Where We Go Wrong In Distillation Towers

Dick Hawrelak

Presented to ES317Y in 1999 at UWO

Introduction

3% of large property damage losses are caused by failures in process towers.

Average Trended Loss was $53.8MM, the largest of all unit operations.

Distillation Tower Problems The following list of problems have

been drawn from my personal experience as a design engineer with Dow Chemical for 33 years (1960 - 1993).

Poor Mass Balances

Normal mass balance. Start-up, shut down or upset. Re-run of off-spec material.

Tower Variables

Poor understanding of tower variables, reflux, downcomers, reboilers, condensers.

VLE data can be confusing and misleading during design.

Tower configuration not the best - Seadrift tower explosion kills one operator.

Tower Controls

Process controls not adequate for upset conditions.

Often dual level controls are required on bottoms of towers.

Reboilers

Poor reboiler selection. High rate of fouling due to high

steam temperatures. Reboiler stall due to loss of level in

tower - Seadrift tower explosion. Reboiler lines too small and choke

recirculation.

•Condensers

Pressure drops higher than design due to fouling.

Hydrates form and plug condenser. Entrainment from tower not

considered in design. Inerts blanket condenser, failure to

remove heat cause PSVs to blow.

VCM Upset

VCM Process

VCM mb

HCL Dispersion

VCM Dispersion

Mechanical Design of Tall Towers

Tower shell thickness not suitable to prevent sway in high winds, tower topples.

Impurities In Tower Feed

Impurities not considered in design, exothermic reaction destroys tower - Seadrift

Flammable Inventory

Designers specify excessive inventory in reflux drum when tower logistics not well understood.

Large bottoms inventory may promote formation of unwanted polymerization products.

Tower Operation at Low Rates

Ballast tray units experience high rate of wear due to low unit reference. Broken valves plug downcomers or end up in pump suction causing further upsets.

Vacuum Operation

Designers too optimistic about air leakage. Vacuum pump or steam jets too small to pull required vacuum.

Poor management of pressure drop.

Recessed Sumps On Trays

Vendors often specify recessed sumps to save 6 inches on tower diameter.

Recessed sumps are perfect traps to collect solids and plug downcomers.

Packed Towers

Poor vapor and liquid distribution affects HETP. Dow wins $3MM lawsuit.

Vendors predict high HETP over wide range of flow. Customers beware.

Poor rangeability in packed tower operation.

Data Collection

Grab samples not suitable to check tower performance.

Require sample range over 24 hour period.

Troubleshooting difficult because of poor instrumentation.

Tower Internals

Flashing tower feeds not considered in feed tray design.

Feed distributors do not work. Demisters often required but are

ignored. Vortex breakers omitted in tower

bottoms.

Tower Optimization

Select a process sequence. Mass balances to match product

specs. Thermal condition of the feed. Minimum reflux ratio. Minimum no. of trays. No. trays versus reflux ratio.

Optimization Cont’d

Feed tray location. Tray efficiency. Valve tray tower design. Select tower internals. Repeat optimization for a packed

tower.

UC Configuration

Dow Configuration

Previous Exam Problem

The students were presented a paper on Union Carbide’s Seadrift, Texas, EO tower explosion. They were asked to comment on the explosion with respect to what they had learned in this safety course.

Other Possible Exam Questions Why do designers often specify

abnormally large flammable inventory in the reflux drum?

Why should the designer minimize the reboiler steam temperature in a flammable distillation tower?

What are the main problems with packed towers?

Summary

This short list is indicative of some of the problems caused by poor engineering discipline in distillation tower design.

Recommend you obtain a copy of the Chemical Plant Design programs and follow the procedures built into the spreadsheets.