Question

1.(a)Explain an application of closed loop temperature control and pressure controls system in an oil refinery (b)Describe the two processes with block diagrams. (c). Select the industrial standard components used to develop the above systems and specification of each components

Answer 1

Currently, the most of domestic enterprises move to the modern automation equipment and work with

advanced process control systems in practice using by technical abilities of microprocessor

technologies for the improvement technological and economical effect. Advanced process

control systems are wide class of system from extended regulators such as compensators, ratio control

systems, Smith predictors and others to multivariable control systems for huge technological objects. The last one includes in a lot of virtual analyzers all of that allow controlling the quality of end

products in automatic mode. The accumulated experience of application of advanced process control

systems suggests more than 50% decrease the typical deviation of the product specification in

comparison with control systems based on the classical control methods.

However, now a day classical control methodologies have not lost their relevance as the simplest in

the implementation and well-studied way of regulating technological parameters and are widely used

in control systems on the operational level. In work the problem of calculating the parameters of

typical regulators for technological processes with several control actions and several output

parameters is considered. An automated way of PI and PID regulators setting in the control system

with a centralized structure for dynamic objects characterized by the arbitrary dimension, cross links

and various delay in the control channels is proposed.

Description of the technological process in an oil-gas separator:

The crude-oil emulsion is a mixture of oil, associated petroleum gas and deposit water. It inputs in an

oil-gas separator either directly from a production oil well, but more often from an oil-dehydration

plant. The apparatus heats the emulsion for dehydrating of crude oil and then it knocks down the

oil.The target (output) parameter of a heating system in the the oil-gas separator is the oil emulsion

temperature, which is measured in a typical scheme of the control system and regulated by changing

the consumption of fuel gas. The total amount of oil emulsion suppling to the plant is controlled and

varies widely. As a rule, this parameter is not stabilized due to the specifics of the technological

process of a crude-oil treatment plant. It is one of the main perturbation influences for the heater

control system. The ratio of the amount of water and oil in the crude-oil emulsion is set by the

hydrometric content and also widely varies, affecting on the quality of the regulation.

In order to heat the crude oil emulsion in the oil-gas separator, gas is supplied to the gas-fired

burners (in practice, oil-well gas is given off the crude oil in the oil-gas separator and then it supplies

to the gas burners). The flow, temperature and pressure of the gas are measured, the flow rate is

regulated. Due to an uncontrolled change in the gas composition, the calorific value of the product will

be to undergo change.

The air necessary for combustion of fuel gas supplies to the combustion zone naturally. Air

parameters namely temperature and humidity affect the process heating the crude oil emulsion, but, as

a rule, these parameters are not included in the regulating scheme and treated as additional

uncontrolled perturbation influences.

The listed (the list is not complete) factors lead to lower quality of the regulation the oil emulsion

heating and also the dehydration process too in traditional automatic control systems. As a result, in

general the efficiency of the oil production and treating processes is reduced. Thus, the application of

modern control methodologies for the control of the oil-gas separator becomes relevant.

Dynamics of the heating process the crude oil emulsion:

The dynamics of thermal processes in the heater is quite complicated. A lot of assumptions and

simplifications have been done in the mathematical model synthesis of the technological process of the

crude oil emulsion heating.

The process of heat transfer from combustion gases to the crude oil emulsion flow is carried out

through the thickness and surface of the flame tubes, therefore, the regularities of thermal conductivity

and heat transfer will be characteristic for this process. A thermocouple measuring the output

temperature of the emulsion is installed in the immediate proximity to the flame tube. For this reason,

the regularities of convective heat transfer in the heating zone will not be taken into account for

simulation the heating dynamics along the fuel gas control channel.

As opposed to the fuel gas, the emulsion flow supplying the preheater performs is heated through

the convective heat transfer exchange, i.e. mixing with the already warmed-up emulsion layers. The

direct contact of the input streams with the flame tubes is constructively excluded. As a result, the

mixing intensity factors, estimated by the period of time of the particles in the plant, acquire additional

significance.

Taking into account the short analysis of the technological process of the emulsion heating and

recommendations, the dynamics of the oil-gas separator along the emulsion heating channel will

be realized as a sequence of aperiodic dynamic elements of the first order.

The value of the target parameter namely the temperature of the crude oil emulsion at the output of the

heating section is determined from the equation:

tem out=tem in+0.01Vgas real n RELcaloric / Gem real Cem

The following input signals support to the simulation model:

– Volume flow of fuel gas in the heating section V_gas_real, m3/h;

– Mass consumption of oil emulsion in the heating section G_em_real, t/h;

– Temperature of oil emulsion on the input to the heating section t_em_in (Твх), °С;

– Relative calorific value of fuel gas Rel_caloric, MJ/m3;

– Heat capacity of the oil emulsion C_em is the, MJ/t °С.

Two-loops cascade control system of the oil emulsion temperature

In order to improve the control quality of the oil-gas separator, we offered stabilizing the

ration of flows with correction to the target parameter of the control object. In contrast to the

traditional regulating scheme of the crude oil emulsion temperature in the heating section of the plant,

we propose to stabilize a ratio of the virtual values. The leading control channel in this system is set a

ratio which is connected with the heat flow necessary for the emulsion heating. The slave control

channel is minimized the unbalanced signal regulating the heat flow associating with the supply rate of

the fuel gas.

The functional diagram of the advanced process control system for regulating the crude oil

temperature T_em_out in the oil-gas separator is shown in Fig. In the discussed system the main

perturbation influences are the change in the composition and density of the fuel gas, the fluctuations

in the flow rate and the hydrometric content of the emulsion. Data is transmitted through two control

channels, namely the calorific value of the gas and the heat content of the emulsion. And then the

control system compensates the listed perturbation influences. The errors of virtual analyzers and

uncontrolled disturbances for example, daily fluctuations associated with changes in ambient

temperature, are taken into account by the main regulator.

Oligas separator fuel gas emulsion Error Ratio Slave Master controller Calculation of the ratio Ratio

Advanced process control system structure

To stabilize the ratio REgas/Eem, the main controlled perturbations such as the rate flow and

hydrometric content of the emulsion, and also the gas density should not affect the emulsion

temperature at the output of the oil-gas separator. If, due to an uncontrolled change in the thermal

characteristics of the plant, the output variable T_em_out is deviated from the set value T_em_set, the main

controller automatically connects to the control process and changes the set point Ratio_set .

To exclude the "rocking" of the proposed cascade control system, it is necessary to prohibit the

simultaneous start-up of regulators during the simulation. When the oil-gas separator control system

starts up, the ratio controller operates the process to the set point given by the master controller. To

realize this requirement, a different sampling time for master and slave regulators was used in the

simulation model of the control object.

Finally,

1. The virtual analyzers for calculating the qualitative indicators of input flows, namely the total

heating value of the fuel gas flow; the heat content (amount of heat consumed) of the oil emulsion and

also the ratio of these parameters.

2. The slave regulator controls the ratio between the total heating value of the fuel gas flow and

the amount of heat necessary to heat the emulsion to a given temperature. The output signal from this

regulator operates the valve installed on the fuel gas pipeline.

3. The master regulator generates a set point for the slave regulator.

The proposed advanced process control system is intended for the two-loop cascade regulation the

oil emulsion heat through the ratio of heat flows. The adequacy of the described above solutions has

been verified through the simulations in the Matlab software. The maximum effect is achieved under

the condition that perturbations affecting the master regulator will appear much less frequently than

disturbances of the slave regulator. For example, diurnal fluctuations in the thermal characteristics of

the oil-gas separator are taken place more rarely than the hourly pulsations of the load of the emulsion

discharge. The settings of the master and slave regulators should be selected in such a way that the

upper regulator "does not swing" the lower one.