Naphtha Cracker Unit Deploys Aspen HYSYS Dynamics Software Solution to Reduce Ethylene Losses

Customer Profile:

Reliance Industries Limited (RIL) is an Indian conglomerate holding company, with its headquarters in Mumbai, India. Reliance owns businesses across India engaged in energy, petrochemicals, textiles, natural resources, retail, and telecommunications. The company is ranked 215th on the Fortune Global 500 list of the world's biggest corporations as of 2016; and 8th among the Top 250 Global Energy Companies (Platts, 2016).

Naphtha Cracker Technology:

Globally, naphtha (a mixture of many different hydrocarbon compounds) is the predominant feedstock, including straight-run naphthas (SRN) from refinery crude units and naphtha derived from condensates during natural gas production. Cracking is a process in which large hydrocarbon molecules are broken down into smaller and more useful ones; it is conducted at high temperatures by two processes:

• Steam cracking which produces high yields of alkenes (unsaturated and have a double bond)

• Catalytic cracking in which a catalyst is employed and which produces high yields of branched and cyclic alkanes (saturated, with single bond)

   

The Process at RIL:

RIL’s Vadodara Manufacturing Division (VMD) is the oldest complex (commissioned in 1973) in the Reliance family and the first fully integrated petrochemical complex in India. The Naphtha Cracker plant is the mother plant of RIL-VMD, which produces ethylene and propylene as a product and the same is used as a raw material in downstream plants.

The cracking of naphtha in the naphtha cracker plant is done in short residence time (SRT) heaters by splitting it into fractions of pure olefinic molecules, aromatics, and polymerized heavies. There are six SRT heaters, with five being used for cracking naphtha and one used for processing recycled ethane. Cracked gases are cooled to 375o C in the transfer line exchanger by producing super-high-pressure steam at 100 kg/cm2g; then it is quenched to 175o C by circulating quench oil. Quench oil and heater effluent enter the gasoline fractionator, and the heavy oil gets stripped in this column. The cracked gases are then further quenched in a quench-water tower, where three streams of water are sprayed at different temperatures. Separation of hydrocarbon from water is done in a quench settler, which has three chambers: the first chamber separates circulating quench water, the second chamber has the process water, and the third chamber separates the liquid hydrocarbon (pyrolysis gasoline) from water.

The quench-water tower overhead vapors are compressed by a four-stage charge gas compressor. The condensed hydrocarbons from fourth-stage suction and discharge are recycled to the condensate stripper to strip off C2s (ethylene and ethane) and lighter components. These are again recycled to the fourth-stage suction drum.

The Problem Areas:

The feed quantity depends on the naphtha feed throughput of the plant - that is, the number of furnaces in operation. The product quality depends on naphtha quality as well as furnace operating conditions. In this process, the prime function of the condensate stripper is to ensure consistent C2s in the bottom stream by varying the column reboiling load. With the existing control philosophy, change in feed rate or composition was resulting in unstable operation of the condensate stripper, leading to higher slippage of C2s in the bottom stream. To improve operational stability and reduce C2 losses, RIL selected AspenTech’s Aspen HYSYS software solution.

Solution Deployment:

A steady-state model was developed to overcome the problem and the design and operating data were simulated to predict and improve performance. However, as steady-state simulation models cannot predict behavior with respect to time, a dynamic model was developed in Aspen HYSYS Dynamics software to study the process behavior for the existing control philosophy and proposed philosophy. Columns are an essential part of most processes and can have many variables that rapidly change during plant operation. Utilizing Aspen HYSYS Dynamics software, RIL could set up models to view and control the dynamic response of columns to ensure process safety and operability.

A new control philosophy was developed based on the response of controlled variables after disturbances in feed rate and composition. The responses to different disturbances indicated that with the existing control philosophy, the column becomes unstable for a longer time and product quality continues to deteriorate. Therefore, dynamic simulation proved to be useful for troubleshooting, testing the control philosophy, developing the new control philosophy, optimizing column operation, and ultimately ensuring the required product quality.

The End Result:

Without any major modifications in the system, the plant’s problem areas were satisfactorily addressed.

The Aspen HYSYS Dynamics software solution:

• Improved column stability

• Simulated scenarios for operator training

• Reduced wastage

• Helped troubleshoot plant performance problems

   

Financial Benefit:

Deploying this solution has helped RIL to reduce the loss of C2s in the bottom stream by about 700 ppm (parts per million), translating into savings of about $100,000 per year.

Source: This content has been derived from a technical paper contributed by KK Parmar and SK Patel, Reliance Industries Limited.

Keywords: Reliance Industries Limited (RIL), Aspen HYSYS Dynamics, Columns, Control Philosophy, Naphtha Cracker, ARC Advisory Group.