Melamine Control Scheme
Melamine plant uses low-pressure urea one-stepprocess. With urea as raw materials, through melting, washing, reaction,crystallization, filtration, packing and other steps, exhaust gas is returnedto urea plant after being recovered, which not only saves energy but alsoreduce pollution.
II.Introduce to Main Device Process
Solid-state urea is transported to urea melttank V101 by conveying fan, and it goes into urea washing tower after beingheated and melted, then it is transported to reactor R201 by pump for reaction,and some is returned to the top of T102 and washed with process gas from V304. Afterbeing separated, some of the process gas generated in T102 goes into crystallizerR301 as crystalline cooling air; the other part goes into C102and then goes into carrier gas distribution tank after being compressed to 0.15MPA,some of which becomes exhaust gas, and some becomes the carrier gas of reactor R201.Melting urea in R201 reacts under the action of catalysts, the generated melaminegas goes into hot gas filter V301Aand B with NH3, CO2 and other impurities for impurity filtering, and then itgoes into crystallizer R301 to crystallize triamine into solid state, goes intocyclone separator V304 for gas-solid separation, and then goes into finishedproduct bin V503 for finished product packing.
III.Process of Exhaust Part
The exhaust gas from triamine main devicesfirst goes into three-level unbalanced absorption system, absorb most of theammonia and a small part of CO2 into absorption liquid, and flows into storagetank V701 of absorption liquid. The gas not absorbed goes into washing tower T601and it is washed by desorption liquid from medium-pressure separation system.The liquid in tower reactor goes into three-level absorption liquid storagetank V606. The absorption liquid sent by unbalanced system goes into carbonseparation tower T703, most of CO2 is desorbed to enter gas phase anddischarged to the atmosphere from the top of the tower, and absorption liquidgoes into desorption tower T701.
Desorption tower T701 heatsammonia and CO2 into gas state with steam and then condense them through fractionalcondenser. Condensed liquid goes into absorber E708B, and gas flows intoabsorber E208A.Gas flows through E708A, E708B,E708C and ammonia rectificationtower T702 in order, liquid flows into it from the top of T702 tower inreserve direction, flowing through E708C,E708B and E708Aand going into V705. Ammonia from the top of T702 tower flows into liquidammonia storage tank V704. The gas not condensed flows into T704 washing tower T704and it is washed by desorption waste water from the top of the tower, most ofliquid in tower reactor reflows to the middle part of T704, and a small partbecomes the return flow liquid of rectification tower T702. Desorption towerT701 desorbs waste water, some of which is transported to T703 and T704, andthe other part returns to T601, and excess desorption waste water isdischarged.
IV. KeyControl Loop
The control loop of triamine system isrelatively simple, mainly including 65 sets of single-loop control, 8 sets of cascadecontrol loop, a set of feedforward feedback control and aset of two-impulse control.
1. Feedforward feedback control
The concept of feedforward control is tomeasure the interference into the process, create proper control action accordingto its signal to change manipulated variables, so as to maintain the controlledvariables at given values. To make it combine with feedback control and makecontrol system more stable.
Triamine system uses this kind of control infeeding. Feeding amount FI0202 is taken as interference signal, and controlsystem pressure is PI0119.
2. Two-impulse molten salt steam drum levelcontrol.