Polymerizer Control Scheme
I. Introductionof the Production Process
The production process of PVC polymerizerstarts from quantitatively adding various materials participating intopolymerization in closed conditions (closed feeding quantitative control), thatis, to take AIBN and IPP as initiator, polyvinyl alcohol as a dispersant, andwater as dispersion and heat transfer medium, and add VC to heat up throughcold mixing, and then enter normal stages of polymerization. During thereaction period, the temperature in the polymerizer shall be kept constant (polymerizationtemperature control), it is polymerized to PVC after 6 to 8 hours,and then the pressure is reduced for material discharge, and the entirereaction is exothermic. During the batch production control process, from rawmaterial feeding to the product discharge, each step of operation shall becarried out in accordance with the requirements, each step of transfer shallmeet certain conditions, it is difficult to control the process objects, thefunction of control system shall be strong, the integrity of control programshall be good, the entire control process can run through the previous and nextsteps, and it can automatically identify production status and push it forwardby step to complete the entire production process of polymerization.
The main devices of DOS system control in theentire polymerization section include: polymerizertemperature control, closed feeding quantitative control, recycling system,public system and so on.
Conventional control scheme includes: hot watertank temperature control, main pipe pressure control, recovery systemtemperature, pressure, level control, etc.; complex control scheme is mainly polymerizertemperature control.
II.Polymerization Temperature Control
In PVC polymerization process,it is the key to producing qualified PVC to effectively control polymerizerfeeding and polymerization temperature, andpolymerization temperature is particularly important, so the requirements fortemperature control in actual production process are very strict. The controllaws required at different stages are different, and the flexible controlscheme of distributed control system is very suitable.
The PVC polymerizer temperaturecontrol during the reaction basically uses cascade regulation scheme oftemperature in polymerizerandjacket temperature, taking the reaction temperature in polymerizer as mainparameters to be adjusted and jacket temperature as secondary parameters, asshown in Figure 2. The use of jacket temperature auxiliary loop can quicklyovercome upstream pressure, temperature and other perturbations of cold and hotcarriers, that is, temperature auxiliary loop can quickly play the role of“coarse adjustment”, and temperature main loop can play the role of “fineadjustment”. However, as for the temperature control of polymerizer, thedifficulty lies in the process from heating up to reaction, which is atransition control from heat absorption to heat release. When being close toreaction point temperature, the objects are extremely sensitive to cold or hotconditions, if the transition process is not controlled well, substantialfluctuations will be caused in temperature in the long period of time atinitial reaction. If the transition process is controlled by artificial heatingup and artificial transition, the control method of cascade regulation is inputafter the reaction is stable, large fluctuations will be caused in thetransition process of the regulation method, and the quality of PVC may bedirectly affected sometimes.
In order to shorten the heating time to shortenthe polymerization cycle and increase the quantity of production, the heatingspeed is faster, it is better. Thus, step-type control is implemented at thebeginning of heating, so as to make the temperature rise as soon as possible.
In order to avoid over-adjustment and make thetemperature in polymerizer have a smooth transition, the measures to changegiven values and adjusting laws shall be taken. At heating stage, cascadecontroller uses less control action (especially the integral action is weak),the given values increase at a certain rate, because constant value controlledis implemented at the beginning, the deviation is very large, and adjustoroutput is quickly saturated, affecting the normal control. After going into thenormal polymerization stage, in order to eliminate the residual error oftemperature control, integral action and proportional action must bestrengthened, and DCS will automatically adjust the parameters of cascade controller.
DCS temperature control process of PVC polymerizeris summarized below. After feeding and cold mixing of polymerizer, the systemwill go into heating stage, DCS system will automatically open hot water inletvalve (fully open), hot water above 90 ℃will be supplied by hot water tank, which will be pumped intopolymerizer jacket to heatthe materials in the polymerize, so as to temperature of materials in thepolymerizer to rise at the fastest rate to induce polymerization.
When the temperature in thepolymerizerrises to a certain value, hotwater inlet valve will be gradually closed, and jacket and baffle inlet valveremain closed, at that time hot water in jacket and baffle is in internalcirculation, the temperature in the polymerizer continuesto rise, but the heating rate slows down. With the rise of temperature, it goesinto transition stage. At this stage, the given values of cascade controlmodule will be provided by program setting module, because it needs to bechanged from heating absorption to heat release at the beginning ofpolymerization, so the control action shall be weak, and weak integral actionshall be used to prevent over-adjustment. In this process, the position ofjacket inlet valve will be slowed opened in accordance with the changes inwater temperature of jacket and reaction temperature in polymerizer,and a certain amount of cold water will go into it,so as to control that the temperature rise rate in the polymerizer isconsistent with the heating curve requested.
If the adjustment action of jacket inlet valveis insufficient to control the temperature, due consideration can be given tothe adjustment action of baffle valve according to the specific conditions ofthe site. When the temperature of polymerizer rises to the value whichis 0.2 ℃ different to theset reaction temperature (related to PVC resin model), the control action ofmain ring in cascade control module will be automatically strengthened, thegiven value remains unchanged, and it is the set temperature.
After the system enters the normal reactionstage, DCS will remove the excess heat of reaction through adjusting thecooling water going into jacket and baffle, so as to ensure a constant reactiontemperature in the polymerizer and the quality of PVC products. At differentreaction stages of constant temperature process, if it is needed at the site,P, I, D parameters may be changed to meet the requirements.