how to compensate pressure changes in temperature control

7/17/2019 How to Compensate Pressure Changes in Temperature Control

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How to Compensate Pressure Changes in Temperature ControlTemperature control is an essential conditions of many industrial processes. Pressure changes, nevertheless can quite

drastically interface with temperature monitoring. Here’s a look at some controls that when used correctly give the desired

results...

In the operation of distillation columns, feedback controllers controlling compositions of product streams are employed to operate

columns at desired product specifications. The process variable for these feedback composition controllers, are actual stream

compositions measured either on-line or in laboratories. Main drawback of this technique is high dynamic lag and large time

constant of analyser system loop.

Alternatively, Tray temperature is often used as process variable to calculate stream compositions rather than complex analyser

systems. Temperature control is an easy and inexpensive way of composition control as it uses high reliability and low

maintenance measuring element. However, temperature control can suffer from pressure variations in the column, as column

temperature can change due to variations in column pressure at fixed composition.

Changing Pressure Affects Temperature ControlTemperature controller may interpret change in column pressure as change in composition and would send a corrective signal.

This false signal could lead to disturbances in the column operation. Generally with columns operating under high pressures,

change in pressure doesn’t have considerable impact on temperature control. Effect of change of column pressure on temperature

control is more prominent in low pressure columns and especially in columns operating under vacuum.

lso, effect of pressure change in case of close separation columns is more pronounced as temperature variation with composition is

small and even small effect of pressure change on temperature can appear relatively large. Thus, in such cases pressurecompensation is provided to temperature control. The article illustrates pressure compensated temperature calculations with

industrial examples of Deisobutanizer and Debutanizer columns in refinery applications.

When pressure compensation is provided, it is required to generate an equation which can give satisfactory correlation betweencolumn pressure and control temperature i.e. vapour pressure and saturation temperature. Vapour pressure is, in general, a

nonlinear function of temperature and composition. Most vapour-pressure equations are empirical equations derived from

integration of the Clausius-Clapeyron equation. These equations are for pure components and coefficients are obtained by

experimental procedures.

Use of linear equation is also reported for this purpose; however, this has a low accuracy and is correct for small variations only.For this particular case study, well known Antoine’s equation is used. The methodology described in the article by Jerald Lin slley

is used here i.e. vapour pressure (Antoine’s) equation for pure component is applied directly to a mixture and empirica l

coefficients are determined as if the mixture were some pseudo- pure component. In the current text, Antoine’s equation islinearized and coefficients are estimated using multivariable linear regression data analysis tool of MS Excel 2010.

System Description: There are four distillation columns in one of the refinery units, operating at various pressures. Out of thesefour columns pressure compensation is provided for two columns only, i.e. Deisobutanizer and Debutanizer, which operate at

comparatively lower pressure. These columns are explained in detail below.

Temperature Control in Isobuatne Separation

1. Deisobutanizer: Deisobutanizer (DIB) is a superfractionator that separates isobutane (i-C4) from normal butane (n-C4) plus

heavier products. The DIB tower has 80 trays which are required to separate the close-boilingpoint components, isobutane and

normal butane. Ideally, it is desired to produce the highest purity isobutane in the distillate product and minimize isobutane losses

to the bottoms. DIB has three feeds from various sources. DIB has an air-cooled total condenser and thermosyphon reboiler. Refer

figure 1 for schematic of DIB. Operating conditions of Deisobutanizer are mentioned in table 1.

DIB Control Schemes



Pressure control: The tower pressure is controlled by partially flooding tube bundle of an air cooled condenser by operating a

butterfly valve located at the condenser outlet. If the overhead pressure increases above the set point, the butterfly valve opens and

exposes more heat transfer surface allowing more overhead gas to condense; conversely, if the overhead pressure falls below theset point, the butterfly valve closes, submerging more tubes and allowing less gas to condense, which leads to increase in the

pressure.

Pressure Control is Essential

Temperature control: Tower heat balance is maintained by controlling the pressure compensated temperature of the vapour

stream of the column bottom tray. This controller resets the flow of low pressure steam to reboiler via direct acting flowcontroller.

This controller in turn achieves proper reboiler duty by adjusting the set point of condensate pot level controller, which exposes

proper number of tubes and thus the proper amount of heat transfer surface for condensation in the reboiler.

Deisobutanizer Column – PCT – ControlProblem statement – Inferential temperature control is employed in Deisobutanizer which is separating isobutane from the mixture

of butane and heavier hydrocarbons. Temperature transmitter is located at the first tray from the bottom in the column. Pressure

compensation is provided to eliminate the effects of pressure variations in the column.

Pressure transmitter used for pressure compensation is also located at the bottom most tray. Correlation of pressure – temperature

has to be developed and values of coefficients in the equation are to be determined.

Analysis: As explained above, in the current scenario, Antoine’s equation for pure component is applied to the mixture of

hydrocarbons. Vapour pressure and corresponding saturation temperature data for the tray composition is required to determinethe coefficients of the equation. Here, since the temperature transmitter is located on the first tray, bottom product composition is

considered for calculations. Since this is a grass root set up and no operating data is available, the required bubble point - pressure

data is generated for the bottom product composition using Pro-II (table 2) simulation.

The Effect of Steam Pressure on Temperature Control

Effect of changes in Deisobutanizer column pressure on temperature is studied. It is observed from table 2 that column

temperature may vary by about 70C i.e. from 121 to 1280C even if pressure varies by (+/-) 0.5 Kg/cm2 from operating pressure.

The pressure variations could disturb the column temperature control system. Thus pressure compensation is essential to nullify

the effect of pressure change.

Antoine’s equation is given by

ln P = A - B / (T + C) (1)

Antoine’s equation is linearized to eq. 2

ln P = A + (AC-B) / T - C (ln P)/T (2)

Coefficients of the above equation are determined using multivariable linear regression data analysis tool of MS Excel 2010.

Antoine’s equation can be re-written in the form mentioned below

WherePatm = Atmospheric Pressure

Pmeasured = Measured column Pressure (kg/cm2g)Tcorrected = Saturation Temperature corresponding to Pmeasured (0C)

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Results: Equation 3 is used to get corrected temperature after pressure compensation. Temperature controller compares this

corrected temperature with actual measured temperature and takes corrective action. The values of the coefficients of the equation

are calculated and mentioned in table 3.

Table 3: Antoine’s equation coefficients for Deisobutanizer column PCT control (Source: Aker Solutions)

The values of the coefficients are derived from the simulated data in the absence of actual plant operating data. Thus thesecoefficients need to be fine-tuned during operation.

Vapour pressure vs. Temperature data from PRO II and from equation 3 with coefficients from table 3 are plotted in figure 2.

From the plot it is evident that the vapour pressure data calculated from equation 3 is in close agreement with PRO II simula tion.

Vapour Pressure vs. Temperature

2. Debutanizer: The Debutanizer separates normal butane (n-C4) from C6 plus heavier hydrocarbons. Normal butane is producedas an overhead product and is sent to tankage. The large difference in boiling points makes this a relatively easy separation.

Debutanizer has an air-cooled total condenser and thermosyphon reboiler. Refer figure 3 for schematic of Debutanizer column.

Operating conditions of Debutanizer are mentioned in table 4.

Table 4: Debutanizer column operating conditions (Source: Aker Solutions)

Debutanizer control schemes: Control scheme of debutanizer is similar to that of Deisobutanizer column.

Temperature control: Tower heat balance is maintained by controlling the pressure compensated temperature of the downcomer

liquid from the sixth tray from bottom. This controller resets the flow of medium pressure steam to the reboiler via direct actingflow controller. This controller in turn achieves proper reboiler duty by adjusting the set point of condensate pot level controller,

which exposes proper number of tubes and thus the proper amount of heat transfer surface for condensation in the reboiler.

Debutanizer column- PCT Control: Problem statement – Inferential temperature control is employed in Debutanizer column

which is separating butane from the mixture of heavier hydrocarbons. Temperature transmitter is located at the sixth tray from the

bottom in the column. Pressure compensation is provided to eliminate the effects of pressure variations in the column. Pressure

transmitter used for pressure compensation is located at the bottom most tray. Pressure – temperature correlation is to be

developed and values of coefficients in the equation are to be determined.

Analysis: In this scenario, tray composition where the temperature transmitter is located (i.e. sixth tray from bottom) would be

different than bottom product composition. Thus Debutanizer column is simulated using PRO II to get the composition at sixth

tray. For this composition, required bubble point - pressure data is generated using Pro II and is mentioned in table 5.

Effect of changes in Debutanizer pressure on temperature is studied. It is observed from the table 5 that column temperature may

vary by about 80C i.e. from 102 to 1100C even if pressure varies by (+/-) 0.5 Kg/cm2 from operating pressure. Thus pressure

compensation is essential to nullify the effect of pressure change.



Table 5: Bubble point – Pressure data for Debutanizer column from Pro II (Source: Aker Solutions)

Pressure Drops Within the Column

Antoine’s equation (eq. 3) can be updated for debutanizer PCT as below –

Controlling process parameters is vital to satisfactory operation of critical fractionators (Source: Aker Solutions)

Where, dp = Pressure drop from pressure measurement to temperature point (kg/cm2) (assumed constant) (It is subtracted fromPmeasured as pressure measurement point is below the temperature point in the column).

When the control temperature and control pressure are measured at two different locations in the column, pressure drop between

the measuring points needs to be added or subtracted depending on the location of respective transmitters. E.g. if the pressure

transmitter is located below the temperature transmitter, pressure drop across respective transmitters shall be subtracted to get thecolumn pressure at the tray where temperature transmitter is located. In case of Deisobutanizer column, pressure transmitter and

temperature transmitter were located at the same tray, thus dp term was set to zero.

Fine Tuning Required for Pressure Control

Results: Antoine’s equation (eq. 4) is used to get corrected temperature after pressure compensation.

Temperature controller compares this corrected temperature with actual measured temperature and takes corrective action. The

values of the coefficients of the equation are determined from regression data analysis tool of MS Excel 2010 and are mentioned

in table 6. Pressure drop term “dp” is worked out from actual vendor data. Since values of these coefficients are derived from thesimulated data and not from actual plant operating data, these coefficients need to be fine-tuned during actual operation of

column.

Controlling process parameters is vital to satisfactory operation of critical fractionators (Source: Aker Solutions)

Vapour pressure vs. Temperature data from PRO II and from Antoine’s equation with coefficients from table 6 are plotted in

figure 4. From the plot it is evident that the vapour pressure data calculated f rom Antoine’s equation is in close agreement with

PRO II simulation.

Calcuation withA ntoine's Equation – An Inexpensive Solution

Major drawback associated with inferential temperature control is misinterpretation of temperature change as a composition

change due to pressure variations in distillation columns. This drawback can be overcome by implementing an inexpensive

pressure compensated temperature control technique as illustrated in this text. Antoine’s equation can be used effectively for

calculation of pressure compensated temperature for hydrocarbon systems.

how to compensate pressure changes in temperature control

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