San Francisco Refinery, Rodeo DISTILLATION CONTROL Dr. Prakash Karpe Control & Elec. Eng. Supt. ConocoPhillips San Francisco Refinery, Rodeo

Distillation Column Control Control Objectives Qc V L D Rectification Stages F R = L/D Stripping Stages QH B Two Control objectives Inventory control Composition control

Degrees of Freedom Analysis From control perspective, degrees of freedom of a process is defined as number of variables that can or must be controlled. Helps to avoid over- or under-control of processes. Degrees of freedom (to control) = No. of rationally placed control valves A control valve represents a manipulated variable (MV)

Degrees of Freedom Analysis Flash Vessel (Separator) F,T,P,xi Disturbances B

Inventory Control For steady state operation of a process, all inventories must be controlled Vapor inventories are maintained by pressure control Liquid inventories are maintained by level control

Degrees of Freedom Analysis Flash Vessel (Separator) PC F F,T,P,xi Disturbances LC B

Degrees of Freedom Analysis Typical Distillation Column Inventory Control PC V LD L D TD F TB QH LB LC B Degrees of Freedom = 3

Liquid Inventory Control Level Control Reflux drum level control LD - L or LD – D? Richardson’s rule: Use the largest stream to control level. Guidelines: L/D < = 1 : Use LD – D pairing L/D > = 5 : Use LD – L pairing For 1 < L/D < 5, use scheme proposed by Rysjkamp (L+D) – D and L/D – L pairings

Two Common Level Control Schemes Level control dilemma Tight flow control? Oscillating level Tight level control? Oscillating product flow Averaging or nonlinear level control Tight level control

Common Level Control Schemes Averaging (nonlinear) level control Used when product is a feed to a downstream process Examples Train of lightends columns Reflux drum level control Tight level control Used when product goes to tankage or a surge drum or process requires low hold up Use P-only controller with KC = 4 Reboiler level control FCC Main Frac and Vacuum column bottoms (coking concern) Dirty wash oil draw level control Control hydrostatic P in the draw line

Vapor Inventory Control Common Pressure Control Schemes Partial Condensers Off gas rate > 0 Common Problems If off gas is routed to a compressor, reflux drum P is controlled leading to tower P swings.

Common Pressure Control Schemes Partial Condensers Off gas rate > 0 or = 0 Common Problems If off gas is routed to a compressor, reflux drum P is controlled leading to tower P swings. Inert gas, typically noncondesables, can cause downstream process problems

Common Pressure Control Schemes Total Condensers Flooded Condenser Off gas rate = 0 Common Problems If P equalizing line is not used, P in the reflux drum swings. If condensed liquid is introduced into the drum from top w/o dip leg, vapor in the drum can collapse.

Common Pressure Control Schemes Total Condensers Hot Vapor Bypass Off gas rate = 0 Common Problems Bypass line inadequately sized If drum top surface is not insulated, P can swing with ambient changes. The effect is less pronounced for high P columns.

Degrees of Freedom Analysis Typical Distillation Column Composition Control PC V LD L D TD F TB QH LB LC B Degrees of Freedom = 3

Composition Control Problem Number of MV’s = 3 Reflux flow: L Distillate flow: D Reboiler heat: QH Reflux ratio Product/ feed ratio Steam/ feed ratio Need three controlled variables (CV’s) Possible CV’s Reflux drum level: LD Distillate composition: xD Appropriate temperature in rectification section (TD) Bottoms composition: xB Appropriate temperature in stripping section (TB) Control problem How do we pair CV’s and MV’s?

Composition Control Fundamental manipulated variables Feed split or cutpoint variable Fraction of the feed that is taken overhead of out of the bottom Increasing distillate flow will increase bottom purity and decrease distillate purity, etc. Fractionation variable Energy that is put into the column to achieve separation Increasing the reflux ratio or the reboiler duty will increase both distillate and bottoms purity Feed split has more pronounced impact on product purity than fractionation variable (exception low purity, < 90%, products) It is almost impossible to control any composition in the column if the feed split is fixed.

Manipulation of Fundamental Variables for Composition Control Fractionation Variables L/D QH/ F (steam to feed ratio) L/F High purity columns or dual product purity columns DeC3’s, DeC4’s, DIB’s, etc. Feed Split Variables D or B flow (direct control scheme) FCC Main Fracs, Crude and Vacuum column side cuts L or QH (indirect control scheme) Level adjusts the product flow indirectly

Controlled Variables for Composition Control Stage temperature (Inferential control) Useless for aij < 1.2 Online analyzer High economic gains aij < 1.2 Temperature control – Special cases Difficult separations ( 1.2 < aij < 1.5) Flat temperature profiles Use differential temperatures ( DT = Tm – Tk) between stages for control Example – HVGO quality control Extremely easy separations (high aij) Nonlinear in nature Steep temperature profile Use temperature profile control Tavg = (Tk + Tm)/ 2 , etc.

Composition Control Temperature Sensor Location Locate TI on the stage whose temperature shows maximum sensitivity to one of the available MV’s From simulation calculate (dTi / dD)L,B, (dTi / dL)D,B , (dTi / dB)L,D and (dTi / dQ)L,D where Ti is the temperature of stage i. Locate TI at the stage where (dTi / dD)L,B , etc., is maximum. For calculating the derivatives, vary B, D, L and Q in the column specs only by small amount, e.g., by +0.5% and -0.5%. Calculate average derivative. Scale each variable by dividing it by its span in order to calculate the derivatives. The derivative will be a dimensionless number. Use high precision numbers

Optimum Temperature Sensor Location Most common Mistake! TC

Optimum TI Location for Columns with Side Draws Locate the TI in the vapor space one – two stages below the product draw for product EP control This temperature (P-compensated) correlates well with the product EP Example Atmos column diesel 95% pt control

TI Location for Side Draw TC

Special Cases Draw Tray Control Total Draw Tray Control tray level by product draw Control pumpback on flow control Control p/a on flow control p/a duty as CV In fuel vacuum columns maximize duty LC FC LT

Special Cases Draw Tray Control Partial Draw Tray Level on the tray is fixed by the outlet weir height. There is no level control FC FC LT FC

Special Cases Stripping Steam Flow Bottom stripping steam Maximize to 8 – 12 lb stm per bbl of product Fixed flow control Side stripping steam Minimize to meat front end spec Use steam/ product ratio control

Distillation Control Case Study: Deisobutanizer Control Joyce Kaumeyer Sr. Consulting Engineer Prakash Karpe Control & Elec. Eng. Supt. ConocoPhillips San Francisco Refinery, Rodeo

Deisobutanizer

Tower Pressure Control Tower Temperature Control Composition Control Tower Operation Tower Pressure Control By Overhead Product Rate Tower Temperature Control Tray 45 By Condensate Level (Steam) Composition Control Operator Adjusts Reflux Rate Based on Lab / On-line Analyzer Tower Feed from Various Upstream Units Large Rate Swings

Deisobutanizer Control Objectives Control IC4 Product, IC4 Concentration Reduce Variability & Control Closer to Specification Improve Tower Pressure Control Reflux / Product Rate = 5 / 1 Change Existing Temperature / Composition Control Reduce NC4 Product, IC4 Concentration

Deisobutanizer Modified Controls

IC4 Product On-line Analyzer Vs. Delta Temperature Correlation

IC4 Product IC4 / Delta Temperature Correlation Process Dynamics Deadtime: 19 minutes Lagtime: 102 minutes

Modified Tower Operation Tower Pressure Control By Reflux Rate Tower Heat Input Control By Condensate Level (Steam) Composition Control Operator Adjusts TDIC Setpoint Based on Lab / On-line Analyzer Tower Feed from Various Upstream Units Large Rate Swings

Tower Pressure Control Before and After

IC4 Product %IC4

NC4 Product %IC4

Future ARC -OR- DMC Add AIC Cascaded to TDIC Requires improved analyzer performance Add Heat Input Feed-Forward to AIC -OR- DMC Hold for DCS platform conversion to Refinery Standard