example thermocouples, determine the temperature of the flowing refrigerant medium and provide a value for transmission to a control appliance.
In a further embodiment according to the invention, a temperature sensor or a plurality of temperature sensors is/are situated in the return of the secondary cycle, and the bypass line is provided with an actuator, using which the flow temperature to the process coolers in the secondary cycle is controllable.
Suitable actuators permit the flow through the bypass line to be set between a minimum value and a maximum value. Preferably, the minimum value is equal to zero, which means a completely closed bypass line. The maximum value preferably corresponds to a completely open bypass line. Between the extreme values any desired values for the flow may be set, preferably continuously without steps. Suitable actuators are known to those skilled in the art, e.g. flaps, ball valves or three-way valves.
In a preferred embodiment, a control appliance is present which has a preset valve for the flow temperature. From a comparison of the preset value with the flow temperature determined by the temperature sensor, the control appliance generates an output signal for transmission to the actuator. The temperature of the refrigerant medium which flows through the bypass line is higher than the temperature of the refrigerant medium which flows from the primary heat exchangers to the process coolers. The controller therefore, in the case of a flow temperature that is too high compared with the preset value, makes it possible to reduce the flow through the bypass line and correspondingly, in the case of a flow temperature which is too low compared with the preset value, makes it possible to increase the flow through the bypass line.
In a further embodiment according to the invention, a control appliance is present which has a preset value for the return temperature. From a comparison of the preset value with the return temperature determined by the temperature sensor, the control appliance generates an output signal for transition to the actuator. In this embodiment also, the controller makes it possible, in the case of a return temperature that is too high compared with the preset value, to reduce the flow through the bypass line, and correspondingly in the case of a return temperature which is too low in comparison with the preset value, to increase the flow through the bypass line.
The flow of the refrigerant medium through the bypass line can also be affected in that actuators are present in the refrigerant medium conduits to the primary heat exchangers or from the primary heat exchangers and can be adjusted appropriately in their degrees of opening. Controlling the bypass stream by influencing an actuator in the bypass line, however, is simpler to implement and is therefore preferred.
The control appliance can be an independent instrument, for example a compact controller which is connected by information technology to the temperature sensor and the actuator. The control appliance can also be implemented in combination with the actuator, for example in the form of a control valve. Alternatively, the control appliance can also be integrated in a higher level system for process control, for example in a process control system.
In a preferred embodiment of the method according to the invention, the primary heat exchangers are designed with respect to number and dimensioning for a high load case. In the low load case, the cooling capacity of the primary cycle is adapted by shutting off one or more of the primary heat exchangers, wherein at least one primary heat exchanger remains in operation. The expressions “high load” and “low load” are taken to mean operating states as defined above.
In an additionally preferred variant, the primary heat exchangers are designed in such a manner that, in the low load case, one heat exchanger is sufficient in order to cool the refrigerant medium to the desired flow temperature, utilizing a maximally permissible temperature difference between primary water intake and outlet. For the high load case, correspondingly more heat exchangers are provided which are preferably individually likewise designed for the maximally permissible temperature difference between primary water intake and outlet, and in sum are suitable for the minimum temperature difference in the high load case. The maximally permissible temperature difference between primary water intake and outlet is frequently officially regulated, for example to a value of 15 K. By this measure, the primary water can be utilized efficiently over the entire seasonally variable load range and the required minimum amount of primary water can be markedly reduced.
The bypass line is preferably designed with respect to its capacity in such a manner that the control range of the bypass line is sufficient in order to control smoothly the flow temperature in the secondary cycle when a primary heat exchanger is shut off and switched in.
In the design of the bypass line, in addition, it is preferably taken into account that daily fluctuations of the primary water temperature can be compensated for by controlling the flow through the bypass line alone. Switching in or shutting off primary heat exchangers in this event is not provided.
Particularly preferably, the flow of primary water through the primary heat exchanger or heat exchangers is kept substantially constant. The flow in this case is not controlled actively, but results from the pressure difference between inlet and outlet of the primary heat exchangers on the primary water side. In the event of fluctuations of this pressure difference, fluctuations in the flow rate also result. If the temperature of the primary water falls, or if the amount of heat to be removed from the refrigerant medium decreases, e.g., owing to a decrease in the flow rate of refrigerant medium through a primary heat exchanger or due to a decrease of the refrigerant medium temperature on exit from the process coolers, the refrigerant medium temperature falls at the exit from this primary heat exchanger. Correspondingly, the exit temperature of the refrigerant medium increases in the event of a temperature elevation of the primary water and/or an increase of the amount of heat to be withdrawn from the refrigerant medium.
In an additionally preferred embodiment of the invention, the capacity is adapted by switching in or shutting off primary heat exchangers in such a manner that the pressure drop of the primary water on flow through the primary heat exchangers in operation is in each case at least 300 mbar, particularly preferably at least 800 mbar. This markedly reduces the probability that deposits form on the primary water side.
The at least two primary heat exchangers can be connected in different ways. Preferably, the primary heat exchangers are connected in parallel both on the part of the primary cycle and also on the part of the secondary cycle.
Primary heat exchangers which can be used are all heat exchangers which are known for this purpose to those skilled in the art, preferably, plate heat exchangers or tube-bundle heat exchangers are used, particularly preferably sealed or welded plate heat exchangers. The particularly preferred plate heat exchangers are usually designed for a high pressure drop. This is advantageous if the bypass is to be implemented without additional transport appliances such as pumps.
In a preferred embodiment of the method according to the invention, the primary water is return cooling water, river water, sea water or brackish water. “Return cooling water” is taken to mean here water which has been cooled by an appliance such as a cooling tower or a recooler in process engineering plants.
Compared with the method known from the prior art, the invention offers a plurality of advantages. The provision of at least two primary heat exchangers which together provide the required cooling capacity in the high load case, but in the low load case can be in part shut off, makes it possible to operate the individual primary heat exchangers with virtually constant flow on the primary water side, which prevents premature fouling. In addition, this offers the possibility of alternately switching in and shutting off the primary heat exchangers in the low load case, which makes possible simple inspection and optionally maintenance or cleaning. In addition, the minimum amount of primary water required for providing the cooling capacity is markedly reduced. A further advantage is considered to be that a control of the flow temperature is considerably simpler, faster and more robust to effect using the bypass stream than control via the flow rate of primary water, as is practiced in the prior art.
The invention will be described in more detail hereinafter with reference to the drawings, wherein the drawings are to be understood as outline depictions. They do not represent any restriction of the invention, for example with respect to the number, type and connection of heat exchangers. In the drawings:
FIG. 1: shows an outline diagram of a cooling system according to the prior art
FIG. 2: shows an outline diagram of a cooling system according to the invention
FIG. 3: shows an outline diagram of a cooling system according to the invention having secondary-side series-connection of the primary heat exchangers
FIG. 4: shows an outline diagram of a cooling system according to the invention having primary-side flexible connection of the primary heat exchangers
FIG. 5: shows a time course of the primary water temperature and the number of primary heat exchangers in operation
FIG. 1 shows a cooling system according to the prior art, in which a refrigerant medium flows in a secondary cycle 20 to process coolers 22, takes up heat there and gives off heat in a primary heat exchanger 12 to primary water in a primary cycle 10, before it flows back to the process coolers 22. The process coolers can be of different types, for example plate, tube-bundle, spiral heat exchangers or jackets of tubes or containers for cooling thereof. The flow temperature of the refrigerant medium upstream of the process coolers 22 is determined using a temperature sensor and controlled by a control appliance 24 to a certain preset value. The amount of primary water in the primary cycle 10 acts as the control variable for the control.
In FIG. 2, a first preferred embodiment of the method according to the invention is shown. The refrigerant medium which leaves the process coolers 22 is passed through two primary heat exchangers 12, 14, where it gives off heat to primary water in a primary cycle 10. In the preferred case shown, the primary heat exchangers are connected in parallel both on sides of the primary cycle and also on sides of the secondary cycle. Between the exit of the refrigerant medium from the process coolers 22 and its entry into the primary heat exchangers 12, 14, a bypass line 26 branches off which opens out again into the secondary cycle 20 downstream of the exit of the refrigerant medium from the primary heat exchangers. The flow temperature of the refrigerant medium to the process coolers 22 is controlled 24 via the setting of the flow rate in the bypass line. In the high load case, both primary heat exchangers 12 and 14 are in operation, whereas in the low load case, the capacity of one primary heat exchanger is sufficient in order to cool the refrigerant medium in the secondary cycle 20 adequately. In this case, one of the primary heat exchangers is shut off by closing the corresponding valves in the secondary cycle.
In FIG. 3, a further preferred embodiment of the method according to the invention is shown. In this example, the primary heat exchangers 12 and 14 are parallel-connected on the side of the primary cycle and series-connected on the side of the secondary cycle. In order to be able to shut off the primary heat exchanger 12 or 14 in the low load case, in the secondary cycle, bypasses are provided which can be switched in and shut off via valves.
FIG. 4 shows a further preferred embodiment of the method according to the invention, in which the primary heat exchangers 12, 14 are parallel-connected on the secondary side. On sides of the primary cycle, the connection is kept flexible. By corresponding opening and closing of the valves shown by way of example, a series connection or a parallel connection may be effected on the primary side. In addition, the primary heat exchangers 12, 14 can be alternately shut off by closing the corresponding valves in the secondary cycle.
The figures serve only for illustration. Configurations and connections deviating from the depictions of course likewise come under the invention provided that the control of the flow temperature in the secondary cycle is performed by adjusting the bypass stream.
In particular, the number of the heat exchangers shown in the figures is only an example and is not restricted thereto. It is advantageous if more than two primary heat exchangers are present. The more primary heat exchangers are available, the more flexibly may individual primary heat exchangers be switched in and shut off, in order to perform optimal matching to the load state currently occurring. Secondly, this also increases the capital costs. Preferably, two to three primary heat exchangers are provided.
A selection criterion for the number of primary heat exchangers may be derived from the temperature gradients of the primary water. Preferably, the optimum number of primary heat exchangers is estimated by the quotient of the maximally permissible temperature difference and the typical temperature difference in the high load case. For the location of Ludwigshafen on the Rhine, Germany, for example, the maximum temperature difference permitted by regulation between primary water intake and outlet is 15 K when river water is used as primary water. In addition, the water returned to the river must not exceed a value of 33° C. In the high load case in the summer months, in which the river water can reach temperatures of 28° C., therefore, temperature differences between primary water intake and outlet of 5 K are usual. This gives a quotient of 15 K/5 K=3. Therefore, advantageously, three primary heat exchangers are provided which are designed in such a manner that each of the three heat exchangers alone can withdraw the required amount of heat from the refrigerant medium in the secondary cycle when the maximally permissible temperature difference of 15 K is exploited fully.
In FIG. 5, the time course of the primary water temperature T (dashed curve) and the number of primary heat exchangers N in operation (continuous lines, right-hand scale) is shown schematically over a time period of 12 months. In this exemplary depiction, it is assumed that the amount of heat to be removed from the secondary cycle remains constant over the time period under consideration. The primary water used is river water which has the lowest temperature in the winter months December and January, e.g. 4° C. The maximally permissible temperature difference can be fully exploited and so one primary heat exchanger is sufficient in order to cool adequately the refrigerant medium in the secondary cycle. As soon as the river water increases above a value from which, taking into account a certain range of fluctuation, the maximally permissible temperature difference can no longer be ensured, a further primary heat exchanger is put in operation. At an assumed range of fluctuation of 3 K and a maximum value of 33° C. for the water given off into the river, a value of 15° C. is given, from which the second primary heat exchanger is put into operation. In the example according to FIG. 5, this is the case in the middle of April. From the start of June, the temperature of the river water increases to a value from which the third primary heat exchanger is required in order to remove reliably, firstly the required amount of heat, and secondly, not to exceed the maximum value of 33° C. Over the summer months of June, July and August, three primary heat exchangers are in operation until the river water temperature has again fallen to the extent that two primary heat exchangers are sufficient, in the example at the start of September. At the end of October, the river water temperature has fallen further, e.g. to below 15° C., such that one primary heat exchanger is again sufficient.
The method according to the invention has the effect that cooling capacities can be provided flexibly adapted to the current requirements. In the example shown in FIG. 5, for a period of five and a half months, one primary heat exchanger is in operation, for a period of three and a half months, two primary heat exchangers are in operation, and for a period of three months, three primary heat exchangers are in operation. In comparison with a pure design for the high load case, the amount of primary water required by the method according to the invention can be reduced drastically.
On the primary water side, a substantially constant amount of water flows through each primary heat exchanger, which prevents fouling. Apart from in the summer months, the heat exchangers which are not yet in operation can be maintained and cleaned without problem, without adversely affecting the operation of the plants in the secondary cycle. Under the assumption that in the case of plants according to the prior art, in which only one primary heat exchanger designed for the high load case is present, in which in the low load case the amount of primary water is reduced, once a year a fouling-related plant shutdown of approximately 3 days is required, using the method according to the invention, the plant capacity can be increased by approximately 1%. In the case of more frequent or longer shutdown times, the economic advantage increases correspondingly.

Claims

1. A method for providing a refrigerant medium having a controlled flow temperature in a secondary cycle (20), wherein the refrigerant medium in the secondary cycle (20) takes up heat from one or more process coolers (22) and then gives off heat to primary water in a primary cycle (10) before it flows back to the process coolers (22), wherein at least two primary heat exchangers (12, 14) are present for cooling the refrigerant medium, in addition a bypass line (26) in the secondary cycle (20) branches off after exit from the process coolers (22) for bypassing the primary heat exchangers (12, 14), and the temperature in the secondary cycle (20) in the flow to the process coolers (22) is controlled via the setting of the bypass stream.

2. The method according to claim 1, wherein number and dimensioning of the primary heat exchangers (12, 14) are designed for a high load case, and in a low load case the refrigeration capacity of the primary cycle (10) is adapted by shutting off one or more of the primary heat exchangers (12, 14), wherein at least one primary heat exchanger remains in operation.

3. The method according to claim 2, wherein the capacity is adapted in such a manner that the pressure drop of the primary water across the primary heat exchangers (12, 14) in operation is in each case at least 300 mbar, preferably at least 800 mbar.

4. The method according to claim 1, wherein the primary heat exchangers (12, 14) are designed for a maximally permissible temperature difference between primary water intake and primary water outlet.

5. The method according to claim 1, wherein the primary heat exchangers (12, 14) are connected in parallel both on the part of the primary cycle (10) and also on the part of the secondary cycle (20).

6. The method according to claim 1, wherein the primary heat exchangers (12, 14) are plate heat exchangers or tube-bundle heat exchangers, in particular sealed or welded plate heat exchangers.

7. Apparatus for carrying out the method according to claim 1, comprising one or more process coolers (22) in the secondary cycle (20) and also at least one temperature sensor in the flow to the process coolers (22), wherein at least two primary heat exchangers (12, 14) are present in which the refrigerant medium of the secondary cycle (20) can release heat to the primary water of the primary cycle (10), in addition, a bypass line (26) is present which, in the secondary cycle (20), branches off after exit from the process coolers (22) for bypassing the primary heat exchangers (12, 14) and is provided with an actuator, using which the temperature in the secondary cycle (20) is controllable in the flow to the process coolers (22).

8. The apparatus according to claim 7, wherein the primary heat exchangers (12, 14) are connected in parallel not only on sides of the primary cycle (10) but also on sides of the secondary cycle (20).