AS_6

CONTROLLO

tecnica

Settembre 2017

Automazione e Strumentazione

ity’s in the system. In

Figure 3

there is an example of a fuel

gas header pressure control before and after deploying the APC

solution. Benefits audited in this small 90 KBPD refinery were

more than $ 2.1 M per year.

Sulphur Complex Control

Sulphur complex, which is made up of multiple lines and equip-

ment is a very important part of the refinery and must work

properly. Any upsets in the complex, like ammine absorbers/

regenerators, sour water strippers (SWS), sulphur recovery

units (Claus), tail gas treaters (TGT) and incinerators, has a sig-

nificant and costly effect on the overall refinery and on the envi-

ronment. This is where APC applications can provide benefits.

For example, a constrained Claus unit should maximize feed

whenever possible as it permits increasing load to profitable

desulphurization units or even the overall refinery. The unit

should work ‘well’, recovering as much sulphur as possible and

maximizing conversion through a Dew Point Margin minimi-

zation. The unit should also minimize O2 consumption and all

other specific utilities consumptions (fuel, EE, and downstream

TGT unit load).

A key point is the

Dew Point Margin

reduction as this requires

explicit

Dew Point control

. Conversion is limited by reaction

equilibrium. The liquid sulphur forms at dew point in condens-

ers, but if it happens on catalyst it can reduce activity (reduces

conversion), increase pressure drop (increases pressure in

the boiler which is usually one of the major constraints) and

condense Sulphur in catalyst pores (shortens catalyst life and

requires expensive regeneration or rejuvenation procedures).

On the other hand, a Dew Point Margin that’s too high

will reduce conversion, increase energy consumptions

(temperature is too high) as well as increase down-

stream TGT consumptions and loads.

Best in class refiners are able to maintain around a

10-15 °C Dew Point Margin but it is not uncommon

to see sites where the margin is greater than 35 °C.

This is a large margin, paid heavily with reduced con-

version and increased consumptions. Not to mention

the case where a refinery is sulphur limited, causing

a loss in money measured in multiple millions of dol-

lars per year. Building first principle Dew Point infer-

entials permits to address these issues. Not an easy

task since sulphur condenses in catalyst pores and a

capillarity condensation effect occurs. The Dew Point

Limit is a function of many variables, including feed

quality, air flow, air humidity, pressure of the system,

reactor temperatures and more. These variables affect the par-

tial pressure of sulphur in a vapour phase over the catalyst and,

consequently, the related saturation temperature that is already

reduced by the presence of inert gases (N

, H

O, not reacted

S, etc.). The accurate first principles Dew Point inferential

can be built, but it requires appropriate competence and integra-

tion of both APC and kinetic modeling technologies.

Achievable benefits can vary significantly depending on the

specific layout and constraints distribution, but benefits are typ-

ically between $ 0.8 and 1.1 million per year for a small 100

KBPD refinery (not considering the case when the refinery is

sulphur limited where benefits can be measured in multi-mil-

lion dollars per year).

Steam Networks Control

A Steam Network can be quite complex with multiple pressure

headers. The major components include steam headers, steam

imports/exports, boilers/HRSGs, gas turbines, steam turbines,

let-down valves and vent valves.

By implementing APC technologies, facilities can experience a

number of benefits. These include:

- Improved steam headers pressure control by manip-

ulating boilers, turbines, and let-downs in order to

stabilize operations and maintain the required steam

production levels;

- Minimized downgrading steam through let-down

valves mainly by manipulating turbine loads, extrac-

tions and, whenever possible, boiler loads;

- Minimized vent flows by manipulating turbine loads

and extractions and, whenever possible/convenient,

boiler loads;

- Control of all constraints, including emission lim-

its during the transition, while optimizing boiler load

changes;

Figure 4 - Claus reactors conversion curve

Figure 3 - Fuel Gas pressure control before and after deploying the APC solution