Purge column

Step 3 Feed flows into column 1 at high pressure with a portion of the product used to purge column 2 at reduced pressure. 

For impact copolymer production, a second reactor (4) in series is required. A reliable and effective gas-lock system (3) transfers powder from the first (homopolymer) reactor to the second (copolymer) reactor, and prevents cross contamination of reactants between reactors. This is critically important when producing the highest quality impact copolymer. In most respects, the operation of the second reactor system is similar to that of the first, except that ethylene in addition to propylene is fed to the second reactor. Powder from the reactor is transferred and depressurized in a gas/powder separation system (5) and into a purge column (6) for catalyst deactivation. The deactivated powder is then pelletized (7) with additives into the final products. 

The powder is released periodically to a gas-powder separation system (4). It is depressurized to a purge column (5) where moist nitrogen deactivates the catalyst and removes any remaining monomer. The monomer is concentrated and recovered. The powder is converted into a variety of pelletized resins (6) tailored for specific market applications. 

The bottoms from the DIB contains most of the nC4, along with some iC4 impurity and all of the heavy isopentane impurity. Since this heavy component will build up in the process unless it is removed, a second distillation column is used to purge out a small stream that contains the isopentane. Some n C4 is lost in this purge stream. The purge column has 20 trays and is 6 ft in diameter. The distillate product from the second column is the recycle stream to the reactor, which is pumped up to the required pressure and sent through a feed-effluent heat exchanger and a furnace before entering the reactor in the vapor phase. 

Step 2. This process has 14 control degrees of freedom. They include fresh feed valve DIB column steam, cooling water, reflux, distillate, and bottoms valves purge column steam, cooling water, reflux, distillate, and bottoms valves furnace fuel valve flooded condenser cooling water valve and DIB column feed valve. 

To avoid the high-pressure safety constraint, we must control reactor pressure. We can use the distillate valve from the purge column, the flooded condenser cooling water valve, or the DIB column feed valve. The most logical variable to use for control of the flooded condenser (.reactor) pressure is the DIB column feed valve (as shown in Fig. 5.5). Based upon the discussion in Step 3, we would then use the flooded condenser cooling water valve to keep the liquid level in a good control range. 

Step 6. We have only two choices, DIB column base valve or purge column distillate valve, for fixing a flow in the recycle loop. Either of these would work. The rationale for picking one is based upon avoiding disturbances to the unit downstream of the fixed flow" location. Since the purge column is not critical from the viewpoint of product quality we elect to fix the flow upstream of reactor (purge column distillate flow") so that we minimize disturbances in reactor temperature and pressure. 

There are four liquid levels to be controlled. DIB column reflux drum level is controlled by manipulating distillate product flowrate. We must also control the levels in the DIB column base and in the purge column reflux drum and base. 

Having made the choice to fix the purge column distillate flow, we are faced wdth the problem of how to control purge column reflux drum level. We have two primary choices reflux flow or heat input. We choose the latter because the flowrate of the purge column reflux is small relative to the vapor coming overhead from the top of the column. Remember the Richardson rule, which says we select the largest stream. 

The flowTate of the purge stream from the base of the purge column is quite small, so it would not do a good job in controlling base level. This is especially true when the large steam flow has been selected to control the reflux drum level. Base level in the purge column can. however, be controlled by manipulating the bottoms flowrate from the DIB column. 

Had we started to assign the DIB column base level control first, we would have ended up with the same inventory control structure. The reason is as follows. Assume we had chosen the DIB column base valve to control base level. After resolving the purge column inventory loops, we would have found that we needed to control the purge column base or reflux drum level with the fresh feed flow to the DIB column. The dynamic lags associated with these loops would have forced us back to the control strategy as described above. 

Step 9. When we use reactor inlet temperature for production rate control (irreversible case), the only remaining degrees of freedom for optimization are the reflux flows for the two columns and the setpoint of the distillate flowrate from the purge column (recycle flow). 

In the reversible case, when the base composition of the DIB column or the recycle flowrate is used for production rate control, the remaining degrees of freedom are purge column reflux flow and the reactor inlet temperature. 

In the HYSYS simulation, the flooded condenser is simulated as a cooler with a temperature control loop manipulating cooling water rate. The reactor inlet pressure is set by the exit of the liquid recycle pump on the purge column distillate stream. 

To prepare column for storage purge column of buffers and leave in appropriate solvent. Cap tightly. 

Only in a purge column (a very small stream is removed to get rid of an inert component) can a product stream be fixed (or ratioed to feed flow rate). The distillate stream can be manipulated to control a composition (or temperature), or it could be manipulated to maintain a RR. In this structure, the reflux flow rate is measured, the flow signal is send to a multiplier, and the output signal of the multiplier is the set point of a distillate flow controller. 

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