2 replies to this topic

[mehul]

as we have distillation column of 1,3 BD , have pressure control on cooling water flow at inlet of overhead condenser, now problems is due to jerking operation of PIC in every 1min. of cycle from 40% opening to 10%opening having overhead pressure indication of 3.4kg/cm2 to 3.37kg/cm2,tell me what will be the effect to bottom re boiler (vertical thermosyphon with limited bottom purge rate product) heating profile and column PDI (flooding) and overhead product purity. feed contain 99.0% 1,3 BD. 0.9%cis butene , 0.1% 1,2 BD.

[aransari85]

Hi Mehul,

Interseting problem... Are you facing this problem in operation?

How the pressure control loop is working, is it controlling the condenser bypass line to reflux drum, or some other way please explain.

In any column when column pressure drops down then direction effects are:

bottom pressure will also drop down, and How is the reboiler control working? is it the column tray temperature control? for this case heaviers will starts boiling up, in ur case Cis-butene will start coming to the overhead of column, inturn column vapor rate increases and may lead to flooding if the loads of vapor crosses hydraulic design of column.

But in ur case i dont see 3.4 and 3.37kg/cm2 is big change in pressure in the column top.
i guess there might be problem with column top pressure control tunning as its opening is changing from 40 to 10% which is drastic change in OP in 1min if its regular.

You can try putting this pressure control in the manual mode and keep in between 10-40% say 25% and monitoring the pressure. if it ready fluctuates in 1 min from 3.4 to 3.37 if it doesnt then these is controller tunning issue. if its still fluctuating then process issue is there.. need to cusult expert to resolve.. may be there problem of flow regin in the condenser outlet line.

Best Regards,
ARA

[mehul]

first of all let me tell you that we have hot vapor bypass type controlling system over column and reflux drum is mount over the condenser in other word condenser is at ground level and reflux drum is elevated.now come to the point yes we have problems of overhead purity and column flooding phenomena at a given condition , now coming to bottom reboiling operation , we have operated VT reboiler managing with hot condensate of system at 97 oC i/l and 60 oC o/l on shell side and H/C is at tube side having bottom tray temp control with 48 oC means at given set point flow control valve opened and bottom purge of heavy component started to withdrwal, now what we do we try to roused PIC which was stuck up at a certain opening , to achieve desire set point it will open or close visversa
at certain level of controller pressure it was suddenly open or close with jerking operation.
we just try to roused this control valve ( butterfly) and minimized jerk operation and we have notice from manual stroke checking that control valve is working smoothly between 50% to 75 % , so will try to keep maintaining opening of control valve in between this range by pinching o/l of cooling water valve, and finally have some relief and got desire purity with range of pressure drop indication (PDI) , tell me how it will happened? what will be the vapor velocity profile to overhead and bottom reboiling circulation rate and why it was deteriorated the condition , why bottom level of column not maintain before action taken,? tell me one things we have heating reboiler with hot condensate with FRC controlling at o/l of reboiler.